U.S. patent application number 11/811945 was filed with the patent office on 2008-11-13 for time division duplex wireless network and associated method using connection modulation groups.
This patent application is currently assigned to Access Solutions Ltd.. Invention is credited to Michael S. Eckert, Kirk J. Griffin, Russell C. McKown, Paul F. Struhsaker.
Application Number | 20080279123 11/811945 |
Document ID | / |
Family ID | 25487192 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080279123 |
Kind Code |
A1 |
Struhsaker; Paul F. ; et
al. |
November 13, 2008 |
Time division duplex wireless network and associated method using
connection modulation groups
Abstract
A wireless network is provided that includes a base station and
subscriber stations that communicate with the base station using
radio frequency (RF) time division duplex (TDD) signaling. The base
station may establish medium access control (MAC) connections with
each station. The base station monitors communications with the
stations and, in accordance, assigns stations or MAC connections to
modulation groups. The base station transmits signals on MAC
connections or to stations in a modulation group in adjacent TDD
slots within a TDD frame. The base station may receive access
requests from the stations, evaluate traffic requirements for the
stations, and determine a longest downlink portion for the
stations. The base station then allocates downlink and uplink
portions of a TDD frame according to the length of the longest
downlink portion.
Inventors: |
Struhsaker; Paul F.;
(Austin, TX) ; Griffin; Kirk J.; (Columbia,
MD) ; McKown; Russell C.; (Richardson, TX) ;
Eckert; Michael S.; (Austin, TX) |
Correspondence
Address: |
DOCKET CLERK
P.O. DRAWER 800889
DALLAS
TX
75380
US
|
Assignee: |
Access Solutions Ltd.
Dallas
TX
|
Family ID: |
25487192 |
Appl. No.: |
11/811945 |
Filed: |
June 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09948059 |
Sep 5, 2001 |
7230931 |
|
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11811945 |
|
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Current U.S.
Class: |
370/280 |
Current CPC
Class: |
H04M 3/42 20130101; H04B
7/0617 20130101; H04L 1/0005 20130101; H04L 1/0083 20130101; H04L
1/0006 20130101; H04J 3/1694 20130101; H04L 5/1453 20130101; H04L
1/007 20130101; H04W 72/0446 20130101; H04L 1/16 20130101; H04L
2001/125 20130101; H01Q 25/00 20130101; H04W 28/04 20130101; H01Q
1/246 20130101; H04L 5/1469 20130101; H04L 12/10 20130101; H04J
3/14 20130101; H04L 1/0042 20130101; H04B 7/022 20130101; H04L
12/403 20130101; H04L 12/66 20130101; H04W 84/12 20130101; H04W
88/021 20130101; H04W 16/28 20130101; H04M 2242/04 20130101; H04W
88/08 20130101; H04L 1/004 20130101; H04L 1/0009 20130101; H04M
2207/206 20130101; H04M 2242/06 20130101; H04L 2001/0098 20130101;
H04W 28/18 20130101; H04W 84/042 20130101; H04W 84/14 20130101;
H04W 74/00 20130101; H04L 1/0017 20130101 |
Class at
Publication: |
370/280 |
International
Class: |
H04J 3/00 20060101
H04J003/00 |
Claims
1-29. (canceled)
30. A method for use in a wireless network comprising a base
station and a plurality of subscriber stations in wireless, radio
frequency (RF), time division duplex (TDD) communication with the
base station, the method comprising: for each of the subscriber
stations, establishing a plurality of associated medium access
control (MAC) connections on a RF communication link between the
base station and the subscriber station; assigning a connection
identifier (CID) to each of the MAC connections; monitoring
communication traffic on the MAC connections; setting for each of
the MAC connections a first controllable characteristic of the RF
communication link from the base station to the associated
subscriber station based upon the monitored communication traffic;
assigning each of the MAC connections to a modulation group
according to the first controllable characteristic for the MAC
connection; and transmitting signals from the base station on MAC
connections in a modulation group in adjacent TDD slots within a
TDD frame.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority to
copending U.S. patent application Ser. No. 09/948,059 filed on Sep.
5, 2001 and entitled "WIRELESS ACCESS SYSTEM USING SELECTIVELY
ADAPTABLE BEAM FORMING IN TDD FRAMES AND METHOD OF OPERATION". The
present application may share common subject matter and figures
with the above United States patent application, which is
incorporated herein by reference for all purposes as if fully set
forth herein
[0002] This application claims priority to: provisional U.S. Patent
Application Ser. No. 60/262,712 filed on Jan. 19, 2001 and entitled
"WIRELESS COMMUNICATION SYSTEM USING BLOCK FILTERING AND FAST
EQUALIZATION DEMODULATION AND METHOD OF OPERATION"; provisional
U.S. Patent Application Ser. No. 60/262,825 filed on Jan. 19, 2001
and entitled "APPARATUS AND ASSOCIATED METHOD FOR OPERATING UPON
DATA SIGNALS RECEIVED AT A RECEIVING STATION OF A FIXED WIRELESS
ACCESS COMMUNICATION SYSTEM"; provisional U.S. Patent Application
Ser. No. 60/262,698 filed on Jan. 19, 2001 and entitled "APPARATUS
AND METHOD FOR OPERATING A SUBSCRIBER INTERFACE IN A FIXED WIRELESS
SYSTEM"; provisional U.S. Patent Application Ser. No. 60/262,827
filed on Jan. 19, 2001 entitled "APPARATUS AND METHOD FOR CREATING
SIGNAL AND PROFILES AT A RECEIVING STATION"; provisional U.S.
Patent Application Ser. No. 60/262,826 filed on Jan. 19, 2001 and
entitled "SYSTEM AND METHOD FOR INTERFACE BETWEEN A SUBSCRIBER
MODEM AND SUBSCRIBER PREMISES INTERFACES"; provisional U.S. Patent
Application Ser. No. 60/262,951 filed on Jan. 19, 2001 entitled
"BACKPLANE ARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS
SYSTEMS"; provisional U.S. Patent Application Ser. No. 60/262,824
filed on Jan. 19, 2001 entitled "SYSTEM AND METHOD FOR ON LINE
INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS AND WIRELINE ACCESS
SYSTEMS"; provisional U.S. Patent Application Ser. No. 60/263,101
filed on Jan. 19, 2001 entitled "SYSTEM FOR COORDINATION OF TDD
TRANSMISSION BURSTS WITHIN AND BETWEEN CELLS IN A WIRELESS ACCESS
SYSTEM AND METHOD OF OPERATION"; provisional U.S. Patent
Application Ser. No. 60/263,097 filed on Jan. 19, 2001 and entitled
"REDUNDANT TELECOMMUNICATION SYSTEM USING MEMORY EQUALIZATION
APPARATUS AND METHOD OF OPERATION"; provisional U.S. Patent
Application Ser. No. 60/273,579 filed Mar. 5, 2001 and entitled
"WIRELESS ACCESS SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK AND
DOWNLINK OF TDD FRAMES AND METHOD OF OPERATION"; provisional U.S.
Patent Application Ser. No. 60/262,955 filed Jan. 19, 2001 and
entitled "TDD FDD AIR INTERFACE"; provisional U.S. Patent
Application Ser. No. 60/262,708 filed on Jan. 19, 2001 and entitled
"APPARATUS, AND AN ASSOCIATED METHOD, FOR PROVIDING WLAN SERVICE IN
A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM"; Ser. No. 60/273,689,
filed Mar. 5, 2001, entitled "WIRELESS ACCESS SYSTEM USING MULTIPLE
MODULATION FORMATS IN TDD FRAMES AND METHOD OF OPERATION";
provisional U.S. Patent Application Ser. No. 60/273,757 filed Mar.
5, 2001 and entitled "WIRELESS ACCESS SYSTEM AND ASSOCIATED METHOD
USING MULTIPLE MODULATION FORMATS IN TDD FRAMES ACCORDING TO
SUBSCRIBER SERVICE TYPE"; provisional U.S. Patent Application Ser.
No. 60/270,378 filed Feb. 21, 2001 and entitled "APPARATUS FOR
ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS
COMMUNICATION SYSTEM"; provisional U.S. Patent Application Ser. No.
60/270,385 filed Feb. 21, 2001 and entitled "APPARATUS FOR
REALLOCATING COMMUNICATION RESOURCES TO ESTABLISH A PRIORITY CALL
IN A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM"; and provisional
U.S. Patent Application Ser. No. 60/270,430 filed Feb. 21, 2001 and
entitled "METHOD FOR ESTABLISHING A PRIORITY CALL IN A FIXED
WIRELESS ACCESS COMMUNICATION SYSTEM. The above Provisional U.S.
patent applications are incorporated herein by reference for all
purposes as if fully set forth herein.
[0003] The present application may share common subject matter and
figures with the following United States patent applications, which
are incorporated herein by reference for all purposes as if fully
set forth herein: [0004] 1) Copending Ser. No. 10/042,705, filed on
Nov. 15, 2000, entitled "SUBSCRIBER INTEGRATED ACCESS DEVICE FOR
USE IN WIRELESS AND WIRELINE ACCESS SYSTEMS"; [0005] 2) Ser. No.
09/838,810, filed Apr. 20, 2001, entitled "WIRELESS COMMUNICATION
SYSTEM USING BLOCK FILTERING AND FAST EQUALIZATION-DEMODULATION AND
METHOD OF OPERATION", now U.S. Pat. No. 7,075,967; [0006] 3) Ser.
No. 09/839,726, filed Apr. 20, 2001, entitled "APPARATUS AND
ASSOCIATED METHOD FOR OPERATING UPON DATA SIGNALS RECEIVED AT A
RECEIVING STATION OF A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM",
now U.S. Pat. No. 7,099,383; [0007] 4) Copending Ser. No.
09/839,729, filed Apr. 20, 2001, entitled "APPARATUS AND METHOD FOR
OPERATING A SUBSCRIBER INTERFACE IN A FIXED WIRELESS SYSTEM";
[0008] 5) Ser. No. 09/839,719, filed Apr. 20, 2001, entitled
"APPARATUS AND METHOD FOR CREATING SIGNAL AND PROFILES AT A
RECEIVING STATION", now U.S. Pat. No. 6,947,477; [0009] 6) Ser. No.
09/838,910, filed Apr. 20, 2001, entitled "SYSTEM AND METHOD FOR
INTERFACE BETWEEN A SUBSCRIBER MODEM AND SUBSCRIBER PREMISES
INTERFACES", now U.S. Pat. No. 6,564,051; [0010] 7) Copending Ser.
No. 09/839,509, filed Apr. 20, 2001, entitled "BACKPLANE
ARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS SYSTEMS";
[0011] 8) Ser. No. 09/839,514, filed Apr. 20, 2001, entitled
"SYSTEM AND METHOD FOR ON-LINE INSERTION OF LINE REPLACEABLE UNITS
IN WIRELESS AND WIRELINE ACCESS SYSTEMS", now U.S. Pat. No.
7,069,047; [0012] 9) Ser. No. 09/839,512, filed Apr. 20, 2001,
entitled "SYSTEM FOR COORDINATION OF TDD TRANSMISSION BURSTS WITHIN
AND BETWEEN CELLS IN A WIRELESS ACCESS SYSTEM AND METHOD OF
OPERATION", now U.S. Pat. No. 6,804,527; [0013] 10) Ser. No.
09/839,259, filed Apr. 20, 2001, entitled "REDUNDANT
TELECOMMUNICATION SYSTEM USING MEMORY EQUALIZATION APPARATUS AND
METHOD OF OPERATION", now U.S. Pat. No. 7,065,098; [0014] 11) Ser.
No. 09/839,457, filed Apr. 20, 2001, entitled "WIRELESS ACCESS
SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK AND DOWNLINK OF TDD
FRAMES AND METHOD OF OPERATION", now U.S. Pat. No. 7,002,929;
[0015] 12) Ser. No. 09/839,075, filed Apr. 20, 2001, entitled "TDD
FDD AIR INTERFACE", now U.S. Pat. No. 6,859,655; [0016] 13)
Copending Ser. No. 09/839,499, filed Apr. 20, 2001, entitled
"APPARATUS, AND AN ASSOCIATED METHOD, FOR PROVIDING WLAN SERVICE IN
A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM"; [0017] 14) Ser. No.
09/839,458, filed Apr. 20, 2001, entitled "WIRELESS ACCESS SYSTEM
USING MULTIPLE MODULATION" (Docket No. WEST14-00026); [0018] 15)
Ser. No. 09/839,456, filed Apr. 20, 2001, entitled "WIRELESS ACCESS
SYSTEM AND ASSOCIATED METHOD USING MULTIPLE MODULATION FORMATS IN
TDD FRAMES ACCORDING TO SUBSCRIBER SERVICE TYPE", now U.S. Pat. No.
6,891,810; [0019] 16) Copending Ser. No. 09/838,924, filed Apr. 20,
2001, entitled "APPARATUS FOR ESTABLISHING A PRIORITY CALL IN A
FIXED WIRELESS ACCESS COMMUNICATION SYSTEM"; [0020] 17) Ser. No.
09/839,727 filed Apr. 20, 2001 and entitled "APPARATUS FOR
REALLOCATING COMMUNICATION RESOURCES TO ESTABLISH A PRIORITY CALL
IN A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM", now U.S. Pat. No.
7,031,738; [0021] 18) Ser. No. 09/839,734, filed Apr. 20, 2001,
entitled "METHOD FOR ESTABLISHING A PRIORITY CALL IN A FIXED
WIRELESS ACCESS COMMUNICATION SYSTEM", now U.S. Pat. No. 7,035,241;
and [0022] 19) Ser. No. 09/839,513, filed Apr. 20, 2001, entitled
"SYSTEM AND METHOD FOR PROVIDING AN IMPROVED COMMON CONTROL BUS FOR
USE IN ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS AND
WIRELINE ACCESS SYSTEMS", now U.S. Pat. No. 6,925,516.
[0023] The above provisional and non-provisional applications are
commonly assigned to the assignee of the present invention.
TECHNICAL FIELD
[0024] The present disclosure is directed, in general, to wireless
access systems and, more specifically, to a burst packet
transmission media access system for use in a wireless access
network.
BACKGROUND
[0025] Telecommunications access systems provide for voice, data,
and multimedia transport and control between the central office
(CO) of the telecommunications service provider and the subscriber
(customer) premises. Prior to the mid-1970s, the subscriber was
provided phone lines (e.g., voice frequency (VF) pairs) directly
from the Class 5 switching equipment located in the central office
of the telephone company. In the late 1970s, digital loop carrier
(DLC) equipment was added to the telecommunications access
architecture. The DLC equipment provided an analog phone interface,
voice CODEC, digital data multiplexing, transmission interface, and
control and alarm remotely from the central office to cabinets
located within business and residential locations for approximately
100 to 2000 phone line interfaces. This distributed access
architecture greatly reduced line lengths to the subscriber and
resulted in significant savings in both wire installation and
maintenance. The reduced line lengths also improved communication
performance on the line provided to the subscriber.
[0026] By the late 1980s, the limitations of data modem connections
over voice frequency (VF) pairs were becoming obvious to both
subscribers and telecommunications service providers. ISDN
(Integrated Services Digital Network) was introduced to provide
universal 128 kbps service in the access network. The subscriber
interface is based on 64 kbps digitization of the VF pair for
digital multiplexing into high speed digital transmission streams
(e.g., T1/T3 lines in North America, E1/E3 lines in Europe). ISDN
was a logical extension of the digital network that had evolved
throughout the 1980s. The rollout of ISDN in Europe was highly
successful. However, the rollout in the United States was not
successful, due in part to artificially high tariff costs which
greatly inhibited the acceptance of ISDN.
[0027] More recently, the explosion of the Internet and
deregulation of the telecommunications industry have brought about
a broadband revolution characterized by greatly increased demands
for both voice and data services and greatly reduced costs due to
technological innovation and intense competition in the
telecommunications marketplace. To meet these demands, high speed
DSL (digital subscriber line) modems and cable modems have been
developed and introduced. The DLC architecture was extended to
provide remote distributed deployment at the neighborhood cabinet
level using DSL access multiplexer (DSLAM) equipment. The increased
data rates provided to the subscriber resulted in upgrade DLC/DSLAM
transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps) to
high speed DS3 and OC3 interfaces. In a similar fashion, the entire
telecommunications network backbone has undergone and is undergoing
continuous upgrade to wideband optical transmission and switching
equipment.
[0028] Similarly, wireless access systems have been developed and
deployed to provide broadband access to both commercial and
residential subscriber premises. Initially, the market for wireless
access systems was driven by rural radiotelephony deployed solely
to meet the universal service requirements imposed by government
(i.e., the local telephone company is required to serve all
subscribers regardless of the cost to install service). The cost of
providing a wired connection to a small percentage of rural
subscribers was high enough to justify the development and expense
of small-capacity wireless local loop (WLL) systems.
[0029] Deregulation of the local telephone market in the United
States (e.g., Telecommunications Act of 1996) and in other
countries shifted the focus of fixed wireless access (FWA) systems
deployment from rural access to competitive local access in more
urbanized areas. In addition, the age and inaccessibility of much
of the older wired telephone infrastructure makes FWA systems a
cost-effective alternative to installing new, wired infrastructure.
Also, it is more economically feasible to install FWA systems in
developing countries where the market penetration is limited (i.e.,
the number and density of users who can afford to pay for services
is limited to small percent of the population) and the rollout of
wired infrastructure cannot be performed profitably. In either
case, broad acceptance of FWA systems requires that the voice and
data quality of FWA systems must meet or exceed the performance of
wired infrastructure.
[0030] Wireless access systems must address a number of unique
operational and technical issues including: [0031] 1) Relatively
high bit error rates (BER) compared to wire line or optical
systems; and [0032] 2) Transparent operation with network protocols
and protocol time constraints for the following protocols: [0033]
a) ATM; [0034] b) Class 5 switch interfaces (domestic GR-303 and
international V5.2); [0035] c) TCP/IP with quality-of-service QoS
for voice over IP (VoIP) (i.e., RTP) and other H.323 media
services; [0036] d) Distribution of synchronization of network time
out to the subscribers; [0037] 3) Increased use of voice, video
and/or media compression and concentration of active traffic over
the air interface to conserve bandwidth; [0038] 4) Switching and
routing within the access system to distribute signals from the
central office to multiple remote cell sites containing multiple
cell sectors and one or more frequencies of operation per sector;
and [0039] 5) Remote support and debugging of the subscriber
equipment, including remote software upgrade and provisioning.
[0040] Unlike physical optical or wire systems that operate at bit
error rates (BER) of 10.sup.-11, wireless access systems have time
varying channels that typically provide bit error rates of
10.sup.-3 to 10.sup.-6. The wireless physical (PHY) layer interface
and the media access control (MAC) layer interface must provide
modulation, error correction and ARQ (automatic request for
retransmission) protocol that can detect and, where required,
correct or retransmit corrupted data so that the interfaces at the
network and at the subscriber site operate at wire line bit error
rates.
[0041] Wireless access systems, as well as other systems which
employ a shared communications media, must also provide a mechanism
for allocating available communications bandwidth among multiple
transmitting and receiving groups. Many wireless systems employ
either a time division duplex (TDD) time division multiple access
(TDMA) or a frequency diversity duplex (FDD) frequency division
multiple access (FDMA) allocation scheme illustrated by the timing
diagram of FIGS. 10A and 10B. TDD 1000 shares a single radio
frequency (RF) channel F1 between the base and subscriber,
allocating time slices between the downlink 1001 (transmission from
the base to the subscriber) and the uplink 1002 (transmission from
the subscriber to the base). FDD 1010 employs two frequencies F1
and F2, each dedicated to either the downlink 1011 or the uplink
1012 and separated by a duplex spacing 1013.
[0042] For wireless access systems which provide Internet access in
addition to or in lieu of voice communications, data and other Web
based applications dominate the traffic load and connections within
the system. Data access is inherently asymmetric, exhibiting
typical downlink-to-uplink ratios of between 4:1 and 14:1.
[0043] TDD systems, in which the guard point (the time at which
changeover from the downlink 1001 to the uplink 1002 occurs) within
a frame may be shifted to alter the bandwidth allocation between
the downlink 1001 and the uplink 1002, have inherent advantages for
data asymmetry and efficient use of spectrum in providing broadband
wireless access. TDD systems exhibit 40% to 90% greater spectral
efficiency for asymmetric data communications than FDD systems, and
also support shifting of power and modulation complexity from the
subscriber unit to the base to lower subscriber equipment
costs.
[0044] Within the spectrum allocated to multichannel multipoint
distribution systems (MMDS), however, some spectrum is regulated
for only FDD operation. Since the total spectrum allocated to MMDS
is relatively small (2.5-2.7 GHz, or about 30 6 MHz channels), some
service providers may desire to utilize the FDD-only spectrum,
preferably utilizing the TDD-based equipment employed in other
portions of the MMDS spectrum.
[0045] The wide range of equipment and technology capable of
providing either wireline (i.e., cable, DSL, optical) broadband
access or wireless broadband access has allowed service providers
to match the needs of a subscriber with a suitable broadband access
solution. However, in many areas, the cost of cable modem or DSL
service is high. Additionally, data rates may be slow or coverage
incomplete due to line lengths. In these areas and in areas where
the high cost of replacing old telephone equipment or the low
density of subscribers makes it economically unfeasible to
introduce either DSL or cable modem broadband access, fixed
wireless broadband systems offer a viable alternative. Fixed
wireless broadband systems use a group of transceiver base stations
to cover a region in the same manner as the base stations of a
cellular phone system. The base stations of a fixed wireless
broadband system transmit forward channel (i.e., downstream)
signals in directed beams to fixed location antennas attached to
the residences or offices of subscribers. The base stations also
receive reverse channel (i.e., upstream) signals transmitted by the
broadband access equipment of the subscriber.
[0046] Media access control (MAC) protocols refer to techniques
that increase utilization of two-way communication channel
resources by subscribers that use the channel resources. The MAC
layer may use a number of possible configurations to allow multiple
access. These configurations include: [0047] 1. FDMA--frequency
division multiple access. In a FDMA system, subscribers use
separate frequency channels on a permanent or demand access basis.
[0048] 2. TDMA--time division multiple access. In a TDMA system,
subscribers share a frequency channel but allocate spans of time to
different users. [0049] 3. CDMA--code division multiple access. In
a CDMA system, subscribers share a frequency but use a set of
orthogonal codes to allow multiple access. [0050] 4. SDMA--space
division multiple access--In a SDMA system, subscribers share a
frequency but one or more physical channels are formed using
antenna beam forming techniques. [0051] 5. PDMA--polarization
division multiple access--In a PDMA system, subscribers share a
frequency but change polarization of the antenna.
[0052] Each of these MAC techniques makes use of a fundamental
degree of freedom (physical property) of a communications channel.
In practice, combinations of these degrees of freedom are often
used. As an example, cellular systems use a combination of FDMA and
either TDMA or CDMA to support a number of users in a cell.
[0053] To provide a subscriber with bi-directional (two-way)
communication in a shared media, such as a coaxial cable, a
multi-mode fiber (optical), or an RF radio channel, some type of
duplexing technique must be implemented. Duplexing techniques
include frequency division duplexing (FDD) and time division
duplexing (TDD). In FDD, a first channel (frequency) is used for
transmission and a second channel (frequency) is used for
reception. To avoid physical interference between the transmit and
receive channels, the frequencies must have a separation know as
the duplex spacing. In TDD, a single channel is used for
transmission and reception and specific periods of time (i.e.,
slots) are allocated for transmission and other specific periods of
time are allocated for reception.
[0054] Finally, a method of coordinating the use of bandwidth must
be established. There are two fundamental methods: distributed
control and centralized control. In distributed control,
subscribers have a shared capability with or without a method to
establish priority. An example of this is CSMA (carrier sense
multiple access) used in IEEE802.3 Ethernet and IEEE 802.11
Wireless LAN. In centralized control, subscribers are allowed
access under the control of a master controller. Cellular systems,
such as IS-95, IS-136, and GSM, are typical examples. Access is
granted using forms of polling and reservation (based on polled or
demand access contention).
[0055] A number of references and overviews of demand access are
available including the following: [0056] 1. Sklar, Bernard.
"Digital Communications Fundamentals and Applications," Prentice
Hall, Englewood Cliffs, N.J., 1988. Chapter 9. [0057] 2. Rappaport,
Theodore. "Wireless Communications, Principles and Practice,"
Prentice Hall, Upper Saddle River, N.J., 1996. Chapter 8. [0058] 3.
TR101-173V1.1. "Broadband Radio Access Networks, Inventory of
Broadband Radio Technologies and Techniques," ETSI, 1998. Chapter
7. The foregoing references are hereby incorporated by reference
into the present disclosure as if fully set forth herein.
[0059] In 1971, the University of Hawaii began operation of a
random access shared channel ALOHA TDD system. The lack of channel
coordination resulted in poor utilization of the channel. This lead
to the introduction of time slots (slotted Aloha) that set a level
of coordination between the subscribers that doubled the channel
throughput. Finally, the researchers introduced the concept of a
central controller and the use of reservations (reservation Aloha).
Reservation techniques made it possible to make trade-offs between
throughput and latency.
[0060] This work was fundamental to the development of media access
control (MAC) techniques for dynamic random access and the use of
ARQ (automatic request for retransmission) to retransmit erroneous
packets. While the work at the University of Hawaii explored the
fundamentals of burst transmission and random access, the work did
not introduce the concept of a frame and/or super-frame structure
to the TDD/TDMA access techniques. One of the more sophisticated
systems developed in the 1970s and in current use is Joint Tactical
Information Distribution System (JTIDS). This system was based on
the joint use of TDMA and time duplexing over frequency-hopping
spread-spectrum channels. This was the culmination of research to
allow flexible allocation of bandwidth to a large group of users.
The key aspect of the JTIDS system was the introduction of dynamic
allocation of bandwidth resources and explicit variable symmetry
(downlink vs. uplink bandwidth) in the link.
[0061] IEEE 802.11 Wireless LAN equipment provides for a centrally
coordinated TDD system that does not have a specific frame or
slotting structure. IEEE 802.11 did introduce the concept of
variable modulation and spreading inherent in the structure of the
transmission bursts. A significant improvement was incorporated in
U.S. Pat. No. 6,052,408, entitled "Cellular Communications System
with Dynamically Modified Data Transmission Parameters." This
patent introduced specific burst packet transmission formats that
provide for adaptive modulation, transmit power, and antenna beam
forming and an associated method of determining the highest data
rate for a defined error rate floor for the link between the base
station and a plurality of subscribers assigned to that base
station. With the exception of variable spreading military systems
and NASA space communication systems, this was one of the first
commercial patents that address variable transmission parameters to
increase system throughput.
[0062] Another example of TDD systems is digital cordless phones,
also referred to as low-tier PCS systems. The Personal Access
Communications (PAC) system and Digital European Cordless Telephone
(DECT, as specified by ETSI document EN 300-175-3) are two examples
of these systems. Digital cordless phones met with limited success
for their intended use as pico-cellular fixed access products. The
systems were subsequently modified and repackaged for wireless
local loop (WLL) applications with extended range using increased
transmission (TX) power and greater antenna gain.
[0063] These TDD/TDMA systems use fixed symmetry and bandwidth
between the uplink and the downlink. The TDD frame consists of a
fixed set of time slots for the uplink and the downlink. The
modulation index (or type) and the forward error correction (FEC)
format for all data transmissions are fixed in these systems. These
systems did not include methods for coordinating TDD bursts between
systems. This resulted in inefficient use of spectrum in the
frequency planning of cells.
[0064] While DECT and PAC systems based on fixed frames with fixed
and symmetric allocation of time slots (or bandwidth) provides
excellent latency and low jitter, and can support time bounded
services, such as voice and Nx64 Kbps video, these systems do not
provide efficient use of the spectrum when asymmetric data services
are used. This has lead to research and development of packet based
TDD systems based on Internet protocol (IP) or asynchronous
transfer mode (ATM), with dynamic allocation of TDD time slots and
the uplink-downlink bandwidth, combined with efficient algorithms
to address both best efforts and real-time low-latency service for
converged media access (data and multi-media).
[0065] One example of a TDD system with dynamic slot and bandwidth
assignment is the ETSI HYPERLAN II specification based on the
Dynamic Slot Assignment algorithm described in "Wireless ATM:
Performance Evaluation of a DSA++ MAC Protocol with Fast Collision
resolution by Probing Algorithm," D. Petras and A. Kramling,
International Journal of Wireless Information Networks, Vol. 4, No.
4, 1997. This system allows both contention-based and
contention-free access to the physical TDD channel slots. This
system also introduced the broadcast of resource allocation at the
start of every frame by the base station controller. Other wireless
standards, including IEEE 802.16 wireless metropolitan network
standards, use this combination of an allocation MAP of the uplink
and downlink at the start of the dynamic TDD frame to set resource
use for the next TDD frame.
[0066] A further improvement to this TDD system was described in
"Multiple Access Control Protocols for Wireless ATM: Problem
Definition and Design Objectives," O. Kubbar and H. Mouftah, IEEE
Communications, November 1997, pp. 93-99. This system expanded on
the packet reservation multiple access (PRMA) method developed in
1989 at Rutgers University WINLAB for ATM and IP based transport
[see "Packet Reservation Multiple Access for Local Wireless
Communications," Goodman et al., IEEE Transaction on
Communications, Vol. 37, No. 8, pp. 885-890]. Like PRMA, this
system logically arranged all the downlink transmissions in the
start of a fixed duration TDD frame and all uplink transmissions at
the end of the TDD frame. This eliminated the inefficiencies in the
DCA++ Hyperlan II protocol. Adaptive allocation of uplink and
downlink bandwidth is supported. The system provided for fixed,
random, and demand assignment mechanisms. Priority is given to
quality of service (QoS) applications with resources being removed
from best efforts demand access users as required.
[0067] The above-described prior art concern the allocation of
services in an individual sector of a cell. A cell may consist of M
sectors, wherein each sector generally covers a 360/M degree arc
around the cell site. Each sector serves Nm subscribers, where m=1
to M. These references did not expressly provide protocol
mechanisms or rules for the operation of a given system.
[0068] U.S. Pat. No. 6,016,311 expressly addresses one possible
implementation to the TDD bandwidth allocation problem. The system
described continuously measures and adapts the bandwidth
requirements based on the evaluation of the average bandwidth
required by all the subscribers in a cell and the number of times
bandwidth is denied to the subscribers. Changes to the bandwidth
allocation are applied based on a set of rules described in U.S.
Pat. No. 6,016,311. While measurements of multiple sectors are
performed and recorded at a central base station controller, no
global coordination of bandwidth allocation of multiple sectors in
a cell or across multiple cells is provided.
[0069] Thus, the prior art does not address two very important
factors in allocation of bandwidth. First, bandwidth allocation
must contemplate stringent bandwidth availability requirements for
specific groups of services based on planning of the network. For
example, consider life-line toll quality voice service. Toll
quality voice requires that a system guarantee a specific maximum
blocking probability for all voice users based on peak busy hour
call usage. A description of voice traffic planning is provided in
"Digital Telephony--2.sup.nd Edition," by J. Bellamy, John Wiley
and Sons, New York, N.Y., 1990. If a TDD system is designed to meet
life-line voice requirements, the allocation protocol must be able
to rapidly (i.e., less than 100 msec) reallocate bandwidth
resources up to the capacity necessary to meet the call blocking
requirements. Another service group example is a guaranteed service
level agreements (SLA). Again, bandwidth must be rapidly restored
to meet the SLA conditions. More generally, one may consider G
possible service groups having a set of weighted priority level and
associated minimum and maximum levels. The weighted priority levels
and minimum and maximum levels may be used to bound the bandwidth
dynamics of the TDD bandwidth allocation. Minimum levels set a
floor for bandwidth allocation and maximum levels set a ceiling.
Then averaging can be applied.
[0070] Second, the TDD bandwidth allocation must consider adjacent
and co-channel interference from both modems and sectors within a
cell and between cells. Cell planning tools can be used to
establish the relationships for interference. For systems that
operate below 10 GHz, antennas and antenna placement at a cell site
will not provide adequate signal isolation. These co-channel
interference issues are well documented in "Frequency Reuse and
System Deployment in Local Multipoint Distribution Service," by V.
Roman, IEEE Personal Communications, December 1999, pp. 20 to
27.
[0071] Therefore, there is a need in the art for a fixed wireless
access network that maximizes spectral efficiency between the base
stations of the fixed wireless access network and the subscriber
access devices located at the subscriber premises. In particular,
there is a need for a fixed wireless access network that implements
an air interface that minimizes uplink and downlink interference
between different sectors within the same base station cell site.
There also is a need for a fixed wireless access network that
implements an air interface that minimizes uplink and downlink
interference between different cell sites within the fixed wireless
access network. More particularly, there is a need in the art for a
fixed wireless that efficiently allocates bandwidth to individual
subscribers according to dynamically changing applications used by
the individual subscribers.
SUMMARY
[0072] Aspects of the disclosure may be found in a wireless network
that includes a base station in wireless communication with
subscriber stations using radio frequency (RF) time division duplex
(TDD) signaling. For each of the subscriber stations, the base
station establishes a plurality of medium access control (MAC)
connections on a RF link between the base station and the station.
The base station monitors communication traffic on the MAC
connections. Based on the monitored traffic, the base station sets
for each MAC connection a controllable characteristic of the RF
communication with the associated subscriber station and assigns
the MAC connection to a modulation group based on the controllable
characteristic. The base station further transmits signals on MAC
connections in a modulation group in adjacent TDD slots within a
TDD frame.
[0073] The foregoing has outlined rather broadly the features and
technical advantages of the present disclosure so that those
skilled in the art may better understand the detailed description
of the disclosure that follows. Additional features and advantages
of the disclosure will be described hereinafter that form the
subject of the claims of the invention. Those skilled in the art
should appreciate that they may readily use the conception and the
specific embodiment disclosed as a basis for modifying or designing
other structures for carrying out the same purposes of the present
disclosure. Those skilled in the art should also realize that such
equivalent constructions do not depart from the spirit and scope of
the disclosure in its broadest form.
[0074] Before undertaking the DETAILED DESCRIPTION below, it may be
advantageous to set forth definitions of certain words and phrases
used throughout this patent document: the terms "include" and
"comprise," as well as derivatives thereof, mean inclusion without
limitation; the term "or," is inclusive, meaning and/or; the
phrases "associated with" and "associated therewith," as well as
derivatives thereof, may mean to include, be included within,
interconnect with, contain, be contained within, connect to or
with, couple to or with, be communicable with, cooperate with,
interleave, juxtapose, be proximate to, be bound to or with, have,
have a property of, or the like; and the term "controller" means
any device, system or part thereof that controls at least one
operation, such a device may be implemented in hardware, firmware
or software, or some combination of at least two of the same. It
should be noted that the functionality associated with any
particular controller may be centralized or distributed, whether
locally or remotely. Definitions for certain words and phrases are
provided throughout this patent document, those of ordinary skill
in the art should understand that in many, if not most instances,
such definitions apply to prior, as well as future uses of such
defined words and phrases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0075] For a more complete understanding of the present disclosure,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
wherein like numbers designate like objects, and in which:
[0076] FIG. 1 illustrates an exemplary fixed wireless access
network according to one embodiment of the present disclosure;
[0077] FIG. 2 illustrates in greater detail an alternate view of
selected portions of the exemplary fixed wireless access network
according to one embodiment of the present disclosure;
[0078] FIG. 3 illustrates an exemplary time division duplex (TDD)
time division multiple access (TDMA) frame according to one
embodiment of the present disclosure;
[0079] FIG. 4 illustrates the timing recovery and distribution
circuitry in an exemplary RF modem shelf according to one
embodiment of the present disclosure;
[0080] FIG. 5 illustrates an interface tray in an exemplary RF
modem shelf according to one embodiment of the present
disclosure;
[0081] FIG. 5A illustrates an exemplary time division duplex (TDD)
frames according to one embodiment of the present disclosure;
[0082] FIG. 5B illustrates an exemplary transmission burst
containing a single FEC block according to one embodiment of the
present disclosure;
[0083] FIG. 5C illustrates an exemplary transmission burst
containing multiple FEC blocks according to one embodiment of the
present disclosure;
[0084] FIG. 6 is a flow diagram illustrating the adaptive
modification of the uplink and downlink bandwidth in the air
interface in wireless access network according to one embodiment of
the present disclosure;
[0085] FIG. 7 is a flow diagram illustrating the adaptive
assignment of selected link parameters, such as modulation format,
forward error correction (FEC) codes, and antenna beam forming, to
the uplink and downlink channels used by each subscriber in the
exemplary wireless access network according to one embodiment of
the present disclosure;
[0086] FIG. 8 is a flow diagram illustrating the adaptive
assignment of selected link parameters to the different service
connections used by each subscriber in the wireless access network
according to one embodiment of the present disclosure;
[0087] FIGS. 9A and 9B depict cell and sector layouts for a
wireless access coverage area according to various embodiments of
the present disclosure;
[0088] FIGS. 10A through 10E are comparative high level timing
diagrams illustrating the bandwidth allocation among sectors and
cells according to the prior art and according to one embodiment of
the present disclosure;
[0089] FIG. 11 depicts in greater detail a frame structure employed
within the exemplary bandwidth allocation scheme according to one
embodiment of the present disclosure;
[0090] FIG. 12 is functional diagram of filtering employed for
wireless communication within each cell and sector in accordance
with one embodiment of the present disclosure;
[0091] FIG. 13 illustrates a spectral response for filtering
employed for wireless communication within each cell and sector in
accordance with one embodiment of the present disclosure; and
[0092] FIG. 14 is functional diagram of filtering employed for
wireless communication within each cell and sector in accordance
with another embodiment of the present disclosure.
DETAILED DESCRIPTION
[0093] FIGS. 1 through 14, discussed below, and the various
embodiments used to describe the principles of the present
disclosure in this patent document are by way of illustration only
and should not be construed in any way to limit the scope of the
disclosure. Those skilled in the art will understand that the
principles of the present disclosure may be implemented in any
suitably arranged wireless access system.
[0094] FIG. 1 illustrates exemplary fixed wireless access network
100 according to one embodiment of the present disclosure. Fixed
wireless network 100 comprises a plurality of transceiver base
stations, including exemplary transceiver base station 110, that
transmit forward channel (i.e., downlink or downstream) broadband
signals to a plurality of subscriber premises, including exemplary
subscriber premises 121, 122 and 123, and receive reverse channel
(i.e., uplink or upstream) broadband signals from the plurality of
subscriber premises. Subscriber premises 121-123 transmit and
receive via fixed, externally-mounted antennas 131-133,
respectively. Subscriber premises 121-123 may comprise many
different types of residential and commercial buildings, including
single family homes, multi-tenant offices, small business
enterprises (SBE), medium business enterprises (MBE), and so-called
"SOHO" (small office/home office) premises.
[0095] The transceiver base stations, including transceiver base
station 110, receive the forward channel (i.e., downlink) signals
from external network 150 and transmit the reverse channel (i.e.,
uplink) signals to external network 150. External network 150 may
be, for example, the public switched telephone network (PSTN) or
one or more data networks, including the Internet or proprietary
Internet protocol (IP) wide area networks (WANs) and local area
networks (LANs). Exemplary transceiver base station 110 is coupled
to RF modem shelf 140, which, among other things, up-converts
baseband data traffic received from external network 150 to RF
signals transmitted in the forward channel to subscriber premises
121-123. RF modem shelf 140 also down-converts RF signals received
in the reverse channel from subscriber premises 121-123 to baseband
data traffic that is transmitted to external network 150. In an
exemplary embodiment of the present disclosure in which external
network 150 is the public switched telephone network (PSTN), RF
modem 140 transmits baseband data traffic to, and receives baseband
data traffic from, access processor 165, which is disposed in
central office facility 160 of the PSTN.
[0096] It should be noted that network 100 was chosen as a fixed
wireless network only for the purposes of simplicity and clarity in
explaining a subscriber integrated access device according to the
principles of the present disclosure. The choice of a fixed
wireless network should not be construed in any manner that limits
the scope of the present disclosure in any way. As will be
explained below in greater detail, in alternate embodiments of the
present disclosure, a subscriber integrated access device according
to the principles of the present disclosure may be implemented in
other types of broadband access systems. In one embodiment of the
present disclosure, such access systems may include wireline
systems (i.e., digital subscriber line (DSL), cable modem, fiber
optic, and the like) in which a wireline connected to the
subscriber integrated access device carries forward and reverse
channel signals.
[0097] RF modem shelf 140 comprises a plurality of RF modems
capable of modulating (including up-converting) the baseband data
traffic and demodulating (including down-converting) the reverse
channel RF signals. In an exemplary embodiment of the present
disclosure, each of the transceiver base stations covers a cell
site area that is divided into a plurality of sectors. In an
advantageous embodiment of the present disclosure, each of the RF
modems in RF modem shelf 140 may be assigned to modulate and
demodulate signals in a particular sector of each cell site. By way
of example, the cell site associated with transceiver base station
110 may be partitioned into six sectors and RF modem shelf 140 may
comprise six primary RF modems (and, optionally, a seventh spare RF
modem), each of which is assigned to one of the six sectors in the
cell site of transceiver base station 110. In another advantageous
embodiment of the present disclosure, each RF modem in RF modem
shelf 140 comprises two or more RF modem transceivers which may be
assigned to at least one of the sectors in the cell site. For
example, the cell site associated with transceiver base station 110
may be partitioned into six sectors and RF modem shelf 140 may
comprise twelve RF transceivers that are assigned in pairs to each
one of the six sectors. The RF modems in each RF modem pair may
alternate modulating and demodulating the downlink and uplink
signals in each sector.
[0098] RF modem shelf 140 is located proximate transceiver base
station 110 in order to minimize RF losses in communication line
169. RF modem shelf 140 may receive the baseband data traffic from
external network 150 and transmit the baseband data traffic to
external network 150 via a number of different paths. In one
embodiment of the present disclosure, RF modem shelf 140 may
transmit baseband data traffic to, and receive baseband data
traffic from, external network 150 through central office facility
160 via communication lines 166 and 167. In such an embodiment,
communication line 167 may be a link in a publicly owned or
privately owned backhaul network. In another embodiment of the
present disclosure, RF modem shelf 140 may transmit baseband data
traffic to, and receive baseband data traffic from, external
network 150 directly via communication line 168 thereby bypassing
central office facility 160.
[0099] Central office facility 160 comprises access processor shelf
165. Access processor shelf 165 provides a termination of data
traffic for one or more RF modem shelves, such as RF modem shelf
140. Access processor shelf 165 also provides termination to the
network switched circuit interfaces and/or data packet interfaces
of external network 150. One of the principal functions of access
processor shelf 165 is to concentrate data traffic as the data
traffic is received from external network 150 and is transferred to
RF modem shelf 140. Access processor shelf 165 provides data and
traffic processing of the physical layer interfaces, protocol
conversion, protocol management, and programmable voice and data
compression.
[0100] FIG. 2 illustrates in greater detail an alternate view of
selected portions of exemplary fixed wireless access network 100
according to one embodiment of the present disclosure. FIG. 2
depicts additional transceiver base stations, including exemplary
transceiver base stations 110A through 110F, central office
facilities 160A and 160B, and remote RF modem shelves 140A through
140D. Central office facilities 160A and 160B comprise internal RF
modems similar to RF modem shelves 140A through 140D. Transceiver
base stations 110A, 110B, and 110C are disposed in cells sites 201,
202, and 203, respectively. In the exemplary embodiment, cell sites
201-203 (shown in dotted lines) are partitioned into four sectors
each. In alternate embodiments, sites 201, 202, and 203 may be
partitioned into a different number of sectors, such as six
sectors, for example.
[0101] As in FIG. 1, RF modem shelves 140A-140D and the internal RF
modems of central office facilities 160A and 160B transmit baseband
data traffic to, and receive baseband data traffic from, access
processors in central office facilities 160A and 160B of the PSTN.
RF modem shelves 140A-140D and the internal RF modems of central
office facilities 160A and 160B also up-convert incoming baseband
data traffic to RF signals transmitted in the forward (downlink)
channel to the subscriber premises and down-convert incoming RF
signals received in the reverse (uplink) channel to baseband data
traffic that is transmitted via a backhaul network to external
network 150.
[0102] Baseband data traffic may be transmitted from remote RF
modem shelves 140A-140D to central office facilities 160A and 160B
by a wireless backhaul network or by a wireline backhaul network,
or both. As shown in FIG. 2, baseband data traffic is carried
between central office facility 160A and remote RF modem 140A by a
wireline backhaul network, namely wireline 161, which may be, for
example, a DS3 line or one to N T1 lines. A local multipoint
distribution service (LMDS) wireless backhaul network carries
baseband data traffic between central office facilities 160A and
160B and remote RF modem shelves 140B, 140C, and 140D. In a LMDS
wireless backhaul network, baseband data traffic being sent to
remote RF modem shelves 140B, 140C, and 140D is transmitted by
microwave from microwave antennas mounted on transceiver base
stations 110A, 110C, and 110F to microwave antennas mounted on
transceiver base stations 110B, 110D, and 110E. Baseband data
traffic being sent from remote RF modem shelves 140B, 140C, and
140D is transmitted by microwave in the reverse direction (i.e.,
from transceiver base stations 110B, 110D, and 110E to transceiver
base stations 110A, 110C, and 110F).
[0103] At each of transceiver base stations 110B, 110D, and 110E,
downlink data traffic from central office facilities 160A and 160B
is down-converted from microwave frequencies to baseband signals
before being up-converted again for transmission to subscriber
premises within each cell site. Uplink data traffic received from
the subscriber premises is down-converted to baseband signals
before being up-converted to microwave frequencies for transmission
back to central office facilities 160A and 160B.
[0104] Generally, there is an asymmetry of data usage in the
downlink and the uplink. This asymmetry is typically greater than
4:1 (downlink:uplink). Taking into account the factors of data
asymmetry, channel propagation, and available spectrum, an
advantageous embodiment of the present disclosure adopts a flexible
approach in which the physical (PHY) layer and the media access
(MAC) layer are based on the use of time division duplex (TDD) time
division multiple access (TDMA). TDD operations share a single RF
channel between a transceiver base station and a subscriber
premises and use a series of frames to allocate resources between
each user uplink and downlink. A great advantage of TDD operation
is the ability to dynamically allocate the portions of a frame
allocated between the downlink and the uplink. This results in an
increased efficiency of operation relative to frequency division
duplex (FDD) techniques. TDD operations typically may achieve a
forty to sixty percent advantage in spectral efficiency over FDD
operations under typical conditions. Given the short duration of
the transmit and receive time slots relative to changes in the
channel, TDD operations also permit open loop power control,
switched diversity techniques, and feedforward and cyclo-stationary
equalization techniques that reduce system cost and increase system
throughput.
[0105] To aid with periodic functions in the system, TDD frames are
grouped into superframes (approximately 10 to 20 milliseconds). The
superframes are further grouped into hyperframes (approximately 250
to 1000 milliseconds). This provides a coordinated timing reference
to subscriber integrated access devices in the system. FIG. 3
illustrates an exemplary time division duplex (TDD) time division
multiple access (TDMA) framing hierarchy according to one
embodiment of the present disclosure. At the highest level, the
TDD-TDMA framing hierarchy comprises hyperframe 310, which is X
milliseconds (msec.) in length (e.g., 250
msec..ltoreq.X.ltoreq.1000 msec.). Hyperframe 310 comprises N
superframes, including exemplary superframes 311-316. Each of
superframes 311-316 is 20 milliseconds in duration.
[0106] Superframe 313 is illustrated in greater detail. Superframe
313 comprises ten (10) TDD frames, including exemplary TDD frames
321-324, which are labeled TDD Frame 0, TDD Frame 1, TDD Frame 2,
and TDD Frame 9, respectively. In the exemplary embodiment, each
TDD frame is 2 milliseconds in duration. A TDD transmission frame
is based on a fixed period of time during which access to the
channel is controlled by the transceiver base station.
[0107] Exemplary TDD frame 321 is illustrated in greater detail.
TDD frame 321 comprises a downlink portion (i.e., base station to
subscriber transmission) and an uplink portion (i.e., subscriber to
base station transmission). In particular, TDD frame 321
comprises:
[0108] Frame header 330--Frame header 330 is a broadcast message
that synchronizes the start of frame and contains access control
information on how the remainder of TDD frame 321 is configured.
The modulation format of frame header 330 is chosen so that all
subscribers in a sector of the transceiver base station can receive
frame header 330. Generally, this means that frame header 330 is
transmitted in a very low complexity modulation format, such as
binary phase shift keying (BPSK or 2-BPSK), or perhaps quadrature
phase shift keying (QPSK or 4-BPSK).
[0109] D downlink slots--The D downlink slots, including exemplary
downlink slots 341-343, contain transceiver base
station-to-subscriber transmissions of user traffic and/or control
signals. The modulation format of each slot is optimized for
maximum possible data transmission rates. Downlink slots may be
grouped in blocks to form modulation groups as shown in FIG. 5A.
Subscribers who receive data using the same modulation format (or
modulation index) and the same forward error correction (FEC) codes
are grouped together in the same modulation group. In some
embodiment of the present disclosure, two or more modulation groups
may have the same modulation format and FEC codes. In alternate
embodiments of the present disclosure, downlink slots may be
grouped in blocks based on physical beam forming, rather than on
modulation format and FEC codes. For example, a transceiver base
station may transmit data to several subscribers that are
directionally along the same antenna beam in consecutive bursts. In
still other embodiments of the present disclosure, downlink slots
may be grouped in blocks based on any combination of two or more
of: 1) physical beam forming, 2) modulation format, and 3) FEC
codes. For the purpose of simplicity, the term "modulation group"
shall be used hereafter to refer to a group of downlink slots that
are transmitted to one or more subscribers using a common scheme
consisting of one or more of modulation format, FEC codes, and
physical beam forming.
[0110] U uplink slots--The U uplink slots, including exemplary
uplink slots 361-363, contain subscriber-to-transceiver base
station transmissions of user traffic and/or control signals.
Again, the modulation format (modulation index) is optimized for
maximum possible data transmission rates. Generally, the modulation
format and FEC codes in the uplink slots are less complex than in
the downlink slots. This moves complexity to the receivers in the
base stations and lowers the cost and complexity of the subscriber
access device. Uplink slots may be grouped in blocks to form
sub-burst groups as shown in FIG. 5A. Subscribers who transmit data
using the same modulation format (or modulation index) and the same
forward error correction (FEC) codes are grouped together in the
same sub-burst group. In some embodiments of the present
disclosure, two or more sub-burst groups may have the same
modulation format and FEC codes. In other embodiments of the
present disclosure, uplink slots may be grouped in blocks based on
physical beam forming, rather than on modulation format and FEC
codes. In other embodiments, uplink slots may be grouped in blocks
based on any combination of two or more of: 1) physical beam
forming, 2) modulation format, and 3) FEC codes. For the purpose of
simplicity, the term "sub-burst group" shall be used hereafter to
refer to a group of uplink slots that are transmitted to one or
more subscribers using a common scheme consisting of one or more of
modulation format, FEC codes, and physical beam forming.
[0111] Contention slots 360--Contention slots 360 precede the U
uplink slots and comprise a small number of subscriber-to-base
transmissions that handle initial requests for service. A fixed
format length and a single modulation format suitable for all
subscriber access devices are used during contention slots 360.
Generally, this means that contention slots 360 are transmitted in
a very low complexity modulation format, such as binary phase shift
keying (BPSK or 2-BPSK), or perhaps quadrature phase shift keying
(QPSK or 4-BPSK). Collisions (more than one user on a time slot)
result in the use of back-off procedures similar to CSMA/CD
(Ethernet) in order to reschedule a request.
[0112] TDD transition period 350--TDD transition period 350
separates the uplink portion and the downlink portion and allows
for transmitter (TX) to receiver (RX) propagation delays for the
maximum range of the cell link and for delay associated with
switching hardware operations from TX to RX or from RX to TX. The
position of TDD transition period 350 may be adjusted, thereby
modifying the relative sizes of the uplink portion and the downlink
portion to accommodate the asymmetry between data traffic in the
uplink and the downlink.
[0113] Exemplary downlink slot 342 is shown in greater detail.
Downlink slot 342 comprises burst header 371, encapsulated packet
data unit (PDU) 372, and forward error correction check sum value
373. The length of downlink slot 342 varies according to the
modulation format used communicate with the subscriber access
device to which downlink slot 342 is transmitted. The other
downlink slots and uplink slots in TDD frame 321 are similar in
structure to downlink slot 342.
[0114] A key aspect of the present disclosure is that the timing of
the downlink and uplink portions of each TDD frame must be
precisely aligned in order to avoid interference between sectors
within the same cell and/or to avoid interference between cells. It
is recalled from above that each sector of a cell site is served by
an individual RF modem in RF modem shelves 140A-140D and the
internal RF modem shelves of central office facilities 160A and
160B. Each RF modem uses an individual antenna to transmit and to
receive in its assigned sector. The antennas for different sectors
in the same cell site are mounted on the same tower and are located
only a few feet apart. If one RF modem (and antenna) are
transmitting in the downlink while another RF modem (and antenna)
are receiving in the uplink, the power of the downlink transmission
will overwhelm the downlink receiver.
[0115] Thus, to prevent interference between antennas in different
sectors of the same cell site, an embodiment of the present
disclosure may use a highly accurate distributed timing
architecture to align the start points of the downlink
transmissions. An embodiment of the present disclosure may also
determine the length of the longest downlink transmission and
ensure that none of the uplink transmissions begin, and none of the
base station receivers begin to receive, until after the longest
downlink is completed.
[0116] Furthermore, the above-described interference between uplink
and downlink portions of TDD frames can also occur between
different cell sites. To prevent interference between antennas in
different cell sites, an embodiment of the present disclosure may
also use the highly accurate distributed timing architecture to
align the start points of the downlink transmissions between cell
sites. An embodiment of the present disclosure may also determine
the length of the longest downlink transmission among two or more
cell sites and ensure that none of the base station receivers in
any of the cells begins to receive in the uplink until after the
longest downlink transmission is completed.
[0117] Within a cell site, a master interface control processor
(ICP), as described below in FIG. 4, may be used to align and
allocate the uplink and downlink portions of the TDD frames for all
of the RF modems in an RF modem shelf. Between cell sites, the
access processor may communicate with several master ICPs to
determine the longest downlink. The access processor may then
allocated the uplinks and downlinks across several cell sites in
order to minimize interference between cell sites and may designate
on master ICP to control the timing of all of the master ICPs.
[0118] FIG. 4 illustrates the timing recovery and distribution
circuitry in exemplary RF modem shelf 140 according to one
embodiment of the present disclosure. RF modem shelf 140 comprises
front panel interface 410 having connectors 411-414 for receiving
input clock references and transmitting clock references. Exemplary
connector 411 receives a first clock signal from a first external
source (External Source A) and exemplary connector 414 receives a
second clock signal from a second external source (External Source
B). Connector 412 outputs an internally generated clock signal
(Master Source Out) and connector 413 receives an external one
second system clock signal (External 1 Second Clock).
[0119] RF modem shelf 140 also comprises a plurality of interface
control processor (ICP) cards, including exemplary ICP cards 450,
460, 470 and 480. ISP card 450 is designated as a master ICP card
and ICP card 480 is designated as a spare ICP card in case of a
failure of master ICP card 450. Within RF modem shelf 140, the ICP
cards provide for control functions, timing recovery and
distribution, network interface, backhaul network interface,
protocol conversion, resource queue management, and a proxy manager
for EMS for the shelf. The ICP cards are based on network
processor(s) that allow software upgrade of network interface
protocols. The ICP cards may be reused for control and routing
functions and provide both timing and critical TDD coordinated
burst timing for all the RF modems in RF modem shelf 140 and for
shelf-to-shelf timing for stacked frequency high density cell
configurations.
[0120] The timing and distribution architecture in RF modem shelf
140 allows for three reference options:
[0121] Primary--An external input derived from another remote modem
shelf acting as a master. BITS (Building Integrated Timing Supply)
reference is a single building master timing reference (e.g.,
External Source A, External Source B) that supplies DS1 and DS0
level timing throughout an office (e.g., 64K or 1.544/2.048
Mbps).
[0122] Secondary--A secondary reference may be derived from any
designated input port in RF modem shelf 140. For remote RF modem
shelf 140, this is one of the backhaul I/O ports. An ICP card is
configured to recover a timing source and that source is placed on
a backplane as a reference (i.e., Network Reference (A/B)) to
master ICP card 450.
[0123] Tertiary--An internal phase locked loop (PLL) may be
used.
[0124] By default, two ICP cards are configured as a master ICP
card and a spare ICP card. The active master ICP card distributes
timing for all of RF modem shelf 140. The timing distribution
architecture of RF modem shelf 140 meets Stratum 3 levels of
performance, namely a free-run accuracy of +/-4.6 PPM (parts per
million), a pull-in capability of 4.6 PPM, and a holdover stability
of less than 255 slips during the first day.
[0125] There are three components to the timing distribution for
the access processor backplane:
[0126] 1. Timing masters (ICP cards 450 and 480).
[0127] 2. Timing slaves (ICP cards 460 and 470).
[0128] 3. Timing references.
[0129] The timing masters are capable of sourcing all clocks and
framing signals necessary for the remaining cards within the AP
backplane. Within a backplane, there are two timing masters (ICP
cards 450 and 480), which are constrained to the slots allocated as
the primary and secondary controllers. The timing masters utilize
the redundant timing references (External Source A, External Source
B, External 1 Second Clock) found on the backplane to maintain
network-qualified synchronization. ISP card 450 (and ISP card 480)
comprises backhaul network input/output (I/O) port 451, multiplexer
452 and PLL-clock generator 453. MUX 452 selects anyone of External
Source A, External Source B, Network Reference (A/B), and the
signal from I/O port 451 to be applied to PLL-clock generator 453.
The timing master has missing clock detection logic that allows it
to switch from one timing reference to another in the event of a
failure.
[0130] Timing is distributed across a redundant set of clock and
framing signals, designated Master Clock Bus in FIG. 4. Each timing
master (i.e., ICP cards 450 and 480) is capable under software
control of driving either of the two sets of clock and framing
buses on the backplane. Both sets of timing buses are
edge-synchronous such that timing slaves can interoperate while
using either set of clocks.
[0131] The timing supplied by the timing master (e.g., ICP card
450) consists of a 65.536 MHZ clock and an 8 KHz framing reference.
There is a primary and secondary version of each reference. To
generate these references, the primary and secondary timing masters
are provisioned to recover the timing from one of the following
sources:
TABLE-US-00001 Table of Clock Source Interface Definitions Source
Connector Frequency External BITS 75/120 Ohm BNC 64K, 1544K, 2048K
(EXT REF A) External BITS/GPS 75/120 Ohm, DB9 64K, 1544K, 2048K
(EXT REF B) External GPS Sync 75/120 Ohm, DB9 1 sec pulse Pulse On
card Reference Digital Logic Level Per I/O reference Network I/O
derived Digital Logic Level Per I/O reference Reference A Network
I/O derived Digital Logic Level Per I/O reference Reference B
[0132] To simplify clock distribution and to provide redundancy all
the clocks are derived from a common clock source. The following
table summarizes the backplane reference clocks as well as the
clock rates of the various backplane resources and how they are
derived from these references.
TABLE-US-00002 Table of Buses and Associated Clocks Clock Frequency
Division or Ratio Common Reference 65.536 MHZ Not Applicable Clock
Common Sync Pulse 1 Hz Not Applicable Framing Reference 8 KHz
Free-run framing (125 usec) provided by Primary or Secondary Clock
Masters Referenced to Common Reference Clock Cell/Packet Clock
32.768 MHZ Reference Clock/2 Rate TDM Bus Rate 8.192 MHZ Reference
Clock/8 RF Reference Clock 10.000 MHZ Free-run RF reference clock
Communications Bus 100 MHZ Derived from free- run Bus Reference
Clock High-speed Serial 1.31072 GHz Ref Clock .times.20 Links
[0133] Timing slaves (i.e., ICP cards 460 and 470) receive the
timing provided by redundant sets of clock and framing buses. Under
software control, timing slaves choose a default set of clocks from
either the A-side or B-side timing buses. They also contain failure
detection logic such that clock and framing signal failures can be
detected. Once a clock or framing failure is detected, the timing
slave automatically switches to the alternate set of timing buses.
ICP cards 460 and 470 contain backhaul I/O ports 461 and 471,
respectively, which may be used to bring in external timing signals
from other RF modem shelves in the network. The timing masters
(i.e., ICP cards 450 and 480) also contain the timing slave
function insofar as they also utilize the timing provided on the
backplane clock and framing buses.
[0134] A qualified timing reference is required for the timing
master to derive backplane timing and to maintain synchronization
within network 100 and with any outside network. Under software
control, an access processor card can be assigned to derive this
timing and to drive one of the two timing reference buses. Ideally,
a second, physically separate card will contain a second qualified
timing source and drive the second backplane timing reference.
[0135] In the event that no qualified timing is present from trunk
interfaces, the access processor backplane has connections which
allow external reference timing (e.g., a GPS-derived clock) from
the interface tray to be applied to the backplane. A one
pulse-per-second (1 PPS) signal is distributed to all system cards
for time stamping of system events and errors. Installations
involving multiple access processor shelves require the timing
reference to be distributed between all access processor
backplanes. In this scenario, the timing reference for a given
backplane is cabled to the remaining backplanes through external
cabling. Multiple remote modem shelves are utilized to distribute
high-capacity backhaul traffic to one or more additional co-located
modem shelves. Traffic is distributed among the shelves through T1,
T3, OC3 and/or other broadband telecommunication circuits. To
maintain network timing, the additional shelves are slaved to these
distribution links and recover timing through the same PLL
mechanisms as the head-end shelf.
[0136] FIG. 5 illustrates exemplary interface tray 1500 associated
with RF modem shelf 140 according to one embodiment of the present
disclosure. Interface tray 1500 comprises signal conditioning-10
MHz oscillator circuitry 1505, alarm conditioning circuitry 1510,
RF circulator-power divider circuitry 1515, and 6:1 switches 1520
and 1525. Exemplary interface tray 1500, located above remote modem
shelf 140, is the junction at which the cell site antennas and the
RF modems interconnect. Interface tray 1500 provides N+1 redundancy
among the RF modems in RF modem shelf 140, using an RF distribution
circuit housed within interface tray 1500. In addition to the
antenna feeds, all external alarms, the BITS and GPS timing
signals, control signals, and power supplies (not shown) are
interfaced through interface tray 1500. Access processor shelf 165
shares the same interface tray design.
[0137] All access to the cell tower antennas, alarms, power,
I.sup.2C, and BITS timing and GPS signals are accomplished through
rear panel 1501 of interface tray 1500. RF signals supplied to the
RF modem cards are received through front panel 1502 of the tray.
All communications and control with interface tray 1500 are done
via discrete connections. Control functions with interface tray
1500 via the remote modem ICP cards are:
[0138] 1. Switching of antennas to the redundant RF Modem
[0139] 2. Alarm indications from external alarms
[0140] 3. CO Output Alarm Indication
[0141] Any external alarms that are detected are conditioned as
necessary by alarm conditioning circuitry 1510 for output to the
primary and secondary master ICP cards in remote RF modem shelf 140
via the discrete interconnections. For CO alarm requirements, the
system will output an alarm to the facility switching equipment via
relay contact closure.
[0142] Interface tray 1500 serves three timing input sources,
namely the BITS signal, the GPS signal, and the GPS 1 PPS signal.
These timing signals are conditioned by signal conditioning-10 MHz
oscillator circuitry 1505, as required, before being transmitted
out front panel 1502 for interfacing to RF modem shelf 140.
Interface tray 1500 supports diversity reception required by the RF
modems. One channel of the diversity pair is dedicated to
transmission. That channel is fed by one of the RF circulators in
RF circulator-power divider circuitry 1515 to allow for
transmission and reception and to support redundant switchover. The
second channel is a receive-only channel. One of the RF power
dividers in RF circulator-power divider circuitry 1515 feeds the
receive only channel.
[0143] To provide N+1 redundancy in the remote modem shelf 140, a
switchover scheme must be devised. For the purposes of discussion,
a six sector cell site is assumed. In this scheme, both RF feeds
for each RF modem channel must be fed to one of 6:1 switches 1520
and 1525. Switching is chosen over power division to reduce the
path loss through the channel versus a power division scheme. All
of the TX/RX signals are fed to 6:1 switch 1520 and all of the RX
only signals are to 6:1 switch 1525. Upon detection of an RF modem
failure, master ICP card 450 is notified and the spare modem is
switched in.
[0144] There is a stable 10 MHz oscillator circuit in signal
conditioning-10 MHz oscillator circuitry 1505 in interface tray
1500. The 10 MHz signal is used to phase reference all of the RF
modem cards. A low-cost backup oscillator is available in interface
tray 1500 in the event of failure of the primary oscillator. The
backup oscillator is phased locked with the GPS signal to allow for
enough stability to operate until maintenance can be performed on
interface tray 1500.
[0145] FIG. 5A illustrates exemplary time division duplex (TDD)
frame 500 according to one embodiment of the present disclosure.
FIG. 5B illustrates exemplary transmission burst 520 containing a
single FEC block according to one embodiment of the present
disclosure. FIG. 5C illustrates exemplary transmission burst 530
containing multiple FEC blocks according to one embodiment of the
present disclosure.
[0146] TDD frame 500 comprises a downlink portion containing
preamble field 501, management field 502, and N modulation groups,
including modulation group 503 (labeled Modulation Group 1),
modulation group 504 (labeled Modulation Group 2), and modulation
group 505 (labeled Modulation Group N). As explained above in FIG.
3, a modulation group is a group of downlink slots transmitted to
one or more subscribers using a common scheme of one or more of: 1)
modulation format, 2) FEC codes, and 3) physical beam forming.
[0147] TDD frame 500 also comprises an uplink portion containing
transmitter-transmitter guard (TTG) slot 506, 0 to N registration
(REG) minislots 506, 1 to N contention (CON) request minislots 508,
N sub-burst groups, including sub-burst group 509 (labeled
Sub-Burst 1) and sub-burst group 510 (labeled Sub-Burst N), and
receiver-transmitter guard (RTG) slot 511. As explained above in
FIG. 3, a sub-burst group is a group of uplink slots transmitted to
one or more subscribers using a common scheme of one or more of: 1)
modulation format, 2) FEC codes, and 3) physical beam forming.
[0148] Each modulation group and each sub-burst group comprises one
or more transmission bursts. Exemplary transmission burst 520 may
be used within a single modulation group in the downlink and covers
one or more downlink slots. Transmission burst 520 also may be used
within a single sub-burst group in the downlink and covers one or
more uplink slots. Transmission burst 520 comprises physical media
dependent (PMD) preamble field 521, MAC header field 522, data
packet data unit (PDU) field 523, and block character redundancy
check (CRC) field 524. Transmission burst 530 comprises physical
media dependent (PMD) preamble field 531, MAC header field 532,
data PDU field 533, block CRC field 534, data PDU field 535, block
CRC field 536.
[0149] The start of every frame includes a Start-Of-Frame (SOF)
field and a PHY Media Dependent Convergence (PMD) field. PMD
preambles are used to assist in synchronization and time-frequency
recovery at the receiver. The SOF field allows subscribers using
fixed diversity to test reception conditions of the two diversity
antennas.
[0150] The SOF PMD field is 2.sup.N symbols long (typically 16, 32,
64 symbols long) and consists of pseudo-random noise (PN) code
sequences, Frank sequences, CAZAC sequences, or other low
cross-correlation sequences, that are transmitted using BPSK or
QPSK modulation. The SOF field is followed by downlink management
messages broadcast from the base station to all subscribers using
the lowest modulation or FEC index and orthogonal expansion.
Management messages are transmitted both periodically (N times per
hyperframe) and as required to change parameters or allocate
parameters. Management messages include: [0151] 1. DownLink Map
indicating the physical slot (PS) where downstream modulation
changes (transmitted every frame); [0152] 2. UpLink MAP indicating
uplink subscriber access grants and associated physical slot start
of the grant (transmitted when changed and at a minimum of one
second hyperframe periods (shorter periods are optional)); [0153]
3. TDD frame and physical layer attributes (periodic at a minimum
of one second hyperframe period); and [0154] 4. Other management
messages including ACK, NACK, ARQ requests, and the like
(transmitted as required).
[0155] The downlink management messages are followed by multi-cast
and uni-cast bursts arranged in increasing modulation complexity
order. The present disclosure introduces the term "modulation
group" to define a set of downstream bursts with the same
modulation and FEC protection. A subscriber continuously receives
all the downstream data in the TDD frame downlink until the last
symbol of the highest modulation group supported by the link is
received. This allows a subscriber maximum time to perform receive
demodulation updates.
[0156] The downlink-to-uplink transition provides a guard time
(TTG) to allow for propagation delays for all the subscribers. The
TTG position and duration is fully programmable and set by
management physical layer attribute messages. The TTG is followed
by a set of allocated contention slots that are subdivided between
acquisition uplink ranging mini-slots and demand access request
mini-slots. The Uplink MAP message establishes the number and
location of each type of slot. Ranging slots are used for both
initial uplink synchronization of subscribers performing net entry
and for periodic update of synchronization of active subscribers.
Contention slots provide a demand access request mechanism to
establish subscriber service for a single traffic service flow. As
collisions are possible, the subscriber uses random back-off, in
integer TDD frame periods and retries based on a time out for
request of service. Contention slots use the lowest possible
modulation, FEC, and orthogonal expansion supported by the base
station.
[0157] The contention slots are followed by individual subscriber
transmissions (sub-bursts) that have been scheduled and allocated
by the base station in the uplink MAP. Each subscriber transmission
burst is performed at the maximum modulation, FEC, and orthogonal
expansion supported by the subscriber. Finally, the subscriber
transmissions are followed by the uplink-to-downlink transition
which provides a guard time (RTG) to allow for propagation delays
for all the subscribers. The RTG duration is fully programmable and
set by management physical layer attribute messages.
[0158] In the downlink, the Physical Media Dependent (PMD) burst
synchronization is not used. The transmission burst begins with the
MAC header and is followed by the packet data unit (PDU) and the
associated block CRC field that protects both the PDU and the
header. The PDU may be a complete packet transmission or a fragment
of a much larger message. When a channel requires more robust FEC,
the PDU may be broken into segments that are protected by separate
FEC CRC fields. This avoids wasting bandwidth with additional MAC
headers.
[0159] One significant difference between the uplink and the
downlink is the addition of the PMD preamble. The PMD preamble
length and pattern can be programmed by transceiver base station
110. Like the SOF field at the beginning of the TDD Frame, the
preamble provides a synchronization method for the base station
receiver. Uplink registration and ranging packet bursts use longer
PMD preambles.
[0160] The medium access control (MAC) layer protocol is connection
oriented and provides multiple connections of different quality of
service (QoS) to each subscriber. The connections are established
when a subscriber is installed and enters operation fixed wireless
access network 100. Additional connections can be established and
terminated with the base station transceivers as subscriber
requirements changes.
[0161] As an example, suppose a subscriber access device supports
two voice channels and a data channel. The quality of service (QoS)
on both of the voice channels and data can set based on the service
structure set by the wireless service provider. At installation, a
subscriber may start with two service connections: a toll quality
voice channel and a medium data rate broadband data connection. At
a later point in time, the subscriber may order and upgrade service
to two toll quality voice channels and high speed data connection
(a total of three connections).
[0162] The maintenance of connections varies based on the type of
connection established. T1 or fractional T1 service requires almost
no maintenance due to the periodic nature of transmissions. A
TCP/IP connection often experiences bursty on-demand communication
that may be idle for long periods of time. During those idle
periods, periodic ranging and synchronization of the subscriber is
required.
[0163] In an exemplary embodiment of fixed wireless access network
100, each subscriber maintains a 64-bit EUI for globally unique
addressing purposes. This address uniquely defines the subscriber
from within the set of all possible vendors and equipment types.
This address is used during the registration process to establish
the appropriate connections for a subscriber. It is also used as
part of the authentication process by which the transceiver base
station and the subscriber each verify the identity of the
other.
[0164] In the exemplary embodiment, a connection may be identified
by a 16-bit connection identifier (CID) in MAC header 522 or MAC
header 532. Every subscriber must establish at least two
connections in each direction (upstream and downstream) to enable
communication with the base station. The basic CIDs, assigned to a
subscriber at registration, are used by the base station MAC layer
and the subscriber MAC layer to exchange MAC control messages,
provisioning and management information.
[0165] The connection ID can be considered a connection identifier
even for nominally connectionless traffic like IP, since it serves
as a pointer to destination and context information. The use of a
16-bit CID permits a total of 64K connections within the
sector.
[0166] In an exemplary embodiment of fixed wireless access network
100, the CID may be divided into 2 fields. Bits [16:x] may be used
to uniquely identify a subscriber. In a cyclo-stationary receiver
processing at a base station, this would set the antenna,
equalizer, and other receiver parameters. Bits [x:1] may be used to
indicate a connection to a type of service. Each subscriber service
can have individual modulation format, FEC, and ARQ. Thus, within a
single sub-burst group transmitted by a subscriber, the voice data
may use one type of modulation format, FEC, and ARQ, and the
broadband internet service may use a different modulation format,
FEC, and ARQ. Similarly, within a single modulation group
transmitted to the subscriber, the voice data may use one type of
modulation format, FEC, and ARQ, and the broadband internet service
may use a different modulation format, FEC, and ARQ.
[0167] As an example, bits [16:7] of the CID may identify 2 10 (or
1024) distinct subscribers and bits [6:1] may identify 2 6=64
possible connections. An apartment building could be given a set of
subscriber ports [16:9] so that bits [9:7] allow 2 8 connections or
256 connections.
[0168] Requests for transmission are based on these connection IDs,
since the allowable bandwidth may differ for different connections,
even within the same service type. For example, a subscriber unit
serving multiple tenants in an office building would make requests
on behalf of all of them, though the contractual service limits and
other connection parameters may be different for each of them.
[0169] Many higher-layer sessions may operate over the same
wireless connection ID. For example, many users within a company
may be communicating with TCP/IP to different destinations, but
since they all operate within the same overall service parameters,
all of their traffic is pooled for request/grant purposes. Since
the original LAN source and destination addresses are encapsulated
in the payload portion of the transmission, there is no problem in
identifying different user sessions.
[0170] Fragmentation is the process by which a portion of a
subscriber payload (uplink or downlink) is divided into two or more
PDUs. Fragmentation allows efficient use of available bandwidth
while maintaining the QoS requirements of one or more of services
used by the subscriber. Fragmentation may be initiated by a base
station for a downlink connection or the subscriber access device
for the uplink connection. A connection may be in only one
fragmentation state at any given time. The authority to fragment
data traffic on a connection is defined when the connection is
created.
[0171] The MAC layer protocol in wireless access network 100 also
supports concatenation of multiple PDUs in a single transmission in
both the uplink and the downlink, as shown in FIG. 5C. Since each
PDU contains a MAC header with the CID, the receiving MAC layer can
determine routing and processing by higher layer protocols. A base
station MAC layer creates concatenated PDUs in the uplink MAP.
Management, traffic data, and bandwidth may all be concatenated.
This process occurs naturally in the downlink. In the uplink,
concatenation has the added benefit of eliminating additional PMD
preambles.
[0172] FIG. 6 depicts flow diagram 600, which illustrates the
adaptive modification of the uplink and downlink bandwidth in the
air interface in wireless access network 100 according to one
embodiment of the present disclosure. Initially, an RF modem shelf,
such as RF modem shelf 140A, receives new access requests from
subscriber access devices in fixed wireless access network 100 and
determines traffic requirements for each new and existing
subscriber in each sector of a single cell site (process step 605).
The traffic requirements of each subscriber may be established in a
number of ways, including minimum QoS requirements, service level
agreements, past usage, and current physical layer parameters, such
as modulation index, FEC codes, antenna beam forming, and the like.
The RF modem shelf then determines from the subscriber traffic
requirements the longest downlink portion of any TDD frame in each
sector of a single cell site (process step 610).
[0173] Next, the access processor for the RF modem shelf (or the RF
modem shelf itself) determines the appropriate allocation of
downlink and uplink portions of TDD frames for a single cell site
in order to minimize or eliminate interference within the cell site
(process step 615). Bandwidth is allocated, and TDD transition
period 350 is positioned, such that the longest downlink
transmission is complete before any receiver in the cell site
starts to listen for the uplink transmission.
[0174] Next, if global allocation of downlink and uplink bandwidth
across multiple cell sites is being implemented (generally the
case), the access processor determines the longest downlink portion
of any TDD frame across several closely located cell sites stations
(process step 620). The access processor then determines the
allocation of uplink and downlink bandwidth for all TDD frames
across several closely located cell sites in order to minimize or
eliminate cell-to-cell interference (process step 625). Again,
bandwidth is allocated, and TDD transition period 350 is
positioned, such that the longest downlink transmission is complete
before any receiver in any of the closely located cell sites starts
to listen for uplink transmissions. Finally, the downlink portions
of the TDD frames are launched simultaneously using the highly
accurate clock from the distributed timing architecture (process
step 630).
[0175] The dynamic application of TDD bandwidth allocation is
bounded by set minimum and maximum boundaries set by the service
provider, based on traffic and network analysis. Further, the
bandwidth bounds may be allocated in sub-groupings based on
established quality of service (QoS) requirements (e.g., voice
data) and Service Level Agreements (SLA) (e.g., broadband data
rate) as the primary consideration and with best efforts, non-QoS
data, and IP traffic as secondary considerations. The bandwidth
bounds may be allocated based on the fact that a subscriber may
support more that one interface and thus more than one modulation
format in order to achieve required error rates for one or more
services provided to the subscriber.
[0176] FIG. 7 depicts flow diagram 700, which illustrates the
adaptive assignment of selected link parameters, such as modulation
format, forward error correction (FEC) codes, and antenna beam
forming, to the uplink and downlink channels used by each
subscriber in wireless access network 100 according to one
embodiment of the present disclosure. The RF modem shelf monitors
data traffic between subscribers and base station and determines
for each subscriber the most efficient combination of modulation
format, FEC code, and/or antenna beam forming for the uplink and
downlink.
[0177] The selected combination is based at least in part on the
error rates detected by the RF modem shelf when monitoring the data
traffic. If the error rate for a particular subscriber is too high
in either the uplink or the downlink, the RF modem shelf can
decrease the modulation format complexity and use a higher level of
FEC code protection in either the uplink or the downlink in order
to reduce the error rate. Conversely, if the error rate for a
particular subscriber is very low in either the uplink or the
downlink, the RF modem shelf can increase the modulation format
complexity and use a lower level of FEC code protection in either
the uplink or the downlink in order to increase the spectral
efficiency, provided the error rate remains acceptably low.
Different modulation formats and FEC codes may be used for
different services (e.g., voice, data) used by a subscriber
(process step 705).
[0178] Next, the RF modem shelf assigns subscribers to modulation
groups in the downlink and to sub-burst groups in the uplink
(process step 710). The base station transceiver then transmits
media access fields (e.g., signaling, ACK & NACK) using the
lowest modulation format/FEC code complexity. The base station
transceiver then transmits the remaining modulation groups in the
downlink to the subscribers in increasing order of modulation
format/FEC code complexity (process step 715). When the downlink is
complete, the base station transceiver receives registration &
contention minislots transmitted by the subscriber access devices
using the lowest modulation format/FEC code complexity. The base
station transceiver then receives the remaining sub-burst groups
transmitted by the subscribers in increasing order of modulation
format/FEC code complexity (process step 720).
[0179] The use of adaptive link parameters improves the link
throughput and correspondingly affects the bandwidth allocation
described above in FIG. 6. Link parameters apply not only to the
transmitter but to the receiver as well. Some embodiments of the
present disclosure may use a bounded (finite) set of modulation
formats to maximize bandwidth utilization to each subscriber in a
channel or sector. In an exemplary embodiment of the present
disclosure, the low complexity (low bandwidth efficiency)
modulation formats used for media access fields (e.g., signaling,
ACK, NACK) are binary phase shift keying (BSPK or 2-PSK) and
quadrature phase shift keying (QPSK or 4-PSK). Some embodiments of
the present disclosure may also use multiple-code orthogonal
expansion codes in conjunction with the low complexity modulation
formats for extremely robust communication. The higher complexity
(higher efficiency) modulation formats used for the modulation
groups and the sub-burst groups may be 8-PSK, 16 quadrature
amplitude modulation (QAM), 32 QAM, 64 QAM, 128 QAM, and the
like.
[0180] Some embodiments of the present disclosure may also use a
bounded set of FEC codes to maximize bandwidth utilization to each
subscriber in a channel or sector. The level of FEC code protection
is based on the services provided. Each subscriber may support
multiple services.
[0181] In an advantageous embodiment of the present disclosure, the
RF modem shelf may use packet fragmentation to transport data in
either the uplink or the downlink. Fragmentation is the division of
larger packets into smaller packets (fragments) combined with an
ARQ (automatic request for retransmission) mechanism to retransmit
and recover erroneous fragments. The RF modem shelf automatically
reduces fragment size for high error rate channels. Fragmentation
is applied for guaranteed error-free sources. The degree of
fragmentation and ARQ is based on the service provided, since each
subscriber may support multiple services.
[0182] FIG. 8 depicts flow diagram 800, which illustrates the
adaptive assignment of selected link parameters to the different
service connections used by each subscriber in wireless access
network 100 according to one embodiment of the present disclosure.
The RF modem shelf assigns connection identification (CID) values
to the uplink and to the uplink connections used by a subscriber.
If a subscriber uses more than one service (e.g., two voice, one
data), the RF modem shelf assigns separate CID values to each
uplink connection and separate CID values to each downlink
connection (process step 805). As noted above, the CID comprises a
bit field with the uppermost bits identifying the subscriber and
the lowermost bits identifying a specific connection to the
subscriber. While many sets of adaptive transmission and reception
parameters are possible, there are a finite number of combinations
that make logical sense. These combinations are grouped into
physical layer usage codes that are broadcast to subscribers as
part of the general header of TDD superframe or frame header on a
periodic basis. These apply to both the base station transmissions
and the subscriber transmissions.
[0183] The RF modem shelf monitors data traffic between subscriber
and base station and determines for each connection the most
efficient combination of modulation format, FEC code, and/or
antenna beam forming for the uplink and downlink (process step
810). The RF modem shelf then assigns each subscriber connection to
a modulation group in the downlink and to a sub-burst group in the
uplink (process step 815). The base station transmits media access
fields (e.g., signaling, ACK & NACK) using the lowest
modulation format/FEC code complexity. Then base station then
transmits modulation groups to subscribers in increasing order of
modulation format/FEC code complexity (process step 820). Finally,
the base station receives registration & contention minislots
using the lowest modulation format/FEC code complexity. Then base
station then receives sub-burst groups from subscribers in
increasing order of modulation format/FEC code complexity (process
step 825).
[0184] Physical layer usage codes are bound to subscriber CID
values by a service establishment protocol. If there is a
degradation or improvement in the channel between a subscriber and
the base station, a protocol exists so the subscriber access device
and the base station may revise the physical layer usage code and
subscriber CID code. The codes and bindings can be added and
deleted based on services requirements of the subscriber.
[0185] FIG. 9A depicts a cell and sector layout for a wireless
access coverage area according to one embodiment of the present
disclosure. Coverage area 900 is logically divided into cells 910,
920, 930 and 940 each logically divided into a number of sectors
911-916, 921-926, 931-936 and 941-946, respectively. Each cell 910,
920, 930 and 940 includes a transceiver base station 110 as
depicted in FIG. 1 at a central location 917, 927, 937, and 947,
respectively, as well as subscriber premises 121-123 within the
coverage area of the respective cell.
[0186] Sectors 911-916, 921-926, 931-936 and 941-946 are logically
divided into two categories: those designated sector type "A" and
those designated sector type "B", with sector categories
alternating within a cell so that no two adjacent cells fall in the
same category and with cells arranged so that no two adjacent
sectors from adjoining cells fall in the same category. Each sector
is falls within a different category than all other adjacent
sectors with which the respective sector shares a common linear
boundary.
[0187] FIGS. 10C through 10E are high level timing diagrams
illustrating bandwidth allocation among sectors according to one
embodiment of the present disclosure, and are intended to be read
in conjunction with FIG. 9A. An embodiment of the present
disclosure may incorporate FDD operation, with dedicated downlink
and uplink channels, within a TDD system by introducing a frequency
change at the normal TDD guard point. Transmission time on the
dedicated downlink frequency F1 and the dedicated uplink frequency
F2 are divided between adjacent sectors within categories A and B.
Thus, the TDD FDD system 1020 of the embodiment of the present
disclosure shown in FIG. 10C allocates both a downlink period 1021,
1022 on the downlink frequency F1 and an uplink period 1023, 1024
on the uplink frequency F2 to each of the sectors within categories
A and B.
[0188] The allocated periods 1012/1022 and 1023/1024 are offset in
both time and frequency, then overlaid so that the sector A
downlink period 1021 does not coincide in time or frequency with
the sector A uplink period 1024 and the sector B downlink period
1022 does not coincide in time or frequency with sector B uplink
period 1023. Instead, downlink transmission 1021 in each sector
within category A occurs at the same time as uplink transmission
1023 within each sector within category B, while downlink
transmission 1022 in each sector within category B occurs
concurrently with uplink transmission 1024 for each sector within
category A.
[0189] In this manner, the dedicated downlink frequency F1 and the
dedicated uplink frequency F2 are time-shared by adjacent sectors,
but remain dedicated to downlink or uplink transmission and may
utilize FDD-only bandwidth within the MMDS spectrum. Duplex spacing
1013 between downlink and uplink frequencies F1 and F2 (typically
50-70 MHz) is also maintained.
[0190] FIG. 11 depicts in greater detail a frame structure employed
within the exemplary bandwidth allocation scheme according to one
embodiment of the present disclosure, and is intended to be read in
conjunction with FIGS. 9 and 10C through 10E. The frame 1100
depicted corresponds to each of the sectors within category A
described above and depicted in FIGS. 9A and 10C through 10E,
although each sector within category would utilize a similar frame,
as described in further detail below.
[0191] Frame 1100 includes a frame header 1110, an downlink
sub-frame 1120, and an uplink sub-frame 1130, with the downlink and
uplink sub-frames logically divided into a number of physical slots
1140. The frame header 1110 includes a preamble 1111 containing a
start-of-frame field, which allows subscribers using fixed
diversity to test reception conditions of the two diversity
antennas, and a physical layer (the air interface is layered as a
physical layer and a media access layer) media dependent
convergence field, utilized to assist in synchronization and
time/frequency recovery at the receiver. The preamble 1111 is
followed by media access management information 1112, which
includes a downlink MAP identifying the physical slot where the
downlink ends and the uplink begins, an uplink MAP indicating
uplink subscriber access grants and the associated physical slot
start of the grant, and other management messages such as
acknowledge (ACK) response, etc.
[0192] During the downlink sub-frame 1120, the base transmitter and
the subscriber receiver are both set to the downlink frequency F1.
The downlink sub-frame 1120 terminates with a frequency change
physical slot 1121, during which multi-stage digital filters within
both the base and the subscriber unit are altered to switch to the
uplink frequency F2, followed by a transmitter transition guard
time 1122, during which no transmission occurs to allow for
propagation delays for all subscriber units. The transmitter
transition guard time 1122, depicted as occupying three physical
slots in FIG. 11, is fully programmable both in position and
duration, set by management physical layer attribute messages.
[0193] During the downlink sub-frame 1130, the base receiver and
the subscriber transmitter(s) are both set to the uplink frequency
F2. The first physical slots within the uplink sub-frame 1130 are
subscriber registration or acquisition uplink ranging slots,
utilized for both initial uplink synchronization of subscribers
performing entry into the network and periodic update of
synchronization of active subscribers, followed by contention
slots, providing a demand access request mechanism to establish
subscriber service for a single traffic service flow. When
collisions occur within the contention slots, the subscriber
employs a random back-off in integer frame periods and retries
based on a time out for request of service. Contention slots use
the lowest possible modulation, forward error correction (FEC), and
orthogonal expansion supported by the base. The number and position
of registration and contention slots within the uplink sub-frame
1130 is set by the uplink MAP message within the media access
management information portion 1112 of the frame header 1110.
[0194] The contention slots within the uplink sub-frame 1130 are
followed by individual subscriber transmissions which have been
scheduled and allocated by the base in the uplink MAP, with each
subscriber transmission burst performed at the maximum modulation,
FEC and orthogonal expansion supported by the subscriber unit. The
uplink sub-frame 1130 terminates with a frequency change physical
slot 1131, during which both the base and the subscriber unit
switch to the downlink frequency F1, followed by a receiver
transition guard time 1132, which is also programmable.
[0195] Frames for sectors falling within category B will have a
similar structure, but will be offset so that the downlink
sub-frame of each category B sector corresponds in time with the
uplink sub-frame of each category A sector, and the uplink
sub-frame of each category B sector corresponds in time with the
downlink sub-frame of each category A sector. The boundary between
downlink and uplink sub-frames is adaptive utilizing block
equalization and burst timing coordination. Accordingly, uplink and
downlink allocations to sectors in categories A and B may be
divided equally as shown in FIG. 10C. or may be split to allow
greater time within a particular frame to the downlink for sectors
in category A, as shown in FIG. 10D, or to the downlink for sectors
in category B, as shown in FIG. 10E. Spectral efficiency is
therefore improved by adapting to the instantaneous traffic
requirements among various sectors.
[0196] While the exemplary embodiment is described above with six
sector cells and only two sector categories, the present disclosure
may be extended to any number of sector categories equal to a power
of 2 (e.g., 2, 4, 8, . . . , etc.), and preferably employs four
sector categories. Where more than two sector categories are
employed, downlink and uplink frequencies may be reused in pairs or
in staggered offsets (e.g., each sector A shares a downlink
frequency F1 with one adjacent sector B but shares an uplink
frequency F2 with a different adjacent sector C, etc.). FIG. 9B
depicts a cell and sector layout for a wireless access coverage
area according to an alternative embodiment of the present
disclosure. Coverage area 950 is logically divided into cells 960,
970, 980 and 990 each logically divided into four sectors 961-964,
971-974, 981-984 and 991-994, respectively. Each cell 960, 970, 980
and 990 includes a transceiver base station 110 as depicted in FIG.
1 at a central location 965, 975, 985, and 995, as well as
subscriber premises 121-123 within the coverage area of the
respective cell.
[0197] Sectors 961-964, 971-974, 981-984 and 991-994 in the
alternative embodiment are logically divided into four categories,
designated sector type "A", "B", "C" and "D", with sector
categories arranged within a cell and between cells so that no two
adjacent cells fall in the same category and no cell adjoins two or
more cells in the same category. Each sector falls within a
different category than all other adjacent sectors with which the
respective sector shares a common linear boundary.
[0198] FIG. 12 is functional diagram of filtering employed for
wireless communication within each cell in accordance with one
embodiment of the present disclosure, and is intended to be read in
conjunction with FIGS. 1, 9A, 9B, 10C-10E, and 11. The filtering
system 1200 depicted is implemented within each transceiver base
station 110 and each subscriber access device on subscriber
premises 121-123. The parameters for filtering system 1200
implemented within each subscriber premises 121-123 will be
described, although those skilled in the art will recognize that
the filtering systems within each transceiver base station 110 will
simply have the transmission and reception frequencies (i.e.,
downlink or uplink frequencies F1 and F2) reversed or otherwise
changed.
[0199] Wireless signals at the appropriate downlink and uplink
frequencies F1 and F2 for the subject cell and sector are
transmitted and received via antenna 1201 and separated by a
diplexer 1202. Signals received from or passed to diplexer 1202 are
filtered utilizing filters 1203 and 1204 tuned to downlink and
uplink frequencies F1 and F2, respectively. The signal received
from filter 1203 is mixed with a signal from a local oscillator
1205 tuned to the downlink frequency F1, while the signal
transmitted to filter 1204 is mixed with a signal from a local
oscillator 1206 tuned to the uplink frequency F2. If direct
conversion is utilized, the output of mixer 1207 may be connected
directly to analog-to-digital (A/D) converter 1208, and the input
to mixer 1209 may be connected directly to digital-to-analog (D/A)
convert 1210.
[0200] If super heterodyne conversion is employed, as is
preferable, filtering system 1200 includes a second (optional)
conversion stage 1211. Within conversion stage 1211, the output of
mixer 1207 passes to a filter 1212 tuned to an image frequency
based on the downlink frequency F1, with the filtered output being
mixed with a signal from a local oscillator 1213 also tuned to the
image frequency based on downlink frequency F1 before being passed
to A/D converter 1208. Similarly, signals from D/A converter 1210
are mixed with a signal from a local oscillator 1214 tuned to an
image frequency based on the uplink frequency F2 and is passed
through a filter 1215 also tuned to the image frequency based on
the uplink frequency F2 before being passed to mixer 1209.
[0201] A/D and D/A converters 1208 and 1210 are coupled to a
digital modulator/demodulator 1216 which decodes and generates the
digital signals from the wireless communications downlinks and
uplinks. Additional digital filtering 1217 may optionally be
employed between A/D converter 1208 and modulator/demodulator 1216.
The filters 1203, 1204, 1212 and 1215, mixers 1207, 1209, 1218 and
1219, A/D/ and D/A converters 1208 and 1210, digital filter 1217,
and digital modulator/demodulator 1216 may be implemented in either
hardware or software, collectively, individually, or in any
combination of the individual elements.
[0202] Filtering system 1200 should have two essential
characteristics for successful implementation of a TDD FDD system
in accordance with the present disclosure. First, the frequency
switching time between the uplink and downlink frequencies for the
filtering system 1200 within all transceivers (within each
transceiver base station 110 and each subscriber premises 121-123)
must be sufficiently fast to complete during the frequency change
physical slots 1121 and 1131. Frequency change physical slots 1121
and 1131, together with guard times 1122 and 1132, insure that
transmission of an uplink/downlink sub-frame is completed
successfully before transmission of the next sub-frame is started.
Frequency switching should preferably take no longer than 1/4 to
1/10 the duration of physical slots 1121 and 1131. Physical slots
1121 and 1131 and/or guard times 1122 and 1132 may alternatively be
extended in duration to accommodate longer frequency switching
times within a transceiver between the downlink and uplink
frequencies.
[0203] Second, filtering system 1200 must filter transmitted and
received signals in depth to ensure, in conjunction with the duplex
spacing employed between the downlink and uplink frequencies F1 and
F2, that spurious out-of-band transmission products do not
interfere with the receiver. FIG. 13 illustrates a spectral
response for filtering employed for wireless communication within
each cell and sector in accordance with one embodiment of the
present disclosure. A signal strength 1300 at which unacceptable
interference prevents successful communication may be identified or
defined for a particular system. Filtering system 1200 should pass
signals within the band 1301 allocated to downlink frequency F1 and
within the band 1302 allocated to uplink frequency F2. By virtue of
duplex spacing 1013 between the downlink and uplink frequencies F1
and F2, together with the in-depth filtering performed by filtering
system 1200, out-of-band signals are sufficiently rejected to
prevent the signal strength from approaching interference level
1300.
[0204] FIG. 14 is functional diagram of filtering employed for
wireless communication within each cell and sector in accordance
with another embodiment of the present disclosure. Filtering system
1400 receives wireless signals at the appropriate downlink and
uplink frequencies F1 and F2 for the subject cell and sector via
antenna 1201. Signals received from or passed to antenna 1201 are
filtered utilizing filter 1401, which covers the full FDD band
employed for the subject sector. A switch 1402 selective connects
the filter 1401 to a power amplifier (PA) 1403 for transmission or
to a low noise amplifier (LNA) 1404 for reception.
[0205] In the embodiment depicted in FIG. 14, the conversion stages
coupled to power amplifier 1403 and low noise amplifier 1404 are
bidirectional, and as a result of the TDD aspect of the signal
pattern employed may be reused for both transmitting and receiving
signals. Local oscillator 1405 coupled to mixer 1406 should be
capable of switching frequencies, converting signals at either the
downlink frequency F1 or the uplink frequency F2 to an image
frequency. Optional second stage 1407 for superheterodyne
conversion includes a filter 1408 and local oscillator 1409 both
tuned to the image frequency and a mixer 1410. A/D converter 1208
and D/A converter 1210 are both connected to mixer 1410.
[0206] An FDD TDD strategy according to the present disclosure
permits filtering and conversion to be performed along a single,
bi-directional signal path which is reused for both the downlink
and the uplink, eliminating the need for separate paths and
reducing the system costs. The spectral performance illustrated in
FIG. 13 should be implemented by filtering system 1400, with the
frequency switching time for local oscillator 1405 within the first
conversion stage being critical to meeting the timing requirements
imposed by the FDD TDD system of some embodiments of the present
disclosure.
[0207] It is important to note that while the present disclosure
has been described in the context of a fully functional data
processing system and/or network, those skilled in the art will
appreciate that the mechanism of the present disclosure is capable
of being distributed in the form of a computer usable medium of
instructions in a variety of forms, and that the present disclosure
applies equally regardless of the particular type of signal bearing
medium used to actually carry out the distribution. Examples of
computer usable mediums include: nonvolatile, hard-coded type
mediums such as read only memories (ROMs) or erasable, electrically
programmable read only memories (EEPROMs), recordable type mediums
such as floppy disks, hard disk drives and CD-ROMs, and
transmission type mediums such as digital and analog communication
links.
[0208] Although the present disclosure has been described in
detail, those skilled in the art should understand that they can
make various changes, substitutions and alterations herein without
departing from the spirit and scope of the disclosure in its
broadest form.
* * * * *