U.S. patent application number 11/982459 was filed with the patent office on 2008-10-23 for system for coordination of communication within and between cells in a wireless access system and method of operation.
This patent application is currently assigned to Raze Technologies, Inc.. Invention is credited to Paul F. Struhsaker.
Application Number | 20080259826 11/982459 |
Document ID | / |
Family ID | 39872725 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080259826 |
Kind Code |
A1 |
Struhsaker; Paul F. |
October 23, 2008 |
System for coordination of communication within and between cells
in a wireless access system and method of operation
Abstract
Apparatus, and an associated method, for providing WLAN
(wireless local area network) service through a fixed wireless
access communication system. WLAN transceivers are fixed in
position at subscriber stations of the fixed wireless access
communication system. Each WLAN transceiver defines a coverage
area. Through appropriate positioning of the WLAN transceivers at
the subscriber stations, overlapping coverage areas are formable
and between which handovers of communications with a mobile station
are effectuated when a mobile station travels out of one coverage
area and into another coverage area.
Inventors: |
Struhsaker; Paul F.; (Plano,
TX) |
Correspondence
Address: |
DOCKET CLERK
P.O. DRAWER 800889
DALLAS
TX
75380
US
|
Assignee: |
Raze Technologies, Inc.
Plano
TX
|
Family ID: |
39872725 |
Appl. No.: |
11/982459 |
Filed: |
October 31, 2007 |
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Current U.S.
Class: |
370/280 |
Current CPC
Class: |
H04W 88/08 20130101;
H04W 72/042 20130101; H04W 84/14 20130101; H04W 84/12 20130101;
H04W 88/02 20130101 |
Class at
Publication: |
370/280 |
International
Class: |
H04J 3/06 20060101
H04J003/06 |
Claims
1-24. (canceled)
25. For use in a wireless access network comprising a plurality of
base stations capable of bidirectional time division duplex (TDD)
communication with wireless access devices disposed at a plurality
of subscriber premises, a TDD frame transmission synchronization
apparatus comprising: a frame allocation controller capable of
receiving from a first radio frequency (RF) modem shelf associated
with a first base station access requests generated by a first
group of wireless access devices communicating with said first base
station and determining from traffic requirements associated with
said access requests a time duration of one of the longest downlink
portion of TDD frames used by a first one of a plurality of RF
modems in said RF modem shelf to communicate with a first wireless
access device, wherein said frame allocation controller further
determines a frame allocation of the downlink portion and the
uplink portion of TDD frames used by said plurality of RF modems to
communicate with said first group of wireless access devices.
26. For use in a wireless access network comprising a plurality of
base stations, each of said plurality of base stations capable of
bidirectional time division duplex (TDD) communication with
wireless access devices disposed at a plurality of subscriber
premises in an associated cell site of said wireless access
network, a method of transmitting a beam, the method comprising:
transmitting directed scanning beam signals in a sector of a cell
site associated with a first base station of the wireless access
network, wherein said transmit path circuitry transmits at a start
of a TDD frame a broadcast beam signal comprising a start of frame
field and subsequently transmits downlink data traffic in a
downlink portion of said TDD frame to at least one of said wireless
access devices using at least one directed scanning beam signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority to
copending U.S. patent application Ser. No. 09/839,499 filed on Apr.
20, 2001 and entitled "APPARATUS, AND AN ASSOCIATED METHOD, FOR
PROVIDING WLAN SERVICE IN A FIXED WIRELESS ACCESS COMMUNICATION
SYSTEM". 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) Ser.
No. 09/839,458, filed Apr. 20, 2001, entitled "WIRELESS ACCESS
SYSTEM USING MULTIPLE MODULATION FORMATS IN TDD FRAMES AND METHOD
OF OPERATION", now U.S. Pat. No. 7,173,916. [0017] 14) 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; [0018] 15) Copending Ser. No. 09/838,924, filed Apr. 20,
2001, entitled "APPARATUS FOR ESTABLISHING A PRIORITY CALL IN A
FIXED WIRELESS ACCESS COMMUNICATION SYSTEM"; [0019] 16) 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; [0020] 17) 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;
[0021] 18) 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. Nos. 6,925,516; and [0022]
19) Ser. No. 09/948,059, filed Sep. 5, 2001, entitled "WIRELESS
ACCESS SYSTEM USING SELECTIVELY ADAPTABLE BEAM FORMING IN TDD
FRAMES AND METHOD OF OPERATION", now U.S. Pat. No. 7,230,931.
[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 fixed 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] 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.
[0042] A common protocol used in the air interface between the base
stations and the subscriber fixed wireless access (FWA) devices is
time division duplex transmission. In TDD systems, the same channel
is used for both transmitting and receiving. During a downlink time
slot, the base stations transmit and the subscriber FWA devices
receive. During an uplink time slot, the subscriber FWA devices
transmit and the base station receive.
[0043] However, the difference between transmitted power and
received power at a base station cell site can be on the order of
100 db. For a cell with multiple sectors collocated at the same
cell site, if the TDD transmit and receive time slots are not
properly synchronized, the system suffers from self interference.
While low side lobe antennas and large separation of frequencies
can minimize the self interference at the cell, these techniques
increase antenna cost and reduce spectral efficiency, since fewer
frequencies can be used in a given cell. This problem extends to
cell-to-cell interference where uncoordinated transmissions
increase the level of interference (carrier-to-interference ratio,
C/I) for a receiver.
[0044] Therefore, there is a need in the art for time division
duplex (TDD) fixed wireless access (FWA) systems that minimize
interference between cell sites and between sectors within a single
cell site. In particular, there is a need for time division duplex
(TDD) fixed wireless access systems that implement improved timing
and synchronization circuitry that coordinates the transmission and
reception of TDD data bursts in the uplink and downlink time slots.
More particularly, there is a need for timing and synchronization
circuitry that provides highly accurate synchronization of the
uplink and downlink time slots within sectors in a single cell site
and between multiple cells sites in order to reduce both self
interference and cell-to-cell interference.
[0045] 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:
[0046] 1. FDMA--frequency division multiple access. In a FDMA
system, subscribers use separate frequency channels on a permanent
or demand access basis.
[0047] 2. TDMA--time division multiple access. In a TDMA system,
subscribers share a frequency channel but allocate spans of time to
different users.
[0048] 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.
[0049] 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.
[0050] 5. PDMA--polarization division multiple access--In a PDMA
system, subscribers share a frequency but change polarization of
the antenna.
[0051] 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.
[0052] 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.
[0053] 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).
[0054] A number of references and overviews of demand access are
available including the following: [0055] 1. Sklar, Bernard.
"Digital Communications Fundamentals and Applications," Prentice
Hall, Englewood Cliffs, N.J., 1988. Chapter 9. [0056] 2. Rappaport,
Theodore. "Wireless Communications, Principles and Practice,"
Prentice Hall, Upper Saddle River, N.J., 1996. Chapter 8. [0057] 3.
TR101-173V1.1. "Broadband Radio Access Networks, Inventory of
Broadband Radio Technologies and Techniques," ETSI, 1998. Chapter
7.
[0058] 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 are 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 N.times.64 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] An 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 access networks
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 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 wireless access network that implements an air interface
that minimizes uplink and downlink interference between different
cell sites within the access network. More particularly, there is a
need in the art for an access network that efficiently allocates
bandwidth to individual subscribers according to dynamically
changing applications used by the individual subscribers.
SUMMARY
[0072] To address the above-discussed deficiencies of the prior
art, the present disclosure provides a system and method for use in
access networks to minimize interference within a cell and between
cells. The present disclosure is also directed, in general, to
communication network access systems and, more specifically, to
access equipment for use in both wireless and wireline
telecommunications systems.
[0073] A wireless access network includes a plurality of base
stations capable of bidirectional time division duplex (TDD)
communication with wireless access devices disposed at a plurality
of subscriber premises. A TDD frame transmission synchronization
apparatus for use in the wireless access network includes a frame
allocation controller that is operable to receive from a first
radio frequency (RF) modem shelf associated with a first base
station access requests generated by a first group of wireless
access devices communicating with the first base station. The frame
allocation controller is also operable to ascertain from traffic
requirements associated with the access requests a time duration of
a longest downlink portion of TDD frames used by a first one of a
plurality of RF modems in the RF modem shelf to communicate with a
first wireless access device. The frame allocation controller is
also operable to determine a frame allocation of a downlink portion
and an uplink portion of TDD frames used by the plurality of RF
modems to communicate with the first group of wireless access
devices.
[0074] A method of transmitting a beam, for use in the wireless
access network, includes transmitting directed scanning beam
signals in a sector of a cell site associated with a first base
station of the wireless access network. Transmit path circuitry is
operable to transmit at a start of a TDD frame a broadcast beam
signal comprising a start of frame field. The transmit path
circuitry is further operable to subsequently transmit downlink
data traffic in a downlink portion of the TDD frame to at least one
of the wireless access devices using at least one directed scanning
beam signal.
[0075] 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
[0076] 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:
[0077] FIG. 1 illustrates an exemplary fixed wireless access
network according to one embodiment of the present disclosure;
[0078] 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;
[0079] FIG. 3 illustrates an exemplary time division duplex (TDD)
time division multiple access (TDMA) frame according to one
embodiment of the present disclosure;
[0080] FIG. 4 illustrates the timing recovery and distribution
circuitry 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. 5D illustrates an interface tray in an exemplary RF
modem shelf according to one embodiment of the present disclosure;
and
[0085] 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;
[0086] 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;
[0087] 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;
[0088] FIG. 9 illustrates selected portions of the receive path of
an exemplary conventional analog beam-forming system.
[0089] FIG. 10 represents an exemplary spatial response of the
receive path of the exemplary analog beam-forming system in FIG.
9;
[0090] FIG. 11 illustrates selected portions of the transmit path
and the receive path of an exemplary conventional digital
beam-forming system;
[0091] FIG. 12A illustrates an exemplary beam scanning pattern
according to one embodiment of the present disclosure;
[0092] FIG. 12B represents the spatial response of the transmit and
receive scanning beams in the exemplary beam scanning pattern in
FIG. 12A;
[0093] FIG. 13A illustrates an exemplary broadcast pattern
according to one embodiment of the present disclosure;
[0094] FIG. 13B represents the spatial response of the broadcast
beam in FIG. 13A; and
[0095] FIG. 14 illustrates the use of broadcast beams and scanning
beams in exemplary time division duplex (TDD) frames according to
one embodiment of the present disclosure.
DETAILED DESCRIPTION
[0096] 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 access system.
[0097] 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. The present disclosure may be implemented in
any type of broadband access systems, including 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.
[0098] 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. The principles of the present disclosure may be
used in those systems that are not considered fixed wireless system
such as mobile, portable or battery-powered wireless systems that
serve as emergency backups for fixed systems in case of a power
blackout or natural disaster.
[0099] 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.
[0100] RF modem shelf 140 comprises a plurality of RF modems
capable of modulating (i.e., up-converting) the baseband data
traffic and demodulating (i.e., 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.
[0101] 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.
[0102] 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.
[0103] 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. Although FIG. 2 illustrates a portion of a
fixed wireless access network, other systems such as mobile,
portable or battery-powered wireless systems may also be used.
[0104] 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.
[0105] 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).
[0106] 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.
[0107] 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.
[0108] 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.
[0109] 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.
[0110] 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:
[0111] 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).
[0112] 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.
[0113] 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.
[0114] 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, such as inter-cell 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.
[0115] 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.
[0116] Exemplary downlink slot 341 is shown in greater detail.
Downlink slot 341 comprises burst header 371, encapsulated packet
data unit (PDU) 372, and forward error correction check sum value
373. The length of downlink slot 341 varies according to the
modulation format used communicate with the subscriber access
device to which downlink slot 341 is transmitted. The other
downlink slots and uplink slots in TDD frame 321 are similar in
structure to downlink slot 341.
[0117] 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.
[0118] Thus, to prevent interference between antennas in different
sectors of the same cell site, the present disclosure uses a highly
accurate distributed timing architecture to align the start points
of the downlink transmissions. The present disclosure also
determines the length of the longest downlink transmission and
ensures that none of the uplink transmissions begin, and none of
the base station receivers begin to receive, until after the
longest downlink is completed.
[0119] 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, the present disclosure also uses the highly
accurate distributed timing architecture to align the start points
of the downlink transmissions between cell sites. The present
disclosure also determines the length of the longest downlink
transmission among two or more cell sites and ensures 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.
[0120] 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.
[0121] 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).
[0122] 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.
[0123] The timing and distribution architecture in RF modem shelf
140 allows for three reference options:
[0124] 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).
[0125] 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.
[0126] Tertiary--An internal phase locked loop (PLL) may be
used.
[0127] 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.
[0128] There are three components to the timing distribution for
the access processor backplane:
[0129] 1. Timing masters (ICP cards 450 and 480).
[0130] 2. Timing slaves (ICP cards 460 and 470).
[0131] 3. Timing references.
[0132] 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.
[0133] 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.
[0134] 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
Frequency External BITS (EXT REF A) 64K, 1544K, 2048K External
BITS/GPS (EXT REF B) 64K, 1544K, 2048K External GPS Sync. Pulse 1
second pulse On-Card Reference Per I/O reference Network I/O
Derived Reference A Per I/O reference Network I/O Derived Reference
B Per I/O reference
[0135] 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 1 Hz Not Applicable Pulse Framing Reference 8 KHz
Free-run framing provided (125 usec) 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 10.000 MHZ Free-run RF reference clock Clock
Communications 100 MHZ Derived from free-run Bus Reference Clock
High-speed 1.31072 GHz REF. Clock x 20 Serial Links High-speed
2.62144 GHz REF. Clock x 40 Serial Links High-speed TBD REF. Clock
x N Serial Links
[0136] 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.
[0137] 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.
[0138] 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.
[0139] 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.
[0140] 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.
[0141] 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.
[0142] 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.
[0143] 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.
[0144] 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:
[0145] 1. DownLink Map indicating the physical slot (PS) where
downstream modulation changes (transmitted every frame);
[0146] 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));
[0147] 3. TDD frame and physical layer attributes (periodic at a
minimum of one second hyperframe period); and
[0148] 4. Other management messages including ACK, NACK, ARQ
requests, and the like (transmitted as required).
[0149] 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.
[0150] 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.
[0151] 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.
[0152] 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.
[0153] 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.
[0154] 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.
[0155] 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).
[0156] 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.
[0157] 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.
[0158] 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.
[0159] 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.
[0160] 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.
[0161] 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.
[0162] 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.
[0163] 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.
[0164] 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.
[0165] 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.
[0166] FIG. 5D illustrates exemplary interface tray 500d associated
with RF modem shelf 140 according to one embodiment of the present
disclosure. Interface tray 500d comprises signal conditioning-10
MHz oscillator circuitry 505d, alarm conditioning circuitry 510d,
RF circulator-power divider circuitry 515d, and 6:1 switches 520d
and 525d. Exemplary interface tray 500d, located above remote modem
shelf 140, is the junction at which the cell site antennas and the
RF modems interconnect. Interface tray 500d provides N+1 redundancy
among the RF modems in RF modem shelf 140, using an RF distribution
circuit housed within interface tray 500d.
[0167] 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 500d. Access
processor shelf 165 shares the same interface tray design.
[0168] All access to the cell tower antennas, alarms, power, 12C,
and BITS timing and GPS signals are accomplished through rear panel
501 of interface tray 500d. RF signals supplied to the RF modem
cards are received through front panel 502 of the tray. All
communications and control with interface tray 500d are done via
discrete connections. Control functions with interface tray 500d
via the remote modem ICP cards are:
[0169] 1. Switching of antennas to the redundant RF Modem
[0170] 2. Alarm indications from external alarms
[0171] 3. CO Output Alarm Indication
[0172] Any external alarms that are detected are conditioned as
necessary by alarm conditioning circuitry 510d 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.
[0173] Interface tray 500d 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 505d, as required, before being transmitted
out front panel 502d for interfacing to RF modem shelf 140.
Interface tray 500d 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 515d 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 515d feeds the
receive only channel.
[0174] 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 520d
and 525d. 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 520d and all of the RX
only signals are to 6:1 switch 525d. Upon detection of an RF modem
failure, master ICP card 450 is notified and the spare modem is
switched in.
[0175] There is a stable 10 MHz oscillator circuit in signal
conditioning-10 MHz oscillator circuitry 505d in interface tray
500d. 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 500d 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 500d.
[0176] 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).
[0177] 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.
[0178] 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).
[0179] 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.
[0180] 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.
[0181] 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).
[0182] 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).
[0183] 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. The present disclosure
uses 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). 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.
[0184] The present disclosure also uses 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.
[0185] 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.
[0186] 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 combination
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.
[0187] 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).
[0188] 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.
[0189] The present disclosure applies beam forming in the transmit
and receive paths of wireless access network 100 for a number of
reasons, including greatly improved to carrier-to-interference
ratio, which allows frequency reuse patterns that require less
spectrum, two to four times the amount of spatial reuse using
multiple simultaneous beams, and increased cell radius,
particularly for the band where the uplink from the subscriber to
the base can use higher receive gain to overcome uplink loss
problems. The use of advanced antenna technology introduces an
additional level of media access control (MAC) complexity.
[0190] The MAC layer and physical (PHY) layer have an added
spatial/beam component that must be factored into MAC layer
coordination of the PHY layer. On a subscriber-by-subscriber basis
(i.e., link-by-link basis), the MAC layer and PHY layer must
coordinate the following parameters:
[0191] 1) Communications burst duration [0192] a) Individual uplink
or downlink for TDD system; and [0193] b) Joint uplink and downlink
for FDD system;
[0194] 2) Modulation complexity;
[0195] 3) FEC rate; and
[0196] 4) Beam/combining parameters.
[0197] Beam forming and advanced antennas change the basic paradigm
that all subscribers have the capability of simultaneously
receiving broadcast information from the base station. Transmit and
receive beams are formed to optimize communications with a given
subscriber with a channel response H.sub.n(t) and beam parameters
B.sub.n(t). The base station forms the beam and either sends or
receives from the subscribers in an order determined by the MAC
layer.
[0198] To support advanced antenna systems both FDD and TDD links
must be designed to provide transmissions based on self-contained
bursts. Conceptually, TDD is easy to understand. A beam is formed
for each transmitted burst in either the upstream or the
downstream. These simple sequential cases can be expanded to
advanced beam forming techniques to provide simultaneous multiple
access to spatially independent users. A beam-forming network can
create two or more independent beams with low self-interference
that allow simultaneous communications using the same frequency.
While beam-forming complexity is increased, spectral reuse is also
increased. The complexity of PHY layer hardware and MAC layer
scheduling software also increase proportionally with the number of
beams created. The MAC and PHY also need to perform burst
scheduling and transmission based on spatial concatenation. One or
more subscribers can be supported by a single set of beam-forming
parameters due to close physical proximity.
[0199] A beam-forming network is a series of antenna elements
combined with a delay-weight network that combines the RF energy of
a wave front incident on the antenna elements. This enhances gain
in a given direction or steers a null in a specific direction. A
beam-forming system consisting of N antenna elements is generally
arranged in a line or in a rectangular array. The N element array
can theoretically steer N-1 nulls. Beam-forming networks are much
less efficient at creating gain patterns for distinct beams.
[0200] The array elements are spaced by W/N, where N is an integer
value 1, 2, 4, 8, . . . , and W is the wavelength of the signal.
Typically, the elements are spaced at W/4. Each of these elements
is connected to a circuit that can be programmed with a variable
delay and phase to each element. The signal products are summed
together to form the final beam (e.g., the ideal constructed beam
diagram). Conceptually this is very much like a FIR signal filter.
The array coherently sums the wave front components from a given
direction by forming a time/phased delay at each element.
[0201] Beam forming can be performed at RF frequencies using analog
delay and phase elements or at baseband frequencies using digital
techniques. FIG. 9 illustrates selected portions of the receive
path of conventional analog beam-forming system 900. Analog
beam-forming system 900 comprises beam-forming network 910,
antennas 931-936, and modem/MAC layer control block 950.
Beam-forming network 910 comprises programmable delay and phase
controller 920, delay (D) elements 921-926, and signal combiner
940. An incident wave front is detected by antenna 931 at time T1,
by antenna 932 at time T2, by antenna 933 at time T3, by antenna
934 at time T4, by antenna 935 at time T5, and by antenna 936 at
time T6.
[0202] As FIG. 9 illustrates, the delay of the signal wave at each
element for a given angle of signal arrival is given by:
t=L/c
where c is the speed of light. The distance, L, between wave fronts
to each element is based on the angle of arrival of the signal
relative the antenna elements. As an example, if the delay, D, for
the antenna elements are all set to be equal, then the antenna has
maximum gain at 0 degrees. If the delays are incrementally set from
delay element 926 to delay element 921 to have the values 0, W/4c,
2W/4c, 3W/4c, . . . , 5W/4c, then the beam that is formed at the
output of signal combiner 940 has maximum gain at 90 degrees (i.e.,
to the right).
[0203] FIG. 10 represents an exemplary spatial response of the
receive path of conventional analog beam-forming system 900. Power
lobe 1010 represents the received beam formed by conventional
analog beam-forming system 900. The signal gain maximum occurs at
an angle of approximately +40 degrees to the right of an arbitrary
0 degree reference in the sector covered by the antennas of
conventional analog beam-forming system 900.
[0204] Analog beam forming has numerous benefits. Analog
beam-forming capabilities can be retrofitted to existing systems by
adding telemetry and time coordination. There is a much lower cost
relative to the baseband configuration. Analog beam-forming systems
also have excellent back-lobe/front-to-back emission
characteristics to meet ETSI TM4 standards. However, analog beam
forming systems are limited in the number of simultaneous beams
according to the maximum number of RF cables up a tower and RF
combiner losses.
[0205] FIG. 11 illustrates selected portions of the transmit path
and the receive path of conventional digital beam-forming system
1100. Digital beam-forming system 1100 comprises modem processors
1110, 1120 and 1130, transmit signal combiner 1140, digital signal
bus 1145 and a plurality of transceiver front-end elements. Modem
processor 1110 is representative of modem processors 1120 and 1130.
Modem processor 1110 comprises modem 1111, programmable transmitter
(TX) weight/delay controller 1112, and programmable receiver (RX)
weight/delay controller 1113.
[0206] Three exemplary transceiver front-end elements are
illustrated. A first transceiver front-end element comprises
antenna 1151, radio frequency/intermediate frequency (RF/IF)
converter 1152, analog-to-digital converter (ADC) 1153, and
digital-to-analog (DAC) 1154. A second transceiver front-end
element comprises antenna 1161, radio frequency/intermediate
frequency (RF/IF) converter 1162, analog-to-digital converter (ADC)
1163, and digital-to-analog (DAC) 1164. Finally, the first
transceiver front-end element comprises antenna 1171, radio
frequency/intermediate frequency (RF/IF) converter 1172,
analog-to-digital converter (ADC) 1173, and digital-to-analog (DAC)
1174.
[0207] As FIG. 11 illustrates, each antenna has a RF/IF
down-converter and up-converter (i.e., RF/IF converters 1152, 1162,
and 1172), an A/D converter, and a D/A converter. The receive path
signals are distributed on high-speed digital signal bus 1145 to be
processed by programmable RX weight/delay controller 1113 and modem
1110 in each baseband modem processor 1110. The transmit path
differs slightly from the receive path. Programmable TX
weight/delay controller 1112 performs the delay and phase
adjustments to the signal to be feed to each antenna element. Then,
transmit signal combiner 1140 (generally an individual card) sums
the transmit signals from the multiple simultaneous channels if
more than one channel is to be transmitted. This summed signal is
presented to the RF/IF converter for each antenna.
[0208] Digital baseband beam forming has the numerous benefits.
Digital baseband beam-forming systems provide the most flexible
configuration and provide all antenna data to the receiver circuits
to allow for rapid calculation of adaptive cancellation. Digital
baseband beam-forming systems also provide all-digital processing
of weights and delay values and reduce the size of the RF
amplifier. However, digital baseband beam-forming systems have a
very high up-front cost. To limit costs, sparse arrays (as opposed
to array panel configurations) are sometimes used, which causes
beam pattern back-lobe problems and sometimes results in failure to
consistently meet ETSI TM4 (i.e., cell-to-cell interference)
standards. Also, critical tower loading is still a problem. High
performance (large number of antenna element) configurations must
have full demodulation and electro-optic interfaces at the
tower.
[0209] The present disclosure uses a combination of the following
techniques to maximize frequency re-use and the number of
subscribers per base station in wireless access network 100:
[0210] 1) beam scanning within a cell sector; and
[0211] 2) spread spectrum transmission to allow down-link broadcast
of synchronization and beam maps for the MAC layer with a cell
sector.
[0212] The present disclosure uses pre-programmed sets of directed
beam patterns to cover a cell in an angular fashion. This is
referred to herein as "beam scanning". FIG. 12A illustrates
exemplary beam scanning pattern 1210 according to one embodiment of
the present disclosure. Beam scanning pattern 1210 covers a cell
sector that is approximately 90 degrees wide. Beam scanning pattern
1210 is covered by nine directed scanning beams, each approximately
10 degrees wide. The base station establishes a set of beams to
cover the cell. Based on physical location in the sector, a
specific beam is used to communicate with one or more subscribers
that lie in that sector. The nine scanning beams may be transmitted
or received in any order.
[0213] FIG. 12B represents exemplary spatial response 1220 of the
transmit and receive scanning beams in beam scanning pattern 1210.
Nine signal gain maximums occur, one for each scanning beam. The
leftmost signal gain maximum is formed at an angle of approximately
-40 degrees to the left of an arbitrary 0 degree reference in the
sector. The rightmost signal gain maximum is formed at an angle of
approximately +40 degrees to the right of an arbitrary 0 degree
reference in the sector. The nine signal gain maximums are spaced
10 degrees apart.
[0214] An important issue when using a spatial/beam component in
wireless access network 100 is subscriber acquisition and
maintaining system synchronization. If wireless access network 100
could guarantee equal distribution of subscribers into the sector
beam set, a simple round robin polling method could be used.
However, uniform distribution is not guaranteed and wireless access
network 100 may use a large number of beams to cover a sector. The
revisit time to any specific beam may result in subscribers falling
out of synchronization.
[0215] One alternative to a guaranteed minimum revisit time is to
have a full-sector coverage broadcast beam and use of spread
spectrum processing gain in the air interface. The broadcast beam
is shown in FIGS. 13A and 13B. FIG. 13A illustrates exemplary
broadcast pattern 1310 according to one embodiment of the present
disclosure. Broadcast pattern 1310 covers all of the 90 degree cell
sector in FIG. 12A. FIG. 13B represents exemplary spatial response
1320 of the broadcast beam. The broadcast beam covers the 90 degree
sector fairly uniformly.
[0216] By introducing spread spectrum and a broadcast beam
operating on the Start of Frame (SOF) field of every frame, the
present disclosure provides improved performance for acquisition
and synchronization as opposed to either equal duration round robin
polling per scan beam or dynamic weighted polling per scan beam
based on the throughput per cell.
[0217] Some elements for the air interface are:
[0218] 1) subscriber receiver (RX) acquisition and
synchronization;
[0219] 2) transmitter (TX) acquisition and ranging;
[0220] 3) map information for beam scheduling and up/down link maps
for a given frame; and
[0221] 4) demand access mechanisms.
[0222] According to an advantageous embodiment of the present
disclosure, the air interface is implemented using a TDD frame
format. According to one embodiment, beam switching occurs on a
frame-by-frame basis so that for a complete TDD frame, only one
beam is active. However, in alternate embodiments of the present
disclosure, more than one beam may be used in each TDD frame,
particularly in the downlink due to typical 4:1 asymmetry. The
uplink may be limited to a single scanning beam per frame.
[0223] To implement the added complexity of switched beam access,
the exemplary TDD frame in FIG. 14 may be used. FIG. 14 illustrates
the use of broadcast beams and scanning beams in exemplary time
division duplex (TDD) signal 1400 according to one embodiment of
the present disclosure. In FIG. 14, single beam per frame mode is
depicted.
[0224] TDD signal 1400 comprises a first TDD frame having a
downlink portion comprising a first start-of-frame field (SOF1), a
first beam map field (Beam Map 1), and a first scan map field (Scan
Map 1). The remainder of the downlink portion comprises preamble
field 501, management field 502, and N modulation groups, including
exemplary modulation groups 503 and 505, which were described above
in greater detail in FIG. 5. The first TDD frame also has an uplink
portion comprising a transmitter-transmitter guard (TTG) slot, a
plurality of registration (REG) minislots, a plurality of
contention (CON) request minislots, and N sub-burst groups,
including sub-burst groups 509 and 510, and receiver-transmitter
guard (RTG) slot 511, which were described above in greater detail
in FIG. 5.
[0225] The first TDD frame is followed by a second TDD frame in
which portions of the downlink are shown. The second TDD frame
comprises a second start-of-frame field (SOF2), a second beam map
field (Beam Map 2), and a second scan map field (Scan Map 2).
[0226] According to one embodiment of the present disclosure, an
exemplary spread spectrum Broadcast Beam is used to transmit the
start-of-frame fields (SOF1 and SOF2) and the beam map fields (Beam
Map 1 and Beam Map 2) in each TDD frame, and distinct scan beams
(i.e., Scan Beam A and Scan Beam B) are used to transmit the scan
map fields (Scan Map 1 and Scan Map 2) and the remainder of the
uplink and downlink portions of each TDD frame.
[0227] The Broadcast Beam Maps provide data indicating which
scanning beam (or beams) are used at which time (measured in
symbols or other baud-oriented time unit) for the frame. For each
scanning beam used, an uplink map and a downlink map must be
provided. The Scanning Beam Map provides the Uplink and downlink
maps. The Scanning Beam Map states at which time in the frame,
using the scanning beam described in the Broadcast Beam Map, the
downlink modulation groups and specific uplink slots with
associated CIDs are allocated for the frame using the specific
scanning beam.
[0228] According to an exemplary embodiment, the Broadcast Beam
uses DSSS spectrum spreading to make up antenna gain loss and
transmits as few fields and frame information as possible to
maintain efficiency. The Broadcast Beam transmits the SOF
synchronization field so that all subscribers, even those that are
not actively communicating (i.e., idle subscribers) can maintain
synchronization.
[0229] Beam Map 1 and Beam Map 2 comprise a Frame Beam Map and
Super-Frame Beam Map containing information that defines scanning
beams that cover the sector, the adjoining sectors of the same base
station, and the sectors of adjoining base stations. The Frame Beam
Map defines the number of downlink beams in the remainder of the
current frame and lists a beam number and frame time (physical
slot) in which each beam starts. The Frame Beam Map also defines
the Uplink beam numbers for each subscriber. The Super-Frame Beam
Map defines the number of beams in the relevant sector.
[0230] Scan Beams A and B provides 802.16 MAP/MAC and standard
packet communications using standard modulation formats. The
remainder of the TDD frame is processed using the Uplink/Downlink
Map information provided by the Broadcast Beam. In such an
embodiment, the Broadcast Beam and the associated Super-Frame and
Frame Beam Maps inform all subscriber wireless access devices which
beams are used. Once the scanning beam is activated, communications
are limited to the subscribers associated with that beam.
[0231] As noted above, the Scanning Beam Map used in the scanning
beams are downlink maps and uplink maps. The downlink map indicates
the modulation format and forward error correction code (FEC) and
time slots enabled on the downlink (i.e., modulation group). The
uplink map indicates the specific subscriber, modulation format,
FEC, and equalizer (cyclo-stationary processing) for the allowed
uplink data bursts.
[0232] The acquisition steps are as follows:
[0233] 1) SOF and Beam Map fields allow initial acquisition
identical to a conventional single beam system, including coarse
and fine frame alignment, baud and frequency NCO lock, and
equalizer initialization;
[0234] 2) access equipment at the subscriber premises creates a
histogram of received signal strength and quality on a per beam
basis;
[0235] 3) subscriber access equipment selects the best scan beam
based on the histogram data; and
[0236] 4) uplink acquisition and ranging initiated by subscriber on
the best scan beam.
[0237] 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.
* * * * *