U.S. patent number RE45,757 [Application Number 11/510,861] was granted by the patent office on 2015-10-13 for cellular wireless internet access system using spread spectrum and internet protocol.
This patent grant is currently assigned to NVIDIA CORPORATION. The grantee listed for this patent is Alan Edward Jones, William John Jones, Roger Phillip Quayle, Shirley Claire Quayle. Invention is credited to Alan Edward Jones, William John Jones, Roger Phillip Quayle.
United States Patent |
RE45,757 |
Quayle , et al. |
October 13, 2015 |
Cellular wireless internet access system using spread spectrum and
internet protocol
Abstract
A cellular wireless internet access system which operates in the
2.5 to 2.68 GHz band and which must comply with complex government
regulations on power levels, subscriber equipment and interference
levels yet which provides high data rates to users and cell sizes
of 11/2 miles radius or more from base stations with subscriber
equipment and antennas mounted indoors. Such base stations are
mounted low and use spread-spectrum transmission to comply with
interference rules with respect to adjacent license areas. An
unidirectional tear-drop coverage pattern is used at multiple cells
to further reduce interference when required. Time division duplex
is used to allow the system to operate on any single channel of
varying bandwidth within the 2.5 to 2.68 GHz band. Backhaul
transmission from base stations to the Internet is provided using
base station radio equipment, operating either on a different
frequency in the band or on the same frequency using a
time-division peer-to-peer technique. Different effective
data-rates are provided by a prioritization tiering technique.
Inventors: |
Quayle; Roger Phillip
(Burlingame, CA), Jones; William John (Chippenham,
GB), Jones; Alan Edward (Wiltshire, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Quayle; Roger Phillip
Quayle; Shirley Claire
Jones; William John
Jones; Alan Edward |
Burlingame
Paraparaumu
Chippenham
Wiltshire |
CA
N/A
N/A
N/A |
US
NZ
GB
GB |
|
|
Assignee: |
NVIDIA CORPORATION (Santa
Clara, CA)
|
Family
ID: |
23717734 |
Appl.
No.: |
11/510,861 |
Filed: |
August 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09432824 |
Nov 2, 1999 |
6865169 |
Mar 8, 2005 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
63/083 (20130101); H04W 12/06 (20130101); H04L
63/0853 (20130101); Y10S 370/908 (20130101); Y10S
370/913 (20130101); H04W 8/265 (20130101); H04L
67/14 (20130101); H04W 84/042 (20130101); H04L
69/329 (20130101) |
Current International
Class: |
G06F
15/177 (20060101); H04W 4/00 (20090101); H04M
3/00 (20060101); G06F 15/16 (20060101); H04L
29/06 (20060101); H04L 29/08 (20060101); H04W
12/06 (20090101); H04W 8/26 (20090101) |
Field of
Search: |
;370/335,216,235,231,332,479,328 ;455/561,446,137 |
References Cited
[Referenced By]
U.S. Patent Documents
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May 1999 |
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Aug 1999 |
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Apr 1999 |
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GB |
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Sep 1998 |
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JP |
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Apr 1999 |
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JP |
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WO-9901969 |
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WO |
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WO-0046963 |
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WO |
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WO-0128168 |
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WO-0131843 |
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WO-0141470 |
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WO-0167706 |
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WO-0167716 |
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Sep 2001 |
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WO |
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WO-0169858 |
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WO |
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WO |
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Feb 2002 |
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May 2002 |
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WO |
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Primary Examiner: Wendell; Andrew
Assistant Examiner: Trandai; Cindy
Claims
What is claimed is:
.[.1. A cellular wireless internet access system comprising: a
plurality of portable subscriber terminals each having a directly
attached antenna for communicating in a predetermined frequency
band with a predetermined nearby cellular base station; a plurality
of cellular base stations each transmitting and receiving in said
predetermined frequency band at a single frequency with a
predetermined said plurality of said subscriber terminals; and
means for operating said base station on a small frequency
allocation obtainable anywhere within the designated frequency band
using a single frequency channel of varying bandwidth between 6 and
24 MHz using different spread spectrum transmission chip rates; and
means for operating said base station in a time-division-duplex
mode to enable said transmitting and receiving at said single
frequency channel thus avoiding the need for separate channels
spaced apart for transmit and receive and including means for
allocating the ratio of time for transmitting and receiving on a
predetermined basis said time division as a function of expected
traffic demand; means for providing high net data rates of 1.5-3.0
Mbps using a plurality of data bearer subchannels on said single
frequency channel, orthogonal downlink spreading codes for CDMA
transmission, and successive interference cancellation or
simultaneous uplink spreading codes..].
.[.2. A system as in claim 1 where each band is divided in the time
domain into frames and each frame has a predetermined number of
time slots allocated to control, uplink, and downlink
communications between said cellular base stations and subscriber
terminals..].
.[.3. A system as in claim 2 where some of said frames are
dedicated to backhaul communication between base stations on a
peer-to-peer basis..].
.[.4. A system as in claim 2 where the data transmission rate is
increased during time domain frames used for backhaul communication
by switching to directional antennas during these timeslots thus
providing an improved radio channel quality to support such
increased data rate..].
.[.5. A system as in claim 1 where said means for using different
transmission chip rates provides net data rates of 1.5-3.0 Mbps on
said small frequency allocation..].
.Iadd.6. A cellular wireless base station, comprising: a
transmitter and a receiver for communicating with at least one
portable subscriber terminal; and a processor for providing a
baseband signal, the baseband signal comprising a plurality of data
bearing sub channels and representing a wireless RF signal for
transmission by the transmitter in a time-division mode on a single
frequency channel, the processor further comprising a bandwidth
selector configured to select a radio bandwidth of the single
frequency channel, based upon an integer multiple of a first
bandwidth, wherein the radio bandwidth selected has a width of the
first bandwidth or an integer multiple of the first bandwidth;
wherein: the processor is further operable to inverse multiplex the
baseband signal into multiple downlink data bearing signals, and
the transmitter is operable to transmit the multiple downlink data
bearing signals simultaneously on the single frequency channel in
the time-division mode at individual data rates that combine to an
aggregate data rate. .Iaddend.
.Iadd.7. The base station of claim 6, wherein the first bandwidth
is no greater than about 6 MHz..Iaddend.
.Iadd.8. The base station of claim 7, wherein the radio bandwidth
is selectable from a group consisting of 6, 12, 18 and 24 MHz.
.Iaddend.
.Iadd.9. The base station of claim 8, wherein the baseband signal
has a net data rate in a range of 1.5-6.0 Mbps. .Iaddend.
.Iadd.10. The base station of claim 6, wherein the transmitted
signal conforms to the UMTS standard and the single frequency
channel is a microwave frequency. .Iaddend.
.Iadd.11. The base station of claim 6, wherein the receiver is
operable to apply interference cancellation to uplink data bearing
signals received by the receiver. .Iaddend.
.Iadd.12. The base station of claim 6, wherein the base station
resides in a first service area adjoining a second service area,
and, at a predefined boundary between the first service area and
second service area, an aggregate signal level on the frequency
channel transmitted from the first service area is below a
predetermined threshold. .Iaddend.
.Iadd.13. The base station of claim 12, wherein the predetermined
threshold is -65 dB relative to a level of a signal that is
transmitted by a base station in the second service area within the
frequency channel and measured 35 miles from the base station in
the second service area. .Iaddend.
.Iadd.14. The base station of claim 6, wherein the base station
resides in a first service area adjoining a second service area,
and an aggregate signal level transmitted from the first service
area relative to a signal transmitted on an adjacent channel in the
first or second service area is below a predetermined threshold.
.Iaddend.
.Iadd.15. The base station of claim 14, wherein the predetermined
threshold is 0 dB. .Iaddend.
.Iadd.16. The method of claim 6, wherein the bandwidth selector is
configured to select the radio bandwidth of the single frequency
channel for the communicating with the at least one subscriber
terminal via both the transmitter and the receiver. .Iaddend.
.Iadd.17. A method for communicating over a cellular wireless
network, the method comprising, at a base station: selecting a
radio bandwidth of a single frequency channel based upon an integer
multiple of a first bandwidth, wherein the radio bandwidth selected
has a width of the first bandwidth or has a width that is an
aggregation of multiples of the width; and communicating on the
single frequency channel with at least one portable subscriber
terminal using a signal comprising a plurality of data bearing sub
channels in a time-division mode, wherein the communicating
comprises: inverse multiplexing the signal into multiple downlink
data bearing signals; and transmitting the multiple downlink data
bearing signals simultaneously in the sub channels at individual
data rates that combine to an aggregate data rate. .Iaddend.
.Iadd.18. The method of claim 17, wherein the first bandwidth is no
greater than about 6 MHz. .Iaddend.
.Iadd.19. The method of claim 18, wherein the radio bandwidth is
selectable from a group consisting of 6, 12, 18 and 24
MHz..Iaddend.
.Iadd.20. The method of claim 19, wherein the signal has a net data
rate in a range of 1.5-6.0 Mbps. .Iaddend.
.Iadd.21. The method of claim 17, wherein the signal conforms to
the UMTS standard. .Iaddend.
.Iadd.22. The method of claim 17, further comprising applying
interference cancellation to uplink data bearing signals received
by the receiver. .Iaddend.
.Iadd.23. The method of claim 17, wherein the base station resides
in a first service area adjoining a second service area, and, at a
predefined boundary between the first service area and the second
service area, an aggregate signal level on the frequency channel
transmitted from the first service area is below a predetermined
threshold. .Iaddend.
.Iadd.24. The method of claim 23, wherein the predetermined
threshold is -65 dB relative to a level of a signal that is
transmitted by a base station in the second service area within the
frequency channel and measured 35 miles from the base station in
the second service area. .Iaddend.
.Iadd.25. The method of claim 17, wherein the base station resides
in a first service area adjoining a second service area, and an
aggregate signal level transmitted from the first service area
relative to a signal transmitted on an adjacent channel in the
first or second service area is below a predetermined
threshold..Iaddend.
.Iadd.26. The method of claim 25, wherein the predetermined
threshold is 0 dB. .Iaddend.
.Iadd.27. A portable user equipment (UE), comprising: a transmitter
and a receiver for communicating with at least one base station;
and a processor for receiving a baseband signal, the baseband
signal comprising at least one data bearing sub channel and
representing a wireless RF signal for transmission by the
transmitter in a time-division mode on a single frequency channel,
the processor further comprising a bandwidth selector configured to
select a radio bandwidth of the single frequency channel based upon
an integer multiple of a first bandwidth, wherein the radio
bandwidth selected has a width of the first bandwidth or an integer
multiple of the first bandwidth and wherein the processor is
further operable to inverse multiplex the baseband signal into at
least one uplink data bearing signal, and the transmitter is
operable to transmit the at least one uplink data bearing signal
simultaneously on the single frequency channel in the time-division
mode using at least one data rate that combine to an aggregate data
rate. .Iaddend.
.Iadd.28. The UE of claim 27, wherein the first bandwidth is no
greater than about 6 MHz. .Iaddend.
.Iadd.29. The UE of claim 28, wherein the radio bandwidth is
selectable from a group consisting of 6, 12, 18 and 24
MHz..Iaddend.
.Iadd.30. The UE of claim 29, wherein the baseband signal has a net
data rate in a range of 1.5-6.0 Mbps. .Iaddend.
.Iadd.31. The UE of claim 27, wherein the transmitted signal
conforms to the UMTS standard. .Iaddend.
.Iadd.32. A method for communicating over a cellular wireless
network, the method comprising, at a portable user equipment (UE):
determining a selected radio bandwidth of a single frequency
channel based upon an integer multiple of a first bandwidth,
wherein the radio bandwidth selected has a width of the first
bandwidth or an integer multiple of the first bandwidth: and
communicating on the single frequency channel with at least one
base station using a signal comprising at least one data bearing
sub channel in a time-division mode, wherein the communicating
comprises: inverse multiplexing the signal into multiple downlink
data bearing signals; and transmitting the multiple downlink data
bearing signals simultaneously in the sub channels at individual
data rates that combine to an aggregate data rate. .Iaddend.
.Iadd.33. The method of claim 32, wherein the first bandwidth is no
greater than about 6 MHz..Iaddend.
.Iadd.34. The method of claim 33, wherein the selected radio
bandwidth is selectable from a group consisting of 6, 12, 18 and 24
MHz. .Iaddend.
.Iadd.35. The method of claim 34, wherein the signal has a net data
rate in a range of 1.5-6.0 Mbps. .Iaddend.
.Iadd.36. The method of claim 32, wherein the signal conforms to
the UMTS standard. .Iaddend.
Description
The present invention is directed to a Cellular Wireless Internet
Access System using spread spectrum and Internet Protocol (IP) and
more specifically to a system which typically operates in the 2.5
to 2.68 GigaHertz (GHz) frequency band in the U.S.A., but which is
also capable of operating in other bands in the U.S.A. or other
countries.
BACKGROUND OF THE INVENTION
Serving the mass market of high-speed Internet access to small
business and residential consumers with wireless technology
requires either a large amount of radio spectrum or radio
transmission techniques which efficiently use the radio spectrum or
both. Especially in the United States it is difficult to identify a
frequency band with a large amount of spectrum that is sufficiently
free and designated by the Federal Communications Commission for
such use. Also, the frequency band must have suitable propagation
characteristics for the geography being served as well as being
available and licensed for the specific application.
Another significant factor is that, as in present cellular
telephone systems, power and signal levels must be restricted and
reuse of frequencies managed to prevent interference amongst the
spectrum users and to neighboring frequencies.
Finally, in order to be able to practically and efficiently serve a
very large number of subscribers in a given geographic area (those
subscribers with personal computers needing high-speed Internet
connections on a wireless basis), it is necessary to provide
technology that is able to be installed by the subscriber and to
operate inside a building without an external antenna, provide
coverage of all buildings within an area and furthermore to utilize
base stations which can be easily deployed without delays due to
site acquisition and environmental or zoning approvals.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
improved Cellular Wireless Internet Access System that meets the
above requirements. Specifically, the object is to provide the
optimum combination of high data rate, cell size, ubiquitous
in-building coverage, regulatory compliance, interference avoidance
and management, and overall quality of service.
In accordance with the above object, there is provided a Cellular
Wireless Internet Access System comprising a plurality of cellular
base stations located at low ground level for transmitting and
receiving in a predetermined frequency band. Such frequency band
has interference sources and recipients in other license areas, the
signals from or to low ground level base stations causing or
suffering the interference are attenuated by foliage, building
penetration, building clutter and terrain losses.
A plurality of portable subscriber terminals each having a
directly-attached antenna communicates in the frequency band with a
nearby cellular base station. A substantial proportion of the
plurality of portable subscriber terminals are located in
buildings. The cellular base stations have low-to-ground level
mounting for reduced environmental impact but a high enough system
gain and a geographically frequent location in close proximity to
any one of the portable subscriber terminals to overcome the above
mentioned losses and both transmit and receive to and from
subscriber terminals in the buildings.
Other features of the system include techniques for operating in
small allocations of radio spectrum, providing high system
capacity, providing high speed service to subscriber terminals
located inside of buildings, routing of backhaul transmissions
through adjacent or nearby base stations, interference reduction
techniques, distributed core network functions, tiering of
subscriber service speeds and enhanced time division duplex modes
to allow operation for both transmission and reception on a single
frequency. All of the foregoing is built on the foundation of a
direct-sequence spread-spectrum wideband
Code-Division-Multiple-Access (CDMA) type of system which provides
the highest performing combination of coverage, in-building signal
penetration, data transmission rates and subscriber capacity, and
allows the use of techniques to reduce the effects of
interference.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of an overview of the Cellular Wireless
Internet Access System of the present invention.
FIG. 2 is a block diagram of a base station.
FIG. 3 is a block diagram of a subscriber terminal or user
equipment.
FIG. 4 is a block diagram of a core portion of FIG. 1.
FIG. 5 is an illustration of interference between a Cellular
Wireless Internet Access System service area and an adjoining
service area of another operator as specified by the Federal
Communications Commission.
FIG. 6 is an illustration of interference avoidance by the specific
mounting of base stations and is based on FIG. 5.
FIG. 7 is a prior art cellular coverage pattern.
FIG. 8 is a variation of FIG. 5 illustrating interference reduction
a coverage pattern achieved using directional antennas.
FIG. 9 is a frequency utilization diagram.
FIG. 10 is a multiple bearer transmission diagram.
FIG. 11 is a schematic illustrating backhaul transmission.
FIG. 12 is a schematic illustrating backhaul transmission of
another type.
FIG. 13 is a diagram showing core network functions.
FIG. 14 is a block diagram showing service tiering.
FIG. 15 is a timing diagram and an illustration of Internet packets
showing a time division duplex feature of the present
invention.
FIG. 16 is a flowchart of channel measurement used in transmission
rate adaption.
FIG. 17 is a diagram showing delay spread caused by multipath
signals
FIG. 18 is a diagram showing successive interference
cancellation
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIG. 1, the wireless aspects of the cellular
Wireless Internet Access System of the present invention is broadly
based on a wideband code division multiple access (W-CDMA) system
which is derived from European third-generation (3G) cellular
wireless standards and also known by the acronyms UMTS for
Universal Mobile Telephone Service and UTRAN for Universal
Terrestrial Radio Access Network but which has enhancements. Using
such a standard as a basis in the present system to meet the
application requirements of high speed wireless Internet access
provides a very flexible wireless or "air interface" supporting a
range of net subscriber data rates typically up to 6 megabits per
second. Also, radio channel bandwidths from 5 to 20 MHz are
supported. By choosing to use the CDMA spread spectrum concept the
present system provides for lower error rates, higher speed
communication and also immunity to several types of interference,
especially the ability to deal with multi-path signals which would
otherwise be detrimental to the system performance.
However, as will be discussed in greater detail below, the present
cellular wireless Internet access system has been specifically
designed to meet the special and particular requirements of the
selected frequency band (especially for the United States) which
has the large amount of available spectrum that is required for
high speed mass market Internet access is at 2.5-2.68 GHz. This
band is known as the "MMDS" (for Multi-channel Multi-point
Distribution Service) and "ITFS" (for Instructional Television
Fixed Service) bands (hereinafter referred to collectively as
"MMDS"). In the United States, a Federal Communications Commission
(FCC) rule making in 1998 opened these bands for two-way
communication services. A subsequent FCC "Report and Order on
Reconsideration" of Jul. 29, 1999 made further changes to the
requirements for operation in these bands. But as will be discussed
below, through the rules imposed by the FCC, there are complex
requirements regarding interference between licenses in adjacent
markets; in other words, the service operator must be able to deal
with interference with adjacent license areas, and at the same time
minimize interference that he originates into such adjacent license
areas.
The present invention will operate in this MMDS frequency band
using a technology optimized for packet data based on a modified
version of the Time-Division-Duplex (TDD) version of the UMTS UTRAN
air interface standard. The optimizations and modifications to the
TDD UTRAN standard are listed below. 1. Modification for operation
in the MMDS Band and compliance with the FCC regulations
(Frequency, bandwidths and radio transmission requirements). 2.
Optimization of the UTRAN protocols (user polling and allocation)
and bearer channel improvements to efficiently support packet data.
3. Modifications to the UTRAN air interface to support higher data
rates (up to 6 Mbps) to subscriber equipment with antennas mounted
inside buildings and cell sizes larger than 1.5 miles in
radius.
As such, the present system is wireless telecommunications access
technology providing low cost, high quality, and high speed
Internet services to residential and small to medium business
customers with net packet data rates up to 6 Mbps, the gross burst
data rate being up to 30.72 Mbps of coded data. Through the use of
Time-Division-Duplex, the system can operate on any discrete
channel of between 6 MHz and 24 MHz (including necessary guard
band) anywhere in the MMDS band in contrast to the prior art of
Frequency-Division-Duplex that requires paired channels separated
by a predetermined guard band. The system operates in a
non-line-of-site multipath radio environment and provides indoor
coverage. The subscriber equipment is user installable. The system
supports user portability and roaming in its total coverage
area.
The system provides "tiering" of service that allow subscribers to
receive different data rate throughput based on the type of service
they have subscribed to. For example, the definition of the lowest
tier of service may provide an equivalent throughput to a dedicated
channel of 384 kbps per second whilst the highest tier of service
may be a 1536 kbps per second equivalent.
End-to-end connectivity is built on the TCP/IP protocol suite. User
data is carried over the radio network using the PPP protocol and
tunneled to an ISP (for Internet Service Provider) using the Layer
2 Tunneling Protocol (L2TP).
The system has the capacity to serve a large number of subscribers
in urban and suburban areas, for example more than 1000 subscribers
per square kilometer.
Referring now specifically to FIG. 1, a network of base stations
11, one of which is illustrated, provides coverage of the target
market. The base station is also referred to as "NODE B" which is
nomenclature from the UMTS standard. Each low-mounted base station
11 with an omni directional antenna 13 provides a radius of
coverage of approximately 0.5 miles. Alternatively, three base
stations 11 may be configured in a sectored coverage pattern with
directional antennas to serve a 1 to 1.5 mile radius. The radius of
coverage may be further increased depending on the radio
environment and the height of the base station antenna. Located
within the base station coverage configuration are hundreds of
subscriber terminals 17 also designated user equipment (UE in UMTS
nomenclature). The subscriber terminal 17 has an attached omni
directional antenna 12 (suitable for in-building use) which
communicates via a code division multiple access (CDMA) radio link
18 to the antenna 13 of base station 11. The subscriber terminal 17
typically connects to the user's personal computer via standard
computer interfaces. It generally requires no external antenna and
is user installable. Base stations 11 are assigned to a particular
network controller 10 by three types of connections, using another
technology such as point to point radio, direct landline etc. 21, a
CDMA radio link 22, or a peer-to-peer routing link 23 using other
NODE B type radio stations 11. Peer-to-peer routing will be
discussed below. The subscriber terminal may also provide a voice
over IP service (VOIP), a digitized packetized voice service for
provision of local telephone access service.
In general, network controller 10 controls user traffic to the
Internet Service Providers (ISP) using the layer 2 tunneling
protocol (L2TP) on top of User Datagram Protocol (UDP) and Internet
Protocol (IP) and/or Asynchronous Transfer Mode (ATM) over a high
speed fiber or microwave link 25. This is aggregated and routed
over a private ATM network via an Internet router 24. The microwave
radio or land-line link 26 is then connected to the
Internet/intranet global communication network 27.
Network controller 10 as illustrated in FIG. 1 has three units
designated RNC (Radio Network Controller) 14, SGSN (Serving General
Packet Radio Service Node) 15 and the Layer 2 Tunneling Protocol
Access (LAC) 16 which tunnels traffic through the Internet to the
Internet Service Provider (ISP) or other destination. All of these
are illustrated in FIG. 4 in greater detail. Also, as illustrated
in FIG. 4, there are macro-diversity links supporting a diversity
transmission scheme (not fully discussed) which allow a subscriber
in the overlap area of two or more base stations using the same
channel (who would otherwise suffer excessive interference) to
receive simultaneously from the two base stations and likewise to
transmit to two base stations simultaneously combating the
detrimental effect of interference and radio path fading in both
links.
Specifically the RNC 14, controls and allocates the radio network
resources and provides reliable delivery of user traffic between
the base station and subscriber terminal, the SGSN 15 provides
session control, and lastly, LAC 16 provides the gateway
functionality to the Internet service provider and to the
registration, location and authentication registers using a layer 2
tunneling protocol.
In-building operation without an external antenna is achieved
through the use of the base stations located typically within 1 to
1.5 miles of each subscriber terminal location to allow a signal
margin for building penetration signal loss and other losses; in
other words, the base stations have a high enough system gain,
mounting at building roof top level (on utility poles) and a
geographically frequent location in proximity to a selected
portable subscriber terminal to provide a high enough power to
overcome building penetration attenuation of the signal in both
directions to and from subscriber terminals in the building.
Secondly, the implementation of spread spectrum wideband CDMA
transmission technology in the present system mitigates the
detrimental effect of multiple reflected signals or multipaths.
Cellular base stations 11 are designed so that they can be mounted
on utility poles or on buildings thus avoiding zoning/environmental
approvals and leasing delays typically associated with traditional
cellular telephone towers. Such mounting is enabled by the
relatively low power of the base stations 11 which is made possible
by the use of spread spectrum transmission which inherently allows
low power operation but with a high data rate and low error rate,
the cellular structure where the base stations are in close
proximity to subscriber terminals, and the use of peer-to-peer
routing through other cellular base stations to the network
controller to facilitate the deployment and interconnection of base
stations.
Thus, in summary the subscriber terminal 17 functions to provide a
connection between the subscriber's computer(s) and also any
voice-over-IP (VoIP) connection and the network controller, which
then connects to the Internet or Intranets as desired.
Similarly, each cellular base station 11 provides a radio
connection to multiple subscriber terminals within its coverage
area and the connection to the network controller 10.
A typical NODE B base station 11 is illustrated in FIG. 2 and
conforms in part to UTRAN/UMTS standards. A transmit/receive switch
31 is connected both to a subscriber access antenna 18, and also
the backhaul directional antenna 22 as illustrated in FIG. 1. The
subscriber access diversity receive antenna 32 is a specialized
antenna configuration to improve system performance. The antenna
inputs are amplified in the RF stage 33, demodulated at 34 or
modulated as the case may be and converted from digital to analog,
or vice versa. At the stage 36 they are digitally processed by the
processor units 37 and 38 and then connected to the network
controller 10 and then to router 24 on line 25 as illustrated in
FIG. 1 and eventually the Internet. The various interfaces
illustrated at 39 use industry standard formats.
FIG. 3 illustrates the subscriber terminal user equipment 17 where
the antenna 12 communicates with a base station 11 and these
communications are processed through transmit and receive radio
frequency stages 41, modulator, demodulator stages 42, digital to
analog and analog to digital converters 33, digital processing unit
44 and finally the processor 46 which interfaces at 47 with various
computer user interfaces. Again, some aspects of the user equipment
comply with the standards discussed above.
FIG. 5 illustrates United States Federal Communication's
Commission's interference rules for an MMDS licensee who must
comply with FCC co-channel and adjacent channel interference rules
in order to protect other licensees on adjacent or nearby channels.
Unlike normal cellular or PCS license allocation, the MMDS licensee
is usually allocated individual channel frequencies on a per-city
basis within a 35-mile radius service area around that city. In
conventional operation in the MMDS band, large cells are used with
a radius up to 35 miles with a base station mounted on a high tower
or hilltop. The present system is designed to operate with smaller
cells of particularly 0.5 to approximately 1.5 miles radius in a
normal urban or suburban environment.
Finally, traditional technologies using the MMDS band use
line-of-sight transmission whereas the present system is designed
to operate without line-of-sight and with a signal to penetrate
buildings.
FIG. 5 relates to interference between the cellular service areas
designated by the circle 51 and an adjoining service area by
another operator designated by the circle 52 which, as illustrated,
has a 35 mile radius around the transmit site 53. As discussed
above, the 2.5-2.68 GHz frequency band at least in the United
States, already has interfering or other remote interference
sources and/or recipients, for example, 53. Thus, under the rules
that have been provided by the United States Federal Communications
Commission for any new cellular service area, indicated as 51, both
subscriber terminals 17 and base stations 11 must have a low enough
power so as not to interfere with the adjoining service area 52.
And, as indicated by the arrows 54, 55 and 56, such possibly
interfering transmissions must obey the following three rules: 1.
Aggregate signal level no greater than the -65 dB relative to
"reference level" on a co-channel (same channel as an adjoining
service area). The "reference Level" is a field strength level
determined by the level of signal transmitted by the operator in
the adjoining area when measured at the 35 mile boundary. 2.
Aggregate signal level no greater than 0 dB relative to the wanted
signal level on an adjacent channel (channel adjacent in frequency
to that used by another licensee). 3. Absolute aggregate signal
level no greater than -73 dBW/m2 at the boundary of an adjoining
service area
Furthermore, the FCC "Report and Order on Reconsideration" of Jul.
29, 1999 1. Removes the requirement for the use of directional
antennas at subscriber equipment (referred to by the FCC as
"response stations" where the transmitted power is less than -6 dBW
Effective Isotropic Radiated Power (EIRP) (250 milliwatts) 2.
Removes the need for professional installation of subscriber
equipment in a situation where the transmitted power is less than
-6 dBW EIRP (250 milliwatts). In situations where the transmitted
power is greater than -6 dBW EIRP (250 milliwatts) but less than 18
dBW (63 watts) professional installation is only required within
150 feet of an existing ITFS Instructional Television Fixed Service
receive site 3. (Power levels specified above are for a 6 MHz
channel in the case of a channel wider or narrower than 6 MHz the
allowable power is adjusted proportionately.)
The invention complies with the above described and other FCC
requirements. In general, the techniques used by the invention to
comply with these FCC regulations (and to combat interference)
include the following. 1. The use of spread spectrum wideband
modulation as implemented in the present system reduces the
transmitter power level required for a given base station to
subscriber terminal path and service data rate. Thus, for example,
the maximum power for a subscriber terminal is approximately 0.25
watts, which is significantly below the applicable FCC limit of 2
watts and is compliant with the FCC "Report and Order on
Reconsideration." The effective radiated power of a cellular base
station is substantially below the applicable FCC limit of 2,000
watts. 2. Dynamic power control is used which sets and continually
adjusts the transmitted power levels to the minimum required to
maintain a viable link between subscriber and base station (not
shown). 3. The location of both subscriber terminals and base
stations at low elevations above the surrounding ground level on
average so that the surrounding building and foliage attenuation
and terrain losses reduce the signal originated towards its distant
receiver locations in adjacent service areas. 4. The location of
subscriber terminal transmit antennas typically inside the
subscriber premises such that building penetration losses further
attenuate the signal originated toward distant receiver locations
in adjacent or joining service areas. 5. High system gain defined
as a high permissible path loss between transmitter and receiver
which is achieved by the use of multiple simultaneous data bearers
each operating at a lower rate than the required aggregate data
rate, the use of orthogonal spreading codes on the downlink, and
successive interference cancellation of multiple codes used on the
uplink.
Now referring to FIG. 6, this illustrates the reduction of signal
due to foliage penetration losses 61, that is, trees, etc.,
building penetration losses 65, building clutter losses 62 and
terrain losses 63.
In situations where interference needs to be reduced further (to be
discussed below), directional antennas are used at cellular base
stations all pointing in directions away from the adjacent service
area but with an overlapping pattern. Referring to FIG. 7, in the
prior art the typical coverage pattern of a network of sectored
cell sites was a repetitive "cloverleaf" pattern. If this pattern
were to be applied to the situation in the MMDS bands (as per FIG.
5), some directional sector antennas would be pointing towards
adjacent/nearby license areas and causing/suffering interference as
a result. Referring to FIG. 8, the present system provides coverage
of the target market 51 using an overlapping pattern of
"unidirectional teardrop" antenna patterns 81 of multiple base
stations. The unidirectional teardrop patterns are oriented away
from the source/recipient of interference in the adjacent license
area 52. This technique may also be used to reduce interference
between adjacent/nearby cells operating on the same frequency.
Thus, in summary, with the subscriber terminal or user equipment
designed for low power, that is less than 250 milliwatts in 6 MHz,
this facilitates compliance with the FCC regulations to thus avoid
the requirement that exact location of users be recorded and
notified, while still providing effective coverage allowing for
building penetration, clutter, foliage and other attenuation of the
signal. This also removes the need for professional installation of
subscriber equipment and avoids the requirement that directional
antennas be used. Also, as will be described below, the present
system is designed to operate with varying channel bandwidths which
may vary for example from 6 MHz, to 12 MHz to 18 MHz and to 24 MHz
Due to the proportionate increase in power permitted when using the
broader bandwidth channels (2 times, 3 times and 4 times with
respect to the 6 MHz bandwidth system) the invention complies with
FCC regulations in all cases.
Referring back to FIG. 6 that illustrates the reduction of signal
due to foliage penetration losses 61, building clutter losses 62,
and terrain losses 63, a subscriber terminal 17 is illustrated as
being contained within the building 64. The base stations 11 are
designed to be mounted on utility poles or buildings that are
within the building and foliage clutter. Thus, they may be mounted
in small, unobtrusive enclosures with integral omni-directional or
directional antennas. Standard household power is obtained from the
utility pole or building either on a metered or unmetered basis
depending on utility requirements. This is facilitated by Node B
low power consumption as a result of the use of spread spectrum
transmission techniques.
Inference avoidance is also provided, if necessary, by dynamic
power control 50. By techniques already known the transmit power
levels of both base stations 11 and subscriber terminals 17 may be
set to the minimum level required to maintain viable
communication.
The total amount of spectrum available to an operator of wireless
Internet access services in the MMDS band may be limited to only a
few 6 MHz channels (for example 4 channels). The technology must
therefore be capable of providing high data and subscriber capacity
in such a small amount of spectrum.
Unlike the normal cellular band, an MMDS licensee does not
necessarily have a large contiguous block of channels. MMDS
channels are allocated to licensees as individual channels of 6 MHz
bandwidth, or blocks of several non-contiguous channels, usually
spaced two channels apart (every second channel).
Thus, an MMDS licensee does not necessarily have "paired" blocks of
frequency separated by a predetermined and fixed spacing, with one
block for transmit (such as from a base station) and one block for
receive. Therefore, an operator providing high-speed wireless
Internet access services in the MMDS bands may need to operate in
both a small total amount of spectrum, and with a small number of 6
MHz channels (contiguous or non-contiguous).
Referring to FIG. 9, the present technology is designed to allow
such operation using the following techniques: 1. The UMTS CDMA
radio technology is designed to operate in channels of 6 MHz, 12
MHz, 18 MHz and 24 MHz bandwidth 91, these being multiples of the
standard 6 MHz MMDS/ITFS channels. The base station and associated
User Equipment is able to support "chip" rates (wideband spread
spectrum transmitted bit-rates) after spreading of 3.84, 7.68 and
15.36 Mchips/sec (Mcps), and the appropriate rate is selected
according to the channel bandwidth available. 2. The system is
designed to allow universal frequency reuse for adjacent base
stations, such that a channel (6 MHz, 12 MHz, 18 MHz or 24 Hz
bandwidth) can be reused (92) on every base station radio in a
network serving a given geographic area (including on every sector
of a sectored base station). Universal frequency reuse enables the
systems to provide high subscriber capacity in a limited amount of
spectrum (such as a single 24 MHz channel). The system achieves
this through the nature of the spread spectrum CDMA technology, and
the use of "Macro-diversity." 3. The system uses
"time-division-duplex" (TDD) transmission 93. In contrast to
traditional "frequency division" duplex (FDD) which uses separate
sets of frequencies for transmit and receive, TDD allows the system
to operate in any channel (or block of up to 4 contiguous channels)
anywhere in the MMDS band. TDD is where transmit and receive occur
on the same channel/frequency but in alternate or separate time
intervals. This allows the present system to operate in a single 6
MHz, 12 MHz, 18 MHz and 24 MHz channel, unlike conventional
cellular wireless systems which use FDD and require the acquisition
of two separate channels, spaced apart in frequency to prevent a
transmitter interfering with its co-located receiver.
In a wireless system, coverage (radius of a cell) and the data
rates provided to customers usually have to be traded off against
one another. The present system is required to provide data rates
of "T1" speeds (1.5 Mbps) and up to 6 Mbps, and to provide coverage
up to a maximum of 21 miles from a base station. A major factor in
the tradeoff is the "delay spread" (see FIG. 17) which increases
with the distance of the subscriber from the base station when
operating in the multipath signal environment that is
characteristic of a radio system that is designed for in-building
coverage and simple user installability. If the delay spread time
is approximately greater than a tenth of the time period of each
symbol of transmitted information, then corruption of the
transmission occurs.
Referring to FIG. 10, the present system solves this problem by: 1.
Transmitting for example 4 "bearers" 110-1 simultaneously on the
same RF channel 110-2, separated by different CDMA spreading codes.
Each bearer in this example is 1/4 of the data rate required by
users. By using a lower data rate, the symbol period for each bit
(time to transmit or receive one bit) is increased, allowing for
greater delay spread (and therefore greater distance) before bits
delayed by multipath arrive during the symbol periods of later
bits, causing corruption of data. Orthogonal spreading codes are
used on such bearers to minimize interference between them and
maximize system gain. 2. The 4 bearers are aggregated or "inverse
multiplexed" 110-3 by interleaving of bits to and from each bearer
at both the base station and the User Equipment to provide
aggregate user data rates of 4 times the bearer rate, (for example
4.times.384 kbps bearers are aggregated to provide a user data rate
of 1.536 Mbps). 3. Implementation of Interference Cancellation (see
FIG. 18) in the receiver of the base station to provide similar
system gain on the uplink as achieved by orthogonal codes on the
downlink to increase data transmission capacity and cell radius
under loaded conditions, as described below.
The present system is required to provide maximum cell coverage (up
to 1.5 miles radius in a typical suburban environment) taking into
account a non line-of-sight radio path and building penetration to
a indoor subscriber terminal with a directly-attached
omni-directional antenna, and moreover to meet these requirements
with the same data transmission rates in both the downlink and
uplink directions.
In the prior art of CDMA wireless systems, the spreading codes that
are transmitted in the downlink (base station to subscriber
equipment) are orthogonal, that is, the pattern of each code is
selected such that their interference relative to each other is
zero. This is then degraded by multipath and implementation issues.
However, in the prior art, the codes utilized in the uplink
(subscriber unit to base station) tend to be uncorrelated rather
than orthogonal, that is they appear as interference or noise to
other users. This results in reduced capacity on the uplink and
reduction in uplink cell radius as the cell is loaded, and in
therefore precludes equal uplink and downlink data transmission
rates for the same cell radius.
In the present system, Successive Interference Cancellation (SIC)
is used in the Base Station receiver to improve the performance of
the uplink such that the same data rates can be supported as in the
downlink for the same cell radius and equivalent other factors. SIC
reduces the effect of interference between non-orthogonal codes,
due to independent time offsets, in the uplink and as such can be
viewed as having the same effect as that of orthogonal codes in the
downlink. Referring to FIG. 18, the receiver 13 receives all the
simultaneously transmitted user codes 1, 2 and 3 etc. The received
codes are all ranked from highest to lowest with respect to Signal
to Interference (SIR). The Rake processing unit 18-1 selects the
code with the highest SIR and processes that code. Once rake
processed Code 1 is regenerated in the regeneration unit 18-3, the
output is now equivalent to what was received in code 1 only, prior
to rake processing. The incoming signal comprising all codes is
also delayed in the delay unit 18-2, and the regenerated code 1 is
then subtracted from the delayed input at the adder 184 producing
an output to code 2's rake processor that does not include code 1,
therefore the interference from code 1 with respect to code 2 has
been cancelled. This process is repeated for each code successively
resulting in the full cancellation of own cell interference to the
last code in the process. Because the codes were ranked with
respect to SIR this process guarantees that the codes with the
lowest SIR benefit the most from SIC.
To be able to serve a large mass market of Internet subscribers, it
is very important to make the equipment easy to install by
providing ubiquitous coverage and service inside of buildings,
while avoiding the need for the installation of a rooftop antenna
at the subscriber's premises. At the same time it is very important
to provide high system capacity to ultimately have the capability
to provide service to a high proportion of homes and businesses in
a given geographic area.
Referring to FIG. 11, the present system provides in-building
coverage and high capacity simultaneously by: 1. Use of a radio
transmission technique (Wideband CDMA modulation and Rake
Receivers) capable of operating with multipath signals (direct and
reflected signals following different paths and arriving at
slightly different times); 2. Covering the service area with a
number of radio cells of small diameter to reduce signal losses,
thus allowing more signal margin to penetrate inside buildings; 3.
Location of microcell base stations at approximately rooftop level
to increase the building penetration of the signal (building
penetration can be maximized by the signal-arriving horizontally);
4. Use of macro-diversity, where a building is served
simultaneously on the same frequency by two or more cells, from
different directions. Macro-diversity increases the probability of
reliable coverage at any point within the building. 5. Multiple
bearer transmission as described above and in FIG. 10. 6.
Interference Cancellation in the receiver of the base station as
described-above and in FIG. 18.
A system with a large number of cells (i.e. micro cells) can result
in a high cost for so-called "backhaul" transmission equipment.
Backhaul is a term in the wireless and cellular telephone art
whereby voice or data that is transmitted from the base station
to/from the central office switch or core network equipment, which
is normally carried by a line-of-sight microwave radio or landline
link. And then of course the core network equipment must in turn
transmit this information to the public switched telephone network,
or to the Internet or Intranets in the context of the present
system. Such backhaul is a major component of system cost,
especially as the volume of data is increased in a high speed
Internet access system. FIGS. 11 and 12 illustrate two different
techniques of backhaul transmission, both of which use a type of
routing through adjacent base stations finally to one of the base
stations locations which incorporates core network functions and
provides access to the Internet. These techniques reduce the cost
of backhaul and facilitate rapid deployment.
FIG. 11 illustrates a system where each cell site designated A, B
and C has both a base station radio 11-1 dedicated to a subscriber
access network 11-2 (that is, communication with a user or
subscriber terminal) and a separate backhaul radio 11-5 with a
directional antenna with a path being indicated as 11-4 to a
similar backhaul radio, for example from cell site A to cell site
B. By using two separate radio transmission and receiver units,
that is 11-1 and 11-5 of the same design type at each base station,
this simplifies and lowers cost and installation time. At the same
time, the backhaul radios 11-5 include the associated directional
antennas, 11-6. Specifically in FIG. 11, cell site A for its
backhaul transmits and receives to cell site C via the intermediate
station cell site B. Cell site C connects the core network
functions and thence to the Internet. Line of sight is not required
for the backhaul links. The backhaul transmission operates in the
same 2.5-2.68 GHz band but on a different channel frequency 11-4 as
compared to the subscriber access network. Thus, additional radio
spectrum outside of the normal MMDS band is not required. The links
11-4 may be operated at a higher bearer data rate due to the
superior radio channel conditions including reduced delay spread
and greater signal strength created by the use of directional
antennas which allows the use of a lower spreading factor or
channel coding, resulting in, for example, twice the throughput bit
rate compared to the subscriber access network.
FIG. 12 illustrates peer-to-peer backhaul routing, for example,
from cell site A to adjacent cell site B, to cell site C which
connects to the core network functions and thence to the Internet,
with the use of a common base station radio transmitter and
receiver. That is the same radio resource at a base station is used
for both subscriber access and peer-to-peer backhaul. Here the
allocation of resources as shown by radio frame diagram 12-1, is by
a time division method, which allocates radio frames in the time
domain on the radio resource between the subscriber access using
the omni directional antenna 83 and the backhaul using the
directional antenna 84. Thus, as illustrated by the frame timing
diagram 12-1, cell site A may transmit and receive traffic with its
subscribers in the uplink and downlink radio frames T1 and T2 using
the omnidirectional antenna and in radio frames T3 and T4 it
switches to the directional antenna 84 aimed toward cell site B
(and vice versa) to transmit and receive backhaul traffic with cell
site B using the backhaul uplink and downlink radio frames. An
identical process occurs between cell site B and cell site C.
Again, by the use of the directional radio path with reduced delay
spread and greater signal strength, the spreading factor can be
reduced, allowing a higher data rate.
Cellular wireless systems normally consist of a number of base
stations connected to a centralized "core network" typically
consisting of switches, base station controllers and related
functions, analogous to a telephone company "central office". The
negative consequences of this are: 1. The (relatively) high cost of
the core network equipment makes it difficult to scale the system
down to a small market (e.g. a small city), where the cost of such
equipment must be spread over a relatively low number of
subscribers. 2. All base stations served by a set of core network
equipment must be connected by backhaul transmission typically to a
single core network point serving the entire network. This is
costly, and can be difficult logistically. 3. A centralized core
network generally implies one connection point to the Internet.
The present system avoids these problems by the following
techniques: 1. Distribution of the core network functions 12 in
FIG. 4 across the network of base stations. Core network functions
are provided for each group of 3 base stations (i.e. 3
single-channel micro cells or 3 sectors of a sectored cell site).
2. Providing for each group of 3 base stations (through the core
network functions therein) to connect directly to the Internet.
This enables the network of base stations to connect to multiple
Internet "points-of-presence," and thereby reduces backhaul
transmission costs and the logistics of providing backhaul.
Referring to FIG. 13, a "Core Network Unit" (network controller) 10
is associated with a cluster of up to 3 base stations. These base
stations may be either at 3 separate cell sites or 3 sectors of a
single sectored cell site.
In addition, an ATM (asynchronous transfer mode) network connection
is provided between core network functions controlling base
stations with overlapping coverage 10 rather than via the Internet
to ensure a constant latency (delay) in the transfer of time
critical macro-diversity data which has been discussed briefly
above.
In the deployment of the system embodying the present system, cost
economies can be realized by using shared packet data channels of
the highest possible speed (data rate) on a base station,
regardless of the speed of service desired by the various
subscribers served by that base station. It is then desirable to
deliver different speeds of service to different customer types at
different prices. This is referred to in the present system as
"service tiering." When multiple users are sharing a single
wideband channel, a service tier is defined as a rate in kilobits
per second (kbps) approximately equivalent to a dedicated channel
of the same speed, as perceived by the typical Internet user. For
example, a 384 kbps tier (provided via a shared packet data channel
of 1536 kbps) is perceived by the Internet user to be similar in
speed to a dedicated channel of 384 kbps.
The approach is based on the difference between peak data rates
(the actual speed of packet data transmission) and average data
rates over a period of time. A typical Internet user perceives the
average data rate over a period of time. For example, a typical
Internet user (while engaged for example in Web browsing) is only
sending or receiving data for 10% of the time. If that user is
allocated 10% of the time of a 1536 kbps channel, he will perceive
a data rate of approximately 1536 kbps. However, if he is allocated
5% of the time on the same channel, he will perceive a rate of half
the channels speed, or 768 kbps. Tiering is thus achieved by the
percentage of the time allocated on the channel to each user
according to his tier of service.
The invention provides tiering by prioritization of packets
according to the defined tier of service of the sending/receiving
customer in the media access control protocol operating between the
radio network controller 14 and the subscriber terminal 17 via the
base station 11.
Referring to FIG. 14, if a customer is using his computer to access
the Internet, that computer is connected to the user equipment, the
UE, and the traffic he wishes to send is buffered and queued in his
user equipment waiting for resources on the radio network to be
allocated. At the same time the radio network controller 14, the
RNC, routinely sends a polling request message to all of the user
equipment. The logical channels used to transmit this information
are derived from a benefit obtained from Wideband CDMA where
transmission of multiple logical data channels can operate
simultaneously on the same radio channel using different spreading
codes. When the user equipment sees the Polling Request Message
given that it has traffic queued to send, it will respond with a
the polling response message with, firstly, the fact that it has
information to send and, secondly, with how much data it wishes to
send. That is received by the RNC, by its scheduler. The scheduler
receives these requests from all of the user equipment and the role
of the scheduler is to allocate time on a traffic channels via time
slots in accordance with the amount of data each user equipment has
to send and also in accordance with the tier priority assigned to
that subscriber's user equipment. Additionally, its tier of
service, providing further accuracy to the prioritization scheme,
determines the polling frequency of each subscriber terminal.
The scheduler operates as follows for inbound traffic from
subscriber terminals 17 to the Internet 27 via the base station 11
and core network functions 10: The request from user equipment from
the subscribers with the highest tiers of service, for example,
tier 8 will get priority so the request will go towards the front
of the scheduler queue. Another factor taken into account in the
scheduler is how long a request from a user's equipment has been
waiting to be serviced. After the scheduler has determined the
schedule, it sends out information in an allocation message to the
user equipment which tells each of them what time slots on the
traffic channels they are being assigned to transmit their
information and the user equipment on receiving it sends its
traffic on the allotted time slot. In the reverse direction
outbound traffic from the Internet 27 to the subscriber terminal 17
is queued for transmission in the radio network controller 4. The
downlink scheduler in the radio network controller 14 prioritizes
traffic in its packet queue for transmission in accordance with the
tier of service of the destination subscriber terminal 17, the
amount of data and the time a packet has been waiting in the queue.
An allocation message is sent to the user equipment by the
scheduler in the radio network controller 14 to indicate the
allocation of timeslots on the downlink.
In summary, the overall purpose of the tiering is that with merely
one radio resource at the base station and one channel in each
direction which has a maximum speed of for example 3 Mbps, a tier 2
subscriber for example may easily be accommodated who has a 384
kbps service. However, data is actually being sent and received to
that subscriber at the full speed of 3 Mbps which is the actual net
burst rate that is being transmitted. But by allocating only a
limited proportion of the time on the channel, that subscriber has
the appearance of the average speed of approximately 384 kbps.
In a conventional time-division-duplex system, alternate timeslots
are allocated to each direction of transmission. This is not the
optimum when the system is used for applications such as Internet
access, where the data traffic is asymmetric, or where peer-to-peer
routing of backhaul traffic between base stations is required.
The present system uses "enhanced time-division duplex (TDD) to
solve these problems: 1. The system is designed to typically
provide a total of 15 timeslots per radio frame between the base
station and the subscriber terminals. Two of these are for
signaling and the remaining 13 for base station-subscribers
downlink and subscribers-base station uplink. Backhaul is supported
by the allocation of radio frames for this function, in fact
stealing them from the CDMA air interface. The overall time
allocation ratio between the three (CDMA downlink, CDMA uplink and
backhaul) can be set according to the traffic asymmetry and the
backhaul requirement. 2. In timeslots used for backhaul, higher
transmission rates may be used (with less coding or spreading),
taking advantage of the use of directional antennas between base
stations.
Referring to FIG. 15, the first level of time division ("T") is
between radio frames 115-1. Frames are allocated either to
subscriber access (base station to UE) 115-2 or to peer-to-peer
backhaul between base stations 115-3. A higher transmission rate
115-5 is used on radio frames allocated to backhaul, allowing each
backhaul radio frame to carry the volume of traffic to/from two
consecutive subscriber access radio frames.
The second level of time division is within each radio frame 115-4,
which is divided into 15 timeslots. Timeslots 0 and 1 are reserved
for common control/signaling purposes. The remaining 13 timeslots
can be allocated to uplink and downlink traffic on any ratio. For
example, for typical asymmetric Web browsing traffic the ratio
could be 3 slots uplink and 11 slots downlink. The ratio can be
changed over time to reflect changing traffic patterns.
In the prior art, the data rate transmitted in a digital cellular
wireless system is determined by the worst case position of a
subscriber in the coverage area of a cell and other worst case
radio channel parameters. However, subscribers close in to the base
station (typically around 30%-50% of subscribers) would be able to
transmit and receive at a higher data rate if the system could
determine the channel conditions to and from each subscriber unit
and set transmission speeds accordingly.
The present system solves this problem by allowing the transmission
rate to be selected for each subscriber unit according to the
channel conditions applicable to such subscriber unit both in the
uplink and downlink directions. This technique is called rate
adaption, and is made possible by the use of direct sequence spread
spectrum transmission and the use of time-division-duplex.
By using direct sequence spread spectrum transmission in which the
user data rate (D) can be varied by changing the spreading factor
(SF) within a fixed transmitted chip rate (C) on the air interface
according to the formula C=D.times.SF. Thus for example the user
data rate can be doubled by halving the spreading factor. Referring
to FIG. 17 the usable spreading factor for the radio path to and
from the base station 11 to each subscriber terminal 17 is
determined by the delay spread on the signal caused by multipath
propagation 17-1 resulting from the combination of a direct path
signal 17-2 and indirect path signals 17-3 at the receiver and by
the ratio of the wanted signal at the receiver to the combined
noise 17-4 and interference 17-5 present.
By using time-division-duplex transmission, the multipath and other
characteristics of the radio channel in both the uplink (subscriber
terminal to base station) and downlink (base station to subscriber
terminal) are identical, allowing the base station to measure the
channel characteristics at its receiver and set the transmission
data rates for both uplink and downlink transmissions. Referring to
FIG. 16, for transmission from each subscriber terminal the base
station measures excess delay spread caused by multipath and
reports this information to the radio network controller which sets
the spreading factor and therefore the user data rate to be used on
subsequent transmissions in both the uplink and downlink directions
according to the relationship between the excess delay spread time
and the symbol period of each transmitted bit of data For example,
if the excess delay is below a set threshold value a spreading
factor of 8 is set for that particular user equipment resulting in
twice the transmitted bit rate, otherwise a spreading factor of 16
is indicated. The base station also measures the ratio of wanted
signal to noise plus interference and reports the resulting signal
to interference figure (S/I) to the radio network controller which
compares this with a predetermined threshold. If the ratio is below
the threshold and a spreading factor of 8 is indicated by excess
delay measurement as described above, the spreading factor will be
increased to 16 and the user data rate reduced according to permit
more reliable communication. A flowchart demonstrating this process
is show in FIG. 16.
The radio network controller signals the spreading factor
determined by such measurement in respect to a particular
subscriber terminal to both the base station and that subscriber
terminal, which spreading factor is then used for subsequent
transmissions to and from such subscriber terminal until such time
as the radio network controller determines new parameters.
Thus, an improved cellular Wireless Internet Access System has been
provided.
* * * * *
References