(12) United States Patent
`(10) Patent N0.:
`US 6,791,952 B2
`
`Lin et al.
`(45) Date of Patent:
`*Sep. 14, 2004
`
`U5006791952B2
`
`(54) ASYMMETRIC DATAACCESS SCHEME
`
`(75)
`
`Inventors: Jie Lin, Torquay (GB); Keith Russell
`Edwards Paignton (GB) Richard
`.’
`’
`JOhn Dmcou’ Totnes (GB)
`(73) Assignee: Nortel Networks Limited, St. Laurent
`(CA)
`
`..................... 370/345
`5,970,062 A * 10/1999 Bauchot
`5,991,311 A * 11/1999 Long et al.
`370/524
`
`23000 Bkaeftetal- --
`455/277-2
`690239615 A *
`6,038,455 A *
`3/2000 Gardner et al.
`455/447
`
`.....
`6,122,263 A *
`9/2000 Dahlin et al.
`370/329
`6,219,547 B1 *
`4/2001 Qaddoura et al.
`455/432
`........... 370/331
`6,359,866 B1 *
`3/2002 Svensson et al.
`FOREIGN PATENT DOCUMENTS
`
`(*) Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent
`term provisions of 35 U.S.C.
`154(a)(2)
`'
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`EP
`EP
`GB
`GB
`JP
`W0
`W0
`
`W0
`W0
`
`0 720 329
`0 732 834
`2 232 562
`2 304 499
`07/079 245
`WO 95/20852
`WO 96/26588
`
`WO 96/27835
`WO 97/12456
`
`7/1996
`9/1996
`12/1990
`3/1997
`3/1995
`8/1995
`8/1996
`
`9/1996
`4/1997
`
`OTHER PUBLICATIONS
`
`21 A 1. No.: 08/962 287
`)
`pp
`,
`
`(
`
`Digital Audio Broadcasting, EUREKA—147 Project, Aug.
`1997.
`
`Filed:
`
`(22)
`(65)
`
`Oct. 31, 1997
`Prior Publication Data
`Us 2001/0006517 A1 Jul. 5, 2001
`7
`
`Int. Cl.
`(51)
`(52) us. Cl.
`
`.............................. H04J 1/00; H04Q 7/00
`....................... 370/281; 370/329; 370/344;
`370/352
`
`(58) Field of Search ................................. 370/280, 281,
`370/276, 294, 295, 328, 329, 330, 334,
`341, 344, 347, 431, 436, 442, 468, 352,
`353, 354, 401, 455/403, 422.1, 450
`
`(56)
`
`References Clted
`U.S. PATENT DOCUMENTS
`
`5,113,339 A
`5/1992 Komatsu etal.
`59485578 A
`“1996 Sweazey
`3:25:32): 2 * 2133: $01“ t~~~1~~~~~~~~~~~~~~~~~~ 395/2001
`,
`,
`an e a.
`5,936,580 A *
`8/1999 VangPuijenbroek ......... 343/700
`5,949,763 A *
`9/1999 Lund ...................... 370/261
`
`5,953,659 A *
`..... 455/422
`9/1999 Kotzin et al.
`
`............... 375/347
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`9/1999 Morris et al.
`
`(List continued on next page.)
`Primary Examiner—Brian Nguyen
`(74) Attorney, Agent, or Firm—Barnes & Thornburg
`
`ABSTRACT
`(57)
`The disclosure relates to an asymmetric data access scheme
`eg Internet access scheme, over a fixed wireless access
`network. The disclosure provides apparatus for bandwidth
`efficient provision of asymmetric data services operating a
`method of communicating between a radio base station and
`a plurality of subscriber terminals, each subscriber terminal
`comprising a subscriber antenna. Each sector of a tri-
`sectored cellular arrangement operates a plurality of. fre-
`quency d1v151on duplex pa1rs, plus one or more add1tronal
`unpaired downlink frequencies. On the uplink, packet
`switched data is transmitted on the conventional access
`channel, eg ALOHA, but on the downlink, packet switched
`-
`-
`-
`-
`2213;333:3122:(ggaargunpalred downhnk frequency over
`'
`
`11 Claims, 5 Drawing Sheets
`
`
`
`1
`
`APPLE 1014
`
`APPLE 1014
`
`1
`
`

`

`US 6,791,952 132
`Page 2
`
`OTHER PUBLICATIONS
`
`“The Slotted Aloha Protocol”, BYTE Articles Bytemarks
`Facts Links VPR Talk, Apr. 1996/ReViews/E—Mail Without
`Wires, downloaded from: http://www.byte.corn/art/9604/
`sec12/art7.htrn, Sep. 18, 1997, 1 page.
`“CSMA—CD”, downloaded from: http://www.seas.upenn.
`edu/~ross/lectures/ethernet/csrna cd.htrn, Sep. 18, 1997, 3
`PPS-
`“Reservation Aloha”, downloaded from: http://rnars.rnc-
`s.kent.edu/rnac/node58.htrnl, Sep. 18, 1997, 2 pps.
`“CS143 HW #3”, downloaded from: http://das—www.har-
`vard.edu/cs/acadernic/courses/cs143/hw/hw3.htrnl, Sep. 18,
`1997, 2 pps.
`“Contention Protocols”, downloaded from: http://cs.uregi-
`na.ca/~holroyd/CS306/lectnote.htrnl, Sep. 18, 1997, 7 pps.
`
`Liu, “Performance analysis of DS/CDMAwith slotted Aloha
`Randorn access for packet PCNs”, Wireless Networks, vol. 1,
`No. 1, Feb. 1995, pps. 1—16, downloaded from: http://
`www.acrn.org/pubs/toc/abstracts/wireless/207630.htrnl.
`
`Lee et al., “Slotted Aloha in High Speed Bidirectional Bus
`Networks”, LIDS—P—2097, Mar. 1, 1992, 1 page, down-
`loaded
`from:
`http://donald—duck.rnit.edu/lidspubs/
`2097.htrnl.
`
`Lee et al., “Space—Tirne Characteristic of Aloha Protocols in
`High Speed Bidirectional Bus Network”, LIDS—P—2153,
`Dec. 1, 1992, downloaded from: http://donald—duck.rnit.
`edu/lids/pubs/2153.htrnl, 1 page.
`
`* cited by examiner
`
`2
`
`

`

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`US 6,791,952 B2
`
`1
`ASYMMETRIC DATA ACCESS SCHEME
`
`FIELD OF THE INVENTION
`
`The present invention relates to a method and apparatus
`for data access over a wireless link, in which a greater data
`rate may be provided in one direction of a link than in an
`opposite direction of the link.
`BACKGROUND OF THE INVENTION
`
`In a fixed wireless access (FWA) telecommunications
`system, subscribers are connected to a backbone telecom-
`munications network by means of radio links in place of
`traditional copper wires. Each of a plurality of subscribers is
`provided with a subscriber radio terminal at their subscriber
`premises. A base station provides cellular coverage, typi-
`cally in urban environments over a 5 km radius, with the
`plurality of subscriber radio terminals. Each base station
`may be connected to a backbone network, eg a Public
`Switched Telecommunications Network (PSTN) switch via
`a conventional transmission link, known as a backhaul link
`thereby providing the plurality of subscribers with access to
`the PSTN. A single base station can serve of the order of up
`to two thousand subscribers, making the installation and
`maintenance cost of a fixed wireless access system lower
`than that of an equivalent copper wire access network.
`Referring to FIG. 1 herein, there is illustrated schemati-
`cally a radio base station and subscriber terminal of a prior
`art fixed wireless access system. A plurality of subscriber
`radio terminals 100 each comprising a transceiver 101 and
`an antenna 102 communicate with a radio base station 103
`having a base station antenna 104 and a base station trans-
`ceiver apparatus 105. Aplurality of such radio base stations
`103 each communicate with a central office switch 106 to
`gain access to a backbone telecommunications network, eg
`a Public Switched Telephone Network (PSTN). In a geo-
`graphical area, a plurality of base stations are distributed to
`provide coverage in a cellular pattern. Each base station 103
`is connected to a local exchange switch 106 via a backhaul
`transmission line 107 which may comprise for example a
`terrestrial line eg fiber optic cable or coaxial cable, or a
`microwave transmission link. Communication between the
`subscriber radio terminal and the base station is via a
`wireless radio link 108. Each local wireless link 108
`between radio base station 103 and subscriber radio terminal
`100 comprises an uplink from the subscriber antenna to the
`radio base station antenna, and a downlink transmitting from
`the radio base station antenna to the subscriber antenna and
`transceiver. Each radio base station operates either an omni-
`directional beam or a plurality of broad sectorized beams
`encompassing all subscribers in a cell or sector for receive
`and transmit, whereas each subscriber radio terminal oper-
`ates a directional pencil beam directed at the base station for
`receive and transmit. In a prior art fixed wireless access
`deployment, although each nominally hexagonal cell
`is
`served by a base station located nominally at the center of
`the cell, current technologies permit the base station antenna
`to be located non-centrally within a cell area.
`In the prior art fixed wireless access system a frequency
`spectrum allocation for the uplink is typically of a same
`bandwidth as a frequency spectrum allocation for the down-
`link. For example, the uplink may be allocated 15—17 MHZ
`bandwidth in an available radio spectrum, and the downlink
`may be allocated a further 15—17 MHZ bandwidth of fre-
`quency spectrum. The uplink and downlink spectrums are
`spaced apart by typically around 50 MHZ, referred to as
`duplex spacing.
`Typically, the uplink frequency allocation of 15 MHZ is
`subdivided into a plurality of 300 KHZ slots each occupied
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`by a separate carrier frequency, giving 48 uplink carriers.
`For a 17 MHZ uplink band, divided into a plurality of 300
`KHZ uplink frequency slots, 54 uplink carriers are available.
`Similarly,
`the allocated downlink frequency spectrum is
`subdivided into a plurality of 300 KHZ downlink frequency
`slots, being symmetric with the uplink frequency allocation.
`The 300 KHZ frequency slots are allocated to a plurality
`of radio base stations over a geographical area according to
`a repeating frequency reuse pattern. To minimize the like-
`lihood of interference, adjacent cells within a fixed wireless
`access network, or sectors within each such cell are allocated
`distinct groups of radio frequencies selected so as to mini-
`mize the likelihood of a transmission with any cell (or sector
`of a cell) causing interference in any other cells or sectors
`nearby. On the uplink, in a three of nine reuse pattern, every
`ninth frequency is reused, so although only 18 of the 54
`available carrier frequencies are used per cell, the frequency
`pattern can be reused indefinitely, and an allocation of
`subscriber radio terminals to base stations giving service to
`around 2000 subscribers per cell can be replicated indefi-
`nitely over a geographical area.
`Thus, typically in a 17 MHZ uplink case, each base station
`may operate 18 carriers, 6 per sector,
`in a tri-sectored
`arrangement. Each carrier frequency is separated into 10
`bearer time slots, providing 60 uplink bearer time slots per
`sector (180 bearers per cell). Of these, 2 to 6 bearer time
`slots per sector are reserved for an access channel, through
`which subscriber radio terminals request access to the radio
`base station leaving 54 bearer time slots per sector available
`for subscriber usage. Each subscriber radio terminal oper-
`ates two subscriber lines, so taking account of the bearers
`reserved for access channels, up to a maximum of 27 radio
`subscriber terminals in a sector can communicate with a
`base station at the same time. However, as usage of sub-
`scriber terminals is statistical in nature, up to approximately
`600 to 700 subscribers per sector can be accommodated
`since not all subscribers communicate at once.
`
`Similarly, in the 17 MHZ downlink band, the downlink
`frequency allocation at each base station is 18 carriers per
`cell, each downlink carrier corresponding to an uplink
`carrier in a frequency division duplex pair. In each sector,
`there are 6 downlink carrier frequencies, corresponding with
`the 6 uplink frequencies,
`to form 6 frequency division
`duplex pairs per sector. As with the uplink carrier
`frequencies, the downlink carrier frequencies are time divi-
`sion multiplexed into a plurality of bearer timeslots. Some of
`those bearer timeslots are used as a downlink broadcast
`channel which advertises available bearer timeslots to all
`subscribers within a sector.
`For circuit switched services carried over the wireless
`link, where those services are characterized by having
`symmetric constant data rate traffic both on the uplink and
`downlink, eg voice traffic, the prior art symmetric allocation
`of frequency spectrum between the uplink and downlink
`beams is relatively efficient. However, for services which
`entail an asymmetric data rate requirement as between the
`uplink and the downlink, for example where the volume of
`traffic data on the uplink differs greatly from a volume of
`traffic data on the downlink, a symmetric frequency spec-
`trum allocation for
`the uplink and downlink beams is
`inefficient. For example, taking an instance of a subscriber
`making Internet communications on a user terminal 109,
`connected to a subscriber radio terminal 100, a request for
`data sent to an Internet service provider 110 on the uplink
`may comprise a transmission of packets of tens or hundreds
`of Bytes. On the other hand, service data provided by the
`Internet service provider may comprise data units of the
`order kBytes or MBytes. Such data is downloaded from the
`local exchange 106 over the backhaul system 107 through
`the base station 103 and over the downlink. In a circuit
`
`8
`
`

`

`US 6,791,952 B2
`
`3
`switched application, the bandwidth is reserved and avail-
`able for use for uplink and downlink communications
`throughout the duration of a communications session. Dur-
`ing the download of data from the Internet, the uplink path
`remains reserved for use by the subscriber, although no data
`traffic may be actually flowing on that uplink.
`In a fixed wireless access network deployment having a
`plurality of subscribers each communicating with a base
`station, under conditions of services of asymmetric data rate,
`having a symmetric frequency spectrum allocation for a
`downlink and uplink path for each subscriber represents an
`inefficient use of frequency spectrum.
`However, in many applications, a symmetric frequency
`allocation is all that is available, due to prior allocation of
`frequencies by license. Frequencies may become available
`due to de-commissioning of legacy equipment using sym-
`metric uplink and downlink frequencies and any replace-
`ment equipment must make use of the symmetric frequen-
`cies allocations becoming available.
`In addition to the wasteful allocation of spectrum on the
`uplink, while data is being transmitted only on the downlink
`in the conventional circuit switched FWA system, there is
`also an additional
`inefficiency due to a time overhead
`associated with connection setup and tear-down, which adds
`further to the overall
`inefficiency of symmetric circuit
`switched fixed wireless access networks when carrying data
`traffic having an asymmetric downlink/uplink data flow.
`SUMMARY OF THE INVENTION
`
`One object of the invention is to provide an improved
`apparatus and method for fixed wireless access network
`operating a symmetric uplink and downlink frequency spec-
`trum allocation, enabling such a network to transport ser-
`vices characterized by having an asymmetric data rate as
`between uplinks and downlinks, more efficiently over the
`symmetrically allocated uplink/downlink frequency spec-
`trums.
`
`According to a first aspect of the present invention there
`is provided in a cellular communications system comprising
`a plurality of base stations, each communicating with a
`plurality of subscriber radio terminals, a method of commu-
`nicating between a radio base station and a said plurality of
`subscriber radio terminals, said method comprising the steps
`of:
`
`at said base station:
`
`receiving packet switched data carried on an uplink
`frequency of a frequency division duplex pair; and
`transmitting packet switched data on a downlink distri-
`bution channel carried on an unpaired downlink fre-
`quency.
`Preferably said uplink packet switched data is carried on
`a contention access channel.
`Said contention access channel may comprise a slotted
`ALOHA channel.
`Preferably said uplink packet switched data shares said
`contention access channel with a plurality of connection
`requests for circuit switched traffic.
`Said uplink packet switched data may comprise a request
`for services having an asymmetric data rate as between a
`source and a destination of said service.
`According to a second aspect of the present invention
`there is provided a method of communicating between a
`radio base station and a plurality of subscriber radio
`terminals, said method comprising the steps of:
`receiving a plurality of service requests from said plural-
`ity of subscriber radio terminals at said radio base
`station;
`recognizing said service requests received at said radio
`base station;
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`in response to a said service request, forwarding said
`service requests to a service provider apparatus;
`receiving service data in response to said service request
`signal; and
`transmitting said service data to said plurality of said
`subscriber radio terminals.
`
`Preferably said step of receiving a said service request
`comprises receiving said service request over a directional
`uplink transmission beam.
`Preferably said step of transmitting said service data
`comprises transmitting said service data on a sectorized
`downlink beam.
`Preferably said service requests are received on an uplink
`access channel and said service data is transmitted on an
`unpaired downlink frequency.
`Preferably said downlink distribution channel carries data
`traffic in response to one or a plurality of service requests
`received on an uplink access channel.
`According to a third aspect of the present invention there
`is provided a method of allocating frequency spectrum slots
`at a radio base station, said radio base station configured for
`communicating with a plurality of subscriber radio
`terminals, said method comprising the steps of:
`allocating a plurality of frequency division duplex pairs
`for communication between said subscriber radio ter-
`
`minal and said plurality of radio base stations, each said
`frequency division duplex pair comprising an uplink
`frequency and a downlink frequency; and
`allocating an unpaired frequency for communicating on a
`downlink from said base station to said plurality of
`subscriber radio terminals.
`
`Preferably said unpaired downlink frequency is operated
`to carry packet switched data.
`Preferably said radio base station operates a plurality of
`sectorized beams, and a said unpaired downlink frequency is
`operated for each said sectorized beam.
`The invention includes a radio base station configured for
`operating the method as described in the third aspect.
`According to a fourth aspect of the present invention there
`is provided a method of allocating bearer channels at a radio
`base station, configured for communicating with a plurality
`of subscriber radio terminals, said method comprising the
`steps of:
`allocating a plurality of paired bearer channels for carry-
`ing circuit switched data, each said bearer channel pair
`comprising an uplink channel carried on an uplink
`frequency slot, and a downlink bearer channel carried
`on a downlink frequency slot; and
`allocating packet switched data to a plurality of unpaired
`downlink bearer channels.
`
`Preferably said unpaired bearer channels are carried on an
`unpaired downlink frequency, and said paired bearer chan-
`nels are carried on one or a plurality of frequency division
`duplex pairs.
`According to a fifth aspect of the present invention there
`is provided a radio base station comprising:
`a first antenna and transceiver apparatus configured to
`operate a plurality of frequency division duplex pairs,
`each said frequency division duplex pair comprising an
`uplink frequency and a downlink frequency;
`a second antenna and a transmitter apparatus configured
`for transmitting at least one unpaired downlink fre-
`quency.
`Said first antenna and transceiver apparatus may comprise
`means for distinguishing between service request signals for
`packet switched data services and connection request signals
`for circuit switched services received on said uplink fre-
`quencies of said frequency division duplex pairs; and
`
`9
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`

`US 6,791,952 B2
`
`5
`means for routing service data received in response to said
`service requests, to said second antenna and transmitter
`for broadcast of said service data by said second
`antenna and transmitter.
`Preferably the base station operates to transmit packet
`switched data on said second antenna and transmitter.
`Said subscriber radio terminal may further comprise:
`an antenna for receiving and transmitting over a fre-
`quency division duplex pair comprising an uplink fre-
`quency and a downlink frequency;
`a transceiver apparatus configured for transmitting on said
`uplink frequency and receiving on said downlink fre-
`quency of said frequency division duplex pairs; and
`a receiver configured for receiving signals on an unpaired
`downlink frequency.
`Said subscriber radio terminal may further comprise a
`multiplexer, wherein said receiver operates to receive sig-
`nals from said antenna via said multiplexer, said multiplexer
`operating to multiplex between signals assigned to said
`transceiver and signals assigned to said receiver.
`Said subscriber radio terminal may comprise a further
`antenna adapted for receiving said signals on said unpaired
`downlink frequency, wherein said receiver operates to
`receive signals from said further antenna.
`The invention includes an asymmetric data access
`scheme, wherein short message uplink traffic is carried on a
`shared access uplink channel.
`The uplink channel may carry Internet data requests.
`The invention includes a fixed wireless access base station
`operable to provide asymmetric data access and operable to
`receive and route uplink traffic received via a shared access
`channel. The uplink channel may comprise Internet data
`requests.
`The invention includes a fixed wireless access subscriber
`apparatus operable to provide asymmetric data access and
`operable to receive and route uplink traffic received via a
`shared access channel.
`The fixed wireless access system may comprise a proxy
`server. The system may comprise a fixed wireless access
`base station wherein the proxy server is located at said base
`station.
`The invention includes a radio transmission system com-
`prising a local subscriber interface and a radio interface and
`adapted to communicate with a remote base station by
`means of an access channel, wherein an Internet request is
`emitted from the radio interface, encoded on the access
`channel responsive to receipt of the Internet request on the
`local subscriber interface.
`The invention includes a satellite base station operable to
`provide asymmetric data access and operable to receive and
`route uplink traffic received via a shared access channel. The
`uplink traffic may comprise Internet data request.
`The invention includes a satellite subscriber apparatus
`operable to provide asymmetric data access and operable to
`receive and route uplink traffic received via a shared access
`channel.
`Said uplink traffic may comprise Internet data requests.
`The invention includes a satellite system comprising a
`proxy server. The system may comprise a base station
`wherein said proxy server is located within said satellite.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a better understanding of the invention and to show
`how the same may be carried into effect, there will now be
`described by way of example only, specific embodiments,
`methods and processes according to the present invention
`with reference to the accompanying drawings in which:
`FIG. 1 illustrates schematically a radial base station and
`subscriber terminal of a prior art fixed wireless access
`system;
`
`6
`FIG. 2 illustrates schematically a prior art cellular cov-
`erage arrangement in an un-sectored fixed wireless access
`network, including an example frequency allocation plan
`suitable for services having symmetric uplink and downlink
`data rates;
`FIG. 3 illustrates schematically a prior art cellular cov-
`erage arrangement and frequency allocation plan for a prior
`art sectored fixed wireless access network, having 3 sectors
`per cell;
`FIG. 4 illustrates schematically an example of a fixed
`wireless access system according to a first specific embodi-
`ment of the present invention, adapted for supporting ser-
`vices having asymmetric data transmission rates as between
`uplink and downlink;
`FIG. 5 illustrates schematically the fixed wireless access
`system as illustrated in FIG. 4 in logical view; and
`FIG. 6 illustrates schematically a frequency re-use plan
`for a plurality of base stations and a plurality of subscribers
`according to a specific method of the present invention.
`DETAILED DESCRIPTION OF THE BEST
`MODE FOR CARRYING OUT THE INVENTION
`
`There will now be described by way of example the best
`mode contemplated by the inventors for carrying out the
`invention. In the following description numerous specific
`details are set forth in order to provide a thorough under-
`standing of the present
`invention.
`It will be apparent
`however, to one skilled in the art, that the present invention
`may be practiced without using these specific details. In
`other instances, well known methods and structures have not
`been described in detail so as not to unnecessarily obscure
`the present invention.
`Further, the techniques disclosed herein are applicable in
`a number of frequency division duplex systems, eg North
`American AMPS (Advanced Mobile Phone System), PCS
`(Personal Communications System), or CDMA (Code Divi-
`sion Multiple Access). Further, it will be realized that whilst
`the specific embodiments and methods disclosed herein
`describe specifically a fixed wireless access application, the
`general techniques employed are equally suitable for appli-
`cation in satellite links.
`
`Referring to FIG. 2 herein, there is illustrated an example
`of a downlink frequency allocation plan for a prior art
`arrangement of hexagonal cells within a fixed wireless
`access network, having a deployment of omni-directional
`base station antennas. Each of a plurality of base stations
`provides coverage for a nominally hexagonal cell area. A
`cluster pattern of seven frequencies F1—F7 covering seven
`cells is duplicated across the network. In order to avoid
`interference between adjacent base stations, and subscriber
`transmissions, distinct frequency groups are allocated as
`between adjacent cells. No two adjacent cells utilize a same
`or like carrier frequency. Carrier frequencies are reused
`between base stations which are sufficiently far apart from
`each other, so as not to cause interference with each other,
`thereby increasing the overall capacity of the fixed wireless
`access network by reuse of carrier frequencies.
`Referring to FIG. 3 herein there is illustrated a downlink
`frequency plan for a prior art tri-sectored center excited
`cellular arrangement, in which each base station at the center
`of a corresponding respective nominally hexagonal cell
`radiates three beam patterns per hexagonal cell, each beam
`pattern having different frequencies. Greater
`frequency
`reuse is achieved in the tri-sectorized arrangement as com-
`pared with the omni-directional cells of FIG. 2 herein. In the
`tri-sectored center excited cellular arrangement of FIG. 3
`each base station is adapted to transmit and receive on
`distinct frequencies over directional downlink and uplink
`beams within the cell.
`In FIG. 3,
`the symbols A1—A3,
`
`10
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`15
`
`20
`
`25
`
`30
`
`35
`
`40
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`45
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`50
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`55
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`60
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`65
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`10
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`10
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`

`

`US 6,791,952 B2
`
`7
`B1—B3, and C1—C3 are used to indicate distinct frequency
`groups allocated to individual sectors. Carrier frequencies
`are chosen to be sufficiently far apart from each other so as
`not
`to interfere with adjacent sectors or cells.
`In each
`nominally hexagonal cell, there are 18 carrier frequencies.
`Each frequency group comprises a set of 6 carrier frequen-
`cies. For example, frequency group A1 of first sector 300
`comprises frequencies fl—f6. The same frequency group A1
`is re-used in spaced apart first tier frequency re-use sector
`301.
`
`Referring again to FIG. 2 herein, the following example
`illustrates an imbalance between uplink and downlink inter-
`ference. For a prior art arrangement of a plurality of, for
`example, 6 subscriber transceivers communicating with a
`single base station in an omni-directional cellular base
`station layout, on a base station to subscriber downlink,
`assuming a frequency reuse of 1, there is only one possible
`base station interferer 200 for a subscriber radio terminal
`having a directional beam 201 aimed at base station 202.
`However, for a base station receiving uplink frequencies
`omni-directionally, there are potentially 6 cells from which
`interference can occur around each base station, these being
`the first tier frequency reuse cells. On the uplink, since each
`base station has 6 subscriber transceivers in its surrounding
`cell, there are potentially six interferers surrounding each
`base station, one in each of the adjacent first tier frequency
`reuse cells to that base station. Thus, there is an in-built
`interference imbalance between the downlink and the uplink
`which manifests itself as a difference in carrier signal to
`noise and interference ratio (CNIR) between downlink and
`uplink paths.
`The CNIR imbalance is mitigated due to the statistical
`nature of transmissions. Whilst the downlink beams are
`non-statistical, ie always on, the uplink beams are statistical,
`only transmitting when connections are made. However,
`under circumstances where many subscribers communicate
`simultaneously, for a single subscriber wireless link,
`the
`worst case interference on the uplink exceeds the worst case
`interference on the downlink.
`
`Taking as a comparison, an example of a prior art center
`excited hexagonal cell having a trisected uplink beam pat-
`tern as shown in FIG. 3 herein,
`instead of an omni-
`directional uplink beam pattern,
`the uplink interference
`position improves compared to the omni-directional case,
`since the trisected uplink beam receives interference from
`only two first tier frequency reuse cells, assuming a fre-
`quency reuse factor 1. However, there is still an imbalance
`in potential worst case interference between the downlink
`and the uplink for communication between a subscriber and
`a base station.
`
`levels of
`traffic types can tolerate different
`Different
`interference. For circuit switched traffic, for example carry-
`ing voice transmission, a high integrity of transmission is
`required since the nature of the traffic is such that intermit-
`tence of a connection cannot be tolerated. However, for
`packet switched data a higher degree of connection inter-
`mittence can be tolerated since packet switched data is less
`delay sensitive than voice data, and can often be retrans-
`mitted.
`Communication is said to be circuit switched where a
`transmission resource between a source and a destination is
`guaranteed reserved for the use of a user for the duration of
`a session, whether that resource is used continuously or not
`(for example a normal voice telephone call). An example of
`this is where a specific timeslot or code is reserved for the
`duration of a call, whether or not a person is speaking.
`Conversely, communication is said to packet switched
`where transmission resources are not guaranteed to be
`reserved for the duration of a user’s session, but are allo-
`cated only as and when required during a user session (for
`
`8
`
`example in a data packet network). Circuit switched chan-
`nels may be described as being deterministic, whereas
`packet switched channels may be described as being non-
`deterministic.
`Whilst
`in the case of fixed wireless access a circuit
`switched symmetric approach to allocation of transmission
`resources makes acceptable use of available radio bandwidth
`for largely continuous services having substantially sym-
`metric data rate in either direction, (for example voice traffic,
`where a volume of traffic in one direction may be approxi-
`mately equal to a volume of traffic in an opposite direction)
`it would nevertheless be highly wasteful of the limited radio
`bandwidth across a fixed wireless access link to operate a
`circuit switched mode where data transfer is intermittent, or
`where data transfer is asymmetric in nature, having a large
`amount of data in one direction and a smaller amount of data
`in an opposite direction across the fixed wireless access link.
`An example of such an asymmetric intermittent usage
`pattern arises in subscriber access to the transmission control
`protocol/Internet protocol (TCP/IP) Internet, for example
`access to the World Wide Web (WWW), User Groups, file
`transfer protocol (FTP), or Bulletin Board. Such Internet
`interactions typically take the form of a relatively short
`information request transmitted by a subscriber on an uplink
`(eg of the order of a few tens of bytes) each information
`request potentially resulting much larger amounts of data
`being downloaded to the subscriber via a downlink (eg many
`K/Bytes or even M/Bytes of data).
`Specific embodiments and methods according to the
`present
`invention may take advantage of a difference
`between the carrier to noise and interference ratio quality as
`between the uplinks and downlinks arising from the use of
`highly directional antennas at the subscriber radio terminals
`whereby the worst