(12) United States Patent
`Wilson et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7.826471 B2
`Nov. 2, 2010
`
`US007826471 B2
`
`(54) MULTI-BEAMCELLULAR
`COMMUNICATION SYSTEM
`
`OTHER PUBLICATIONS
`Giuliano R. MaZZenga F. Vatalaro F. “Smart cell sectorization for
`third generation CDMA Svstems’” —Wireless Communications and
`(75) Inventors: into St.p Simon Mi Computing 2002
`253-267, XPO0240 1304.
`(Continued)
`Primary Examiner Robert W Wilson
`(74) Attorney, Agent, or Firm Barnes & Thornburg LLP
`
`(73) Assignee: Nortel Networks Limited (CA)
`(*) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1322 days.
`
`(21) Appl. No.: 10/385,792
`(22) Filed:
`Mar 11, 2003
`
`(65)
`
`Prior Publication Data
`US 2004/0179544A1
`Sep. 16, 2004
`
`(51) Int. Cl.
`(2006.01)
`H04B 7/22
`(52) U.S. Cl. ........ grgrrr. 37Of442
`(58) Field of Classification Search ............... ... None
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`5,029, 183 A * 7, 1991 Tymes ........................ 375,141
`5.565,873 A 10, 1996 Dean.
`6,005,854. A * 12/1999 Xu et al. ..................... 370/335
`6,356,767 B2 * 3/2002 Froula ........................ 455,512
`6,377.559 B1 * 4/2002 Haardt ....................... 370,326
`6,477,158 B1 * 1 1/2002 Take .......................... 370,335
`6,542,485 B1* 4/2003 Mujtaba ..................... 370,335
`
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`(57)
`
`ABSTRACT
`
`A cellular communication system comprising a plurality of
`geographically spaced base stations (2) each of which com
`prises an antenna arrangement (4, 6, 8) per base station sector,
`each of which antenna arrangements has an antenna element
`for generating an array of narrow beams (10, 12, 14) coverin
`the gets E. a AS R t
`each of the beams so as to generate Successive sets of simul
`taneously transmitted timeslots per sector. The timeslots are
`each split into multiple orthogonal codes, for example Walsh
`codes. The communication system additionally comprising a
`scheduling device (31) for allocating for successive sets of
`timeslots common overhead channels, including a common
`pilot channel, which are allocated to the same Sub-set of codes
`of each timeslot in the set. For successive sets of timeslots
`different data traffic is allocated to the same sub-set of codes
`of each timeslot in the set. This effectively generates a sector
`wide antenna beam carrying the common Overhead channels
`and a plurality of narrow beams each of which carry different
`data traffic. Inter-beam interference is addressed by the appli
`cation of Adaptive Modulation and Coding and by an inter
`beam handoffscheme. The handoff scheme ensures that when
`an end user equipment is located in a cusp region between
`adjacent beams the antenna arrangement simultaneously
`transmits data traffic to that mobile station on at least both of
`the adjacent beams.
`
`GB
`
`2378857
`
`2, 2003
`
`41 Claims, 4 Drawing Sheets
`
`30
`
`
`
`
`
`
`
`
`
`CUSP
`ETERMIN
`ATION
`
`duplexers
`
`
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`& i 2.
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`antenna
`
`
`
`1
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`GM 1010
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`

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`US 7,826.471 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`5/2005 Feria et al. .................. 370,329
`6,891,813 B2 *
`6/2005 Roy ........................ 455,562.1
`6,907.272 B2 *
`7,095,987 B2* 8/2006 Brothers et al.
`550
`7,133,379 B2 * 1 1/2006 Yoshii et al. ................ 370,328
`2002/0072393 A1* 6/2002 McGowan et al. .......... 455,562
`2002/0105929 A1
`8, 2002 Chen et al.
`
`
`
`Motorola Inc. “Technical Overview of 1xEV-DV - Global Telecom
`Solution Sector, Sep. 6, 2002, XP002427030.
`Dennet S: “The cdma2000 ITU-R RTT Candidate Submission” Jul.
`27, 1998, XP002427031.
`
`* cited by examiner
`
`2
`
`

`

`U.S. Patent
`
`Nov.2, 2010
`
`Sheet 1 of 4
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`US 7,826,471 B2
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`[317
`
`Buusyue
`
`suaxajdnp
`
`“NIW3L30
`
`NOILV
`
`TA
`
`vO
`
`ONTNGSHIS
`
`3SIAIG
`
`3
`
`
`
`
`
`

`

`U.S. Patent
`
`Nov. 2, 2010
`
`Sheet 2 of 4
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`US 7.826471 B2
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` • • [-|-|-|- • No. EEEE e º ETBE
`
`ig. 2C
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`4
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`

`

`U.S. Patent
`
`Nov. 2, 2010
`
`Sheet 3 of 4
`
`US 7.826471 B2
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`
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`F-POCCHS which
`this MS must
`monitor
`
`F-PDCCHS Which
`may illuminate this
`beam
`
`5
`
`

`

`U.S. Patent
`
`Nov. 2, 2010
`
`Sheet 4 of 4
`
`US 7.826471 B2
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`A. base station (2) determines
`a mobile system is located in a
`cusp region between two beams (10, 12)
`
`B. scheduling device (31) schedules
`same data traffic to be transmitted to
`that mobile system on the two beams
`(10, 12) simultaneously
`
`Fig. 6
`
`
`
`
`
`Y. measure carrierlinterface
`level on sector wide
`pilot channel
`
`
`
`Y. measure carrierlinterface
`level on the unique auxiliary
`pilot channel of a beam
`
`Z. apply appropriate modulation
`and coding to signals transmitted
`on the beams covering the sector
`
`Z. apply appropriate modulation
`and coding to that beam
`
`Fig. 7A
`
`Fig. 7B
`
`6
`
`

`

`1.
`MULT-BEAMCELLULAR
`COMMUNICATION SYSTEM
`
`FIELD OF THE INVENTION
`
`This invention relates to multi-beam cellular communica
`tion systems and in particular to methods and apparatus for
`increasing the capacity of Such systems.
`
`5
`
`BACKGROUND OF THE INVENTION
`
`10
`
`Digital cellular systems, such as Code Division Multiple
`Access (CDMA) and Universal Mobile Telephony System
`(UMTS) systems, are currently used for providing telecom
`munications to stationary and mobile end user equipments. 15
`The mobile end user equipments may be mobile stations, for
`example mobile telephones and mobile computing devices.
`Such digital cellular systems divide a geographical area to be
`covered by the communication system into a plurality of cell
`areas. Within each cell is positioned a base station with which 20
`a plurality of end user equipments located within the cell
`communicate.
`In general it is desired to have as few base stations as
`possible, as they are expensive, require extensive effort in
`obtaining planning permission and in Some areas Suitable 25
`base station sites may not be available. In order to have as few
`base stations as possible, each base station should ideally
`have as large a capacity as possible in order to service as large
`a number of end user equipments as possible.
`One method of achieving an increase in capacity in a 30
`CDMA system is described in U.S. patent application 2002/
`0072393 of McGowan et al. This involves the replacement of
`a sector wide beam width antenna with an antenna array that
`allows the formation of a number of narrower beams that
`cover the area of the original wide beam. The signal for a 35
`particular mobile station can then be sent and received only
`over the beam or beams that are useful for that mobile station.
`Such multiple narrow beams can be used for traffic channels,
`whereas overhead channels can be transmitted over the sector
`wide beam so that the overhead channels, including a pilot 40
`channel, are common to all the narrow beams in the sector.
`McGowan etal provides an antenna array for providing fixed
`narrow beams for transmitting and receiving traffic channels
`and for broadcasting common overhead channels over the
`sector using the same antenna array. This is achieved by 45
`providing appropriate frequency or phase offsets between the
`multiple fixed beams.
`Significant optimisation of the system taught in McGowan
`etal is required for it to be deployed in evolving third gen
`eration (3G) communications systems, such as 1xEV-DV 50
`(1xEVolution for Data and Voice), 1xEV-DO (1xEVolution
`for Data Only) and HSDPA (High Speed Data Packet Access
`an extension to UMTS (Universal Mobile Telephony Sys
`tem). Also, there is a further need to increase base station
`capacity.
`
`55
`
`SUMMARY OF THE INVENTION
`
`The present invention relates generally to a cellular com
`munication system in which each cell sector is covered by a 60
`multi-beam antenna array in which coded timeslots, which
`may be orthogonally coded timeslots, are transmitted on each
`beam in the sector wherein the codes are re-used in the beams
`in the sector to simultaneously send different data traffic to
`end user equipments on the different beams. A pilot channel 65
`common to all the beams is sent simultaneously to end user
`equipments in each beam on the same code.
`
`US 7,826,471 B2
`
`2
`According to a first aspect of the present invention there is
`provided a method for use in a cellular communication sys
`tem comprising at least one base station, the or each of which
`comprises an antenna arrangement per base station sector for
`generating an array of narrow beams covering the sector,
`which method comprising the steps of:
`simultaneously transmitting timeslots over each beam;
`generating multiple codes in each time slot;
`transmitting the same overhead channels including a pilot
`channel to end user equipments on each beam in a sector
`using the same Sub-set of codes; and
`transmitting different data traffic to end user equipments on
`different beams in a sector by re-using the same codes in
`each beam.
`The simultaneous transmission of timeslots over each
`beam generates Successive sets of simultaneously transmitted
`timeslots and the step of transmitting the same overhead
`channels may comprises the step of allocating the same Sub
`set of codes in each timeslot of a set of timeslots to common
`overhead channels, including a pilot channel, and the step of
`transmitting different data traffic may comprise the step of
`allocating a Sub-set of codes in each timeslot of a set of
`timeslots to different data traffic.
`According to a second aspect of the present invention there
`is provided a cellular communication system comprising at
`least one base station the or each of which comprises an
`antenna arrangement per base station sector, each of which
`antenna arrangements has a beam forming device for gener
`ating an array of narrow beams covering the sector, over each
`of which beams timeslots are simultaneously transmitted
`which timeslots are each split into multiple codes, wherein
`the or each base station is arranged to transmit the same
`overhead channels including a pilot channel to end user
`equipments in each beam in a sector using the same Sub-set of
`codes and to transmit different data traffic to end user equip
`ments on different beams in a sector by re-using the same
`codes in each beam.
`The transmission of timeslots by the antenna arrangement
`generates Successive sets of simultaneously transmitted
`timeslots persector and the communication system may addi
`tionally include a base station controller for allocating the
`common overhead channels to the same Sub-set of codes of
`each timeslot in a set of timeslots and for allocating different
`data traffic to a sub-set of codes of each timeslot in a set of
`timeslots.
`According to a third aspect of the present invention there is
`provided computer readable media for installation on a base
`station controlling device of a cellular communication system
`for controlling a base station which comprises an antenna
`arrangement per base station sector for generating an array of
`narrow beams covering the sector, in which each antenna
`arrangement simultaneously transmits timeslots over each
`beam to generate Successive sets of simultaneously transmit
`ted timeslots and in which each time slot is split into multiple
`codes; which computer readable media carries out the steps
`of:
`allocating the same Sub-set of codes in each timeslot of a
`set of timeslots to common overhead channels including
`a common pilot channel; and
`allocating a sub-set of codes in each timeslot of a set of
`timeslots to different data traffic so as to transmit differ
`ent data traffic on different beams by reusing the same
`codes in each beam.
`According to a fourth aspect of the present invention there
`is provided a base station of a cellular communication system
`comprising an antenna arrangement per base station sector,
`each of which antenna arrangements has a beam forming
`
`7
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`

`

`3
`device for generating an array of narrow beams covering the
`sector, over each of which beams timeslots are simulta
`neously transmitted which timeslots are each split into mul
`tiple codes, wherein each base station is arranged to transmit
`common overhead channels including a pilot channel to end
`user equipments in each beam in a sector using the same
`sub-set of codes and to transmit different data traffic to end
`user equipments on different beams in a sector by re-using the
`same codes in each beam.
`The transmission of timeslots by the antenna arrangement
`generates Successive sets of simultaneously transmitted
`timeslots persector and the base station may include a sched
`uling device for allocating the common overhead channels to
`the same sub-set of codes of each timeslot in a set of timeslots
`and for allocating different data traffic to a sub-set of codes of
`each timeslot in a set of timeslots.
`The first to fourth aspect of the present invention enable
`data traffic to be transmitted simultaneously on all beams of a
`multi-beam antenna arrangement within a cell sector, without
`reuse of pilot channel codes. Therefore, a large increase in
`capacity is achieved without introducing pilot channel inter
`ference problems. The present invention relies on the spatial
`separation of the beams for an end user equipment to dis
`criminate between the beams.
`The multiple codes may be orthogonal codes such as Walsh
`codes which are suitable for use in CDMA systems or OVSF
`(Orthogonal Variable Spreading Factor) codes which are suit
`able for use in UMTS systems. Also, Gold or other pseudo
`random codes could be used.
`Due to the reliance on the spatial separation of the beams to
`discriminate between the beams, it is desirable to operate an
`inter beam handoff scheme to reduce the loss of data traffic
`when an end user equipment enters a region of high carrier/
`interference levels, such as a cusp region between adjacent
`beams. In this case when it is determined, for example by a
`processor of the base station, that an end user equipment is
`located in Such a region, in response to Such a determination
`data traffic may be sent to that end user equipment on the same
`codes simultaneously on multiple beams, for example by a
`base station scheduling device.
`In addition because spatial separation is relied upon to
`discriminate between beams it is useful to apply adaptive
`modulation and coding (AMC) to the data traffic transmitted
`over the beams. AMC can be effectively exploited according
`by the present invention because traffic is sent on all of the
`beams in a sector simultaneously and so a stable interference
`environment is provided. In one embodiment the carrier/
`interference level on the common pilot channel is measured
`and in response to the measured level appropriate modulation
`and coding is applied to the signals transmitted on the beams.
`In an alternative embodiment, which has the advantage of not
`requiring an inter-beam handoff scheme in which data is sent
`on multiple beams, a unique auxiliary pilot signal may be
`transmitted on each beam in the sector and the carrier/inter
`ference level on the auxiliary pilot signal of a beam can be
`determined and in response to the determined level appropri
`ate modulation and coding can be applied to the signals trans
`mitted on that beam. The carrier/interference level of the best
`beam in a sector for an end user equipment, ie. the beam on
`which data traffic to that end user equipments is sent, can be
`determined by measuring the carrier/interference level for the
`auxiliary pilot in the best beam and then subtracting the
`measured carrier level of the auxiliary pilots in the other
`beams in that sector.
`Due to the reliance on spatial separation to discriminate
`between beams in a sector, it may be advisable to allocate
`sub-sets of codes to different data traffic which sub-sets
`
`25
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`US 7,826,471 B2
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`5
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`10
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`15
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`4
`include a different number of codes in simultaneously trans
`mitted timeslots of adjacent beams. Then if an end user equip
`ment mistakenly demodulates data traffic sent on a different
`beam from that intended, the data packet will not be of the size
`expected by that end user equipment and so will fail its
`physical layer error checking. Thus, any Such error can be
`quickly corrected.
`According to a fifth aspect of the present invention there is
`provided a handoff method for use in a base station of a
`cellular communication system comprising at least one
`antenna arrangement for generating an array of narrow beams
`for covering a sector wherein an end user equipment located
`in the sector discriminates between the beams in the sector
`only by the spatial separation of the beams and in which
`different data traffic signals are simultaneously transmitted to
`end user equipments on each beam, comprising the steps of
`determining when an end user equipment is located in a
`high interference region of the sector, and
`in response to Such a determination simultaneously trans
`mitting data traffic to that end user equipment on at least
`two beams of the sector.
`According to a sixth aspect of the present invention there is
`provided a base station of a cellular communication system
`comprising at least one antenna arrangement for generating
`an array of narrow beams for covering a sector wherein an end
`user equipment located in the sector discriminates between
`the beams only by the spatial separation of the beams in the
`sector and in which different data traffic signals are simulta
`neously transmitted to end user equipments on each beam,
`wherein the base station includes a processing device for
`determining when an end user equipment is located in a high
`interference region of the sector and in response to such a
`determination the antenna arrangement is arranged to simul
`taneously transmit data traffic to that end user equipment on at
`least two beams of the sector.
`According to a seventh aspect of the present invention there
`is provided computer readable media for installation on a base
`station of a cellular communication system which base sta
`tion comprises at least one antenna arrangement for generat
`ing an array of narrow beams for covering a sector wherein an
`end user system located in the sector discriminates between
`the beams only by the spatial separation of the beams in the
`sector and in which different data traffic signals are transmit
`ted simultaneously to end user equipments on each beam,
`wherein the computer readable media carries out the steps of:
`determining when an end user equipment is located in a
`high interference region between two adjacent beams;
`and
`in response to Such a determination simultaneously sched
`uling data traffic to that end user equipment on at least
`two beams in the sector.
`According to the fifth, sixth and seventh aspects of the
`present invention the high interference region of the sector
`may be a cusp region between two adjacent beams, in which
`case data traffic may be simultaneously transmitted to the end
`user equipment on at least both of the adjacent beams.
`The present invention according to the fifth, sixth and
`seventh aspect of the present invention provides a scheme for
`inter-beam handoff for reducing the loss of traffic to end user
`systems where different traffic is sent simultaneously on the
`different beams and only spatial separation is used by the end
`user equipments to distinguish between the beams. This
`handoffscheme is of particular use where different data traffic
`signals are transmitted simultaneously to end user equip
`ments on all beams in a sector.
`According to an eighth aspect of the present invention there
`is provided a method for use in a base station of a cellular
`
`8
`
`

`

`5
`communication system which base station comprises at least
`one antenna arrangement for generating an array of narrow
`beams for covering a sector wherein an end user system
`located in the sector discriminates between the beams in the
`sector only by the spatial separation of the beams, comprising
`the steps of:
`simultaneously transmitting different data traffic packets to
`end user equipments on each beam, and
`simultaneously transmitting data traffic packets of differ
`ent lengths on adjacent beams.
`According to a ninth aspect of the present invention there is
`provided a base station of a cellular communication system
`comprising at least one antenna arrangement for generating
`an array of narrow beams for covering a sector wherein an end
`user system located in the sector discriminates between the
`beams only by the spatial separation of the beams in the
`sector, the base station comprising a scheduling device for
`simultaneously scheduling different data traffic packets on
`each beam and for simultaneously scheduling data traffic
`packets of different lengths on adjacent beams.
`The present invention according to the eighth and ninth
`aspects of the present invention provides an arrangement
`whereby wrongly demodulated packets can be identified
`quickly by the physical layer error checking where the data
`packet from an adjacent beam to the servicing beam is
`demodulated. The error checking is failed because the data
`packet will be of a different length from the expected packet.
`Timeslots may be simultaneously transmitted over each
`beam and multiple codes may be generated in each time slot,
`in which case the data packets may be formed from Sub-sets
`of codes, which codes may be orthogonal codes such as Walsh
`codes or OVSF codes. Two data packets may be formed from
`each of two sub-sets of Walsh codes in each timeslot in which
`the two subsets include a different number of Walsh codes.
`Other aspects and features of the present invention will
`become apparent to those ordinarily skilled in the art upon
`review of the of the following description of specific embodi
`ments of the invention in conjunction with the accompanying
`Figures.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In order that the present invention is more fully understood
`and to show how the same may be carried into effect, refer
`ence shall now be made, by way of example only, to the
`Figures as shown in the accompanying drawing sheets,
`wherein:
`FIG. 1 shows an antenna arrangement for generating three
`narrow beams overlaid with a sector wide beam for providing
`coverage for a sector of a tri-sectored cell in accordance with
`the present invention;
`FIG. 2A shows the partitioning of the Walsh coding in three
`time slots sent simultaneously on the three beams of the
`antenna arrangement of FIG. 1, which time slots carry data to
`six mobile stations with two of the mobile stations located in
`the centre of each fixed beam;
`FIG. 2B shows the partitioning of the Walsh coding in three
`time slots sent simultaneously on the three beams of the
`antenna arrangement of FIG. 1, which time slots carry data to
`five mobile stations with one of the mobile stations located in
`the cusp between two fixed beams;
`FIG.2C shows the partitioning of the Walsh coding in three
`time slots sent simultaneously on the three beams of the
`antenna arrangement of FIG. 1, which time slots carry data to
`four mobile stations with one of the mobile stations located in
`each of the two cusps between the three fixed beams:
`
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`40
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`US 7,826,471 B2
`
`6
`FIG. 3 shows a scheme for allocating packet data control
`channel codes to the beams of the antenna arrangement of
`FIG. 1:
`FIG. 4 shows schematically, the main beam pattern for the
`three fixed beams (in full lines) and the main beam pattern for
`the sector wide beam (in dottedlines) for the antenna arrange
`ment of FIG. 1;
`FIG. 5 shows the partitioning of the Walsh coding in three
`time slots sent simultaneously on the three beams of the
`antenna arrangement of FIG. 1, which partitioning is similar
`to that of FIG. 2A except that the two blocks of Walsh codes
`for data traffic have different breakpoints in adjacent beams:
`FIG. 6 shows a flow chart of the steps in an inter-beam
`handoff scheme according to the present invention;
`FIG. 7A shows a flow chart of the steps of a first adaptive
`modulation and coding scheme applied to the signals trans
`mitted on the beams of the antenna system of FIG. 1; and
`FIG.7B shows a flow chart of the steps of a second adaptive
`modulation and coding scheme applied to the signals trans
`mitted on the beams of the antenna system of FIG. 1.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`There will now be described by way of example the best
`mode contemplated by the inventor for carrying out the inven
`tion. In the following description, numerous specific details
`are set out in order to provide a complete understanding of the
`present invention. It will be apparent, however, to those
`skilled in the art that the present invention may be put into
`practice with variations of the specific.
`FIG. 1 shows an antenna arrangement for providing cov
`erage in one sector of a three sector cell of a cellular commu
`nication system, which cell is serviced by a base station (2).
`The base station (2) is equipped with three such antenna
`arrangements (4, 6, 8) off-set in azimuth angle by 120°, so that
`each antenna arrangement covers a sector of the cell. The
`antenna arrangement defines a first narrow transmit/receive
`beam (10a) and diversity receive beam (10b), a second nar
`row transmit/receive beam (12a) and diversity receive beam
`(12b), a third narrow transmit/receive beam (14a) and diver
`sity receive beam (14b) and a full sector overlay transmit/
`receive beam (44). The generated antenna beam has three
`dual polar 22-degree beams (10, 12, 14) per sector with a full
`sector beam (44) overlayed. The beams (10a, 12a, 14a, 44)
`are formed by six columns of antenna elements (16) fed by a
`beam former (18) and the beams (10b, 12b. 14b, 44) are
`formed by six columns of antenna elements (20) which teed a
`receive only beam former (22).
`The beam former (18) has three ports each connected to a
`corresponding line (24, 26, 28). The respective lines carry the
`data traffic to be transmitted by corresponding beams (10a,
`12a, 14a) and also the signal received by corresponding
`beams (10a, 12a, 14a). The traffic to be transmitted on the
`respective beams is allocated and fed to each line by a mul
`tiplexer (30). Then the overhead channels, such as the pilot,
`paging and synchronisation channels which are common to
`all the beams are added to each of the three lines (24, 26, 28)
`by a multiplexer (32). Care must be taken to ensure phase
`coherency between the overhead signals and the traffic sig
`nals. A phase offset is applied to two of the input lines (26,28)
`by phase shifters (34,36) so as to reduce pilot interference, in
`accordance with the teaching of McGowan et al referred to
`above. The signals on the lines (24, 26, 28) are then upcon
`verted by chanellizer elements and then amplified by power
`
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`

`US 7,826,471 B2
`
`15
`
`7
`amplifiers (38, 40, 42). At the duplexers, the transmit signals
`are multiplexed onto the same line as the receive signal for
`each beam.
`The multiplexers (30, 32) are controlled by a scheduling
`device (31). The scheduling device (31) may be part of the
`base station and Schedules overhead channels, Voice channels
`and data traffic channels to codes of timeslots to be transmit
`ted over the beams (10, 12, 14). The scheduling device (31)
`may be a digital signal processor on which appropriate com
`puter readable material or software is installed for controlling
`10
`scheduling.
`One pilot signal is common to the sector, with a single
`scrambling code modulated onto the pilot. This is achieved by
`transmitting the same pilot signal on all the narrow beams (10.
`12, 14) simultaneously to effectively create a pilot signal
`which is transmitted on the sector wide beam (44). Accord
`ingly, no changes are required to pilot planning and no
`changes in pilot pollution are experienced as compared to
`conventional tri-cellular networks. Pilots have two functions.
`The first is that they act as a beacon identifying the base
`station by the scrambling code which is modulated onto the
`pilot. The mobile station monitors every pilot in its active list,
`generally eight of them, and reports carrier/interference of the
`best pilot signal and the Scrambling code of the best pilot
`signal to the base station transmitting that pilot signal. In this
`way a Suitable cell sector for Voice and data users is selected.
`The mobile station (1) also uses the pilot of the serving base
`station sector as a phase reference for demodulation. For this
`purpose, the pilot must have good carrier/interference levels
`and must be in phase with the narrow beams (10, 12, 14) on
`which the data traffic is carried.
`The reverse link strategy employed is that the inputs from
`the three narrow beams (10b, 12b. 14b) and the three narrow
`beams (10b, 12b. 14b) are fed to a modem (not shown). The
`modem performs maximal ratio combining on the six inputs.
`FIGS. 2A to 2C each show one set of three timeslots which
`are transmitted on the three beams (10, 12, 14) simulta
`neously. Each time slot is typically split into 32 orthogonal
`Walsh codes, although an alternative is to use 16 Walsh codes
`in each timeslot. The same 32 Walsh codes are re-used in each
`beam. Referring now to FIGS. 2A to 2C, a sub-set P. for
`example 2 of the 32 used orthogonal Walsh codes are allo
`cated for the overhead channels and are common to the three
`fixed beams (10,12, 14). For example, as described above, the
`same pilot signal is sent on an overhead channel simulta
`neously on all the narrow beams in the sector.
`A second sub-set V, for example 2 of the 32 used orthogo
`nal Walsh codes are allocated for power-controlled channels,
`Such as Voice channels and are transmitted on one, two or all
`three of the narrow beams, depending on whether the mobile
`station assigned that channel is in handoff between beams, ie.
`there is no re-use of these Walsh codes for other mobile
`stations in the three fixed beams. That is, the same voice
`signal is sent on the same Voice channel on one, two or all
`three of the narrow beams in the sector. Thus, voice traffic to
`a mobile station is transmitted on one or more of the narrow
`beams, but the code the Voice traffic is assigned to is not
`re-used elsewhere in the sector.
`The re-use of Walsh codes according to the present inven
`tion is unable to Support Voice communications because of the
`increased levels of interference that result.
`The remaining Walsh codes, for example 28 of the 32 used
`Walsh codes are allocated to data traffic. Theses remaining
`Walsh codes may be split, for example into two blocks of 14
`Walsh codes.
`This allocation of Walsh codes in timeslots to different
`traffic, or scheduling, is carried out by the scheduling device
`
`45
`
`8
`(31) of the base station (2). The scheduling device may com
`prise a digital computing device on which appropriate com
`puter readable media or software for controlling scheduling
`has been installed.
`With the allocation shown in FIG. 2A, it is possible to
`simultaneously, ie. within the same time slot period, transmit
`data to six mobile stations 1 to 6 within the sector covered by
`the three beams (10, 12, 14). In the beam (10) two mobile
`stations 1 and 2 are centrally located and each mobile station
`is allocated one of the two blocks of Walsh codes. Thus, data
`traffic to mobile station 1 is transmitted in a first block of the
`Walsh codes (identified by 1 in FIG. 2a) in the time slot on
`beam (10) and data traffic to mobile station 2 is transmitted in
`a second block of the Walsh codes (identified by 2 in FIG.
`2a) in the time slot on beam (10). In the beam (12) two mobile
`stations 3 and 4 are centrally located and each mobile station
`is allocated one of the two blocks of Walsh codes. In the beam
`(14) two mobile stations 5 and 6 are centrally located and each
`mobile station is allocated one of the two blocks of Walsh
`codes.
`With the allocation shown in FIG. 2B, it is possible to
`simultaneously, ie. within the same time slot period, transmit
`to five mobile stations 1 to 5 within the sector covered by the
`three beams (10, 12, 14). In the beam (10) one mobile station
`2 is centrally located and is allocated one of the two blocks of
`Walsh codes. The other block of Walsh codes is allocated to
`one mobile station 1 which is located in the cusp between the
`beam (10) and the beam (12). In the beam (12), in accordance
`with the handoff procedures described below between the
`three beams, the same one of the two blocks of Walsh code is
`allocated to the mobile station 1 located within the cusp
`between the beam (10) and the beam (12). In t