Multi-beam cellular communication system

Abstract

A cellular communication system comprising a plurality of geographically spaced base stations (2) each of which comprise 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) covering the sector. Timeslots are simultaneously transmitted over each of the beams so as to generate successive sets of simultaneously 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 application of Adaptive Modulation and Coding and by an inter-beam handoff scheme. 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.

Claims

1. A method performed by a user equipment of a cellular communications system, the method comprising: receiving, from a base station, a common pilot signal on a first subset of orthogonal resources; receiving, from the base station and on a second subset of orthogonal resources, a packet data control channel including an allocation of a third subset of orthogonal resources, wherein the packet data control channel is different from the common pilot signal; and receiving, within a time slot and on the third subset of orthogonal resources, data from the base station using both a first beam and a second beam simultaneously, wherein the first, second, and third subsets of orthogonal resources are respective first, second, and third subsets of tones within an Orthogonal Frequency Division Multiplexing (OFDM) system.

2. The method of claim 1, wherein a scheduler of the base station assigns the third subset of orthogonal resources on a per time slot basis.

3. The method of claim 1, wherein the packet data control channel is transmitted across a region spanned by both of the first beam and the second beam.

4. The method of claim 1, wherein the base station transmits, within the time slot, data to a second user equipment within at least one of the first beam and the second beam using a fourth set of orthogonal resources.

5. The method of claim 1, further comprising: receiving a first auxiliary pilot signal on the first beam; and receiving a second auxiliary pilot signal on the second beam.

6. The method of claim 5, wherein the first and second auxiliary pilot signals facilitate channel estimation of the respective first and second beams at the user equipment.

7. The method of claim 1, wherein the base station, in response to feedback received from the user equipment, adapts a modulation or coding format of subsequent transmissions of user data to the user equipment.

8. A user equipment, comprising: a memory; and at least one hardware processor communicatively coupled with the memory and configured to: receive, from a base station, a common pilot signal on a first subset of orthogonal resources; receive, from the base station and on a second subset of orthogonal resources, a packet data control channel including an allocation of a third subset of orthogonal resources, wherein the packet data control channel is different from the common pilot signal; and receive, within a time slot and on the third subset of orthogonal resources, data from the base station using both a first beam and a second beam simultaneously, wherein the first, second, and third subsets of orthogonal resources are respective first, second, and third subsets of tones within an Orthogonal Frequency Division Multiplexing (OFDM) system.

9. The user equipment of claim 8, wherein a scheduler of the base station assigns the third subset of orthogonal resources on a per time slot basis.

10. The user equipment of claim 8, wherein the packet data control channel is transmitted across a region spanned by both of the first beam and the second beam.

11. The user equipment of claim 8, wherein the base station transmits, within the time slot, data to a second user equipment within at least one of the first beam and the second beam using a fourth set of orthogonal resources.

12. The user equipment of claim 8, wherein the at least one hardware processor is further configured to: receive a first auxiliary pilot signal on the first beam; and receive a second auxiliary pilot signal on the second beam.

13. The user equipment of claim 12, wherein the first and second auxiliary pilot signals facilitate channel estimation of the respective first and second beams at the user equipment.

14. The user equipment of claim 8, wherein the base station, in response to feedback received from the user equipment, adapts a modulation or coding format of subsequent transmissions of user data to the user equipment.

15. A non-transitory computer readable medium storing instructions to cause one or more processors to perform operations comprising: receiving, from a base station, a common pilot signal on a first subset of orthogonal resources; receiving, from the base station and on a second subset of orthogonal resources, a packet data control channel including an allocation of a third subset of orthogonal resources, wherein the packet data control channel is different from the common pilot signal; and receiving, within a time slot and on the third subset of orthogonal resources, data from the base station using both a first beam and a second beam simultaneously, wherein the first, second, and third subsets of orthogonal resources are respective first, second, and third subsets of tones within an Orthogonal Frequency Division Multiplexing (OFDM) system.

16. The non-transitory computer readable medium of claim 15, wherein a scheduler of the base station assigns the third subset of orthogonal resources on a per time slot basis.

17. The non-transitory computer readable medium of claim 15, wherein the packet data control channel is transmitted across a region spanned by both of the first beam and the second beam.

18. The non-transitory computer readable medium of claim 15, wherein the base station transmits, within the time slot, data to a second user equipment within at least one of the first beam and the second beam using a fourth set of orthogonal resources.

19. The non-transitory computer readable medium of claim 15, further comprising: receiving a first auxiliary pilot signal on the first beam; and receiving a second auxiliary pilot signal on the second beam.

20. The non-transitory computer readable medium of claim 19, wherein the first and second auxiliary pilot signals facilitate channel estimation of the respective first and second beams at a user equipment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order that the present invention is more fully understood and to show how the same may be carried into effect, reference shall now be made, by way of example only, to the Figures as shown in the accompanying drawing sheets, wherein:

(2) 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;

(3) 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;

(4) 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;

(5) 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;

(6) FIG. 3 shows a scheme for allocating packet data control channel codes to the beams of the antenna arrangement of FIG. 1;

(7) 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 dotted lines) for the antenna arrangement of FIG. 1;

(8) 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;

(9) FIG. 6 shows a flow chart of the steps in an inter-beam handoff scheme according to the present invention;

(10) FIG. 7A shows a flow chart of the steps of a first adaptive modulation and coding scheme applied to the signals transmitted on the beams of the antenna system of FIG. 1; and

(11) FIG. 7B shows a flow chart of the steps of a second adaptive modulation and coding scheme applied to the signals transmitted on the beams of the antenna system of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(12) There will now be described by way of example the best mode contemplated by the inventor for carrying out the invention. 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.

(13) FIG. 1 shows an antenna arrangement for providing coverage in one sector of a three sector cell of a cellular communication 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 narrow transmit/receive beam (12a) and diversity receive beam (12b), a third narrow transmit/receive beam (14a) and diversity 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 beamformer (18) and the beams (10b, 12b, 14b, 44) are formed by six columns of antenna elements (20) which feed a receive only beamformer (22).

(14) The beamformer (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 multiplexer (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 signals. 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 upconverted by chanellizer elements and then amplified by power amplifiers (38, 40, 42). At the duplexers, the transmit signals are multiplexed onto the same line as the receive signal for each beam.

(15) 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 transmitted over the beams (10, 12, 14). The scheduling device (31) may be a digital signal processor on which appropriate computer readable material or software is installed for controlling scheduling.

(16) 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). Accordingly, 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.

(17) 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.

(18) FIGS. 2A to 2C each show one set of three timeslots which are transmitted on the three beams (10, 12, 14) simultaneously. Each time slot is typically split into 32 orthogonal Walsh codes, although an alternative is to use 16 Walsh codes in each time slot. 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 allocated 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 simultaneously on all the narrow beams in the sector.

(19) A second sub-set V, for example 2 of the 32 used orthogonal 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.

(20) The re-use of Walsh codes according to the present invention is unable to support voice communications because of the increased levels of interference that result.

(21) 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.

(22) This allocation of Walsh codes in timeslots to different traffic, or scheduling, is carried out by the scheduling device (31) of the base station (2). The scheduling device may comprise a digital computing device on which appropriate computer readable media or software for controlling scheduling has been installed.

(23) 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 26 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.

(24) 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 hand-off 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 this way the same data traffic is transmitted simultaneously in the same block of Walsh codes on the same time slot in beam (10) and beam (12) to the mobile station 1. The remaining block of Walsh codes in that time slot for beam (12) is allocated to a mobile station 3 located centrally in the beam (12). In the beam (14) two mobile stations 4 and 5 are centrally located and each mobile station is allocated one of the two blocks of Walsh codes.

(25) With the allocation shown in FIG. 2C, it is possible to simultaneously, ie. within the same time slot period, transmit to four mobile stations 1 to 4 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 this way the same data is transmitted simultaneously in the same block of Walsh codes on the same time slot in beam (10) and beam (12) to the mobile station 1. The remaining block of Walsh codes in that time slot for beam (12) is allocated to a mobile station 3 located in the cusp between the beam (12) and the beam (14). In accordance with the handoff procedures described below between the three beams, the same one of the two blocks of Walsh code for the beam (14) is allocated to the mobile station 3 located within the cusp between the beam (12) and the beam (14). In this way the same data is transmitted simultaneously in the same block of Walsh codes on the same time slot in beam (12) and beam (14) to the mobile station 3. In the beam (14) one mobile station 4 is centrally located and is allocated the remaining one of the two blocks of Walsh codes for beam (14).

(26) Accordingly, it can be seen that the re-use of Walsh codes in the three beams (10, 12, 14) allows significantly more traffic to be carried by the beams. The spatial separation of the beams is relied upon for time slots transmitted on the separate beams to be distinguished. This increases the capacity of a cellular communication network adopting the present invention without causing additional pilot codes to be used and hence without increasing pilot signal interference.

(27) In FIG. 2A, the mobile station 1 is located centrally in the narrow beam (10) and so data traffic is sent to it on blocks of Walsh codes in timeslots transmitted by the beam (10) only. As the mobile statin moves towards the region covered by the beam (12) It enters a cusp region between the beams (10) and (12). The carrier/interference level in the transmissions to the mobile station 1 on the beam (10) will become worse due to interference from the beam (12). With the mobile station located within the cusp region the problem can occurs that the mobile station 1 which should be demodulating data traffic in a designated block of Walsh codes in a time slot of the beam (10) may sometimes demodulate data traffic on the same block of Walsh codes and same time slot but of the beam (12). To avoid this problem, when a base station identifies that the mobile station is entering a cusp region [Box A of FIG. 6] a handoff procedure is initiated in which, as shown in FIG. 2B, the same traffic data is sent to mobile station 1 in the same block of Walsh codes and in the simultaneously transmitted time slots of both adjacent beams (10) and (12) [Box B of FIG. 6]. Then, the mobile station will demodulate the combined signal from the designated block of Walsh codes transmitted on beam (10) and the designated block of Walsh codes transmitted on beam (12). The mobile station is highly likely to receive the data traffic sent on the two beams because the main source of interference (ie. the adjacent beam) now contributes to the wanted signal power. A base station can identify when a mobile station enters a cusp region by measuring the receive power on each beam for signals from that mobile station and finding the power to be similar in two adjacent beams. If the mobile station continues to move in the direction of the region covered by the beam (12), it moves out of the cusp region between the adjacent beams (10) and (12). When the base station identifies that the mobile station 1 has left the cusp region and has entered a central portion of the beam (12), the handoff procedure is halted and the mobile station 1 is sent data traffic on designated blocks of Walsh code on time slots of beam (12) only.

(28) In one known prior art implementation of a multi-beam system in accordance with McGowan et al but where the Walsh codes are not re-used in this way, a maximum of two mobile stations (not including mobile stations using power-controlled channels) can be transmitted to in a sector covered by the multiple beams in a time slot. The first mobile user is allocated a first block of the sub-set of Walsh codes used for carrying traffic within the sector and the second mobile user is allocated the remaining block. This means that traffic cannot be sent on all beams at all times and in the prior art implementation only those beams of the multi-beam system on which traffic is to be sent in a time slot are illuminated for that time slot. This has the advantage of reducing interference within the system. However, Adaptive Modulation and Coding (AMC) cannot be fully utilised in such a system because the level of interference Is not stable over time.

(29) In the examples given above in relation to FIGS. 2A to 2C the sub-set of Walsh codes which are re-used in the three beams (10, 12, 14) are split into two blocks. This is the convention in accordance with current standards and is adopted in order to keep the signalling overhead to an acceptable level, if the sub-set of Walsh codes are further sub-divided, then more signalling messages to mobile stations will be required to notify the mobile stations of their allocations of Walsh code blocks. However, with the development of the technologies involved, it may become advantageous to sub-divide the sub-set of re-used Walsh codes in a time slot into more than two blocks.

(30) As described above in relation to FIGS. 2A to 2C, up to six mobile stations may be scheduled simultaneously within a sector and so in the worst case six length 64 Walsh codes (in accordance with the 1×EV-DV standard) are required for F-PDCCH (Forward-Packet Data Control Channels). This number can be reduced in accordance with FIG. 3 by re-using the same F-PDCCH code in outer beams which have spatial separation. Current standards provide mobile stations that can support two F-PDCCH codes simultaneously. In the scheme shown in FIG. 3, four F-PDCCH codes are required per sector, which codes are labelled 1 to 4 in FIG. 3. Each F-PDCCH code is transmitted on two of the three beams (10, 12, 14). The F-PDCCH codes 3 and 4 transmit the same data from adjacent beams, ie. beams pairs (10, 12) and (12, 14) and can be used to control packet data traffic to a mobile system in handover between adjacent beams. The F-PDCCH codes 1 and 2 transmit different data (ie. they control different mobile stations) in the two outer beams (10) and (14) by relying on the spatial separation of the different mobile stations (46, 54) controlled. The set of F-PDCCH codes which the mobile station must monitor is dependent on the position of the mobile station within the sector. For example, in FIG. 3, mobile station (46) which is central to the beam (10) monitors codes 1 and 2, mobile station (48) which is in handover between beams (10) and (12) monitors code 3, mobile station (50) which is central to the beam (12) monitors codes 3 and 4, mobile station (52) which is in handover between beams (12) and (14) monitors code 4 and mobile station (54) which is central to the beam (14) monitors re-used codes 1 and 2. It should be noted that many alternative schemes for the packet data control channel can be devised.

(31) The system according to the present invention in which a sub-set of Walsh codes are re-used in each of the fixed beams (10, 12, 14) within the sector is optimised for systems employing Adaptive Modulation and Coding (AMC) such as the 3G (third generation) evolution standards 1×EV-DV (1×EVolution-Data and Voice), 1×EV-DO (1×EVolution-Data Only) and HSDPA (High Speed Data Packet Access an extension to UMTS). 1×EV-DV is an evolution of CDMA2000 1× designed to significantly improve system capacity for mixed voice and data traffic. The 1×EV-DV air interface, although entirely compatible with the IS-05 and CDMA2000 1× standards does have a number of key differences. A first is the ‘shared channel’ concept in which, instead of allocating a sub-set of Walsh codes to a mobile station for all time, a sub-set of Walsh codes can be allocated to a mobile station per time slot. A second is Adaptive Modulation and Coding (AMC), which AMC measures the prevailing level of interference and depending on the measured level allocates a level of modulation and coding.

(32) The benefits of AMC can be optimised by the use of a system according to the present invention, because the three beams are illuminated simultaneously and so the interference occurring on the three beams (10, 12, 14) Is approximately stable between the measurement of the interference environment and the application of an appropriate level of modulation and coding. This is not the case in the prior art system in which selected narrow beams in a sector are illuminated or powered down depending on where the mobile station is. The base station determines the carrier/interference level on the sector wide pilot channel based on measures it receives from the mobile stations receiving the pilot channel [Box Y of FIG. 7A]. In response to the determined carrier/interference level appropriate levels of modulation and coding are applied to the signals transmitted on the beams covering the sector [Box Z of FIG. 7A]. Thus, the AMC in the present system relies on a pilot signal which is common to all beams, and so has a beam pattern shown schematically by dotted lines in FIG. 4, to measure the interference environment of the beams (10, 12, 14) the patterns of which are shown in solid lines in FIG. 4. Therefore, the measured interference environment does not take into account adjacent beam interference. Thus, the level of interference measured for mobile stations, in particular where they are located at the cusp between two adjacent beams, is not as accurate as is ultimately desirable. Thus, the handoff procedures between adjacent beams described above have to be implemented in order to adequately support the mobile stations located at the cusps between the beams. The application of AMC, appropriate scheduling and the inter-beam handoff procedures enables potentially poor carrier/interference levels due to the re-use of Walsh codes in adjacent beams to be overcome for data traffic.

(33) The potentially poor carrier/interference levels which can be experienced by the re-use of Walsh codes in adjacent beams is not consistent with voice communications. Also, AMC is not suitable for use with voice traffic. This is why the Walsh codes allocated voice traffic are not r-used across the beams (10, 12, 14) in a sector. For the data traffic transmitted over the three beams (10, 12, 14) it matters little if the wrong beam is chosen for transmission to a mobile station, provided this error is only on an occasional time slot. This is very different from a voice call where a drop would occur.

(34) In the system described above there is no discrimination between re-used Walsh codes on adjacent beams apart from the spatial separation of the beams. If the mobile station is located at a cusp between adjacent beams then there is a risk that the wrong beam transmission will be demodulated by the mobile station. This is mitigated by a handover procedure between adjacent beams in which the same data is sent simultaneously on adjacent beams, just for the time that the mobile station is in the beam cusp region, as is illustrated in FIGS. 2B, 2C and 6.

(35) The risks associated with demodulating a data packet from the wrong beam pose a significant problem because each such data packet would pass physical layer error checking (for example, parity check, cyclic redundancy check, etc.) and thus would not be flagged as an error. The error would thus propagate to higher layers, making the time delay before which the error is detected larger, with the consequent increased loss of packets, which would have to be re-sent. This means that a conservative approach to handoff is appropriate meaning that the mobile stations will often be in handoff. However, if duplicate data has to be sent on adjacent beams for a large proportion of time, then the capacity improvement afforded by the re-use of the Walsh codes is reduced.

(36) This problem can be limited by applying the scheme illustrated in FIG. 5. In FIG. 5 two users are simultaneously scheduled in each of the beams (10, 12, 14), in the same way as is described above in relation to FIG. 2A. The control channel F-DPCCH informs them which block of the 28 Walsh codes allocated to data traffic each should demodulate in each time slot. The length of the block of codes to be demodulated is also notified to the mobile system on the control channel F-DPCCH. The breakpoint (56) between the two blocks of Walsh codes is located in a different position in adjacent beams. For example, in beam (10), codes 1 and 2 are allocated to the control channels, codes 3 and 4 are allocated to voice channels, codes 5 to 19 are allocated to data traffic for mobile station 1 and codes 20 to 32 are allocated to data traffic for mobile station 2. Therefore, the block to be demodulated by mobile station 1 is 15 codes long and the block to be demodulated by mobile station 2 is 13 codes long. Then, in beam (12), codes 1 and 2 are allocated to the control channels, codes 3 and 4 are allocated to voice channels, codes 6 to 17 are allocated to data traffic for mobile station 3 and codes 18 to 32 are allocated to data traffic for mobile station 4. Therefore, the block to be demodulated by mobile station 3 is 13 codes long and the block to be demodulated by mobile station 4 is 15 codes long. Therefore, if mobile station 1 in beam (10) were to decode its specified block of codes from the wrong beam's transmission, ie. the block destined for mobile station 3 in beam (12), the block would be too short, at only 13 codes instead of the expected 15 codes and so will fall its physical layer error checking. Corrective action can be taken immediately, for example by re-transmitting the block of codes that should have been demodulated by mobile station 1 simultaneously on beams (10) and (12) to the mobile station 1. The time delay during which data is lost and has to be re-sent is reduced. Similarly, the breakpoint (56) in the timeslots simultaneously transmitted from beams (12) and (14) is offset. If it is ensured that the breakpoint (56) defining the boundary between block of Walsh codes is always at a different position in simultaneously transmitted time slots of adjacent beams then such errors will be detected quickly.

(37) This allocation of Walsh codes in timeslots to data traffic so as to ensure that simultaneously transmitted data packets of different length are transmitted on adjacent beams, or scheduling, is carried out by the scheduling device (31) of the base station (2). The scheduling device may comprise a digital computing device on which appropriate computer readable media or software for controlling scheduling has been installed. The data packets, as described above are formed from sub-sets of orthogonal codes on timeslots transmitted on the beams.

(38) The use of offset breakpoints is described above in relation to a sub-set of Walsh codes for CDMA based systems. However, the breakpoints could be defined as tones in Orthogonal Frequency Division Multiplexed (OFDM) or frequencies in Frequency Division Multiple Access/Time Division Multiple Access systems. That is, the set of tones or frequencies allocated to the system could be split into two sub-sets of tones or frequencies with the division between the sub-sets always set differently in adjacent beams. The use of such offset breakpoints is relevant to systems where spatial separation alone discriminates between adjacent beams.

(39) As an alternative to the handover procedure described above, it is possible for each of the narrow beams to carry a unique auxiliary pilot signal in addition to a primary pilot signal being sent which is common to all of the beams in a multi-beam sector. The auxiliary pilots are used by the mobile stations to provide an accurate measurement of the interference environment for the individual beams which can be used for AMC and also provide an accurate pilot signal for channel estimation purposes. The mobile station will use the carrier level measured from the other beams within the same sector to correct the carrier/Interference level measured on the strongest beam for that mobile station (ie. the serving beam for that mobile station on which data traffic will be transmitted to it) in order to take account of the interference from adjacent beams within the sector. In this way the carrier/Interference level on the auxiliary pilot channel of a beam is determined by the base station based on responses from the mobile stations services by that beam [Box Y of FIG. 7B]. Then the base station applies appropriate modulation and coding to that beam. With more accurate AMC and channel estimation mobile stations can be handed over between beams without a handoff period in which the same data has to be sent to the mobile station on both beams.

(40) Although, the invention has been described above in relation to a fixed 6 multi-beam system, it can also be applied to include a multi-beam system in which steered narrow beams are dynamically pointed so that the peak of the beam is directed at individual mobile stations. The basic concepts of overlay broadcast and pilot channels, re-use of Walsh codes in the narrow beams and the use of AMC and handover remain the same for steered beams.