Method and apparatus for directional radio communication

[0002] The present invention relates to a method and apparatus for directional radio communication in which signals between a first station and a second station may be transmitted only in certain directions. In particular, but not exclusively, the present invention is applicable to cellular communication networks using space division multiple access.

[0003] With currently implemented cellular communication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by that base transceiver station. However, space division multiple access (SDMA) systems have now been proposed. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in the beam direction from which a signal from the mobile station is received. SDMA may also permit the base transceiver station to determine the direction from which signals from the mobile station are received.

[0004] SDMA systems may allow a number of advantages over existing systems to be achieved. In particular, as the beam which is transmitted by the BTS may only be transmitted in a particular direction and accordingly may be relatively narrow, the power of the transceiver can be concentrated into that narrow beam. It is believed that this results in a better signal to noise ratio with both the signals transmitted from the base transceiver station and the signals received by the base transceiver station. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver station can be achieved. Furthermore, in the transmitting direction, the directionality of the BTS allows energy to be concentrated into a narrow beam so that the signal transmitted by the BTS can reach far away located mobile stations with lower power levels than required by a conventional BTS. This may allow mobile stations to operate successfully at greater distances from the base transceiver station which in turn means that the size of each cell or cell sector of the cellular network can be increased. As a consequence of the larger cell size, the number of base stations which are required can also be reduced leading to lower network costs. SDMA systems generally require a number of antenna elements in order to achieve the required plurality of different beam directions in which signals can be transmitted and received. The provision of a plurality of antenna elements increases the sensitivity of the BTS to received signals. This means that larger cell sizes do not adversely affect the reception of signals by the BTS from mobile stations.

[0005] SDMA systems may also increase the capacity of the system, that is the number of mobile stations which can be simultaneously supported by the system is increased. This is due to the directional nature of the communication which means that the BTS will pick up less interference from mobile stations in other cells using the same frequency. The BTS will generate less interference to other mobile stations in other cells using the same frequency when communicating with a given MS in the associated cell.

[0006] Ultimately, it is believed that SDMA systems will allow the same frequency to be used simultaneously to transmit to two or even more different mobile stations which are arranged at different locations within the same cell. This can lead to a significant increase in the amount of traffic which can be carried by cellular networks.

[0007] SDMA systems can be implemented in analogue and digital cellular networks and may be incorporated in the various existing standards such as GSM, DCS 1800, TACS, AMPS and NMT. SDMA systems can also be used in conjunction with other existing multiple access techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA) techniques.

[0008] One problem with SDMA systems is that the direction in which signals should be transmitted to a mobile station needs to be determined. In certain circumstances, a relatively narrow beam will be used to send a signal from a base transceiver station to a mobile station. Therefore, the direction of that mobile station needs to be assessed reasonably accurately. If a mobile station is moving relative to the base transceiver station, it may move from the region of the cell covered by a first beam to the region of the cell covered by the adjacent beam. A decision needs to be made by the base transceiver station as to when the adjacent beam should be used instead of the first beam for transmitting signals to the mobile station. This decision is complicated by the fact that the first and the adjacent beams will in practice partially overlap.

[0009] As additional problem is that the direction in which a signal is to be transmitted by the BTS to the mobile station is determined on the basis of the uplink signals received by the BTS from the mobile station. However, the frequencies of the down link signals transmitted from the mobile station to the BTS are different from the frequencies used for the signals transmitted by the BTS to the mobile station. The difference in the frequencies used in the uplink and downlink signals means that the behaviour of the channel in the uplink direction may be different from the behaviour of the channel in the downlink direction. Thus the optimum direction determined for the uplink signals will not always be the optimum direction for the downlink signals.

[0010] It is an aim of certain embodiments of the present invention to address these difficulties.

[0011] According to a first aspect of the present invention, there is provided a method of directional radio communication in a mobile communication network between a first station and a second mobile station, the method comprising the steps of:

[0012] identifying at said first station consecutive bursts of communication data transmitted from said second station,

[0013] determining from a plurality of adjacent beam directions respective beam directions from which sequential communication data bursts are received by the first station; and

[0014] selectively selecting one or more of said beam directions for transmitting of signals from said first station to said second station such that when it is determined that the sequential communication data bursts are received by the first station from two different beam directions, said two different beam directions are simultaneously selected for transmission of said signals from said first station to said second station, and wherein when said two different beam directions are not adjacent, the signals are transmitted from the first station to said second station in both of said different beam directions and additionally in the intervening beam directions.

[0015] Thus, with embodiments of the present invention, the ambiguity caused by the second station moving from the area covered by one beam to the area covered by another beam may be compensated for. In particular, the probability that a signal from the first station will reach the second station is increased.

[0016] Preferably, when said two different beam directions are not adjacent, the signals are transmitted from the first station to the second station in both of said different beam directions and additionally in the intervening beam directions. If the different beam directions are not adjacent, it is likely that one of those directions will correspond to an anomalous path being followed by a burst of communication data from the first station. The signal transmitted from the first station to the second station would be unlikely to follow this anomalous path. By also transmitting in all the intervening beam directions, the effects resulting from the detection of a signal following the anomalous path can be reduced.

[0017] Preferably, the first and second beam directions can be used for a predetermined plurality of consecutive signals transmitted from the first station. Alternatively, the first and second beam directions may only be used for a signal transmitted from the first station. It is preferred that whenever possible, the signal from the first station be transmitted only in a single beam direction.

[0018] The determined direction from which each of the sequential data bursts is received may be defined as the beam direction from which a strongest signal is received. This corresponds to the path of least attenuation for a given burst. Alternatively, the determined direction from which each of the sequential data bursts is received may be defined as the beam direction from which the signal following the shortest path is received.

[0019] Preferably, the method includes a step of monitoring a distance parameter representative of the distance between the first and second stations, wherein if the distance between the first and second stations is less than a predetermined value, the signals transmitted to said second station are transmitted with a relatively wide angular spread, regardless of the direction from which the first and second signals are received. In practice, this may mean that a signal is transmitted over a relatively large number of beam directions, if the distance between the first and second stations is less than the critical distance. If the distance between the first and second stations is less than said predetermined value, the signals may be transmitted to the second station with a relatively low power and if the distance is greater than the predetermined value, the signals may be transmitted with a higher power level.

[0020] According to a second aspect of the present invention, there is provided a method of directional radio communication in a mobile communication network between a first station and a second mobile station, the method comprising the steps of:

[0021] identifying at said first station consecutive bursts of communication data transmitted from said second station,

[0022] determining from a plurality of adjacent beam directions respective beam directions from which sequential communication data bursts are received by the first station; and

[0023] selectively selecting one or more of said beam directions for transmission of signals from said first station to said second station such that when it is determined that the sequential communication data bursts are received by the first station from two different beam directions, said two different beam directions are simultaneously selected for transmission of said signals from said first station to said second station, and including the step of monitoring a distance parameter representative of the distance between said first and second stations, wherein if the distance between said first and second stations is less than a predetermined value, the signals transmitted to said second station are transmitted with a relatively wide angular spread, regardless of the directions from which said two signals are received.

[0024] Preferably, the control means is arranged, when said two different directions are not adjacent, to control the transmitter means to transmit communication data in both of said different beam directions and additionally in the intervening beam directions. Preferably, the control means is arranged to control the transmitter means to transmit a predetermined plurality of consecutive signals in said first and second beam directions.

[0025] According to a third aspect of the present invention there is provided a first station for directional radio communication in a mobile communication network with a second mobile station, said first station comprising:

[0026] identifier means for identifying consecutive bursts of communication data transmitted from said second station;

[0027] determining means for determining from a plurality of adjacent beam directions respective beam directions from which sequential communication data bursts are received by said first station;

[0028] transmitter means for transmitting signals to said second station;

[0029] control means for selecting one or more of said beam directions for transmission of signals from the first station to the second station such that when it is determined that the sequential communication data bursts are received by the first station from two different beam directions, said control means is arranged to simultaneously select said two different beam directions for the transmission of signals by said transmitter means, and when said two different beam directions are not adjacent, to control the transmitter means to transmit communication data in both of said different beam directions and additionally in the intervening beam directions.

[0030] According to a fourth aspect of the present invention, there is provided a first station for directional radio communication in a mobile communication network with a second mobile station, said first station comprising:

[0031] identifier means for identifying consecutive bursts of communication data transmitted from said second station;

[0032] determining means for determining from a plurality of adjacent beam directions respective beam directions from which sequential communication data bursts are received by said first station;

[0033] transmitter means for transmitting signals to said second station;

[0034] control means for selecting one or more of said beam directions for transmission of signals from the first station to the second such that when it is determined that the sequential communication data bursts are received by the first station from two different beam directions, said control means is arranged to simultaneously select said two different beam directions for the transmission of signals by said transmitter means and to monitor a distance parameter representative of the distance between the first and second stations such that if the distance between the first and second stations is less than the predetermined value, the signals transmitted to said second station are transmitted with a relatively wide angular spread, regardless of the directions from which said two signals are received.

[0035] The present invention is particularly applicable to cellular communication networks where the first station is a base transceiver station. However, it should be appreciated that embodiments of the present invention may be applicable to non-cellular directional radio communication networks. It should be appreciated that the term “burst” used in this document merely indicates a portion of a signal received from the second station and is not intended to limit the applicability of the invention to a standard system such as GSM in which the term “burst” may have a specific meaning. This burst can correspond to, for example, data bursts used in a GSM TDMA system. Alternatively, the term “burst” could refer to a given portion of the data transmitted from the second station.

[0036] For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:

[0037] FIG. 1 shows a schematic view of a base transceiver station (BTS) and its associated cell sectors;

[0038] FIG. 2 shows a simplified representation of an antenna array and the base transceiver station;

[0039] FIG. 3 shows the fixed beam pattern provided by the antenna array of FIG. 2;

[0040] FIG. 4 shows a schematic view of the digital signal processor of FIG. 2;

[0041] FIG. 5 illustrates the channel impulse response for four channels, out of the eight channels;

[0042] FIGS. 6ato hshow the beam direction in from which eight consecutive data bursts transmitted by the MS to the base transceiver station are deemed to be received; and

[0043] FIGS. 7ato hshow the selected beams in eight consecutive data bursts transmitted by a SDMA base transceiver station to a MS in accordance with an embodiment of the present invention.

[0044] Reference will first be made to FIG. 1 in which three cell sectors 2 defining a cell 3 of a cellular mobile telephone network are shown. The three cell sectors 2 are served by respective base transceiver stations (BTS). Each BTS 4 has a transceiver which transmits and receives signals to and from a respective one of the three cell sectors 2. Thus, one dedicated base transceiver station is provided for each cell sector 2. Each BTS 4 is thus able to communicate with mobile stations (MS) such as mobile telephones which are located in respective cell sectors 2.

[0045] The present embodiment is described in the context of a GSM (Global System for Mobile Communication) network. In the GSM system, a frequency/time division multiple access F/TDMA system is used. Data is transmitted between the BTS 4 and the MS in bursts. The data bursts include a training sequence which is a known sequence of data. The purpose of the training sequence will be described hereinafter. Each data burst is transmitted in a given frequency band in a predetermined time slot in that frequency band. The use of a directional antenna array allows space division multiple access also to be achieved. Thus, in embodiments of the present invention, each data burst will be transmitted in a given frequency band, in a given time slot, and in a given direction. An associated channel can be defined for a given data burst transmitted in the given frequency, in the given time slot, and in the given direction. As will be discussed in more detail hereinafter, in some embodiments of the present invention, the same data burst is transmitted in the same frequency band, in the same time slot but in two different directions.

[0046] FIG. 2 shows a schematic view of one antenna array 6 of one BTS 4 which acts as a transceiver. It should be appreciated that the array 6 shown in FIG. 2 only serves one of the three cells 2 shown in FIG. 1. Another two antenna arrays 6 are provided to serve the other two cell sectors 2. The antenna array 6 has eight antenna elements a₁. . . a₈. The elements a₁. . . a₈are arranged to have a spacing of a half wavelength between each antenna element a₁. . . a₈and are arranged in a horizontal row in a straight line. Each antenna element a₁. . . a₈is arranged to transmit and receive signals and can have any suitable construction. Each antenna element a₁. . . a₉may be a dipole antenna, a patch antenna or any other suitable antenna. The eight antenna elements a₁. . .a₈together define a phased array antenna 6.

[0047] As is known, each antenna element a₁. . . a₈of the phased array antenna 6 is supplied with the same signal to be transmitted to a mobile station MS. However, the phases of the signals supplied to the respective antenna elements a₁. . . a₈are shifted with respect to each other. The differences in the phase relationship between the signals supplied to the respective antenna elements a₁. . . a₈gives rise to a directional radiation pattern. Thus, a signal from the BTS 4 may only be transmitted in certain directions in the cell sector 2 associated with the array 6. The directional radiation pattern achieved by the array 6 is a consequence of constructive and destructive interference which arises between the signals which are phase shifted with respect to each other and transmitted by each antenna element a₁. . . a₈. In this regard, reference is made to FIG. 3 which illustrates the directional radiation pattern which is achieved with the antenna array 6. The antenna array 6 can be controlled to provide a beam b₁. . . b₈in any one of the eight directions illustrated in FIG. 3. For example, the antenna array 6 could be controlled to transmit a signal to a MS only in the direction of beam b₅or only in the direction of beam b₆. As will be discussed in further detail hereinafter, it is possible also to control the antenna array 6 to transmit a signal in more than one beam direction at the same time. For example, a signal may be transmitted in the two directions defined by beam b₅and beam b₅. FIG. 3 is only a schematic representation of the eight possible beam directions which can be achieved with the antenna array 6. In practice, however, there will in fact be an overlap between adjacent beams to ensure that all of the cell sector 2 is served by the antenna array 6.

[0048] The relative phase of the signal provided at each antenna element a₁. . . a₈is controlled by Butler matrix circuitry 8 so that a signal can be transmitted in the desired beam direction or directions. The Butler matrix circuitry 8 thus provides a phase shifting function. The Butler matrix circuitry 8 has eight inputs 10a-hfrom the BTS 4 and eight outputs, one to each antenna element a₁. . . a₈. The signals received by the respective inputs 10a-hcomprise the data bursts to be transmitted. Each of the eight inputs 10a-hrepresents the beam direction in which a given data burst could be transmitted. For example, when the Butler matrix circuitry 8 receives a signal on the first input 10a,the Butler matrix circuitry 8 applies the signal provided on input 10ato each of the antenna elements a₁. . . a₈with the required phase differences to cause beam b₁to be produced so that the data burst is transmitted in the direction of beam b₁. Likewise, a signal provided on input 10bcauses a beam in the direction of beam b₂to be produced and so on.

[0049] As already discussed, the antenna elements a₁. . . a₈of the antenna array 6 receive signals from a MS as well as transmit signals to a MS. A signal transmitted by a MS will generally be received by each of the eight antenna elements a₁. . . a₈. However, there will be a phase difference between each of the signals received by the respective antenna elements a₁. . . a₈. The Butler matrix circuitry 8 is therefore able to determine from the relative phases of the signals received by the respective antenna elements a₁. . . a₈the beam direction from which the signal has been received. The Butler matrix circuitry 8 thus has eight inputs, one from each of the antenna elements a₁. . . a₈for the signal received by each antenna element. The Butler matrix circuitry 8 also has eight outputs 14a-h.Each of the outputs 14ato 14hcorresponds to a particular beam direction from which a given data burst could be received. For example, if the antenna array 6 receives a signal from a MS from the direction of beam b₁, then the Butler matrix circuitry 8 will output the received signal on output 14a.A received signal from the direction of beam b₂will cause the received signal to be output from the Butler matrix circuitry 8 on output 14band so on. In summary, the Butler matrix circuitry 8 will receive on the antenna elements a₁. . . a₈eight versions of the same signal which are phase shifted with respect to one another. From the relative phase shifts, the Butler matrix circuitry 8 determines the direction from which the received signal has been received and outputs a signal on a given output 14a-hin dependence on the direction from which the signal has been received.

[0050] It should be appreciated that in some environments, a single signal or data burst from a MS may appear to come from more than one beam direction due to reflection of the signal whilst it travels between the MS and the BTS 4, provided that the reflections have a relatively wide angular spread. The Butler matrix circuitry 8 will provide a signal on each output 14a-hcorresponding to each of the beam directions from which a given signal or data burst appears to come. Thus, the same data burst may be provided on more than one output 14a-hof the Butler matrix circuitry 8. However, the signal on the respective outputs 14a-hmay be time delayed with respect to each other.

[0051] Each output 14a-hof the Butler matrix circuitry 8 is connected to the input of a respective amplifier 16 which amplifies the received signal. One amplifier 16 is provided for each output 14a-hof the Butler matrix circuitry 8. The amplified signal is then processed by a respective processor 18 which manipulates the amplified signal to reduce the frequency of the received signal to the baseband frequency so that the signal can be processed by the BTS 4. To achieve this, the processor 18 removes the carrier frequency component from the input signal. Again, one processor 18 is provided for each output 14a-hof the Butler matrix circuitry 8. The received signal, which is in analogue form, is then converted into a digital signal by an analogue to digital. (A/D) converter 20. Eight A/D converters 20 are provided, one for each output 14a-hof the Butler matrix circuitry 8. The digital signal is then input to a digital signal processor 21 via a respective input 19a-hfor further processing.

[0052] The digital signal processor 21 also has eight outputs 22a-h, each of which outputs a digital signal which represents the signal which is to be transmitted to a given MS. The output 22a-hselected represents the beam direction in which the signal is to be transmitted. The digital signal is converted to an analogue signal by a digital to analogue (D/A) converter 23. One digital to analogue converter 23 is provided for each output 22a-hof the digital signal processor 21. The analogue signal is then processed by processor 24 which is a modulator which modulates onto the carrier frequency the analogue signal to be transmitted. Prior to the processing of the signal by the processor 24, the signal is at the baseband frequency. The resulting signal is then amplified by an amplifier 26 and passed to the respective input 10a-hof the Butler matrix circuitry 8. A processor 24 and an amplifier 26 are provided for each output 22a-hof the digital signal processor 21.

[0053] Reference will now be made to FIG. 4 which schematically illustrates the digital signal processor 21. It should be appreciated that the various blocks illustrated in FIG. 4 do not necessarily correspond to separate elements of an actual digital signal processor 21 embodying the present invention. In particular, the various blocks illustrated in FIG. 4 correspond to various functions carried out by the digital signal processor 21. In one embodiment of the present invention, the digital signal processor 21 is at least partially implemented in integrated circuitry and several functions may be carried out by the same element.

[0054] Each signal received by the digital signal processor 21 on the respective inputs 19a-his input to a respective channel impulse response (CIR) estimator block 30. The CIR estimator block 30 includes memory capacity in which the received signal is stored and also memory capacity in which the estimated channel impulse response is stored. The channel impulse response estimator block 30 is arranged to estimate the channel impulse response of the channel of the respective input 19a-h.As already discussed an associated channel can be defined for the given data burst transmitted in the selected frequency band, the allocated time slot and the beam direction from which the signal is received for a single data burst. The beam direction from which a signal is received is ascertained by the Butler matrix circuitry 8 so that a signal received at input 19aof the digital signal processor represents mainly the signal that has been received from the direction of beam b₁and so on. It should be appreciated that the signal received at a given input may also include the side lobes of the signal received, for example, adjacent inputs.

[0055] Each data burst which is transmitted from a mobile station MS to the BTS 4 includes a training sequence TS. However, the training sequence TS_RXwhich is received by the BTS 4 is distorted due to noise and also due to multipath effects which leads to interference between adjacent bits of the training sequence. This latter interference is known as intersymbol interference. TS_EXis also affected by interference from other mobile stations, for example mobile stations located in other cells or cell sectors using the same frequency which may cause co-channel interference. As will be appreciated, a given signal from the MS may follow more than one path to reach the BTS and more than one version of the given signal may be detected by the antenna array 6 from a given direction. The training sequence TS_RXwhich is received from input 19ais cross correlated by the CIR estimator block 30 with a reference training sequence TS_REFstored in a data store 32. The reference training sequence TS_REFis the same as the training sequence which is initially transmitted by the mobile station. In practice the received training sequence TS_RXis a signal modulated onto a carrier frequency while the reference training sequence TS_REFis stored as a bit sequence in the data store 32. Accordingly, before cross-correlation is carried out, the stored reference training sequence is similarly modulated. In other words the distorted training sequence received by the BTS 4 is correlated with the undistorted version of the training sequence. In an alternative embodiment of the invention, the received training sequence is demodulated prior to its correlation with the reference training sequence. In this case, the reference training sequence would again have the same form as the received training sequence. In other words, the reference training sequence is not modulated.

[0056] The reference training sequence TS_REFand the received training sequence TS_RXeach are of length L corresponding to L bits of data and may for example be 26 bits. The exact location of the received training sequence TS_RXwithin the allocated time slot may be uncertain. This is because the distance of the mobile station MS from the BTS 4 will influence the position of the data burst sent by the MS within the allocated time slot. For example, if a mobile station MS is relatively far from the BTS 4, the training sequence may occur later in the allotted time slot as compared to the situation where the mobile station MS is close to the BTS 4.

[0057] To take into account the uncertainty of the position of the received training sequence TS_RXwithin the allotted time slot, the received training sequence TS_RXis correlated with the reference training sequence TS_REFn times. Typically, n may be 7 or 9. It is preferred that n be an odd number. The n correlations will typically be on either side of the maximum obtained correlation. The relative position of the received training sequence TS_RXwith respect to the reference training sequence TS_REFis shifted by one position between each successive correlation. Each position is equivalent to one bit in the training sequence and represents one delay segment. Each single correlation of the received training sequence TS_RXwith the reference training sequence TS_REFgives rise to a tap which is representative of the channel impulse response for that correlation. The n separate correlations gives rise to a tap sequence having n values.

[0058] Reference is now made to FIG. 5 which shows the channel impulse response for four of the eight possible channels corresponding to the eight special directions. In other words, FIG. 5 shows the channel impulse response for four channels corresponding to a given data burst received in four of the eight beam directions from the mobile station, the data burst being in a given frequency band and in a given time slot. The x axis of each of the graphs is a measure of time delay whilst the y axis is a measure of relative power. Each of the lines (or taps) marked on the graph represents the multipath signal received corresponding to a given correlation delay. Each graph will have n lines or taps, with one tap corresponding to each correlation.

[0059] From the estimated channel impulse response, it is possible to determine the location of the training sequence within the allotted time slot. The largest tap values will be obtained when the best correlation between the received training sequence TS_RXand the reference training sequence TS_REFis achieved.

[0060] The CIR estimator block 30 also determines for each channel the five (or any other suitable number) consecutive taps which give the maximum energy. The maximum energy for a given channel is calculated as follows: $E=\sum _{j=1}^5{\left(h_j\right)}^2$

[0061] where h represents the tap amplitude resulting from a cross correlation of the reference training sequence TS_REFwith the received training sequence TS_RX. The CIR estimator block 30 estimates the maximum energy for a given channel by using a sliding window technique. In other words, the CIR estimator block 30 considers each set of five adjacent values and calculates the energy from those five values. The five adjacent values giving the maximum energy are selected as representative of the impulse response of that channel.

[0062] The energy can be regarded as being a measure of the relative strength of the desired signal from a given MS received by the BTS 4 from a given direction. This process is carried out for each of the eight channels which represent the eight different directions from which the same data burst could be received. The signal which is received with the maximum energy has followed a path which provides the minimum attenuation of that signal.

[0063] An analysis block 34 is provided which stores the maximum energy calculated by the CIR estimator block 30 for the respective channel for the five adjacent values selected by the CIR estimator block as being representative of the channel impulse response. The analysis block 34 may also analyse the channel impulse responses determined by the CIR estimator block 30 to ascertain the minimum delay. The delay is a measure of the position of the received training sequence TS_RXin the allotted time slot and hence is a relative measure of the distance travelled by a signal between the mobile station and the BTS 4. The channel with the minimum delay has the signal which has travelled the shortest distance. This shortest distance may in certain cases represent the line of sight path between the mobile station MS and the BTS 4.

[0064] The analysis block 34 is arranged to determine the position of the beginning of the window determining the five values providing the maximum energy. The time delay is then determined based on the time between a reference point and the beginning of the window. That reference point may be the common time when all received training sequence in each branch start to be correlated, the time corresponding to the earliest window edge of all the branches or a similar common point. In order to accurately compare the various delays of the different channels, a common timing scale is adopted which relies on the synchronisation signal provided by the BTS 4 in order to control the TDMA mode of operation. In other words, the position of the received training sequence TS_RXin the allotted time slot is a measure of the time delay. It should be appreciated that in known GSM systems, the delay for a given channel is calculated in order to provide timing advance information. Timing advance information is used to ensure that a signal transmitted by the mobile station to the BTS falls within its allotted time slot. The timing advance information can be determined based on the calculated relative delay and the current timing advance information. If the mobile station MS is far from the base station, then the MS will be instructed by the BTS to send its data burst earlier than if the mobile station MS is close to the BTS.

[0065] The results of the analysis carried out by each analysis block 34 is input to a beam selection control block 36 which determines the direction from which a given data burst is deemed to be received. This deemed direction may, for example be the beam direction from which the signal with the greatest strength is received or alternatively may be the direction from which the data burst is first received. That beam direction is then stored by beam selection block 36 and the same steps are carried out in respect of the next burst which is received from the mobile station. If the two determined beam directions are the same in the two consecutive data bursts, then the signal from the BTS to the MS is transmitted in that determined beam direction. In one preferred embodiment, the base transceiver station is controlled so as to in normal circumstance only transmit the signal to the MS in a single beam direction. However, if the beam directions from which the first and second consecutive bursts are received are not the same, then the next signal transmitted by the BTS to the MS will be in the direction from which the second signal is received as well as the direction from which the first signal is received.

[0066] If the beam direction from which the first signal is received by the BTS is not adjacent to the beam direction from which the second data burst is received, then the signal transmitted by the BTS is also transmitted in the intervening beam directions. For example, if the first burst is determined to come from the direction of beam b₁and the second data burst is deemed to come from the direction of beam b₃, then the signal transmitted by the base station to the mobile station will be in the directions of beams b₁, b₂and b₃.

[0067] Reference will now be made to FIGS. 6ato hand 7ato hwhich illustrate the principle used by embodiments of the present invention. FIGS. 6ato hillustrate the beam direction from which eight consecutive data bursts transmitted by a mobile station to the base station are received.

[0068] The beam directions illustrated in FIG. 6 can alternatively be regarded as the beam directions which would be selected for consecutive data bursts transmitted by the BTS to the MS if only a single beam direction were to be selected for each data burst. The first and second data bursts from the mobile station MS are deemed to come from the direction of beam b₃. The third and fourth data bursts are deemed to be received from the direction of beam b₄. The fifth and sixth bursts are deemed to be received from the direction of beam b₃whilst the seventh burst is deemed to be received from the direction of beam b₅. Finally, the eighth data burst from the mobile station is determined to come from the direction of beam b₃.

[0069] The beam directions in which eight consecutive bursts are transmitted by the BTS to MS are shown in FIGS. 7ato h.As the first and second bursts from the mobile station are both deemed to be received from the direction of beam b₃, the first and second bursts transmitted by the BTS 4 to the mobile station are both in the direction of beam b₃. However, since the second and third bursts from the mobile station are deemed to be received respectively from the directions of beam b₃and beam b₄, the third burst transmitted by the BTS to the MS is in the direction of both names b₃and b₄. The third and fourth bursts received from the mobile station are deemed to be received from the same beam direction, that is beam b₄. Accordingly, the fourth burst transmitted by the BTS 4 is transmitted in the direction of beam b₄only. The fourth burst received by the BTS is received from the direction of beam b₄whilst the fifth burst received by the BTS 4 is received from the direction of beam b₃. Thus, the first burst transmitted by the BTS is in the direction of both beams b₃and b₄. The fifth and sixth bursts received by the BTS 4 are both deemed to be received from the direction of beam b₃. Accordingly, the sixth burst transmitted by the BTS is transmitted in the direction of beam b₃. The sixth burst received by the BTS is deemed to be received from the direction of beam b₃whilst the seventh burst is deemed to be received from the direction of the beam b₅. Accordingly, the seventh burst is transmitted to the MS in the direction of beams b₃and b₅as well as the intervening beam, beam b₄. Finally, as the seventh and eighth bursts received by the BTS 4 are deemed to be received from beam directions b₅and b₃respectively, the eight burst transmitted by the BTS is transmitted in the direction of beams b₃, b₄and b₅.

[0070] In one modification to the embodiment described above, where two consecutive data bursts are received from the mobile station from different directions, the subsequent N bursts transmitted by the BTS to the MS will be in those two different directions and any intervening directions. N can be any suitable number of bursts, for example 3 or 4.

[0071] It is found that the above described method is most suitable for those cases where the distance between the BTS and the MS is greater than a given distance. This distance will depend largely on the local environment but may for example be greater than around 0.5 kilometers. If the mobile station is closer to the BTS than this 0.5 kilometer distance, then the signal which is sent to the MS is transmitted over a wide angular spread. This may involve the selection of a number of the possible beam directions, for example 4 or more directions. Typically, the power level used for each beam will be lower if a relatively large number of beams are selected.

[0072] One way in which the distance between the MS and the BTS is determined is from the timing advance information outlined above which provides a measurement of the distance between the BTS 4 and the MS. This first method may be preferred in some embodiments of the invention as it gives reasonably accurate results. Alternatively, the angular spread of the received signal at the BTS can be used as a measurement of the distance between the MS and the BTS. When the distance between the BTS and the MS is greater than the critical distance, the bulk of the energy received from the MS is distributed among a relatively few beam directions. In particular, the energy will be mainly concentrated in one or two beams, or possibly three beam directions. However, when the distance between the mobile station and the BTS is less than the critical distance, the received desired energy will appear to be distributed among a much greater number of beams. To determine the angular spread, the beam selection control block 36 compares the channel impulse response obtained for each of the possible beam directions for a given signal. If most of the received energy is distributed in three or less beam directions, then it is assumed that the distance between the BTS and MS is greater than the critical distance. Alternatively, if most of the received energy is received from four or more beam directions, then it is assumed that the distance between the MS and the BTS is less than the critical distance.

[0073] Accordingly, in embodiments of the present invention, the use of two preceding signals received from the MS to determine the or each direction in which a signal is to be transmitted to the MS may only be used in those situations where the distance between the MS and the BTS 4 is greater than the critical distance. When the distance between the MS and the BTS is less than the critical distance, the BTS 4 will transmit signals to the MS over a relatively large number of beam directions, for example 4 or more. The power level used when transmitting over a relatively wide angular spread will generally be lower than the power used for the or each beam when the distance between the MS and the BTS 4 is greater than the critical distance. The required power level is selected by the beam selection block 36.

[0074] Generating block 38 is responsible for generating the signals which are to be output from the digital signal processor 21. The generating block 38 has an input 40 representative of the speech and/or information to be transmitted to the mobile station MS. Generating block 38 is responsible for encoding the speech or information to be sent to the mobile station MS and includes a training sequence and a synchronizing sequence within the signals. Block 38 is also responsible for production of the modulating signals. Based on the generated signal and determined beam direction input from the beam selection block 36, generating block 38 provides signals on the respective outputs 22a-hof the digital signal processor 21. The generating block 38 also provides an output 50 which is used to control the amplification provided by amplifiers 24 to ensure that the signals in the beam directions have the required power levels. As mentioned above, the power level for a given beam is determined by the beam selection block 36 based on whether or not the distance between the MS and the BTS is greater or less than the critical distance.

[0075] The output of the channel impulse response block 30 is also used to equalise and match the signals received from the mobile station MS. In particular, the effects of intersymbol interference resulting from multipath propagation can be removed or alleviated from the received signal by the matched filter and equalizer block 42. It should be appreciated that the matched filter (MF) and equalizer block has an input (not shown) to receive the received signal from the MS. The output of each block 42 is received by recovery block 44 which is responsible for recovering the speech and/or the information sent by the MS. The steps carried out by the recovery block include demodulating and decoding the signal. The recovered speech or information is output on output 48.

[0076] It should be appreciated that whilst the above described embodiment has been implemented in a GSM cellular communication network, it is possible that the present invention can be used with other digital cellular communication networks as well as analogue cellular networks. The above described embodiment uses a phased array having eight elements. It is of course possible for the array to have any number of elements. Alternatively, the phased array could be replaced by discrete directional antennae each of which radiates a beam in a given direction. The Butler matrix circuitry can be replaced by any other suitable phase shifting circuitry, where such circuitry is required. The Butler matrix circuitry is an analogue beam former. It is of course possible to use a digital beam former DBF or any other suitable type of analogue beam former. The array may be controlled to produce more than eight beams, even if only eight elements are provided, depending on the signals supplied to those elements.

[0077] It is also possible for a plurality of phased arrays to be provided. The phased arrays may provide a different number of beams. When a wide angular spread is required, the array having the lower number of elements is used and when a relatively narrow beam is required, the array having the larger number of elements is used.

[0078] As will be appreciated, the above embodiment has been described as providing eight outputs from the Butler matrix circuitry. It should be appreciated that in practice a number of different channels will be output on each output of the Butler matrix at the same time. Those channels may be different frequency bands. The channels for different time slots will also be provided on the respective outputs. Whilst individual amplifiers, processors, analogue to digital converters and digital to analogue converters have been shown, these in practice may be each provided by a single element which has a plurality of inputs and outputs.

[0079] The determination of the direction from which the two sequential signals are received need not be carried out in relation to two consecutive signals. For example, the direction from which a data burst is deemed to be received may be determined every N bursts where N is a given integer. For example, the BTS may determine the direction from which a first burst is received from the MS and the direction from which a third burst is received. If these two directions are not the same, then the signal transmitted by the BTS will be in both these directions.

[0080] Likewise, the directions used for transmitting a signal by the first station may not be in response to immediately preceding communication data bursts received from the MS. In some embodiments, the different beam directions selected for transmission by the BTS may be based on the sequential communication data bursts received one or more data bursts previously from the MS.

[0081] Embodiments of the present invention may have applications other than just in cellular communication networks. For example, embodiments of the present invention may be used in any environment which requires directional radio communication. For example, this technique may be used in PRN (Private Radio Networks) or the like.