GB2427789A

GB2427789A - Repeater comprising directional antenna arrays to reduce received interference

Info

Publication number: GB2427789A
Application number: GB0512956A
Authority: GB
Inventors: Yong Sun
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2005-06-24
Filing date: 2005-06-24
Publication date: 2007-01-03
Anticipated expiration: 2025-06-24
Also published as: GB0512956D0; GB2427789B

Abstract

A repeater apparatus 44 for use in a communications network comprises a receiver 54 for receiving signals 58 from a first communications device, such as a base station (BS), and a transmitter 56 for re-transmitting the signal 60 to a second communications device, such as a mobile station (MS), wherein the transmitter 56 and receiver 54 include directional antenna arrays. The repeater 44 may also add a compensation signal to the received signals 58 to remove a frequency component caused by direct interference from the transmitter 56. A filter may be used to estimate multipath interference at the receiver 54 due to transmitted signals 60 from the transmitter 56. The filter may adaptively set a plurality of weights and delays operable over the frequency band of the received signal 58 so as to estimate the multipath interference.

Description

Repeater Apparatus This invention relates to a communications device and a

method of operating a communications device and in particular relates to a repeater apparatus having a receiver and transmitter used to relay messages in a communications system.

In a communications system simplex operation refers to point to point communication that takes place on a single frequency. Users in a simplex environment may transmit and receive but never do both at the same time. Furthermore, only one person in a group can successfully transmit at one time.

A duplex channel in contrast refers to "two way" traffic flow on the network. A half- duplex channel is one that can carry information in both directions, but not at the same time. A full-duplex channel is a channel which can carry information in both directions at once.

Conventional cellular networks that use a hub-and-spoke model require that all the wireless devices that are part of the network must be within broadcast range of a central access point ("base station") in order to communicate. Such a network has limited network coverage and also limited edge quality of service.

An alternative network model is a "mesh" network in which network traffic is routed from the base station through relay stations to network users. In a mesh network different parts of a message can follow different paths through the network. Networks based on the mesh architecture have a number of advantages over the hub-and-spoke model such as the ability to provide redundant paths that can route around localised network failures.

The relay station (or alternatively the repeater apparatus) in such a network has the purpose of relaying communications traffic from one communications device to other devices/stations in the network. Since a repeater is designed to both transmit and receive there can potentially be coupling interference between the transmitter and receiver. In order to avoid or mitigate against interference problems there are two general modes of operation for conventional repeaters.

Frequency division duplex (FDD) is the application of frequency-division multiple access to separate outward and return signals. FDD repeater stations therefore can transmit on one frequency and receive on another frequency simultaneously. Such a repeater has the advantage that by changing the frequency the coupling signal interference problem is largely avoided.

Furthermore, the relaying delay, i.e. the time delay between the repeater receiving a signal on frequencyfi and re-transmitting the same signal on frequencyf2, will be relatively small. However, a repeater operating in FDD mode will require that each relay link operative in the network uses more than one frequency. Consequently, such repeaters have a low frequency efficiency.

* Time division duplex (TDD) is the application of time-division multiple access to separate outward and return signals. A TDD repeater station will generally use a single frequency for both transmission and reception. Use of a single frequency gives rise to an unavoidable coupling interference problem between the receiver and transmitter on the repeater and consequently the repeater does not relay the transmitted signal simultaneously with the received signal. A drawback therefore of a relay station using a TDD mode of operation is the large relaying delay compared with FDD systems.

It is therefore an object of the present invention to provide a repeater apparatus that overcomes some of the above mentioned problems with the prior art and which is less susceptible to coupling interference between the transmitter and receiver.

According to a first aspect of the invention there is provided a repeater apparatus for use in a communications network comprising: a receiver for receiving a signal from a first communications device, and; a transmitter for re-transmitting the signal from the first communications device to a second communications device wherein the receiver and transmitter comprise directional antenna arrays The first aspect of the present invention proposes the use of antenna arrays that are capable of sending and receiving directionally directed signals. This will reduce the likelihood of the transmitted signals reflecting off of surrounding structures and will consequently reduce the amount of interference the repeater station experiences. It is noted that the "first communications device" will typically, but not necessarily, be a base station and the "second communications device" will typically, but not necessarily, be a wireless device (such as a mobile telephone or PC).

Conventional communications networks utilise omni-directional antenna arrays. To avoid interference between neighbouring relay stations conventional networks require that there is no overlap between transmission/reception frequencies. For example, in the case of a network comprising two relay stations which relay signals from a base station to one network user each there will be four separate frequencies in operation - base station to relay station 1 onfj; relay station to user I onf2; base station to relay station 2 onf3; relay station to user 2 onf4.

In the present invention however there will be little or no interference between the two relay stations and network users because the signals will be directionally targeted. This therefore allows the operating frequencies to be re-used, for example using the repeater of the first aspect of the present invention allows the base station to transmit to relay station I onfj; relay station to user 1 onf2; base station to relay station 2 onfj; relay station to user 2 onf2.

Therefore, in addition to reducing multipath interference, a repeater station according to the first aspect of the present invention also increases the efficiency of a network by reducing the number of different frequencies required for substantially interference free operation.

For a repeater apparatus that relays a signal simultaneously (or virtually simultaneously) there will be coupling interference between the receiver and the transmitter if the same frequency is used for transmission and reception. Specifically, the receiver will experience direct interference in which it receives a proportion of the re- transmitted signal from the transmitter itself. The receiver will also experience multipath interference in which the signal from the transmitter is reflected off of surrounding structures (e.g. buildings) and is received as interfering signals at the receiver.

Although the direct and multipath (indirect) interference may be relatively weak compared to the re-transmitted signal it will still be significant when compared to the weak signal that is generally received from the base station. There therefore exists a need to compensate for both the direct and indirect interference at the receiver.

Conveniently therefore the repeater apparatus further comprises means to add a compensation signal to the signal received at the receiver in order to remove frequency components that are caused due to direct interference from the transmitter. For example, the signal transmitted by the transmitter may be subtracted by an adder component from the received signal. More preferably the magnitude and phase of the compensation signal are varied to more closely mimic the component of the transmitted signal received at the receiver (and the magnitude/phase compensated signal is then subtracted from the received signal).

The above compensation signal will remove or reduce the direct coupling interference between the transmitter and receiver. However, the received signal will still comprise frequency components arising from multipath interference.

Therefore, preferably the repeater apparatus further comprises filter means to model the multipath interference signal. The multipath signal that is derivedlestimated by the filter can then be subtracted from the received signal.

The filter means to reduce multipath interference may also be incorporated into existing repeater architectures (for example US6807399). Therefore in a second aspect of the present invention there is provided a repeater apparatus for use in a communications network comprising a receiver for receiving a signal from a first communications device, and; a transmitter for re-transmitting the signal from the first communications device to a second communications device wherein the repeater apparatus further comprises filter means to estimate multipath interference arriving at the receiver as a result of the transmission signal from the transmitter The first and second aspects of the present invention may conveniently comprise means to remove, from the received signal, the frequency components caused by multipath interference. Therefore preferably, in addition to the filter means, the invention further comprises means to add a compensation signal to the signal received at the receiver in order to remove frequency components caused due to multipath interference from the transmitter.

A repeater apparatus comprising a filter means in accordance with either the first or second aspects of the present invention provides for a relay station that overcomes or mitigates many of the problems with prior art repeater stations. In particular it is noted that the filter means allows the station to receive and transmit simultaneously using only a single frequency.

Although a communications network transmits at defined frequencies in reality a signal will be transmitted within a frequency band (i.e. transmission will be centred on a frequencyf but will actually lay within a bandf ôj). Since multipath interference is a wideband effect different phase, amplitude and delay effects will occur across the frequency band. Conveniently therefore the filter means comprises a number of weighting components to compensate for phase and amplitude variations in the received signal and also a number of delay elements to compensate for arrival time variations in the multipath components of the received signal across the frequency band.

A filter means as described above will need to be trained to estimate the multipath interference effects and so preferably the repeater will initially transmit a training message from the transmitter to the receiver in order to derive suitable values for the weighting and delay means within the filter.

The weighting and delay components within the filter can be fixed during the reception/transmission of a data packet or alternatively can be adaptively updated by the inclusion of training/pilot signals within the message signal.

As noted above interference signals can be large relative to the signal received from the base station. Preferably therefore the repeater apparatus further comprises amplitude limiting means arranged to avoid saturation of the electronic components within the repeater caused by interference signals.

According to a third aspect of the present invention there is provided an operating program which, when loaded into a communications device, causes the device to become one according to the first or second aspects of the present invention.

The above-described operating program to implement the above-described OFDM transmitters and methods may be provided on a data carrier such as a disk, CD- or DVD-ROM, programmed memory such as read-only memory (Firmware), or on a data carrier such as optical or electrical signal carrier. For many applications embodiments of the above-described transmitters, and transmitters configured to function according to the above-described methods will be implemented on a DSP (Digital Signal Processor), ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). Thus code (and data) to implement embodiments of the invention may comprise conventional program code, or microcode or, for example, code for setting up or controlling an ASIC or FPGA. Similarly the code may comprise code for a hardware description language such as Verilog (Trade Mark) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.

According to a fourth aspect of the present invention there is provided a communications network comprising a repeater apparatus according to the first or second aspects of the present invention.

According to a fifth aspect of the present invention there is provided a method of relaying a signal comprising the steps of: i) Receiving a signal from a base station ii) Re-transmitting the received signal from an antenna array to a further communications device wherein the base station transmits a directional signal and the antenna array is a directional antenna array.

According a sixth aspect of the present invention there is provided a method of relaying a signal comprising the steps of: i) Receiving signal from a base station ii) Re-transmitting the received signal from an antenna array to a further communications device wherein an estimate of multipath interference arriving at receiver from the re- transmitted signal is generated and used to compensate the received signal.

The present invention will now be described with reference to the following non- limiting preferred embodiments in which: Figure 1 is a schematic diagram illustrating a typical relay link Figure 2 is a schematic diagram illustrating the two general modes of operation of a repeater station Figure 3 illustrates a first known architecture for reducing coupling interference Figure 4 illustrates a second known architecture for reducing coupling interference Figure 5 is a schematic diagram illustrating a multipath environment Figure 6 is a schematic diagram illustrating the present invention Figure 7 illustrates a repeater architecture according to the present invention Figure 8 illustrates a training structure for the architecture depicted in Figure 7 Figure 1 depicts a typical communications scenario in which a base station (BS) 1 is intending to transmit to a mobile station (MS) 3 that is outside of its effective transmission area. A relay link is formed between the base station and mobile station by means of the repeater or relay station 5. The repeater comprises a receiver antenna array R (labelled 7 in Figure 1) and a transmit antenna array T (labelled 9).

In use, the base station 1 transmits along link "a" to the receive antenna array 7 of the repeater 5. The message is relayed either simultaneously in the case of frequency division duplex repeaters or with a delay in the case of time division duplex repeaters and the repeater re- transmits the message along link "b" to the mobile station 3.

For repeater systems that re-transmit on the same frequency as the base station interference, labelled "c" in Figure 1, can arise between the transmit and receive antennas on the repeater.

It is noted that conventionally repeater stations are located in high positions in order to reduce unwanted interference from other structures. Such repeaters are of limited use in modem mobile services and in general are more suitable to fixed relay systems instead of wireless access systems.

Figure 2 is a schematic diagram comparing the operation of FDD and TDD repeaters.

The Figure shows a repeater 11 with a receiver R and transmitter T. For an FDD receiver it can be seen that the message is received on a first frequencyfi and re- transmitted on a second frequencyf2. For a TDD repeater the message is received at a first time ti and re-transmitted at a second time t2.

The operation of the repeater over time for both modes is also shown. For FDD it can be seen that the receive and transmit operations (13 and 15 respectively) occur simultaneously whereas for TDD the receive and transmit operations (17 and 19 respectively) occur at different times.

Each repeater mode has drawbacks. In the case of FDD two frequencies must be used for a relay link. In the case of TDD there will relaying delays since the receive and transmit antennas do not operate simultaneously.

Two common repeater architectures that are designed to reduce coupling interference between the receiver and transmitter on a repeater are discussed in US 6807399. In both architectures it is assumed that transmission and reception occur simultaneously (or with a small relaying delay) and that a single frequency is used across the relay link.

The first architecture is shown in Figure 3 which shows the receive portion 20 of a repeater station comprising a receiver antenna 22 for receiving a signal transmitted from a base station (not shown), a band pass filter 24, a low noise amplifier 26 and a local oscillator (LO), 28. The output of the local oscillator 28 is mixed with the output of the amplifier 26 by means of a mixer 30. The architecture further comprises an adder 32 which is used to add a compensation signal to the signal received by the receiver 22 in order to remove interference from the repeater's transmitter (not shown).

In use, the antenna 22 receives the signal transmitted by the base station, S_Rx. The received signal also comprises a component of the retransmitted signal that is transmitted from the transmitter of the repeater station itself, S_Tx, i.e. the total signal detected by the receiver is S_Rx + S_Tx. This received signal is filtered by the band pass filter 24 and input to the adder 32.

The adder 32 is designed to remove the signal component relating to the re-transmitted signal, i.e. -S_Tx.

The output of the adder is input into the low noise amplifier 26 and the output of the amplifier 26 is mixed by the mixer 30 with a local signal from the local oscillator 28 to obtain a downwardly converted intermediate frequency, IFRx.

The architecture depicted in Figure 3 helps to reduce coupling interference. However, there are drawbacks to the architecture such as the phase difference caused by the physical separation of the transmitter and receiver, propagation losses and receiver gain.

As a consequence a base noise is generated within the architecture from the outputs of the low noise amplifier and the frequency mixer.

Figure 4 represents a variation of the architecture of Figure 3 which is designed to overcome some of the above disadvantages. Like numerals have been used to denote like features between Figures 3 and 4.

The architecture of Figure 4 additionally comprises a phase converter 34 and a variable amplifier 36. In use, the converter 34 and amplifier 36 serve to adjust the phase and magnitude of the compensation signal -S_Tx that is input into the adder 32 such that it more closely matches the frequency component S_Tx generated in the receiver.

Although the architecture of Figure 4 achieves higher performance than the architecture of Figure 3 it is noted that the both architectures have been designed only to counteract the interference directly arising from the transmitter antenna array within the repeater.

In a typical operational scenario however interference will arise from other sources such as reflections from buildings, i.e. there will be multipath interference in addition to direct interference from the transmitter.

Figure 5 shows a similar communications scenario to Figure 1 in which a base station 1 communicates with a mobile station 3 via a repeater station 5. Like numerals have been used to denote like features between this Figure and Figure 1.

In contrast to Figure 1 however Figure 5 additionally depicts the presence of two buildings 40 and 42. These two buildings represent two sources of interference in which signals from the transmitter are detected at the receiver of the repeater. Path "d" represents that portion of the transmitted signal that is reflected off of building 40 to the receiver and path "e" represents the transmitted signal that is reflected from building 42 to the receiver.

As noted above the repeater architectures of Figures 3 and 4 are designed only to be counteract direct interference between the transmitter and receiver on a repeater station (i.e. the path "c" interference in Figure 1 and 5). As a consequence such repeaters are generally located in high locations to avoid further interference that might arise from surrounding buildings or other structures.

Although multipath interference from surrounding obj ects will be much smaller than the direct interference from the transmitter itself it will still be significant when compared to the signal received from the base station. It is therefore noted that locating a repeater according to Figures 3 or 4 in the environment depicted in Figure 5 would adversely affect the operational efficiency of such a relay station.

A repeater station in accordance with the first aspect of the present invention is shown in Figure 6. This Figure shows a repeater 44 that is surrounded by four objects (46, 48, 50, 52) that are all potential sources of interference between the receiver 54 and transmitter 56 on the repeater. The repeater receives a signal 58 from a base station (not shown) and re-transmits 60 it to a mobile station (also not shown).

In accordance with the present invention the receiver 54 and transmitter 56 comprise anteima arrays that are capable of directional antenna processing, i.e. the receiver receives a directionally transmitted signal from the base station and the transmitter re- transmits the signal as a directional signal to the mobile station.

The use of directional antennas within the repeater greatly reduces the interference that the receiver 54 experiences from the surrounding objects (46, 48, 50, 52).

It is clear from the Figure that a prior art receiver could potentially detect interfering signals from any of the objects (46, 48, 50, 52). For the sake of clarity only five possible interference paths are depicted in Figure 6.

Path I represents interference that is directly transmitted from transmitter to receiver.

Path 2 represents an interfering signal that has travelled to the receiver via objects 48, 46 and 52. Path 13 represents an interfering signal that has travelled to the receiver via objects 50 and 52. Path 14 represents an interfering signal that has travelled to the receiver via object 48 and path 15 represents an interfering signal that has travelled to the receiver via object 50.

As noted above, the present invention utilises directional antennas. It can be seen that this naturally reduces the effects of multipath interference, for example paths 14 and 15 in Figure 6 are blocked by the configuration of the antennas. The use of directional antennas also improves power transmission efficiency.

Therefore, it is noted that a repeater station incorporating the features of the first aspect of the present invention provides a relay station with greater power efficiency than prior art systems and a lower susceptibility to multipath interference.

The interference effects of surrounding buildings can be further reduced by the second aspect of the present invention. Figure 7 illustrates a repeater architecture that incorporates both the first and second aspects of the present invention. It is noted that the first aspect of the invention may be used in combination with the second aspect or alternatively the second aspect may be incorporated into prior art repeaters in order to improve their efficiency in a multipath environment.

It is further noted that like features between Figures 3 and 4 and Figure 7 are denoted by like numerals. Specifically the repeater of Figure 7 has the following components that are also present in Figure 4 - a band pass filter 24, a low noise amplifier 26, a local oscillator (LO) 28, mixer 30, adder 32, phase converter 34 and variable amplifier 36.

The antenna 22' in Figure 7 is a directional antenna as opposed to the onmi-directional antenna in Figure 4 (and other prior art repeater systems).

The repeater architecture of Figure 7 additionally comprises an amplitude limiter 62 which is located between the adder 32 and the low noise amplifier 26. This component limits the input signal to the amplifier 26 such that it is not saturated by interference signals that the directional antenna 22' is not able to block.

The architecture of Figure 7 also includes a filter 64 which is designed to reduce and compensate for the effects of multipath interference detected at the receiver (such as the interference from paths 2 and 13 in Figure 6). This filter 64 represents the second aspect of the present invention and is described in more detail below.

As well as filter 64, the architecture of Figure 7 also comprises a switching element 66 which is located after the adder 30. The switching element 66 is connected to the filter 64 and a second adder 68. The second adder 68 in turn is connected to a second low noise amplifier 70.

A second mixer 72 receives inputs from the local oscillator 28 and the signal component relating to the re-transmitted signal, -S_Tx. The output of this further mixer 72 is fed into the filter 64.

The operation of the architecture of Figure 7 and also the second aspect of the present invention will now be described.

The multipath effects described above are all wideband effects which will affect the phase, amplitudes and delays of signals reaching the receiver of the repeater. Although the intermediate frequency output by the adder 30 will be centred on a single frequency (e.g. 140 MHz) the signal will, in reality, have a bandwidth (e.g. 20MHz) around this central value. Therefore, different parts of this intermediate frequency band will experience different mulitpath effects.

The filter 64 is designed to compensate for these interference effects in the received signal and comprises a number of weighting elements 74 and delay elements 76 (see insert to Figure 7) which are designed to approximate the multipath interference that the signal transmitted from the transmitter will experience. The filter therefore will output an approximation of the multipath component that is being received at the receiver. This approximation can then be subtracted from the received signal (in much the same way as the direct interference component is removed at the adder 32) in order to recover the actual signal that was transmitted from the base station.

In use, the switching element 66 directs the output of the adder 30 directly to the second adder 68 (connection paths F and G in Figure 7).

The signal component relating to the re-transmitted signal, -S_Tx., is fed into the phase converter 34 as described in relation to Figure 4. This component is adjusted in phase and amplitude by the variable amplifier 36 and the interference arising directly from the receiver is subtracted from the received signal at adder 32. This part of the architecture of Figure 7 is identical to that of Figure 4.

However, additionally the signal component relating to the re-transmitted signal, - S_Tx., is also passed to the second mixer 72 (path Nm Figure 7) where the output of the local oscillator is used to down-convert it to an intermediate frequency or baseband signal. This IF/BB signal is passed by the adder 70 to the filter 64 (path 1). The output of the filter is an approximation of the multipath interference signal that arrives at the receiver from the transmitter and this is fed (path J) to the further adder 68 which acts to subtract the interference signal from the IF output of the adder 30.

It can therefore be seen that a repeater station according to the present invention compensates for both direct and indirect interference between the receiver and transmitter and that therefore simultaneous (or near simultaneous) relaying of a signal may be achieved wherein the same frequency is used on both the receive and transmit sides of the relay station.

Before the filter can compensate for multipath effects it will, however, require training.

During training a training message is transmitted from the repeater's transmitter and subsequently detected by the receiver 22'. The received signal passes through the architecture as described in relation to Figure4. The intermediate frequency output by the adder 30 is then passed to the filter 64 (via path H as opposed to path G in normal use) by means of the switching element 66. The filter 64 is then trained (by the introduction of appropriate weighting of the elements 74 and appropriate delays to the elements 76) such that the training signal can be retrieved. The architecture is now ready for normal use.

Figure 8 depicts a proposed training/data transmission structure for the present invention. In Figure 8 a signal is received from the base station by the receiver (Rx) during the frame period, tj to t,. During this period the signal is re-transmitted from the transmitter (Tx) to another repeater or mobile station.

Prior to time tj, i.e. from t0 to tj, there is a training period during which the filter is trained and the weighting coefficients of the filter are set. During this training period the transmitter transmits a known training message which is detected by the repeater's receiver array and used to train the IF/BB filter. The weights/delay coefficients of the filter can either be calculated by zero forcing or alternatively by adaptive filtering.

At the end of the training period the filter weights/coefficients may be fixed for the duration of the following frame period. Alternatively, the insertion of one or more pilot symbols into the signal (e.g. in-band tone insertion) would allow the filter to be adaptively updated during operation.

Claims

CLAIMS: 1. A repeater apparatus for use in a communications network

comprising a receiver for receiving a signal from a first communications device, and; a transmitter for re-transmitting the signal from the first communications device to a second communications device wherein the receiver and transmitter comprise directional antenna arrays
2. A repeater apparatus as claimed in claim 1 further comprising means to add a compensation signal to the signal received at the receiver in order to remove a frequency component caused due to direct interference from the transmitter.
3. A repeater apparatus as claimed in any preceding claim further comprising filter means to estimate multipath interference arriving at the receiver as a result of the transmission signal from the transmitter.
4. A repeater apparatus for use in a communications network comprising a receiver for receiving a signal from a first communications device, and; a transmitter for re-transmitting the signal from the first communications device to a second communications device wherein the repeater apparatus further comprises filter means to estimate multipath interference arriving at the receiver as a result of the transmission signal from the transmitter
5. A repeater apparatus as claimed in claim 4 further comprising means to add a compensation signal to the signal received at the receiver in order to remove a frequency component caused due to direct interference from the transmitter
6. A repeater apparatus as claimed in any of claims 3 to 5 wherein the apparatus comprises means to add a compensation signal to the signal received at the receiver in order to remove frequency components caused due to multipath interference from the transmitter.
7. A repeater apparatus as claimed in any of claims 3 to 6 wherein the filter means comprises a plurality of weighting and delay means operable over the frequency band of the received signal, the weighting and delay means being set so as to estimate multipath interference from the transmitter.
8. A repeater apparatus as claimed in any of claims 3 to 7 wherein the repeater initially transmits a training message from the transmitter to the receiver in order to derive suitable values for the weighting and delay means.
9. A repeater apparatus as claimed in either of claims 7 or 8 wherein the weighting and delay means are adaptively updated.
10. A repeater apparatus as claimed in any of claims 3 to 9 wherein transmission and reception of the signal is on the same frequency.
ii. A repeater apparatus as claimed in any preceding claim further comprising amplitude limiting means arranged to avoid saturation of electronic components within the apparatus caused by interference signals.
12. An operating program which, when loaded into a communications device, causes the device to become one as claimed in claims 1 to 11.
13. An operating program as claimed in claim 12 carried on a carrier medium.
14. An operating program as claimed in claim 13, wherein the carrier medium is a transmission medium.
15. An operating program as claimed in claim 13, wherein the carrier medium is a storage medium
16. A communications network comprising a repeater apparatus according to any of claims ito 11.
17. A method of relaying a signal comprising the steps of: i) Receiving a signal from a base station ii) Re-transmitting the received signal from an antenna array to a further communications device wherein the base station transmits a directional signal and the antenna array is a directional antenna array.
18. A method of relaying a signal comprising the steps of: i) Receiving signal from a base station ii) Re-transmitting the received signal from an antenna array to a further communications device wherein an estimate of multipath interference arriving at receiver from the re- transmitted signal is generated and used to compensate the received signal.