GB2428538A

GB2428538A - Directional distributed relay system with efficient use of frequency resource

Info

Publication number: GB2428538A
Application number: GB0512955A
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: 2007-01-31
Anticipated expiration: 2025-06-24
Also published as: GB0512955D0; GB2428538B

Abstract

A communications network having n operating frequencies, e.g. f1, f2, comprises a base station (BS) 1 and a plurality of relay stations (RS1, RS2) 3, 7. Each relay station 3, 7 is capable of receiving signals transmitted at a first frequency and re-transmitting the received signals at a second frequency. The base station 1 and relay stations 3, 7 comprise directional antennas so that the network is arranged to support n communication devices, e.g. MS1 5, MS2 9, in direct or indirect communication with the base station 1. The directional antennas may comprise beam-forming or adaptive antenna arrays wherein the base station 1 is capable of beam-forming up to n different transmission signals simultaneously and each relay station 3, 7 is capable of simultaneously receiving and transmitting a plurality of beam-formed signals. The base station 1 may also be able to monitor and control the power of signals transmitted from the relay stations 3, 7.

Description

M&C Folio: GBP92192 Document: 1080915 Directional Distributed Relay System

This invention relates to a communications system and method and in particular a communications system that utilises a mesh architecture.

Conventional cellular networks use a hub-and-spoke model in which 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 the 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 are more flexible than the hub-and-spoke model.

Mesh networks use relay transmitters to route messages. In order to avoid interference a relay terminal cannot relay information by having its transmitter use the same frequency band as its receiver at the same time. Consequently a relay station can operate in one of two modes. In a frequency division multiple access (FDMA) relay station different frequencies are used on the reception and transmission parts of the relay link. In time- division multiple access (TDMA) configurations the receiver and transmitter within a relay station do not operate simultaneously, i.e. the relay station first receives a message and then later transmits it on the same frequency.

A simple FDMA (frequency division multiple access) relay transmission is depicted in Figure 1. Figure 1 shows a single relay link comprising at one end a base station (BS) 1 which is in communication via a relay station (RS) 3 with a communication device 5 at the other end of the link. The end-user's communications device may, for example, be a mobile telephone or a laptop computer with wireless networking capabilities.

The relay link depicted uses a first frequencyfj to transmit between the base station 1 and the relay station 3 and a second frequencyf2 to transmit between the relay station and the communication device 5 (also referred to in the following discussion as the mobile station (MS)).

The use of two separate frequencies (fj andf2) enables the relay station to simultaneously receive from the base station and transmit to the communications device and also avoids interference between the message it receives (from the base station) and the message it transmits (to the communications device). It is clear, however, that the relay station in Figure 1 halves the network resource efficiency compared to a hub-andspoke model as two separate frequencies are now being used to support a single user in the system.

In reality a communications system will be a multi-user environment with a number of communications devices (mobile stations) and a number of relay stations within the network.

Figure 2 depicts a network with two end users and shows a base station 1, a first relay station 3, a first communications device, MS1 (5), a second relay station 7 and a second communications device, MS2 (9) (Note: like numerals are used to denote like features with Figure 1).

The network of Figure 2 has two relay links. Relay link I is from the base station to communications device 5 via the first relay station 3 and relay link 2 is from the base station 1 via relay station 7 to the second communications device 9.

As in Figure 1 frequencyfi is used to transmit from the base station 1 to the first relay station 3 which then transmits on frequencyf2 to the first communications device 5.

The base station transmits on a separate frequencyf to the second relay station 7 which itself transmits on another frequencyf to the second communications device 9.

Conventionally,f andf are different fromfj andf2 and alsoJ!=f so that the various signal traffic does not interfere with each other. Therefore, in conventional mesh architectures the system depicted in Figure 2 requires four separate frequencies to operate -f', f, f andf. (see for example Raif Pabst et al, "Relay-Based Deployment Concepts for Wireless and Mobile Broadband Radio", IEEE Communications Magazine, September 2004).

It is therefore an object of the present invention to provide a communications system that has greater resource efficiency than prior art systems.

Accordingly in a first aspect the present invention provides a communications network having n operating frequencies comprising a base station and a plurality of relay stations, each relay station capable of receiving signals transmitted at a first frequency and re-transmitting said received signals at a second frequency wherein the base station and relay stations comprise directional antennas and the network is arranged in use to support n communications devices in direct or indirect communication with the base station The present invention proposes the use of directional distribution of network traffic by the use of directional antennas within the base station and relay stations. The use of directional antennas enables the network to use only a single frequency resource, on average, per end user regardless of the route taken through the network, i.e. regardless of the number of frequency hops used. This represents a significant increase in efficiency compared to prior art mesh architecture configurations. Therefore, in a network capable of transmitting at n distinct frequencies the present invention provides for upto n different relay links.

It is noted that the invention may be implemented with either fixed relay stations or mobile relay stations and may also be used in an FDMA scheme or combination schemes such as FDMAITDMA or FDMAICDMA (Code-Division Multiple Access).

Preferably the relay stations should comprise two sets of antenna arrays, a first set for reception of a directional signal and a second set for onward transmission of the signal.

The second antenna array should also be capable of directional transmission.

Preferably the communications devices used in the network (e.g. a mobile telephone) should be configured to be able to receive directionally transmitted signals.

Furthermore, the communications devices should be capable of directionally transmitting signals to a base station or relay station.

Preferably the directional antennas comprise beam-forming or adaptive antenna arrays.

Furthermore, if, for example, a base station utilises m relay stations at any one time its beamforming or adaptive antenna array should be capable of beam-forming m transmission signals which are targeted on each of the m relay stations. Similarly, each relay station should be capable of simultaneously transmitting a plurality of beam- formed signals.

It is noted that by using directional antennas and in particular beamforming or adaptive antenna arrays the co-channel interference arising from other relay stations and communications devices is reduced as compared to traditional omni-directional antenna arrays.

Each link in a relay link should preferably use different frequencies to neighbouring sections in the link. For example, for a relay link comprising base station (BS) to first relay station (RS1) to second relay station (RS2) to communications device/mobile station (MS), the frequencies used could be: BS- RS1 -fj; RS1-' RS2 -f2; R52-' MS - fi Co-channel interference in the present invention can also conveniently be reduced by controlling the transmission signal power from the base station and also the relay stations. Preferably therefore the base station can monitor transmission power from the base station itself and also from each relay station. Furthermore, the base station can preferably control the power it transmits to the relay stations and can also assign transmission power at each relay station. By monitoring and controlling transmission power in this way the network can develop a trade off between received signal strength and co-channel interference.

Preferably a training sequence is inserted into the transmitted signals to allow both relay stations and the communications devices of end users within the network to measure the received power from the base station. This information can in turn be transmitted back to the base station, for example by means of a feedback mechanism.

Preferably each communication device (mobile station) is assigned a specific training sequence wherein the training sequences are orthogonal to one another.

Conveniently the training sequence can also be used by the relay stations and communications devices to estimate the direction of arrival (DoA) of signals transmitted from the base station andlor relay stations. Preferably, this DoA information is fed back to the base station by means of the feedback mechanism.

As an alternative to using training sequence information the relay stations and communications devices connected to the network could transmit standard message sequences to the base station so that it could measure power levels across the network and also the direction of arrival of signals at any device or station within the network.

The communications network according to the first aspect of the invention provides a network having n available operating frequencies which can, on average, support n relay links. In the event that the number of communications devices exceeds the number, n, of available relay links then the network may conveniently utilise a time division multiple access scheme in order to host a greater number of users.

Therefore, according to a second aspect of the present invention there is provided a communications system having n operating frequencies for communicating with >n communication devices comprising a communications network according to the first aspect of the present invention wherein the conimunications network additionally is capable of transmitting signals using a time division multiple access scheme.

In a corresponding aspect of the present invention there is provided a method of operating a communications network, the network having n operating frequencies and comprising a base station and a plurality of relay stations, each relay station capable of receiving signals transmitted at a first frequency and re-transmitting said received signals at a second frequency wherein the base station and relay stations are arranged to directionally transmit signals such that the network can, in use, support n communications devices in direct or indirect communication with the base station The present invention will now be described with reference to the following non- limiting preferred embodiments in which: Figure 1, discussed hereinbefore, is a schematic diagram illustrating a typical relay transmission system; Figure 2, discussed hereinbefore, illustrates a relay system which utilises two relay links; Figure 3 illustrates an embodiment of the present invention Figure 4 illustrates an application environment in which individual components of the network have large spatial separations Figure 5 illustrates a further embodiment of the present invention Figure 6 illustrates a further embodiment of the present invention comprising a FDMAITDMA combination As described above a traditional FDMA mesh network communications system assigns different frequencies to different relay links in order to avoid interference between transmitted messages on different relay links. Such networks can be characterised as orthogonal regenerative FDMA-based relays. However, even though such a system may optimise frequency allocation such that the network capacity is maximised there will be some reduction in the efficient use of resources, e.g. frequency usage.

The present invention substantially reduces the resource problems associated with conventional systems by employing directional transmission and reception of messages between devices in the network such that, on average, only a single frequency is assigned per end-user.

An embodiment of the present invention is depicted in Figure 3. It is noted that the network environment is virtually identical to that depicted in Figure 2 and as a consequence like numerals have been used to denote like features.

In contrast to Figure 2 however it is noted that the base station comprises an antenna array that is capable of directional transmission to the relay stations 3 and 7. The relay stations, which are capable of receiving directionally transmitted signals, are in turn capable of onward directional transmission to the communications devices of the end users, in this case the mobile stations MS1 5 and MS2 9.

Two relay links are shown to be in simultaneous operation in Figure 3. However, as a result of the use of directional antennas in the various components of the network (base station, relay station and mobile stations) only two separate frequencies are used to transmit signals to the two end users.

Specifically, Figure 3 shows two relay links. In the first relay link (base station to the first mobile station 5) frequencyfj is used to transmit from the base station to the first relay station 3. Transmission from the relay station to the end user utilises frequencyf2.

In other words, for relay link 1: - BS - RS; f2 - RS- MS.

In the second relay link (base station to the second end user 9) frequencyf2 is used to transmit from the base station to the second relay station 7. Transmission from the relay station to the end user utilises frequencyfi. In other words, for relay link 2:f2 - BS -+ RS; f' - RS-' MS.

It can be seen that compared to the network configuration of Figure 2 the present invention provides for a more efficient use of resources.

Applied to a more general system the present invention provides a network having n different frequency resources which may support n relay links. The number of frequency resources used in a given situation is dependent on the number of users active in the network at that time. The number of hops in a routing path therefore does not affect the allocation of resources to the system.

In the present invention the base station comprises an adaptive beamfonning antenna array which has the facility to transmit directionally to the various relay stations and end user devices that are active in the network. Each relay station comprises two antenna arrays, one which is capable of directionally transmitting a signal to another relay station or end user and one which is capable of receiving a directional transmission from either the base station or another relay station.

The end user communication devices also comprise an antennas array capable of receiving a directional transmission.

In the network illustrated in Figure 3 there are four potential sources of interference between the various components of the system, namely: 1) I: interference from the base station 1 to the second communication device 9 on frequencyfi 2) 12: interference from the base station 1 to the first communication device 5 on frequencyf2 3) 13: interference from the first relay station 3 to the second relay station 7 on frequencyf2 4) 14: interference from the second relay station to the first relay station onfj The above interference mechanisms can be controlled and reduced in a number of ways.

Each of the base stations, relay stations and communication devices (mobile stations) utilise directional antenna arrays which are capable of directional signal transmissionlreception. Since signals are directionally transmitted this naturally reduces the amount of interference between component devices within the system.

Furthermore, the use of adaptive antennas within the relay stations and end user devices in conjunction with adaptive antenna array processing techniques allows co-channel interference to be reduced further.

Still further, it is noted that by monitoring and controlling received and transmitted signal strengths within the system the network can reduce interference by trading off received signal strength and interference signal strength.

Interference may be reduced further across a network by ensuring, where possible, that potentially interfering relay links are not set up in close proximity with one another.

This embodiment is illustrated in Figure 4. The network shown in Figure 4 is essentially similar to the networks shown in Figures 2 and 3 in that two relay links (out of a possible n links that can be provided by the present invention) are shown. Each link comprises the base station, a relay station and a communications device. Since the components of Figure 4 are similar to the earlier Figures the components have been labelled as 1', 3', 5', 7' and 9'.

The two relay links of Figure 4 both use the frequenciesfj andf2. However, in order to minimise potential interference between the various components of the system the base station has chosen to allocate these frequencies to relay links which terminate at communication devices that have a large spatial separation from one another (e.g. as depicted within the Figure the two mobile stations are located either side of the base station).

Figure 5 shows a further example of the present invention. In the network illustrated in the Figure there are four communications devices and four relay stations in communication with the base station.

Figure 5 shows a base station I, first and second relay stations (3, 7) and first and second mobile devices (5, 9) as in Figure 3.

Additionally however there is a third relay station 11 and third and fourth mobile devices 13, 15. It is also noted in this example that first mobile device 5 is now in communication with the base station 1 via two relay stations 3 and 17.

In accordance with the present invention all the devices in the network of Figure 5 utilise directional antennas such that they are capable of directional transmission and reception of signals. This means that although there are four end user communications devices (5, 9, 13 and 15) and four relay stations (3, 7, 11 and 17) in the network the system only needs to assign four different frequencies (J}, f2, f3 andf4) to allow simultaneous transmission and reception to all devices inthe network. In other words on average only one frequency resource per end user has been used and the fixed frequency resources have been effectively shared between the base station, four communications devices and four relay stations.

As noted above the present invention provides a communications network with n operating frequencies which is capable of supporting, on average, n relay links. This equates to n separate communications devices that can be in contact with the base station.

In the event, however, that the number of communications devices exceeds the capability of the network to support (i.e. when there are more than n devices) then the present invention may be combined with time division multiple access in order to allow the communications devices to share the available resources. This situation is illustrated in Figure 6 in which relay links are established according to available time and frequency slots.

For the sake of clarity the network shown in Figure 6 is capable of supporting only two operating frequencies,fj andf2. There are however three relay stations and four communications devices in communication with the base station (in reality however the number of frequencies and devices connected to the network would be much greater).

Figure 6 shows a base station 1, first and second relay stations (3, 7) and first and second mobile devices (5, 9) as in Figure 3. Additionally in Figure 6 there is a third relay station 19 and third and fourth mobile devices (21, 23).

In order to reduce interference between the devices relay links are established according to the time/frequency slots detailed in Table 1 below. In the table below the various components of the network are referenced as follows: Base station 1 = BS First Relay station 3 = RS1 Second relay station 7 = RS Third relay station 19 = RS3 First communications device 5 MS1 Second communications device 9 MS2 Third communications device 21 = MS3 Fourth communications device 23 = MS4

TABLE 1

_ ___fi___ __f2___ t1 BS-)RS1 RS2->MS2 BS-RS2 RS1-MS1 t2 RS1-)BS MS2-RS2 RS2-)BS MS1-RS1 t3 BS-)MS4 RS3-MS3 BS-)RS2 t4 MS4-)BS MS3-RS3

Claims

CLAIMS: 1. A communications network having n operating frequencies

comprising a base station and a plurality of relay stations, each relay station capable of receiving signals transmitted at a first frequency and re- transmitting said received signals at a second frequency wherein the base station and relay stations comprise directional antennas and the network is arranged in use to support n communications devices in direct or indirect communication with the base station
2. A communications network as claimed in any preceding claim wherein each relay station comprises two antenna arrays, one for transmission and one for reception.
3. A communications network as claimed in any preceding claim wherein communications devices connected to the network are capable of detecting and receiving directionally transmitted signals.
4. A communications network as claimed in any preceding claim wherein communications devices connected to the network comprise directional antennas for transmission.
5. A communications network as claimed in any preceding claims wherein the directional antennas comprise beam-forming or adaptive antenna arrays.
6. A communications network as claimed in claim 5 wherein the base station is capable of beam-forming up to n different transmission signals simultaneously.
7. A communications network as claimed in claims 5 or 6 wherein each relay station is capable of simultaneously receiving a plurality of beam- formed signals and transmitting a plurality of beam formed signals.
8. A communications network as claimed in any preceding claim wherein the network is in communication with at least one communications device via one or more relay stations and wherein the one or more relay stations in the so-formed relay link receives and transmits signals at different frequencies.
9. A communications network as claimed in any preceding claim wherein the base station can monitor the power of the signals it transmits.
10. A communications network as claimed in any preceding claim wherein the base station can monitor the power of signals transmitted from the plurality of relay stations.
11. A communications network as claimed in any preceding claim wherein the base station can control the power of signals transmitted at any of the plurality of relay stations.
12. A communications network as claimed in any preceding claim wherein training sequences are inserted into signals transmitted from the base station.
13. A communications network as claimed in claim 12 wherein the base station assigns specific training sequences for each communications device connected to the network, the training sequences being arranged to be orthogonal to one another.
14. A communications network as claimed in either of claims 12 or 13 wherein the relay stations and communications devices use the training sequences to estimate the direction of arrival of signals that are transmitted to them.
15. A communications network as claimed in any of claims 12 to 14 wherein the relay stations and communications devices transmit power reception levels and direction of arrival information back to the base station.
16. A communications network as claimed in any preceding claim wherein relay stations and other communications devices connected to the network transmit standard message sequences to the base station to enable signal power levels across the network to be monitored.
17. A communications system having n operating frequencies for communicating with >n communication devices comprising a communications network according to any of claims 1 to 17 wherein the communications network additionally is capable of transmitting signals using a time division multiple access scheme
18. Processor control code to, when running, implement the communications network of any one of claims ito 16.
19. A carrier carrying the processor control code of claim 18.
20. A method of operating a communications network, the network having n operating frequencies and comprising a base station and a plurality of relay stations, each relay station capable of receiving signals transmitted at a first frequency and re-transmitting said received signals at a second frequency wherein the base station and relay stations are arranged to directionally transmit signals such that the network can, in use, support n communications devices in direct or indirect communication with the base station