GB2500180A

GB2500180A - Determining a duration of a recruitment slot in a wireless relay network

Info

Publication number: GB2500180A
Application number: GB1204223.0A
Authority: GB
Inventors: Yuanyuan Zhang; Mahmoud Hadef
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2012-03-09
Filing date: 2012-03-09
Publication date: 2013-09-18
Also published as: GB2501365B; KR101909488B1; GB201303370D0; KR20130103276A; GB2501365A; GB201204223D0

Abstract

A wireless network comprising a source node (2), a destination node (8) and at least a first relay node (4, 6) is arranged to determine a first measure of signal quality relating to a direct link between the source node (2) and the destination node (8), and a second measure of signal quality relating to a link between the source node (2) and the destination node (8) via at least the first relay node (4, 6). At a relay node (4, 6), a duration of a recruitment slot (12, 14) is determined, within which the relay node (4, 6) may transmit a recruitment message (38, 40) indicating availability of the relay node (4, 6) to relay. The recruitment slot duration is determined on a basis comprising the first measure of signal quality. In this way, the recruitment of cooperative relays (4, 6) may be dependent on the signal quality of the direct link from the source node (2) to the destination node (8). Physical (PHY) layer cooperative Medium Access Control (MAC) for Wireless Networks.

Description

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Medium Access Control for Wireless Networks

Technical Field

The present invention relates generally to wireless networks, and more 5 specifically, but not exclusively, to a method and apparatus relating to Medium Access Control (MAC) for Wireless Local Area Networks.

Background

Wireless networks, such as Wireless Local Area Networks (WLAN) 10 according to the IEEE 802.11 standard, are widely deployed, and typically provide the benefits of low cost, simple deployment and high speed data communications. In a WLAN network, the physical layer of IEEE 802.11 a/b/g/n is typically used to transmit and receive data packets over a shared wireless medium. The IEEE 802.11 Medium Access Control (MAC) typically 15 provides a reliable delivery mechanism for user data over wireless channels which may be subject to interference and fading. IEEE 802.11 DCF (Distributed Coordination Function) is a typical MAC protocol based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), in which a handshaking mechanism is typically used to combat the effects of collisions and 20 facilitate transmission of large data packets. In such a DCF scheme, when a source node is ready to transmit a packet, it first senses the activity on the transmission channel until an idle period equal to DIFS (Distributed Inter Frame Space) is detected. In this instance, the source waits for another random backoff interval before transmission to avoid collision with other nodes. Then the source 25 starts the transmission by sending a RTS (Request-To-Send) control packet. If the control packet is received correctly, the destination sends a CTS (Clear-to-Send) control packet after a SIFS (Short Inter Frame Space) interval. Once the CTS frame is received, the source transmits its data packet after a SIFS interval. If the data packet is received correctly, the destination responds by sending an 30 acknowledgement (ACK) packet after SIFS interval. IEEE 802.11 DCF also makes use of a network allocation vector (NAY) for virtual carrier sensing. The

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NAV is typically maintained by nodes that are not currently involved in any transmission or reception of packets, and tracks the remaining time of any ongoing data transmission and updates according to information received in the control/data packets.

5 With the expanding use of wireless mesh networks employing ad-hoc routeing between nodes via other nodes which are used as relay nodes, MAC schemes such as IEEE 802.11 DCF have been extended to so called cooperative MAC schemes that allow cooperation between nodes at the MAC layer to enable routeing via beneficial multi-hop routes, so that, for example, a slow 10 single hop transmission may be replaced by a fast two or more hop transmission. MAC protocols for cooperative communications include CoopMAC (Cooperative MAC), rDCF (relay enabled DCF) and Robust Cooperative Relaying.

However, prior art cooperative MAC protocols may be inefficient in 15 terms of complexity, signalling overhead and data capacity. For example, a data base of the signal qualities of links between nodes may have to be maintained, with associated signalling, in order to select an appropriate multi-hop route. Furthermore, prior art cooperative MAC protocols may be limited in their capabilities by a need to limit interference between simultaneous transmissions 20 of data that may interfere, so that selection of routes may be mutually exclusive due to potential interference. This may limit achievable bandwidth.

It is an object of the invention to mitigate the problems with the prior art systems.

25 Summary

In accordance with a first aspect of the present invention, there is provided a method of transmitting signals in a wireless network, the wireless network comprising a source node, a destination node and at least a first relay node, the wireless network being arranged to determine a first measure of signal 30 quality relating to a direct link between the source node and the destination

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node, and a second measure of signal quality relating to a link between the source node and the destination node via at least the first relay node,

the method comprising, at a relay node:

determining a duration of a recruitment slot within which the relay node 5 may transmit a recruitment message indicating availability of the relay node to relay, the recruitment slot duration being determined on a basis comprising the first measure of signal quality.

This has an advantage that the recruitment of a relay may be dependent on the signal quality of the direct link from the source node to the destination 10 node. A relay may be less likely to be recruited if the signal quality of the direct link is good than if it is poor.

In an embodiment of the invention, the method comprises, if it is determined to transmit the recruitment message, determining a delay period for transmission of the recruitment message from the first relay in dependence on 15 the second measure of signal quality.

This has an advantage that recruitment of a relay node may be dependent on the signal quality of a link via the relay from the source node to the destination node. The relay may be more likely to be recruited if the signal quality of the link from the source node to the destination node is good than if it 20 is poor.

In an embodiment of the invention, data may be transmitted from the source node to the destination node in at least a first mode or a second mode, the first mode comprising transmission directly from the source node to the destination mode, and the second mode comprising transmission from the source 25 node to the destination mode by a first path via one relay, the method comprising:

selecting the second mode of operation on a basis comprising the first and second measures of signal quality.

This has an advantage that data may be transmitted in a mode 30 appropriate for transmission from the source node to the destination mode by a first path via one relay by comparison of a measure of signal quality of the direct

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link with a measure of quality of the link from the source to the destination via the one relay, in a format appropriate for the second mode with, for example, appropriate addressing.

In an embodiment of the invention, the method comprises: 5 sending a first message from the source node indicating that data is ready to send;

receiving the first message at the destination node; and determining the first measure of signal quality from the signal quality of the first message as received at the destination node.

10 This has an advantage that the first measure of signal quality may be determined on the basis of the receipt of existing signalling without the need to provide additional signalling.

In an embodiment of the invention, the method comprises:

receiving the first message at the first relay node; and 15 determining the second measure of signal quality from at least the signal quality of the first message as received at the first relay node.

This has an advantage that the second measure of signal quality may be determined on the basis of the receipt of existing signalling without the need to provide additional signalling.

20 In an embodiment of the invention, the method comprises:

sending a second message from the destination mode comprising an indication of a duration of a recruitment slot within which a relay node may transmit a message, said duration being determined on the basis of the signal quality of the received first message.

25 This has an advantage that the duration of a recruitment slot may be conveyed to other nodes of the network efficiently, using the second message, which may be a modification of a message that needs to be sent for another purpose, such as to indicate Clear to Send (CTS).

In an embodiment of the invention, the method comprises: 30 receiving the second message at the first relay node,

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wherein said determining the second measure of signal quality is in dependence on at least the signal quality of the second message as received at the first relay node.

This has an advantage that the second measure of signal quality may be 5 determined on the basis of the receipt of existing signalling.

In an embodiment of the invention, the method comprises:

transmitting a third message, the third message being a recruitment message, from the first relay node after the determined delay period, the determined delay period starting from the end of receipt of the second message 10 at the first relay node, in dependence on the determined delay period being less than or equal to the duration of the recruitment slot.

This has an advantage that the recruitment message may not be sent if the delay period is longer than the recruitment slot, so that the recruitment of relays may be inhibited on the basis of the signal quality in the direct link from 15 the source node to the destination node.

In an embodiment of the invention, the third message comprises an indication of the second measure of signal quality.

This has an advantage that the second measure of signal quality may be conveyed to the source node using the recruitment message, so providing 20 efficient use of signalling resources.

In an embodiment of the invention, the method comprises:

selecting the second mode of operation on a basis comprising the receipt of the third message.

In an embodiment of the invention, the method comprises: 25 determining a data rate for transmission of data from the source node on a basis comprising the indication of the second measure of signal quality in the third message.

This has an advantage that the data rate may be determined to be appropriate for the link from the source node to the destination node via the 30 relay node.

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In an embodiment of the invention, data may be transmitted from the source node to the destination node in a third mode, the third mode comprising transmission from the source node to the destination mode via a combination of the first path via the first relay node and a second path via a further relay node, 5 the method comprising:

receiving the first message at the further relay node;

determining a fourth measure of signal quality of the first message as received at the further relay node;

receiving the second message at the further relay node and determining a

10 fifth measure of signal quality, the fifth measure of signal quality being a measure of signal quality of the second message as received at the further relay node;

determining a second delay period for transmission of a fourth message in dependence on the fourth and fifth measures of signal quality;

15 receiving the third message from the first relay node; and transmitting the fourth message after the after the determined second delay period, the determined second delay period starting from the end of receipt of the third message at the further relay node, in dependence on the determined delay period being less than or equal to the duration of the recruitment slot.

20 This has an advantage that the data capacity of the wireless network may be increased by simultaneous transmission of data via at least two relays.

In an embodiment of the invention, the fourth message comprises an indication of received signal quality based on the fourth and fifth measures of signal quality.

25 This has an advantage that signalling may be conveyed efficiently without requiring additional messages to be sent.

In an embodiment of the invention, the method comprises:

selecting the third mode of operation for transmission of data from the source node on a basis comprising the receipt of the third and fourth messages;

30 and

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determining a data rate for transmission of data from the source node on a basis comprising the indication in the third and fourth message of received signal quality based on the second, third, fourth and fifth measures of signal quality.

5 This has an advantage that the source node may set a data rate appropriate to the link over which the data may be sent.

In an embodiment of the invention, the method comprises comprising transmitting data simultaneously via the first and second paths.

This has an advantage that the data capacity of the wireless network may 10 be increased.

In an embodiment of the invention, the method comprises relaying data at the first and further relay nodes according to a Quantise Map and Forward (QMF) protocol.

This has an advantage of providing efficient transmission in combination 15 with medium access control implemented according to embodiments of the invention.

In an embodiment of the invention, the first message may be a Ready To Send (RTS) message, the second message may be a Clear To Send (CTS) message, the third message may be a Ready To Relay (RTR) message, and the 20 the fourth message may be a Ready To Relay (RTR) message.

In an embodiment of the invention, the basis for determining the recruitment slot duration comprises an allowed modulation scheme.

This has an advantage that the recruitment slot duration may be set according to an achievable data rate taking into account an allowed modulation 25 or coding scheme.

In an embodiment of the invention, the method comprises receiving an indication of said allowed modulation or coding scheme from the source node.

This has an advantage that the source node may communicate the allowed modulation scheme to other nodes.

30 In an embodiment of the invention, the wireless network operates according to IEEE 802.11.

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In accordance with a second aspect of the present invention, there is provided a relay node for transmitting signals in a wireless network, the wireless network comprising a source node, a destination node and at least the relay node, the wireless network being arranged to determine a first measure of signal 5 quality relating to a direct link between the source node and the destination node, and a second measure of signal quality relating to a link between the source node and the destination node via at least the first relay node,

the relay node being arranged to:

determine a duration of a recruitment slot within which the relay node 10 may transmit a recruitment message indicating availability of the relay node to relay, the recruitment slot duration being based on the first measure of signal quality.

In an embodiment of the invention, the relay node is arranged to:

if it is determined to transmit the recruitment message, determine a delay 15 period for transmission of the recruitment message from the first relay in dependence on the second measure of signal quality.

Further features and advantages of the invention will be apparent from the following description of preferred embodiments of the invention, which are given by way of example only.

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Brief Description of the Drawings

Figure 1 is a schematic diagram showing a network with two relay nodes according to an embodiment of the invention;

Figure 2 is a timing diagram showing messages flow in a scenario in 25 which two relays are recruited according to an embodiment of the invention;

Figure 3 is a timing diagram showing messages flow in a scenario in which one relay is recruited according to an embodiment of the invention;

Figure 4 is a timing diagram showing messages flow in a scenario in which no relay is recruited according to an embodiment of the invention; 30 Figure 5 is a flow diagram showing processes at the source node according to an embodiment of the invention;

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Figure 6 is a flow diagram showing processes at the destination node according to an embodiment of the invention;

Figure 7 is a flow diagram showing processes at the relay node according to an embodiment of the invention;

5 Figure 8 is a diagram showing messaging according to an embodiment of the invention;

Figure 9 is a schematic diagram showing RTS and CTS signalling fields according to an embodiment of the invention;

Figure 10 is a schematic diagram illustrating selection of transmission 10 mode when source-destination link is an advantageous choice in an embodiment of the invention;

Figure 11 is a schematic diagram illustrating selection of transmission mode when cooperative transmission via 1 relay is an advantageous choice in an embodiment of the invention; and 15 Figure 12 is a schematic diagram illustrating selection of transmission mode when cooperative transmission via 2 relays is an advantageous choice in an embodiment of the invention.

Detailed Description

20 By way of example, embodiments of the invention will now be described in the context of a Wireless Local Area Network System operating according to IEEE 802.11 protocols.

However, it will be understood that this is by way of example only and that other embodiments may involve other wireless systems; embodiments are 25 not limited to the use of Wireless Local Area Network (WLAN) Systems.

Figure 1 shows an embodiment of the invention in which a wireless network, in this example a WLAN network operating according to IEEE 802.11, comprises a source node 2, a destination node 8, a first relay node 4, and a second relay node 6. Data, typically in the form of data frames, is to be sent 30 from the source node to the destination node. As shown in Figure 1, the data may be sent directly from the source node 2 to the destination node 8, and/or

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sent via one or both relay nodes 4, 6. The network is arranged to operate, according to the embodiment of the invention, so that the route taken by the data depends on signal quality in the links via each relay and on the signal quality in the direct link from the source node to the destination node. The link or 5 combination of links via which the data is sent may be determined on a frame by frame basis, as the nodes move or as the radio frequency environment changes, for example by a change in interference or fading characteristics. Relay nodes are either recruited or not recruited to be used in a link from the source node to the destination node, typically on the frame-by-frame basis. This is typically 10 arranged as illustrated in Figure 2.

Figure 2 is a timing diagram showing message flow between the source node, the destination node, and two relay nodes in a scenario in which two relays nodes are recruited, for example for the sending of one or more data frames. The wireless network is arranged to determine a first measure of signal 15 quality relating to a direct link between the source node 2 and the destination node 8, and a second measure of signal quality relating to a link between the source node 2 and the destination node 8 via at least the first relay node 4. The first measure of signal quality may be determined from the signal quality of a message as received at the destination node from the source node, for example 20 from a first message send from the source node indicating that data is ready to send, such as a RTS (Ready To Send) message 34. The first message may also be received at the first relay node, and the second measure of signal quality may be determined from at least the signal quality of the first message as received at the first relay node.

25 Figure 2 shows two recruitment slots, recruitment slot 1 12 and recruitment slot 2 14, within the illustrated frame format. A recruitment slot is a period within which a relay node may transmit a recruitment message indicating availability of the relay node to relay, such as a Ready to Relay (RTR) signal RTR1 38, RTR2 40. The relay node may be sent data from the source node to 30 relay to the destination node on the basis of the receipt of the recruitment message at the source node.

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The duration of a recruitment slot is determined, in an embodiment of the invention, on the basis of the quality of the direct link from the source node to the destination node, that is to say the first measure of signal quality. Typically, the higher the quality of the direct link, the shorter the recruitment 5 slot.

Each relay node backs off, that is to say delays, transmission of the recruitment message by a delay period, also referred to as a back-off period, according to the quality of the link from the source node to the destination node via the relay node, that is to say the second measure of signal quality. As a 10 result, the relay node with the best link transmits its recruitment message first. On receiving the recruitment message, the other relays inhibit transmission of their own recruitment message in that recruitment slot; they may have another attempt in a second recruitment slot, in which the already-recruited relay will be silent.

15 In this way, the relay node having the highest quality link is recruited first, and a relay node is typically only recruited if the direct link is sufficiently poor in comparison with the link via a relay that it is advantageous to use the relay.

So, if the direct link is poor, the recruitment slot will be long, so that 20 even relays that transmit a recruitment message after a long period have a chance to be recruited. However, if the direct link is good, the recruitment slot will be short, so that only relays that transmit a recruitment message after a short period have a chance to be recruited. This prevents relays with poor links being recruited and saves in signalling time overhead. The duration of the recruitment 25 slot may be zero if the quality of the direct link is sufficiently good. In this case no relay nodes will be recruited, since they will be expected to offer a less efficient use of radio resources than the direct link, even if the signal quality of the link to the relay node is good, since the link via the relay node would comprise two or more hops rather than the single hop from the source node to 30 the destination node.

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Data may be transmitted from the source node to the destination node in at least a first mode or a second mode, the first mode comprising transmission directly from the source node to the destination node, and the second mode comprising transmission from the source node to the destination node by a first 5 path via one relay. For example, the destination address of the data may be indicated in a header appropriate to the mode, that is appropriate to the route between the source node and destination node. The second mode of operation may be selected, instead of the first mode on a basis comprising the first and second measures of signal quality. So, the mode of operation in which data is 10 relayed is selected on the basis of the measures of quality of the direct link and the link via the relay.

An indication of the duration of the recruitment slot may be sent in a second message from the destination mode, such as a Clear To Send (CTS) message 36, the duration of the recruitment slot being determined on the basis of 15 the signal quality of the received first message at the destination node, that is to say the quality of the direct link. At the relay node, on receipt of the message carrying the indication, the duration of a recruitment slot within which the relay node may transmit a recruitment message may be determined from the received indication.

20 The receipt of the second message, which may be a CTS message, at the relay node may be put to a further use. The second measure of signal quality, which relates to the link via the relay node, may be determined in dependence on at least the signal quality of the second message as received at the first relay node.

25 The recruitment message, that is to say a third message, may be transmitted from the relay node after the determined delay period, the determined delay period starting from the end of receipt of the second message, which may be the CTS message, at the relay node, in dependence on the determined delay period being less than or equal to the duration of the 30 recruitment slot. If the determined delay period is longer than the duration of recruitment slot, the recruitment message may not be transmitted.

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The third message, for example the RTR message, may comprise an indication of the second measure of signal quality, which relates to the link via the relay node. Then, the second mode of operation may be selected on a basis comprising the receipt of the third message, for example at the source node.

5 Also, a data rate may be selected for transmission of data from the source node on a basis comprising the indication of the second measure of signal quality in the third message. A modulation and/or coding scheme may also be selected on the basis of receipt of the third message at the source node.

A physical layer (PHY) transmission scheme, such as space-time coding, 10 that allows simultaneous transmission and reception of different data streams 44, 46 on a shared medium can be used to advantage with a MAC scheme according to an embodiment of the invention, such as that illustrated in Figure 2. This may enable data to be relayed by two or more relays at once, so that two or more links between the source and destination nodes may be set up in parallel, 15 increasing the potential data capacity and throughput rate. In an embodiment of the invention, the method comprises relaying data at the first and further relay nodes according to a Quantise Map and Forward (QMF) protocol, that allows simultaneous transmission and simultaneous reception of different data streams 44, 46 on a shared medium.

20 So, data may be transmitted from the source node to the destination mode in a third mode, via a combination of a first path via a first relay node and a second path via a further relay node. In this mode, the first message, in this example the RTS message, is received at the further relay node, and a fourth measure of signal quality is determined, being a measure of signal quality of the 25 first message as received at the further relay node. The second message, in this case the CTS message, is also received at the further relay node and a fifth measure of signal quality is determined, the fifth measure of signal quality being a measure of signal quality of the second message as received at the further relay node, which relates to the quality of the link via the further relay.

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A second delay period is determined for transmission of a fourth message, in this example RTR2 40 message, in dependence on the fourth and fifth measures of signal quality.

The third message, in this example RTR1 38 from the first relay node, is 5 received at the second relay node, and the fourth message, in this example RTR2 40, is transmitted after the determined second delay period 32, the determined second delay period starting from the end of receipt of the third message at the further relay node, in dependence on the determined delay period being less than or equal to the duration of the recruitment slot. The fourth message, in this case 10 RTR2 40, may comprise an indication of received signal quality based on the fourth and fifth measures of signal quality.

The third mode of operation for transmission of data from the source node may be selected on a basis comprising the receipt of the third and fourth messages, in this case the RTR1 38 and RTR2 40 messages, and the indications 15 of signal quality that each carries.

A data rate for transmission of data from the source node may be determined on a basis comprising the indication in the third and fourth message of received signal quality based on the second, third, fourth and fifth measures of signal quality.

20 In an embodiment of the invention, the basis for determining the recruitment slot duration comprises an allowed modulation and/or coding scheme. In this way, the signal quality, for example signal to noise ratio, may be used to select an allowable modulation and/or coding scheme , and the slot duration may be made dependent on the data rate that may be achieved using the 25 allowable modulation and/or coding scheme. Similarly, the delay in transmission of the recruitment message from a relay node may be dependent on an allowable modulation and/or coding scheme, so that the delay may be made dependent on the data rate that may be achieved using the allowable modulation and/or coding scheme. This may make the operation of the MAC scheme in 30 selecting relays to recruit more efficient. An indication of an allowed modulation and/or coding scheme may be received from the source node. The

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allowed coding scheme may be a network coding scheme, for example QMF (Quantise Map Forward), AF (amplify and Forward) or CF (Compress and Forward).

As shown in Figure 2, a SIFS (Short Inter Frame Space) interval 10, 16, 5 18. 20 is typically provided between the RTS 34 and CTS 36, between the RTR2 40 and data 42 transmitted by the source node, between the data 44, 46 transmitted by the relay nodes, and between the data 44, 46 transmitted by the relay nodes and the acknowledgement ACK 48 from the destination node. The SIFS interval allows for data transmission times to avoid clashes between 10 signals.

Figure 3 illustrates a situation similar to that of Figure 2, except that Relay 2 is experiencing a poor quality link from the source node via Relay 2 to the destination node. As a result, the delay period 32 for the recruitment message is greater than the duration of the recruitment slot 2 14, so that the 15 RTR2 message is not transmitted and Relay 2 is not recruited. Hence the data 42 sent by the source node is not addressed to Relay 2, only Relay 1, and accordingly the data is only relayed by Relay 1 as transmitted data 44.

Figure 4 illustrates a situation similar to that of Figure 3, except that both Relay 1 and Relay 2 experience poor quality links from the source node via the 20 respective relay to the destination node. As a result, the delay period 30 for the recruitment message is greater than the duration of the recruitment slot 1 12, so that the RTR1 message is not transmitted and neither Relay 1 nor Relay 2 is recruited. Hence the data 42 sent by the source node is not addressed to Relay 1 or Relay 2, and accordingly the data 42 is received in the direct link to the 25 destination node.

In embodiments of the invention, a Medium Access Control (MAC) protocol of a Wireless Local Area Networks (WLAN), such as a IEEE 802.11 WLAN, is provided that allows cross layer cooperative communications. The MAC protocol according to the embodiment of the invention, an embodiment of 30 which is illustrated by Figures 2, 3 and 4, is referred to here as Adaptive Cross-layer Cooperative MAC (ACCMAC), and this ACCMAC exploits the

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capabilities of PHY protocols, in particular the existing Quantise Map and Forward (QMF) protocol, which allow allows relaying with simultaneous transmission and reception of different data streams on a shared medium. In this way, multiple relay paths may be used in parallel, increasing the data capacity of 5 the network. Furthermore, the ACCMAC scheme according to an embodiment of the invention is efficient in terms of signalling overhead, using existing transmissions such as RTS (Ready to Send) and CTS (Clear to Send) as the basis of link quality measurements that enable automatic recruitment of relays that provide the best quality links without the need to maintain a database of link 10 quality measurements and without the need for a high signalling overhead; relay recruitment is automatic, based on a recruitment slot that has a duration dependent on the signal quality in a direct link between a source and destination node, and a back-off period for a recruitment message from each relay that is dependent on the signal quality, and allowable modulation scheme, in a link 15 from the source to the destination via the respective relay. Signal level cooperation at the PHY layer exploits cooperative diversity to improve the outage performance and bit error rate (BER). Packet level cooperation at the MAC layer relates to the selection of one or more relays to serve as intermediate nodes to forward data between a source and a destination in a two-hop approach, 20 with which each hop can provide a higher transmission rate than the direct link between the transmitter and the receiver. In embodiments of the invention, a MAC protocol is provided to enable cross-layer cooperative communications between the PHY and MAC layers, allowing signal level cooperation at PHY layer and adaptive relay selection at MAC layer. Firstly, the MAC protocol is 25 designed to 'recruit relays in the air' which avoiding the overhead of keeping a relay table at each node. Secondly, the MAC layer header has been modified to accommodate information exchange among nodes for adaptive transmission mode selection among direct transmission, one-relay or multiple-relay transmission according to channel conditions. Thirdly, once there are multiple 30 relays available, the MAC design enables simultaneous transmission from the relays instead of sequential transmission from each relay. As has been shown,

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the first, second and third aspects of the design have been integrated into a new Cooperative MAC for WLAN systems, or for other distributed networks to enable cross layer cooperative communications, potentially providing higher transmission rate and reduced power consumption by making use of both PHY 5 and MAC layer cooperative transmission.

Embodiments of the invention may use cooperative communications to take advantage of the broadcast nature of wireless medium which may limit the capabilities of prior art systems due to the potential for interference between parallel links. Broadcast and spatial diversity may be utilized improve the 10 performance of wireless links. Cooperative communications at PHY layer may be implemented by relaying protocols such as, Amplify-and-Forward (AF), Decode-and-Forward (DF), Compress-and-Forward (CF) and Quantize-Map-Forward (QMF). QMF has been found to be particularly suitable in embodiments of the invention, supporting multiple relays transmitting at the 15 same time and furthermore does not require relays to fully decode the received data, allowing efficient implementation. However, prior art MAC systems do not support QMF based relaying, since a typical prior art MAC layer of a WLAN is designed to avoid simultaneous transmission from different nodes in case of collisions. In embodiments of the invention, the broadcast nature of 20 signalling messages and the capability for simultaneous transmission at the PHY layer are exploited.

In embodiments of the invention, the carrier sensing scheme of IEEE802.il MAC is redesigned to allow relays able to support good quality links to join a network with simultaneous transmissions. Also, the carrier 25 sensing and handshaking mechanism for the source and the destination nodes is modified to recruit relays "in the air", that is without keeping a relay table at each node. The MAC header and framework have been redesigned to support adaptive relay selection among direct transmission, single relay and multiple relay modes.

30 Referring again to Figure 2, in an embodiment of the invention, the destination node sends back CTS 36 after a SIFS interval 10. Information of the

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conditions for relays to provide better performance than the direct transmission is carried in the CTS. This information will be used at both the source and the destination to set the 'recruitment slot' for the following relay recruitment procedure. The maximum recruitment slot duration may be arranged to be equal 5 to the SIFS duration 10; this duration corresponds to the relay back-off corresponding to the minimum conditions for a relay to provide better performance than direct transmission. The minimum recruitment slot is 0, which may apply when the direct link can achieve the maximal supported transmission rate of the system, and no relay will join the transmission. 10 The relays may start the back-off period 30 after receiving CTS 36, while both the source and the destination wait for a recruitment slot period to recruit relays. The back-off time at each relay may be set according to the overall achievable data rate of transmission from source to destination via the relay, a relay with higher achievable data rate having a shorter back-off time. The 15 achievable data rate may be indicated by measures of signal quality of the link, and by allowable modulation and/or coding schemes of the link, which may also be indicated by signal quality such as signal to noise ratio. The achievable data rate may also be used as an indication of signal quality.

If a relay may complete a back-off period within the recruitment slot, it 20 may send a RTR1 (Ready to Relay) to respond to the recruitment opportunity. In this RTR1, an indication of the achievable transmission rate for a link using the relay, that may be an indication of signal quality, is broadcast to the nearby nodes.

In the case that multiple-relay is supported, based on the current 25 information exchange, the nodes may start another recruitment slot in the same way as for the first recruitment slot. A further relay may send out a RTR2 40. Once the relay recruitment period has completed, the source transmits a data frame 42 after a SIFS interval 16. The recruited relays receive the data frame 42 from the source, and re-transmit the data frame 44, 46 to the destination after 30 signal level processing according to a PHY layer cooperation scheme that is supported, for example QMF, simultaneously after a SIFS interval 18. The

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destination sends back an ACK 48 after a SIFS interval 20 if it can receive the data correctly by jointly processing the data received from the source and/or the relays.

According to the scheme illustrated in Figure 2, the destination node and 5 the source node recruit relays in the air to avoid the overhead of keeping a relay table at each node, and the cross layer design supports PHY cooperative schemes, such as Quantize-Map-Forward relaying schemes and randomized distributed space time coding schemes which allow simultaneous transmission from relays at PHY layer. For example, in the wireless system illustrated by 10 Figure 1, with one source node 2, one destination node 8 and two relays 4, 6, the system can adaptively switch among three transmission modes, in which the source node sends data to the destination directly, or via 1 relay, or via 2 relays providing it is advantageous to do so. That is, if the direct transmission can achieve the maximal supported rate, the source sends data to the destination 15 directly without any help from the relay, in the first mode. If transmission via one of the relays can provide performance improvement, the source sends data to the destination first, and the relay sends a copy to the destination afterwards in the second mode. If transmission via two relays can achieve further performance gain, the relays can send the data to destination simultaneously 20 afterwards, in the third mode. In embodiments of the invention, the destination and the source recruit relays in the air, which may avoid the need to keep a relay table at each node. In an embodiment of the invention, QMF based PHY layer relaying protocol is used, since it may provide good performance compared with AF and DF. Other embodiments of the invention may use alternative PHY layer 25 relaying protocols such as randomized distributed space time coding. In an embodiment of the invention, the wireless system can support more than 2 relays simultaneously by providing further recruitment slots.

In an embodiment of the invention, the transmission power for the nodes may be fixed, the RTS and CTS may be overheard by other nodes besides the 30 transmitter and intended receiver node, and channel state information (CSI or received SNR) is available at the receiver side and exchanged via RTS and CTS.

20

Typically, the transmission in two directions between two nodes uses the same frequency and the channels are symmetric, that is to say the channels have the same characteristics for transmissions in both directions.

Figure 5 is a flow diagram showing processes at the source node 5 according to an embodiment of the invention. In the embodiment illustrated, if there is at least one packet buffered in the queue, the source node starts a random back-off after an interval of DIFS before sending out RTS to reserve the channel. If CTS from the destination has been received, the source may reserve the channel for transmission. In the CTS control frame, the destination also 10 informs the source about duration of the recruitment slot to recruit relays. The source may stay idle and listening to the channel for a period of a recruitment slot to recruit a relay until a RTR control frame is received or the recruitment slot expires. This recruitment may repeat to recruit more relays depending on the system requirements. Once the recruitment procedure is completed, the source 15 starts to transmit the data frame. If an ACK has been received, the source goes back to the idle status, or it goes back to the standard IEEE 802.11 DCF random back-off stage to find next opportunity to transmit data.

Figure 6 is a flow diagram showing processes at the destination node according to an embodiment of the invention. In the embodiment illustrated, 20 upon receiving the RTS from the source node, the destination estimates the channel condition between the source and the destination. Based on the channel conditions, it further calculates the conditions at which the cooperative transmission via relays could help to improve the performance of direct transmission. The destination sends out the CTS control frame that also 25 includes the settings of recruitment slot to recruit relays in the air. If the destination can receive RTR(1,2) control frames from the relays before the recruitment slot expires, the relays will join the transmission to provide better performance than the direct transmission, or the destination stays idle until the end of the recruitment slot. After receiving the data frame from the source 30 and/or relays and physical layer signal processing, detection and CRC (Cyclic Redundancy Check), the destination sends back ACK upon correct reception.

21

Figure 7 is a flow diagram showing processes at the relay node according to an embodiment of the invention. In the embodiment illustrated, a potential relay node, after receiving the CTS, starts to back-off, that is to say starts a delay period. The back-off time may be set according to channel 5 conditions from the relay to the source node and destination node. The relay node may estimate the channel quality based on the received RTS and CTS. The back-off time may be designed to make sure relays can join the network, when by doing so the overall achievable rate can be increased, before the recruitment slot expires. If the back-off time at the relay is shorter than the 10 recruitment slot, the relay may send RTR (RTR1 or RTR2 according to whether the relay is the first or second to be recruited) to indicate that the relay will participation in transmission. The relay receives the data frame from the source, and sends data to the destination after some processing according to the PHY relaying schemes used.

15 A carrier sensing and handshaking scheme has been shown in Figure 2,

and this is described in more detail in an exemplary embodiment of the invention as follows. When a channel is not reserved by other nodes and there is data in the buffer at the source node, the source may wait for the period of DIFS and may perform random back-off, as in the legacy IEEE 802.11 DCF 20 scheme. Then the source may reserve the channel by sending the control frame RTS. After waiting for a SIFS period, the destination may broadcast the CTS control frame. In this frame, it may also delivers the information about the setting of recruitment slot. Upon receiving the CTS, both the source and destination may start the recruitment slot to recruit relays. At the same time, 25 relays may start back-off individually according to channel conditions, channel quality, achievable data rate, and/or channel state information relating to one or more links from the source to the relay and from the relay to the destination. The back-off time may be arranged so that the better the channels quality is, the shorter is the back-off time. If a relay finishes the back-off before the end of the 30 recruitment slot, it may send out a control frame RTR (Ready to Relay). This may be repeated to recruit more relays in the same manner. In the exemplary

22

case with 2 relays, after the second recruitment, the control frame handshaking procedure is completed. The source node may transmit the data frame after a SIFS and the relays resend the received data from the source after another SIFS. The destination may acknowledge the successful reception with an ACK to 5 complete this transmission. Hidden nodes, for example nodes other than the source node, destination node and relay nodes, may update the NAV (network allocation vectors) each time they receive control frames from the wireless network. Control frames may reserve the channel with a duration as shown in the following table.

10

Frame

NAV Reservation Duration

RTS

T cts+2 * SIFS+RS1+RS2

CTS

RS1+RS2 +2*Trtr+T dadas+SIFS

RTR1

RS2+T datas+T datat+SIFS

RTR2

T |)AT/Ys+T |)AT/\r+3*SIFS+ACK

DATAs

TDATAr+2*SIFS+ACK

DATAr

SIFS+ACK

ACK

0

In the above table, RS denotes 'Recruitment slot', TCts, TRTr denote the transmission duration of the control frame CTS and RTR, Tdatas and Tdatai are the transmission duration of the data frame from the source and the relays, 15 respectively. The Recruitment slot may set up the minimum conditions under which the relays could provide better performance.

In an embodiment of the invention, the duration of the recruitment slot may be set in the following way. For WLAN, the maximum transmission supported at PHY layer with cooperation may be it,,.-.,. Based on the channel 20 quality of the source-destination link, the achievable rate of this direct link is SfChg), where is the received SNR of the source-destination link. So duration of the recruitment slot is set to be:

23

Recruitment slot 1 = {1 — * * SIFS .

In this example, the worse the source-destination channel quality is, the longer 5 the recruitment slot is, and the maximum duration of the recruitment slot equals SIFS. In this case, the maximum back-off limit is the duration of SIFS and the minimum could be 0 when the direct link is good enough. This ensures to minimize delay of the transmission and help other relays to set NAV. By listening to the CTS frame, each potential relay gets to know the channel 10 conditions between itself to the source and the destination node. It can then calculate the achievable rate if it joins the transmission as a relay. If the achievable rate is smaller than }, it keeps idle, for example by setting the back-off to 3*SIFS, which is typically longer than the recruitment slot. If the achievable rate is greater than S ), the relay starts back-off with a period of 15 the following:

backoff dwatian 1 = (i - * SIFS .

20 By this process, the higher the achievable rate the relaying transmission can achieve, the shorter the back-off time is. So that:

R (jsd } < R fcr> >W >• Tsd ), Recruitment slot 1 > backoff time 1.

25 So the back-off time is set as below:

' (i - * sifs if r(ysd) < ffCav, ra)

3 if ^ 8(ysr>¥r&-ys&y

, , .. t (a - *SIFS if < Riy^Yr^Yvi)

actio t1 aurai-ian.. t = ,

i 3* SIFS

backof f dicrat ion 1 =

24

In the case of multiple relays, each relay may reset their back-off duration whenever it hears a RTR from another relay. The reset back-off period may start once transmission from another relay has ceased. In the RTR1 control frame, the relay also broadcast information about (for simplicity,

5 we use to denote it). In the case of multiple relays, based on this information, the 2nd recruitment started with duration:

Recruitment slot 2 = (1 —* SIFS.

10 And each relay reset their duration time with:

if R2 <

if Rz > Rt

15 where R- is the achievable rate of this relay participating the transmission as a 2nd relay.

Figure 8 shows a messaging diagram for the communication of control messages, data and acknowledgement messages between a source, relays and destination in a manner consistent with an embodiment of the invention. 20 In an embodiment of the invention, in the case that the source-

destination link can achieve the maximal supportive rate, the actual recruitment slot is set to 0. In this case, the source will sends the data frame after one SIFS slot. There is no extra time wasting on waiting for responses from relays. This is a difference from other protocols, in which the system needs to allocate fixed 25 time to wait for the relays' response.

In embodiments of the invention, the RTS may be the same as with the legacy IEEE802.il. The CTS may have a format as illustrated in Figure 9. In this embodiment, eight extra bits have been used in the duration field in the 802.11 frame and the duration field has been changed to include both the i (1 ^2-) * SIFS

backoff duration 2 = | " Rm®x'

I 2 * SIFS

25

duration and the recruitment slot information to inform the source to recruit relays. The RTR may have a format as illustrated in Figure 9. In this embodiment, an extra 16 bits have been used in the duration field to set the transmission rate for both source-relay and relay-destination link.

5 Figure 10 is a schematic diagram illustrating selection of transmission mode when source-destination link is an advantageous choice in an embodiment of the invention. In the example of Figure 10, the highest transmission rate of the system, limited by the coding and modulation rate, is 11Mbps. The direct transmission can support the highest rate and cooperative transmission is not 10 needed. In an embodiment of the invention, the recruitment slot duration will be set to 0 in this case, and the source will transmit without recruiting any relays.

Figure 11 is a schematic diagram illustrating selection of transmission mode when cooperative transmission via 1 relay is an advantageous choice in an embodiment of the invention. In the example of Figure 11, since the source-15 destination can only support 2 Mbps, the recruitment slot is set to a period appropriate to that rate. The Relay 1 can provide an overall rate higher than 2 Mpbs, and it sends a RTR to respond to the recruitment. Relay 2 can provide a better rate than the direct link but worse than Relay 1, so it will back-off for a longer time than Relay 1 so that relay that may provide the higher rate link may 20 join the transmission first. If multiple-relay mode is allowed, then Relay 2 may respond in the second recruitment slot, and both relays may transmit the data frame to the destination simultaneously.

Figure 12 is a schematic diagram illustrating selection of transmission mode when cooperative transmission via 2 relays is an advantageous choice in 25 an embodiment of the invention. In the example of Figure 11, Relay 1 joins the transmission first as it is the relay offering the highest rate link, and Relay 2 joins the transmission in the second recruitment slot since a higher transmission rate can be provided with 2 relays.

In an embodiment of the invention, alternative relaying protocols at the 30 PHY layer may be used, such as Decode and Forward (DF). In this case, the destination informs the source/relays the CTS control frame about the specific

26

modulation/coding combinations instead of the transmission rate requirements. By receiving this information, relays can judge themselves if they are able to transmit in this kind of coding/modulation format to ensure reliable transmission. Qualified nodes can transmit in the modulation/code, enabling 5 straightforward signal detection.

The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of 10 any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

27

Claims

1. A method of transmitting signals in a wireless network, the wireless network comprising a source node, a destination node and at least a first relay node, the wireless network being arranged to determine a first measure of signal quality relating to a direct link between the source node and the destination node, and a second measure of signal quality relating to a link between the source node and the destination node via at least the first relay node,

the method comprising, at a relay node:

determining a duration of a recruitment slot within which the relay node may transmit a recruitment message indicating availability of the relay node to relay, the recruitment slot duration being determined on a basis comprising the first measure of signal quality.

2. A method according to claim 1, the method comprising:

if it is determined to transmit the recruitment message, determining a delay period for transmission of the recruitment message from the first relay in dependence on the second measure of signal quality.

3. A method according to claim 1 or claim 2, wherein data may be transmitted from the source node to the destination node in at least a first mode or a second mode, the first mode comprising transmission directly from the source node to the destination mode, and the second mode comprising transmission from the source node to the destination mode by a first path via one relay, the method comprising:

28

4. A method according to any preceding claim, the method comprising:

sending a first message from the source node indicating that data is ready to send;

5. A method according to claim 4, the method comprising:

receiving the first message at the first relay node; and determining the second measure of signal quality from at least the signal quality of the first message as received at the first relay node.

6. A method according to claim 5, the method comprising:

7. A method according to claim 6, the method comprising:

receiving the second message at the first relay node,

8. A method according to claim 7, the method comprising: transmitting a third message, the third message being a recruitment message, from the first relay node after the determined delay period, the determined delay period starting from the end of receipt of the second message at the first relay node, in dependence on the determined delay period being less than or equal to the duration of the recruitment slot.

29

9. A method according to claim 8, wherein the third message comprises an indication of the second measure of signal quality.

5

10. A method according to claim 9, the method comprising:

11. A method according to claim 9 or claim 10, the method 10 comprising:

determining a data rate for transmission of data from the source node on a basis comprising the indication of the second measure of signal quality in the third message.

15

12. A method according to any of claims 9 to 11, the method comprising:

determining a modulation scheme for transmission of data from the source node on a basis comprising the indication of the second measure of signal quality in the third message.

20

13. A method according to any of claims 4 to 12, wherein data may be transmitted from the source node to the destination node in a third mode, the third mode comprising transmission from the source node to the destination mode via a combination of the first path via the first relay node and a second 25 path via a further relay node, the method comprising:

receiving the first message at the further relay node;

receiving the second message at the further relay node and determining a 30 fifth measure of signal quality, the fifth measure of signal quality being a

30

measure of signal quality of the second message as received at the further relay node;

5 receiving the third message from the first relay node; and transmitting the fourth message after the determined second delay period, the determined second delay period starting from the end of receipt of the third message at the further relay node, in dependence on the determined delay period being less than or equal to the duration of the recruitment slot.

10

14. A method according to claim 13, wherein the fourth message comprises an indication of received signal quality based on the fourth and fifth measures of signal quality.

15 15. A method according to claim 14, the method comprising:

selecting the third mode of operation for transmission of data from the source node on a basis comprising the receipt of the third and fourth messages; and determining a data rate for transmission of data from the source node on

20 a basis comprising the indication in the third and fourth message of received signal quality based on the second, third, fourth and fifth measures of signal quality.

16. A method according to any of claims 13 to 15, comprising

25 transmitting data simultaneously via the first and second paths.

17. A method according to claim 16 comprising relaying data at the first and further relay nodes according to a Quantise Map and Forward (QMF) protocol.

30

31

18. A method according to any of claims 4 to 17, wherein the first message is a Ready To Send (RTS) message.

19. A method according to any of claims 6 to 18, wherein the second 5 message is a Clear To Send (CTS) message.

20. A method according to any of claims 8 to 19, wherein and the third message is a Ready To Relay (RTR) message.

10

21. A method according to any of claims 13 to 20, wherein the fourth message is a Ready To Relay (RTR) message.

22. A method according to any preceding claim, wherein the basis for determining the recruitment slot duration comprises an allowed modulation

15 or coding scheme.

23. A method according to claim 22, the method comprising receiving an indication of said allowed modulation or coding scheme from the source node.

20

24. A method according to any preceding claim, wherein the wireless network operates according to IEEE 802.11.

25. A relay node for transmitting signals in a wireless network, the

25 wireless network comprising a source node, a destination node and at least the relay node, the wireless network being arranged to determine a first measure of signal quality relating to a direct link between the source node and the destination node, and a second measure of signal quality relating to a link between the source node and the destination node via at least the first relay

30 node,

the relay node being arranged to:

32

determine a duration of a recruitment slot within which the relay node may transmit a recruitment message indicating availability of the relay node to relay, the recruitment slot duration being based on the first measure of signal quality.

5

26. A relay node according to claim 25, the relay node being arranged to:

if it is determined to transmit the recruitment message, determine a delay period for transmission of the recruitment message from the first relay in 10 dependence on the second measure of signal quality.