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RELAYING PROTOCOLS AND RELAY SELECTION FOR MULTI-HOP AND

COOPERATIVE COMMUNICATION NETWORKSAALTO UNIVERSITY

School of Science and Technology

Wang Wei

2010-5-11

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The transmission rates for 4G systems are two orders of magnitude higher than those of 3G systems. This creates power concerns

The spectrum for 4G systems will be located above 2GHz band used by the 3G systems. The radio propagation is obviously more vulnerable to non-LOS

The system complexity becomes disproportional regarding to the evolution of wireless network technologies.

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Issues of 4G systems

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An impractical solution to these problems is to significantly increase the density of Base Stations.

It will result in high deployment cost and high transmission cost. The cost would be feasible if the number of subscribers also would be increased at the same rate.

On the other hand, the much higher demand in transmission rates makes the aggregate throughput rate the bottleneck.

 

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An impractical solution

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Some new network elements

Automatic Channel Selection: the ACS is in charge of synchronizing APs attached to it.

Mobile Access Point: the MAP provides access to the fixed network part of the RAN.

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Adoption of MIMO technology realize the concept of cooperative communications due to fact that multiple single antenna nodes can form a distributed virtual MIMO system.

Three basic scenarios: They are RC, CBC and CMAC

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Radio range extension

Combat shadowing at high radio frequencies

Reduce infrastructure deployment costs and admit rapid deployment

Enhance capacity in cellular networks

Higher bandwidth due to shorter hops

Greater range due to multi-hop forwarding

The Relay-UE links can use unlicensed spectrum than the BS-UE links that should be licensed spectrum

Don’t need complicated distributed routing algorithms

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Benefits via relays are listed:

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Focus on decode-and-forward (DF), amplify-and-forward (AF)

DF: the signal is first decoded to remove the noise and then re-encoded for forwarding. This strategy is more appropriate for dedicated relays, which can have higher power and processing capabilities.

AF: Both signal and noise are amplified before forwarding. This strategy is more appropriate for UT relays due to the reduced complexity.

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Relaying strategy

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both signal and noise are amplified before forwarding to the destination.

The relay amplifies the signal from the source then forwards it to the destination to equalize the effect of the channel fade.

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Amplify-and-forward relaying protocol

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Outage probability of AF relaying

If the SNR is not high enough, the outage probability of fixed AF relaying is larger than that of direct transmission.

outage probability of fixed AF relaying degrades faster than that of direct transmission, the SNR is high enough the outage probability of fixed AF relaying is lower.

the diversity order 2, achieved by fixed AF relaying, can be obtained from the curve slope.

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Outage probability of AF relaying II

The performances of fixed AF relaying and direct transmission decay with increasing spectral efficiency and

The performance of fixed AF relaying degrades faster than that of direct transmission.

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the signal is first decoded to remove noise and then re-encoded it. After re-encoding the signal it is retransmitted to the receiver.

The decoded signal at the relay may be incorrect and if the incorrect signal is transmitted to the destination the decoding at the destination is difficult.

Therefore, this strategy is more appropriate for dedicated relays, which can have higher power and processing capabilities.

According to this, the diversity order of this strategy is one, and the performance is limited by the worst link from the source to relay links and the source to destination links.

The mutual information between the source and destination is limited by that of the worst link.

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Decode-and-forward relaying protocol

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Performance of fixed DF relaying is worse than that of direct transmission.

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Fixed relaying is easy for implementation but the spectral efficiency is lost since half of the bandwidth is allocated to the relay for transmission. The overall rate is reduced especially when the source to the destination channel is good.

In spite of that, the performance of DF relaying is limited by the weakest source-relay and relay-destination channels which make the diversity order to one.

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Issues of fixed relaying protocol

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Adaptive relaying protocols are proposed to overcome these problems. The proposed adaptive relaying protocols comprise two strategies, selective relaying and incremental relaying.

Selective DF relaying Incremental relaying

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Adaptive relaying

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In selective relaying, the relay and the source are assumed to know the channel state between them.

If the SNR of the signal received at the relay exceeds a certain threshold, the relay performs DF operation. If the channel between the source and the relay falls below the threshold, the relay idles.

Selective relaying improves the performance of the DF relay strategy, since the SNR threshold at the relay can be designed to overcome the inherent problem in the fixed DF relaying strategy that the relay forwards all decoded signals to the destination even though some decoded signals are incorrect.

If the SNR in the source-relay link exceeds the threshold, the SNR of the combined MRC signal at the destination is the sum of the received SNR from the source and the relay.

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Selective relaying protocol

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It is assumed that there is a feedback channel from the destination to the relay. The destination feedbacks an acknowledgement to the relay if it was able to receive the source’s message correctly in the first transmission phase and the relay does not need to transmit.

This protocol has the best spectral efficiency among the other protocols due to the fact that relay does not need to transmit always, the second transmission phase becomes opportunistic depending on the channel state on the direct channel between the source and the destination.

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Incremental relaying

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Outage probability of IAF relaying is lower than that of direct transmission. The performance of IAF is better .

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Performances of different relaying protocols

As can be seen from the figure, the performance of IAF relaying still is the best among these four different transmission methods.

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In the future wireless networks, the relay-based communication will be supported.

There are well-placed relay nodes in the networks to process the message from the source node and forward it to the destination node. Especially in the dense wireless networks, there are typically several fixed relay nodes in the region between the source and the destination.

Which of these potential relays should be selected needs to be considered. Meanwhile, the higher bandwidth efficiency has to be achieved and the diversity order cannot be discounted.

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Relay selection

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By using single-relay DF scenario, the question “when to cooperate” can be answered.

The source decides when to cooperate by taking the ratio between the source-destination channel gain and the relay’s metric then comparing it with a threshold, which is referred to as the cooperation threshold.

If the ratio is larger or equal to the cooperation threshold, the source transmits the information directly to the destination without the relay nodes.

Otherwise, the source needs the relay to forward the information to the destination.

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Single-relay DF scenario

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The source-relay channel variance is increased gradually. The symbol error rate decreases with increasing SNR.

The simulated exact SER decrease faster with increasing the source-relay channel variance. This also means that the performance is better if the source-relay channel is stronger.

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Can let us know when to cooperate and which relay to cooperate with.

Assume that there are N relays between the source and the destination and each relay receives the symbols from the previous one, applies MRC on the received output then re-transmits the symbols if the relay decoded the information correctly.

Increasing the number of relays increases the probability of the having the optimal relay is higher.

If there are N relays, it needs N+1 phases and the bandwidth efficiency is 1/(N+1) SPCU.

The aim of the relay-selection is to increase the bandwidth efficiency and also achieve full diversity order.

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Multi-node scenario

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Once the optimal relay is decided then the multi-node relay-selection cooperative system will behaves like a single-relay selection system.

Thus, the source can compare the source-destination channel gain with the metric of the optimal relay to decide whether need to cooperate with the relay or not.

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Multi-node scenario II

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Multi-node scenario III

As can be seen from this figure, the curve slope increases with increasing number of relays. Moreover, the full diversity order is achieved.

According to the upper bound SER curve slope since the exact SER curves are constrained by the upper bounds. This also means that the performance improves faster when the number of relays is increased.

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The advantage of fixed relaying protocol is easy implement action.

The common problem of the fixed relaying protocol is low bandwidth.

For the fixed AF relaying protocol, the primary issue is the noise effect, because the noise is also amplified in the relay when the relay transmission

is adopted.

The performance of fixed DF relaying is limited by the weakest channel.

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Recall the issues of fixed relaying protocol

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The adaptive relaying overcomes the main problems that the fixed relaying protocols have. The performance of adaptive relaying is improved comparing with the fixed relaying.

The performance of selective DF is the same as for fixed AF relaying, it no longer has the inherent problem the fixed DF relaying has due to the threshold scheme. So, it will not be limited by the weakest channel.

The incremental AF relaying is the most efficient relaying protocol among all of the relaying protocols that have been discussed previously. It overcomes the problems that the other relaying protocols have due to the second phase becomes opportunistic depending on the channel state information.

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Recall advantages of adaptive relaying protocols

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The single-relay relay-selection scheme is used to explain when to cooperate with the relay node in order to complete the relay transmission. The performance is better if the source-relay channel is stronger.

the multi-node relay-selection cooperative scenario is the same as that of the single relay scenario but utilizing the optimal relay. The probability of choosing the best relay is higher if the number of candidate relay nodes is increased. Moreover, the performance improves faster when the number of relays is increased.

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Recall relay selection

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For the future works, it is proposed that the SER analysis for different relaying protocols would be studied in depth.

Moreover, the multi-node cooperative communication should be studied and analyzed in the future, because it is more practical than single-relay communication scheme.

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Future works