Optimal Resource Allocation for Energy-constrained Cooperative Networks
Most of the existing work in cooperative diversity focuses on improving the peer-to-peer link quality in the single-user scenario by using coding or power and rate allocation. In ad-hoc networks, how to efficiently and fairly allocate resources among multiple users and their relays is still unknown. In particular, fairness in the resource allocation process is an important issue that has not been addressed. Usually, a user may regard itself as unfairly treated if its throughput is much lower than others. In cooperative ad-hoc networks, the issue is more complex since unfairness would exist even if all the users achieve a similar throughput. For instance, if some node always acts as a relay but its own throughput is not improved accordingly, it may simply refuse to cooperate. In sensor networks, this means some nodes may consume their power very quickly, which could lead to routing failure and decreased network throughput.
Several cooperative protocols for medium-access control have been proposed in . These symmetric and fixed protocols require that a group of users relay the signals for each other. In cellular networks, where all users transmit to the same destination (the base station), fairness and efficiency can be achieved simultaneously, for example, by carefully grouping the users with similar channel gains. However, in ad-hoc networks, each node may transmit to a different destination. So each node should have its own relay set in order to improve the spectral efficiency. As a result, there will probably be some nodes that have more opportunities to act as relays. As such, their power might be used up more quickly and an unfair situation could then occur.
To address this issue, we proposed a unified cross-layer framework for resource allocation in energy-constrained cooperative networks in . Our objective is to guarantee that the lifetime of each node can be equal to a target lifetime and that the energy used in transmitting and/or relaying each node¡¯s signal is equal to its total energy. Moreover, each node can efficiently use the available energy to optimize its performance such as throughput or outage probability. The proposed framework is applied into cooperative beamforming and selection relaying. Numerical results show that without an appropriate resource allocation, unfair cooperation will result in a significant decrease in the lifetime of heavily-used nodes. In contrast, the proposed framework cannot only guarantee fairness, but also provide significant throughput or diversity gain over the conventional cooperation schemes.
To implement the fair resource allocation, a distributed multiuser cooperative protocol, the Fair Cooperative Protocol (FCP), was proposed in , where a power reward is adopted by each node to evaluate the power contributed to and by others. The analysis also showed that the great improvement in fairness achieved by the proposed FCP can further lead to substantial throughput gains over the direct transmission and full cooperation cases.
 J. N. Laneman and G. W. Wornell, ¡°Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks,¡± IEEE Trans. Inf. Theory, vol. 49, no. 10, pp. 2415-2425, Oct. 2003.
 Wei Chen, Lin Dai, Khaled B. Letaief and Zhigang Cao, "A Unified Cross-layer Framework for Resource Allocation in Cooperative Networks," IEEE Trans. Wireless Commun., vol. 7, no. 8, pp. 3000-3012, Aug. 2008. (won the 2009 IEEE Marconi Prize Paper Award)
 Lin Dai, Wei Chen, Leonard J. Cimini, Jr. and Khaled B. Letaief, "Fairness Improves Throughput in Energy-constrained Cooperative Ad-hoc Networks," to appear in IEEE Trans. Wireless Commun.