Research On The Resource Allocation Algorithms In Cooperative Communication Systems

Cooperative communication is a novel space diversity technology, which has a wide application prospects in the cellular mobile communication systems, ad hoc network, wireless sensor networks and wireless LAN. Differing from traditional single-hop networks, a transmission of cooperative systems is implemented cooperatively by the source and its relays, which determines that the system resource need to be allocated between the source and the relays based on the channel status of the links among the source, the relays and the destination. In cooperative communication systems, power and bandwidth are the primary resources. Besides, the cooperative relays are important resources too, so it is necessary how to choose the relay from enormous users in a cooperative communication system. In addition, with the introduction of OFDM in cooperative systems, the resorce allocation becomes more important and challenging. In OFDM-Based cooperative systems, on the one hand the power need to be allocated not only between the source and the relalys but also among their subcarriers, on the other hand the subcarrier resource also need to be allocated between the source and the relays. Besides the relay selection is more complicated in OFDM-Based cooperative systems. This dissertation researched the resource allocation problems in cooperative systems and proposed several algorithms for narrowband and widthdand OFDM cooperative systems.Firstly, the power allocation problem for coded cooperative systems was considered. Two power allocation strategies of minimization of the power consumption and maximization of the achievable rate were developed in a frequency-division half-duplex coded cooperative system, respectively. Based on these power allocation strategies, the optimal cooperation level was evaluated through simulations. Simulation results show that the performances of power consumption and achievable rate can be improved significantly by adopting the optimal cooperative level and power allocation.Secondly, the power allocation and subcarrier pairing for cooperative OFDM systems were researched and the outage proformance was analysed. Under separate power constraints at the source and the relay, two associated waterfilling algorithms of power allocation for the AF and DF cooperative OFDM systems were proposed respectively. When the source power is equal to the relay, the achievable rate of associated waterfilling algorithms outperforms that of the equal power allocation. If we combined the associated waterfilling power allocation algorithm with the power distribution method, the achievable rate can be further improved. Then two optimal subcarrier pairing methods, i.e. order pairing and reverse order pairing were derived under two extreme conditions of the source being far away the destination and the relay being near the source, respectively. The outage performance of subcarrier pairing with two subcarriers was analysed. The analyses results show that the subcarrier pairing can not improve the diversity gains, but reduce the outage probability.Thirdly, the relay selection problems for OFDM-based opportunistic cooperative systems and multi-relay selective cooperative systems were studied. The outage probabilities of the two cooperative systems were derived respectively. The results show that they can all possess full diversity gains, and the outage performance of multi-relay selective cooperative system is superior to that of the opportunistic cooperative systems. A single-relay selection algorithm based on the number of the best or the worst subcarrier pairing at each relay for opportunistic cooperative OFDM systems was proposed. When there are many subcarriers and few relay, the performance of the proposed algorithm can be compared to that of the exhausting algorithm. Then a multi-relay selection algorithm based on subcarrier pairing was developed for multi-relay selective cooperative OFDM systems, which can improve the achievable rate by allocating appropriate subcarrier to the relays.Finally, the OFDMA technology was introduced into cooperative systems and the subcarrier allocation was researched. The dissertation proposed two frame structures with two cooperative users respectively for the two occasions that only one user and two users all have data to transmit. Then two subcarrier alloction algorithms based on round robin algorithm and greedy algorithm for repeat forward cooperative systems and coded cooperative systems, repectively. The simulations results show that cooperative OFDMA system can improve the system achievable rate greatly through rational subcarrier allocation between the source and the relay.