Resource Optimization And Cooperation Strategies In Wireless Relay System

The aspiration of communicating anywhere at any time boosts the evolution of wireless network. The relay based wireless networks, such as ad-hoc, mesh, have solved many problems that exist in the traditional wireless networks, e.g. lack of expansibility and robustness, and thus the researches on these systems become flourishing. Cooperative diversity, as an efficient approach of enhancing channel capacity and reliability, is a newborn technique under this prosperity. To further improve the transmission rate, resource optimization at the data link layer is vital. Different from single-hop networks, resource optimization in relay system must take the speciality of multi-hop into account and assign resources suitably among the hops. In view of the frequency selective fading of wireless channel in broadband systems, Orthogonal Frequency Division Multiplexing (OFDM) will be widely adopted in the future communication systems. The problem of resource optimization becomes more complicated in OFDM based wireless relay system where the subcarrier and power resources are required to be jointly allocated to each hop for the maximization of system throughput. Since researches in this area are not mature, we take the problem of resource optimization in OFDM based wireless relay system as a challenge of this dissertation.The transmission of relay system is accomplished by the cooperation among nodes; hence cooperation strategy is crucial to the system performance. Cooperation strategy comprises selection of cooperation mode and cooperative partner; the former one determines the cooperation condition and pattern, and the latter one determines the cooperation object. While designing cooperation strategy, system throughput as well as fairness and performances of nodes are required to be comprehensively considered due to the current application instance. Cooperative strategy is of great significance for its tremendous influence on the link transmission rate and the entire system throughput. Therefore, the other challenge of this dissertation is to propose effective cooperation strategies in wireless relay system.OFDM based wireless relay system is the object of our research on wirelessresource optimization. Firstly, the problem of subcarrier and power allocation in regenerative multi-hop system is studied. In order to heighten transmission rate and reliability, three resource allocation strategies based on greedy algorithm are proposed for ergodic channel, which enhance the link capacity by effectively assigning resources. On non-ergodic channel condition, in order to maximize instantaneous transmission rate, two adaptive resource optimization algorithms are presented for OFDM-TDMA and OFDM-FDMA manners respectively. Both two algorithms improve the link throughput comparing with fixed resource allocation strategy. Secondly, the problems of power allocation and subcarrier pairing are addressed in the relay system with cooperative diversity. The problem is divided into four types according to the power limitation and the channel information knowledge at nodes. Given the subcarrier pairing result, optimal power allocation algorithms are proposed for those four types. Combining Hungary algorithm, a joint power allocation and subcarrier pairing scheme is presented, which boosts the channel capacity effectively.The researches on cooperation strategy in this dissertation is based on general wireless cooperative relay system. Firstly, partial channel state information based cooperation-strategy is studied. A selection relaying strategy which aims at minimizing link outage probability is proposed. Since only the geographical information of nodes are required in the proposed strategy, heavy and complicated channel estimation is avoided. Further, this strategy is extended into large-scale networks, we propose a min-max criteria based partner selection scheme that can successfully achieve the goal of minimizing the maximum outage probability among all the nodes. Secondly, the game theory method of cooperation strategy is studied. A system model constructed by two peer to peer users and one access point is presented, in which each node is source as well as potential relay to the other. Then, a cooperative bandwidth allocation strategy based on Nash bargaining solution is proposed, which effectively increases the utility of user and the entire system.