Partner Selection Methods For Multi-Antenna Terminals In Wireless Cooperative Systems

Cooperative transmission, as a newly arisen transmission technology, lets the antennas be shared between users, making a virtual Multiple-Input-Multiple-Output system. It adequately utilizes both the network and the spcial resources, obtains diversity gains without any complexity increase. Thus multi-path fading could be efficiently suppressed and the system performance is highly improved. Now it is gaining popularity in the research of wireless access.Based on deep understanding of the conventional cooperative models, typical transmission schemes as well as cooperative protocol and partner selection algorithms, this paper considers the multi-antenna cooperative scenario, where all the nodes are equipped with multiple antennas.First, we construct the cooperative system model in multi-antenna setup, where Zero-forcing technology is used at both the source and the relay, and Maximum Ratio Combining technology at the destination. We present expressions for signals at each node and we also provide the basic partner selection progress. Besides, possible approaches for partner selection are presented and analyzed.Second, we extend the use of selecting Decode-and-Forward protocol into the multi-antenna setup. We give the detailed communication progress. And we derive the expressions of the outage probability as well as diversity order. We also provide the simulation results to verify previous analysis. By doing this, we get the relations of diversity order versus the transmit/receive antenna number and diversity order versus the possible partner number.Then, to solve the problems lying within the selecting DF protocol under multi-antenna condition, we propose a improved scheme based on cooperative threshold and the harmonic function of the channel parameters. By adopting a predefined threshold, the source decides to cooperate only when it needs help. The rationale behind this is that there is no need for the relay to forward the source's information if the direct link, between the source and destination, is of high quality. The source picks only one relay to cooperate with, and the optimal relay is the one which has the maximum instantaneous value of the relay's metric, which is a modified version of the harmonic mean function of its source-relay (S-R) and relay-destination (R-D) channel gains. We also presented some simulation results to verify the proposed relay selection scheme. It is confirmed that the proposed cooperative scheme shows better outage performance than the conventional DF protocol. The new selection criterion combines both the S-R link and the R-D link, which avoids adopting cooperation when the S-R channel is in good condition while the R-D is quite not. Meantime, the introduction of the cooperation threshold prevents unnecessary cooperation when the direct path is of high quality.