Wireless Communication Antenna Cooperative Diversity Method

Nowadays, with the development of high data transmission service such as Internet and multimedia,the demands for high speed wireless access have been rapidly increasing. However, high speed communication has two severe challenges in wireless communication environment: one is how to improve the bandwidth efficiency of limited radio spectrum; and the other is how to cope with more severe channels fading, such as frequency selective fading and time selective fading to improve the transmission quality of signals in the high speed communication ,which is also very difficult task .Because it is very difficult to solve the problem by traditional methods, a new approach should be developed to improve the performance and capacity of the wireless communication system. Recent years, a latest technique using multiple antenna elements at both the transmitter and the receiver, called Multiple-Input and Multiple-Output ( MIMO ),has being developed. A large amount of research shows that a MIMO system can communicate in several independent fading channels, the transmission capacity of the system can increase linearly with the number of antennas and the error probability can decrease exponentially with the number of antennas. So more and more attention has been paid to MIMO.A spatial diversity is formed when there is one receive antenna or one transmit antenna. Multiple-path fading is the main factor that effects the system capacity and reliability in wireless communications. Diversity can combine the independent source samples to antagonize fading and symbol interference. Diversity usually can be divided into temporal diversity, frequency diversity and spatial diversity. The main idea of spatial diversity is to use spatial independent information samples transmitted or received to antagonize multiple-path fading and it can be used with other diversity forms. Then we want to apply diversity to wireless communications, but due to the limitation of size that we can not input several antennae in one mobile terminal to achieve diversity order. So a new kind of method called cooperative diversity is proposed. In a cooperative diversity system the terminals form a virtual antenna matrix to achieve diversity order. Research shows that cooperative diversity can be applied with coding to achieve coding gains and with OFDM to antagonize frequency selective fading.First we show that a MIMO system can improve the performance of capacity and error rate compared to a single antenna system, then we introduce the concept of cooperative diversity and compare different cooperative diversity methods. Then we propose two improved cooperative diversity approaches. Based on the Naf and the opportunistic diversity, we make the source transmit during all the coherence interval composed of 2 orthogonal time slots, the proposed method is particularly attractive in sending high-rate data. After theoretic analysis and Montecalo simulation, we compare the proposed approach with the opportunistic diversity and the stc diversity at different data rates, reach the conclusion that when the system transmits at a high data rate, at a certain outage probability, the proposed method can get a SNR gain. Through changing the relay selection rule we improve the multi-node decode-and-forward relaying method. The simulation shows that though it brings more complexity, the SER performance of the system is improved. Last we pay some attention to the research of cooperative diversity methods in OFDM systems. This kind of research is just initialized, has much potential in improving the system performance and needs more work to do.