Asynchronous Space-Time Cooperative Communications In Wireless Relay Systems

With the rapid development of wireless communication technology and mobile terminal technology,high-speed data services have been on the rise,which makes the research on capacity of communication systems and reliability of data transmission attract considerable attention.Since relay cooperative technique can enhance communication capacity,improve transmission quality and increase the service coverage,it has become one of the key techniques in the fourth generation(4G)mobile communication system,and plays a vital role on the evolution of mobile network architecture in the fifth generation(5G)mobile communication system.The most distinctive characteristic of cooperative communication systems is its distributed nature: multiple nodes in cooperation are spatially separated in several different physical locations,resulting in the problem of time asynchronization.If the relay nodes forward signals by using space-time cooperative technique,the space-time structure of the cooperative signals received at the destination node will be destroyed,which degrades the performance of data transmission deeply.To resolve the problem,asynchronous transmission schemes based relay space-time cooperative technique are analyzed in this paper.The main contents are as follows:Firstly,for two-relay cooperative systems with the amplify-and-forward(AF)protocol,an improved scheme based on the existing Alamouti asynchronous space-time cooperative scheme is proposed.The improved scheme was obtained by combining traditional orthogonal frequency division multiplexing(OFDM)technique with the classical Alamouti coding for an asynchronous cooperative system,where double OFDM symbols were broadcasted at the source node,and amplification and forwarding were implemented at the relay nodes,and therefore the received signals at the destination node have the Alamouti code structure.Compared with the existing scheme,the improved scheme has low processing complexity at the relay nodes,and can achieve higher diversity gain,thus improving the bit error rate(BER)performance of system.Secondly,for multi-relay cooperative systems with AF protocol,in order to solve the loss on transmission rate caused by using orthogonal space-time block coding(OSTBC)at relay nodes,two improved schemes are proposed: asynchronous space-time cooperative scheme based quasi-orthogonal space-time block coding(QOSTBC)and constellation rotation QOSTBC.In the asynchronous space-time cooperative scheme based QOSTBC,the received signals were amplified and forwarded with the structure of the QOSTBC,and therefore full transmission rate can be attained.Moreover,the BER performance of the scheme is better than the existing scheme at intermediate and low Signal-to-Noise Ratio(SNR)under high speed transmission environment.However,the improved scheme based QOSTBC can not achieve full diversity gain.To overcome this drawback,by introducing constellation rotation when symbols were modulated at the source node,the received signals at the destination node has the structure of constellation rotation QOSTBC,and therefore asynchronous space-time cooperative scheme based constellation rotation QOSTBC is proposed.Compared with the improved scheme based QOSTBC,the scheme based constellation rotation QOSTBC can achieve full diversity gain while maintaining full transmission rate,leading to substantially improved BER performance.