Research Of Combining Techniques On Broadband Mobile Cooperative Communications

As the key techniques of broadband mobile communication systems, Multiple-Input Multiple-Output(MIMO) and cooperation technique can be joined for combating fading, realizing high-efficient transmission and increasing boosting system performance. In cooperative communications, using combining technique can achieve diversity gains without extra feedback links and changing present system frame and protocols, which play a key role in theory research and applications. However, there are many factors which call for high-quality combining technique. On the one hand, with the increase of bandwidth and motion, the distortions of uncombined signals become more serious due to the fading of wireless channel. On the other hand, the dimensions of uncombined signals increase rapidly because of applications MIMO technique. In this thesis, combining technique of broadband mobile cooperative communications will be focus on. The main work is organised as follows:Firstly, the basic structures and cooperative protocols are presented in detail. The combining algorithms of Single Input Single Output(SISO) relay systems with Amplify and Forward(AF) and Decode and Forward(DF) protocols are explored, which is the basis of combining algorithms’ study in Multi-Input Multi-Output(MIMO) relay systems. Simulations are carried out to compare the identities of different algorithms.Secondly, a combining algorithm based on Maximum Likelihood(ML) rule is proposed for AF MIMO relay system. Take the advantage of Cholesky decomposition, multi-path signals is combined into traditional MIMO detected form. Compared with the mostly used vector expand combining algorithm, the proposed algorithm achieves prominence performance gains with lower detected complexity in high-level modulation. An upper bound of SER for the proposed algorithm is also derived, which gives a reference for system design.Thirdly, a low-detected dimension ML combining(LDD-MLC) algorithm is proposed for DF MIMO relay systems. With the proper approximation of relay detected error, the proposed algorithm performs an equilibrated process of detected error distance, reducing the detected searching complexity at the cost of small performance loss.Finally, aiming at increasing the communication quality under broadband mobile condition, an OFDM-MIMO relay system model and two combining strategies are proposed. C-MLC and LDD-MLC algorithms used in MIMO systems are transplanted into the OFDM-MIMO relay systems. Simulations show that LDD-MLC strategy brings evident performance gains, while C-MLC strategy can only ameliorate the performance in special channel conditions.