Research On Channel Estimation And Multiuser Access For MC-CDMA Modulated Two-Way Relay Systems

Two-way relay technique, applying physical layer network coding (PLNC), possesses all the advantages that relay technique has, which include providing wireless diversity gain, and improving the reliability of wireless transmission. Besides, compared with one-way relay, two-way relay has a higher system capacity without sacrificing extra hardware cost and channel resource, which makes two-way relay attract many researchers’interests.In this work, we focus our interests on the signal processing of physical layer of two-way relay, including channel estimation and multiuser access.As a key technique of wireless physical layer, channel estimation technique is first introduced in this work. We first summarize two basic channel estimation algorithms of OFDM modulated two-way relay systems, and then we propose a code-aided maximum likelihood iterative algorithm for two-way relay multicarrier code division multiple access (MC-CDMA) systems. The core idea of this iterative receiver is exploiting the soft information from decoder to help channel estimation, so that the training cost can be cut down to improve the frequency efficiency. In addition, MC-CDMA modulation discussed in this work can separate the signal from two terminals, which benefits data detection, and then increases the convergence speed. This iterative algorithm has been discussed both in single antenna and multiple antenna scenarios in this work.Multiuser access is needed in most wireless communication systems. In order to deal with the interference caused by multiuser access in two-way relay systems, we propose a multiuser access method which is suitable for MC-CDMA modulated two-way relay systems. This method utilizes the CDMA nature of MC-CDMA modulation, which supports multiuser access by allocating every terminal a spread sequence which is orthogonal with each other. This is in perfect accordance with PLNC of symbol superposition when synchronization is ideal. Also, terminals’ pre-equalization and diversity receiver are discussed to combat frequency selective fading channels. In this work, three diversity receive schemes are introduced. They are maximal ratio combining (MRC), equal gain combining (EGC), orthogonality restoring combining (ORC). And their performance is analyzed by numerical simulation.At the end of this work, we propose some ideas for further research.