Superposition coding and modulation technique for wireless relay systems

Abstract

Cooperative communication has been considered a promising technique used in wireless network to introduce diversity. Based on distributed architecture, cooperative network enables single-antenna users to share their information as a virtual multiple antenna array, which makes it possible to enhance the user data rates and obtain the diversity gain. Depending on the ways users share and process the information, practical user cooperation is mainly grouped in three modes: Amplify and Forward (AF), Decode and Forward (DF) and Compress and Forward (CF). Generally, DF outperforms AF in most practical implementation and is alternately the best by theoretical analysis. In particular, it is shown in theory that DF outperforms CF when the inter-user link is in good condition and vice versa. Considered as a capacity achieving approach, superposition encoding technique produces approximate Gaussian output signals. It is the power and spectral efficiency of the encoding scheme that make it useful for several wireless communication systems such as MIMO system, relay channels, and so on. The main goal of the thesis is to study how much superposition coding can help when applied to DF user cooperation. In particular, a new DF scheme with soft information relaying technique is proposed to overcome the performance degradation happening when the quality of the inter-user channel is bad. Based on the scheme, soft log-likelihood ratio (LLR) of the relay decoder is quantized, encoded, modulated by superimposition and then forwarded to the destination. The destination receiver employs an iterative superposition decoder and modified maximum a posteriori probability (MAP) decoder to recover the transmitted signal from the source. The performance of the scheme is evaluated by simulation. We show that the new scheme can significantly outperform the conventional DF scheme even in a poor inter-user channel. Furthermore, a new cooperative transmission strategy based on superposition and multiuser detection techniques is also investigated in the thesis. The new scheme is an improved DF scheme in which the two users take turns in being the relay for each other and the signal forwarded by each user is the superimposed data of both users. Here, interleavers are introduced in the superposition process as an efficient user separation tool, providing a large improvement in error performance and also facilitating the decoding process at the destination. Different types of iterative receivers are examined. The performance of the scheme is evaluated by simulation. It is shown that the new transmission scheme outperforms the conventional one in terms of both spectral and power efficiency.