Cooperative Communication Systems Using Distributed Turbo Coding
Rui Lin · UC Research Repository (University of Canterbury) · 2011
Cooperative and relay wireless communication networks use one or more distinct wire- less nodes as relays to combat performance impairments, which include signal atten- uation due to power loss over long distances and the signal power fluctuation due to multi-path fading. The second one is the primary focus of this thesis. Diversity is an effective weapon to combat the fading effects and using multiple antennas at both the transmitter and receiver is a way of achieving it. However, using multiple antennas may not be practical for some mobile terminals due to the limited physical size, battery power and computational capability of these mobile terminals. Therefore, cooperative schemes have been proposed in which several wireless nodes cooperate with each other to achieve diversity even though each node has only one an- tenna. Many cooperative schemes have been proposed including Amplify-and-Forward (AF) and Decode-and-Forward (DF) [1] to achieve this diversity. This thesis primarily proposes and studies a DF scheme. For the DF schemes, imperfect decoding at the relay is one of the major limitations to the achievable performance. To address this problem and yet remain simple to implement, a novel DF based cooperative scheme is proposed in this thesis. This proposed scheme uses a distributed Turbo coding (DTC) scheme and is resilient to relay errors. It is analyzed by using the approximate Gaussian density evolution method. The analysis shows how the iterative Turbo decoding performs with different bit error rates (BERs) at the relay. Based on the results, a novel scheme, which adaptively changes the code used at the relay, is then proposed to further improve performance. The diversity offered by the proposed scheme is achieved at the cost of decreased spectral efficiency compared to a non-cooperative network. To recover some of the spectral efficiency loss due to cooperation, the proposed single source node cooperative scheme is extended to a two source nodes (users) scheme. The relay node forwards a combined packet which is formed by combining multiple users' information packets into a single packet. The results show that combining multiple users' information packets using superposition modulation can achieve better performance than that of using "XOR" operation (network coding) when two users have different channel qualities, which is very common in a wireless communication environment.