| A wireless ad hoc network consists of multiple wireless nodes that can move around and communicate without a fixed network infrastructure. The broadcasting nature of the wireless medium allows neighboring nodes to cooperate in information transmission, creating multiple communication routes and offering cooperative diversity for fading mitigation. Cooperative diversity is considered particularly useful in wireless ad hoc and sensor networks, where power/bandwidth/size restrictions of the mobile nodes may prevent use of spatial diversity to combat channel fading.; We examine the modulation and signal processing aspect in cooperative communications. We consider cases when the channel state information (CSI) can be conveniently and accurately estimated as well as when it cannot. CSI estimation, typically performed at the receiving nodes via training, is challenging and costly, especially in fast fading environments. The difficulty is exacerbated in multi-antenna or multi-node wireless systems since the amount of training grows with the number of links. The need for non-coherent or differential modulation techniques, which circumvent CSI estimation, in multi-antenna systems is well recognized. It is imperative to develop similar techniques for cooperative wireless systems since their burden of CSI estimation is even more, severe than multi-antenna systems.; The modulation techniques discussed are distributed in nature since the signal is modulated at the source and, subsequently; at the relays for transmission to the destination. We examine both distributed coherent and distributed differential modulation schemes, which are the counterparts of the conventional non-cooperative coherent and differential modulation techniques. Within each category, we consider non-regenerative schemes involving amplify-and-forward (AF) as well as regenerative schemes involving decode-and-forward (DF) operation performed at the relays. This gives four distributed modulation schemes dubbed as coherent amplify-and-forward (CAF), coherent decode-and-forward (CDF), differential amplify-and-forward (DAF), and differential decode-and-forward (DDF), respectively. We discuss the implementation, detection, and combining issues for each scheme, and analyze the performance in terms of bit error rate (BER), outage probability, and diversity order. We bring out the connections between cooperative systems and non-cooperative multichannel communication systems, and highlight the challenges in the analysis and design of the former imposed by several unique characteristics of wireless relay channels. |
