Introduction

Recently, cooperative communications have gained attention as an emerging transmit strategy for future wireless networks. Cooperative communications efficiently take advantage of the broadcasting nature of wireless networks. The basic idea is that users or nodes in a wireless network share their information and transmit cooperatively as a virtual antenna array, thus providing diversity that can significantly improve system performance. In cooperative transmission, relays are assigned to help a sender in forwarding its information to its receiver. Thus the receiver gets several copies of the same information via independent channels.

The pioneer work on cooperative transmission can be found, e.g., in [53], where a general information theoretical framework about relaying channels is established. In [272, 273], a CDMA-based two-user cooperative modulation scheme has been proposed. The main idea is to allow each user to retransmit estimates of their partner's received information such that each user's information is transmitted to the receiver at the highest possible rate. This work is extended in [175], where the outage and the ergodic capacity behavior of various cooperative protocols, e.g., decode-and-forward (DF) and amplify-and-forward (AF) cooperative protocols, are analyzed for a three-user case under quasi-static fading channels. In [298], the authors provided SER performance analysis and optimum power allocation for DF cooperative systems in a narrowband Rayleigh fading environment. The work in [138] analyzes the schemes based on the same channel without fading, but with more complicated transmitter cooperative schemes involving dirty paper coding. In [199], a cooperative broadcast strategy was proposed with an objective of maximizing the network lifetime.