I. Introduction
Cooperative diversity has emerged as a promising technique to combat multi-path fading in wireless communications. This technique has made it possible for single-antenna users in a local area to form virtual antenna array and to achieve spatial diversity in a distributed way. Reference [1] has firstly introduced this idea into the existing CDMA systems. Various repetition-based cooperation schemes have been proposed in [2]. And the authors in [3] have proposed coded cooperation scheme that integrates cooperation into channel coding. These previous works all base on the basic cooperation model with two users and one destination.