Differential modulation schemes for decode-and-forward cooperative diversity

In this paper, we develop a cooperative diversity scheme that supports decode-and-forward cooperative diversity in the absence of channel state information (CSI) at either user or destination terminals. The proposed scheme employs differential modulation in the broadcast phase, whereas in the relay phase, the information is retransmitted from relay terminals using differential space-time codes. The proposed scheme has a simple structure. In forming a differential space-time code in the relay phase, in addition to the information to be relayed, a relay terminal requires only the portion of the previous codeword transmitted from the same terminal. When different users have different channel quality to the destination, it is pointed out that unitary codes remain the optimum in high signal-to-noise ratio (SNR) scenarios.