A Distributed Layered Modulation for Relay Downlink Cooperative Transmission

For downlink relay system, the conventional straight forward scheme named decode-and-forward (DF) protocol is used at relay station (RS) to decode the data from base station (BS) and forward the decoded data to mobile station (MS) to provide spatial diversity gain. However, it is not efficient and optimal since the channels between BS-to-RS, BS-to-MS, and RSto-MS are asymmetric, i.e., different link quality for three links in the practical scenario. In this paper, a distributed layered modulation scheme is proposed to resolve this problem. With this proposed scheme, at BS and RS transmitters, they transmit a layered modulated signal comprising a higher-order modulation and a lower-order modulation component according to the different channel conditions of BS-to-RS and RS-to-MS links, respectively, and, therefore, the channel capacity of each link can be utilized efficiently. In addition, a two-layer demodulation is proposed to obtain the signals by exploiting the two packets from BS and RS. In order to further enhance performance, a soft combining method is used at MS receiver, in which the bit Log Likelihood Ratio (LLR) of the signals during the broadcast phase and the relay phase are combined before turbo decoding. Simulation results show that the proposed distributed layered modulation scheme has a significant gain compared to the existing DF relay.