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A distributed layered modulation for relay downlink cooperative transmission

In downlink relay systems, the conventional straightforward relaying scheme named decode-and-forward (DF) protocol is used at relay station (RS) to decode the data received from base station (BS) and forward the decoded data to the mobile station (MS) to improve the spatial diversity gain. However, conventional DF mode is spectral inefficient and not capacity optimal since the wireless channels in BS-to-RS, BS-to-MS, and RS-to-MS links are asymmetric, i.e., different channel qualities are obtained in different links in the practical scenario. In this paper, a distributed layered-modulation scheme is proposed to improve the DF mode performance in the presence of asymmetric links. With this proposed scheme, BS and/or RS transmit/re-transmit a layered modulated signal comprising a higher-order and a lower-order modulation components according to the different channel conditions in 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 at the destination by exploiting the two copies of transmitted packet from BS and RS, respectively. In order to further enhance the decoding performance, a soft combining method, in which the bit Log Likelihood Ratio (LLR) of the signals that received during the broadcast phase and the relaying phase are combined before turbo decoding, is used at MS receiver. Simulation results show that the proposed distributed layered modulation scheme obtains a significant performance gain in terms of block error rate (BLER) compared to the existing DF relaying scheme.