Spectrum-Efficient Coded Modulation Design for Two-Way Relay Channels

In this paper, we present and analyze spectrum and energy efficient coded modulation schemes for two-way relay channels. Depending on how the side information is utilized, two relay-to-destination decode-and-forward approaches are investigated. The first one, termed joint modulation relaying (JMR), jointly modulates two source messages by applying side information at the modulator and the demodulator. The second one, termed bit-cooperative coded modulation relaying (BCCMR), is a new joint coded modulation scheme for coded relay systems. In particular, a new constellation mapping for the JMR is proposed. The proposed constellation mapping is a many-to-one function and can be considered as a new form of network coding operation, in which the network coding is performed on the redundancy labeling of each constellation point. We establish the corresponding asymptotic optimal constellation labeling criterion by formulating the constellation constrained capacity with side information. For the BCCMR, a novel transceiver structure based on systematic low-density parity-check (LDPC) codes is developed. The joint coding-modulation design introduces the benefits of increased minimum Euclidean distance and the Hamming distance by jointly exploiting side information in the demapping and the decoding. The advantages of the LDPC-coded BCCMR and the JMR are analyzed using the density evolution method.