Fundamental Limits of Two-pair Two-Way Relay Channels

• Other Authors: Xin, Haiyang (author.)
• Format: Others
• Language: English; Chinese
• Published: 2016
• Online Access: http://repository.lib.cuhk.edu.hk/en/item/cuhk-1292194

In this thesis, we study the fundamental limits of the multipair two-way relay channel (TWRC) system, consisting of one relay and multiple pairs of users. Each user only exchanges information with the other user of the same pair via help of the relay. The information exchange consists of two phases. In the uplink phase, all the users simultaneously transmit signals to the relay; in the downlink phase, the relay broadcasts the processed signals to all the users. For simplicity and concreteness, we focuses on the multipair TWRC system with two pairs; the general principles expounded here can be extended to the case of more pairs. === The first part of this thesis investigates the downlink transmission phase. For the downlink phase, each user is only interested in its partner’s message and treats the message for the other pair as interference. Suppose that the relay encodes messages for user pairs sequentially, i.e., message for one pair followed by message for the other pair. Then, the first message becomes noncausal interference for the remaining message, i.e., the encoder knows such interference prior to the encoding of the remaining message. Hence, the downlink transmission can be modeled as a broadcast channel with noncausal interference at the transmitter (i.e., the relay) and side information at the receivers (i.e., the users). We first obtain an achievable rate region of this system by extending Gelfand and Pinsker’s (GP’s) random binning method. We then apply the result to Gaussian scalar and vector channels, and consider the design of the auxiliary random variable used in the GP’s approach. Asymptotic analysis shows that our scheme is asymptotically capacityachieving at high signal-to-noise ratio (SNR) for both scalar and vector channels. We further investigate the system optimization in the finite SNR regime. Numerical results === show that our scheme can achieve a sum-rate close to the cut-set bound over a wide SNR range, and significantly outperforms the schemes that do not exploit the knowledge of noncausal interference at the transmitter. === The second part of this thesis studies the capacity of the Gaussian two-pair TWRC system, where each node is equipped with single antenna. For the Gaussian TWRC, the noises at the relay and the user nodes are Gaussian and independent of each other. Our main contribution is a message-reassembling strategy to decouple the coding design for the uplink and downlink transmissions. This strategy provides flexibility to fully exploit the channel randomness of both uplink and downlink. For the uplink transmission, each user transmits a superposition of Gaussian random codeword and nested lattice codeword, and the relay employs successive interference cancellation to decode signals. For the downlink transmission, the relay first reassembles and re-encodes messages, and then transmits a superposition of the resulting codewords. Finally, each user decodes the other user’s message of the same pair with the help of self-information. The analytical results show that the capacity region is achievable to within 1/2 bit per user. === The third part of this thesis extends our studies to the Gaussian two-pair multiinput multi-output (MIMO) TWRC system, where each node is equipped with multiple antennas. In the uplink phase, the relay’s received signal space is divided into two subspaces for each pair: in one subspace, the signals of the two users are near-parallel, and physical-layer network coding (PNC) decoding is applied to retrieve network-coded messages; in the other subspace, the two user signals are near-orthogonal, and complete decoding (CD) is applied to retrieve individual user messages. In the downlink phase, the relay’s transmit signal space is split into orthogonal subspaces for each pair, and in each subspace a common message is broadcasted to the two users. Our main contribution is to establish a critical link between the transmission scheme and the concept of principal angle in linear algebra. In particular, principal angle provides a way to quantify the degree of orthogonality between the user signals and helps determining how to partition the relay’s received signal space into PNC decoding subspace and CD decoding subspace. Building on that, we provide a near-optimal design of user/relay precoders for both uplink and downlink phases to maximize the asymptotic sum-rate of our scheme at high signal-tonoise ratio (SNR). The analysis and numerical results indicate that our scheme performs close to the cut-set bound and significantly outperforms existing schemes in the literature. === Xin, Haiyang. === Thesis Ph.D. Chinese University of Hong Kong 2016. === Includes bibliographical references (leaves ). === Abstracts also in Chinese. === Title from PDF title page (viewed on …).