Browsing by Subject "Multiuser systems"

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    Code design for interference channels
    (2019-11) Herfeh, Mahdi Shakiba
    As the number of wireless devices dramatically increases, they experience more interference in their communications. As a result, managing interference in wireless networks is an important challenge in future wireless communication systems, which can be tackled in di erent layers of communications. Designing good channel codes, which can enable reliable communication close to the information theoretic limits in the presence of interference, is one of the ways to increase the quality of service. With the above motivation, in this research, we focus on code design for interference channels (ICs). We, rst consider classical two-user fading IC and study implementation of di erent encoding/decoding schemes with low-density paritycheck (LDPC) codes for both quasi-static and fast fading scenarios. We adopt the Han-Kobayashi (HK) type encoding, derive stability conditions on the degree distributions of LDPC code ensembles, and obtain explicit and practical code designs. In order to estimate the decoding thresholds, a modi ed form of the extrinsic information transfer (EXIT) chart analysis based on binary erasure channel (BEC) approximation for the incoming messages from the component LDPC decoders to state nodes is developed. The proposed code design is employed in several examples for both fast and quasi-static fading cases. A comprehensive set of examples demonstrates that the designed codes perform close to the achievable information theoretic limits. Furthermore, multiple antenna transmissions employing the Alamouti scheme for fading ICs are studied; a special receiver structure is developed, and speci c codes are explored. Finally, advantages of the designed codes over point-to-point (P2P) optimal ones are demonstrated via both asymptotic and nite block length simulations. Next, we consider cognitive interference channels (CICs), a variant of classical two-user ICs in which one of the transmitters (cognitive transmitter) has non-causal knowledge of the other's (private user's) message. Prompted by the information theoretical results, we design an explicit coding scheme for CIC in the primary decodes cognitive regime. We present a novel joint decoder and design LDPC codes for our set-up. Simulation results demonstrate that the proposed joint decoder and the designed codes outperform the conventional maximum ratio combining type decoder and the point-to-point optimal codes, respectively. Later, we propose and evaluate the idea of exible modulation for P2P communication with available channel side information at the transmitter. This technique does not perform as well as dirty paper coding (DPC); however, its simplicity is a major advantage. Also the exible modulation technique shows more robustness to inaccuracy in the channel state information. Finally, we consider a multiple access channel (MAC) in which the non-causal knowledge of one of the users' message is available at the other user. We consider both Gaussian channel and fading scenarios. We propose the idea of joint encoding, and study its performance via simulations demonstrating that the proposed approach outperforms the classical coding scheme.