Power Allocation in Wireless Relay Networks

This thesis is mainly concerned with power allocation issues in wireless relay networks where a single or multiple relays assist transmission from a single or multiple sources to a destination. First, a network model with a single source and multiple relays is considered, in which both cases of orthogonal and non--orthogonal relaying are investigated. For the case of orthogonal relaying, two power allocation schemes corresponding to two partial channel state information (CSI) assumptions are proposed. Given the lack of full and perfect CSI, appropriate signal processing at the relays and/or destination is also developed. The performance behavior of the system with power allocation between the source and the relays is also analyzed. For the case of non-orthogonal relaying, it is demonstrated that optimal power allocation is not sufficiently effective. Instead, a relay beamforming scheme is proposed. A comprehensive comparison between the orthogonal relaying with power allocation scheme and the non-orthogonal relaying with beamforming scheme is then carried out, which reveals several interesting conclusions with respect to both error performance and system throughput. In the second part of the thesis, a network model with multiple sources and a single relay is considered. The transmission model is applicable for uplink channels in cellular mobile systems in which multiple mobile terminals communicate with the base station with the help of a single relay station. Single-carrier frequency division multiple access (SC-FDMA) technique with frequency domain equalization is adopted in order to avoid the amplification of the multiple access interference at the relay. Minimizing the transmit power at the relay and optimizing the fairness among the sources in terms of throughput are the two objectives considered in implementing power allocation schemes. The problems are visualized as water-filling and water-discharging models and two optimal power allocation schemes are proposed, accordingly. Finally, the last part of the thesis is extended to a network model with multiple sources and multiple relays. The orthogonal multiple access technique is employed in order to avoid multiple access interference. Proposed is a joint optimal beamforming and power allocation scheme in which an alternative optimization technique is applied to deal with the non-convexity of the power allocation problem. Furthermore, recognizing the high complexity and large overhead information exchange when the number of sources and relays increases, a relay selection scheme is proposed. Since each source is supported by at most one relay, the feedback information from the destination to each relay can be significantly reduced. Using an equal power allocation scheme, relay selection is still an NP-hard combinatorial optimization problem. Nevertheless, the proposed sub-optimal scheme yields a comparable performance with a much lower computational complexity and can be well suited for practical systems

Power allocation in MMSE relaying over frequency-selective rayleigh fading channels

This paper develops an amplify-and-forward relaying scheme for multiuser wireless cooperative networks under frequency-selective block-fading. Single-carrier frequency division multiple-access with frequency-domain equalization technique is employed at both the relay and destination to combat the inter-block and inter-symbol interference caused by multipath propagation. With the assumption that the full channel state information (CSI) is available at the destination, the relay only knows the uplink channels while no CSI is available at the sources, two power allocation schemes are developed for the relay: (i) to minimize the total transmit power at the relay while maintaining the signal-to-interference-plus-noise ratio (SINR) for each user at the destination above a certain level, and (ii) to maximize the worst SINR among all the users subject to a constraint on total relay power. In the first problem, it is shown that SINR adaptation is needed not only to guarantee a feasible solution but also to significantly reduce the transmit power at the relay for certain channel conditions. In the second problem, a flexible multi-level water-filling scheme is developed, which can be easily modified to adapt to the different channel conditions as well as different quality-of-service provision strategies. © 2006 IEEE

Two–Way Relaying Communications with OFDM and BICM/BICM-ID

Relay-aided communication methods have gained strong interests in academic community and been applied in various wireless communication scenarios. Among different techniques in relay-aided communication system, two-way relaying communication (TWRC) achieves the highest spectral efficiency due to its bi-directional transmission capability. Nevertheless, different from the conventional point-to-point communication system, TWRC suffers from detection quality degradation caused by the multiple-access interference (MAI). In addition, because of the propagation characteristics of wireless channels, fading and multipath dispersion also contribute strongly to detection errors. Therefore, this thesis is mainly concerned with designing transmission and detection schemes to provide good detection quality of TWRC while taking into account the negative impacts of fading, multipath dispersion and multiple-access interference. First, a TWRC system operating over multipath fading channels is considered and orthogonal frequency-division multiplexing (OFDM) is adopted to handle the inter-symbol interference (ISI) caused by the multipath dispersion. In particular, adaptive physical-layer network coding (PNC) is employed to address the MAI issue. By analyzing the detection error probability, various adaptive PNC schemes are discussed for using with OFDM and the scheme achieving the best trade-off among performance, overhead and complexity is suggested. In the second part of the thesis, the design of distributed precoding in TWRC using OFDM under multipath fading channels is studied. The objective is to design a distributed precoding scheme which can alleviate MAI and achieve multipath diversity to combat fading. Specifically, three types of errors are introduced when analyzing the error probability in the multiple access (MA) phase. Through analysis and simulation, the scheme that performs precoding in both time and frequency domains is demonstrated to achieve the maximum diversity gains under all types of errors. Finally, the last part of the thesis examines a communication system incorporating forward error correction (FEC) codes. Specifically, bit-interleaved code modulation (BICM) without and with iterative decoding (BICM-ID) are investigated in a TWRC system. Distributed linear constellation precoding (DLCP) is applied to handle MAI and the design of DLCP in a TWRC system using BICM/BICM-ID is discussed. Taking into account the multiple access channel from the terminal nodes to the relay node, decoding based on the quaternary code representation is introduced. Several error probability bounds are derived to aid in the design of DLCP. Based on these bounds, optimal parameters of DLCP are obtained through analysis and computer search. It is also found that, by combining XORbased network coding with successful iterative decoding, the MAI is eliminated and thus DLCP is not required in a BICM-ID system