The Practical Challenges of Interference Alignment

Interference alignment (IA) is a revolutionary wireless transmission strategy that reduces the impact of interference. The idea of interference alignment is to coordinate multiple transmitters so that their mutual interference aligns at the receivers, facilitating simple interference cancellation techniques. Since IA's inception, researchers have investigated its performance and proposed improvements, verifying IA's ability to achieve the maximum degrees of freedom (an approximation of sum capacity) in a variety of settings, developing algorithms for determining alignment solutions, and generalizing transmission strategies that relax the need for perfect alignment but yield better performance. This article provides an overview of the concept of interference alignment as well as an assessment of practical issues including performance in realistic propagation environments, the role of channel state information at the transmitter, and the practicality of interference alignment in large networks.Comment: submitted to IEEE Wireless Communications Magazin

Wireless transmission protocols using relays for broadcast and information exchange channels

Relays have been used to overcome existing network performance bottlenecks in meeting the growing demand for large bandwidth and high quality of service (QoS) in wireless networks. This thesis proposes several wireless transmission protocols using relays in practical multi-user broadcast and information exchange channels. The main theme is to demonstrate that efficient use of relays provides an additional dimension to improve reliability, throughput, power efficiency and secrecy. First, a spectrally efficient cooperative transmission protocol is proposed for the multiple-input and singleoutput (MISO) broadcast channel to improve the reliability of wireless transmission. The proposed protocol mitigates co-channel interference and provides another dimension to improve the diversity gain. Analytical and simulation results show that outage probability and the diversity and multiplexing tradeoff of the proposed cooperative protocol outperforms the non-cooperative scheme. Second, a two-way relaying protocol is proposed for the multi-pair, two-way relaying channel to improve the throughput and reliability. The proposed protocol enables both the users and the relay to participate in interference cancellation. Several beamforming schemes are proposed for the multi-antenna relay. Analytical and simulation results reveal that the proposed protocol delivers significant improvements in ergodic capacity, outage probability and the diversity and multiplexing tradeoff if compared to existing schemes. Third, a joint beamforming and power management scheme is proposed for multiple-input and multiple-output (MIMO) two-way relaying channel to improve the sum-rate. Network power allocation and power control optimisation problems are formulated and solved using convex optimisation techniques. Simulation results verify that the proposed scheme delivers better sum-rate or consumes lower power when compared to existing schemes. Fourth, two-way secrecy schemes which combine one-time pad and wiretap coding are proposed for the scalar broadcast channel to improve secrecy rate. The proposed schemes utilise the channel reciprocity and employ relays to forward secret messages. Analytical and simulation results reveal that the proposed schemes are able to achieve positive secrecy rates even when the number of users is large. All of these new wireless transmission protocols help to realise better throughput, reliability, power efficiency and secrecy for wireless broadcast and information exchange channels through the efficient use of relays

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

Vandermonde-subspace Frequency Division Multiplexing for Two-Tiered Cognitive Radio Networks

Vandermonde-subspace frequency division multiplexing (VFDM) is an overlay spectrum sharing technique for cognitive radio. VFDM makes use of a precoder based on a Vandermonde structure to transmit information over a secondary system, while keeping an orthogonal frequency division multiplexing (OFDM)-based primary system interference-free. To do so, VFDM exploits frequency selectivity and the use of cyclic prefixes by the primary system. Herein, a global view of VFDM is presented, including also practical aspects such as linear receivers and the impact of channel estimation. We show that VFDM provides a spectral efficiency increase of up to 1 bps/Hz over cognitive radio systems based on unused band detection. We also present some key design parameters for its future implementation and a feasible channel estimation protocol. Finally we show that, even when some of the theoretical assumptions are relaxed, VFDM provides non-negligible rates while protecting the primary system.Comment: 9 pages, accepted for publication in IEEE Transactions on Communication