Hybrid Processing Design for Multipair Massive MIMO Relaying with Channel Spatial Correlation

Massive multiple-input multiple-output (MIMO) avails of simple transceiver design which can tackle many drawbacks of relay systems in terms of complicated signal processing, latency, and noise amplification. However, the cost and circuit complexity of having one radio frequency (RF) chain dedicated to each antenna element are prohibitive in practice. In this paper, we address this critical issue in amplify-and-forward (AF) relay systems using a hybrid analog and digital (A/D) transceiver structure. More specifically, leveraging the channel long-term properties, we design the analog beamformer which aims to minimize the channel estimation error and remain invariant over a long timescale. Then, the beamforming is completed by simple digital signal processing, i.e., maximum ratio combining/maximum ratio transmission (MRC/MRT) or zero-forcing (ZF) in the baseband domain. We present analytical bounds on the achievable spectral efficiency taking into account the spatial correlation and imperfect channel state information at the relay station. Our analytical results reveal that the hybrid A/D structure with ZF digital processor exploits spatial correlation and offers a higher spectral efficiency compared to the hybrid A/D structure with MRC/MRT scheme. Our numerical results showcase that the hybrid A/D beamforming design captures nearly 95% of the spectral efficiency of a fully digital AF relaying topology even by removing half of the RF chains. It is also shown that the hybrid A/D structure is robust to coarse quantization, and even with 2-bit resolution, the system can achieve more than 93% of the spectral efficiency offered by the same hybrid A/D topology with infinite resolution phase shifters.Comment: 17 pages, 13 figures, to appear in IEEE Transactions on Communication

Energy E fficiency Oriented Full Duplex Wireless Communication Systems

Full-duplex (FD) transmission is a promising technique for fifth generation (5G) wireless communications, enabling significant spectral efficiency (SE) improvement over existing half-duplex (HD) systems. However, FD transmission consumes higher power than HD transmission, especially for millimetre wave band. Therefore, energy efficiency (EE) for FD systems is a critical yet inadequately addressed issue. This thesis addresses the critical EE challenges and demonstrates promising solutions for implementing FD systems, as detailed in the following contributions. In the first contribution, a comprehensive EE analysis of the FD and HD amplify-and-forward (AF) relay-assisted 60 GHz dual-hop indoor wireless systems is presented. An opportunistic relay mode selection scheme is developed, where FD relay with different self-interference (SIC) techniques or HD relay is opportunistically selected. Together with transmission power adaptation, EE is maximised with given channel gains. A counter-intuitive finding is shown that, with a relatively loose maximum transmission power constraint, FD relay with two-stage SIC is preferable to both FD relay with one-stage SIC and HD relay, resulting in a higher optimised EE. A full range of power consumption sources are considered to rationalise the analysis. The effects of imperfect SIC at relay, drain efficiency and static circuit power on EE are investigated. Simulation results verify the theoretical analysis. In the second contribution, EE oriented resource allocation for FD decode-of-forward (DF) relay-assisted 60 GHz multiuser systems is investigated. In contrast to the existing SE oriented designs, the proposed scheme maximises EE for FD relay systems under cross-layer constraints, addressing the typical problems at 60 GHz. A low-complexity EE-orientated resource allocation algorithm is proposed, by which the transmission power allocation, subcarrier allocation and throughput assignment are performed jointly across multiple users. Simulation results verify the analytical results and confirm that the FD relay systems with the proposed algorithm achieve a higher EE than the FD relay systems with SE oriented approaches, while offering a comparable SE. In addition, a much lower throughput outage probability is guaranteed by the proposed resource allocation algorithm, showing its robustness against channel estimation errors. In the third contribution, it is noticed that in wireless power transfer (WPT)-aided relay systems, the SE of the source-relay link plays a dominant role in the system SE due to limited transmission power at the WPT-aided relay. A novel asymmetric protocol for WPT-aided FD DF relay systems is proposed in multiuser scenario, where the time slot durations of the two hops are designed to be uneven, to enhance the degree of freedom and hence the system SE. A corresponding dynamic resource allocation algorithm is developed by jointly optimising the time slot durations, subcarriers and transmission power at the source and the relay. Simulation results con rm that, compared to the symmetric WPT-aided FD relay (Sym-WPT-FR) and the time-switching based WPT-aided FD relay (TS-WPT-FR) systems in the literature, the proposed asymmetric WPT-aided FD relay system achieves up to twice the SE and higher robustness against the relay's location and the number of users. In the final contribution, to strike the balance between high SE and low power consumption, a hybrid duplexing strategy is developed for distributed antennas (DAs) systems, where antennas are capable of working in hybrid FD, HD, and sleeping modes. To maximise the system EE with low complexity, activation/deactivation of transmit/receive chain is first performed, by a proposed channel-gain-based DA clustering algorithm, which highlights the characteristics of distributed deployment of antennas. Based on the DAs' con figuration, a novel distributed hybrid duplexing (D-HD)-based and EE oriented algorithm is proposed to further optimise the downlink beamformer and the uplink transmission power. To rationalise the system model, self-interference at DAs, co-channel interference from uplink users to downlink users, and multiuser interference in both uplink and downlink are taken into account. Simulation results confirm that the proposed system provides significant EE and SE enhancements over the colocated FD MIMO system, showing the advantages in alleviating high path loss as well as in cutting the carbon footprint. Compared to the sole-FD DA system, the proposed system shows much higher EE with marginal loss in SE. Also, the SIC operation in the proposed system is much more simplified compared to the two benchmarks

Robust Beamforming Designs for Nonregenerative Multipair Two-Way Relaying Systems

In this paper, we consider nonregenerative multipair two-way relaying systems, where a relay node supports K pairs of two-way communications in the presence of imperfect channel state information (CSI). We employ a stochastic approach to model the channel uncertainties and study a robust beamforming design at the relay. It is shown that the proposed beamformer provides a reduction in the computational complexity, as well as good robustness against channel errors, compared with conventional designs. To reduce the estimation overhead at the users, we also suggest an efficient downlink training method to inform the users of the channel-dependent self-interference cancellation (SIC) parameters. With the training method, the additive noise may incur a malfunction of the SIC. To address this issue, we develop an intelligent selection criterion that decides whether the SIC should be adopted or not. Finally, from simulation results, we demonstrate the efficiency of our proposed schemes