Source-relay optimization for a two-way MIMO relay system

We consider the Simultaneous Wireless Information and Power Transfer strategy optimization in the two-way transmission relay system under Power Splitting (PS) receiver. In such case, we propose two operational strategies for two-way transmission under PS receiver structure, (1) Delay-Limited-Relay strategy (PS-LDR) and (2) Delay-Tolerated-Relay strategy (PS-TDR). PS-LDR strategy is that both sending node and receiving node need synchronization of information transmission, so communication systems have higher Signal Noise Ratio (SNR) threshold for channels, or communication may break down. In order to explore the performance of PS-LDR strategy, we use outage probability to calculate the analytical expression of the actual maximum throughput (AMT). PS-TDR strategy allows that the information received by the receiving node delays with the information sent by the sending node. Communication system in PS-TDR strategy has lower SNR threshold than that in PS-LDR strategy. The analytical expression of the AMT is calculated by the ergodic capacity in PS-TDR strategy. The influences of system parameters on the optimal throughput in two strategies are discussed.

Transceiver Designs for Multiuser Two-Way Relay Systems

A new structure for a regenerative two-way relay (TWR) system, called a wired TWR (WTWR) system is introduced. The WTWR consists of two relay nodes that are connected through an optical link, and it operates as a single conventional TWR node. For the WTWR system, beamforming vectors maximising the sum rate of two source/destination nodes (i.e. users) are designed. The WTWR exploiting the proposed beamforming is applicable to various systems that can employ a TWR protocol, such as heterogeneous networks, distributed antenna systems, and coordinated multipoint transmission systems.

This paper investigates the outage performance of simultaneous wireless information and power transfer (SWIPT) in two-way relay channels, where two sources exchange their information via an energy harvesting (EH) relay with digital network coding (NC) employed. The relay harvests energy from the received radio-frequency (RF) signals transmitted from the two sources and then use it to forward the received signals. For such a system, we study the power splitting-based two-way relay (PS-TWR) transmission strategy and analytically derive an explicit expression of the system outage probability. In order to explore the system outage performance limit, we formulate an optimization problem to minimize the system outage probability by jointly optimizing the power splitting ratio and the time assignment ratio. Since the problem is non-convex and have no known solution method, we present a genetic algorithm (GA) to solve it. Numerical results demonstrate our theoretical analysis and show that the power splitting ratio and the time assignment ratio greatly affect the system outage performance. Moreover, it is also shown that for the SWIPT-enabled two-way relay system, the relay node should be deployed closer to the source with better channel quality and when it is positioned on the middle point between the two sources, the system gets the worst outage performance, which is very different from traditional non-SWIPT two-way relay systems.

Two-way relaying promises considerable improvements on spectral efficiency in wireless relay networks. While most existing works focus on physical layer approaches to exploit its capacity gain, the benefits of two-way relaying on upper layers are much less investigated. In this paper, we study the cross-layer design and optimization for delay quality-of-service (QoS) provisioning in two-way relay systems. Our goal is to find the optimal transmission policy to maximize the weighted sum throughput of the two users in the physical layer while guaranteeing the individual statistical delay-QoS requirement for each user in the datalink layer. This statistical delay-QoS requirement is characterized by the QoS exponent. By integrating the concept of effective capacity, the cross-layer optimization problem is equivalent to a weighted sum effective capacity maximization problem. We derive the jointly optimal power and rate adaptation policies for both three-phase and two-phase two-way relay protocols. Numerical results show that the proposed adaptive transmission policies can efficiently provide QoS guarantees and improve the performance. In addition, the throughput gain obtained by the considered three-phase and two-phase protocols over direct transmission is significant when the delay-QoS requirements are loose, but the gain diminishes at tight delay requirements. It is also found that, in the two-phase protocol, the relay node should be placed closer to the source with more stringent delay requirement.

In this paper, we investigate the linear processing at either base station or relay station in cellular network with a non-regenerative MIMO relay assisting. The purpose of linear processing is to equalize the two-hop fading channel in order to support multiuser transmission in downlink and uplink. Under the transmit power constraint at each station, the closed-form expressions of the suboptimal MMSE processing matrix in downlink and the optimal MMSE processing matrix in uplink are derived when the linear processing is performed at either base station or relay station. Simulation results demonstrate that the proposed MMSE strategies significantly outperform the corresponding ZF strategies in terms of BER.

When two wireless terminals exchange information through an intermediate relay, physical-layer network coding (PNC) improves the spectrum efficiency by allowing concurrent message transmission. In two-way relay systems, the performance gain of PNC under practical limits has been investigated. The results show that generally the performance would be significantly degraded by channel impairments like asymmetric channel gain at the multiple-access phase. As an alternative interference exploitation scheme, successive interference cancellation (SIC) works well when multiple sources’ signals have different power levels at the destination to successfully decoding. In this paper, an adaptive transmission scheme combing PNC and SIC is proposed for the two-way relay systems. Separate sum-rate simulation results for PNC and SIC show the opposite tendency with the channel impairment in two-way relaying, which means combining them would get a smooth sum-rate gain and let the system be more robust to dynamic scenarios.

In this paper, while considering the impact of antenna correlation and the interference from neighboring users, we study the problem of channel estimation and training sequence design in multi-input multi-output (MIMO) two-way relaying (TWR) systems. To this end, we propose to decompose the bidirectional transmission links into two phases, i.e., the multiple access (MAC) and the broadcast (BC) phases. By deriving the optimal linear minimum mean-square-error estimators, the corresponding training design problems for the MAC and BC phases are formulated and solved. Subsequently, algorithms and, in some special cases, closed-form solutions for obtaining the optimal training sequences for channel estimation in TWR systems are derived. Moreover, to further reduce channel estimation overhead, the minimum required length of the training sequences are determined. Simulation results verify the effectiveness of the proposed training designs in improving channel estimation performance in TWR systems.

In this chapter, two-way relaying networks using Physical-Layer Net- work Coding (PLNC) are considered with practical constraints. In a Multiple Access (MA) phase, two users transmit messages simultaneously on the same physical resources. The relay constructs a relay message based on the superimposed receive signal of both users. This message is broadcasted to the users which are able to extract the desired data from the other user. However, as the relay is not able to resolve shifts in time and frequency for the individual channels, the transmission scheme needs to be adjusted, if perfect synchronization cannot be ensured. This chapter deals with the proper selection of waveforms for multi-carrier systems in the Two-Way-Relay Channel (TWRC) using PLNC. We address Generalized Frequency Division Multiplexing (GFDM) compared to Orthogonal Frequency Division Multi-plexing (OFDM) under practical constraints like Carrier Frequency Offsets (CFOs) and Timing Offsets (TOs) within double dispersive channels. For OFDM rectangular transmit filters are used, which have a broad spectrum shape and are sensitive to shifts, especially in frequency domain. GFDM applies Gaussian waveforms that are inherently less susceptible to synchronization offsets than theirs rectangular counterparts, but additional interference is introduced even in a perfectly synchronized and flat fading channel. Thus, interferences caused by the channel or by the non-orthogonal Gaussian impulse shape are treated by an equalizer at the relay. The simulation results show that the Gaussian waveform outperforms the conventional OFDM system if the nodes are not perfectly synchronized.

The Joint Relay Selection and Power Allocation (JRSPA) for Amplify-and-Forward (AF) relaying (JRSPA-AF) combines Power Allocation (PA) and Relay Selection (RS) very well in Two-Way Relaying (TWR) system. In order to explore the system performance of JRSPA-AF protocol, the outage probability and Diversity-Multiplexing Trade-off (DMT) of JRSPA-AF protocol are analysed. One upper bound, two lower bounds and approximate expressions of outage probability along with the DMT are derived. Based on this, the idea of JRSPA is introduced into Decode-and-Forward (DF) relaying TWR system, and the JRSPA-DF protocol is proposed. Moreover, the exact expressions of outage probability and DMT for JRSPA-DF are derived. Simulations are developed to validate the analytical results, the results show that the approximation outage probability of JRSPA-AF can be used well to evaluate system outage probability over the entire transmission SNR region, and the analysis results of outage probability of JRSPA-DF are consistent with its simulation results. In addition, the simulation results indicate that JRSPA-DF can achieve lower outage probability than JRSPA-AF and other protocols.

In wireless networks, node cooperation has been proposed as a strategy to improve communication reliability and system capacity. This paper investigates employing linear dispersion codes (LDC) over non-regenerative MIMO relay system with gain matrix. Gain matrix at the relay is found that increases the capacity between the source and the destination. The relay stations exploit the knowledge of the first hop and second hop channel to adaptively assign power among the relays. We deduce the capacity of the proposed system and compare the capacity with that of employing LDC over conventional amplify and forward (AF) system through simulation. Improvement of capacity can be achieved.

This paper address the problem of designing jointly the linear transmitters and receiver in a non-regenerative MIMO relay system, where linear processing is applied at the source, relay and destination node, using minimum mean square error (MMSE) criterion. Under the average power constraints at both the source and relay node, the iterative optimization method is presented and a low-complexity block optimization method is proposed. Simulation results show that the proposed methods perform better than other existing relaying schemes, and the performance gain is very significant when the signal-to-noise ratio (SNR) is high.

In this paper, we consider a decode-and-forward (DF) two-way relay (TWR) system in power line communication (PLC). The DF TWR employs physical- layer network coding (PNC). We derive analytic expressions for the average capacity and the outage probability of the system over a log-normal fading channel. Analytic results are verified through Monte Carlo simulations. From the results, the TWR is able to mitigate the half-duplex (HD) spectral efficiency loss incurred by one-way relaying. The impact of the impulsive noise on the system performance is also highlighted in the simulation results. It is shown that higher impulsive probability degrades system performance.

In this paper we introduce the concept of Adaptive Orthogonal Frequency Division Multiplexing (AOFDM) for two-way relay systems. In order to implement AOFDM at the nodes, the Channel State Information (CSI) is required. Usually CSI is estimated and hence the effect of estimation error should be considered while implementing AOFDM. In the case of two-way relay, channel estimation at the nodes usually result in the overall channel between the source and relay. But for implementing AOFDM, we require the individual channel between node and relay. We propose a simple method to obtain the individual channel assuming that the estimate of combined channel is available. The loading algorithm used is the Levin-Campello loading algorithm. But the general frame work provided by us can be used to implement other loading algorithms for two-way relay systems.

In this paper, we consider energy usage in a three-node, two-way relay system with stochastic data arrivals and Gaussian channels. Specifically, we allow for a set of transmission modes, assuming a multiaccess uplink transmission with the relay as an optimal multiuser receiver, and find the optimal allocation of time and energy resources to the modes. In the downlink, we introduce three transmission strategies, namely, orthogonal transmission of a digital network-coded message and the remainder of the longer source message over time, superposition coding of the digital network-coded message and the remainder of the longer message, and superposition of the two source messages. We then minimize total energy usage subject to queue stability constraints at all nodes for a given pair of average packet arrival rates. The total energy usage of these three strategies in the downlink is compared, and it is found that it is best to use superposition of the network-coded message and the remaining bits in the downlink. To study stability, we present an analysis of the queues at each node under a random scheduling protocol that closely approximates the theoretical design through a Markov chain model.