Yijia Fan

MIMO Configurations for Relay Channels: Theory and Practice

In this paper we discuss and compare different signalling and routing methods for multiple-input multiple-output (MIMO) relay networks in terms of the network capacity, where every terminal is equipped with multiple antennas. Our study for signalling includes the two well known digital (decode and forward) relaying, analogue (amplify and forward) relaying, and a novel hybrid (filter, amplify and forward) relaying. We propose both optimal and suboptimal hybrid relaying schemes which avoid full decoding of the message at the relay. We show that they outperform analogue relaying and give similar performance to digital relaying, particularly when the relay has forward channel state information (CSI) or larger number of antennas than the source and destination. For the routing schemes designed for multiple relay channels, we use relay selection schemes to exploit the spatial diversity, which we call selection diversity of the networks. We propose both optimal and suboptimal relay selection schemes and show that their performance converges when a large number of antennas is deployed at each node in the network. We also compare relay selection routing with a space-time coded relay cooperation protocol and show the performance advantage of selection diversity over cooperative diversity in certain scenarios. Finally, we give a brief discussion on the application of another MIMO structure called single signal beamforming in the relay scenario. Its performance will be compared with that of spatial multiplexing

Recovering Multiplexing Loss through Successive Relaying Using Repetition Coding

In this paper, a transmission protocol is studied for a two relay wireless network in which simple repetition coding is applied at the relays. Information-theoretic achievable rates for this transmission scheme are given, and a space-time V-BLAST signalling and detection method that can approach them is developed. It is shown through the diversity multiplexing tradeoff analysis that this transmission scheme can recover the multiplexing loss of the half-duplex relay network, while retaining some diversity gain. This scheme is also compared with conventional transmission protocols that exploit only the diversity of the network at the cost of a multiplexing loss. It is shown that the new transmission protocol offers significant performance advantages over conventional protocols, especially when the interference between the two relays is sufficiently strong.

Threshold Selection for SNR-based Selective Digital Relaying in Cooperative Wireless Networks

This paper studies selective relaying schemes based on signal-to-noise-ratio (SNR) to minimize the end-to-end (e2e) bit error rate (BER) in cooperative digital relaying systems using BPSK modulation. In the SNR-based selective relaying, the relay either retransmits or remains silent depending on the SNRs of the source-relay, relay-destination, and source-destination links. Different models assuming the availability of different sets of instantaneous and average SNR information at the relay are studied. For each model, the optimal strategy to minimize the e2e BER is a different threshold rule on the source-relay SNR, if the link SNRs are uncorrelated in time and space. Approximations for the optimal threshold values that minimize the e2e BER and the resulting performance are derived analytically for BPSK modulation. Using the derived threshold the e2e BER can be reduced significantly compared to simple digital relaying. By studying the performance under different models, it is shown that knowledge of the instantaneous source-destination SNR at the relay can be exploited. The gain from this knowledge is higher when the average source-destination SNR is large. However, knowledge of the instantaneous relay-destination SNR at the relay does not change performance significantly.

Buffering in a Three-Node Relay Network

This paper explores two buffering relay models to improve the capacity of relay networks in slow fading environments. Throughput, average packet delay, and information loss are considered as functions of buffer size and signal-to-noise ratio (SNR). It is shown that, for any fading statistics, both a fixed buffering relay model and a dynamic buffering relay model offer significant performance advantages in terms of capacity over existing methods at the expense of increased delay. It is also seen that, of the two models, dynamic buffering provides a smaller average delay.

Performance of selection relaying and cooperative diversity

In this paper, a two-hop cooperative multi-relay communication network is considered. Selection relaying schemes are attracting considerable attention due to their prudent bandwidth utilization and ability to provide full diversity. The recent developments in selection relaying (SR) have largely focused on information theoretic analyses such as outage performance. Some of these analyses are accurate only in high SNR regimes. This paper provides exact outage and capacity performance expressions for selection relaying and tight approximation over a sufficiently wide range of SNR regimes for selection cooperative relaying. The outage capacity for SR is also provided. The motivation for this work is that practical systems operate at far lower SNR values than those supported by the high SNR analysis. Therefore, designers should be able to evaluate system performance to a reasonable degree of accuracy over practical SNR values. Simulations are used to corroborate the analytical results and close agreement is observed.

Optimal Single-Port Matching Impedance for Capacity Maximization in Compact MIMO Arrays

A complete multiple-input multiple-output (MIMO) system model with compact arrays at both link ends containing arbitrary matching networks is presented based on a Z-parameter approach. The complete channel matrix including the coupling effect is also presented. Utilizing this system model, the optimum single-port matching impedance for capacity maximization is derived for a 2 times 2 MIMO system with coupling at the receivers only. A closed-form result for the optimum matching impedance in high signal-to-noise ratio scenarios is given and proved to be equal to the input impedance of the receive end. Simulation of ideal dipoles verifies our analytical results and demonstrates the superiority of the optimum matching to other matching conditions in improving MIMO system performance. Experimental data for monopoles is also presented to further confirm our numerical findings and validate the accuracy of our derivation.

Base Station Location Optimization for Minimal Energy Consumption in Wireless Networks

This paper studies the combined problem of base station location and optimal power allocation, in order to optimize the energy efficiency of a cellular wireless network. Recent work has suggested that moving from a network of a small number of high power macrocells to a larger number of smaller microcells may improve the energy efficiency of the network. This paper investigates techniques to optimize the number of base stations and their locations, in order to minimize energy consumption. An important contribution of the paper is that it takes into account non-uniform user distributions across the coverage area, which is likely to be encountered in practice. The problem is solved using approaches from optimization theory that deal with the facility location problem. Stochastic programming techniques are used to deal with the expected user distributions. An example scenario is presented to illustrate how the technique works and the potential performance gains that can be achieved.

Asymptotic BER analysis of threshold digital relaying schemes in cooperative wireless systems

Threshold relaying is an effective technique to achieve cooperative diversity in uncoded cooperative wireless networks, which suffer from error propagation due to detection errors at the relays. This paper analyzes the asymptotic end-to-end (e2e) bit error rate (BER) of threshold digital relaying. A three node network with a source, destination and relay and with links experiencing independent Rayleigh fading is considered. It is shown that, as the average link signal-to-noise ratios (SNR) are increased simultaneously, the optimal threshold that minimizes the e2e BER increases as log(SNR). The resulting e2e BER decreases as log(SNR)/SNR2. Moreover, any threshold of the form log(cSNR), where c is a positive constant, achieves the same order of e2e BER as the one achieved by the optimal threshold and provides dual diversity. A value of c that performs very close to the optimal threshold is also proposed.

Optimum Threshold for SNR-Based Selective Digital Relaying Schemes in Cooperative Wireless Networks

We study selective digital relaying schemes where the relay may choose to retransmit or to remain silent based on the qualities of the links between the source, relay and the destination. We first analyze a baseline scheme, called static relaying, where the relaying decisions are based only on the average signal-to-noise ratio (SNR) values of all the links. The second scheme, dynamic relaying, allows the relay to make decisions based on the instantaneous SNR of the source-relay link and average SNRs of the relay-destination and source-destination links. We show that, in dynamic relaying the optimal strategy to minimize the average end-to-end bit error rate is a threshold rule on the instantaneous SNR of the source-relay channel. In this case, the optimal threshold value is a function of average SNR of relay-destination and source-destination channels. We derive closed-form expressions for the optimal threshold and the bit error performance achieved by this threshold. We show that dynamic relaying can provide significant performance advantage over static relaying.

On the Performance of Selection Relaying

Interest in selection relaying is growing. The recent developments in this area have largely focused on information theoretic analyses such as outage performance. Some of these analyses are accurate only at high SNR regimes. In this paper error rate analyses that are sufficiently accurate over a wide range of SNR regimes are provided. The motivations for this work are that practical systems operate at far lower SNR values than those supported by the high SNR analysis. To enable designers to make informed decisions regarding network design and deployment, it is imperative that system performance is evaluated with a reasonable degree of accuracy over practical SNR regimes. Simulations have been used to corroborate the analytical results, as close agreement between the two is observed.