Himal A. Suraweera

Capacity Limits and Performance Analysis of Cognitive Radio With Imperfect Channel Knowledge

Cognitive radio (CR) design aims to increase spectrum utilization by allowing the secondary users (SUs) to coexist with the primary users (PUs), as long as the interference caused by the SUs to each PU is properly regulated. At the SU, channel-state information (CSI) between its transmitter and the PU receiver is used to calculate the maximum allowable SU transmit power to limit the interference. We assume that this CSI is imperfect, which is an important scenario for CR systems. In addition to a peak received interference power constraint, an upper limit to the SU transmit power constraint is also considered. We derive a closed-form expression for the mean SU capacity under this scenario. Due to imperfect CSI, the SU cannot always satisfy the peak received interference power constraint at the PU and has to back off its transmit power. The resulting capacity loss for the SU is quantified using the cumulative-distribution function of the interference at the PU. Additionally, we investigate the impact of CSI quantization. To investigate the SU error performance, a closed-form average bit-error-rate (BER) expression was also derived. Our results are confirmed through comparison with simulations.

Full-Duplex Relay Selection for Amplify-and-Forward Cooperative Networks

This paper focuses on the relay selection problem in amplify-and-forward (AF) cooperative communication with full-duplex (FD) operation. Different relay selection schemes assuming the availability of different instantaneous information are studied. We consider optimal relay selection that maximizes the instantaneous FD channel capacity and requires global channel state information (CSI) as well as several sub-optimal relay selection policies that utilize partial CSI knowledge such as a) source-relay and relay-destination links b) loop interference c) source-relay links and loop interference. To facilitate comparison, exact outage probability expressions and asymptotic approximations of these policies that show a zero diversity order are derived. In addition, an optimal relay selection procedure that incorporates a hybrid relaying strategy, which dynamically switches between FD and half-duplex relaying according to the instantaneous CSI, is also investigated.

Performance analysis of beamforming in two hop amplify and forward relay networks with antenna correlation

The performance of beamforming with antenna correlation in a two hop amplify and forward (AF) multiple input multiple-output (MIMO) relay network is analyzed. This network consists of a single relay which is used to amplify and forward the signal from the source to the destination. The source and destination are both equipped with multiple antennas, which are correlated in space, while the relay has a single antenna. In this paper, we derive new closed form expressions for the outage probability and probability density function of the received signal-to-noise ratio (SNR) at the destination. We also present exact symbol error rate expressions for the two hop AF MIMO relay network, and show that the full spatial diversity order can be achieved. Our results also indicate that spatial correlation is detrimental to the outage probability and symbol error rate at high SNR, and beneficial at low SNR.

Performance Analysis of Two Hop Amplify-and-Forward Systems with Interference at the Relay

We analyze the performance of a two hop channel state information (CSI)-assisted amplify-and-forward system, with co-channel interference at the relay. The systems outage probability and the average bit error rate (BER) in the presence of Rayleigh faded multiple interferers are investigated. We derive an exact expression for the outage probability and an accurate bound for the systems average BER. Simulation results show the validity of the analysis and point out the effect of interference.

Multipair Full-Duplex Relaying with Massive Arrays and Linear Processing

We consider a multipair decode-and-forward relay channel, where multiple sources transmit simultaneously their signals to multiple destinations with the help of a full-duplex relay station. We assume that the relay station is equipped with massive arrays, while all sources and destinations have a single antenna. The relay station uses channel estimates obtained from received pilots and zero-forcing (ZF) or maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To significantly reduce the loop interference effect, we propose two techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the relay station. We derive an exact achievable rate expression in closed-form for MRC/MRT processing and an analytical approximation of the achievable rate for ZF processing. This approximation is very tight, particularly for a large number of relay station antennas. These closed-form expressions enable us to determine the regions where the full-duplex mode outperforms the half-duplex mode, as well as to design an optimal power allocation scheme. This optimal power allocation scheme aims to maximize the energy efficiency for a given sum spectral efficiency and under peak power constraints at the relay station and sources. Numerical results verify the effectiveness of the optimal power allocation scheme. Furthermore, we show that, by doubling the number of transmit/receive antennas at the relay station, the transmit power of each source and of the relay station can be reduced by 1.5 dB if the pilot power is equal to the signal power, and by 3 dB if the pilot power is kept fixed, while maintaining a given quality of service.

Application of smart antenna technologies in simultaneous wireless information and power transfer

Simultaneous wireless information and power transfer (SWIPT) is a promising solution to increase the lifetime of wireless nodes and hence alleviate the energy bottleneck of energy constrained wireless networks. As an alternative to conventional energy harvesting techniques, SWIPT relies on the use of radio frequency signals, and is expected to bring some fundamental changes to the design of wireless communication networks. This article focuses on the application of advanced smart antenna technologies to SWIPT, including multiple-input multiple-output and relaying techniques. These smart antenna technologies have the potential to significantly improve the energy efficiency and also the spectral efficiency of SWIPT. Different network topologies with single and multiple users are investigated, along with some promising solutions to achieve a favorable trade-off between system performance and complexity. A detailed discussion of future research challenges for the design of SWIPT systems is also provided.

Wireless Information and Power Transfer With Full Duplex Relaying

In this paper, we propose an innovative spatial-modulation (SM) based full-duplex (FD) decode-and-forward (DF) relaying protocol where the energy-constrained dual-antenna relay is powered by the radio frequency (RF) energy from the single-antenna source using the time-switching (TS) architecture. In this system, either one or both of the relay antennas receive the energy signal from the source in the energy harvesting phase. In the information transmission phase, one of the two relay antennas is selected to be active to decode and forward the information transmitted from the source and the other relay antenna receives the information from the source at the same time. In this way, the throughput of the information transmission between the relay and the destination can be significantly improved by the additional information mapped to the active antenna index which consequently leads to the improvement of the overall system throughput. Since the current SM capacity solution is not in a closed-form, we propose two tight SM capacity upper bounds and present the solution of the optimal time split ratio for the maximum system throughput according to the proposed upper bound. Monte-carlo simulations are conducted to verify the analysis and reveal the throughput gain of the proposed SM-FD relaying protocol in comparison with conventional FD relaying protocol.

Amplify-and-Forward Relay Selection with Outdated Channel Estimates

We study the effect of outdated channel state information on the outage and error rate performance of amplify-and-forward (AF) relay selection, where only one out of the set of available relays is activated. We consider two variations of AF relay selection, namely best relay selection and partial relay selection, when the selection is based upon outdated channel estimates. For both these variations, closed-form expressions for the outage probability are obtained, along with approximate expressions for the symbol error rate in the medium to high signal-to-noise-ratio (SNR) regime. The diversity gain and coding gain of the above schemes are also explicitly derived. Numerical results manifest that the outage performance of AF relay selection is highly dependent on the level of correlation between the actual channel conditions and their corresponding (outdated) estimates. This result has a significant impact on the deployment of relay selection in practical applications, implying that a high feedback rate may be required in practice in order to attain the full benefits of relay selection. It is further shown that it may be preferable, in terms of outage and symbol error probability, not to include links in the relay selection process that experience a sufficiently high maximum Doppler shift, since in those cases partial relay selection outperforms best relay selection.

Low-Complexity End-to-End Performance Optimization in MIMO Full-Duplex Relay Systems

In this paper, we deal with the deployment of full-duplex relaying in amplify-and-forward (AF) cooperative networks with multiple-antenna terminals. In contrast to previous studies, which focus on the spatial mitigation of the loopback interference (LI) at the relay node, a joint precoding/decoding design that maximizes the end-to-end (e2e) performance is investigated. The proposed precoding incorporates rank-1 zero-forcing (ZF) LI suppression at the relay node and is derived in closed-form by solving appropriate optimization problems. In order to further reduce system complexity, the antenna selection (AS) problem for full-duplex AF cooperative systems is discussed. We investigate different AS schemes to select a single transmit antenna at both the source and the relay, as well as a single receive antenna at both the relay and the destination. To facilitate comparison, exact outage probability expressions and asymptotic approximations of the proposed AS schemes are provided. In order to overcome zero-diversity effects associated with the AS operation, a simple power allocation scheme at the relay node is also investigated and its optimal value is analytically derived. Numerical and simulation results show that the joint ZF-based precoding significantly improves e2e performance, while AS schemes are efficient solutions for scenarios with strict computational constraints.

Outage probability of cooperative relay networks in Nakagami-m fading channels

It is well known that the cooperation among nodes can improve the performance of a wireless network. In this letter we analyze the outage probability behaviour of a relay network in Nakagami-m fading channels. A closed-form solution for the outage probability is derived. When m=1, the results are applicable for Rayleigh fading. Computer simulations confirm the presented mathematical analysis