Tung T. Pham

Power Allocation in MMSE Relaying over Frequency-Selective Rayleigh Fading Channels

This paper develops an amplify-and-forward relaying scheme for multiuser wireless cooperative networks under frequency-selective block-fading. Single-carrier frequency division multiple-access with frequency-domain equalization technique is employed at both the relay and destination to combat the inter-block and inter-symbol interference caused by multipath propagation. With the assumption that the full channel state information (CSI) is available at the destination, the relay only knows the uplink channels while no CSI is available at the sources, two power allocation schemes are developed for the relay: (i) to minimize the total transmit power at the relay while maintaining the signal-to-interference-plus-noise ratio (SINR) for each user at the destination above a certain level, and (ii) to maximize the worst SINR among all the users subject to a constraint on total relay power. In the first problem, it is shown that SINR adaptation is needed not only to guarantee a feasible solution but also to significantly reduce the transmit power at the relay for certain channel conditions. In the second problem, a flexible multi-level water-filling scheme is developed, which can be easily modified to adapt to the different channel conditions as well as different quality-of-service provision strategies.

Optimization of Cooperative Beamforming for SC-FDMA Multi-User Multi-Relay Networks by Tractable D.C. Programming

This paper addresses the optimal cooperative beamforming design for multi-user multi-relay wireless networks in which the single-carrier frequency division multiple access (SC-FDMA) technique is employed at the terminals. The problem of interest is to find the beamforming weights across relays to maximize the minimum signal-to-interference-plus-noise ratio (SINR) among source users subject to individual power constraints at each relay. Such a beamforming design is shown to be a hard nonconvex optimization problem and therefore it is mathematically challenging to find the optimal solution. By exploring its partial convex structures, we recast the design problem as minimization of a d.c. (difference of two convex) objective function subject to convex constraints and develop an effective iterative algorithm of low complexity to solve it. Simulation results show that our optimal cooperative beamforming scheme realizes the inherent diversity order of the relay network and it performs significantly better than the equal-power beamforming weights.

Power Allocation in Orthogonal Wireless Relay Networks With Partial Channel State Information

Wireless amplify-and-forward relay networks in which the source communicates with the relays and destination in the first phase and the relays forward signals to the destination in the second phase over orthogonal and uncorrelated Rayleigh fading channels are considered. Convex programming is used to obtain optimal and approximately optimal power allocation schemes to maximize the average signal-to-noise ratios at the output of the receiver filters under two different assumptions of partial channel state information (CSI). Analysis and simulation results demonstrate the superiority of the proposed power allocation schemes over the equal-power allocation scheme. Performance comparison to the extreme cases of i) direct transmission between the source and destination and ii) having full CSI is made to illustrate the gain and loss, respectively, of the proposed schemes. The impact of power allocation between the source and relays is also investigated by computer simulation.

Distributed Beamforming in Multiuser Multi-Relay Networks with Guaranteed QoS

This paper considers optimal distributed beamforming designs in a multi-relay network with multiple sources and multiple destinations. It is assumed that all source-destination pairs operate in orthogonal channels to avoid inter-user interference at the destinations. The distributed beamforming designs are carried out to minimize the sum relay power with guaranteed quality of service (QoS) in terms of signal-to-noise-ratio (SNR) at the destinations. Considered are optimization problems with and without per-relay power constraints. Although the two optimization problems can be readily transformed into convex second-order conic programs (SOCPs), the paper proposes simple and fast iterative algorithms to efficiently solve them.

Relay Assignment for Max-Min Capacity in Cooperative Wireless Networks

This paper is concerned with the problem of relay assignment in cooperative wireless networks having multiple sources, multiple relays, and a single destination. With the objective of maximizing the minimum capacity among all sources in the network, a mixed-integer linear programming (MILP) problem is formulated, which can be solved by standard branch-and-bound algorithms. To reduce computational complexity, a greedy solution in the form of a lexicographic bottleneck assignment algorithm is proposed. Simulation results obtained for the IEEE 802.16j uplink scenarios show that the greedy algorithm performs very close to the optimal solution but at a much lower computational cost. The proposed greedy solution can also be tailored to provide further improvements on other network performance criteria.

Optimization of Hierarchical Modulation for Decode-and-Forward Wireless Relay Networks

This paper presents two designs of optimal nonuniformed constellations for decode-and-forward wireless relay networks with an orthogonal space-time block code (STBC). The first design is concerned with the unequal error protection issue, in which two data streams to be transmitted from a source to a destination are protected at two different levels. The design is to minimize the bit error rate (BER) of one stream while maintaining the BER of the other stream to be no larger than a given threshold. Based on a simple approximation of the average BER, a design parameter is obtained in closed form. The second design focuses on minimizing the average BER of the combined data stream at the destination. Developed are two near-optimal designs for 4/16-quadrature amplitude modulation (QAM) and 4/64-QAM hierarchical modulation (HM) schemes. The designs are also extended to two-way relay networks where both the source and the destination have data to transmit to each other. Simulation results confirm the superiority of the proposed designs and the advantages of relay-assisted transmission employing HM over conventional point-to-point transmission. A comparison with the existing relay-assisted transmission model is also discussed.

Design of Hierarchical Modulation for Wireless Relay Networks

Two designs of un-uniformed constellations for wireless relay networks are considered. The first design is concerned with the unequal error protection issue, in which two data streams to be transmitted from a source to a destination are protected at two different levels. Based on a simple approximation of the average bit-error-rate (BER), a design parameter is obtained in a closed form. The second design focuses on minimizing the average BER of the combined data stream at the destination. Developed are two near optimal designs for 4/16-QAM and 4/64-QAM hierarchical modulation schemes. Simulation results confirm the superiority of the proposed designs as well as the advantages of the relay-assisted transmission with a hierarchical modulation over the conventional point-to-point transmission.

Non-Orthogonal Amplify-And-Forward Relaying with Partial Channel State Information

Wireless amplify-and-forward relay networks in which the source communicates with the relays and destination in the first phase and the relays simultaneously forward signals to the destination in the second phase over uncorrelated Rayleigh fading channels are considered. We examine the scenario in which each relay only knows the perfect information of its source-relay channel while the destination knows the exact information of the relay-destination channels and the statistics of the source-relay channels. Based on a combiner developed at the destination, we propose an efficient beamforming scheme at the relays and develop its quantized version using Lloyds algorithm to work with a limited-rate feedback channel. Simulation results show that the non-orthogonal relaying with the proposed beamforming scheme outperforms the orthogonal relaying with power allocation in terms of the ergodic capacity. In terms of the signal-to-noise ratio, the non-orthogonal scheme also becomes superior to the orthogonal scheme when the number of quantization regions increases.

Power Allocation in Wireless Relay Networks with Partial Channel State Information

Amplify-and-forward (AF) wireless relay networks in which the source communicates with the relays and destination in the first phase and the relays forward signals to the destination in the second phase over orthogonal and uncorrelated Rayleigh fading channels are considered. Convex programming is used to obtain optimal and approximately optimal power allocation (OPA) schemes to maximize the average signal-to-noise ratios (SNRs) at the output of the receiver filters under two different assumptions of partial channel state information (CSI). Analysis and simulation results demonstrate the superiority of the proposed power allocation schemes over the equal-power allocation scheme. Performance comparison to the extreme cases of (i) direct transmission between the source and destination and (ii) having full CSI is made to illustrate the gain and loss, respectively, of the proposed schemes. The impact of power allocation between the source and the relays is also investigated by computer simulation.

Beamforming in Nonorthogonal Amplify-and-Forward Relay Networks

This paper considers wireless amplify-and-forward (AF) relay networks in which the source communicates with the relays and destination in the first phase and the relays simultaneously forward signals to the destination in the second phase over uncorrelated Rayleigh fading channels. We examined one scenario in which each relay only knows the perfect information of its source-relay channel, whereas the destination knows the exact information of the relay-destination channels and the statistics of the source-relay channels. Based on a combiner that was developed at the destination, we propose an efficient beamforming scheme at the relays and develop its quantized version using Lloyds algorithm to work with a limited-rate feedback channel. Simulation results show that the nonorthogonal relaying with the proposed beamforming scheme outperforms the orthogonal relaying with power allocation in terms of the ergodic capacity. In terms of the signal-to-noise ratio (SNR), the nonorthogonal scheme also becomes superior to the orthogonal scheme when the number of quantization regions increases.