Ha H. Nguyen

Fast Global Optimal Power Allocation in Wireless Networks by Local D.C. Programming

Power allocations in an interference-limited wireless network for global maximization of the weighted sum throughput or global optimization of the minimum weighted rate among network links are not only important but also very hard optimization problems due to their nonconvexity nature. Recently developed methods are either unable to locate the global optimal solutions or prohibitively complex for practical applications. This paper exploits the d.c. (difference of two convex functions/sets) structure of either the objective function or constraints of these global optimization problems to develop efficient iterative algorithms with very low complexity. Numerical results demonstrate that the developed algorithms are able to locate the global optimal solutions by only a few iterations and they are superior to the previously-proposed methods in both performance and computation complexity.

On the capacity of Rayleigh fading cooperative systems under adaptive transmission

In this letter, the use of adaptive source transmission with amplify-and-forward relaying is proposed. Three different adaptive techniques are considered: (i) optimal simultaneous power and rate adaptation; (ii) constant power with optimal rate adaptation; (iii) channel inversion with fixed rate. The capacity upper bounds of these adaptive protocols are derived for the amplify-and-forward cooperative system over both independent and identically distributed (i.i.d.) Rayleigh fading and non-i.i.d. Rayleigh fading environments. The capacity analysis is based on an upper bound on the effective received signal-to-noise ratio (SNR). The tightness of the upper bound is validated by the use of a lower bound and by Monte Carlo simulation. It is shown that at high SNR the optimal simultaneous power and rate adaptation and the optimal rate adaptation with constant power provide roughly the same capacity. Channel inversion is shown to suffer from a deterioration in capacity relative to the other adaptive techniques.

Resource Allocation for OFDMA-Based Cognitive Radio Multicast Networks With Primary User Activity Consideration

This paper considers the primary user activity or the subchannel availability in optimally distributing the available resources for an orthogonal frequency-division multiple-access (OFDMA) cognitive radio multicast network. For this purpose, a risk-return model is presented, and a general rate-loss function, which gives a reduction in the attainable throughput whenever primary users reoccupy the temporarily accessible subchannels, is introduced. Taking the maximization of the expected sum rate of secondary multicast groups as the design objective, an efficient joint subcarrier and power-allocation scheme is proposed. Specifically, the design problem is solved via a dual optimization method under constraints on the tolerable interference thresholds at individual primary users frequency bands. It is shown that as the number of subcarriers gets large (which is often the case in practice), the dual-domain solution becomes globally optimum with regard to the primal problem. More attractively, the ¿practically optimal¿ performance of this approach is achieved with a substantially lower complexity, which is only linear in the total number of subcarriers as opposed to exponential complexity typically required by a direct search method. Our proposed design is valid for unicast and multicast transmissions and is applicable for a wide range of rate-loss functions, among which, the linear function is a special case. The superiority of the dual scheme is thoroughly verified by numerical examples.

Design and performance of BICM-ID systems with hypercube constellations

This paper introduces new mappings of QPSK symbols, viewed as a multi-dimensional hypercube, to improve the performance of bit-interleaved coded modulation with iterative decoding (BICM-ID). By evaluating the upper bound of the bit error rate performance of BICM-ID, a condition to find the best mapping of a hypercube constellation in terms of the asymptotic performance under different channel models is established. A general and simple algorithm to construct the best mapping of a hypercube is then proposed. Analytical and simulation results show that the use of the proposed mappings together with very simple convolutional codes can offer significant coding gains over the conventional BICM-ID systems for all the channel models considered. Such coding gains are achieved without bandwidth or power expansion and with a very small increase in the system complexity.

Signal mappings of 8-ary constellations for bit interleaved coded modulation with iterative decoding

Bit interleaved coded modulation with iterative decoding (BICM-ID) is a spectral efficiency coded modulation technique. This technique is therefore very attractive for many broadcasting services where transmission bandwidth is a primary concern. It has been shown that when signal constellation, interleaver and error-control code are fixed, signal mapping has a critical influence to the error performance of a BICM-ID system. Based on the technique of mutual information, good mappings of different 8-ary constellations are presented for BICM-ID systems over an additive white Gaussian noise (AWGN) channel. It is also shown that, compared to free Euclidean distances, mutual information is a more useful technique to find the good signal mappings for BICM-ID systems.

Efficient Resource Allocation for OFDMA Multicast Systems With Spectrum-Sharing Control

This paper considers the important problem of efficient allocation of available resources (such as radio spectrum and power) in orthogonal frequency-division multiple-access (OFDMA)-based multicast wireless systems. Taking the maximization of system throughput as the design objective, three novel efficient resource-allocation schemes with reduced computational complexity are proposed under constraints on total bandwidth and transmitted power at the base station (BS). Distinct from existing approaches in the literature, our formulation and solution methods also provide an effective and flexible means to share the available radio spectrum among multicast groups by guaranteeing minimum numbers of subcarriers to be assigned to individual groups. The first two proposed schemes are based on the separate optimization of subcarriers and power, where subcarriers are assigned with the assumption of uniform power distribution, followed by water filling of the total available transmitted power over the determined subcarrier allocation. In the third scheme, which is essentially a modified genetic algorithm (GA), each individual of the entire population represents a subcarrier assignment, whose fitness value is the system sum rate computed on the basis of the power water-filling procedure. Numerical results show that with a flexible spectrum-sharing control mechanism, the proposed designs are able to more flexibly and fairly distribute the total available bandwidth among multicast groups and, at the same time, achieve a high system throughput.

Performance Analysis of Adaptive M-QAM for Rayleigh Fading Cooperative Systems

The use of constant-power, rate-adaptive M-QAM transmission with an amplify-and-forward cooperative system is proposed. The upper bound expressions are derived for the outage probability, achievable spectral efficiency, and error rate performance for the amplify-and-forward cooperative system over both independent and identically distributed (i.i.d.) and non-i.i.d. Rayleigh fading environments. The analysis is based on an accurate upper bound on the total effective signal-to- noise ratio SNR at the destination. Adaptive continuous rate M-QAM achieves a capacity that comes within a constant gap of the Shannon capacity of the channel, but adaptive discrete rate M-QAM suffers additional performance penalties.

Blind Spectrum Sensing for OFDM-Based Cognitive Radio Systems

Given the ever-growing demand for radio spectrum, cognitive radio has recently emerged as an attractive wireless technology. Since orthogonal frequency-division multiplexing (OFDM) is one of the major wideband transmission techniques, detection of OFDM signals in low-signal-to-noise-ratio scenario is an important research problem. In this paper, it is shown that cyclic prefix (CP) correlation coefficient (CPCC)-based spectrum sensing, which was previously introduced as a simple and computationally efficient spectrum-sensing method for OFDM signals, is a special case of the constrained generalized likelihood ratio test (GLRT) in the absence of multipath. As such, the performance of this algorithm degrades in a multipath scenario, where OFDM is usually implemented. Furthermore, by considering multipath correlation in the GLRT algorithm and employing the inherent structure of OFDM signals, a simple and low-complexity algorithm called the multipath-based constrained-GLRT (MP-based C-GLRT) algorithm is obtained. The MP-based C-GLRT algorithm is shown to outperform the CPCC-based algorithm in a rich multipath environment. Further performance improvement can be achieved by simply combining both the CPCC- and MP-based C-GLRT algorithms. A simple GLRT-based detection algorithm is also developed for unsynchronized OFDM signals, whose performance is only slightly degraded when compared with the synchronized detection in a rich multipath environment.

Joint Optimization of Source Precoding and Relay Beamforming in Wireless MIMO Relay Networks

This paper considers joint linear processing at multi-antenna sources and one multiple-input multiple-output (MIMO) relay station for both one-way and two-way relay-assisted wireless communications. The one-way relaying is applicable in the scenario of downlink transmission by a multi-antenna base station to multiple single-antenna users with the help of one MIMO relay. In such a scenario, the objective of join linear processing is to maximize the information throughput to users. The design problem is equivalently formulated as the maximization of the worst signal-to-interference-plus-noise ratio (SINR) among all users subject to various transmission power constraints. Such a program of nonconvex objective minimization under nonconvex constraints is transformed to a canonical d.c. (difference of convex functions/sets) program of d.c. function optimization under convex constraints through nonconvex duality with zero duality gap. An efficient iterative algorithm is then applied to solve this canonical d.c program. For the scenario of using one MIMO relay to assist two sources exchanging their information in two-way relying manner, the joint linear processing aims at either minimizing the maximum mean square error (MSE) or maximizing the total information throughput of the two sources. By applying tractable optimization for the linear minimum MSE estimator and d.c. programming, an iterative algorithm is developed to solve these two optimization problems. Extensive simulation results demonstrate that the proposed methods substantially outperform previously-known joint optimization methods.

Bit-Interleaved Coded OFDM With Signal Space Diversity: Subcarrier Grouping and Rotation Matrix Design

This paper investigates the application of bit-interleaved coded modulation and iterative decoding (BICM-ID) in orthogonal-frequency-division-multiplexing (OFDM) systems with signal space diversity (SSD) over frequency-selective Rayleigh-fading channels. Correlated fading over subcarriers is assumed. At first, a tight bound on the asymptotic error performance for the general case of precoding over all N subcarriers is derived and used to establish the best achievable asymptotic performance by SSD. It is then shown that precoding over subgroups of at least L subcarriers per group, where L is the number of channel taps, is sufficient to obtain this best asymptotic error performance, while significantly reducing the receiver complexity. The optimal joint subcarrier grouping and rotation matrix design is subsequently determined by solving the Vandermonde linear system. Illustrative examples show a good agreement between various analytical and simulation results