Peter Wing Chau Chan

The evolution path of 4G networks: FDD or TDD?

Frequency-division duplexing and time-division duplexing are two common duplexing methods used in various wireless systems. However, there are advantages and technical issues associated with them. In this article we discuss in detail the features, and the design and implementation challenges of FDD and TDD systems for 4G wireless systems. In particular, we present a number of advantages and flexibilities an TDD system can bring to 4G systems that an FDD system cannot offer, and identify the major challenges, including cross-slot interference, in applying TDD in practice. Due to the fact that cross-slot interference is one of the critical challenges to employing TDD in cellular networks, we also provide a quantitative analysis on its impact on co-channel and adjacent channel interfering cells

Adaptive Resource Allocation and Capacity Comparison of Downlink Multiuser MIMO-MC-CDMA and MIMO-OFDMA

In this paper, we examine and compare the potential maximum sum capacity of downlink multiple-input-multiple-output orthogonal frequency division multiple access (MIMO-OFDMA) and multiple-input-multiple-output multicarrier code division multiple access (MIMO-MC-CDMA) in a single-cell multiuser environment with channel side information at the transmitter, with and without a fairness constraint. The resource allocation is formulated as a cross-layer optimization framework and optimal power allocation and user selection algorithms are proposed for both scenarios. We find that for delay-sensitive applications, where fairness is imposed, the performance gain of OFDMA over MC-CDMA is quite large at moderate path loss exponents and number of antennas. However, for delay-insensitive applications, the benefits of OFDMA over MC-CDMA are significantly reduced when the path loss exponent or the number of antennas is large

Capacity Maximization for Zero-Forcing MIMO-OFDMA Downlink Systems with Multiuser Diversity

This paper considers the optimal resource allocation for a multiuser MIMO-OFDMA downlink system using zero-forcing SDMA, with the objective of maximizing the system capacity. We provide the general mathematical formulation of the capacity maximization problem whose resource allocation includes user selection, power allocation and receive signal subspace selection. While the optimal solution of this problem is difficult to obtain, we consider two special cases: user selection based only and antenna selection based only optimization, and develop the optimal solution by reformulating the combinatorial optimization problem into a convex problem. Our proposed algorithms for these special cases can be extended to develop a suboptimal algorithm for the original general optimization problem which allows subspace selection. Moreover, we develop reduced-complexity algorithms for these problems which incur negligible capacity loss. Numerical results and analysis are provided to evaluate the system capacity and to study the effect of the number of users and the receive antennas per user on the performance. Comparisons on the capacity achieved by user selection only, antenna selection only and suboptimal subspace selection are discussed

Effect of Carrier Frequency Offset on Channel Estimation for SISO/MIMO-OFDM Systems

Few works have addressed the effect of CFO (carrier frequency offset) on channel estimation performance. In this paper, compact analytical expressions on the mean square error of channel estimation in the presence of CFO are derived for OFDM (orthogonal frequency division multiplexing) systems employing single and multiple antennas. Concise upper bounds are also derived for SISO-OFDM systems. It is revealed that channel estimation MSE (mean square error) increases at the rate of approximately the square of the CFO and increasing the number of pilots does not contribute to better suppression of the MSE caused by CFO. In addition, it is observed that LMMSE (linear minimum mean square error) channel estimation is more resistant to CFO compared to LS (least square) in terms of channel estimation MSE. Furthermore, from the results derived herein, a clue for the pilot design of OFDM systems with CFO can be obtained

Performance Comparison of Downlink Multiuser MIMO-OFDMA and MIMO-MC-CDMA with Transmit Side Information - Multi-Cell Analysis

Orthogonal frequency division multiple access (OFDMA) and multicarrier code division multiple access (MC-CDMA) have recently drawn much attention for being potential candidates of future generation cellular systems. In single-cell scenario, while MC-CDMA is good at achieving frequency diversity when there is no channel state information available at the transmit side (CSIT), OFDMA achieves higher capacity than MC-CDMA due to its finer resolution in exploiting multiuser diversity with CSIT. Whether multiple-input-multiple-output (MIMO) MC-CDMA or OFDMA is a better option in multi-cell system remains unjustified in the literature. In this paper, we study the ergodic capacity and the goodput of MIMO-MC-CDMA and MIMO-OFDMA downlink systems with CSIT in multi-cell scenario assuming that the base station has the knowledge of the average inter-cell interference level only. Several types of users modeling different interference patterns are considered: (I) high data rate delay-insensitive users, (II) high data rate delay-sensitive .users, and (III) voice users (low data rate and bursty). Optimal resource allocation algorithms are used to compute the capacities of the systems, while a simple heuristic is used to obtain the achievable goodputs for both systems. The effects of path loss, number of antennas and different user types are studied and insightful results are obtained. We find that OFDMA has a higher system goodput for both Type I and Type II high data rate users, while MC-CDMA has a higher goodput for Type III users. Compared to MC-CDMA, the goodput of an OFDMA system is more sensitive to the activity factor of the voice users and suffers from noticeable loss. This demonstrates the superiority of the two systems in different practical situations.

Hybrid visible light communications in Intelligent Transportation Systems with position based services

This paper proposes a hybrid communication system for Intelligent Transportation System (ITS) utilizing visible light and radio communications for position-based services. The directionality of light communication is used to distribute position-based keys to vehicles such that they can extract information related only to their desired lanes. The extended coverage of radio communication is used to provide stable data communication complementing the visible light communications (VLC) systems. This paper provides system models and important parameters for designing a hybrid ITS system. The system performance is evaluated by simulation, and the results demonstrate a considerable increase of the effective communication area and receivable information using the proposed hybrid ITS system compared with a VLC system.

Reduced-complexity power allocation in zeroforcing MIMO-OFDM downlink system with multiuser diversity

This paper considers the power allocation for a multiuser MIMO-OFDM downlink system using zero-forcing multiplexing, with the objective to maximize the system capacity. An optimal power allocation algorithm has been developed and is presented. While the optimal power allocation algorithm achieves promising capacity, its complexity increases exponentially with the number of users. We develop two power allocation algorithms whose complexities increase only linearly with the number of users, with only negligible loss of capacity compared to the optimal algorithm. Numerical results and analysis are provided to evaluate the system capacities of the optimal and the reduced-complexity algorithms. At SNR of 10 dB, the optimal and the suboptimal schemes attain 40% capacity gain over a round-robin scheme with water filling and 60% capacity gain over a round-robin scheme with equal power allocation

Low-complexity antenna diversity receivers for WCDMA handsets

A low-complexity antenna diversity architecture using a single receiver chain together with a novel combining algorithm suitable for WCDMA systems is designed to improve handset performance in the mobile downlink. Simulation results are presented comparing different diversity receiver configurations under several channel conditions. System sensitivity to the Doppler frequency and correlation between diversity branches are investigated. Both selection diversity (SD) and a receiver using two front-ends (2FE) are compared with the combining algorithm. The results show that the combining architecture outperforms SD, which is a proposed candidate for WCDMA handset by a gain of about 1.25 dB, while being inferior to 2FE by only 0.75 dB.

Angular-Domain Channel Model and Channel Estimation for MIMO System

A new MIMO channel model based on angular-domain, double-directional channel model is proposed. Angular domain is a more appropriate coordinate to describe the MIMO channel, since the incoming rays and outgoing rays appear in clusters in the real world. This model is developed based on linear space and sampling theory and is applicable to any practical antenna geometry and includes the actual radiation pattern of the antenna array. It is compared with other existing models, including i.i.d., Kronecker, virtual channel representation and eigenbeam model, using channel estimation based model validation. Both simulation and experimental results show that it outperforms other angular-domain based models in terms of channel estimation error. A more accurate channel model not only facilitates better performance analysis, but also channel estimation in practical communication system.

Low-complexity antenna diversity receivers with SINR enhancement for WCDMA handsets

This paper introduces a combining algorithm for WCDMA handsets using a single front-end circuitry to enhance signal-to-interference-and-noise ratio (SINR). The proposed algorithm can have a gain of about 6 dB over selection diversity, the conventional design for multiple antenna system for handsets. Analytical equations based on a multipath channel model are derived and simulation results and performance evaluation comparing several diversity receivers are also presented.