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Orthogonal frequency division multiple access (OFDMA) and multicarrier code division multiple access (MCCDMA) 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.

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. © 2007 IEEE.

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.

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.

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.

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.

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.

In wireless communications, spectrum is a scarce resource and hence imposes a high cost on the high data rate transmission. Fortunately, the emergence of multiple antenna system has opened another very resourceful dimension – space, for information transmission in the air. It has been demonstrated that multiple antenna system provides very promising gain in capacity without increasing the use of spectrum, hence leading to a breakthrough in the data rate of wireless communication systems. Since then, multiple-input multiple-output (MIMO) system has become one of the major focuses in the research community of wireless communications and information theory. The study of the performance limits of MIMO system becomes very important since it gives a lot of insights in understanding and designing the practical MIMO systems. There has been lots of literature studying the performance limits of MIMO system. Most of them are investigated in terms of channel capacity. However, channel capacity only characterizes the system performance when the block length of the code is infinite. While it sheds some light on how the ultimate throughput of the system vary with different system parameters, it does not tell us the error probability when the block length is finite, which is an important measure of interest in practical system design. On the other hand, the study on the error probability or performance limits of MIMO system for finite block length is however relatively less extensive. In this thesis, we aim to study on the performance limits of some MIMO systems, in terms of achievable throughput subject to a given error probability and a given finite block length. The achievable throughput is sort of an extension of capacity to the finite block length case. We focus in MIMO system with continuous channel inputs, i.e. the modulation is not discrete. To obtain the achievable throughput, we study the error probability of the optimal code, given the data rate, the finite block length

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 10dB, 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.

We present an online algorithm to track multiple narrowband burst-type interferers in wideband communication systems. The arrivals/departures and the strengths of the interferers are identified with the use of HMM. This algorithm is targeted for wideband wireless systems which are overlaid on top of multiple narrowband services whose traffic can be bursty. This algorithm can be used to suppress or avoid interference for various kinds of communication systems including both single-carrier systems and multi-carrier systems such as OFDM. As an example, we apply the algorithm in a Direct-Sequence Spread-Spectrum system for interference suppression. Simulation results and analysis are presented. The simulation results show that our algorithm achieves very satisfactory performance in interference suppression compared to other NBI suppression methods in terms of both BER and robustness to the burst rate of the interferers.

Nowadays, cellular mobile phone system is one of the most popular applications of wireless communications. The demand of mobile phone service is very large and such demand is still growing. The technical challenges of a cellular phone system include large capacity, satisfactory voice quality, small size and low power consumption of handsets. A lot of wireless communications techniques have been suggested and applied in mobile phone systems. Such techniques include various multiple access methods like FDMA, TDMA and CDMA, powerful coding, smart antennas and multiuser interference cancellation. Smart antenna is the technique of interest in this thesis. Though the cellular system can be improved by enhancing the performances of both basestations and mobiles, relatively less literature talks about the latter. This thesis, however, focuses on a smart antenna receiver design on the handset, which emphasizes on the improvement of signal quality and system capacity with the least expense of the circuit complexity and size, which is a key concern of handset as mentioned before.