This letter addresses the problem of robust transceiver design for the multiuser multiple-input-multiple-output (MIMO) downlink where the channel state information at the base station (BS) is imperfect. A stochastic approach which minimizes the expectation of the total mean square error (MSE) of the downlink conditioned on the channel estimates under a total transmit power constraint is adopted. The iterative algorithm reported in [2] is improved to handle the proposed robust optimization problem. Simulation results show that our proposed robust scheme effectively reduces the performance loss due to channel uncertainties and outperforms existing methods, especially when the channel errors of the users are different.

In this letter, we discuss the problem of receive antenna selection in the downlink of multiuser multiple-input multiple-output (MIMO) systems with Tomlinson-Harashima precoding (THP), where the number of receivers is assumed equal to that of transmit antennas. Based on the criterion of maximum system sum-capacity, a per-layer receive antenna selection scheme is proposed. This scheme, which selects one receive antenna for each receiver, can well exploit the nonlinear and successive characteristics of THP. Two models are established for the proposed per-layer scheme and the conventional per-user scheme. Both the theoretical analysis and simulation results indicate that the proposed scheme can greatly improve the equivalent channel power gains and the system sum-capacity.

In this letter, we propose an efficient user selection algorithm aiming to select users with less spatially correlation and meet the user number limit of zero-forcing beamforming in downlink multiuser MIMO systems. This algorithm yields a considerable complexity reduction with only a small loss in performance and it only needs partial users' CSI feedback. Coupled with the algorithm, a null space updating method in O(K2) time and a modified proportional fair scheduling algorithm are also proposed.

In this letter, we propose an adaptive turbo coded modulation scheme for orthogonal frequency division multiplexing (OFDM), and three power allocation algorithms with transmit power variable over subcarriers, variable over subbands and constant over subcarriers. Our object is to improve the overall throughput under target bit-error-rate (BER). Simulation results show that our fixed power adaptation scheme exhibits a more than 7 dB signal-noise-ratio (SNR) gain relative to non-adaptive turbo coded modulation, and an about 2 dB additional gain can be achieved with our variable power algorithm employed. We also discuss the effect of the number of subbands on throughput performance in our adaptive scheme.

The exact calculation of the ergodic and outage capacity for Rayleigh fading single-input multiple-output (SIMO) channels in the presence of unequal-power Rayleigh fading interferers is mathematically quite challenging due to the complicated distribution of the capacity. In this paper, a SIMO system with M receive antennas and N interferers is considered. Based on some statistical results, the closed-form upper and lower bound for the ergodic and outage capacity are derived respectively. These bounds are shown to be simple to compute and appear to be quite tight.

Spatial multiplexing (SM) offers a linear increase in transmission rate without bandwidth expansion or power increase. In SM systems, the LMMSE receiver establishes a good tradeoff between the complexity and performance. The performance of the LMMSE receiver would be degraded by MIMO channel estimation errors. This letter focus on obtaining the asymptotic convergence of output interference power and SIR performance for the LMMSE receiver with channel uncertainty. Exactly matched simulation results verify the validity of analysis in the large-system assumption. Furthermore, we find that the analytical results are also valid in the sense of average results for limited-scale system in spite of the asymptotic assumption used in derivation.

In orthogonal frequency-division multiplex access (OFDMA) downlink systems, the carrier-frequency offset (CFO) between the multiple transceivers introduces inter-carrier interference (ICI). In this letter, we propose an iterative precoding scheme to suppress the ICI due to CFO. This scheme is applied at the transmitter, and can jointly cancel the ICI for all the receivers. Moreover, by the studies of the convergence behavior of the iterations, a sufficient condition for the convergence is presented. The theoretical analysis and simulation results both show that this iterative scheme is equivalent to the zero-forcing (ZF) scheme in function, but with much lower complexity.

Downlink multiuser MIMO system has attracted considerable attention recently for its potential to increase the system capacity. However, due to the limitation on the number of transmit antennas, when there are more users than can be supported simultaneously in a cell, other multiple access schemes, such as TDMA, must be applied in combination with multiuser MIMO. In this paper, we aim to design practical user scheduling algorithms to maximize the system capacity. Because the brute-force search for optimal user allocation is computationally prohibitive, we propose three low complexity suboptimal scheduling algorithms that offer both low complexity and high performance.

Two important lemmas on the determinant of random matrixes are deduced in this paper. Then based on these results, expression for the mean capacity of MIMO system over Rayleigh fading channels is obtained. This expression requires little calculation and is simple and efficient compared with conventional methods, and furthermore, it gives an explicit relation on the mean capacity of MIMO systems with antenna numbers and the relation of mean capacity with signal to noise ratio (SNR). Accuracy of this theoretic formula has been verified by computer simulation.

This letter proposed a linear precoding scheme for the V-BLAST system that requires only knowledge of the statistical CSI; the transmitter does not need the instantaneous CSI. Power allocation on the eigenmodes of the transmit correlation matrix is one way to minimize bit error rate (BER). Simulation results show that the proposed precoding V-BLAST system provides a significant reduction in the BER compared with the conventional V-BLAST systems.

Vector precoding is a nonlinear broadcast precoding scheme in the downlink of multi-user MIMO systems which outperforms linear precoding and THP (Tomlinson-Harashima Precoding). This letter discusses the problem of joint receive antenna selection in the multi-user MIMO downlink with vector precoding. Based on random matrix analysis, we derive a simple heuristic selection criterion using singular value decomposition (SVD) and carry out an exhaustive search to determine for each user which receive antenna should be used. Simulation results reveal that receive antenna selection using our proposed criterion obtains the same diversity order as the optimal selection criterion.

This letter introduces a novel multi-user detection method, successive interference cancellation based on the order of log-likelihood-ratio(LLR-SIC), for code division multiple access (CDMA) systems. Unlike the conventional successive interference cancellation (SIC) based on the order of correlation, LLR-SIC operates on the fact that the user with the largest absolute value of log-likelihood ratio (LLR) should be first detected and cancelled from received signal. Simulation results show that LLR-SIC significantly outperforms the conventional SIC and partial parallel interference cancellation (P-PIC) over Rayleigh fading channels, and that LLR-SIC performance is not sensitive to channel estimation error at medium Eb/N0.

In this letter, a fixed-power variable-rate turbo coded modulation scheme for orthogonal frequency division multiplexing (OFDM) is proposed and subband adaptation algorithm based on capacity evaluation is presented. Our object is to improve the overall throughput under target bit-error-rate (BER). Simulation results show that our adaptive OFDM scheme exhibits an about 2.5-5 dB signal-noise-ratio (SNR) gain with target BER of 10-4 and subband number of 16 relative to fixed threshold adaptive turbo coded modulation. Moreover, unlike fixed threshold adaptation, with the number of subband decreased, throughput performance is not degraded due to our capacity evaluation algorithm.

In this letter, the performance improvement by the deployment of multiple antennas in cognitive radio systems is studied from a system-level view. The term opportunistic spectrum efficiency (OSE) is defined as the performance metric to evaluate the spectrum opportunities that can actually be exploited by the secondary user (SU). By applying a simple energy combining detector, we show that deploying multiple antennas at the SU transceiver can improve the maximum achievable OSE significantly. Numerical results also reveal that the improvement comes from the reduction of both the detection overhead and the false alarm probability.

In spectrum-sharing systems where the secondary user (SU) opportunistically accesses the primary user (PU)'s licensed channel, the SU should satisfy both the transmit power constraint of the SU transmitter and the received power constraint at the PU receiver. This letter studies the ergodic capacity of spectrum-sharing systems in fading channels. The ergodic capacity expression along with the optimal power allocation scheme is derived considering both the average transmit and received power constraints. The capacity function in terms of the two power constraints is found to be divided into transmit power limited region, received power limited region and dual limited region. Numerical results in Rayleigh fading channels are presented to verify our analysis.

In this letter, Multicarrier Code Division Multiple Access (MC-CDMA) technique combined with Space Time Block Code (STBC) is analyzed in the case of two transmit antennas. A multicarrier transmit diversity scheme with antenna selection is proposed. The transmission power is allocated onto the antenna which has larger channel gain instead of the two antennas uniformly. Simulation results show that the new scheme has considerable performance gain compared to the Alamouti's scheme.

Based on the minimum mean square error (MMSE) detection with iterative soft interference cancellation (SoIC), we propose an adaptive MMSE (A-MMSE) algorithm which acts as an MMSE operator at the beginning of iteration and a maximum ratio combination (MRC) when the interference is nearly cancelled. In our algorithm, a modified metric matrix based on the reliability of soft information from the decoder output is multiplied by the interference part of channel correlation matrix to update the detection operator. The simulation results have shown that this A-MMSE iterative SoIC algorithm can achieve significant performance advantage over the traditional MMSE iterative SoIC algorithm.

This letter focuses on the simplified capacity evaluation for the downlink of a distributed antenna system (DAS) with random antenna layout. Based on system scale-up, we derive a good approximation of the downlink capacity by developing the results from random matrix theory. We also propose an iterative method to calculate the unknown parameters in the approximated expression of the downlink capacity. The approximation is illustrated to be quite accurate and the iterative method is shown to be quite efficient by Monte Carlo simulations.

In this letter, we address the problem of downlink power allocation for the generalized distributed antenna system (DAS) with cooperative clusters. Considering practical applications, we assume that only the large-scale channel state information is available at the transmitter. The power allocation scheme is investigated with the target of ergodic achievable sum rate maximization. Based on some approximations and the Rayleigh Quotient Theory, the simple selective power allocation scheme is derived for the low SNR scenario and the high SNR scenario, respectively. The methods are applicable in practice due to their low complexity.

In this letter, we investigate the sum-rate capacity of a power-controlled multi-user distributed antenna system (DAS) with antennas deployed symmetrically on a circle. The sum-rate capacity, when divided by user number, is proved to converge to an explicit expression as user number and antenna number go to infinity with a constant ratio. We further show how this theoretical result can be used to optimize antenna deployment. Simulation results are also provided to demonstrate the validity of our analysis and the applicability of the asymptotic results to a small-scale system.