Ayman Assra

EM-Based Joint Channel Estimation and Data Detection for MIMO-CDMA Systems

In this paper, we present an iterative joint channel-estimation and data-detection technique for multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) systems over Rayleigh fading channels. The proposed receiver performs channel estimation and data detection using the expectation-maximization (EM) algorithm. We derive a closed-form expression for the optimized weight coefficients of the EM algorithm, which is shown to provide large performance improvement relative to the conventional equal-weight EM-based signal decomposition. Our results show that the receiver can achieve near-optimum performance with modest complexity using very few training symbols. Furthermore, our simulation results confirm that the proposed receiver is near-far resistant and offers fast convergence in severe near-far scenarios.

An EM Approach for Cooperative Spectrum Sensing in Multiantenna CR Networks

In this paper, a cooperative wideband spectrum sensing scheme based on the expectation-maximization (EM) algorithm is proposed for the detection of a primary user (PU) system in multiantenna cognitive radio (CR) networks. Given noisy signal observations from N secondary users (SUs) over multiple subbands at a fusion center (FC), prior works on cooperative spectrum sensing often use the set of received subband energy as decision statistics over the sensing interval. However, to achieve satisfactory performance, knowledge of the channel state information (CSI) and the noise variances at all the SUs is required by these algorithms. To overcome this limitation, our proposed method, which is referred to as joint detection and estimation (JDE), adopts the EM algorithm to jointly detect the PU signal and estimate the unknown channel frequency responses and noise variances over multiple subbands in an iterative manner. Various aspects of this proposed EM-JDE scheme are investigated, including a reliable initialization strategy to ensure convergence under practical conditions and a distributed implementation to reduce communication overhead. Under the assumption of perfect estimation for the channel frequency responses and noise variances, we further show that the proposed EM-JDE converges to the maximum-likelihood (ML) solution, which serves as an upper bound on its performance. Monte Carlo simulations over Rayleigh fading channels show that the proposed scheme significantly improves the performance of spectrum detection by exploiting the diversity of the spatially distributed SUs with multiple antennas.

BER analysis of space-time diversity in CDMA systems over frequency-selective fading channels

The performance of direct-sequence code division multiple access (DS-CDMA) using space-time spreading system, over frequency-selective fading channels, is investigated. The underlying transmit diversity scheme, previously introduced in the literature, is based on two transmit and one receive antenna. It was shown that when employed in flat fast-fading channels, the received signal quality can be improved by utilising the spatial and temporal diversities at the receiver side. We study the problem of multiuser interference in asynchronous CDMA systems that employ transmit/receive diversity using space-time spreading. To overcome the effects of interference, a decorrelator detector is used at the base station. Considering binary phase-shift keying transmission, we analyse the system performance in terms of its probability of bit error. In particular, we derive the probability of error over frequency-selective Rayleigh fading channels for both fast and slow-fading channels. For the fast-fading channel, both simulations and analytical results show that the full system diversity is achieved. On the other hand, when considering a slow-fading channel, we show that the scheme reduces to conventional space-time spreading schemes where the diversity order is half of that of fast-fading.

A Channel-Estimation and Data-Detection Scheme for Multiuser MIMO-CDMA Systems in Fading Channels

In this paper, we examine the effect of channel estimation errors on the performance of multiple-input-multiple-output (MIMO) systems that employ code-division multiple-access (CDMA) transmission. Channel estimation based on training techniques has widely been considered throughout the literature. However, employing these training techniques in MIMO-CDMA systems degrades system performance due to multiuser interference. This degradation is clear as the diversity advantage of the MIMO system diminishes with the increased level of interference. As a remedy to this problem, we propose a channel-estimation and data-detection scheme based on the superimposed training technique for space-time spreading systems. The proposed scheme enhances the performance of the space-time system by eliminating the interference effect from both the channel and data estimates using two decorrelators: channel and data decorrelators. We investigate the performance of the proposed estimation technique considering an asynchronous CDMA uplink transmission over frequency-selective slow-fading channels. In particular, we analyze the bit-error-rate (BER) performance of the multiuser system with two-transmit-antenna and V-receive-antenna configuration over Rayleigh fading channels. Compared with other conventional estimation techniques, our results show that the proposed estimation technique is more robust to channel-estimation errors. Furthermore, both simulations and analytical results indicate that full system diversity is achieved.

Performance of Space-Time Diversity in CDMA Over Frequency-Selective Fading Channels

Asynchronous direct-sequence code-division multiple-access (DS - CDMA) using space-time spreading system is investigated over frequency-selective fast-fading channels. The underlaying transmit diversity scheme, previously introduced in the literature, is based on two transmit and one receive antenna. It was shown that when employed in flat fast-fading channels, the received signal quality can be improved by utilizing the spatial and temporal diversities at the receiver side. Here, the bit-error-rate (BER) performance of the underlying system is investigated for an uplink transmission where a decorrelator detector is used at the base station receiver. In particular, we derive a closed form expression for the probability of error over frequency-selective fast-fading channels. The analytical BER is derived as a function of both the number of multipath and users. The BER results show that the space-time spreading scheme exploits both the temporal and spatial diversity in a time-varying multipath channel. The results also show the accuracy of the derived expression when compared with simulation results for different number of users.

Iterative joint channel estimation and data detection for MIMO-CDMA systems over correlated fading channels

In this paper, we introduce an iterative joint channel estimation and data detection technique based on the expectation-maximization (EM) algorithm for multiple-input multiple-output (MIMO) code-division multiple-access (CDMA) systems over correlated fading channels. Based on the Kronecker model, we study the effect of channel correlation on the space-time (ST) EM-based joint detection and estimation (JDE) receiver assuming that there is no prior knowledge of channel correlation at the receiver. Then, we derive a new ST EM-based JDE receiver, which adapts the correlation between different MIMO channel coefficients. Different from the uncorrelated MIMO channel, the optimized weight coefficient are derived jointly based on the minimum mean-square error (MMSE) criterion. Considering fixed correlation channel matrix, our results show that our proposed receiver attains the diversity gain of uncorrelated MIMO channels. In addition, the effect of channel correlation is only interpreted as signal-to-noise-ratio (SNR) loss, especially in high receive correlation scenarios.

Joint Decorrelating Channel and Data Estimation for Space-Time Spreading Systems

In this paper, we propose a new joint channel and data detection scheme based on the superimposed training technique for space-time spreading systems. The proposed scheme enhances the performance by eliminating the multiple access interference from both the channel and data estimation by employing two decorrelators: channel and data decorrelators. On a frequency-selective fading channel, our simulation results show that the proposed scheme outperforms other conventional joint channel and data estimation techniques. Moreover, unlike other conventional detection techniques, our proposed estimation technique is shown to achieve the full system diversity.

EM Channel Estimation and Data Detection for MIMO-CDMA Systems over Slow-Fading Channels

In this paper, we present an iterative joint channel estimation and data detection technique for multiple-input multiple-output (MIMO) code-division multiple-access (CDMA) systems over Rayleigh fading channels. The proposed receiver performs the channel estimation and data detection using the expectation-maximization (EM) algorithm. We derive a closed-form expression for the optimized weight coefficients of the EM algorithm which is shown to provide a large performance improvement relative to the conventional equal-weight EM-based signal decomposition. Our results show that the receiver can achieve near-optimum performance with modest complexity using very few training symbols. Furthermore, our simulation results confirm that the proposed receiver is near-far resistant and offers fast convergence in severe near-far scenarios.

Joint Iterative Channel Estimation and Data Detection for MIMO-CDMA Systems over Frequency-Selective Fading Channels

In this paper, we present an iterative joint channel estimation and data detection technique for multiple-input multiple-output (MIMO) code-division multiple-access (CDMA) systems over frequency-selective fading channels. Based on the expectation-maximization (EM) algorithm, the proposed iterative receiver achieves a performance close to the optimum maximum-likelihood (ML) receiver. In addition, the performance of the proposed receiver is optimized through weight coefficients using the minimum mean-square error (MMSE) criterion. Compared to the single-user bound, our results show that the proposed receiver can mitigate the multiple-access interference and attain the full system diversity. Furthermore, our simulation results confirm that the proposed receiver is near-far resistant and offers fast convergence in severe near-far scenarios.

Performance of Superimposed Training-Based Channel Estimation in MIMO-CDMA Systems

In this paper, we investigate the performance of superimposed training-based channel estimation techniques considering an asynchronous code-division multiple-access (CDMA) uplink transmission over frequency-selective fading channels. In that, we analyze the performance of a channel estimation and data detection scheme based on superimposed training for space-time spreading (STS) systems. Our results show that the scheme enhances the performance of the space-time system by eliminating the interference effect from both the channel and data estimates using two decorrelators; channel and data decorrelators. Compared with other conventional estimation techniques, the underlying system is more robust to channel estimation errors. Furthermore, both simulations and analytical results indicate that full system diversity is achieved.