Mohammad Jaber Borran

On design criteria and construction of non-coherent space-time constellations

We consider the problem of digital communication in a Rayleigh flat-fading environment using a multiple-antenna system, when the channel state information is available neither at the transmitter nor at the receiver. It is known that at high signal-to-noise ratio (SNR), or when the coherence interval is much larger than the number of transmit antennas, a constellation of unitary matrices can achieve the capacity of the noncoherent system. However, at low SNR, high spectral efficiencies, or for small values of coherence interval, the unitary constellations lose their optimality and fail to provide an acceptable performance. In this work, inspired by the Steins lemma, we propose to use the Kullback-Leibler (KL) distance between conditional distributions to design space-time constellations for noncoherent communication. In fast fading, i.e., when the coherence interval is equal to one symbol period and the unitary construction provides only one signal point, the new design criterion results in pulse amplitude modulation (PAM)-type constellations with unequal spacing between constellation points. We also show that in this case, the new design criterion is equivalent to design criteria based on the exact pairwise error probability and the Chernoff information. When the coherence interval is larger than the number of transmit antennas, the resulting constellations overlap with the unitary constellations at high SNR, but at low SNR they have a multilevel structure and show significant performance improvement over unitary constellations of the same size. The performance improvement becomes especially more significant when an appropriately designed outer code or multiple receive antennas are used. This property, together with the facts that the proposed constellations eliminate the need for training sequences and are most suitable for low SNR, makes them a good candidate for uplink communication in wireless systems.

An efficient detection technique for synchronous CDMA communication systems based on the expectation maximization algorithm

Maximum likelihood detection of superimposed signals in code-division multiple access (CDMA) communication systems has a computational complexity that is exponential in the number of users, and its implementation is practically prohibitive even for a moderate number of users. Applying the expectation maximization algorithm to this problem, we decompose the multiuser detection problem into a series of single-user problems, and thus present an iterative computationally efficient algorithm for detection of superimposed signals in synchronous direct-sequence CDMA communication systems. The resulting structure includes the well-known multistage detector as one of its special cases. With a proper choice of its parameters, the new detector can achieve the advantages of both the multistage and conventional detector and have good performance for both strong and weak users.

Design of coded modulation schemes for orthogonal transmit diversity

We propose a concatenated orthogonal space-time code structure to decouple the problems of maximizing spatial and temporal diversity gains in fast Rayleigh fading channels. We also introduce the idea of constellation expansion to design the outer coded modulation scheme. It is shown that the code design criteria reduce to the maximization of Hamming and product distances in the expanded constellation.

Wavelet-based denoising using hidden Markov models

Hidden Markov models have been used in a wide variety of wavelet-based statistical signal processing applications. Typically, Gaussian mixture distributions are used to model the wavelet coefficients and the correlation between the magnitudes of the wavelet coefficients within each scale and/or across the scales is captured by a Markov tree imposed on the (hidden) states of the mixture. This paper investigates correlations directly among the wavelet coefficient amplitudes (sign/spl times/magnitude), instead of magnitudes alone. Our theoretical analysis shows that the coefficients display significant correlations in sign as well as magnitude, especially near strong edges. We propose a new wavelet-based HMM structure based on mixtures of one-sided exponential densities that exploits both sign and magnitude correlations. We also investigate the application of this for denoising the signals corrupted by additive white Gaussian noise. Using some examples with standard test signals, we show that our new method can achieve better mean squared error, and the resulting denoised signals are generally much smoother.

EM-based multiuser detection in fast fading multipath environments

We address the problem of multiuser detection in fast fading multipath environments for DS-CDMA systems. In fast fading scenarios, temporal variations of the channel cause significant performance degradation even with the Rake receiver. We use a previously introduced time-frequency (TF) Rake receiver based on a canonical formulation of the channel and signals to simultaneously combat fading and multipath effects. This receiver uses the Doppler spread caused by rapid time-varying channel as another means of diversity. In dealing with multiaccess interference and as an attempt to avoid the prohibitive computational complexity of the optimum maximum-likelihood (ML) detector, we use the expectation maximization (EM) algorithm to derive an approximate ML detector. The new detector turns out to have an iterative structure very similar to the well-known multistage detector with some extra parameters. At the two extreme values of these parameters, the EM detector reduces to either one-shot TF Rake or generalized multistage detector. For the intermediate values of the parameters, it combines the two estimates to obtain a better decision for the bits of the users. Because of using the EM algorithm, this detector has better convergence properties than the multistage detector; the bit estimates always converge, and if an appropriate initial vector is used, they converge to the global maximizer of the likelihood function. As a result, the new detector provides significantly improved performance while maintaining the low complexity of the multistage detector. Our simulation results confirm the expected performance improvements compared to the base case of the TF Rake as well as the multistage detector used with the TF Rake.

Channel estimation and signal detection for multi-carrier CDMA systems with pulse-shaping filter

We consider the problem of digital communication in a fading environment using the multi-carrier CDMA technology. By incorporating the effect of the pulse-shaping filter in the channel estimation and signal detection modules, we develop new estimator and detector structures which significantly outperform the commonly used time-domain equalizers and matched filter detector. Our simulation results demonstrate the elimination of some of the error floors which one would experience if usual time-domain techniques were used to cancel the effect of non-ideal pulse-shaping filter.

Linear MMSE chip equalization and parallel interference cancellation as applied to 1xEV-DV

1xEV-DV is a 3G CDMA standard, capable of achieving data rates up to 3 Mbps. This standard employs high-order modulation (up to 16-QAM) which makes the detection at the receiver difficult, due to the multipath distortion. In this paper, we investigated the linear minimum mean square error (LMMSE) chip equalizer, as well as the parallel interference canceller (PIC), and their hybrid approaches, as applied to the 1xEV-DV downlink receiver. The results showed that the hybrid receivers perform excellently in all scenarios tested, and outperform previous art LMMSE and PIC receivers.

Partially coherent constellations for multi-carrier systems

For wireless communication systems in high mobility environments, the usual assumption of perfect channel state information at the receiver is not very realistic. In the presence of channel estimation errors, the conventional PSK or QAM constellations, which are designed based on maximizing the minimum Euclidean distance between constellation points, are no longer optimal. Multi-carrier systems, which are the main focus of this work, are no exception to this fact. In this work, we first characterize the estimation variance of a pilot-based channel estimator for multi-carrier systems. Then, based on a previously derived KL-based criterion, we design new constellations for the ITU vehicular A channel at different SNR regions. The proposed constellations show significant performance improvement over the conventional QAM constellations. We show that the error floor due to the residual channel taps can be reduced by as much as one order of magnitude, if the proposed constellations are used in a multi-carrier system.

Performance comparison between MC-CDMA and 1xEV-DV

This paper presents performance comparison between a cellular system implemented according to the IxEV-DV standard, and a MC-CDMA system with similar parameters. The downlink peak data rate of the IxEV-DV system is 3.0912 Mbps. The proposed MC-CDMA system can provide the same data rates with significantly better performance. The MC-CDMA in this paper is comparable to the IxEV-DV system in the sense that same channel coding, modulation and slot structure as defined for IxEV-DV system are used.

Power-efficient non-coherent space-time constellations

We consider the problem of digital communication in a Rayleigh flat-fading environment using a multiple-antenna system, when the channel state information is available neither at the transmitter nor at the receiver. It is known that at high SNR, or when the coherence interval is much larger than the number of transmit antennas, a constellation of unitary matrices can achieve the capacity of the non-coherent system. However, at low SNR, high spectral efficiencies, or for small values of coherence interval, the unitary constellations lose their optimality and fail to provide an acceptable performance. In an earlier work, using a design criterion based on the KL distance between distributions, we have shown, for the case of a single transmit antenna, that at low SNR, a multilevel constellation can provide better error rate performance than the unitary constellation of the same size. In this work, we extend those single antenna results to the case of multiple transmit antennas. The interesting result of this work is that the same idea of multilevel constellations can be generalized to design low-power non-coherent constellations for multiple antennas. The resulting constellations overlap with the unitary constellations at high SNR, but at low SNR they have a multilevel structure and show significant performance improvement over unitary constellations of the same size. The performance improvement becomes especially more significant when a large number of receive antennas are used. This property, together with the facts that the proposed constellations eliminate the need for training sequences and are most suitable for low SNR, makes them a good candidate for uplink communication in wireless systems.