Jian-Kang Zhang

Equal-diagonal QR decomposition and its application to precoder design for successive-cancellation detection

In multiple-input multiple-output (MIMO) multiuser detection theory, the QR decomposition of the channel matrix H can be used to form the back-cancellation detector. In this paper, we propose an optimal QR decomposition, which we call the equal-diagonal QR decomposition, or briefly the QRS decomposition. We apply the decomposition to precoded successive-cancellation detection, where we assume that both the transmitter and the receiver have perfect channel knowledge. We show that, for any channel matrix H, there exists a unitary precoder matrix S, such that HS=QR, where the nonzero diagonal entries of the upper triangular matrix R in the QR decomposition of HS are all equal to each other. The precoder and the resulting successive-cancellation detector have the following properties. a) The minimum Euclidean distance between two signal points at the channel output is equal to the minimum Euclidean distance between two constellation points at the precoder input up to a multiplicative factor that equals the diagonal entry in the R-factor. b) The superchannel HS naturally exhibits an optimally ordered column permutation, i.e., the optimal detection order for the vertical Bell Labs layered space-time (V-BLAST) detector is the natural order. c) The precoder S minimizes the block error probability of the QR successive cancellation detector. d) A lower and an upper bound for the free distance at the channel output is expressible in terms of the diagonal entries of the R-factor in the QR decomposition of a channel matrix. e) The precoder S maximizes the lower bound of the channels free distance subject to a power constraint. f) For the optimal precoder S, the performance of the QR detector is asymptotically (at large signal-to-noise ratios (SNRs)) equivalent to that of the maximum-likelihood detector (MLD) that uses the same precoder. Further, We consider two multiplexing schemes: time-division multiple access (TDMA) and orthogonal frequency-division multiplexing (OFDM). We d

Full-Diversity Codes for MISO Systems Equipped With Linear or ML Detectors

In this paper, a general criterion for space-time block codes (STBC) to achieve full diversity with a linear receiver is proposed for a wireless communication system having multiple transmitter and single receiver antennas [multiple-input-single-output (MISO)]. Particularly, the STBC with Toeplitz structure satisfies this criterion, and therefore, enables full diversity. Further examination of this Toeplitz STBC reveals the following important properties: (1) the symbol transmission rate can be made to approach unity; (2) applying the Toeplitz code to any signalling scheme having nonzero distance between the nearest constellation points results in a nonvanishing determinant. In addition, if quadratic-amplitude modulation (QAM) is used as the signalling scheme, then for independent MISO flat-fading channels, the Toeplitz codes is proved to approach the optimal diversity-versus-multiplexing tradeoff with a zero-forcing (ZF) receiver when the number of channel uses is large. This is, so far, the first nonorthogonal STBC shown to achieve the optimal tradeoff for such a receiver. On the other hand, when maximum-likelihood (ML) detection is employed in a MISO system, the Toeplitz STBC achieves the maximum coding gain for independent channels. When the channel fading coefficients are correlated, the inherent transmission matrix in the Toeplitz STBC can be designed to minimize the average worst case pairwise error probability.

Linear toeplitz space time block codes

In this paper we consider coherent flat fading wireless communication systems with multiple transmitter antennas and single receiver antenna (MISO). We propose a Toeplitz linear space time block code (STBC) that converts an original MISO flat fading channel into a Toeplitz virtual multiple inputs multiple outputs (MIMO) channel. We show that our proposed code has the following main features: (a) The symbol transmission rate is (T-M + 1)/T, where M is the number of transmitter antenna and T is the number of channel uses (T > M). (b) Linear receivers (zero-forcing and minimum mean square error) can extract full diversity. Moreover, when the channel coefficients are independent and the maximum likelihood (ML) detector is employed, our Toeplitz STBC minimizes the exact worst case average pair-wise error probability, (c) When channels are correlated, we design our Toeplitz STBC that minimizes the exact average worst case pair-wise error probability. By transforming this non-convex optimization problem into a convex one, the problem can be solved efficiently by employing an interior point method. In particular, when the design criterion in question is approximated by the Chernoff bound, we obtain a closed form solution, (d) Finally, for the independent MISO flat fading system, we prove that our proposed codes can approach the optimal diversity-vs-multiplexing tradeoff developed by Zheng and Tse with a linear zero-forcing receiver when the number of channel uses is large

A fractionally spaced blind equalizer based on linear programming

We formulate the blind fractionally spaced equalization (FSE) problem as one that minimizes a piecewise linear convex function subject to some linear constraints on the equalizer parameters. We show that this formulation achieves both the interference removal and the carrier phase recovery when the input signal possesses a certain quadrature amplitude modulation (QAM) type symmetry. A fast linear programming implementation is presented to solve the convex minimization problem. Computer simulation results indicate the new linear programming-based FSE is able to accurately equalize channels that are known to be not equalizable by T-spaced (or baud rate) blind equalizers and yields superior performance to other blind FSE methods.

Ergodic Channel Capacities for the Amplify-and-Forward Half-Duplex Cooperative Systems

In this paper, the ergodic channel capacities are established for the amplify-and-forward (AF) half-duplex cooperative systems, which consist of a source node, a destination, and multiple-relay nodes. The relay nodes assist the transmission from the source node to the destination. Since the channel matrices for the cooperative systems involve product of Gaussian random variables, which are no longer Gaussian, the approach in obtaining the ergodic channel capacity for conventional multiple-input-multiple-output (MIMO) Gaussian channels (Telatar, 1999) is not applicable. By using a novel approach, we have arrived at the following conclusions about the ergodic channel capacities for the AF cooperative systems. For the single antenna AF relay systems in which all nodes are equipped with one antenna, the optimal covariance matrix of the input signals to achieve the ergodic channel capacity is diagonal, and the diagonal elements are obtained by solving optimization problems of multidimensional integrals. These diagonal entries are not all equal even if all the channel gains in the cooperative systems are independent and identically distributed (i.i.d.) Gaussian with unit variance. Therefore, a white input signal for the AF cooperative system may not achieve the ergodic channel capacity of the system. This is in direct contrast to the case of conventional multiple-input-single-output (MISO) systems having i.i.d. Gaussian channel gains of unit variance, in which case, the ergodic capacity is achieved if the input covariance matrix is a scaled identity matrix. For the MIMO relay system in which the nodes have multiple antennas, the input covariance matrix to achieve the ergodic capacity is block diagonal and each block diagonalizes the autocorrelation of channel matrix from the source to the destination. This is different from the case of conventional MIMO systems, where the input covariance matrix to achieve the ergodic channel capacity diagonalizes the channel autocorrelation matrix of the MIMO system (Tulina, Lozano, and Verdu, 2006). If the channel gains in conventional MIMO systems are correlated Gaussian random variables, the input covariance matrix is a full matrix, not block diagonal; and if the channel gains are i.i.d. Gaussians, the optimal input covariance matrix is a scaled identity. The observations obtained in this paper reveal useful insights of how the AF cooperative systems ldquomimicrdquo the conventional MISO and MIMO systems from the ergodic channel capacity perspective.

Trace-Orthonormal Full-Diversity Cyclotomic Space–Time Codes

In this paper, we consider the design of full-diversity space-time codes for a coherent multiple-input multiple-output (MIMO) communication system. Starting from both the information theoretic and detection error viewpoints, we first establish that a desirable property for general linear dispersion (LD) codes is to have an interunitary as well as an intraunitary structure-a structure we call trace-orthonormality. By imposing the trace-orthonormal structure on an LD code and applying cyclotomic number theory, we establish, for an arbitrary number of transmitter and receiver antennas, a systematic and simple method to jointly design a unitary cyclotomic matrix, the Diophantine number, and the corresponding constellation for an LD code. As a result, this enables us to construct full-diversity rectangular cyclotomic LD codes with any symbol transmission rate less than or equal to the number of transmitter antennas. In addition, for the case when the number of transmitter antennas is greater than the number of receiver antennas, by taking advantage of the delay, we also arrive at the design of a special trace-orthonormal full-diversity cyclotomic space-time block code which, for the number of transmitter antenna being equal to 2m, can be proved to minimize the worst case pairwise error probability of a maximum-likelihood (ML) detector for a q-ary quadrature amplitude modulation (QAM) signal constellation and, therefore, achieves optimal coding gain. Computer simulations show that these codes have bit-error performance advantages over currently available codes

Optimal precoder for amplify-and-forward half-duplex relay system

In this paper, an optimal unitary precoder is designed for an amplify-and-forward (AF) half-duplex relay system to obtain the maximum coding gain while the original ergodic channel capacity for the relay system is kept unchanged. A closed-form design is derived for quadrature amplitude modulation (QAM) signals by employing the properties of Farey sequence in number theory. Simulation results indicate that the proposed design greatly improves the bit error rate (BER) performance for the relay system.

Design of block transceivers with MMSE decision feedback detection

This paper presents a method for jointly designing the transmitter-receiver pair in a block-by-block transmission system that employs minimum mean square error intra-block decision feedback detection. We provide a recursive closed-form expression for a transceiver which maximizes the Gaussian mutual information and also minimizes the bit error rate at moderate-to-high signal-to-noise ratios (in the absence of error propagation). The proposed design generates uncorrelated inputs to the decision device with equal signal-to-interference-and-noise ratios. These properties suggest that one can approach the capacity of the block transmission system using (independent instances of) the same (Gaussian) code for each element of the block. Our simulation studies indicate that the proposed transceiver performs significantly better than standard transceivers, and that it retains its performance advantages in the presence of error propagation.

Blind Detection with Unique Identification in Two-Way Relay Channel

This paper considers the blind detection for a two-way relay system in which two source nodes exchange information via a relay node by amplify-and-forward relaying. An efficient transmission scheme is first proposed to achieve unique identifications of both the transmitted symbols and channel coefficients at a noise-free receiver using the M-ary phase shift keying modulation. Blind receivers based on the generalized likelihood ratio test are then derived for both the reciprocal and nonreciprocal channels with additive Gaussian noise. The least square error-based receiver is also studied for the case without prior knowledge of the noise power for detection. Moreover, constellation selection algorithms are proposed to achieve a uniform transmission bit rate for the ease of implementation. Finally, numerical results are provided to validate the proposed schemes.

Blind adaptive FRESH filtering for signal extraction

A blind adaptive method called the BA-FRESH (optimum frequency-shift) filtering technique is proposed and its convergence analyzed. The BA-FRESH filter does not require training signals nor knowledge of the statistics of the desired signal. It is capable of separating desired signals from spectrally overlapping interference by knowing only the cycle frequencies of the signals.