Noncoherent receivers for differential space-time modulation

In this paper, noncoherent receivers for differential space-time modulation (DSTM) are investigated. It is shown that the performance of the previously proposed conventional differential detection (DD) receiver is satisfactory only for very slow flat fading channels. However, conventional DD suffers from a considerable loss in performance even for moderately fast fading, especially if more than one transmit antenna is used. In order to overcome this problem, two improved noncoherent receivers are considered. The first one is the multiple-symbol detection (MSD) receiver. Because of the high computational complexity of MSD, also a low-complexity decision-feedback differential detection (DF-DD) receiver is derived. Analytical and simulation results confirm that both receivers perform equally well and can take full advantage of the enhanced diversity provided by multiple transmit antennas even for fast fading     

Noncoherent space-time coding: An algebraic perspective

The design of space-time signals for noncoherent block-fading channels where the channel state information is not known a priori at the transmitter and the receiver is considered. In particular, a new algebraic formulation for the diversity advantage design criterion is developed. The new criterion encompasses, as a special case, the well-known diversity advantage for unitary space-time signals and, more importantly, applies to arbitrary signaling schemes and arbitrary channel distributions. This criterion is used to establish the optimal diversity-versus-rate tradeoff for training based schemes in block-fading channels. Our results are then specialized to the class of affine space-time signals which allows for a low complexity decoder. Within this class, space-time constellations based on the threaded algebraic space-time (TAST) architecture are considered. These constellations achieve the optimal diversity-versus-rate tradeoff over noncoherent block-fading channels and outperform previously proposed codes in the considered scenarios as demonstrated by the numerical results. Using the analytical and numerical results developed in this paper, nonunitary space-time codes are argued to offer certain advantages in block-fading channels where the appropriate use of coherent space-time codes is shown to offer a very efficient solution to the noncoherent space-time communication paradigm.     

Noncoherent equalization of GMSK using complex receiver structures

In this letter, the performance of the HIPERLAN system in an indoor multipath fading channel is considered. Due to the high carrier frequency and high data rate, a simple noncoherent demodulator followed by a nonlinear equalizer, which includes a RAM and a Viterbi decoder, is proposed to cope with intersymbol interference. The novelty of the proposed equalizer is that a complex noncoherent signal is used. Although the complexity of the receiver is doubled, performance is greatly improved with respect to real receivers     

在Grassmann流形上构造非相干酉空时码

研究基于Grassmann流形的非相干酉空时星座图的设计方法。首先定义了非相干空时码在流形上的酉矩阵框架结构;然后在此框架约束下,将已有的Grassmann流形上最优包络分布的最小Frobenius弦距离作为阈值,通过设置合适的步长来改变酉矩阵中各元素的幅值和相位,在流形上搜索最小Frobenius弦距离大于阈值的点,搜索到事先设定的星座图点数,即构成酉空时星座图。数值仿真结果表明本框架结构非相干Grassmannian酉空时码的性能均优于现有的其他形式非相干酉空时码的性能。     

Layered space-time equalization for wireless MIMO systems

We investigate layered space-time equalization (LSTE) architectures for multiple-input multiple-output (MIMO) frequency-selective channels. At each stage or layer of detection, a MIMO delayed decision feedback sequence estimator (MIMO-DDFSE) is used to tentatively detect a group of selected data streams, among which a subgroup of data streams are output and are canceled from the received signals. With the proposed architectures, the numbers of the tentatively detected data streams and output data streams can be different at different LSTE stages, while the MIMO-DDFSE can also reduce to the special cases of multiple-input single-output decision feedback equalizer (MISO-DFE), MISO-DDFSE, and MIMO-DFE, allowing tradeoffs between performance and complexity. We also derive the equalizer coefficients, discuss timing recovery, and consider channel estimation. Simulation results demonstrate the performance of the proposed LSTE structures and the tradeoffs between performance and complexity of the multistage structure and the single-stage version. We also demonstrate the impact of imperfect channel estimation, imperfect interference cancellation, the number of receive antennas, filter length, and oversampling on performance.     

Turbo space-time equalization of TCM for broadband wireless channels

This paper presents a space-time turbo (iterative) equalization method for trellis-coded modulation (TCM) signals over broadband wireless channels. For fixed wireless systems operating at high data rates, the multipath delay spread becomes large, making it impossible to apply trellis-based equalization methods. The equalizer proposed here consists of a broadband beamformer which processes antenna array measurements to shorten the observed channel impulse response, followed by a conventional scalar turbo equalizer. Since the applicability of trellis-based equalizers is limited to additive white noise channels, the beamformer is required to preserve the whiteness of the noise at its output. This constraint is equivalent to requiring that the finite-impulse response (FIR) beamforming filters must have a power complementarity property. The power complementarity property imposes nonnegative definite quadratic constraints on the beamforming filters, so the beamformer design is expressed as a constrained quadratic optimization problem. The composite channel impulse response at the beamformer output is shortened significantly, making it possible to use a turbo equalizer for the joint equalization and decoding of trellis modulated signals. The proposed receiver structure is simulated for two-dimensional TCM signals such as 8-PSK and 16-QAM and the results indicate that the use of antenna arrays with only two or three elements allows a large decrease in the channel signal-to-noise ratio needed to achieve a 10/sup -4/ bit-error rate.     

Interference suppression for space-time coded CDMA via decision-feedback equalization

A single-user receiver structure is proposed for space-time coded code-division multiple-access (CDMA) downlink in a multiuser frequency-selective channel. This structure is a two-dimensional (2-D) decision-feedback equalizer (2D-DFE) whose filters are optimized based on the MMSE criterion to mitigate noise, intersymbol interference (ISI), and multiuser interference (MUI) with a moderate complexity. By modeling the spreading codes of the interfering users as random sequences, system performance was evaluated using the Gaussian approximation. Two models for the desired user's spreading sequence have been considered and compared. Our numerical results show that in both cases the 2D-DFE exhibits significant performance improvement over the standard space-time coded RAKE, especially in interference-limited conditions. It is also observed that the gain obtained by using DFE in a MISO channel is less that in a SISO channel and this problem can be solved by providing diversity at the receiver.     

Blind and semi-blind equalization for generalized space-time block codes

This paper presents a general framework for space-time codes (STCs) that encompasses a number of previously proposed STC schemes as special cases. The STCs considered are block codes that employ arbitrary redundant linear precoding of a given data sequence together with embedded training symbols, if any. The redundancy introduced by the linear precoding imposes structure on the received data that under certain conditions can be exploited for blind or semi-blind estimation of the transmitted sequence, a linear receiver that recovers the sequence, or both simultaneously. Algorithms based on this observation are developed for the single-user flat-fading case and then extended to handle multiple users, frequency-selective fading, as well as situations where the channel is rank deficient, or there are fewer receive than transmit antennas.     

频域均衡空时分组码技术研究

采用翻转矩阵定义了频域均衡空时分组码的编码方式,分析了它的系统结构和编译码过程,并且给出了信道分解的矩阵表达形式,得到了频域均衡空时分组码的一种简明的分析方法。仿真实验表明:与多载波的正交频分复用空时分组码相比,频域均衡空时分组码在频率选择性衰落信道下具有较好的性能。     

Semiblind channel estimation and equalization for MIMO space-time coded OFDM

Based on the special features of space-time coding (STC) and orthogonal frequency-division multiplexing (OFDM), it is mathematically proved that all channel responses of a multiple-input-multiple-output STC-OFDM system can be identified blindly subject to two ambiguity matrices with a subspace-based method. A method is then presented to resolve the two ambiguity matrices by using few pilot symbols. With the estimated channels, a frequency domain approach is presented to recover the transmitted symbols. The presented semiblind algorithms are valid even if the channel transfer functions are not coprime and do not require precise channel order information (only an upper bound for the orders is required).     

MIMO MAP equalization and turbo decoding in interleaved space-time coded systems

Decoding of space-time codes in frequency-selective fading channels is considered. The approach is based on iterative soft-in soft-out equalization and decoding. It is applicable to space-time coded systems that deploy symbol/bit interleavers. We focus on the equalization stage by extending the Ungerboeck equalizer formulation to a multiple-input multiple-output time-variant channel. The resulting structure comprises a bank of matched filters, followed by an a posteriori probabilities calculator that runs the Bahl-Cocke-Jelinek-Raviv/maximum a posteriori algorithm with an appropriate metric. Simulation results are reported for space-time bit-interleaved codes designed over the enhanced data rates for GSM evolution (EDGE) air interface.     

Single-carrier frequency-domain equalization with decision-feedback processing for time-reversal space-time block-coded systems

In this letter, we propose to introduce frequency-domain equalization with decision-feedback processing (FD-DFE) for time-reversal space-time block-coded (TR-STBC) systems with multiple transmit antennas to effectively achieve both spatial diversity and multipath diversity over frequency-selective channels. This letter will also present a theoretical performance analysis result that relates the minimum mean-squared error (MMSE) to a new tight upper bound for the probability of error. It is shown that the new bound is not only valid for the TR-STBC systems, but also valid for single-input single-output, single-input multiple-output, and multiple-input multiple-output systems with MMSE equalization.     

A general scheme for equalization of space-time block-coded systems with unknown CSI

Recently, space-time block codes (STBCs) have gained much attention as an effective transmit diversity technique to increase the capacity of wireless communication systems. In this paper, we consider a general technique for direct equalization of space-time block-coding systems with unknown channel state information (CSI). This technique is suitable for several existing hybrid STBC schemes, such as STBC/orthogonal-frequency-division multiplexing (STBC-OFDM) and STBC/code-division multiple access (STBC-CDMA). We show that by exploiting the redundancy in the structure of STBC, a zero-forcing equalizer can be constructed without channel estimation. The conditions for the identifiability of the zero-forcing equalizer are also derived to ensure correct equalization. To further improve the performance of the proposed method, a new iterative algorithm is developed by incorporating the finite alphabet property of information symbols. Simulation results show that the proposed algorithms can outperform comparative schemes in most simulation conditions.     

Blind equalization for an application of unitary space-time modulation in ISI channels

Transmit diversity schemes have gained attention due to the promise of increased capacity and improved performance. Among these schemes, unitary space-time modulation and differentially encoded unitary space-time modulation allow for simple noncoherent decoding for flat-fading channels. In this paper, a new blind equalization algorithm for these transmission schemes in intersymbol interference (ISI) channels is proposed. A matrix-type constant modulus algorithm that exploits the unitary structure of the space-time codes is developed. The equalizer is paired with a noncoherent decoder, resulting in a completely blind, low-complexity method for decoding in the presence of ISI. A noiseless convergence analysis is conducted and verified via simulation in both noiseless and noisy cases. The performance of the overall system is evaluated via simulation and semi-analytically, and the achieved performance is between that of the ideal zero-forcing and the minimum-mean squared-error equalizers.     

Joint transmit-receive space-time equalization in spatially correlated MIMO channels: a beamforming approach

Multi-input multi-output (MIMO) channels have been shown in the literature to present a significant capacity increase over single-input single-output ones in some situations. To achieve this theoretical capacity, the constituent parallel subchannels arising from the MIMO channel have to be properly used. Many practical schemes are being currently developed to achieve this goal. We first show that, from an information-theoretic point of view, beamforming becomes asymptotically optimal as the spatial correlation of the channel fading increases. In light of this result, wideband beamvectors are jointly derived for both transmission and reception. We allow a controlled partial response and design zero-forcing and minimum mean-squared error transmit-receive filters. Conceptually, the beamforming scheme is shown to decompose into two stages: the first one corresponds to a spatial flattening of the MIMO channel, i.e., choosing the subchannel with the highest gain at each frequency; the second stage depends on the particular design criterion and performs a power distribution at the transmitter and defines the equalizer at the receiver. These methods are further extended to the general case of multiple beamforming, i.e., when more than one subchannel are used. An exact and practical implementation of a modified "waterfilling" solution required for the filter design is proposed. All derived methods are assessed and compared in terms of capacity and bit-error rate.     

Efficient adaptive receivers for joint equalization and interference cancellation in multiuser space-time block-coded systems

This paper develops low-complexity adaptive receivers for space-time block-coded (STBC) transmissions over frequency-selective fading channels. The receivers are useful for equalization purposes for single user transmissions and for joint equalization and interference cancellation for multiuser transmissions. The receivers exploit the rich code structure of STBC codes in order to deliver recursive-least-squares (RLS) performance at least-mean-squares (LMS) complexity. Besides reduced complexity, the proposed adaptive receivers also lower system overhead requirements.     

Noncoherent coded modulation

Trellis coded modulation with two or multidimensional signal constellations, together with coherent maximum-likelihood detection, is considered an attractive solution for communications over the additive white Gaussian noise (AWGN) channel. In this paper a new noncoherent communication system is introduced called noncoherent coded modulation (NCM) as an alternative to coherent coded modulation. NCM achieves almost the same power efficiency, without bandwidth expansion or an extensive increase in complexity. As a noncoherent system, the method does not need carrier phase estimation. Nonetheless, differential encoding is not required. High performance noncoherent detection is achieved by using multiple symbol observations. Unlike previous approaches, a sliding window for the observations is used, with each observation covering several branches of the trellis, such that the observations are time-overlapped. We define a new type of noncoherent maximum-likelihood sequence estimator (MLSE), and analyze its performance over the AWGN channel by numerical calculation of the union bound. We perform a computerized search and present new codes for noncoherent detection with their performance. The new codes cover many useful rates and complexities and achieve higher performance than existing codes for noncoherent detection. The method can also be used for multiple symbol demodulation of MDPSK with better results than existing methods     

Noncoherent block-coded MPSK

For coherent detection, block-coded modulation is a bandwidth efficient scheme. In this paper, we propose theorems about the error performance of block-coded modulation using M-ary phase-shift keying (MPSK) for noncoherent detection. Based on these theorems, we propose a novel block-coded modulation scheme for noncoherent detection called noncoherent block-coded MPSK. The proposed scheme provides flexible designs of noncoherent block codes with different code rate, block length and error performance. Good noncoherent block codes can be easily obtained by properly choosing binary linear block codes as the component codes. Moreover, noncoherent block codes of this new scheme can be decoded by multistage decoding, which has the advantage of low complexity and satisfactory error performance. In this paper, two algorithms of multistage decoding for noncoherent detection are proposed as well. The error performance of some designed codes and decoding algorithms is verified by computer simulation.