Diagonal block space-time code design for diversity and coding advantage over flat fading channels

The potential promised by multiple transmit antennas has raised considerable interest in space-time coding for wireless communications. In this paper, we propose a systematic approach for designing space-time trellis codes over flat fading channels with full antenna diversity and good coding advantage. It is suitable for an arbitrary number of transmit antennas with arbitrary signal constellations. The key to this approach is to separate the traditional space-time trellis code design into two parts. It first encodes the information symbols using a one-dimensional (M,1) nonbinary block code, with M being the number of transmit antennas, and then transmits the coded symbols diagonally across the space-time grid. We show that regardless of channel time-selectivity, this new class of space-time codes always achieves a transmit diversity of order M with a minimum number of trellis states and a coding advantage equal to the minimum product distance of the employed block code. Traditional delay diversity codes can be viewed as a special case of this coding scheme in which the repetition block code is employed. To maximize the coding advantage, we introduce an optimal construction of the nonbinary block code for a given modulation scheme. In particular, an efficient suboptimal solution for multilevel phase-shift-keying (PSK) modulation is proposed. Some code examples with 2-6 bits/s/Hz and two to six transmit antennas are provided, and they demonstrate excellent performance via computer simulations. Although it is proposed for flat fading channels, this coding scheme can be easily extended to frequency-selective fading channels.     

Differential space-time turbo codes

Serial concatenation of simple error control codes and differential space-time modulation is considered. Decoding is performed iteratively by passing symbol-wise a posteriori probability values between the decoders of the inner space-time code and the outer code. An extrinsic information transfer analysis is used to predict thresholds for outer convolutional codes of various memory orders and a simple outer parity-check code. This parity-check code is well matched to the inner differential space-time code and achieves a bit-error rate (BER) of 10/sup -6/ less than 2 dB from the Shannon capacity of the fast fading multiple antenna channel. The differential space-time code can also be used to generate a priori information in the absence of channel knowledge. This information can be exploited by a channel estimator inserted into the decoding iteration. It is demonstrated that the inner space-time code provides soft training symbols from periodically inserted training symbols. The reliability of these soft training symbols does not depend on the speed of the channel variations, but on the structure of the inner code and the signal-to-noise ratio (SNR). Simulation studies confirm these findings and show that the proposed system with no initial channel knowledge achieves a performance very close to that of the system with perfect channel knowledge.     

垂直分层空时码系统下的信道盲估计

对垂直分层空时码系统进行信道估计时，常需采用训练序列，发射天线个数的增加会使得所需的训练符号个数增加，减少了传输数据的有效时间。本文将信号处理中的子空间方法同分层空时码的编码结构相结合，提出了一种垂直分层空时码，实现了无需训练序列的信道估计。Monte Carlo仿真结果表明，在相干时间允许的范围内，利用本文算法估计出的信道进行信号检测时，其性能十分接近使用准确信道信息的检测结果。     

Distributed Space-Time Codes Using Constellation Rotation

Rate and diversity impose a fundamental tradeoff in wireless communication. We propose a novel distributed space-time coding (DSTC) scheme based on constellation rotation (DSTC-CR) for Amplify-and-Forward relay networks. The proposed code can achieve full-diversity or full-rate, and also offers a flexibility for a desired rate-diversity tradeoff. This code can work well with arbitrary signal constellation and any number of relays and achieve minimal-delay. Through analysis of pairwise error probability, coding design criteria, Chernoff bound, decoding strategies and optimal power allocation are provided. Simulation results show that DSTC-CR scheme outperforms diagonal DSTC (DDSTC) and distributed linear dispersion (DLD) code at high power. From the comparison with DDSTC, the DSTC-CR scheme can achieve the same information rate using a lower modulation order.     

A VERTICAL LAYERED SPACE—TIME CODE AND ITS CLOSED—FORM BLIND SYMBOL DETECTION

Vertical layered space-time codes have demonstrated the snormous potential to accommodate rapid flow data.Thus far,vertical layered space-time codes assumed that perfect estimates of current channel fading conditions are available at the receiver.However,increasing the number of transmit antennas increases the required training interval and reduces the available time in which data may be transmitted before the fading coefficients change.In this paper,a vertical layered space-time code is proposed.By applying the subupace method to the layered space-time code,the symbols can be detected without training symbols and channel estimates at the transmitter or the receiver.Monte Carlo simulations show that performance can approach that of the detection method with the knowledge of the channel.     

Multiple symbol differentially detected multilevel codes for theRayleigh-fading channel

Conventional differential detection is known to perform poorly on the fast-fading Rayleigh-fading channel due to the rapid variation of the channel state. The technique of multiple symbol differential detection (MSDD) improves the error performance by detecting over a sequence of symbols and exploiting the inherent correlation of the fading process. To further reduce the error rate, we need to introduce some form of coding. Successful coding for the Rayleigh-fading channel requires interleaving to decorrelate the channel-a conflicting requirement to MSDD. We propose a solution consisting of interleaving blocks of L M-ary phase-shift keying (MPSK) symbols that are multiple symbol differentially detected. The blocks are treated as elements of an M^L symbol alphabet over which a multilevel code based on geometrically uniform partitions is defined     

Multislot estimation of fast-varying space-time communication channels

In mobile communications, coherent detection requires the estimate of an increasing number of channel parameters due to the promising performance of systems that deploy multiple antennas. However, the estimate of space-time channels requires a number of training symbols that grows with the number of unknowns. To overcome this problem, we propose a subspace-based estimation method that exploits the different varying rates in the structure of the space-time channel for moving terminals. Since the channel has some fast-varying (faded amplitudes of the paths) and slowly varying (delays and directions of arrival) features, the multislot (MS) estimate is composed of two terms: the slowly varying spatial and temporal bases estimated from L consecutive slots and the fast-varying amplitudes estimated on a slot-by-slot basis. Performance analysis and simulations confirm the expected benefits of the multislot approach and demonstrate that for large L, the mean square error (MSE) on the channel estimate depends only on the number of fast-varying parameters.     

On code design for the Slepian-Wolf problem and lossless multiterminal networks

A Slepian-Wolf coding scheme for compressing two uniform memoryless binary sources using a single channel code that can achieve arbitrary rate allocation among encoders was outlined in the work of Pradhan and Ramchandran. Inspired by this work, we address the problem of practical code design for general multiterminal lossless networks where multiple memoryless correlated binary sources are separately compressed and sent; each decoder receives a set of compressed sources and attempts to jointly reconstruct them. First, we propose a near-lossless practical code design for the Slepian-Wolf system with multiple sources. For two uniform sources, if the code approaches the capacity of the channel that models the correlation between the sources, then the system will approach the theoretical limit. Thus, the great advantage of this design method is its possibility to approach the theoretical limits with a single channel code for any rate allocation among the encoders. Based on Slepian-Wolf code constructions, we continue with providing practical designs for the general lossless multiterminal network which consists of an arbitrary number of encoders and decoders. Using irregular repeat-accumulate and turbo codes in our designs, we obtain the best results reported so far and almost reach the theoretical bounds.     

A subspace approach to blind space-time signal processing forwireless communication systems

The two key limiting factors facing wireless systems today are multipath interference and multiuser interference. In this context, a challenging signal processing problem is the joint space-time equalization of multiple digital signals transmitted over multipath channels. We propose a blind approach that does not use training sets to estimate the transmitted signals and the space-time channel. Instead, this approach takes advantage of spatial and temporal oversampling techniques and the finite alphabet property of digital signals to determine the user symbol sequences. The problem of channels with largely differing and ill-defined delay spreads is discussed. The proposed approach is tested on actual channel data     

Blind chip-level equalizer for the downlink of cyclic-prefix CDMA systems

The authors present a chip-level blind frequency domain equalizer (FEQ) for the forward-link channel of a cyclic-prefix code division multiple access system. The FEQ coefficients are obtained without the need of training symbols or knowledge of channel state information. The coefficients are instead acquired by solving a constraint energy minimization problem involving the subspace spanned by the active and passive spreading codewords. They also prove that the random scrambling code sequences is required for the operation of the proposed equalization algorithm. Results from computer simulations are provided to verify the performance of the proposed FEQ.     

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.     

A full-rate, full-diversity four-antenna quasi-orthogonal space-time block code

We present a complex, full-rate quasi-orthogonal space-time block code for four transmit antennas. Using carefully tailored constellation phase rotations, we show that this code achieves full diversity for specialized PSK-based constellations. The optimal receiver for the new code decouples the symbol detection problem into pairs of symbols, thus greatly reducing complexity. Finally, we present and compare performance of the new code with several other codes in the literature. The new code is shown to perform as well as the best known code of its class.     

Accurate Distortion Estimation and Optimal Bandwidth Allocation for Scalable H.264 Video Transmission Over MIMO Systems

In this paper, we propose an optimal strategy for the transmission of scalable video over packet-based multiple-input multiple-output (MIMO) systems. The scalable extension of H.264/AVC that provides a combined temporal, quality and spatial scalability is used. For given channel conditions, we develop a method for the estimation of the distortion of the received video and propose different error concealment schemes. We show the accuracy of our distortion estimation algorithm in comparison with simulated wireless video transmission with packet errors. In the proposed MIMO system, we employ orthogonal space-time block codes (O-STBC) that guarantee independent transmission of different symbols within the block code. In the proposed constrained bandwidth allocation framework, we use the estimated end-to-end decoder distortion to optimally select the application layer parameters, i.e., quantization parameter (QP) and group of pictures (GOP) size, and physical layer parameters, i.e., rate-compatible turbo (RCPT) code rate and symbol constellation. Results show the substantial performance gain by using different symbol constellations across the scalable layers as compared to a fixed constellation.      

Errors and erasures decoding of BCH and Reed-Solomon codes for reduced M-ary orthogonal signaling

The paper presents a comparison of communication systems using different signal constellation sizes and Reed-Solomon or Bose-Chaudhuri-Hocquengem codes with different rates so that the overall required bandwidth is the same for each system. In these comparisons, the channel symbol size is smaller than the code symbol size, so that a code symbol contains parts of multiple channel symbols. Thus, the normal assumption of independent code symbols does not apply. Instead, consideration must be taken to obtain the best arrangement of channel symbols in each code symbol. Analytical expressions are developed to compare the bit error probability performance of comparable systems, based on individual codewords using errors-only decoding and errors and erasures decoding with transmission over a Rayleigh fading channel.     

A high-rate orthogonal space-time block code

We present a space-time block code from complex orthogonal designs for 5 transmit antennas, which can send 10 information symbols in a block of 15 channel uses and hence have rate 2/3. Simulation results show that this orthogonal space-time block code with rate 2/3 for five transmit antennas can achieve diversity gain over those orthogonal space-time block codes with higher rates for less number of transmit antennas.     

Channel and Delay Estimation Algorithm for Asynchronous Cooperative Diversity

Wireless Mobile Communications rely on a host of techniques, all related to one goal, sending the most possible information accross a link or a network. In recent years, both spatial and multiuser diversity have proven to be key techniques to achieve this goal. These two diversity dimensions can be exploited by the use of multiple antennas and/or the use of multiple terminals sending at the same time/frequency/code, these terminals can be seen as a multiple antenna emitter. This transmission diversity can be achieved with cooperative space-time encoded transmissions. One of the practical problems with this sort of array of transmitters is that the emitters will be asynchronous to some extent, hence the need for systems that can deal with asynchronicity, both from a signal design point of view and from a signal processing point of view. Having tackled the signal design previously, we take a look at the signal processing aspect and present a channel and delay estimation algorithm for asynchronous cooperative diversity in Block-Flat-Fading channel. The signal design is based on a precoding frame-based scheme with packet-wise encoding. This precoding is based on the addition of a cyclic prefix, implemented as a training sequence. The signal processing takes advantage of the known symbols offered by this cyclic prefix/training sequence and we show that it enables best synchronization and channel estimation which reaches the Cramer-Rao Bound. The BER performances are the same as synchronous MRC case, with full diversity order.     

On design criteria and construction of noncoherent 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 Stein's 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.     

A two-dimensional multipath channel estimator for CDMA systems with timing-offset errors

This letter proposes a two-dimensional pilot-aided channel estimator for multicarrier and single carrier cyclic prefix assisted code division multiple access systems in the presence of timing-offset errors. Using optimal training sequences with special cyclic correlation properties can minimize the timing offset errors. Optimal and binary sub-optimal training sequences are designed in this context. Both theoretic analysis and simulation studies demonstrate significant performance improvement for the proposed method over conventional frequency domain channel estimation methods.     

Joint channel estimation and data detection in space-time communications

The paper considers joint channel estimation and data sequence detection for multipath radio channels with multiple antennas at the transmitter and/or receiver. An iterative space-time receiver based on the expectation-maximization algorithm is proposed. We examine the performance of this receiver for transmit diversity and space-time coding methods over Rayleigh fading channels. Simulation results show that the receiver can often achieve near-coherent performance with modest complexity and using very few pilot symbols.     

基于空时分组编码的差分检测方法

利用正交设计原理提出了通用的差分空时分组码(GDSTBC，general differential space-time block code)。与已有的差分调制方法相比，GDSTBC对信号星图无任何限制，因而可利用幅度和相位同时携带信息提高频谱效率。基于最大似然准则，给出了平坦。Rayleigh衰落信道下的非相干译码器。我们将证明：在高信噪比下，GDSTBC能够以线性复杂度和满天线分集恢复数据符号；在PSK调制方式下，没有信道估计时性能下降3dB；Ganesan基于PSK星图的差分空时分组码、Xia基于APSK星图的差分空时调制技术都可看成GDSTBC的特例。