空时码概述

在第三代移动通信系统中,空时编码(space-time coding)技术是抗信道衰落和提高系统容量的一种新的编码方式.本文介绍了空时编码技术的由来和分类,并着重介绍了空时分组码的基本编解码算法.最后在SPW下对2发1收的正交空时分组码的编解码算法进行了仿真并给出了仿真结果.     

Distributed space-time block coding

In this paper, a new class of distributed space-time block codes (DSTBCs) is introduced. These DSTBCs are designed for wireless networks which have a large set of single-antenna relay nodes /spl Nscr/, but at any given time only a small, a priori unknown subset of nodes S/spl sube//spl Nscr/ can be active. In the proposed scheme, the signal transmitted by an active relay node is the product of an information-carrying code matrix and a unique node signature vector of length N/sub c/. It is shown that existing STBCs designed for N/sub c/2 co-located antennas are favorable choices for the code matrix, guaranteeing a diversity order of d=min{N/sub S/,N/sub c/} if N/sub S/ nodes are active. For the most interesting case, N/sub S//spl ges/N/sub c/, the performance loss entailed by the distributed implementation is analytically characterized. Furthermore, efficient methods for the optimization of the set of signature vectors are provided. Depending on the chosen design, the proposed DSTBCs allow for low-complexity coherent, differential, and noncoherent detection, respectively. Possible applications include ad hoc and sensor networks employing decode-and-forward relaying.     

Universal noiseless coding

Universal coding is any asymptotically optimum method of block-to-block memoryless source coding for sources with unknown parameters. This paper considers noiseless coding for such sources, primarily in terms of variable-length coding, with performance measured as a function of the coding redundancy relative to the per-letter conditional source entropy given the unknown parameter. It is found that universal (i.e., zero redundancy) coding in a weighted sense is possible if and only if the per-letter average mutual information between the parameter space and the message space is zero. Universal coding is possible in a maximin sense if and only if the channel capacity between the two spaces is zero. Universal coding is possible in a minimax sense if and only if a probability mass function exists, independent of the unknown parameter, for which the relative entropy of the known conditional-probability mass-function is zero. Several examples are given to illustrate the ideas. Particular attention is given to sources that are stationary and ergodic for any fixed parameter although the whole ensemble is not. For such sources, weighted universal codes always exist if the alphabet is finite, or more generally if the entropy is finite. Minimax universal codes result if an additional entropy stability constraint is applied. A discussion of fixed-rate universal coding is also given briefly with performance measured by a probability of error.     

Full-diversity full-rate complex-field space-time coding

Exciting developments in wireless multiantenna communications have led to designs aiming mainly at one of two objectives: either high-performance by enabling the diversity provided by multi-input multi-output (MIMO) channels or high-rates by capitalizing on space-time multiplexing gains to realize the high capacity of MIMO fading channels. By concatenating a linear complex-field coder (a.k.a. linear precoder) with a layered space-time mapper, we design systems capable of achieving both goals: full-diversity and full-rate (FDFR), with any number of transmit- and receive-antennas. We develop FDFR designs not only for flat-fading but for frequency-selective, or, time-selective fading MIMO channels as well. Furthermore, we establish the flexibility of our FDFR designs in striking desirable performance-rate-complexity tradeoffs. Our theoretical claims are confirmed by simulations.     

Generalized PSK in space-time coding

A wireless communication system using multiple antennas promises reliable transmission under Rayleigh flat fading assumptions. Design criteria and practical schemes have been presented for both coherent and noncoherent communication channels. In this paper, we generalize one-dimensional (1-D) phase-shift keying (PSK) signals and introduce space-time constellations from generalized PSK (GPSK) signals based on the complex and real orthogonal designs. The resulting space-time constellations reallocate the energy for each transmitting antenna and feature good diversity products; consequently, their performances are better than some of the existing comparable codes. Moreover, since the maximum-likelihood (ML) decoding of our proposed codes can be decomposed to 1-D PSK signal demodulation, the ML decoding of our codes can be implemented in a very efficient way.     

Optimal space-time coding under iterative processing

In this paper, we deal with the design of a full-rate space-time block coding (STBC) scheme optimized for linear iterative decoding over fast fading multiple-input multiple-output (MIMO) channel. A general and simple coding scheme called diagonal threaded space-time (DTST) code is presented for an arbitrary number of transmit and receive antennas. Theoretical analysis shows that DTST code associated with linear iterative decoding tends towards full diversity performance while providing maximum MIMO multiplexing gain. Simulation results confirm the ability of DTST to outperform the state-of-the-art STBC and conventional spatial data multiplexing schemes under iterative processing.     

Universal modeling and coding

The problems arising in the modeling and coding of strings for compression purposes are discussed. The notion of an information source that simplifies and sharpens the traditional one is axiomatized, and adaptive and nonadaptive models are defined. With a measure of complexity assigned to the models, a fundamental theorem is proved which states that models that use any kind of alphabet extension are inferior to the best models using no alphabet extensions at all. A general class of so-called first-in first-out (FIFO) arithmetic codes is described which require no alphabet extension devices and which therefore can be used in conjunction with the best models. Because the coding parameters are the probabilities that define the model, their design is easy, and the application of the code is straightforward even with adaptively changing source models.     

Performance comparison of space-time coding techniques

Simulation results for the comparative performance of a number of space-time coding (STC) schemes in a multiple-input-multiple-output (MIMO) channel are presented and compared with a single-antenna benchmark and two-antenna space-time transit diversity (STTD) scheme. Both the space-time trellis coding (STTC) and BLAST approaches offer high spectral efficiencies, but STTC outperforms BLAST in terms of its power efficiency     

Combined array processing and space-time coding

The information capacity of wireless communication systems may be increased dramatically by employing multiple transmit and receive antennas. The goal of system design is to exploit this capacity in a practical way. An effective approach to increasing data rate over wireless channels is to employ space-time coding techniques appropriate to multiple transmit antennas. These space-time codes introduce temporal and spatial correlation into signals transmitted from different antennas, so as to provide diversity at the receiver, and coding gain over an uncoded system. For large number of transmit antennas and at high bandwidth efficiencies, the receiver may become too complex whenever correlation across transmit antennas is introduced. This paper dramatically reduces encoding and decoding complexity by partitioning antennas at the transmitter into small groups, and using individual space-time codes, called the component codes, to transmit information from each group of antennas. At the receiver, an individual space-time code is decoded by a novel linear processing technique that suppresses signals transmitted by other groups of antennas by treating them as interference. A simple receiver structure is derived that provides diversity and coding gain over uncoded systems. This combination of array processing at the receiver and coding techniques for multiple transmit antennas can provide reliable and very high data rate communication over narrowband wireless channels. A refinement of this basic structure gives rise to a multilayered space-time architecture that both generalizes and improves upon the layered space-time architecture proposed by Foschini (see Bell Labs Tech. J., vol.1, no.2, 1996)     

On the robustness of space-time coding

Space-time (ST) coding has emerged as one of the most promising technologies for meeting the challenges imposed by the wireless channel. This technology is primarily concerned with two-dimensional (2-D) signal design for multitransmit antenna wireless systems. Despite the progress in ST coding, several important questions remain unanswered. In a practical multiuser setting, one would expect different users to experience different channel conditions. This motivates the design of robust ST codes that exhibit satisfactory performance in various environments. In this paper, we investigate the robustness of ST codes in line-of-sight and correlated Rayleigh fading channels. We develop the design criteria that govern the performance of ST codes in these environments. Our analysis demonstrates that full-diversity ST codes are essential to achieving satisfactory performance in line-of-sight channels. We further show that a simple phase randomization approach achieves significant performance gains in the line-of-sight case without affecting the performance in Rayleigh fading channels. In the correlated fading environments, we characterize the achievable diversity order based on the number of diversity degrees of freedom in the channel. This characterization supports experimental observations that suggest that the quasistatic model is not a worst-case scenario and establishes the necessary tradeoff between the transmission rate and performance robustness. Finally, we consider the design of ST codes using some prior knowledge about the channel spatio-temporal correlation function.     

FSO-MIMO中的自适应多层空时编码

为了根据信道的时变特性来选择合适的空时编码方式,结合正交空时分组码与分层空时码的优点,并借鉴天线分组的多层空时编码原理,提出了一种在自由空间光通信多输入多输出中4×4的自适应多层空时编码方案,并用Monte Carlo法进行仿真研究。结果表明,在一定的信噪比范围内,采用自适应多层空时,编码方案在保证一定误比特率的条件下,能使数据传输速率达到最大化;采用自适应调制方式能更有效地利用资源并提高数据的传输速率。     

Differential space-time coding for frequency-selective channels

We introduce two space-time transmission schemes which allow full-rate and full-diversity noncoherent communications using two transmit antennas over fading frequency-selective channels. The first scheme operates in the frequency domain where it combines differential Alamouti (seeIEEE J. Select. Areas Commun., vol.16, p.1451-58, Nov. 1998) space-time block-coding (STBC) with OFDM. The second scheme operates in the time domain and employs differential time-reversal STBC to guarantee blind channel identifiability without the need for temporal oversampling or multiple receive antennas     

Cooperative space-time coding for wireless networks

We consider a cooperative transmission scheme in which the collaborating nodes may have multiple antennas. We present the performance analysis and design of space-time codes that are capable of achieving the full diversity provided by user cooperation. Our codes use the principle of overlays in time and space, and ensure that cooperation takes place as often as possible. We show how cooperation among nodes with different numbers of antennas can be accomplished, and how the quality of the interuser link affects the cooperative performance. We illustrate that space-time cooperation can greatly reduce the error rates of all the nodes involved, even for poor interuser channel quality.     

基于网络编码与空时编码的协作MAC协议

针对无线ad hoc网络中协作造成的中继效率低以及不同QoS需求难以满足等问题,提出了一种联合网络编码和空时编码的协作MAC协议(NSTCMAC)。NSTCMAC将网络编码与空时编码技术相结合,设计出区分业务类型的协作MAC协议传输机制,以满足不同业务类型的QoS需求;进一步通过马尔科夫链模型分析了区分业务类型的协作机制及性能。仿真结果表明,相比传统的DCF、COOPMAC以及CD-MAC协议,NSTCMAC协议能更好地保证不同的QoS需求,并能有效地解决协作造成的中继效率低的问题。     

Universal space-time codes from demultiplexed trellis codes

In broadcast scenarios or in the absence of accurate channel probability distribution information, code design for consistent channel-by-channel performance, rather than average performance over a channel distribution, may be desirable. Root and Varaiya's compound channel theorem for linear Gaussian channels promises the existence of universal codes that operate reliably whenever the channel mutual information (MI) is above the transmitted rate. This paper presents two-dimensional trellis codes that provide such universal performance over the compound linear vector Gaussian channel when demultiplexed over two, three, and four transmit antennas. The presented trellis codes, found by exhaustive search, guarantee consistent performance on every matrix channel that supports the information transmission rate with an MI gap that is similar to the capacity gap of a well-designed additive white Gaussian noise (AWGN)-specific code on the AWGN channel. As a result of their channel-by-channel consistency, the universal trellis codes presented here also deliver comparable, or, in some cases, superior, frame-error rate and bit-error rate performance under quasi-static Rayleigh fading, as compared with trellis codes of similar complexity that are designed specifically for the quasi-static Rayleigh-fading scenario.     

A new differential space-time block coding scheme

We present a new differential space-time block code (DSTBC). The scheme can be represented by a trellis and decoded using the Viterbi algorithm. It provides a differential coding gain of 1 dB due to redundancy introduced in the differential encoding and it is only 2 dB away from the corresponding coherent space-time block code (STBC).     

MIMO antenna subset selection with space-time coding

This paper treats multiple-input multiple-output (MIMO) antenna subset selection employing space-time coding. We consider two cases differentiated based on the type of channel knowledge used in the selection process. We address both the selection algorithms and the performance analysis. We first consider the case when the antenna subsets are selected based on exact channel knowledge (ECK). Our results assume the transmission of orthogonal space-time block codes (with emphasis on the Alamouti (see IEEE J. Select. Areas Commun., vol.16, p.1451-68, Oct. 1998) code). Next, we treat the case of antenna subset selection when statistical channel knowledge (SCK) is employed by the selection algorithm. This analysis is applicable to general space-time coding schemes. When ECK is available, we show that the selection algorithm chooses the antenna set that maximizes the channel Frobenius norm leading to both coding and diversity gain. When SCK is available, the selection algorithm chooses the antenna set that maximizes the determinant of the covariance of the vectorized channel leading mostly to a coding gain. In case of ECK-based selection, we provide analytical expressions for average SNR and outage probability improvement. For the case when SCK-based selection is used, we derive expressions for coding gain. We also present extensive simulation studies, validating our results.     

MIMO-OFDM系统中的空时编码技术研究

近年来,宽带无线通信技术和应用得到了迅猛的发展.人们对无线数据和多媒体业务的需求,促进了用于高速宽带无线通信的诸多新技术的发展和应用.其中,MIMO技术利用多径效应,能够极大改善无线通信的频谱效率和通信可靠性.OFDM技术具有优越的频谱效率,两者的结合即MIMO-OFDM技术被认为是未来移动通信系统的关键技术.本文简要介绍MIMO技术和OFDM的基本原理,MIMO-OFDM系统以及空时编码技术在MIMO-OFDM系统中的应用.     

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.     

Combining beamforming and orthogonal space-time block coding

Multiple transmit and receive antennas can be used in wireless systems to achieve high data rate communication. Efficient space-time codes have been developed that utilize a large portion of the available capacity. These codes are designed under the assumption that the transmitter has no knowledge about the channel. In this work, on the other hand, we consider the case when the transmitter has partial, but not perfect, knowledge about the channel and how to improve a predetermined code so that this fact is taken into account. A performance criterion is derived for a frequency-nonselective fading channel and then utilized to optimize a linear transformation of the predetermined code. The resulting optimization problem turns out to be convex and can thus be efficiently solved using standard methods. In addition, a particularly efficient solution method is developed for the special case of independently fading channel coefficients. The proposed transmission scheme combines the benefits of conventional beamforming with those given by orthogonal space-time block coding. Simulation results for a narrow-band system with multiple transmit antennas and one or more receive antennas demonstrate significant gains over conventional methods in a scenario with nonperfect channel knowledge