Space-Time-Block-Coded Transmissions for Downlink MC-CDMA Transmissions Over Frequency-Selective Fading Channels

Mitigation of multipath fading effects and suppression of multiuser interference (MUI) constitute major challenges in the design of the third generation wireless mobile systems. Space-time (ST) coding offers a attractive solution to cope with mutipath fading, but most existing ST coding schemes assume flat fading channels that may not be realistic for wide-band communications. Especially multiuser ST block-coded transmissions through multipath fading channels present unique challenge in suppressing not only MUI but also intersymbol/chip interference. In this paper, we design ST multiuser transceivers for MC-CDMA quasi-synchronous systems, capable to reliably transmit over frequency-selective multipath downlink channels. The proposed system is able to provide transmit diversity and to guarantee symbol recovery in multiuser environments, regardless of unknown multipath. Unlike existing approaches, the mobile does not need to know the channel of other users. In addition to decoding simplicity, computer simulations show the performance merits of the proposed transceiver.     

Space-time block-coded multiple access through frequency-selectivefading channels

Mitigation of multipath fading effects and suppression of multiuser interference (MUI) constitute major challenges in the design of wide-band third-generation wireless mobile systems. Space-time (ST) coding offers an effective transmit-antenna diversity technique to combat fading, but most existing ST coding schemes assume flat fading channels that may not be valid for wide-band communications. Single-user ST coded orthogonal frequency-division multiplexing transmissions over frequency-selective channels suffer from finite-impulse response channel nulls (fades). Especially multiuser ST block-coded transmissions through (perhaps unknown) multipath present unique challenges in suppressing not only MUI but also intersymbol/chip interference. In this paper, we design ST multiuser transceivers suitable for coping with frequency-selective multipath channels (downlink or uplink). Relying on symbol blocking and a single-receive antenna, ST block codes are derived and MUI is eliminated without destroying the orthogonality of ST block codes. The system is shown capable of providing transmit diversity while guaranteeing symbol recovery in multiuser environments, regardless of unknown multipath. Unlike existing approaches, the mobile does not need to know the channel of other users. In addition to decoding simplicity, analytic evaluation and corroborating simulations reveal its flexibility and performance merits     

Robust space-time codes for correlated Rayleigh fading channels

Space-time (ST) coding has emerged as an effective strategy to enhance performance of wireless communications in fading environments. Many different ST coding schemes have been proposed to achieve reliable communications in independent fading channels. However, a design of robust ST codes for correlated fading channels has not been addressed. We propose a simple robust ST coding scheme that achieves robust performance over a wide range of fading conditions. The key to achieve robust performance is to formulate code design criteria that are not dependent on the channel correlation statistics. A provably robust scheme can be formulated by concatenating a full-rank ST block code with an outer encoder. We derive several robust code examples via the concatenated orthogonal ST block code and TCM construction. The simulation results show that some traditional ST codes perform poorly, whereas the proposed codes achieve robust performance over a broad range of fading conditions.     

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.     

基于空时分组编码的MC-CDMA下行链路合并方案

基于空时分组编码的发射分集技术利用空间和时间分集 ,能有效抗多径 ,增强信道可靠性。将空时分组编码应用到MC -CDMA下行链路中 ,构建了一种新的多载波CDMA系统 (ST -MC -CD MA) ,通过在每个子载波信道中获得空间分集增益来提高系统性能。具体实现时 ,依据传统MC -CDMA信号合并方案 ,提出了ST -MC -CDMA空时译码后相应的四种合并方案。仿真结果验证了这 4种合并方案的优、缺点 ;并进一步证明 :在频率选择性瑞利衰落信道中 ,该系统比采用相同合并方案的传统MC -CDMA有明显的性能改善。     

时变信道中非理想反馈条件下波束成形的误码率分析

在基于信道信息有限反馈的无线多入单出系统中,发射机可采用简单的波束成形技术实现发射分集增益和阵列增益。已有的相关研究大多包含块衰落信道、准确信道估计或无反馈延迟等理想假设。该文建立了更为实际的Jakes时变信道中存在信道估计误差和反馈延迟的系统模型,分析了方形和矩形正交幅度调制星座图的平均误码率。研究表明:误码率的理论分析和仿真结果完全相符;增加反馈比特数可提高阵列增益,但不能增加分集增益;在慢变信道中,波束成形要显著优于正交空时分组码;误码率受信道估计误差和反馈延迟影响,且对后者尤为敏感。     

A New Union Bound on the Error Probability of Binary Coded OFDM Systems in Wireless Environments

Orthogonal Frequency Division Multiplexing (OFDM) systems are commonly used to mitigate frequency-selective multipath fading and provide high-speed data transmission. In this paper, we derive new union bounds on the error probability of a coded OFDM system in wireless environments. In particular, we consider convolutionally coded OFDM systems employing single and multiple transmit antennas over correlated block fading (CBF) channels with perfect channel state information (CSI). Results show that the new union bound is tight to simulation results. In addition, the bound accurately captures the effect of the correlation between sub-carriers channels. It is shown that as the channel becomes more frequency-selective, the performance get better due to the increased frequency diversity. Moreover, the bound also captures the effect of multi-antenna as space diversity. The proposed bounds can be applied for coded OFDM systems employing different coding schemes over different channel models.     

Reduced-Complexity Receiver Structures for Space–Time Bit-Interleaved Coded Modulation Systems

Transmission efficiency in radio channels can be considerably improved by using multiple transmit and receive antennas and employing a family of schemes called space-time (ST) coding. Both extended range and/or improved bandwidth efficiency can be achieved, compared with a radio link with a single transmit and receive antenna. Bit-interleaved coded modulation schemes give diversity gains on fading channels with higher order modulation constellations combined with conventional binary convolutional codes also for the case of a single transmit and receive antenna radio link. In this paper, we study a family of flexible bandwidth-efficient ST coding schemes which combine these two ideas in a narrowband flat-fading channel and single-carrier modems. We address receiver complexity for the case of a large number of transmit antennas and higher order modulation constellations. Especially, we focus on practical configurations, where the number of transmit antennas is greater than that of receive antennas. Simplified receivers using tentative decisions are proposed and evaluated by means of simulations. Tradeoffs between complexity reduction and performance loss are presented. We emphasize systems that are of particular interest in applications where the number of transmit antennas exceeds the number of receive antennas. A system with four transmit antennas with an eight-fold complexity reduction and a performance loss of about 1 dB is demonstrated     

Multiband-OFDM MIMO coding framework for UWB communication systems

The emerging ultrawideband (UWB) system offers a great potential for the design of high speed short-range wireless communications. In order to satisfy the growing demand for higher data rates, one possible solution is to exploit both spatial and multipath diversities via the use of multiple-input multiple-output (MIMO) and proper coding techniques. In this paper, we propose a general framework to analyze the performance of multiband UWB-MIMO systems regardless of specific coding schemes. A combination of space-time-frequency (STF) coding and hopping multiband OFDM modulation is also proposed to fully exploit all of the available spatial and frequency diversities, richly inherent in UWB environments. We quantify the performance merits of the proposed scheme in case of Nakagami-m frequency-selective fading channels. Different from the conventional STF coded MIMO-OFDM system, the performance of the STF coded hopping multiband UWB does not depend on the temporal correlation of the propagation channel. We show that the maximum achievable diversity of multiband UWB-MIMO system is the product of the number of transmit and receive antennas, the number of multipath components, and the number of jointly encoded OFDM symbols. Interestingly, the diversity gain does not severely depend on the fading parameter m, and the diversity advantage obtained under Nakagami fading with arbitrary m parameter is almost the same as that obtained in Rayleigh fading channels. Finally, simulation results are presented to support the theoretical analysis.     

Space-time overlays for convolutionally coded systems

We consider the design of space-time overlays to upgrade single-antenna wireless communication systems to accommodate multiple transmit antennas efficiently. We define the overlay constraint such that the signal transmitted from the first antenna in the upgraded system is the same as that in the single-antenna system. The signals transmitted from the remaining antennas are designed according to space-time coding principles to achieve full spatial diversity in quasi-static flat fading channels. For both binary phase-shift keying (BPSK) and quaternary phase-shift keying modulation systems, we develop an algebraic design framework that exploits the structure of existing single-dimensional convolutional codes in designing overlays that achieve full spatial diversity with minimum additional decoding complexity at the receiver. We also investigate a concatenated coding approach for a BPSK overlay design in which the inner code is an orthogonal block code. This approach is shown to yield near optimal asymptotic performance for quasi-static fading channels. We conclude by offering a brief discussion outlining the extension of the proposed techniques to time-varying block fading channels.     

Space-time bit-interleaved coded modulation for OFDM systems

Space-time coding techniques significantly improve transmission efficiency in radio channels by using multiple transmit and/or receive antennas and coordination of the signaling over these antennas. Bit-interleaved coded modulation gives good diversity gains with higher order modulation schemes using well-known binary convolutional codes on a single transmit and receive antenna link. By using orthogonal frequency division multiplexing (OFDM), wideband transmission can be achieved over frequency-selective fading radio channels without adaptive equalizers. In this correspondence, we combine these three ideas into a family of flexible space-time coding methods. The pairwise error probability is analyzed based on the correlated fading assumption. Near-optimum iterative decoders are evaluated by means of simulations for slowly varying wireless channels. Theoretical evaluation of the achievable degree of diversity is also presented. Significant performance gains over the wireless local area network (LAN) 802.11a standard system are reported.     

Nakagami衰落信道上MIMO WCDMA系统的MQAM性能分析

日益增长的无线业务需求要求提高衰落信道上无线通信的频谱利用率。本文利用Q2(x)的另一种数学表达式和矩生成函数推导了Nakagami-m衰落信道上多输入多输出(MI-MO)WCDMA系统采用M进制正交幅度调制(MQAM)的平均误符号率(ASER)表达式,分析框架可以推广到开环和闭环系统发射和接收天线为任意数目的应用场合,研究了存在多址干扰下MIMOWCDMA系统的平均误符号率性能,数值计算结果表明组合发送分集和接收分集可以显著改善系统的性能。     

Combined turbo coding and unitary space-time modulation

Space-time coding is well understood for high data rate communications over wireless channels with perfect channel state information. On the other hand, channel coding for multiple transmit antennas when channel state information is unknown has only received limited attention. A new signaling scheme, named unitary space-time modulation, has been proposed for the latter case. In this paper, we consider the use of turbo coding together with unitary space-time modulation. We demonstrate that turbo coded space-time modulation systems are well suited to wireless communication systems when there is no channel state information, in the sense that the turbo coding improves the bit error rate (BER) performance of the system considerably. In particular, we observe that the turbo-coded system provides 10-15 dB coding gain at a BER of 10/sup -5/ compared to the unitary space-time modulation for various transmit and receive antenna diversity cases.     

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the system's interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on -steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.     

Cooperative Protocols Design for Wireless Ad-Hoc Networks with Multi-hop Routing

Wireless ad-hoc networks can experience significant performance degradation under fading channels. Spatial diversity has been shown to be an effective way of combating wireless fading with the multiple-input multiple-output (MIMO) technique by transmitting correlated information through multiple antennas. The virtual MIMO technique, which allows multiple wireless stations with single antenna to form a virtual transmission array, is shown to be a viable solution from several recent studies. In this paper, we propose a complete system framework for wireless ad-hoc networks utilizing two different cooperative relaying techniques at the physical layer: the repetition coding and the space-time coding. In the data link layer, two medium access control protocols are proposed to accommodate the corresponding physical layer cooperative diversity schemes. In the network layer, diversity-aware routing protocols are proposed to determine the routing path and the relaying topology. Simulations with both constant bit rate and TCP (transmission control protocol) traffic show significant performance gains of the proposed cooperative relaying schemes.     

Power allocation of cooperative amplify-and-forward communications with multiple relays

Cooperative diversity is one of the most effective ways to mitigate the fading effect of wireless channels and obtain the spatial gain in wireless networks.In this paper,an optimal power allocation(OPA)scheme for a cooperative communication system using the amplify-and-forward(AF)transmit strategy with multiple relay users is proposed by minimizing the bit-error-rate(BER)at the destination under the constraint of the total transmit power of both the source user and the relay users.Simulation results indicate that the proposed power allocation method can achieve significant BER performance improvement than using the equal power allocation(EPA)scheme,while still attains low complexity.The system performance is improved significantly with the increasing of the number of relay users at high signal-noise ratio(SNR).However,at low SNR,the system performance decreases when the relay number increases.Thus,an adaptive relay selection scheme may be used to choose the appropriate relay numbers in different transmission scenarios to provide system performance improvement and keep the power allocation scheme with low complexity.     

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.     

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)     

Space–time coding and signal processing for high data rate wireless communications

The information capacity of wireless communication systems can be increased dramatically by employing multiple transmit and receive antennas [Foschini GJ, Gans MJ. On limits of wireless communications in a fading environment when using multiple antennas. Wireless Communications Magazine 1998; 6 311–335. Telatar E. Capacity of Multi‐Antenna Gaussian Channels, Technical Memorandum, AT&T Bell Laboratories, 1995.] An effective approach to increasing data rate over wireless channels is to employ coding techniques appropriate to multiple transmit antennas, that is space–time coding. Space–time codes introduce temporal and spatial correlation into signals transmitted from different antennas, in order to provide diversity at the receiver, and coding gain over an uncoded system. The spatial–temporal structure of these codes can be exploited to further increase the capacity of wireless systems with a relatively simple receiver structure. This paper provides an overview of space–time coding techniques and the associated signal processing framework. Copyright © 2001 John Wiley & Sons, Ltd.     

瑞利衰落信道采用组合发射机SC/接收机 MRC的MQAM性能分析

日益增长的无线业务需求要求提高衰落信道上无线通信的频谱利用率.本文研究一种使用组合发射机SC/接收机MRC(SC/MRC)的MQAM方案,推导其在平坦瑞利衰落信道上的误符号率,分析无线信道时变特性对系统性能的影响.数值计算结果表明该组合空间分集方案可以通过调整发射天线和接收天线的数目来获得比传统接收机分集接收更大的分集增益.