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)     

A new approach to layered space-time coding and signal processing

The information-theoretic capacity of multiple antenna systems has been shown to be significantly higher than that of single antenna systems in Rayleigh-fading channels. In an attempt to realize this capacity, Foschini (1996) proposed the layered space-time architecture. This scheme was argued to asymptotically achieve a lower bound on the capacity. Another line of work has focused on the design of channel codes that exploit the spatial diversity provided by multiple transmit antennas (Tarokh et al. 1998, Hammons and Gamal 2000). In this paper, we take a fresh look at the problem of designing multiple-input-multiple-output (MIMO) wireless systems. First, we develop a generalized framework for the design of layered space-time systems. Then, we present a novel layered architecture that combines efficient algebraic code design with iterative signal processing techniques. This novel layered system is referred to as the threaded space-time (TST) architecture. The TST architecture provides more flexibility in the tradeoff between power efficiency, bandwidth efficiency, and receiver complexity. It also allows for exploiting the temporal diversity provided by time-varying fading channels. Simulation results are provided for the various techniques that demonstrate the superiority of the proposed TST architecture over both the diagonal layered space-time architecture in Foschini (1996) and the multilayering approach (Tarokh et al. (1999).     

Utilizing Space-Time Diversity for Wireless Communications

In this paper we consider the use of multiple antennas for wireless communication over fading channels.The fading we consider is quasi-static flat Rayleigh fading. For such a scenario optimal SNR schemes are derived when the transmitter knows the channel and when the transmitter does not know the channel. But in both cases the receiver has to estimate the channel. When the SNR during channel estimation is reasonably high we derive expressions to show the impact of channel estimation errors on SNR.     

Performance analysis of adaptive loading OFDM under Rayleigh fading

In this paper, we investigate the performance of adaptive loading orthogonal frequency-division multiplexing (OFDM) under Rayleigh fading with maximal ratio-combining (MRC) diversity at the receiver. We assume that channel-state information is available at both the transmitter and the receiver. Closed-form expressions for the lower bound on the average capacity of OFDM transmission under Rayleigh fading are provided for ideal MRC diversity. Simple approximate expressions for the average capacity of the Rayleigh-fading channel are also provided for the high signal-to-noise ratio (SNR) case. In the second part of this paper, a maximum-rate adaptive-loading strategy is derived for uncoded quadrature-amplitude-modulation modulated OFDM. Simple lower bound expressions and high-SNR approximations are provided for the average spectral efficiency of the maximum-rate adaptive-loaded uncoded OFDM under Rayleigh-fading channel conditions. According to the results, the performance of the uncoded adaptive-loading OFDM is about 8.5 dB inferior to the capacity bound at 10/sup -5/ symbol error probability under frequency-selective Rayleigh fading.     

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.     

On Limits of Wireless Communications in a Fading Environment when Using Multiple Antennas

This paper is motivated by the need for fundamental understanding of ultimate limits of bandwidth efficient delivery of higher bit-rates in digital wireless communications and to also begin to look into how these limits might be approached. We examine exploitation of multi-element array (MEA) technology, that is processing the spatial dimension (not just the time dimension) to improve wireless capacities in certain applications. Specifically, we present some basic information theory results that promise great advantages of using MEAs in wireless LANs and building to building wireless communication links. We explore the important case when the channel characteristic is not available at the transmitter but the receiver knows (tracks) the characteristic which is subject to Rayleigh fading. Fixing the overall transmitted power, we express the capacity offered by MEA technology and we see how the capacity scales with increasing SNR for a large but practical number, n, of antenna elements at both transmitter and receiver.We investigate the case of independent Rayleigh faded paths between antenna elements and find that with high probability extraordinary capacity is available. Compared to the baseline n = 1 case, which by Shannon's classical formula scales as one more bit/cycle for every 3 dB of signal-to-noise ratio (SNR) increase, remarkably with MEAs, the scaling is almost like n more bits/cycle for each 3 dB increase in SNR. To illustrate how great this capacity is, even for small n, take the cases n = 2, 4 and 16 at an average received SNR of 21 dB. For over 99% of the channels the capacity is about 7, 19 and 88 bits/cycle respectively, while if n = 1 there is only about 1.2 bit/cycle at the 99% level. For say a symbol rate equal to the channel bandwith, since it is the bits/symbol/dimension that is relevant for signal constellations, these higher capacities are not unreasonable. The 19 bits/cycle for n = 4 amounts to 4.75 bits/symbol/dimension while 88 bits/cycle for n = 16 amounts to 5.5 bits/symbol/dimension. Standard approaches such as selection and optimum combining are seen to be deficient when compared to what will ultimately be possible. New codecs need to be invented to realize a hefty portion of the great capacity promised.     

Joint source-channel (de-)coding for mobile communications

Real world source coding algorithms usually leave a certain amount of redundancy within the coded bit stream. Shannon (1948) already mentioned that this redundancy can be exploited at the receiver side to achieve a higher robustness against channel errors. We show how joint source-channel decoding can be performed in a way that is applicable to any mobile communication system standard. Considerable gains in terms of bit error rate or signal-to-noise ratio (SNR) are possible dependent on the amount of redundancy. However, an even better performance can be achieved by changing also the transmitter sided source and channel encoders. We propose an encoding concept employing low-dimensional quantization. Keeping the gross bit rate as well as the clean channel quality the same, it decreases the complexity of the source encoder and the decoder significantly. Finally, we give an application of our methods to spectral coefficient coding in speech transmission over a Rayleigh fading channel resulting in channel SNR gains of about 2 dB as compared to state-of-the-art (de-)coding and bad frame handling methods     

Channel estimation for OFDM systems with transmitter diversity inmobile wireless channels

Transmitter diversity is an effective technique to improve wireless communication performance. In this paper, we investigate transmitter diversity using space-time coding for orthogonal frequency division multiplexing (OFDM) systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and we derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as high as 1.475 bits/s/Hz, the required signal-to-noise ratio is only about 7 dB for a 1% bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40-Hz Doppler frequency. In summary, with the proposed channel estimator, combining OPDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels     

异步垂直空时结构中的功率扩展技术

在平坦衰落信道中,针对异步垂直贝尔实验室空时结构(V-BLAST)信号模型下,现有线性最佳检测算法误码率性能随信噪比提高改善缓慢的问题,提出一种基于功率扩展的迭代检测方法:发射端用功率扩展将发射信号扩展到整个空时信号块上,接收端进行基于功率扩展的迭代检测。同时,分析了所提方法在每次迭代检测后的误码率性能。分析和仿真验证了误码率性能的改进。在4发4收场景下,误码率为10-5时,相比于线性最优最小均方误差(MMSE)方法,获得了约6dB信噪比增益。     

A quasi-orthogonal group space-time architecture to achieve a better diversity-multiplexing tradeoff

Most existing MIMO (multiput-input multiput-output) schemes optimize only either the diversity gain or the multiplexing gain. To obtain a good tradeoff between these two, the quasi-orthogonal group space-time (QoGST) architecture is proposed, wherein the transmit stream is subgrouped but encoded via an inter-group space-time block encoder, with group interference suppression at the receiver. This paper also considers another combined space-time coding and layered space-time architecture, which we refer to as group layered space-time (GLST), where space-time block coding is employed within each group. Under the assumption of Rayleigh fading and a prior perfect channel state information at the receiver, a performance analysis will demonstrate that both QoGST and GLST can achieve a good diversity-multiplexing tradeoff. QoGST is even superior to GLST. Simulation results will validate our analysis and further show that compared to the existent layered space-time block code (LSTBC) scheme, both QoGST and GLST can achieve a significant performance gain     

Generalized layered space-time codes for high data rate wireless communications

We present the architecture of generalized layered space-time codes (GLST) as a combination of Bell Labs layered space-time (BLAST) architecture and space-time coding (STC) in multiple-antenna wireless communication systems. This approach provides both spectral and power efficiency with moderate complexity. The framework is to partition all the available transmit antennas into groups and apply STC on each group as component codes. Based on the mappings from coded symbols to transmit antenna groups, we can construct different GLST systems. Particularly, horizontal mapping and diagonal mapping are introduced and referred to as HGLST and DGLST respectively. The basic decoding of GLST, under quasi-static flat Rayleigh fading environments and assuming perfectly known channel state information (CSI) at the receiver, combines group interference suppression and group interference cancellation techniques. As a result, the individual STC on each group is decoded serially. To improve the overall system performance, we derive the optimal power allocation among all space-time codewords without requiring the knowledge of CSI at the transmitter and suitable for all GLST systems. We also derive the optimal serial decoding order based on the channel realizations at the receiver for HGLST systems without power allocation. Simulation results show that both can provide much improvement. To further enhance the system performance, we propose a low complexity hard-decision iterative decoding method. This method efficiently exploits full receive antenna diversity and, hence, dramatically improves the system performance which is confirmed by simulation.     

New Approach for Error Compensation in Coded V-BLAST OFDM Systems

In this paper, we investigate coded layered space-time architectures for frequency-selective fading multiple-input multiple-output orthogonal frequency-division multiplexing (OFDM) channels. By computing outage capacity formulas, we will show that the capacity of the vertical Bell Labs layered space-time (V-BLAST) architecture can closely approach the Shannon capacity in the frequency-selective OFDM environment. Motivated by the capacity analysis, we propose pragmatic approaches which preserve the optimality of the layered space-time concept. We present methods to prevent the error propagation from catastrophically affecting the signal detection in subsequent layers. First, we start with a comprehensive signal modeling which includes error propagation. We derive an improved signal detector and describe the optimal soft-bit log-likelihood ratio value-computation method by taking decision errors into account for soft-input channel decoding. Then, to further enhance the V-BLAST performance, we show that cancellation using decoded decisions from previous layers makes the decision errors almost completely disappear, so that the layered space-time architecture can approach the attainable channel capacity. Finally, simulations confirm that the proposed schemes show a significant performance improvement over the conventional methods     

Distributed differential space-time coding for wireless relay networks

Distributed space-time coding is a cooperative transmission scheme for wireless relay networks. With this scheme, antennas of the distributive relays work as transmit antennas of the sender and generate a space-time code at the receiver. It achieves the maximum diversity. Although the scheme needs no channel information at relays, it does require full channel information, both the channels from the transmitter to relays and the channels from relays to the receiver, at the receiver. In this paper, we propose a differential transmission scheme, which requires channel information at neither relays nor the receiver, for wireless relay networks. As distributed space-time coding can be seen as the counterpart of space-time coding in the network setting, this scheme is the counterpart of differential space-time coding. Compared to coherent distributed space-time coding, the differential scheme is 3dB worse. In addition, we show that Alamouti, square real orthogonal, and Sp(2) codes can be used differentially in networks with corresponding numbers of relays. We also propose distributed differential space-time codes that work for networks with any number of relays using circulant matrices.     

Bit error probability of distributed Turbo coded relay systems over quasi-static Rayleigh fading channels

As is known, distributed Turbo coding (DTC) performs close to the theoretic outage probability bound of a relay channel when correct decoding is assumed at the relay. However, decoding error is inevitable in practical fading channels due to the error-prone feature of radio channels, and the decoding error propagation in DTC scheme will severely degrade the error performance of the relay system. As a result, it is necessary to evaluate the error performance of the DTC scheme in multi-hop relaying wireless systems in practical fading channels. Moreover, the theoretical method of analysis provides an effective tool for obtaining the error performance besides lengthy simulations. In this article, the concept of equivalent signal-to-noise ratio (SNR) of the two-hop relay channel and the method of computing equivalent SNR are developed, and then the upper bound on the bit error probability (BEP) of DTC relay systems is analyzed by use of Turbo code's distance spectrum, the concept of uniform interleaver, the limit-before-averaging technique, and the union bound method. Both theoretical analysis and numerical simulation are implemented for relay systems with DTC scheme over quasi-static Rayleigh fading channels. The results show that the upper bound approaches the simulation results in the medium to high SNR region.     

MC V-BLAST系统的MIMO信道盲估计

本文研究多载波垂直分层空时(MC V-BLAST)系统的下行频率选择性衰落多输入多输出(MIMO)无线信道估计问题.本文首先为MC V-BLAST系统提出了一种新颖的移不变性编码方法.利用上述移不变性性质,本文进一步提出了相应的下行频率选择性衰落MIMO无线信道的盲估计方法.仿真结果表明了本文移不变性编码方法的有效性和信道盲估计方法的性能.     

MIMO系统中空时分组编码的信道容量

在多输入多输出（MIMO）系统中，建立空时分组编码模型，在分析瑞利衰落下正交空时分组码接收信噪比的基础上，得到了正交空时分组编码信遗容量的表达式，并采用数值计算的方法对其进行分析论述，得出在接收天线数一定的情况下，增加发送天线数所能得到的信道容量的上界。同时，分析证明，在发送天线数一定时，正交空时分组编码信道容量随接收天线数的增加而增加。在接收天线数一定时，信遗容量也随着发送天线数的增加而增加，但当发送天线数增加到一定数量时容量的增加就变得不十分明显。     

Nakagami-m信道协作网络编码中断性能分析

无线网络编码是一项能极大提高无线网络流量的有用技术,而协作分集可以有效地克服无线衰落的影响。研究了Nakagami-m信道下协作网络编码系统中断性能,分析了不同协作策略下的互信息表达式,推导了协作网络编码中断概率闭式表达式,并对分析的结果进行了仿真。仿真结果显示,在高SNR和谱效率R〈4条件下,协作网络编码中断概率较分布式空时编码性能SNR增益有接近2 dB的提高,并且随分布参数m值的增大,协作网络编码中断概率性能逐步改善。     

相对时延对ST-FSK系统性能的影响

空时频移键控(ST-FSK)结合了空时分组码和频移键控的优点,在瑞利衰落信道条件下无需信道信息,可采用非相干的最大似然(ML)检测器。该文推导了在接收端已知相对时延条件下的非相干ML判决度量,讨论了在接收端未知相对时延条件下信噪比与二进制、四进制ST-FSK系统误比特率性能的关系。理论和仿真结果都表明:(1)低信噪比弱化了相对时延对ST-FSK系统可靠性的影响,但在高信噪比条件下相对时延会严重制约系统性能;(2)相对时延对系统性能的影响与系统进制数的关系不大。     

LDPC-based space-time coded OFDM systems over correlated fadingchannels: Performance analysis and receiver design

We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels     

On power adaptation in adaptive signaling systems

The Shannon capacity of a fading channel under an average-power constraint with channel side information at the transmitter and receiver is only negligibly larger than the capacity of the same channel when constant-power transmission is employed. However, power adaptation has been shown to be quite useful in practical systems, where it has been conjectured that it allows for compensation of the effect of rate quantization. Here, an average bit-error probability constraint is employed instead of the conventional instantaneous bit-error probability constraint. When the set of rates available to the transmitter is unrestricted in practical systems, necessary conditions for jointly optimal power and rate allocation are derived and used to demonstrate that power adaptation is of limited utility. However, when the rates available to the transmitter are restricted to the nonnegative integers for the example of uncoded quadrature amplitude modulation over frequency-nonselective Rayleigh fading channels, a 0.5-0.75 dB loss in power efficiency is incurred when employing only a single power level for each constellation, and a 0.5-bits/symbol loss in rate is incurred when constant power transmission is employed.