时变衰落信道中同步时间扩展信号的分集接收技术

扩频通信可以区分多径信号进行RAKE分集接收,是一种有效处理多径效应的手段。与此对应,时间扩展通信也可以在频域上进行类似RAKE的分集接收,有效处理时变衰落带来的影响。时间扩展会带来严重的符号间干扰,同步时间扩展可以有效控制和处理符号间干扰。基于信道分解的分集接收技术保证了同步时间扩展在时变衰落信道下的性能。获得10-4误码率,本文分集算法(扩展长度1024)在时变衰落信道中所需的信噪比与AWGN信道不分集处理所需的信噪比只相差1.4dB。     

Differential modulation diversity

In this paper, differential modulation diversity (DMD) is introduced. This diversity scheme is based on diagonal signal constellations which have been previously proposed for differential space-time modulation (DSTM). DMD can exploit both space and time diversity and DSTM, which is a pure space-diversity scheme, results as a special case. Low-complexity noncoherent receivers originally designed for DSTM are adapted to DMD and the power efficiency of DMD for Ricean fading with spatial correlation and imperfect interleaving is investigated. Based on analytical expressions for the pairwise error probability it is shown that space diversity increases the effective fading bandwidth, which has a negative influence on performance. Time diversity does not have this disadvantage and is preferable especially for fast fading channels. If space and time diversity are combined, a robust receiver results that yields high performance for a wide range of fading velocities.     

Adaptive Channel‐Matched Extended Alamouti Space‐Time Code Exploiting Partial Feedback

Since the publication of Alamouti's famous space‐time block code, various quasi‐orthogonal space‐time block codes (QSTBC) for multi‐input multi‐output (MIMO) fading channels for more than two transmit antennas have been proposed. It has been shown that these codes cannot achieve full diversity at full rate. In this paper, we present a simple feedback scheme for rich scattering (flat Rayleigh fading) MIMO channels that improves the coding gain and diversity of a QSTBC for 2ⁿ (n = 3, 4,…) transmit antennas. The relevant channel state information is sent back from the receiver to the transmitter quantized to one or two bits per code block. In this way, signal transmission with an improved coding gain and diversity near to the maximum diversity order is achieved. Such high diversity can be exploited with either a maximum‐likelihood receiver or low‐complexity zero‐forcing receiver.     

Error performance of innovations-based MLSE for Rayleigh fadingchannels

Error performance of maximum likelihood sequence estimation (MLSE) of digital signals transmitted over Rayleigh fading channels is studied in this paper. The application of the innovations approach provides us not only with a general MLSE receiver structure, but also with a tool for analyzing the performance of the receiver. We show that the sequence pairwise error probability of the MLSE receiver is determined by the eigenvalues of a matrix generated from the autocorrelation function of the received signal. For any practical applications, the MLSE for Rayleigh fading channels exhibits an irreducible error floor that depends on the channel characteristics such as the Doppler frequency bandwidth and frequency selectivity. An upper bound on bit error probability can be calculated by using the sequence pairwise error probability. Also, it is shown that diversity reception can significantly improve the MLSE error performance     

A switched diversity receiving system for mobile radio

A low cost switched diversity receiving system has been developed for use in UHF-FM mobile radio. The input of a single receiver is switched back and forth between two antennas upon command from a signal level sensing logic circuit. The system has been measured on simulated Rayleigh fading channels and has been found to give a significant improvement to both voice and data signals.     

Combined Transmitter Diversity and Multi-Level Modulation Techniques

Orthogonal transmitter diversity such as frequency diversity and time diversity is quite simple to implement and, with optimum signal combining, can take full advantage of fading multipath channels. However, such a scheme has a bandwidth efficiency that decreases inversely with the number of diversity branches making it less attractive in wireless communications applications. This paper considers combined orthogonal transmitter diversity and multi-level linear modulation techniques. The idea is to view the signal constellations of the modulation scheme in an augmented signal space formed by the modulation signal dimension and the number of branches of the transmitter diversity scheme. This augmented signal space provides a good spread for the modulation signal points and can be quite efficient for high-level linear modulation techniques. The obtained results show that this combined scheme, not only improves the system performance on both additive white Gaussian noise and fading multipath channels, but also improves the bandwidth efficiency of orthogonal transmitter diversity.     

A Switched Diversity Receiving System for Mobile Radio

A low cost switched diversity receiving system has been developed for use in UHF-FM mobile radio. The input of a single receiver is switched back and forth between two antennas upon command from a signal level sensing logic circuit. The system has been measured on simulated Rayleigh fading channels and has been found to give a significant improvement to both voice and data signals.     

Microdiversity reception of spread-spectrum signals on Nakagamifading channels

An analytical framework to evaluate the performance of different predetection diversity techniques in various mobile radio environments is developed. The average bit-error rate analysis applies to phase coded spread-spectrum systems, over Nakagami multipath fading channels. A simple and practical selection combining rule is considered. Our numerical results reveal that this new low-complexity receiver structure exhibits comparable performance to that of an optimum linear diversity combiner when the channel does not experience severe fading and for small diversity orders, conditioned on the situation that all the diversity branches have identical mean signal strengths. In this study, we also investigate the effect of variations in the mean signal and noise power levels on each of the independent diversity branches. This is an important consideration because in practice equal mean signal strengths rarely occur, which results in loss of diversity gain. We found that the signal-plus-noise-and-interference selection model outperforms the traditional signal-to-interference-plus-noise ratio selection scheme if the discrepancy between the mean signal strengths are small, owing to the statistical nature of the multiple-access interference     

瑞利衰落信道下扩频信号隐分集性能分析

介绍了瑞利衰落信道下采用扩频信号的失真接收机获取隐分集增益机理.在多重分集通道和每个分集通道存在离散多径的瑞利信道模型基础上,分别详细分析推导了单重分集通道和多重分集通道情况下的隐分集性能表达式,给出了不同情况下的误码性能理论曲线,并在硬件系统测试平台上,对扩频信号的隐分集性能进行了实测.测试结果表明该分析方法可行,可用于指导工程实践.     

Performance of antenna diversity multiuser receivers in CDMA channels with imperfect fading estimation

The performance of antenna diversity coherent and differentially coherent linear multiuser receivers is analyzed in frequency-nonselective Rayleigh fading CDMA channels with memory. The estimates of the complex fading processes are utilized for maximal-ratio combining and carrier recovery of the coherent multiuser receiver. To analyze the impact of channel estimation errors on the receiver performance, error probability is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimation as well as the fading-induced error probability floor. The impact of fading dynamics on the differentially coherent decorrelating receiver with equal-gain combining is quantified. While performance of multiuser receivers at lower SNR is determined by both the fading dynamics and the number of active CDMA users, performance at higher SNR is given by an error probability floor which is due to fading only and has the same value as in a single-user case. The comparison of the two receiver structures indicates that the coherent decorrelating receiver with diversity reception may be preferable to the differentially coherent one in nonselective fading CDMA channels with memory.     

A decision feedback equalizer with time-reversal structure

This work describes the use of a receiver with a time-reversal structure for low-complexity decision feedback equalization of slowly fading dispersive indoor radio channels. Time-reversal is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the equalizer is a time-reverse of the actual channel impulse response. Selective time-reversal operation, therefore, allows a decision feedback equalizer (DFE) with a small number of forward filter taps to perform equally well for both minimum-phase and maximum-phase channel characteristics. The author evaluates the theoretical performance bounds for such a receiver and quantifies the possible performance improvement for discrete multipath channels with Rayleigh fading statistics. Two extreme cases of DFE examples are considered: an infinite-length DFE; and a DFE with a single forward filter tap. Optimum burst and symbol timing recovery is addressed and several practical schemes are suggested. Simulation results are presented. The combined use of equalization and diversity reception is considered     

Spectrally Shaped DS–CDMA with Dual Sideband Combining for Increased Diversity on Dispersive Fading Channels

A new modulation scheme is proposed in this paper. This scheme uses sinusoidal chip waveforms to shape the spectrum of a direct sequence spread spectrum (DS-SS) signal such that the transmitted signal has two distinct spectral lobes, one from a lower sideband (LSB) and the other from an upper sideband (USB). By properly selecting the frequency of the sinusoidal chip waveforms, the two sideband signals can be made to undergo independent fading in a dispersive fading channel. These two independent sideband signals, when combined at the receiver, provide diversity gain. Our analysis and simulation results show that the bit error ratio (BER) performance of the proposed scheme is superior to that of the equivalent DS-SS system that uses conventional rectangular chip waveforms for severely faded channels.     

Postdetection phase combining diversity

We propose a postdetection phase combining (PC) scheme for the two branch diversity reception of differential phase shift keying (DPSK) over multipath fading channels. The receiver has a differential phase detector (DPD) in each diversity branch, and the combiner weights each detector output in proportion to the vth power of the signal envelope at the detector's input. For π/4-shift QDPSK over frequency-flat Rayleigh fading channels, we find via computer simulation that the optimum weight factor is v=2, and that our simple, practical combining scheme performs almost as well as postdetection maximal ratio combining (MRC). We demonstrate similar relative performances for frequency-selective fading channels and for channels with co-channel interference (CCI)     

Prediction/cancellation techniques for fading broadcastingchannels. I. PSK signals

An approach for reducing the effects of multiplicative noise, such as fading, on the performance of mobile broadcasting transmission systems is introduced. The proposed method is based on a noise prediction/cancellation technique and is applied to phase-shift keyed (PSK) signals. By exploiting the statistical characteristics of the multipath fading signal and its strong correlation properties, a novel sequential receiver structure is derived that uses a detection algorithm which identifies the most probable transmitted sequence in the maximum-likelihood detection sense. The algorithm has been obtained for both Rayleigh and Rician fading channels. The proposed detection technique has been evaluated for various QPSK schemes operated in channels corrupted by a combination of additive white Gaussian noise (AWGN) and fading. The computer simulation results obtained show that by using a relatively simple receiver with a prediction algorithm of second order, considerable error floor reductions are achieved by the proposed receivers. As a direct result of these error floor reductions substantial bit error rate (BER) improvements of more than three orders of magnitude over conventionally detected schemes are obtained     

DTTB单载波系统的分集接收与均衡性能研究

分析了数字电视地面广播传输系统单载波模式中,将空间分集技术与判决反馈均衡器相结合,采用MMSE合并,在频率选择性衰落信道下的误码性能.可以看到采用分集接收技术,能够极大提高均衡器的输出信噪比,从而抑制误码扩散,使得接收机在对抗频率选择性衰落信道时性能有了很大提高.     

一种新颖的基于子矩阵交织的差分酉空时调制

差分酉空时调制(DUSTM)是一种应用于时变衰落信道下的多天线调制方法。该方法在慢衰落信道下无需知道信道状态信息而能获得全发送分集增益。但是,在快速衰落信道下,其性能明显恶化并且呈现出较高的误码平层。该文通过在差分酉空时调制中引入矩阵分割和子矩阵交织等操作提出了一种基于子矩阵交织的差分酉空时调制(SMI-DUSTM)方案,并对其性能进行了分析。性能分析和相应的计算机仿真证明了SMI-DUSTM 不仅能够继承DUSTM在慢衰落信道下的优点,而且在快速衰落信道下能够保持良好的系统性能。     

Performance of an array receiver with a Kalman channel predictorfor fast Rayleigh flat fading environments

We develop an approach for using an antenna array for tracking fast Rayleigh flat fading channels and suppressing cochannel interference. The fast flat fading process is assumed to be a general autoregressive (AR) process in order to characterize temporal variation of channels and evaluate its effect on the receiver structure and performance. The optimal array receiver structure that minimizes the probability of error for BPSK signals is derived, which includes a Kalman filter to predict the fading channels. A simple integral expression for the probability of error is also derived for the optimal receiver. In particular, we analyze the case with identical shaping filters. An irreducible probability of error is shown to exist due to the prediction error of multiple channels. Another interesting observation from the study is that the diversity gain with m antenna elements in the presence of k interferences is usually greater than (m-k), even in the presence of channel prediction error. Simulations are carried out to verify the theoretical analysis     

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     

Direct-Sequence Spread-Spectrum Receiver for Communication on Frequency-Selective Fading Channels

The reception of direct-sequence spread-spectrum signals on frequency-selective fading communication channels is considered. The fading statistics are described using the wide-sense-stationary uncorrelated-scattering (WSSUS) channel model. It is shown that, under certain assumptions about this channel such as time-invariance over the duration of a data symbol, an orthogonal representation for the received distorted signal can be found. The optimum incoherent receiver can then be realized with reasonable complexity. The analysis shows that exploiting the inherent diversity of a frequency-selective channel can reduce the receiver error probability by several orders of magnitude. The optimum selective channel and the jamming susceptibility of the receiver are discussed.     

Best antenna selection for coded SIMO–OFDM

Multiple receive antennas with optimal combining have been known to improve error performance over fading multipath channels by providing spatial diversity. This benefit is obtained at the cost of greatly increased system complexity due to the need for multiple RF chains and signal combiners. Best antenna selection is a technique that can provide multiple antenna gains with only a single RF chain and no combiners. Best antenna selection is complicated by frequency selectivity in orthogonal frequency division multiplexing (OFDM) as the signal at any one antenna may not be the best at all subcarriers. In this paper, we propose a novel technique for best antenna selection in coded OFDM. To simplify the receiver, we assume a block fading model for the underlying frequency selective channel. The best antenna will then determined based on coding theorems known for block fading channels. Our simulations show significant improvement in coded OFDM performance over existing techniques.