DSSS-MIMO结构的标签延迟发射分集方法

针对频率选择性衰落多输入多输出(Multiple-Input Multiple-Output,MIMO)结构,提出在各发射天线通道引入直接序列扩频(Direct-Sequence Spread-Spectrum,DSSS)操作以便实现信道的矢量化,然后提出了一种标签延迟发射分集方法和相应的实现频率选择性衰落无线信道估计的盲方法.仿真结果表明了上述标签延迟发射分集方法的有效性及其信道盲估计方法的性能.     

二维扩频系统中时频码片级差分检测的性能分析

时变频率选择性衰落信道中,针对二维扩频系统提出了一种时频码片级差分检测方法,并进行了误码率分析,同时进行了仿真验证.分析和仿真结果表明,时频码片级差分检测同时具有抵抗信道频率选择性和时间选择性衰落的能力,在选择性衰落信道中的性能优于平坦衰落信道中的性能.通过性能对比发现,时频码片级差分检测相对多载波扩频码片级差分检测能够更加有效地降低了载波间干扰对系统性能的影响.     

叠加训练序列OFDM优化接收机的误码率性能分析

在时变频率选择性衰落信道下，针对叠加训练序列的正交频分复用系统，在接收端利用训练序列的一阶统计特性进行信道估计，分析了接收机的比特误码率性能．然后利用差错概率最小化准则，给出了优化的训练序列分离因子，计算机仿真结果验证了理论分析的有效性。     

频率选择性慢衰落信道下的快速频率估计

给出了在频率选择性慢衰落信道下适用于突发传输的两种快速频偏估计算法。针对慢衰落选择性信道下的频率估计问题,两种算法均给出了一个简单的闭式解,且算法实现无需任何信道幅度信息。第一种算法要求用于频率估计的训练序列的自相关矩阵具有对角特性,第二种算法使用周期的训练序列,两种算法所需的运算量大大低于最大似然估计算法。仿真结果显示,在较高信噪比下两种算法均具有较好的估计性能。     

几种CDMA技术及其性能比较

CDMA是一种码分多址技术，可提供远远超过传统接入技术(如TDMA和FDMA)的容量并能抗信道的频率选择性衰落。作为一种移动无线通信技术，CDMA技术在不断发展，出现了几种有竞争力的方案，诸如：正交频分复用(OFDM)的多载波CDMA，即所谓的MC—CDMA：直接序列扩频的多载波CDMA(MC—DS—     

无人机视距数据链的信道特性与扩频技术

无人机视距数据链的信道类型主要是一个干涉型平衰落信道,但也可能发生频率选择性衰落、时间选择性衰落。限于无人机的使用特点和条件,视距数据链不同于一般通信系统,不适于极高的通信可靠度,也不要求很大的衰落储备。分析了扩频系统在无人机视距数据链中的特殊性并给出了解决方法。     

一种新的抗远近效应的盲DS-CDMA接收机

本文研究多径频率选择性衰落信道下直接序列扩频码分多址(DS-CDMA)信号的处理问题,提出了一种新型的盲接收机.该接收机不需预知多址干扰用户的扩频码,不需预知信道参数,只需已知期望用户的扩频码和粗略的定时,就可以完成用户的盲检测.同时,该接收机通过降维自适应去相关滤波和基于判决指导的自适应多径合并,来获得低成本和良好的抗衰落、抗远近效应性能.仿真结果验证了本文提出的接收机的优良性能.     

基于SVD的LC_CDMA通信系统信道估计研究

在LC-CDMA通信系统中，由于在多径衰落信道下使用了时变扩频码，使得信道估计非常困难，而对接收机的设计往往需要信道的先验知识。本文建立了长码条件下的CDMA通信系统模型和多径频率选择性衰落信道的FIR滤波器模型，利用子空间分解的方法，推导了长码条件下CDMA通信系统的信道估计算法，并仿真分析算法的性能。     

部分功率训练序列与OFDM承载数据的优化分离

在采用部分功率训练序列同步的OFDM系统中,接收到的OFDM承载的数据信号会受到部分功率训练序列的严重干扰。由于信道估计的误差,使得对训练序列的分离同样存在误差。本文给出了在频率选择性衰落信道中,一种优化的抑制训练序列对OFDM承载数据的干扰的方法。通过理论分析,给出了优化的训练序列对消因子。计算机仿真结果验证了理论分析的合理性。     

频率选择性慢衰落信道下的一种Rake接收技术

通过对无线通信信道传输特性的讨论,以瑞利信道和莱斯信道为模型,研究在QPSK扩频调制下接收端信号的同步、多径分离与按增益合并.仿真实现了频率选择性慢衰落信道下的一种Rake接收技术,结果表明用他来减小多径衰落是有效的.     

Comprehensive evaluation of chip interleaving effect on turbo‐coded DS‐SS in a Rayleigh fading channel with antenna diversity reception

In this paper, we present a comprehensive performance evaluation of chip interleaving effect on turbo coded direct sequence spread spectrum (DS‐SS) system in a frequency non‐selective Rayleigh fading channel with antenna diversity reception. At the transmitter, chip interleaver scrambles the SF (spreading factor) chips associated with a data symbol and transforms the transmission channel into a highly time‐selective or highly memoryless channel at the chip level. The use of chip interleaving is equivalent to using SF‐antenna diversity reception with correlated fading among the branches and with reduced average received signal power per antenna by a factor of SF. We theoretically analyze how chip interleaving alters the received signal statistics. Then, the effect of the various parameters, viz. interleaver size, interleaving depth, information sequence length, spreading factor and the fading maximum Doppler frequency, are also evaluated. It is found that the bit error rate (BER) performance improves with increasing spreading factor and increasing frame length. Chip interleaving is found to be effective in the presence of receive antenna diversity as well. Copyright © 2005 John Wiley & Sons, Ltd.     

Tradeoffs of Source Coding, Channel Coding and Spreading in Frequency Selective Rayleigh Fading Channels

This paper investigates the tradeoffs of source coding, channel coding and spreading in CDMA systems. We consider a system consisting of an image source coder, a convolutional channel coder, an interleaver, and a direct sequence spreading module. With different allocations of bandwidth to source coding, channel coding and spreading, the system is analyzed over a frequency selective Rayleigh fading channel. The performance of the system is evaluated using the cumulative distribution function of peak signal-to-noise ratio. Tradeoffs of different components of the system are determined through simulations. We show that, for a given bandwidth, an optimal allocation of that bandwidth can be found. Tradeoffs among the parameters allow us to tune the system performance to specific requirements.     

An interference avoidance code assignment strategy for downlink multi-rate mc-ds-cdma with tf-domain spreading

Frequency selective fading may affect the orthogonality of the spreading codes in the multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems. In this paper, we define a new performance metric called the multiple access interference (MAI) coefficient for the MC-DS-CDMA system to quantitatively predict the impact of inter-code interference with the time- and frequency-domain spreading in a frequency selective fading channel. With the help of MAI coefficient, a novel interference avoidance code assignment strategy is proposed. By jointly considering the incurred MAI effect and the blocking probability in the code tree structure, the proposed interference avoidance code assignment method can effectively reduce the MAI for the multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.     

直扩系统时变单音干扰抑制的新方法

针对直接序列扩频通信系统中的时变单音干扰,提出了一种频域干扰估计时域对消的新方法。在系统中,将干扰信道建模为时变衰落信道。该算法首先在频域对时变单音干扰信号参数进行估计,然后对时变单音干扰进行重构和对消,推导了理论误码率公式。理论分析和仿真结果表明,该算法性能由干扰功率和多普勒频移等因素决定。在频率选择性衰落信道下,当信噪比为20dB,扩频增益为16,干扰信道多普勒频移为100Hz时,该算法可以抑制20dB(干信比)的单音干扰。      

Single Carrier Frequency Domain Equalization Space–Time Block-Spread CDMA with Multiuser Interference-Free Detection

In this paper, we study a single carrier space–time block-spread (STBS) with frequency domain equalization combined with direct-spread code division multiple access (CDMA) which we term, SCFDE-STBS-CDMA. We propose a novel SCFDE space–time scheme for CDMA that achieves multiuser-interference free reception and performs well in both slow and fast fading frequency selective channel. The orthogonality among the users is preserved at the receiver allowing a multiuser-free MUI-free detection in slow fading channel. In fast fading channel, we proposed a MMSE detector that exploits the time diversity of the fast fading channels. In the conventional counterpart scheme proposed in the literature, the length of the spreading factor affect negatively the performance of the system when the channel is fast fading while in the proposed scheme, the spreading factor is an additional degree of freedom that do not degrade the system performance. Since the maximum number of users supported depends on the spreading factor, the proposed scheme can then maintain more users than the conventional one in fast fading channels. The bit-error rate (BER) performance of the proposed scheme is analyzed and compared with the conventional approach in the literature.     

Performance of multicarrier DS CDMA systems

In this paper, we apply a multicarrier signaling technique to a direct-sequence CDMA system, where a data sequence multiplied by a spreading sequence modulates multiple carriers, rather than a single carrier. The receiver provides a correlator for each carrier, and the outputs of the correlators are combined with a maximal-ratio combiner. This type of signaling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. We use bandlimited spreading waveforms to prevent self-interference, and we evaluate system performance over a frequency selective Rayleigh channel in the presence of partial band interference; we also compare system performance with that of a single-carrier RAKE system     

Performance enhancement of DS/CDMA system using chaotic complex spreading sequence

In this paper, a novel direct sequence/spread spectrum (DS/SS) communication system is proposed. This scheme exploits the two-dimensional complex valued chaotic Ikeda map as the spreading sequences. With this double spreading DS/SS system, the effect of multiple access interference can be mitigated by choosing the spreading sequences with appropriate cross-correlation properties. The performance of the system is assessed and demonstrated in a multiuser environment by means of computer simulation with additive white Gaussian noise, Rayleigh fading, and selective fading channel conditions. These studies reveal that the proposed system significantly outperforms the Gold code DS/SS-BPSK system in synchronous channel conditions. In asynchronous case, the improvement is substantial for low signal-to-noise ratios.     

一种改进的MIMO-OFDM的信道估计算法

针对频率选择性衰落信道下MIMO—OFDM的系统,基于瑞利衰落信道的模型,利用m序列的自相关性,提出了一种基于m序列做梳状导频的多输入多输出正交频分复用最小二乘算法,该算法可以避免对大矩阵求伪逆,以减少计算复杂度,从而提高了信道估计的计算性能。通过对该算法的误码率性能分析和计算复杂度分析,结果表明,相比传统经典最小二乘算法,所提出信道估计算法在中低信噪比下,有效提高了信道估计性能,适合于在实际应用中实现。     

Selection of spreading sequences for direct-sequencespread-spectrum communications over a doubly selective fading channel

Error probabilities are evaluated for direct-sequence spread-spectrum communications over channels with doubly selective fading. The error probability for such a system depends on the spreading sequence, the autocorrelation function of the fading process, and the receiver signal-to-noise ratio. The focus of this paper is on the effect of the spreading sequence on the performance of differentially coherent detection of binary direct-sequence spread-spectrum signals using a correlator receiver. It is shown that significant performance differences result from different choices of spreading sequence. It is also shown that, given a moderate range of delay and Doppler spreads, sequences can be found which yield low bit error probabilities over the entire range. These are found to be robust with respect to a variety of shapes for the channel autocorrelation function and the full range of signal-to-noise ratios     

The effects of sequence selection on DS spread spectrum withselective fading and Rake reception

Error probabilities are evaluated for direct-sequence spread-spectrum communications and Rake reception over channels with doubly selective fading. The error probability for such a system depends on the spreading sequence, the autocorrelation function of the fading process, the received signal-to-noise ratio, and the number of taps in the Rake receiver. The focus of the paper is on the effect of the spreading sequence on the performance of each of two systems. One system employs noncoherent detection of differentially-encoded binary direct-sequence spread-spectrum signals and a post-detection diversity-combining Rake receiver which uses equal-gain combining. The other system employs coherent detection of binary direct-sequence spread-spectrum signals and a post-detection diversity-combining Rake receiver with perfect gain estimates for the channel. A simple sequence selection criterion is introduced, and the sensitivity of the performance of the system to the choice of the spreading sequence is examined. It is shown that significant performance differences result from different choices of the spreading sequence. It is also shown that, given a moderate range of delay spreads, sequences can be found that yield low bit error probabilities over that range. These are found to be robust with respect to the delay spectrum for the channel, the number of taps in the Rake receiver, the Doppler spread, and the signal-to-noise ratio