双瑞利衰落信道下最大比合并性能研究

基于接收信噪比的统计特性,研究了双瑞利衰落条件下各分集支路衰落幅度不平衡时最大比合并MRC接收系统的差错和分集性能。采用基于矩生成函数的方法导出了涵盖多种调制方式的平均误符号率ASER的通用公式,并得到了其Chernoff一致界。仿真结果表明:双瑞利衰落下采用MRC接收技术可以大大减小传输系统的ASER且能获得与瑞利衰落时相同的渐进分集增益,但在实际可接收到的信噪比范围内可获得的分集增益与满分集增益具有明显的差距;相对于单支路接收系统,双瑞利衰落下的MRC系统可近似得到满的相对分集增益。     

无线相干光通信空间分集接收合并技术

空间分集接收能补偿大气湍流造成的信道衰落。在给出相干检测分集接收的系统模型和晴朗大气信道模型的基础上,考虑子孔径间信号相关性,分析了等增益合并分集和最大比合并分集的误码率性能,并就中断概率与选择分集进行了比较。分析结果表明,空间分集接收能够明显改善相干光通信系统的性能,并且接收信号间的空间相关性越小分集接收的性能越好,其中最大比合并分集性能相对其他两种合并方式优势明显,选择分集性能最差,但它与等增益合并分集的差距不大,同时选择分集实现相对容易,在工程应用中要综合考虑实现的难易程度和性能。     

空间激光通信探测器阵列接收合并技术

大气湍流、吸收和散射等因素引起的光强闪烁严重影响了空间激光通信系统的效率,导致激光束能量衰减、信噪比下降,探测器阵列分集接收技术能有效克服这种影响.主要研究了探测阵列分集接收的三种合并方式,即选择式合并、等增益合并和最大比合并.分析对比了三种合并方式在提高功率增益和信噪比方面异同点.在相同大气湍流和中断概率条件下,最大比值合并对信号改善最为明显,等增益合并次之,选择式合并在分集接收中对信号改善最小.当两路分集接收时,最大比合并相对于无分集时信噪比改善因子提高了100％,等增益合并提高了78.5％,选择式合并提高了50％.最大比合并实现复杂,选择式合并实现相对容易,可根据实际工程性能要求,选择合适的合并方式.     

自适应算法在DSSS信号分集接收中的应用

分集接收技术由于能够获得分集增益,可提高多径信号的接收质量,因此在直接序列扩频系统中被广泛采用。常用的信号分集合并方法主要有选择性合并、等增益合并和最大比合并三种,其中最大比合并效果最佳,但需要准确地计算每路信号的信噪比,而采用自适应算法有效解决了准确计算信噪比的问题。因此,提出一种将RLS自适应算法与多径信号分集接收相结合的方法,它首先将接收到的多径信号分离为多路不同延时的信号,然后应用RLS自适应算法根据最小方差准则计算每一路信号的加权系数,通过对加权系数的更新获得最大分集增益,通过对不同的信号合并方式在多径信道下的性能进行仿真比较可知,采用RLS自适应算法的信号合并方式取得的性能最好。     

基于实时载噪比测量的分集增益测试方法

为测试分集接收设备的分集增益,提出了一种基于实时载噪比测量的分集增益测试方法,通过给分集接收设备输入2路不同信噪比、独立衰落的信号,利用功率计监测得到输入、输出信号和噪声的功率值,计算得出分集接收设备的分集增益。应用结果表明,方法能够测试分集接收设备的分集增益,具有较好的有效性和可行性。     

DAB单频网中循环延迟分集方法研究

详细分析了循环延迟分集技术,介绍了其实现方式、延迟的选择和分集增益的原理.进一步提出了在DAB单频网中应用循环延迟分集的方法.结果表明,采用提出的循环延迟发射分集方法能有效提高单频网覆盖交叉区域的接收误码率.     

PPM调制的超宽带差分接收机

提出一种PPM-DIFF-UWB调制方式及其接收机,该接收机采用三条具有不同延时特性的相关支路实现PPM-UWB信号的差分接收,提供了一种关于PPM调制的超宽带信号的低复杂度的接收方式,可用于实现在密集多径信道下的多径分集接收,且不需要复杂的信道估计以及发送额外的参考信号.理论分析和实验仿真结果表明该接收机的误码性能优于常规的PPM-TR-UWB接收机.     

DTMB接收系统中一种最优分集接收技术研究

通过对于DTMB(Digital Television Terrestrial Multimedia Broadcasting,数字电视地面多媒体广播)接收系统的研究,提出了一种最优分集接收结构。本结构结合了时域自适应判决反馈均衡器和空间分集技术,采用LMS(最小均方值)算法进行均衡器抽头自适应调整。相对于现有接收结构的研究,创新的提出了采用双前馈滤波器接收,合并至判决器后再传递到反馈滤波器的均衡器结构。该最佳接收结构相较两支路信号直接合并再输入到均衡器的接收结构,极大的提高了系统鲁棒性,减少了均衡器误码率。系统通过自适应调整判决器中两个前馈滤波器输入信号的权重,更加有效的适应信道的动态变化,较之两路信号等增益合并性能上更加优越,在实际接收机设计中也有很大的应用价值。     

概率方法预测特定环境超宽带信号功率分布

针对超宽带(UWB)信号在无线信道的传播特点,使用随机桥过程产生建模UWB信号多径分量传播轨迹的随机射线,构造出可以描述UWB信道特性的基本随机变量,由此得到预测信道中某个场点接收到的信号功率的概率方法.对一个典型室内传播环境中沿着任意路径接收的UWB信号功率进行仿真计算,结果表明预测功率分布概率方法的算法稳定.对传播环境中某个区域的功率分布进行计算,同时研究了不同分集接收方式对功率预测的影响.仿真结果表明,全Rake分集接收方式并不占优势;在分集接收的抽头数较少时,部分Rake分集接收和选择性Rake分集接收方式具有相似的分集接收效果.最后对接收点信号功率的分布进行统计分析,并对概率方法与射线追踪法进行了比较.     

自适应滤波在扩频信号分集接收中的应用研究

使用自适应滤波器完成对扩频信号的扩频码捕获后,传输信道中由于信号衰落造成的多径信号的幅度就可以从自适应滤波器收敛后的系数权值向量上体现出来.根据分集接收的理论分析,依据多径信号的信号幅度值进行分集接收在理论上是可行的,因此利用自适应滤波器对接收到的多径信号进行隐分集接收是完全可行的.使用自适应滤波器在完成扩频码捕获的同时,在不增加额外硬件支路的情况下就可以实现对多径信号的分集接收功能,从而改善在多径衰落环境下的信道传输性能.     

分集接收技术在海上升空通信中继系统中的应用

海上升空通信中继系统是解决海上广域无线宽带通信的重要手段,但其性能易受海面波动带来的多径衰落影响。为了有效提升中继系统接收信号质量,提出了一种多天线分集接收方法,通过现场可编程门阵列(Field Programmable Gate Array, FPGA)实现相应的接收处理。仿真分析及实验室测试表明,所提出的四天线分集接收技术能够带来近6 dB的合成增益。经海上对比试验验证,采用所提出的多天线分集接收技术的升空通信中继系统,其数据接收完整性提升了10%～20%,系统性能显著改善。     

Free-space optical communication through atmospheric turbulence channels

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers.     

A sectorized beamspace adaptive diversity combiner for multipathenvironments

The beam diversity technique is effective in combating multipath fading in wireless communications. In a beam diversity system, multiple receiving branches are formed with multiple antenna beams with distinctive patterns. These beams are synthesized in such a fashion that the fading phenomena observed at different branches are nearly uncorrelated. The disadvantage of such a system is the lack of adaptivity for cochannel interference (CCI) suppression. In this paper, an adaptive beam diversity combiner is proposed for sectorized signal reception. The diversity branches are formed with several adaptive beamformers whose response patterns encompass an angular sector in the field-of-view of the receiver. With a set of judiciously chosen weight vectors, effective diversity combining can be achieved inside the sector, and out-of-sector CCI can be suppressed via nulling. Simulation results confirm the efficacy of the proposed scheme     

Effect of aperture averaging and spatial diversity on capacity of optical wireless communication systems over lognormal channels

In this paper it is evaluated average channel capacity of an optical wireless communication system with aperture averaging and diversity reception over lognormal channels using a simple approximate closed form expression. The qualitative improvement in channel capacity is compared and investigated for various turbulence mitigation techniques: namely aperture averaging, diversity techniques such as maximal ratio combining and equal gain combining. Based on our study it is found that aperture averaging gives reasonably improved performance as compared to both types of diversity reception beyond certain turbulence strength. However, irrespective of turbulence strength, substantial improvement in capacity may be achieved with an array of direct detection receivers. Results obtained using the proposed expressions are in excellent agreement with those based on Monte Carlo simulations.     

Performance comparison of selection combining schemes for binary DPSK on nonselective Rayleigh-fading channels with interference

This paper is concerned with the error performance analysis of binary differential phase shift keying (DPSK) with differential detection over the nonselective Rayleigh-fading channel with selection diversity reception and with an additive, correlated, Gaussian interference process in each diversity channel. The fading process is assumed to have an arbitrary Doppler spectrum with arbitrary Doppler bandwidth. The selection schemes investigated are: 1) the selection combining (SC) scheme based on signal-to-noise power ratio (SNR); 2) the SC scheme based on signal-plus-noise (S+N); and 3) the SC scheme based on maximum output (MO). New, exact, closed-form bit-error probability (BEP) expressions are derived, and a performance comparison among the three SC schemes and combining diversity reception is given. The results obtained reduce to previously known results when the correlated interference process is absent, and when the fading process does not fluctuate over the duration of several symbol intervals. The results indicate that the performance of each scheme depends on the tradeoff between the number of diversity branches, the SNR, the interference level, and the correlation of the interference process. However, the SC-(S+N) scheme generally performs worse than the SC-SNR scheme, the SC-MO scheme and combining diversity reception scheme. The findings presented here are not only of fundamental theoretical value, but are also of practical interest to the designers of future mobile communication systems.     

Error analysis of noncoherent M-ary FSK with postdetection EGC overcorrelated Nakagami and Rician channels

We analyze the performance for the noncoherent reception of M-ary orthogonal frequency shift keying with postdetection equal gain combining over a correlated fading channel. Two kinds of correlated fading statistics are considered: (1) Nakagami fading in which the diversity branches can have unequal signal-to-noise ratios (SNRs) as well as different m-parameters and (2) Rician fading in which the diversity branches can have unequal SNRs. Using the characteristic function of the combiner output SNR, closed-form expressions for the symbol error probability are obtained     

Adaptive Diversity Combining for Improved Millimeter Wave Communication through Rain

Under severe rain conditions there may be significant amounts of multiply-scattered millimeter wave radiation present in the vicinity of the receiving antenna for a satellite downlink. This paper explores angle diversity reception of the scattered radiation as an alternative to spatial (multiple site) diversity for ameliorating rain attenuation. Results from scalar plane-wave transport theory for the attenuation, angle-of-arrival spread, multipath spread, and Doppler spread of the scattered field are summarized. When combined with a diversity reception communication analysis, these solutions imply that a large performance gain over nondiversity reception can be achieved if adaptive coherent combining can successfully be employed. Although this gain does not require multiple receiver sites, it does entail a transmission bandwidth reduction.     

Performance analysis of linear multisensor multiuser receivers forCDMA in fading channels

Bit-error probability (BEP) analysis for linear multiuser receivers with multiple sensors in frequency selective Rayleigh fading channels is presented. The analysis is applied to evaluate the BEP in antenna diversity reception and in a cellular CDMA system. Diversity and multiuser receivers are compared based on the examples. It is observed that adding new diversity antenna elements improves performance even if the correlation between the antenna elements is relatively large (up to 0.7). However, the large correlation values pose a significant reduction in the diversity gain in comparison to the zero correlation. It is also seen that the macroscopic diversity improves the performance of receivers significantly in cellular CDMA networks. When comparing diversity and multiuser receivers it is concluded that multiuser receivers are necessary to provide low BEPs. It is also highly beneficial to have at least two diversity antennas available, in particular, if there is no multipath diversity provided by the channel. The results also show that the reduction of intercell multiple-access interference yields a significant performance advantage in cellular networks. It is also demonstrated that the combination of spatial diversity and a multiuser receiver provides a significant receiver performance or system capacity gain in comparison to implementing only one of them