大容量散射调制解调技术研究

简要介绍了时域自适应判决均衡器的原理,并根据对流层散射传播特点,利用自适应均衡技术以提高散射系统的抗码间干扰能力。采用四重显分集加自适应均衡的接收技术不仅可以减小码间干扰,收集隐分集能量,而且与显分集结合能够平滑信道选择性深衰落以及空间平坦衰落,大大提高设备通信能力。详细论述了自适应均衡器的技术特点和检测技术原理,并给出了对流层散射自适应均衡解调器的实现方案和抗多径测试结果。     

全数字线性调频信号产生及失真自适应接收机技术研究

对流层散射信道是一种随机多径信道,在该信道上传输数字信息时,首先要克服多径时散引起的码间干扰和快衰落导致的通信性能恶化。本文阐述了分离多径技术方案抗码间干扰、抗快衰落以及失真自适应接收的机理,研究了线性调频信号多径分集调制解调器的数字化实现,给出了系统性能测试的结果,积极有效地解决了散射信道存在的主要问题。     

瑞利衰落信道下带内频率分集性能分析

针对带内频率分集技术应用的有效性以及其性能分析的重要性,在瑞利衰落信道下,以2PSK为例,从分析推导N重空间分集最佳合并系统误码性能入手,结合带内频率分集技术的功率分散问题分析,给出了瑞利衰落信道下N重带内频率分集最佳合并系统的误码性能分析方法,并分析比较了不同带内频率分集重数的性能,得出带内频率分集技术的设计思想。     

多载波频率分集CDMA系统的通用矩阵模型及在瑞利衰落信道中的性能

提出一种通用的多载波频率分集CDMA系统基带发射、瑞利信道以及相关接收的矩阵信号分析模型。通过分析可以看出，多载波频率分集CDMA系统是一种高效的并行调制技术，它使信道和信号之间的卷积关系变成了线性加权关系，有效地抑制了符号间干扰和码间干扰，同时还具有频率分集的优点。对多载波频率分集CDMA系统在BPSK调制下的理论误码性能进行了深入探讨，给出了一种新颖的单用户系统和多用户系统的误码率分析方法。通过高斯近似法和Monte-Carlo模拟方法得到的数值仿真结果表明：瑞利衰落信道下，多载波频率分集CDMA具有良好的抗多径效果：随着用户数的增加，系统性能下降；在无线宽带数据业务传输中的性能大大优于CDMA，应用前景广阔。     

地空信道传输特性与技术分析

从地空通信的原理出发,分析了地空通信的通信覆盖能力与空中平台升空高度以及天线仰角的关系,给出了地面反射二径模型,分析了地空信道的衰落特性、误码性能以及在分集接收条件下的误码性能改善,并进行了相应的仿真,给出了相应的仿真结果。仿真结果表明地空通信信道的衰落十分明显,对信息传输性能造成不可忽视的影响,并且在低莱斯因子条件下,采用分集接收技术可有效对抗信道衰落,且对传输误码性能有明显改善,这些结论对今后的工程设计与实现提供重要参考和指导。     

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。     

失真自适应接收机技术及信号形式选择

对流层散射信道是一种典型的多径衰落信道,多径传播造成了信号的深衰落和多径扩散。而多径扩散现象的存在,会使速率高、码元窄、频谱宽的信号遭受频率选择性衰落的影响,造成传输中的信号码元之间产生码间干扰,严重影响信息传输质量。在此提出了一种基于失真自适应接收技术和3/4占空半余弦信号形式的数字接收机方案,能够自适应地跟踪失真信号的变化而进行最佳相干接收,使系统具有稳定的通信性能。     

低莱斯因子下的时频联合MMSE均衡算法

针对低莱斯因子的慢衰落时变多径信道条件下,难以有效利用信号能量,导致系统误码性能差,单纯的基于最小均方误差准则均衡（Minimum Mean Squared Error, MMSE）技术不能够有效降低误码率的实际问题。在构建信道等效模型的基础上,提出了一种利用硬判决符号重构信道,在两个信道长度内比较改判信号误差信息的时频联合判决两轮次MMSE均衡算法。仿真结果表明:通过均衡处理后的低莱斯因子信道能逼近高莱斯因子的误码特性,当信噪比为20dB时,不同莱斯因子下QPSK信号的误码率均降低至10^(-4)以下,且在一个数量级内。当信噪比为2~6dB时,系统误码率分布在10^(-1)~10^(-3)之间,要低于信号在自由空间传播的误码率,说明MMSE均衡充分利用了多径分量的能量,获得了信噪比增益。      

随机M进制正交码混合DS—SFH CDMA扩频通信系统性能分析

本文研究了随机ＭＦ是制正交码混合ＤＳ－ＳＦＨ扩频码分多址信号经过多径瑞利频率非选择性衰落信道，在非相干ＲＡＫＥ接收机中采用最大输出信噪比选择或最大输出选择接收时系统的性能；给出了两种分集接收情况下差错概率表示式。数值计算研究了分集数，Ｍ值，多用户干扰对系统性能的影响，并且对这两种分集接收情况下系统性能进行了比较。     

低Rician因子信道传输损伤分析及抗衰落措施

分析了低Rician因子多径信道的误码性能。详细讨论了多重分集对Rician信道误码性能的改善程度,并用Matlab进行仿真。研究结果表明,分集接收可以很有效地平滑低Rician因子多径信道的深衰落,并给出了定量的分析结论,这些结论有助于工程设计中技术方案的选择。     

Assessment of a pico-cellular system using propagation measurementsat 1.9 GHz for indoor wireless communications

Narrow-band propagation measurements were conducted at two floors of a multi-storied building at 1.9 GHz to study the behavior of the Rice factor and local mean power. To evaluate the influence of the Rice factor on a pico-cellular system, the outage probability is calculated for two different cases: (i) Rician fading for the desired and interfering signals with different Rice factors; (ii) desired signal with Rician fading and interfering signals with Rayleigh fading with a different local mean power for each interfering signal. Finally, a pico-cellular structure is determined as an illustration     

A Walsh-Hadamard coded spectral efficient full frequency diversity OFDM system

A full frequency diversity COFDM (FD-COFDM) scheme based on sub-channel multiplexing is proposed to improve the performance of COFDM without sacrificing spectral efficiency. Since the channel suffers the frequency selective fading, the received signal contains strong inter-symbol interference (ISI). To obtain the diversity- and code-gains, an iterative receiving algorithm is presented. By jointly using diversity combining, interference cancelation and soft decoding, the proposed iterative receiving algorithm properly decodes the information bits in the strong ISI environment. Consequently, all the subcarriers of FD-COFDM participate in diversity so that the system can achieve full frequency diversity in most practical channels. The new receiving algorithm is relatively simple compared to the maximum likelihood estimation algorithm, and is therefore a better alternative to balance the algorithm complexity and system performance.     

Channel precoding for indoor radio communications using dimensionpartitioning

A new phase precoding technique is developed to combat the intersymbol interference (ISI) resulting from a frequency-selective slowly fading channel in a personal communication system using quadrature phase-shift keying (QPSK). Based on a new dimension partitioning technique, the precoder predistorts only the phase of the transmitted signal to keep a constant transmitted signal amplitude and, therefore, to ensure the stability of the precoder even in equalizing a nonminimum-phase channel. Under the constraint of the constant amplitude, the dimension partitioning method is developed to guarantee the possibility of correct detection for all transmitted information symbols and to further improve the transmission accuracy by increasing the size of the decision regions. Analytical and simulation results demonstrate that over frequency selective Rayleigh and Rician fading channels, the system using the proposed channel precoder can achieve a bit error rate (BER) comparable with that using a conventional decision feedback equalizer (DFE). The precoder can outperform the DFE in an indoor environment where there is a strong direct propagation path. The main advantage of using the precoder is that the impairment of ISI due to multipath propagation on the transmission performance can be mitigated without increasing the complexity of the portable unit receiver. The proposed technique is especially useful for personal communications, where ISI due to multipath fading channels can severely deteriorate the BER transmission performance and where the simplicity of portable units is a vital characteristic of the system     

Subblock processing in MMSE-FDE under fast fading environments

Frequency domain equalization (FDE) has been studied for reducing inter-symbol interference (ISI) caused by frequency selective fading in single carrier systems. When a high-mobility terminal exists in the system, the channel state may change within a DFT block. Then, the ISI reduction performance of FDE degrades since cyclicity of the channel matrix is lost. We propose to divide a received data block into multiple subblocks to decrease the channel transition within the DFT block in fast fading environments. Also, to satisfy periodicity of the received signal in each subblock, we introduce a pseudo cyclic prefix technique. The results of numerical analysis show that the proposed method can effectively decrease the error floor in fast fading environments.     

Power control and diversity in mobile radio cellular systems in thepresence of Ricean fading and log-normal shadowing

The performance of a cellular mobile radio system with frequency reuse is evaluated in terms of outage probability. Deterministic path loss, log-normal shadowing, and Ricean fading are accounted for, and the use of diversity and power control is considered in order to enhance system performance. Both hexagonal and lineal cells are considered. Particular attention is given to the sensitivity of the outage probability to the system parameters, especially those related to the propagation model (fading, shadowing, and path loss). It is seen that diversity and power control can improve the system behavior. The performance is sensitive to the fading parameter (i.e., the Rice factor) of the intended user, but is relatively independent of that of the interferers. Also, a significant dependence is observed on the shadowing parameter, whereas a limited dependence is seen on the outage threshold and the channel utilization. Finally, the presence of a dual path loss law degrades the performance, and the outage probability increases as the breakpoint distance gets larger     

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.     

V-BLAST系统的一种新的分集迭代检测算法

讨论了原V-LAST(垂直Bell实验室分层空间-时间)系统的算法,从获得接收分集的角度给出了一种新的迭代处理算法。该算法利用接收端进行分集接收处理来改善V-BLAST系统每层的接收信号,对进一步抵抗V-BLAST系统的误差传播效应取得很好的效果。该算法可以看做是V-BLAST系统许多算法的一种延伸。仿真结果显示,与原V-BLAST算法相比具有更好的误码性能:在开环2发2收的情况下,信道为瑞利平坦慢衰落,采用QPSK(正交相移键控)调制,误码率为10-2时可获得2 dB的增益。