在CDMA多用户环境下STBC系统性能分析

该文提出了在CDMA多用户环境下对时空分组码(STBC)一种新的分析方法。文章考虑了对CDMA下行链路中有N根发射天线和M根接收天线的情况,提出了STBC(Space-Time Block Coding)的误码率的一种新的表达形式,并将它与具有M*N接收路径的最大比合并(MRC)进行了比较。通过比较,发现STBC与MRC具有相同的分集增益,同时,STBC还能提供部分的额外增益。因此,STBC在多用户环境下具有更好的性能。     

基于频分的MC-CDMA多用户系统

对多载波码分多址系统的多用户接收进行了研究,提出了基于频分的多载波码分多址(FD—MC—CDMA)系统,它是在MC—CDMA基础上结合FDMA的思想,发射端为每个用户分配一个特定的子载波集进行数据传输,减少相同子栽波上所叠加用户的数目;接收端使用最大似然检测器,利用所有接收信号进行多用户联合检测。该系统与传统的MC-CDMA系统相比有保证频率分集增益的同时降低多址干扰、有效利用多用户信息的优点。经过在Matlab平台上的仿真,结果表明,在频率选择性衰落信道和相同接入用户数的情况下,该系统与传统的MC—CDMA系统相比拥有更低的误码率,并且误码率随着用户数的增加呈阶梯状的缓慢递增。     

OFDM快时变信道下的相位旋转调制技术

由于发射端和接收端振荡器之间的频率偏差以及无线信道的时变性,OFDM系统各个子载波之间的正交性将会被破坏,从而产生载波间干扰(ICI)。该文利用发送端的多天线分集,提出了一种新型的相位旋转调制技术。它通过在频域对不同发送天线上的发送信号进行不同角度的旋转,使接收端等效信道频域响应产生的载波间干扰最小,达到抑制载波间干扰的作用。实验证明,这种新型的相位旋转调制技术不会改变OFDM系统的发送功率,能够在不占用额外系统带宽的情况下有效地消除快时变信道环境下系统的载波间干扰,达到较好的误码率性能。     

频谱有效的差分空时编码CDMA系统

该文针对码分多址(Code-Division Multiple-Access,CDMA)系统提出了频谱有效的差分空时传输方案。考虑包含M个同步共道用户的多用户环境,每一用户配备双发射天线。若接收端配备N2个接收天线,该方案将采用解相关检测器和接收天线分集分离M个用户。基于平坦Rayleigh衰落信道,给出了非相干译码器,它可为每一共道用户提供2(N-1)的最小分集增益。与已有的差分空时编码CDMA系统相比,该方案具有两大优势:第一,仅需增加单个接收天线,该方案可在频谱效率提高1/3的条件下显著地改善系统性能;第二,译码仅具有线性复杂度。     

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。     

多径衰落信道中M进制直接序列扩频系统RAKE接收机性能分析

本文分析了存在码片定时误差和载波相位估计误差情况下,采用BPSK调制的M进制正交码直接序列扩频信号在多径衰落信道下的RAKE接收机性能.分别讨论了不使用分集和采用选择式合并、等增益合并和最大比合并三种分集合并策略时接收信号的误码率,并推导出相应的近似计算方法.文章最后在此基础上分析了不同的分集合并办法、码片定时误差和载波相位误差对系统性能的影响.     

相位调制的多用户OFDM-FH通信系统性能分析

 本文研究一种高效的通信方式,即相位调制的OFDM-FH通信系统。为了提高衰落信道下系统误码率性能,接收端采用最大比合并的分集接收技术。利用特征函数方法,推导该系统在加性高斯白噪声信道和瑞利衰落信道时系统误码率性能。给出误比特率与跳频频点数 ,OFDM子载波数N和用户数 等参数之间的关系。理论分析和仿真实验结果表明,在相同参数条件下,基于相位键控调制的OFDM-FH多址接入系统与其它OFDM-FH通信系统比较,具有系统频带宽度占用少、误比特率低等优势。     

SIMO系统分集合并技术的性能分析

由于无线传输环境的复杂性,使得接收端必须采用抗信道衰落的技术,分集技术就是抗信道衰落的最有效措施之一。本文介绍了SIMO系统3种常见分集合并方式的基本原理及合并准则,分析了分集接收天线数与系统性能及系统复杂度之间的关系,并在瑞利衰落信道中采用16-QAM调制的情况下,对3种分集技术的误码率性能进行了仿真。结果表明MRC性能最好,EGC性能稍差,SC性能较差。但3种合并技术的算法复杂度正好相反,在实际应用中可根据需要进行折衷。     

无线宽带数据传输中多载波CDMA系统的性能

在分析同步多载波CDMA系统基带发射、接收模型和多径衰落信道模型的基础上,提出一种新颖的误码率分析方法。对系统在瑞利(Rayleigh)衰落信道中的误码性能进行了理论研究,得到了BPSK调制条件下的系统误码率表达式。数值分析结果表明:多载波CDMA在无线宽带数据传输中性能大大优于CDMA,具有良好的应用前景。     

多径衰落信道下多载波CDMA信号的频率分集接收

本文研究了多载波码分多址(Multi-Carrier CDMA)扩展频谱系统在多径衰落信道下的频率分集接收技术。文中首先研究了理想载波恢复、频率选择性瑞利衰落信道条件下多载波码分多址系统的性能,然后提出了用时间频率窗平滑来改善载波相位估计的算法,并对该算法在衰落信道下的性能进行了仿真。理论推导和计算机仿真结果说明,多载波码分多址系统的频率分集接收方案与理论上最佳的RAKE分集接收相比,性能略有下降,但接收机结构得到了简化。     

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.     

相干DS-SS/MPSK系统中频差影响的简化评估

该文针对单径慢衰落信道模型下的相干DS-SS/MPSK系统,分析了频差对系统性能的影响,得到了误比特率的封闭表达式。首先推导了匹配滤波解扩符号的信噪比特性和相干相位估计误差的分布;然后给出了衰落信道中误比特率的精确积分式,并利用MPSK条件误符号率的麦克劳林级数形式,推导了AWGN,Nakagami-m衰落和瑞利衰落信道中BPSK,QPSK误比特率的简化逼近式。最后的数值结果表明:逼近式的精度非常高,基本没有损失,可应用于实际系统设计和性能评估。     

Comparison of pilot symbol-assisted and differentially detectedBPSK for DS-CDMA systems employing RAKE receivers in Rayleigh fadingchannels

The capacity of a code-division multiple-access (CDMA) system is a function of the bit error rate (BER) performance of individual users. Therefore, it is important to optimize the individual links before proceeding to system level analysis. This is particularly true for operating in a fading channel where the performance without diversity reception is rather poor. This paper compares the BER performance of differential detection and pilot symbol-assisted coherent detection of a direct-sequence (DS) spread-spectrum (SS) signal on a frequency-selective Rayleigh fading channel using RAKE reception. Both equal gain and maximal ratio combining are considered, and the effect of convolutional coding with interleaving is studied. It is shown that in the particular cases considered in this paper, rate 1/8 convolutionally encoded pilot symbol-assisted BPSK performs better than coded differential detection, thus providing a higher system capacity     

Performance of MRC Diversity Systems for the Detection of Signals with Nakagami Fading

Bit error rate (BER) is analyzed theoretically for diversity reception in Nakagami fading environment using anM-branch maximal ratio combiner (MRC). Coherent and incoherent reception of frequency shift keying (FSK) are considered, using the multiple branch diversity system for both identical and different diversity branch fading parameters. The effect of correlation is also considered for the dual diversity case. The results are extended to include coherent phase shift keying (CPSK) and differential phase shift keying (DPSK).     

基于智能天线阵接收的蜂窝CDMA网络性能分析

本文分析了在频率选择性信道中基于智能天线阵接民的蜂窝CDMA网络上行链路的性能,分析基于多个小区的异步蜂窝CDMA系统,系统采用BPSK调制,并采用智能天线阵接收的RAKE合并技术,通过分析最终得到了在该环境下的闭合的CDMA系统误码率公式,结果表明基于智能天线阵接收的蜂窝CDMA性能要比不采用的好得多,并给出了误码率与系统用户数,小区数和衰落模型之间的关系,结果对于分析蜂窝CDMA系统容量具有一定的指导意义。     

弱湍流中紫外光非直视分集接收技术的研究

根据弱湍流信道中对数正态分布模型,建立了紫外光非直视分集接收系统。采用开关键控（OOK）调制,在不同闪烁指数和接收天线数下,分别对比分析了最大比合并（MRC）、等增益合并（EGC）和选择性合并（SC）的误码性能。仿真结果表明,相比于无分集情况,采用三种合并方式的误码率性能有明显提升。在接收天线数相同的情况下,三种合并方式中,MRC的性能最优,其次是EGC,SC的性能最差。对比分析了不同接收天线数时的误码率性能,随着接收天线数的增加,三种合并方式的误码性能得到了较大改善。在弱湍流信道中,采用分集接收技术能够减轻衰落的影响,提高分集增益。     

A digital coherent receiver suitable for land-mobile satellitecommunications

Describes an advanced coherent demodulation technique suitable for land-mobile satellite communications. The proposed technique features a combined narrow/wide band dual open loop (DOL) carrier phase estimator, which effectively enables the coherent receiver to track fast phase fluctuations caused by fading, without degradation in phase slip characteristics. Additionally, an open loop phase estimator has inherent quick recovery performance. Its bit error rate (BER) performance is shown to be superior to that for existing detection schemes, achieving a 10^-2 BER at 6.3 dB E_b/N₀ (0.9 dB greater than the theoretical E_b/N₀ condition for perfect carrier phase tracking) for QPSK over a Rician fading channel with the 10 dB Rician factor and the 1/16 baud rate fading pitch. The paper also describes a quick bit timing recovery scheme, with interpolation, featuring an open loop structure. Further, it presents an experimental digital modem developed through the use of digital signal processors     

All-digital reverse modulation architecture based carrier recoveryimplementation for GMSK and compatible FQPSK

A carrier recovery circuit implementation with an all-digital reverse modulation approach for coherent detection in the GSM/GMSK system as well as the GMSK compatible improved efficiency cross-correlated FQPSK system is presented. The proposed carrier recovery implementation utilizes all-digital reverse modulation circuit in a feedback loop to remove the modulated signal from the received intermediate frequency (IF) signal and to estimate the phase error of this carrier signal using a phase-locked loop (PLL). The digital reverse modulation approach avoids the multipliers required in an analog reverse modulation design, so that it can be implemented in a single chip FPGA. Hardware implementation of the coherent detection demonstrates that cross-correlated FQPSK is completely compatible with GMSK in the system performance and the receiver structure for GSM. Experimental performance evaluations show that the proposed carrier recovery circuit provides a Bit Error Rate (BER) performance within 0.3 dB in a non-linearly amplified channel corrupted by additive white Gaussian noise (AWCN) as compared with the simulated performance of the GSM/GMSK system     

DS-CDMA系统中最大比合并的性能分析

该文推导了存在信道估计误差的DS-CDMA系统在多径Rayleigh衰落信道中的误码率。数值计算结果表明当实际系统中的信道估计存在误差并且信道多径功率不等时,在假设相等噪声功率和理想信道估计下得到的传统最大比合并方法因无法输出最大的信噪比而性能明显下降。