多径衰落信道下频域差分检测OFDM系统的误码率分析

在多径衰落信道条件下，分析了正交频分复用系统中频域差分检测的误码率性能，并进行了仿真验证。分析与仿真结果表明，信道相干时间和相干带宽均影响频域差分检测的误码率性能，信道相干带宽对频域差分检测的影响比信道相干时间的影响更大，频域差分检测适合于多径延迟扩展比较小的信道。     

Frequency-Selective Fading Effects in Digital Mobile Radio with Diversity Combining

we analyze the effects of frequency-selective fading in a cellular mobile radio system that uses 1) phase-shift keying (PSK) with cosine rolloff pulses, and 2) space diversity with maximal-radio combining. The distorting phenomena with which we deal are multipath fading (which produces the frequency selectivity), shadow fading, and cochannel interference. The relevant quality measure is defined to be the bit error rate averaged over the multipath fading, denoted by (BER). The relevant system performance characteristic is defined to be the probability distribution for (BER), taken over the ensemble of shadow fadings and locations of the desired and interfering mobiles. To obtain numerical results, we use a combination of analysis and Monte Carlo simulation, invoke widely accepted models for the multipath and shadow fadings, and assume a cellular system with seven channel sets and centrally located base stations. The outcome is a set of performance curves that reveal the influences of various system and channel parameters. These include: the number of modulation levels (two or four), the diversity order, the shape of the multipath delay spectrum, and the standard deviation (or delay spread, τ0) of the multipath delay spectrum. Practical factors accounted for in these assessments include fading- and interference-related timing recovery errors and combiner imperfections. Our results highlight the importance of the ratiotau_{0}/T, whereTis the digital symbol period. They show that the delay spectrum shape is of no importance fortau_{0}/T leq 0.2, but can have a profound influence fortau_{0}/T geq 0.3. We also find that using 4-PSK leads to better detection performance, in certain cases, than using 2-PSK.     

The effect of sample timing on bit error rate performance in a multipath fading channel

Digital mobile radio often suffers from fatal effects caused by frequency-selective fading, resulting in severe bit error rate (BER) degradation. The feasibility of digital mobile radio largely depends on the multipath-distortion tolerant capability of modulation technique. The BER performance is compared among four digital modulation techniques in a frequency selective fading channel with large multipath delay spread. In particular, taking the timing fluctuation of eye patterns due to delay distortion of a fading channel into account, the effect of sample timing on BER is analyzed. The analyses have revealed that BER performance markedly depends on the choice of sample timing, and also an interesting fact that BPSK is outstanding in the multipath-distortion tolerance, if coupled with adaptive sampling, and its BER curves remain almost the same as that of frequency-flat fading as long as multipath delay difference is less than a bit interval.     

Effects of Power Amplifier Distortion and Carrier Frequency Offset on the Performance of WLAN System

Due to the efficiency of mitigation multipath delay spread, orthogonal frequency division multiplexing (OFDM) is extensively used in the wireless local area network (WLAN) domain, such as the IEEE 802.11a standard defined by the IEEE 802.11 standardization group and the HIPERLAN/2 defined by the European Telecommunication Standards Institute Project on Broadband Radio Access Networks. OFDM based WLAN system is very sensitive to the power amplifier distortion and the carrier frequency offset. The performance of the OFDM based WLAN system in the presence of the power amplifier distortion and the carrier frequency offset under practical fading channel is researched and analyzed in this paper. A closed form of bit error rate (BER) is derived for the OFDM based WLAN system in the presence of the power amplifier distortion and the carrier frequency offset under practical fading channel. The effects of the power amplifier distortion and the carrier frequency offset on the OFDM based WLAN system performance are comparatively studied by the theoretical method and by the simulation method under practical multipath fading channels. Studies show that the theoretical and simulated results match well.     

户内多径信道中 PRF 对 UWB 系统性能影响

首次研究分析脉冲重复频率(PRF)对密集多径衰减信道下采用 THSS 和 PPM 的 UWB 系统 BER 性能的影响。通过使用混合选择／最大比率结合分集接收机(H-S／MRC)并利用虚拟分支技术,首次推导出基于 PRF、选择并结合的多径成分数量 L 以及信道的多径扩频的 UWB 系统准确 BER 表达式,并对 BER 性能进行分析和仿真评估。结果表明：在密集多径衰减环境下,不仅仅脉冲波形和 L,而且 PRF 对 UWB 系统 BER 性能影响极大。当固定 L 和脉冲波形时,增大 PRF 将大大降低 UWB 系统 BER 性能。     

The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation

Frequency-selective fading caused by multipath time delay spread degrades digital communication channels by causing intersymbol interference, thus resulting in an irreducible BER and imposing a upper limit on the data symbol rate. In this paper, a frequencyselective, slowly fading channel is studied by computer simulation. The unfiltered BPSK, QPSK, OQPSK, and MSK modulations are considered first to illustrate the physical insights and the error mechanisms. Two classes of modulation with spectral-shaping filtering are studied next to assess the tradeoff between spectral occupancy and the performance under the influence of time delay spread. The simulation is very flexible so that different channel parameters can be studied and optimized either individually or collectively. The irreducible BER averaged over fading samples with a given delay profile is used to compare different modulation/detection methods, while the cumulative distribution of short-term BER is employed to show allowable data symbol rates for given values of delay spread. It is found that both GMSK and QPSK with a raised-cosine Nyquist pulse are suitable for a TDM/TDMA digital portable communications channel.     

Performance of differential chaos-shift-keying digital communication systems over a multipath fading channel with delay spread

The performance of the noncoherent differential chaos-shift-keying (DCSK) communication system over a multipath fading channel with delay spread is evaluated. Analytical expressions of the bit error rates are derived under the assumption of an independent Rayleigh fading two-ray channel model. Analytical and simulated results are presented and compared. The multipath performance of the DCSK system is also compared with that of the coherent CSK system as well as conventional generic waveform communication schemes.     

Micro- and macrodiversity MDPSK on shadowed frequency-selectivechannels

The performance of M-ary differential phase shift keying (MDPSK) on frequency-selective slow Rayleigh fading, lognormal shadowed channels with diversity combining is analyzed for mobile and portable applications. The use of L-branch equal gain postdetection microdiversity combining to mitigate the effects of fading and P-port macrodiversity to alleviate the effects of shadowing are investigated. Four performance criteria are considered for a frequency-selective multipath fading, intersymbol interference channel. These are, the short term bit error rate (BER), the irreducible BER, the complementary distribution over the lognormal shadowing of the average BER, and the probability that the instantaneous BER exceeds a threshold value, averaged over a spatial environment. Closed-form expressions for the four performance criteria are obtained. The BER and outage performance results show that diversity combining is an effective method for improving the system performance (and hence system reliability), when the normalized delay spread is not large. It is also seen that, in most cases, 4DPSK gives the best performance followed by 8DPSK and 2DPSK, respectively, for a given information throughput     

Analysis of DPSK error rate due to multipath delay spread

The intersymbol interference (ISI) produced by multipath channel delay spread can be approximated as Gaussian noise. The equivalent average signal-to-noise power ratio (SNR) is defined. Very simple expressions for average bit error rate (BER) due to delay spread are derived for linear quaternary DPSK (QDPSK) transmission with differential detection over mobile radio Rayleigh fading channels.<>     

MC-CDMA调制在散射通信信道下的性能分析

为提高对流层散射通信系统容量和抗干扰能力,对散射信道下的多载波扩频方案进行研究.基于所建立的Rayleigh 分布对流层散射信道模型,分析了频域扩频多载波技术MC-CDMA的系统性能,对典型散射信道下的系统误码率进行了分析与仿真.结果表明:MC-CDMA可有效提升散射通信系统容量,单用户时系统误码率性能较好,随着用户数和散射信道多普勒频移的增大,系统误码率随之增大,需采用多址干扰控制和子载波干扰消除技术来改善系统误码率性能.     

QDPSK signal transmission performance with postdetection selectiondiversity reception in land mobile radio

The bit error rate (BER) performance of 16-128 kb/s Nyquist raised-cosine filtered quaternary differential phase-shift keying (RC-QDPSK) signal transmission was experimentally investigated. Two-branch postdetection selection diversity reception was employed. Measured results are presented for the BER performance due to additive white Gaussian noise (AWGN), cochannel interference, and multipath channel delay spread. Field BER measurements were also conducted at a carrier frequency of 1.45 GHz in an area characterized by high-rise buildings and in an area surrounded by mountains. The results show that postdetection selection diversity is a simple yet powerful technique to improve the BER performance in fading mobile radio channels     

空时分组码的多径译码方法

为有效地将空时分组码应用到多径衰落环境下的码分多址系统，以充分利用多个路径的信号能量，提出了一种多径环境下空时分组码译码的新方法。由于空时分组码译码与信道估计紧密相关，本文亦讨论了多径信道估计以及信道估计误差对本方法的影响。仿真结果表明，采用多路径译码方法可明显提高系统的误码率性能。     

Bit error simulation for π/4 DQPSK mobile radio communicationsusing two-ray and measurement-based impulse response models

An accurate software/hardware bit-by-bit error simulator for mobile radio communications is described. Simulation results in indoor and outdoor channels are compared with theoretical results. Bit error rate (BER) results in simulated frequency-selective fading channels generated by several channel models such as two-ray, constant amplitude, and simulated indoor radio channel impulse models (SIRCIMs) are presented. It is shown that BER is not only dependent on the RMS delay spread, but also on the distribution of temporal and spatial multipath components in local areas. An important result is that a two-ray Rayleigh fading model is a poor fit for indoor wireless channels and, if used, can underestimate the BER by orders of magnitude. A real-time bit error simulation of video transmission using the bit-by-bit error simulator is described. The simulator, called BERSIM, is shown to be a useful tool for evaluating emerging data transmission products for digital mobile communications     

Diversity receiver scheme and system performance evaluation for aCDMA system

Performance of a code-division multiple-access (CDMA) system is studied. The frame-error rate (FER) and bit-error rate (BER) of the forward traffic channel are obtained by using a simulation system based on TIA/EIA/IS-95. A diversity receiver structure with an adaptive search engine is developed and analyzed. The adaptive rate of the search engine is optimized to achieve high diversity gain. The diversity gain factor is derived based on a cluster of wide-band tapped delay-line multipath fading models. The relationships between receiver diversity gain, multipath channel characteristics, and the number of distributed antennas are derived. The multipath channel characteristics have a major impact on the system performance. Simulation shows that when channel mean delay spread is less than the chip interval, a distributed antenna system is essential for improving the system capacity. When more users are served in the system, FER is more sensitive to the channel characteristics. Uncaptured energy acts as self-interference, which can increase the BER by an order of magnitude and reduce system capacities, FER could also be improved by reducing the resolution of the diversity receiver branches to one half of the chip interval. The system performance for the indoor and outdoor environments are evaluated by utilizing the proposed diversity receiver over typical personal communications services channel models specified by the Joint Technical Committee (JTC)     

Performance of a Binary PPM Ultra-Wideband Communication System with Direct Sequence Spreading for Multiple Access

In this paper, we present an analysis of the BER performance of an ultra-wideband (UWB) system with pulse position modulation (PPM) for data modulation and direct sequence (DS) spreading for multiple access over indoor lognormal fading channels. A rake receiver is used to combine a subset of the resolvable multipath components using the maximal ratio combining technique. Inter-path and multiple-access interferences are modeled and incorporated into the bit-error-rate expressions. The analytical and simulation results allow one to quantify many critical aspects of a DS-PPM UWB system such as the gain of the optimally spaced signaling scheme over the orthogonal signaling scheme, the potential error floor given a specific channel multipath delay spread and the number of interfering users, tolerance of the system to timing jitter, and impact of user codes.     

PERFORMANCE OF CODED OFDM VIA VARIABLE GUARD INTERVAL

This letter presents the principles of Coded Orthogonal Frequency Division Multiplexing (COFDM) system and focuses on the effect of the Guard Interval (GI) on the system performance. The role of the GI parameter period to solve the problem of interferences that occur at the receiver is discussed. A new COFDM concept with a Variable GI (VGI) is proposed in order to improve the Bit Error Rate (BER) performance in the presence of multipaths with variable delays. A series of simulations have been carried out on the BER performance as a function of GI, maximum delay spread, and Signal-to-Noise Ratio (SNR), with QPSK as modulation scheme over multipath fading and Additive White Gaussian Noise (AWGN). The results show that the optimum values of the GI parameter approximately equal to the maximum delay spread, and the proposed system with VGI provides a better performance compared with the fixed length GI.     

Experimental evaluation of combined postdetection diversity andconcatenated channel coding

The combined effects of postdetection diversity reception and concatenated channel coding are experimentally evaluated for π/4-shift QDPSK signal transmission over a Rayleigh fading channel. Two-branch postdetection diversity reception using maximal ratio combining (MRC) and selection combining (SC) are considered. The concatenated channel coding uses the Reed-Solomon (15,k) code of GP(2 ⁴) as the outer code and the BCH (7,4) code as the inner code (k=9,11,13). Measured bit error rate (BER) performance results are presented under cochannel interference (CCI) and multipath channel delay spread, as well as additive white Gaussian noise (AWGN)     

小波包跳频通信系统的性能分析

介绍了将小波包调制（WPM）技术与跳频技术相结合的高效通信方式,即小波包跳频（WPM-FH）通信系统。利用特征函数法分析和推导了系统在高斯信道、瑞利衰落信道下的误码性能,给出了用户数,跳频点数及子信道数与系统性能之间的关系。理论分析和实验仿真表明,该系统不仅保持了小波包调制技术的优点,提高了频谱利用率,且具有较好的抗多径衰落和抗干扰能力,较好地改善了通信系统性能。同时,为了进一步改善多径衰落对系统性能的影响,在接收端加入了自适应均衡技术,实验结果表明系统性能得到了一定的提高。      

Postdetection optimal diversity combiner for DPSK differentialdetection

Postdetection diversity reception weights and combines all the detector outputs before symbol decision to combat the effects of multipath fading. A theoretical analysis of a postdetection optimal diversity combiner that can minimize the symbol error probability for differential phase shift keying (DPSK) differential detection in the presence of multiplicative Rayleigh fading, and co-channel interference (CCI) is presented. The effect of unequal average powers among diversity branches is taken into account. It is shown that the postdetection maximal-ratio combiner (MRC) described previously by the author is not optimal unless all branches have the same average power. It is also found that the combiner optimized for the effect of CCI (fading induced random FM noise) should weight each branch detector output in inverse proportion to the average CCI power (desired signal power). Assuming two-branch diversity, calculated BER (bit-error-rate) performance of π/4-shift QDPSK due to AWGN, CCI, and random FM is presented. In addition, the BER due to multipath channel delay spread (which is not treated in the theoretical analysis) is also computed to find the optimal combiner     

多径信道下FFH-FMDCSK系统的性能分析

本文提出一种基于差分混沌频率调制(FMDCSK)的快速跳频扩谱通信系统(简称FFH-FMDCSK)。它不但可以解决FMDCSK系统的抗干扰和多址接入问题,而且还可以提高其抗多径衰落的能力,同时降低对其他系统的干扰。本文比较了FFH-FMDCSK和FMDCSK在多径信道下的系统误码率,给出他们的理论公式和Monte-Carlo仿真结果。