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.     

Digital transmission of high bit rate signals using 16DAPSK-OFDMmodulation scheme

Radio transmission signals are distorted by multipath propagation and signal level fading in the channel. In the case of digital transmission systems, the distortion of signals causes a degradation of bit error performance. It is an especially severe problem with digital outside-broadcasting-link (OBL) which has to transmit, for example, digital HDTV signals at more than about 45 Mbps. The OFDM modulation scheme can mitigate the adverse effects of multipath fading. An experimental OFDM modem has been developed to enable stable transmission of digital HDTV sources in an OBL. The experimental modem distributes transmitting digital data to 688 carriers in a bandwidth of about 13.5 MHz using the 16DAPSK for each OFDM carrier. A combination of 8DPSK and 2DASK is adopted as the 16DAPSK and another combination of 8DPSK and coherent 2ASK instead of 2DASK is also considered. Indoor transmission experiments confirmed that the 16DAPSK-OFDM is feasible for transmitting high bit rate signals, even under severe propagation conditions     

Fade margin calculation for channels impaired by Rician fading

Excess path loss due to multipath severely restricts the performance of power limited mobile networks such as those using satellite-aided links. To reduce multipath related losses, the higher elevation angle of the spacecraft can be exploited by utilizing mobile antennas which reduce the strength of the multipath reflections in favor of the line-of-sight signal. The presence of a strong and stable path in a fading link will change the envelope statistics of the received waveform from Rayleigh to a more favorable Rician distribution. It is determined that the excess path loss, or fade margin, of a Rician channel when coherent detection of binary phase shift keying (BPSK) or quaternary phase shift keying (QPSK) signals is considered. The results are presented parametrically such that they can be applied to a wide range of propagation characteristics from heavy fading to nonfading situations. Furthermore, similar results are also given for the case where only limited coverage is provided.     

Behaviour of spectrum-conserving RBQPSK digital radio during multipath propagation

Studies indicate that reduced bandwidth quadrature phase-shift-keyed digital radio relay systems (RBQPSK) employing adaptive decision-feedback techniques have good spectrum utilisation efficiency coupled with robustness in the presence of distortion induced by multipath propagation. The letter discusses the practical confirmation of RBQPSK performance during multipath propagation on a 6 GHz, 51 km long test link in East Anglia.     

Dégradation de la qualité due aux trajets multiples dans un système a transmission numérique a quatre états de phase

A study is made on the effect of propagation on digital transmission based on two series of measurements made with four phase shift keying and differential demodulation equipment at bit rates of 38.8 and 216 Mbit/s. Degradation due to selectivities of attenuation caused by multipath propagation is inferred for both experiments : whereas the degradation is low at 38.8 Mbit/s, it becomes very large at 216 Mbit/s. The results are interpreted and commented upon. By taking a dual beam model, a calculation is made for degradation due to propagation under multipath conditions, as a function of the delay of the reflected beam.     

超宽带Rake接收机在密集多径信道下的性能

超宽带信号在室内环境中将经历密集多径传播,产生严重的时间弥敞。采用Rake接收是提高超宽带接收机性能的重要手段。该文针对采用BPSK调制的超宽带系统,推导出密集多径信道下Rake接收机的误码性能计算公式,揭示了Rake接收机性能与多径信道参数以及脉冲波形自相关系数之间的关系。针对典型信道模型和脉冲给出了计算实例。     

Sector antenna and DFE modems for high speed indoor radiocommunications

This paper examines the outage probabilities of BPSK and BPSK/DFE radio modems with omnidirectional and six-sector antennas. The effects of room size, data rate, transmitted power and sector antenna patterns on the performance of the systems are analyzed. A 2-D ray tracing algorithm is used to model the radio propagation in a typical indoor environment used for wireless LAN applications. The parameters of the radio propagation model are adjusted to fit the results of simulations to the results of empirical measurements with omnidirectional antenna. It is shown that in a line-of-sight (LOS) environment, a six-sector antenna is more effective than a DFE with an omnidirectional antenna at eliminating the effects of multipath. For an obstructed-line-of-sight (OLOS) environment where at least one wall exists between the transmitter and the receiver, the DFE is more effective than the six-sector antenna. It is shown that a BPSK/DFE modem with a six-sector antenna can support data rates close to 20 Mbps     

数字波束相位成形失真分析

数字波束系统中,通常采用相位加权实现波束成形,但这种方式对具有一定带宽的调制信号而言是存在失真的。为了研究这种失真对典型数字通信的影响,首先建立了一维线性均匀加权直线阵数字波束成形模型,介绍了这种阵列天线相位波束成形的原理,随后结合BPSK调制信号,对数字波束相位成形过程进行推导,获得相位成形波束接收信号数学表达式。通过分析表明对于BPSK调制信号而言,相位波束成形会引入码间串扰,并且串扰影响随着阵列规模增大、信息速率提高、波束扫描角度增大而加剧。     

A channel model for multipath interference on terrestrialline-of-sight digital radio

A comprehensive system model for characterizing the effects of multipath propagation on digital radio systems in the 4-6-GHz band is shown in this paper. The effects of terrain-induced multipath propagation in the presence of atmospheric anomalies are studied using data from experimental microwave links in the field and in the laboratory. This technique, which treats multipath propagation as digital signal distortions caused by interference from ground reflections, has not been shown previously. A forward multipath propagation model is used to identify the critical propagation parameters for a quadrature amplitude modulated (QAM) signal. A normalized two-ray channel model is developed to approximate the frequency response produced by interference from a ground reflection in a narrow band. The effectiveness of this channel model is evaluated using measured data from the test radio link in the laboratory and in the field     

Multipath propagation model for line-of-sight street microcells inurban area

This paper proposes a multipath propagation model for line-of-sight (LOS) street microcells with building roof base sites (BS) in urban areas, Multipath propagation characteristics are of great importance in evaluating the performance of digital systems and designing wireless links. Typical delay profiles are measured to clarify their statistical characteristics in LOS street microcells. The channel sounder used is a sliding correlator with 30-Mb/s PN code and a center frequency of 2.6 GHz. The measurements clarify the features of delay profile and mean RMS delay spread. The proposed delay profile model explains one plausible mechanism of multipath propagation. The delay profiles calculated using the model agreed well with the measured profiles. Furthermore, the factors influencing the RMSs delay spread are investigated, and the regression equation of medium RMS delay spread on a sidewalk is established. The proposed model can evaluate the transmission characteristics of wireless digital communication systems in multipath propagation environments     

Effects of beam tilting on bit-rate selection in mobile multipathchannel

With an increase in system bit-rate, unexpected distortion due to the multipath effects may be so serious that the system is rendered inoperative. The reduction of the multipath distortion by beam tilting at a basestation (BS) is theoretically analyzed using multipath propagation models; the system performance, characterized by the critical bit-rate and the probability that multipath distortion can be handled by the system, is evaluated. It is shown that the capability of the system to handle multipath distortion is improved, and the system bit-rate can be selected over a wider range with the introduction of beam tilting than that without beam tilting     

Dirtortion AND Crosstalk Caused by Multipath Propagation in Frequency-Modulation Sound Broadcasting

The characteristics of multipath distortion in FM sound broadcasting were determined through computer analysis and subjective listening tests. Stereophonic multipath distortion is found to be far greater than monophonic multipath distortion, especially for long delay times. Crosstalk into stereophonic main-program reception due to multipath propagation from the SCA (Subsidiary Communications Authorization) using a 67-kHz subcarrier was also analyzed and multiplexing of the SCA is found to have a potential for causing severe crosstalk trouble. The DU ratios required to suppress the distortion and the crosstalk are provided.     

Intersymbol interference effects on BPSK and QPSK carrier trackingloops

The tracking performance of three conventional carrier tracking loops in the presence of intersymbol interference (ISI) is analyzed. The closed-form expressions for the squaring losses of BPSK (binary phase-shift-keyed) low-SNR (signal-to-noise ratio), BPSK high-SNR, and QPSK (quaternary phase-shift-keyed) low-SNR loops are derived. ISI is shown to affect the BPSK high-SNR loop as much as the I-Q loop despite the nonlinear effect of the hard-limiter. For the QPSK carrier tracking loop, the degradation is shown to be greater due to the modulation of both components of the carrier. In this case, ISI, in addition to modifying the coefficients in the squaring loss, adds a new term not present in wideband channels. In all cases, a unity squaring loss could not be achieved because of the irreducible error due to pattern noise. This noise is only present in band-limited channels and is dependent on the data model used     

Analogue and digital transmission using polymer optical fibre

The performance of a hybrid AM/BPSK (amplitude modulated/binary phase shift keying) polymer optical fibre transmission system has been investigated. A BPSK modulated 2 Mbit/s pseudorandom digital channel is substituted for one of the AM channels in a 60-channel cable TV system. For the digital channel, a bit error rate of <10^-9 is obtained after transmission through 200 m of fibre. The results show that distortion caused by laser nonlinearity did not degrade the system at low modulation depth     

Performance of cofdm for satellite digital audio broadcasting

This paper analyses the performance of a coded orthogonal frequency division multiplexing (COFDM) based system for digital audio broadcasting using a highly inclined orbit satellite constellation. The sources of performance degradation considered are non-linear amplification in the satellite transponder and multipath propagation for mobile reception. Despite its strong peak-to-mean amplitude variation, COFDM has been found to be very robust against non-linear amplification. In the multipath channel, practical cases demonstrate low performance losses even in highly populated areas when highly inclined elliptical orbits (HEO) are used.     

Performance of direct-sequence spread-spectrum RAKE receivers withrandom spreading sequences

In this paper, we derive error probability expressions for binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) spread direct-sequence spread-spectrum (DSSS) systems employing random spreading sequences with RAKE receivers. The derived expressions accurately take into account the effect of interpath interference which usually has been neglected in previous analyses. Selection, equal gain, and maximal ratio techniques are considered for diversity combining. Two possible finger assignment strategies, one based on the instantaneous amplitudes and another based on the average powers of the multipath components, are considered for the assignment of multipath components to the available demodulating fingers in the RAKE receiver. Also, various simple, and in many cases, closed-form approximations for the error probabilities are derived and their accuracies are assessed     

Availability Prediction of 8 PSK Digital Microwave Systems During Multipath Propagation

Analysis is presented to evaluate the worst case availability of 90 Mbit/s, 8 PSK digital microwave systems during multipath propagation. The model used in the analysis is based on the assumption that during multipath fading, the microwave energy propagates from the transmitter to the receiver via two paths, thus generating a single echo. Based on this model, three main results are found; that is 1) echo delay decreases exponentially with fade depth, 2) the worst case fade distribution follows a Rayleigh distribution, 3) the unavailability of a typical 6 GHz path during multipath propagation is unacceptably high without the use of an adaptive amplitude slope equalizer.     

A versatile wave propagation model for the VHF/UHF rangeconsidering three-dimensional terrain

A polarimetric wave propagation model for field strength forecasting and coverage prediction in the VHF/UHF frequency range is presented. The model uses a digital terrain data bank and considers multipath propagation. Based on the uniform geometrical theory of diffraction (UTD) and physical optics an approach is described for calculating the propagation effects in natural 3-D terrain, given by topological and morphographical data. The method for field strength forecasting is described and methods for the analysis of the predicted multipath signal are discussed. It is shown how the complex probability density function (PDF) for the receiver field strength and the field strength delay spectrum can be derived. Methods for further evaluation of the transmitting channel characteristics are discussed     

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。