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     

A microcellular ray-tracing propagation model and evaluation of itsnarrow-band and wide-band predictions

Due to the site specific nature of microcellular operational environments, propagation models are required to take into account the exact position, orientation and electrical properties of individual buildings, and hence, ray-tracing techniques have emerged as the dominant methods to predict propagation in such environments. A novel hybrid three-dimensional (3-D) ray-tracing algorithm which can evaluate scenarios incorporating many thousands of objects by utilising the concept of “illumination zones,” is presented. In order to evaluate the accuracy of the presented model, comparisons of narrow-band and wide-band predictions with measurements are performed for a variety of scenarios. First, power comparisons show that very accurate predictions can be achieved (RMS errors less than 3.7 dB). Then, wide-band analysis shows that since the RMS delay spread for systems with finite bandwidth is a function of the multipath phase, only average measured and predicted RMS delay spread values can be compared and as a result, limited averaging can produce large RMS errors. With sufficient averaging the achieved wide-band accuracy in terms of the predicted RMS delay spread, is adequate for most planning purposes     

Pilot-Based LMMSE Channel Estimation for OFDM Systems With Power–Delay Profile Approximation

In linear-minimum-mean-square-error (LMMSE) channel estimation for multicarrier systems, one needs to know the channel correlation function. This poses a problem for systems with a small number of pilots operating in time-varying channels. We propose to approximate the channel power–delay profile (PDP) with a shape that can completely be described in two parameters, namely, the mean delay and the root-mean-square (RMS) delay spread. Furthermore, we develop a simple technique to estimate these delay parameters. The approximate PDP is then used to generate the LMMSE filter coefficients for data-subcarrier channel estimation. Mathematical expressions are derived that can be used to predict the accuracy of the various estimates, and they are verified by simulation. The proposed technique is applicable to both point-to-point communication and multiaccess communication where different users may experience different channel conditions. As a practical application, we also specialize the proposed technique to Mobile WiMAX signals and investigate the resulting performance.      

基于改进射线跟踪法和BP神经网络算法的室外微蜂窝毫米波信道特性研究

在28 GHz与39 GHz毫米波频段室外微蜂窝场景下,基于改进射线跟踪法和反向传播(back propagation, BP)神经网络算法对毫米波单发单收信道及单发多收信道进行建模仿真研究. 在得到的无线信道仿真数据基础上,研究分析了毫米波信道的路径损耗、均方根(root-mean-square, RMS)时延扩展(delay spread, DS)、接收功率等传播特性. 通过与现有文献的测量结果对比分析验证了改进射线跟踪法的正确性与有效性. 通过BP神经网络方法拟合得到的路径损耗模型参数结果与改进射线跟踪法仿真得到的路径损耗参数结果对比发现,两者吻合程度很高,验证了BP神经网络算法能很好地对室外微蜂窝毫米波信道大尺度参数进行预测. 同时,文中给出了一种普遍适用的用来表征室外微蜂窝视距(line-of-sight, LoS)与非视距(non-line-of-sight, NLoS)场景下28 GHz与39 GHz毫米波信道的路径损耗模型. 结果表明:LoS场景下的RMS DS和接收功率都小于NLoS场景下得到的结果;LoS场景与NLoS场景下RMS DS、水平方向到达角、多径簇的个数累积分布函数均服从高斯分布;RMS DS在毫米波频段微蜂窝场景下,随着频率的升高而增大,到达接收端的多径成簇呈现稀疏性.     

Delay-spread considerations of same-frequency repeaters in widebandchannels

In current analog cellular systems, same-frequency repeaters are very often used as gap fillers. With digital wideband systems, the use of a repeater, with its inherent group delay added to the differential propagation delay, may yield a degradation in the performance in view of the resulting delay spread. An approach to estimate the statistics of the delay spread when each link is subjected to lognormal shadowing and each channel's power-delay profile follows an exponential decay is presented. Expressions for the cumulative density function (CDF), mean value, mean-square value, and standard deviation for the RMS delay spread are derived, and their accuracy is verified through simulation     

Microcellular radio-channel propagation prediction

Designers of wireless-communications networks require detailed understanding of radio-propagation in complicated, multi-path channels. Unlike conventional cellular systems, emerging wireless personal-communication networks will most likely operate in confined, urban environments (microcells). The application of broad-band-digital modulation to these networks requires careful consideration of the dispersive nature of the urban radio channel. This paper presents a ray-tracing simulation technique which incorporates site-specific environmental data, such as the location, the orientation, and the electrical properties of buildings, to predict path loss and delay spread in urban microcells. Using simplified geometric-optics assumptions, rays are traced in three dimensions. This determines the paths by which direct, specularly reflected and transmitted, diffusely scattered, and diffracted rays arrive at a receiver. The received rays are combined noncoherently as a function of delay, to estimate the channel power-delay profile. The power-delay profile is used for verification of model accuracy, via qualitative and statistical comparisons of measured and predicted data, for receiver locations on the Virginia Tech campus. The comparisons conclusively show the simulation's ability to accurately model urban microcellular propagation     

OFDM系统基于导频的信道多径时延估计

本文针对宽带正交频分复用(OFDM)系统,分析了多径时延扩展与循环前缀(cP)长度及导频数目的关系.随后利用导频的频域相关性估计得到信道时域功率时延谱(PDP),并根据PDP提出信道均方根(RMS)时延及最大多径时延门限值的估计算法.最终实现OFDM系统对信道多径参量的实时跟踪,有利于系统参数自适应调整.仿真结果表明,本文算法在低复杂度条件下可以有效获知当前传输信道模型多径时延参数的近似统计信息.     

A comparison of two radio propagation channel impulse responsedetermination techniques

A comparison of two radio propagation channel impulse response determination techniques is described. Presented are typical impulse response and transfer functions obtained from each measurement system. Also included for comparison are average impulse response envelopes and cumulative probability distributions for the RMS delay spread of static indoor radio channels calculated from 120 measurements using each system. The comparisons show good agreement between results     

Indoor radiated-mode leaky feeder propagation at 2.0 GHz

This paper presents the results of narrow-band and wide-band propagation measurements carried out at 2.0 GHz in an indoor environment using a radiated-mode leaky feeder as the transmitting antenna. The narrow-band measurements were devised to measure attenuation of radio signals and the wide-band techniques to measure multipath impulse responses and their associated root mean square (RMS) delay spread. Analysis of the narrow-band data files shows that the received signal levels in the direction along the feeder generally decay exponentially due to the feeder-specific attenuation. The received signal levels in the direction radial to the feeder decrease slowly, and the distance-power law exponent is found to be smaller than one. The slow and fast variations of the received signal levels are also examined. The results reveal that the slow variations basically follow the log-normal distribution, while the fast variations fit the Rayleigh distribution in the direction parallel to the feeder and the Rician distribution in the direction radial to the feeder, respectively. Analysis of the wide-band data files reveals that the maximum value of the RMS delay spread is 60.6 ns and the RMS delay spread values are less than 42 ns 50% of the time. One therefore can conclude that the indoor channel excited by the radiated-mode leaky feeder has a broad coherent band-width and can support a data rate of up to 3.3 Mb/s without equalization     

无人机毫米波信道建模及统计特性研究

针对传统移动信道模型不适用于无人机毫米波通信场景的问题,本文基于射线追踪原理构建了一个无人机毫米波三维几何信道模型,并提出了一种针对该模型的信道参数更新算法,该算法考虑了通信距离、信号角度、路径时延和功率等传播参数的随时间演进过程。在此基础上,针对南京航空航天大学校园传播场景,研究分析了时延分布、时延扩展、角度分布和角度扩展等信道统计特性。数值仿真结果表明,本文模型能够有效复现无人机信道的时变非平稳特性,输出信道统计特性与实测结果吻合,可用于无人机毫米波通信系统的方案设计、性能优化和评估验证等领域。      

An analytical calculation of power delay profile and delay spread with experimental verification

Realistic system and link level simulations require specific information about the radio channel. A key role is played by the power delay profile which is widely agreed to be a negative exponential characterized by the delay spread. Since this finding is based on empirical evidence only, the delay spread must be derived from measured data. The power delay profile is rigorously derived by combining ray optical and statistical properties of wave propagation in a single room. We show that the power delay profile is indeed a negative exponential and provide a decay constant which serves as an analytical upper bound for experimentally obtained delay spreads of indoor environments. To the best knowledge of the author, this is the first derivation of the power delay profile from first principles.     

Millimeter-Wave Propagation Channel Characterization for Short-Range Wireless Communications

This paper presents and analyzes the results of millimeter-wave 60-GHz frequency range propagation channel measurements that are performed in various indoor environments for continuous-route and direction-of-arrival (DOA) measurement campaigns. The statistical parameters of the propagation channel, such as the number of paths, the RMS delay spread, the path loss, and the shadowing, are inspected. Moreover, the interdependencies of different characteristics of the multipath channel are also investigated. A linear relationship between the number of paths and the delay spread is found, negative cross correlation between the shadow fading and the delay spread can be established, and an upper bound exponential model of the delay spread and the path loss is developed to estimate the worst case of the RMS delay spread at given path loss. Based on the DOA measurements that are carried out in a room [line of sight (LOS)] and in a corridor with both LOS and nonline-of-sight (NLOS) scenarios, radio-wave propagation mechanisms are studied. It is found that considering the direct wave and the first-order reflected waves from smooth surfaces is sufficient in the LOS cases. Transmission loss is very high; however, diffraction is found to be a significant propagation mechanism in NLOS propagation environments. The results can be used for the design of 60-GHz radio systems in short-range wireless applications.     

The performance of the PACS radio link and its improvements atvehicular speeds

The personal access communication system (PACS) radio interface is the leading low-tier candidate for standardization in North America. This radio interface was originally conceived to serve pedestrian and fixed-distribution applications; there has been significant recent interest in extending this technology into high-mobility environments. In such environments, rapid channel variations significantly degrade the performance of the preselection diversity scheme proposed for use in PACS handsets. The effects of time-delay spread on the PACS radio link was also included in our investigation. It is found that a received signal strength indicator (RSSI) with a short measurement length can better cope with high fading rates than can a quality measure in a preselection diversity system, although a quality measure has better performance than RSSI at low speeds in the presence of time-delay spread. In the preselection diversity scheme, using short-measurement RSSI provides relatively good performance in both low- and high-mobility environments. However, its performance degrades rapidly for RMS delay spreads larger than about 9% of a symbol time. Postselection diversity using two complete receiver chains is more robust than preselection diversity, both to high fading rates and to delay spread. Postselection diversity is relatively insensitive to changes in the fading rate and can tolerate an RMS delay spread up to 12.5% of a symbol time     

Time delay spread measurements of wideband radio signals within a building

Measurements of time delay spread of wideband 850 MHz digital radio signals due to multipath propagation within a large building are described. These measurements show a median RMS time delay spread of 125 ns and a worst case of 250 ns. Consequently, signalling rates above 400 kHz may not be feasible.     

Path-loss and time dispersion parameters for indoor UWB propagation

The propagation of ultra wideband (UWB) signals in indoor environments is an important issue with significant impacts on the future direction and scope of the UWB technology and its applications. The objective of this work is to obtain a better assessment of the potentials of UWB indoor communications by characterizing the UWB indoor communication channels. Channel characterization refers to extracting the channel parameters from measured data. An indoor UWB measurement campaign is undertaken. Time-domain indoor propagation measurements using pulses with less than 100 ps width are carried out. Typical indoor scenarios, including line-of-sight (LOS), non-line-of-sight (NLOS), room-to-room, within-the-room, and hallways, are considered. Results for indoor propagation measurements are presented for local power delay profiles (local PDP) and small-scale averaged power delay profiles (SSA-PDP). Site-specific trends and general observations are discussed. The results for path-loss exponent and time dispersion parameters are presented.     

Analysis of path loss and delay spread at 900 MHz and 2.1 GHz whileentering tunnels

Some propagation characteristics in tunnel environments at 900 MHz and 2.1 GHz are analyzed. Narrow-band parameters are given in terms of the propagation factor and linear regression slopes. Wide-band analysis is considered by calculating delay spread factors along the communication path. A ray-launching technique has been implemented, using ray optics and the uniform theory of diffraction (UTD). The proposed theoretical model has been validated by comparing the results with measurements obtained by other authors. The particular situation when a mobile antenna crosses the tunnel entrance and transmitter is outside the tunnel is studied. Our simulations show that the larger the angular separation of the transmitter with respect to the tunnel longitudinal axis is, the higher values of the delay spread. The more critical point appears around the tunnel entrance (“excitation zone”), where a signal power decay up to 40 dB may be detected. A similar value has been already found when turning a corner in microcellular urban environments     

A ray-tracing propagation model for digital broadcast systems inurban areas

A propagation model is presented for characterizing the channel response for digital systems in urban areas where multiple reflections from buildings are encountered. A deterministic ray-tracing propagation model is used to predict the time delay and fading characteristics for the channel in a hypothetical urban area. The analysis shows that due to multiple reflection and diffraction sources, the RMS delay spread of the channel in urban areas can be several hundred nanoseconds, so that very effective equalizers will be required to achieve successful performance of high-data-rate digital systems such as 20-Mb 16-QAM digital HDTV. The channel response results presented also suggest that polarization diversity may be a useful technique for mitigating some of the channel impairments predicted by the propagation model