Refractive Multipath Model for Line-of-Sight Microwave Relay Links

A major source of propagation outages on microwave line-of-sight (LOS) links is multipath fading caused by steep negative refractive index gradients. This paper develops a complete transfer function characterization of refractive multipath on LOS links. The results clearly show the dependence of discrete path amplitudes and delays on receiver height, transmitter height, refractive index gradient, layer height and ground range.     

Gigabit/s indoor wireless systems with directional antennas

The design of a high speed (>150 Mb/s) wireless local area network (WLAN), requires that many factors be considered, including technical, economic, and regulatory. A major technical factor is the channel response behavior (multipath) in the indoor environment as a function of the frequency band, building type and radio system architecture. The consequences of designing indoor wireless systems with directional antennas at one or both ends of a line-of-sight (LOS) link are investigated. We determine how narrow the beamwidth must be so that the maximum data rate is not limited by multipath effects. For such beamwidths, simple unequalized two-level frequency shift keying (FSK) or phase shift keying (PSK) modems can be used in place of the more costly and complex “anti-multipath” modems, and data rates above 1 Gb/s may be achieved. The channel impulse response in an empty room is estimated using geometrical optics, observing that with directional antennas, multipath rays must arrive from the same direction as the LOS ray. The link outage probability is then estimated as a function of the antenna beamwidth, and guidelines are established for the selection of the frequency band and antenna placement. Experiments using a 19-GHz 622-Mb/s binary phase shift keying (BPSK) link and 15° beamwidth horn antennas in an office building with plaster walls and large metallized windows have demonstrated error-free performance on both LOS and non-LOS (NLOS) links     

5G毫米波频段不同天线形式信道参数测量及对比分析

基于5G毫米波应用频段,开展了全向天线、喇叭天线以及阵列天线的信道测量与信道参数对比分析研究. 利用空间交替广义期望最大化(space-alternating generalized expectation-maximization, SAGE)算法提取了多径信道参数,包括多径时延、到达角、多径强度等,基于多径分量距离（multipath component distance, MCD）对多径分量进行了分簇结果的对比,对比分析了相同测量点处不同天线形式得到的信道参数差异. 结果表明,同场景下不同天线的信道参数测量结果会有较大差别,全向天线路损指数最小,阵列天线均方根(root-mean-square, RMS)时延扩展和RMS角度扩展均最小,喇叭天线可以得到较多的簇. 本文为5G毫米波频段信道建模提供了不同天线形式信道参数结果对比分析数据.     

多天线Rice信道空间衰落相关的近似表达

将von Mises分布作为多径散射接收信号的功率密度分布,在存在稳定直射分量的条件下,推导出相应的多天线Rice信道中的多径角度扩展公式,进而推导出了小尺度Rice衰落信道中紧凑阵元空间相关系数的近似表达式。von Mises分布的集中度参量对多径角度扩展的影响较Rice因子更为明显。只要多径散射是各向同性的,阵元间的空间相关性较弱;只要多径散射是各向异性的,空间相关性将随直射分量功率增大而急剧增加。分析表明,角度扩展和阵元间距是影响接收信号空间衰落相关性的主要因素。     

多径效应对信号接收及方向测量的影响

在论述L阵干涉仪和圆阵相关法测向体制误差形成的基础上,基于电磁波传播的多 径效应,建立了地空无线链路四径传输模型,并将单个多径分量用时延相同的多径簇来描述 ,分析了多径分量和多径簇对单个天线接收信号相位和不同天线接收信号互相关系数造成的 影响。仿真结果表明,多径条件下接收信号的波形、频谱和相位谱较直射信号都将发生显 著的变化,多径效应对俯仰角造成的测量误差要大于对方位角造成的测量误差。     

Experimental characterisation of diffuse multipath at 10.2 GHz using method of multiple windows

Thomson's multiple window method of spectrum estimation is applied to the problem of diffuse multipath characterisation in a low angle tracking radar environment. The method gives optimum power spectrum estimates in both the frequency and wavenumber domains. The results presented are based on data collected by means of a 32-element sampled aperture antenna system operating at 10.2 GHz.<>     

Modulation techniques for high-speed wireless indoor systems usingnarrowbeam antennas

We consider high-speed wireless indoor links in which narrowbeam antennas are used to limit multipath delay spread. For this kind of link, the performances of simple 2- and 4-level modulation schemes are analyzed. The amplitudes and phases of all multipath rays are assumed to be static (no time variation), as expected for fixed links in an indoor environment. We consider coherent PSK with data-directed and pilot tone carrier recovery, differential PSK, and coherent and noncoherent forms of 2-level ASK. Our objective is to find techniques for which factors other than multipath limit the attainable bitrate. Thus for each scheme, analysis is used to identify regions of the multipath parameter set over which data eye closure occurs. Results are given for the 2- and 3-ray multipath channels which can arise in an indoor environment with narrowbeam antennas. Specifically, we assume two parallel walls approximately perpendicular to the direct ray and a narrowbeam antenna at one or both ends of the link. The 2-level schemes are found to be quite robust with respect to 2-ray multipath, and can mostly tolerate a second ray amplitude equal to that of the direct ray. The 4-level schemes are sensitive to 2-ray multipath, showing eye closure at many phases with a second ray power about 7.5 dB below the first (direct) ray. A similar sensitivity is found for 2-level schemes with 3-ray multipath. Finally, 4-level schemes with 3-ray multipath are significantly more sensitive, showing eye closure with a second ray power about 11 db below the direct ray. Thus, considerations of simplicity and robustness favor 2-level PSK schemes (coherent with pilot tone, or differential) for narrowbeam high speed transmission indoors     

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     

Time division adaptive retransmission for reducing signal impairments in portable radiotelephones

Multiple-antenna receiving diversity was shown previously to be effective in mitigating the effects of random angular orientation and multipath radio propagation for portable radiotelephones. It is shown that time-division adaptive retransmission used with appropriate antenna configurations can also mitigate these effects. The retransmission configurations require fewer antennas than the receiving diversity configurations for a given improvement in relative signal-to-noise ratio (S/N). Cumulative distributions of S/N were determined for adaptive retransmission and diversity using random orientation and multipath propagation models. Distributions of S/N for systems with two antennas at the portable set and two appropriately polarized antennas at the portable radiotelephone terminal (PORT) are similar to distributions for two-branch selection diversity in the fixed-orientation mobile radio environment. Systems with one portable antenna and two PORT antennas have distributions with slopes similiar to two-branch mobile radio distributions but the distributions for the portables range from 3 to 7 dB worse.     

Propagation considerations for the design of an indoor broad-bandcommunications system at EHF

This paper reports the measurement and analysis of wideband propagation data for indoor radio channels at 40 GHz. Propagation characteristics are reported for two open-concept office areas of different sizes in two different buildings. Also, the results of measurements in one building are compared for system configurations in which either an omnidirectional or a narrowbeam antenna is employed at a base station for communications to multiple work stations with omnidirectional antennas. It is reported that, on a statistical basis, at the 90th percentile, multipath dispersion is the same for the two base-station antenna radiation patterns. Dispersion was, however, found to be lower in the smaller of the two measurement areas, where transmit/receive ranges were shorter. The 90th percentile of static RMS delay spread for this area was 19 ns compared with 45 ns for the larger area. Multipath spreads at the -25 dB relative power level were about 370 ns, compared with 140 ns in the larger area. In the larger area, global propagation loss was found to be well modeled by the one-way propagation equation with different range exponents (1.5 and 4) before and after a breakpoint at a range of 25 m. In the smaller area, the range exponent was found to be greater, being equal to 3.5. This is considered to be a result of reduced multipath infill, which would accompany the reduced dispersion. Temporal fading on fixed links with omnidirectional antennas was found to have depths such that a 14-dB fade margin is required for 99% reliability. Finally, spatial variations in received power at a given range indicated the requirement for a power margin between 4-7 dB for 99% reliability. These results are used in a link budget example for a broad-band indoor extremely high frequency (EHF) digital communications system     

Antenna resolution as limited by atmospheric turbulence

Spatial variations of the index of refraction of the atmosphere introduce fluctuations in the phase and amplitude of a wave propagating through it. The effect of these fluctuations on the resolution capability of microwave antennas is discussed in this paper. The measure of resolution used in the calculations is the beamwidth, which is defined as the square root of the second moment about the mean of the normalized antenna pattern. This measure simplifies the calculations greatly and results in a simple expression for the beam broadening due to the variation of the refractive index. This broadening is dependent on the distance to the target, the scale of the turbulence, and the variance of the refractive index and is relatively insensitive to the shape of the spatial correlation function of the refractive index. Measurements by others of the nature of the refractive index spatial variation are then used to obtain numerical estimates of the ultimate resolution capability of a microwave antenna as limited by atmospheric turbulence.     

Signal correlation between two normal-mode helical antennas for diversity reception in a multipath environment

The spatial correlation coefficient of signals received by two normal-mode helical antennas in a multipath environment characterized by Clarke's model is rigorously obtained. A new formula relating the signal correlation coefficient to the mutual impedance is derived. The mutual impedance is defined and calculated in a new manner. Our findings show that: 1) the envelope correlation coefficient of two helical antennas is greater than that of two monopole antennas; 2) the envelope correlation coefficient is dependent on the antenna terminal load and the mutual impedance and an optimal antenna load can be determined to produce the smallest correlation coefficient; and 3) the frequency response of the envelope correlation coefficient resembles that of the voltage standing-wave ratio.     

Assessment of MISO time reversal for short-range communications in the 5 GHz ISM band

The purpose of this paper is to investigate the performance of Time Reversal (TR) for short-range Multiple-Input Single-Output (MISO) systems in the Industrial, Scientific, and Medical (ISM) 5 GHz band using actual channel measurements. Such a scenario is considered for low cost user equipment applications, where low-complexity schemes like TR will be an potential candidate. The experiment is conducted with 8 transmit antennas and 1 receive antenna per user. The antennas are arranged in compact and distributed configurations, and the channel is measured in both Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS) dominated scenarios. We investigate the performance of TR in terms of time compression, spatial focusing, and diversity gain, by analyzing the delay spread, the spatial Peak-to-Average-Leakage power ratio and the diversity gain, respectively. The conclusions that can be drawn from the experiment are: TR with multiple transmit antennas shows better performance compared to the single antenna link in terms of time compres- sion, spatial focusing, and diversity gain, even in realistic short-range communications. Second, a distributed 8 Tx antenna configuration achieves lower delay spread and higher diversity gain than a compact antenna array with the same number of elements. In the situation where practical impairments are taken into consideration, the system delay involving propagation and processing time is found to be a crucial parameter degrading the performance of TR.     

A stochastic multipath channel model including path directions for indoor environments

A novel stochastic channel model for the indoor propagation channel is presented. It is especially for, but not limited to future communication systems with multiple antennas like space division multiple access (SDMA), spatial filtering for interference reduction (SFIR), or multiple-input/multiple-output (MIMO). The model is designed for indoor scenarios, straight forward extendable to urban environments. It is based on physical wave propagation. The new approach describes the channel by multipath components, each characterized by its transfer matrix (including loss), delay, direction of arrival, and departure. The appearance and disappearance of multipath components over time is modeled as a birth and death process, a marked Poisson process. This enables first-time the correct modeling of spatial and temporal correlations. In each modeling step, path properties change according to the motion of transmitter and receiver. The changing delay times of propagation paths yield a realistic Doppler behavior of the channel. Deterministic ray tracing results are used to produce the huge data sets required for the statistical evaluation of the parameters of the proposed model. This method enables an automated parameter extraction for new environments or frequencies. The ray tracing tool has been verified by narrowband, wideband, and directional channel measurements. The novel stochastic spatial channel model allows the simulation of third-generation broadband radio systems including arbitrary antenna configurations and patterns. System simulations for the bit-error rate of radio links can be performed including intelligent antenna configurations like SDMA, SFIR, or MIMO. Furthermore, the capacity of complete systems can be investigated.     

Millimeter wave wireless interconnects in deep submicron chips: Challenges and opportunities

On-chip wireless links offer the most promising solution to improve performance over traditional Networks-on-Chip (NoCs). Though, significant advancements are being made to support intra-chip wireless communication, a complete understanding of on-chip wireless channel, that facilitates design optimization of transceivers and antennas is still lacking. In this work, we derive on-chip wireless channel characteristics, taking into account antenna implementation, near field and multipath propagation effects. These observations are then used to study impact on wireless NoC performance, packet energy, delay and bandwidth. The study provides crucial insights for circuit designers to tune transceiver and antenna specifications to achieve desired network performance.     

Effects of turbulence in a planetary atmosphere on radio occultation

There has been considerable interest in the amplitude and phase fluctuations of the radio signal received from a flyby spacecraft during occultation by a planetary atmosphere. For planetary flyby missions, the Fresnel-zone size exceeds the outer scale size of turbulence, and existing formulations for the frequency spectra of the amplitude and phase fluctuations are inadequate because they do not account for the inhomogeneity of the turbulence in the direction transverse to the propagation path. In this paper, the formulation is given for the correlation functions for the log-amplitude and phase fluctuations of both spherical and plane waves propagating in a turbulent medium whose correlation function for refractive index fluctuations is described by the product of a function of the average coordinate and a function of the difference coordinate. The results are applied to radio occultation of a flyby space probe by the atmosphere of Venus, assuming that the turbulence in the atmosphere exists as a layer, that it is localized, isotropic, and smoothly varying, and that the localized turbulence is described by the Kolmogorov spectrum. Closed-form solutions for both variances and frequency spectra of the log-amplitude and phase fluctuations are obtained using Rytov's method, and it is seen that the shape of the frequency spectra depends a great deal on the characteristics and extent of the turbulence.     

Delay spread reduction effect of beam antenna and adaptively controlled beam facing access system in urban line-of-sight street microcells

The paper presents theoretical and experimental investigation results of the delay spread reduction effect of beam antennas in urban line-of-sight (LOS) street microcells. The 95% delay spread cumulative probability in a 600 m LOS cell length is reduced to 70% when the 3 dB beamwidth of the base station (BS) antenna is in the range from 80/spl deg/ to 140/spl deg/, compared with that (420 ns) in omnidirectional antennas. An adaptively controlled beam facing access (ABFA) system in which the beam antennas are used in both the base and mobile stations is further proposed. The beam direction of the BS antenna is fixed along the street, but that of the mobile station (MS) antenna is adaptively controlled as the maximum received power is obtained. Using ABFA, the 95% delay spread cumulative probability in the 300 m LOS microcell is reduced to less than 70 ns, when the antenna beam-width lies in the range from 20/spl deg/ to 140/spl deg/ in the BS and about 80/spl deg/ in the MS.     

Measurements and models for 38-GHz point-to-multipoint radiowavepropagation

This article presents results of a wide-band measurement campaign conducted at 38 GHz. The objective of the research was to determine multipath and time varying channel behavior of short-hop millimeter-wave point-to-multipoint radio links during various weather events. 73963 power delay profiles (PDPs) were captured on three links, each comparable to proposed local multipoint distribution systems (LMDS) in a campus environment. Multipath was observed in unobstructed LOS links during rain but not during clear weather. Short-term variation of the received signal over 1-2 min observation periods is described by a Rician distribution with a K factor which varies as a function of rain rate. Measured rain attenuation exceeds Crane's (1996) model predictions by several decibels. A novel prediction technique is presented that applies canonical antenna patterns and site specific information to estimate worst case multipath channel characteristics including relative power, time of arrival (TOA), and angle of arrival (AOA) of each multipath component. New metrics, the excess delay zone and relative power zone, are defined and contour plots are developed to determine potential reflectors from an area site map. These results and models provide useful guidelines for the design of millimeter-wave wireless communication systems     

Multicarrier Orthogonal-CDMA for Fixed Wireless Access Applications

In this paper, we suggest applying multicarrier spread spectrum (MC-SS) techniques to fixed wireless access applications. Due to the fixed nature of subscribers in these applications, cross-polarization discrimination and directional subscriber antennas can be used to reduce interference in a multicellular system. Power control schemes are applied on both uplink and downlink. The suggested system has a high capacity and uses a simple multiaccess layer. It is robust against multipath effects and can provide service coverage not only to line-of-sight (LOS) but also to nearly LOS subscribers. Only a single carrier frequency is used in the entire network.     

Performance of short- and long-range meteor burst communicationsystems with different antennas

The performance of meteor burst communication at different ranges for a number of antenna configurations is analyzed. Characteristics of ideal antenna patterns for short- and long-range meteor scatter communication are described. An ideal antenna configuration matches its illumination pattern to the spatial arrival pattern of meteor trails. The analysis is used to compare the performance of several commonly used antenna configurations. It will be demonstrated that the characteristics of antennas which perform well at short ranges are different than those that perform well at long ranges