26GHz LMDS在雨区中传输的分析

分析了降雨对26GHzLMDS产生的降雨衰耗和去极化,并给出了有关的公式.介绍了一个26GHzLMDS在新加坡降雨时进行测量的方法和结果.研究指出,降雨对26 GHz LMDS的信号传输有很大的影响,随着降雨率的增加,降雨衰耗的增加接近线性,差分衰减和差分相移的增加变得更加显著.当降雨率接近90mm/h时,会出现3dB的差分衰减和45°的差分相移.     

基于LMDS技术的宽带无线网络设计

LMDS是一种宽带无线接入点对多点通信系统,工作频率在20GHz以上.本文首先介绍了LMDS技术,然后提出了一种基于LMDS技术的宽带无线网络设计方案.最后讨论了RF设计准则、LMDS的发展前景及存在的问题.     

LMDS“最后一公里”无线接入

LMDS全称为本地多点分配系统(Local Multi-point Distribution System),它是一种工作在10GHz~40GHz频段的解决"最后一公里"接入的高速无线接入技术,目前中国采用的LMDS工作在26GHz频段,LMDS能够满足从商业用户到普通个人用户的包括话音、数据和图像等电信业务的宽带综合接入。LMDS具有部署快速、无需挖掘道路的特点,可以解决光纤布放短缺以及布放成本过高地区的宽带接入问题。     

我国开通LMDS接入网

3月23日获悉,中国联通公司广东分公司LMDS宽带无线接入实验网已经顺利通过验收,工作频段定在38～39GHz,可用的频段带宽达到1GHz以上,系统采用ATM技术支持TCP/IP、FR。这标志着我国第一套LMDS接入网络成功开通。该网络采用美国P-COM公司LMDS宽带固定无线接入产品。此次中国联通广东LMDS宽带无线接入实验是为了配合中国联     

LMDS技术应用前景及典型案例

LMDS即本地多点分配业务,是上世纪90年代发展起来的一种宽带无线接入技术。工作在10—40GHz微波频段,频谱资源丰富,可以支持高速率的数据业务,被业界称为“无线光纤”。作为一种无线接入技术,LMDS具有所有无线系统特有的网络部署周期短,提供服务快的优势;应用于固定无线接入环境,LMDS还具备组网灵活、     

Ka波段卫星通信系统降雨衰减的计算

研究基于ITU-R的降雨衰减预测方法,并根据此算法以62°E,Ka波段30GHz的卫星通信系统为例,对我国65个典型站点的降雨衰减值进行了计算,为降雨衰减的进一步研究提供了数据支持。     

基于DDS技术的高精度微波信号发生器

介绍了一种基于DDS和正交上变频技术的微波信号发生器的设计,给出了完整的系统解决方案。它的工作频率范围为0.8￣2.5GHz,从而使DDS技术得以在微波频域获得应用,并且还可以产生调频、调幅、调相等各种调制信号。     

用于DVFS片上系统的全数字SARDLL设计

针对动态电压/频率调整系统芯片中时钟同步问题,设计一个具有宽工作频率范围和固定锁定周期的快速锁定全数字逐次逼近延时锁定环,采用改进的可复位数字控制延时线方法,在减小面积和提高最高工作频率的同时,有效地解决传统全数字逐次逼近延时锁定环的谐波锁定和零延时陷阱问题。整个延时锁定环采用 TSMC-65 nm CMOS工艺标准单元库实现,仿真结果表明,在典型工艺角和25℃情况下,工作频率范围为250 MHz~2 GHz,锁定时间为固定的18个输入时钟周期,当电源电压为1.2 V、输入时钟频率为2 GHz时,功耗为0.4 mW。     

“无线光纤”接到中国

作为全球最大的数字微波生产商的美国P—COM公司在今年的无线通信展上推出了其最新研制开发的宽带无线接入产品LMDS系统(Local MultipointDistribution Service),引起了参观人员的关注。由于这种最新的点对多点的微波设备的容量极大,速率高,可与光纤通信相媲美,而且解决了数据传输的“最后一公里”接入的问题,所以LMDS被誉为“无线光纤”。 LMDS是一种新兴的宽带无线接入技术,它具有更高带宽和双向数据传输的特点,可提供多种宽带交互式数据及多媒体业务,且成本进一步降低,使用更为方便。LMDS工作在28GHzR的波段附近,可用的带宽达到1GHz以上,因此几乎可以提供任何种类的业务,支持话音、数据和图象业务和ATM、TCP/IP和MPEG2等标     

南京地区基于E频段毫米波链路的降雨监测研究

近年来,利用微波链路监测降雨这项新技术越来越受关注,准确的降雨观测数据对气候监测至关重要。该研究利用在南京地区搭建的E频段毫米波链路反演降雨强度信息,对降雨引起的衰减进行分离并实时确定参考基线,基于雨滴尺寸分布计算降雨强度,校正了湿天线效应的影响。实验结果表明,基于E频段链路反演获得的降雨强度在71GHz频段的相关系数约达到0.90,均方误差约为0.11mm/h,在81GHz频段约达到0.91,均方误差约为0.15mm/h。通过与激光雨滴谱仪的实测降雨强度作比较,验证了E频段毫米波链路监测降雨的可靠性。     

A combined passive–active microwave retrieval of quantitative rainfall from Topex/Poseidon radar altimeter and Topex microwave radiometer

Synergistic measurements of both active and passive nadir looking microwave sensors onboard Topex/Poseidon (T/P) satellite are explored for their instantaneous rain estimation capability over tropical oceans. Data of T/P altimeter (the differential radar backscatter at C and Ku band frequencies, i.e. δσ°?=?σ°_C?σ°_Ku), and Topex microwave radiometer (TMR) (brightness temperatures at 18, 21 and 37?GHz frequencies) coincident with special sensor microwave/imager (SSM/I) and tropical rainfall measuring mission (TRMM)–microwave radiometer (TMI) have been analysed for this purpose. The rainfall response has been separated from the surface variability and atmospheric water content (in terms of water vapour and liquid water), by analysing the TMR data in view of the radiative response of its channels under different ocean–atmospheric conditions over the Indian oceanic region. Among a large collocated data on different spatial and temporal scales, the most restrictive criteria (<?10?km spatial, <?30?min temporal difference) produce the better statistics (in terms of correlation and rms errors) from the rainfall rate from SSM/I and TMI with the combined radar and radiometric observations from T/P and TMR respectively. The analysis thus shows the added advantage of optimally integrated active and passive microwave measurements for instantaneous rain estimation as compared to our earlier study (Varma et al. ) by passive TMR measurements alone, for both better estimation of rain and corrections to T/P radar backscatter due to its path attenuation. The equation is further used to estimate monthly averaged global rain rate maps for its qualitative and quantitative assessment. Typical rain rate maps from blended radar and radiometer for two contrasting seasons for the months of January and July for the year 2000 (during the north‐east and south‐west monsoon respectively), are compared with similar maps of TRMM–precipitation radar of monthly rainfall accumulations, showing most of the regional and global tropical features delineated.     

Spatial information of high-frequency brightness temperatures for passive microwave rainfall retrievals

The effect of rainfall inhomogeneity within the sensor field of view (FOV) affects significantly the accuracy of rainfall retrievals causing the so-called beam-filling error. Observational analyses of Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Precipitation Radar (PR) data suggest that the beam-filling error can be classified in terms of the mean rain rate and the rainfall inhomogeneity parameter or coefficient of variation (CV_R, standard deviation divided by mean). The dependence of the beam-filling error on the rain rate and CV_R has been confirmed quantitatively using a single channel at 19.4 GHz. It is also found significantly different beam-filling errors for the two different regions, the East and West Pacific, where the spatial and vertical distributions of rainfalls are different. It is also observed that the vertical distribution of rainfall is related to the spatial variability of rainfall (CV_R) and similarly to the spatial variability of TMI 85.5 GHz brightness temperature (CV _Tb). Based on these findings, this study exploits the CV _Tb to correct the beam-filling error in a direct inversion from a rainfall (R) and brightness temperature (T _b) curve at a single frequency, and to reduce the retrieval error in the context of a Bayesian-type inversion method for multi-frequency rainfall retrievals. Both the experiments suggest that the spatial variability of the high-frequency radiometer data appears to contain useful information for retrievals.     

Passive microwave sensing of soil moisture content: The effects of soil bulk density and surface roughness

Microwave radiometric measurements over bare fields of different surface roughness were made at frequencies of 1.4 GHz, 5 GHz, and 10.7 GHz to study the frequency dependence, as well as the possible time variation, of surface roughness. An increase in surface roughness was found to increase the brightness temperature af soils and reduce the slope of regression between brightness temperature and soil moisture content. The frequency dependence of the surface roughness effect was relatively weak when compared with that of the vegetation effect. Radiometric time-series observations over a given field indicate that field surface roughness might gradually diminish with time, especially after a rainfall or irrigation. The variation of surface roughness increases the uncertainty of remote soil moisture estimates by microwave radiometry. Three years of radiometric measurements over a test site revealed a possible inconsistency in the soil bulk density determination, which is an important factor in the interpretation of radiometric data.     

Compact microstrip antennas with very wide ARBW and triple circularly polarized bands

This article presents two compact circularly polarized microstrip antennas with a very wide 3 dB axial ratio bandwidth and triple circularly polarized bands. A hexagonal stub (circular polarization element) along with tuning element in the ground plane is used for achieving wide 3 dB ARBW in antenna‐1, while a novel approach of using a parasitic strip around the circular polarization element is used in antenna‐2 for introducing band elimination notches in the circularly polarized band of antenna‐1. The antenna‐1 has a ?10 dB impedance bandwidth of 12.34% (3.8‐4.3 GHz), 84.02% (4.9‐12 GHz), and 3 dB ARBW of 79.94% (4.9‐10.9 GHz). The antenna‐2 displays circularly polarized band elimination notch characteristics with ?10 dB impedance bandwidth of 24.80% (3.85‐4.94 GHz), 31.72% (6.1‐8.4 GHz), 25.35% (9.3‐12 GHz), and 3 dB ARBW of 4.84% (4.63‐4.86 GHz), 19.08% (6.02‐7.29 GHz), and 5.7% (9.54‐10.1 GHz). Both the antennas are designed and fabricated on FR4 substrate of dimension (0.52 × 0.52 × 0.04)λ₀ at a frequency of 7.9 GHz.     

Effects Analysis of Two-phase Precipitations on Microwave and Millimeter-wave Attenuation Characteristics

Centimeter\|wave radar and millimeter wave radar have been widely used to detect cloud and precipitation,two\|phase precipitations may be detected when radars detect meteorological targets and the study on the attenuation characteristics of two\|phase precipitations can help us retrieve the micro\|physical parameters of precipitation particles using radar echoes.Assuming precipitation meets M—P distribution,variations in the attenuation coefficient of precipitation as functions of rainfall rate and temperature,the results showed when the transmitting frequency of radar is less than or equal to 35 GHz,the attenuation coefficient of liquid\|phase precipitation is larger than that of ice\|phase precipitation,however this case is reverse when the transmitting frequency of radar is greater than 94 GHz,the reason is oscillating amplitude and times of the attenuation efficiency in the case of ice\|phase precipitation particle are larger than that of liquid\|phase precipitation particle.Whether ice\|phase precipitation particle or liquid\|phase precipitation,the attenuation coefficients are less affected by temperature changes.At last,the empirical formulas were established to describe the relationships amongst attenuation coefficient,rainfall rate and temperature.Assuming the frequencies of radars are 3 GHz、6 GHz、10 GHz(Centimeter\|wave)and 35 GHz、94 GHz、140 GHz、220 GHz(millimeter\|wave),variations in the complex dielectric constant of two\|phase spherical precipitations as functions of temperature and frequency would be studied,the attenuation efficiency of single precipitation particle can be computed by Lorenz\|Mie theory and the result showed that the oscillating amplitude of ice\|phase precipitation particle is larger than that of liquid\|phase precipitation particle though the permittivity of liquid\|phase precipitation particle is larger than that of ice\|phase precipitation particle.     

Detection of rainfall rates utilizing spaceborne microwave radiometers

Over most of the microwave spectrum, raindrops both absorb and scatter radiation producing large changes in brightness temperatures relative to clear or cloudy conditions. Since the structure of rain varies substantially for different rain rates and climatological backgrounds, the raindrop size distribution, the rain-layer thickness and the ice clouds above the rain layer are all important inputs to the model computations. The subsequent modeling involves applying the Mie theory to derive the absorption and scattering effects and the radiative transfer computations to derive the brightness temperature. The radiative transfer calculation is based upon a variational iterative approach which takes account of the multiple scattering effect of the rain layer. Results over both ocean and land backgrounds are demonstrated. It is shown that over ocean background, which is “cold” in the microwave region, lower frequencies are sufficiently sensitive to rain conditions. Over land background, higher frequencies have to be utilized in order to obtain the sensitivity to rain rates. It is also demonstrated that by using discrimination tests of the radiometric data, the rain/no rain decision can be made and the rainfall rate can be retrieved from a statistical inversion technique. The shortcomings of this technique are: (1) the assumption of a homogeneous field of view at different wavelengths can be erroneous, (2) the actual physical structure of the rain layer may be significantly different from the one used in the climatological approach, and (3) the additional uncertainty which may be introduced because of the limitations of the radiative transfer algorithm.     

A compact reconfigurable aperture coupled fed antenna for intelligent transportation system application

This article presents a novel reconfigurable aperture coupled fed antenna with a defected ground plane for an intelligent transportation system (ITS). The antenna design with aperture coupled feed line. The defected ground plane allows variation in reactance of the proposed antenna around the center frequency and coupling of energy. This antenna is multiband and works at resonant frequencies of 2.45, 5.9, and 24 GHz allowing measurable reconfigurability with the help of PIN diode. From the measurement, one can infer that the bandwidth is 0.8, 1, and 8.5 GHz, and gain obtained are 11.7, 4.04, and 4.4 dBi at 2.45, 5.9, and 24 GHz frequencies, respectively. It is circularly polarized with a broadside radiation pattern. The advantage of this design is that it covers all the three bands allocated by the Federal Communications Commission for ITS and vehicular communication applications. It is an electrically small, low profile antenna and has a simple structure thus gives large bandwidth.     

Uncertainty analysis of radar rainfall estimates over two different climates in Iran

Accuracy of rainfall quantification is one of the most important concerns in meteorological and hydrological modelling. Satellites and weather radars can provide meteorological information with higher temporal and spatial resolution than ground stations. Rain gauges measure rain rate directly; however, weather radars estimate rain rate by converting radar reflectivity aloft to rain rate at ground level. This conversion with a power law relation between radar reflectivity and rain rate could be altered from place to place or in various precipitation types. This variety may be the source of errors and uncertainty of radar rainfall estimates. One way to assess the uncertainty of radar rainfall is simulation of rainfall fields. In this article, after calibrating two radars located in the south-western and northern parts of Iran, uncertainty of rainfall estimates of these radars has been analysed using the Gaussian Copula model. Reliability of this model was examined for 10% of the rainfall events that were not included in the simulation process. Obtained results of the current research indicate that recalibration of radars can considerably reduce bias and root mean error. In addition, the Copula-based model can generate rainfall fields with similarly spatial structures to those of observed rainfall data.     

A wideband compact antenna with quad‐circular polarized bands in its operating regions

This article presents the design of an offset CPW‐fed slot antenna which exhibits a narrow impedance bandwidth (IBW; |S₁₁| ≤ ?10 dB) extending from 1.20 GHz to 1.45 GHz and another wide impedance bandwidth from 1.86 GHz to 8.4 GHz thus covering almost all the conventional operating frequencies. The antenna is loaded with semicircular and rectangular stubs and meandered microstrip lines to realize circular polarization at 1.35 GHz, 3.3 GHz, 4.9 GHz, and 7.5 GHz with axial ratio bandwidth (axial ratio ≤ 3 dB) of 19.25% (1.2‐1.46 GHz), 4.24% (3.24‐3.38 GHz), 4.1%(4.8‐5 GHz), and 5.2% (7.3‐7.69 GHz) respectively thus covering the GPS, WiMAX, WLAN, and X‐band downlink satellite communication application bands. The mechanism of generation of CP is discussed using vector analysis of surface current density distribution. The gain is fairly constant in the wide IBW region with maximum fluctuation of 1.2 dB. The structure is compact with an overall layout area of 0.27λ × 0.27λ, where λ is the free‐space wavelength corresponding to the lowest circular polarized (CP) frequency. A comparison of the proposed antenna with previously reported structures is performed with respect to impedance bandwidth, compactness, number of CP bands, LHCP to RHCP isolation and gain to comprehend the novelty of the proposed design. A prototype of the proposed antenna is fabricated and the measured results are in accord with the simulated results.