[1] The paper describes an extensive wideband channel sounding measurement campaign to investigate signal propagation through vegetation. The measurements have been conducted at three frequencies (1.3, 2 and 11.6 GHz) at sites with different measurement geometries and tree species. The data have been used to evaluate current narrowband empirical vegetation attenuation models and study the prevailing propagation mechanisms. Evaluation of the modified exponential decay (MED), maximum attenuation (MA) and nonzero gradient (NZG) models show that on a site by site basis, the NZG model gives the best prediction of excess attenuation due to vegetation. The MA model has been found to be the worst of the three models. The studies have shown that the measurement site used to obtain the NZG model parameter values given in International Telecommunication Union (ITU) [2001] is influenced by metal lampposts and passing traffic, and thus was based on corrupted data. The results show that the leaf state, measurement geometry and vegetation density are more important factors influencing signal attenuation than tree species or leaf shape. Generally, the 11.6 GHz signal was attenuated much more than the 1.3 and 2 GHz signals by vegetation in-leaf, but the differences in attenuation were not significant in the out-of-leaf state. A successful excess attenuation model due to vegetation must consider the measurement geometry and vegetation descriptive parameters as well as any contributions from ground reflection and/ or diffraction over the top or round edges of the trees.
Abstract-This paper is concerned with the statistical investigation of a massive data bank of 49 years of rainfall rate R continuously recorded in Barcelona using a rain-rate gauge with ten seconds response time. With radio communications in mind, the paper addresses and reviews in detail: 1) reliable statistical model for R, 2) number of years required to obtain a database from which to derive a reliable R-distribution, and 3) the CCIR worst-month concept.The research has shown that the generalized Pareto a exp ( -pR)/Rb gives nearly perfect fit for all ranges of R followed closely by the gamma distribution, and the simpler square root (R"*) normal distribution gives excellent fit too. The log-normal distribution was unsatisfactory for R 5 60 mm/h. The spread of the yearly distribution of P ( R ) is cube root normally 'distributed ( [ f ( R ) ] " 3 )and between 7 and 10 years are required before a reliable average distribution P(R) can he obtained. The study of the P(R) return time in years is also presented.High resolution of P ( R ) is presented looking at the evolution of the annual P ( R ) in terms of the hourly and monthly contributing parts revealing statistical features such as the location in time of rain rates above 50 mm/h. Finally, the study shows that the calendar month contribution to P(R) remains at all times well below the synthetic CCIR worst month and recommendations are then given about its use.
The paper is concerned with one aspect of our recent development carried out at Portsmouth Polytechnic (UK) regarding a novel approach for the simultaneous measurement of amplitude and phase nose in oscillators. A description of the measurement system and the associated processing techniques is presented.The measurement system includes a conventional complex (I & Q) demodulator, a high speed sampler and a digital computer. The techniques have been named time-domain phase unwrapping and real amplitude de-enveloping. They permit the reconstruction of the demodulated I and Q signals as a vector rotating at a convenient beat frequency randomised, in phase and magnitude, by noise. The problems encountered in conventional AM and PM detection can be overcome using the afore-mentioned approaches. As a result, direct and true amplitude and phase noise can be measured and characterised.
A theoretical and experimental study of Doppler correction strategies for elliptical orbit satellite communications is presented. Exact analytical expressions for Doppler shift and elevation angle as a function of perigee time of the ground‐station are also presented. It is shown that for a 63° inclined Molnya orbit and a U.K. observer, the Doppler shift remains below about ± 10 ppm and the Doppler rate below about 6 × 10−4 ppm/s. Various strategies for continuous Doppler correction of the on‐board oscillators are reviewed or proposed. These include a closed loop ground–satellite system and an on‐board programmable correction approach. Two detailed theoretical and experimental studies of on‐board encoded Doppler correction distribution systems are presented in the context of the UK CERS/TSAT mobile communications experiment. A frequency lock system which requires the distribution of only one signal and a phase‐lock system requiring a master reference and the time‐variant code word are described. Numerical values for a 1·5 GHz carrier are presented throughout but can be scaled to other frequencies.
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