dc.description.abstract | In our study, we find that the rainfall pattern in Taiwan area is a little different with other model prediction. In rainfall research, we conducted a research to collect three different rainfall regions in Taiwan from over 10 years hourly rainrate data. Because hourly rainrate data is too rough to react on the relation of attenuation, a conversion model convert hourly to minutely rainfall data is necessary. For establish the rain conversion model to get the long-term R0.01. 1 minute integration rainfall data was collected at one metropolis, Chungli and hourly data with all 4 areas in Taiwan in same period were used. Continuously, we obtain the representable cumulative minutely rainrate distribution function using the 12 years hourly data of Taiwan area we converted then compared with ITU recommended empirical cumulative distribution functions. We find that the R0.01 of rainrate distribution is lower than ITU-R prediction. On the other way, in research of rain attenuation, we build a point to point Ka band terrestrial propagation system with optical rain gauge system to measure the rain attenuation in Chungli. According to the attenuation and rainfall data, we find that ITU-R and Crane models both make overestimates in rainrate and attenuation prediction.
In another way, the Ka band signal considered at which the penetration into the canopy is shallow. Thus, the multiple scattering mechanisms can be ignored. A vegetation targets are modeled as a half-space of randomly oriented and spaced finite cylinders, or needles, or disks, or their combinations depending the wavelength. The finite length cylinders stand for tree branch or trunk, the needles for stem or coniferous leaves, and the disks for deciduous leaves. For a plane wave exciting such as canopy simulation and experiment results show that the power distribution scattered or pass through to the receiver from a half-space of needle-shaped leaves follows the Gamma distribution. For disk-shaped leaves, the power returns statistics depend on the leaf orientation. We proved the Weibull distribution provides the best predictions for horizontal oriented leaves, which resemble a surface type scatterer. Gamma distribution is found to best represent the power return from nearly vertical disk-shaped leaves a mixture of branches and leaves and the signal transmit through any vegetation.
We also study the rain effects on the performance of a Ka-band LMDS system in Taiwan. The rain distribution statistics is established based on 2-year measurements with 1 minute interval and 12-year measurements with 1 hour interval. The fading due to rain statistics is established based on over 2-year measurements. The system performance in terms of signal-to-interference ratio (S/I), bit error rate, and channel capacity was analyzed. For cellular network service, light rain events induce better S/I because other BTS interference signals around the main BTS are likely to be blocked by rain. The availability of the QPSK modulation scheme in the presence of rain fading without cellular interference meets the margin of BER=10-6 in 6 km cell coverage. When cellular interference presences, the BTS effective service boundary shrinks 2~3 km and 3~4 km using QPSK and 16QAM, respectively, under BER=10-6. Results also suggests that the service radius of a 64QAM modulation scheme with BER=10-6 is less than 4 km without cellular interference but totally not functional under cellular network environment. In this scenario, the 64QAM service radius should be less than 1km and QPSK service radius should be less than 3km according to the channel capacity in rain fading with cellular interference in 6km cell coverage. In summary, in cellular network environment, when we shorten the cell coverage radius, the BTS effective service range will be correspondingly compressed, indicating that the cellular interference is more serious than rain. For solve the interference problem, we expand the cell planning to 4 frequencies and 2 polarizations. Then, the performance of LMDS using M-QAM in 4 frequency network is better than 2 frequency network. In summary, in Taiwan LMDS cellular network environment, M-QAM modulation is difficult to provide an effective and accurate high speed transmission in 6 km large cell coverage radius unless the system expand the frequency channel to increase the distance between each interference sources. | en_US |