| By the trend of developing global and personal communication technology, satellite communication system has become an essential part of the modern telecommunication, because of its unique advantage in big overlay, large bandwidth and great transmission capacity. While the Ku band satellite communication system has sunch merits:huge available bandwidth, small disturbance and small terminal equipment, it can offer better service for high speed satellite internet, interacting multimedia data tranmission and other new communication services. Thus, the research of Ku band satellite communication system has played an significant role for the satellite communication development in our country.Because of the high frequency of Ku band, rain attenuation has become one of the key factors which influence the transmission quality. Therefore it's necessary to develop the research of rain attenuation characteristics, and a effective compensation method for rain attenuation is one of the best way to solve the rain attenuation problem in order to improve the satellite communication quality.The causation of rain attenuation is discussed firstly in this paper. The effect of rain attenuation in the satellite communication system is studied next. ITU-R rain attenuation prediction model, which has been used widely in satellite communication, is introduced in this paper as well. According to data measured by the equipment in lab, the characteristics of three types of rainfall have been analyzed by comparing the curves of received signal level, noise level and rain attenuation value of their own. A conclusion has been deduced that the more tempestuous the rainfall is, the larger the attenuation is, which cause the noise raise too. All these changes lead to the decline of the communication quality.The trait of rain attenuation variety rate is analyzed within three different rainfall types and different intervals by calculating the rainfall data and here comes the result: the more tempestuous the rainfall is and the longer the interval is, the faster attenuation variety rate is. To know more about the laws of rain attenuation variety rate from the mathematic point of view, curve fitting and mathematical statistics are utilized in this paper.The probability density and cumulative distribution of rain attenuation variety rate for the three rainfall types have been calculated based on the measured data. The result has indicated that the probabilities of rain attenuation variety rate in positive and negative directions are approximated to each other, and moreover, the probabilities of rain attenuation variety rate which has exceeded 2dB in thunderstorm and typhoon are higher than that in plum rains, which means the rain attenuation rate has varied faster in fierce rainfall than that in calm rainfall. According to the tendency of cumulative distribution of rain attenuation variety rate, polynomial, exponential and logarithm function have been chosen as fitting function which is used to fit the curve of cumulative distribution of rain attenuation variety rate. The curve has been fit from two directions, positive and negative direction, and the varieties of fitting curve in three types of rainfall have been compared in three different intervals. The fitting parameters and error sum of squares have been calculated at the same time. The diversities of the goodness of fit are not only among the different types of rainfall, but also among the different fitting functions: logarithm function has the least compensation error, however, when rain attenuation variety rate has exceeded 2dB, exponential function has the best goodness of fit.Rain attenuation variety rate, for the uplink power control compensation method, is an important parameter, which can adjust the output step so that the compensation will neither too short or excessive. The study of the cumulative distribution of rain attenuation variety rate has offered reference information to the compensation algorithm design and improvement.Power control is the most efficient compensation method in common use. Uplink power control compensation method, which has been used frequently, has been emphasized in this paper. The principle of power control is to increase the sending power of terminals to counteract the rain attenuation by a certain algorithm, which can calculate the rain attenuation based on the reference signal. The detailed design of power control compensation method has been started with choosing reference signal, then the compensation algorithm will be presented and discussed.Reference signal could be either beaconing or spatial noise temperature. Different control methods can be used with different kinds of reference signal. In the condition of laboratory, beaconing, which is sent by the earth station built by the lab, has been chosen as reference signal.The compensation algorithm has been designed aiming at the VSAT(Very Small Aperture Terminal). The specific basic-algorithm is that: the present rain attenuation is reckoned by the average of the fore-moments'rain attenuation in certain time. In order to prove the effect of this basic-algorithm and calculate the specific value to the three types of rainfall, the simulation experiments have been conducted in different forecast time and intervals. Comparing the compensation errors of different forecast time we can find that the compensation error will increase by the randomicity between two measured value when the forecast time is too short, which means fewer measured value has been used in reckoning the present rain attenuation value. By contraries, if the forecast time is too long, the compensation effect becomes worse either because the pertinence among the data descend. The experiment result has indicated that the compensation effect of plum rains are not varied obviously in different forecast time. However, the forecast time of thunderstorm or typhoon is supposed to be around 10 seconds which is testified in the simulation experiments. Then comparing the compensation errors of different intervals it can be found that the shorter interval is, the better compensation effect will be. But give attention to efficiency, equipment cost or more things, here comes the experiment results: the compensation effect of plum rains are not influenced evidently by different intervals, and intervals for thunderstorm or typhoon should better not exceed 10 seconds when forecast time is 10 seconds. However, when interval for thunderstorm or typhoon reach 30 seconds, the compensation errors have become unacceptable. Therefore compensation forecast time and interval are two important factors of compensation algorithm.Continual power compensation will aggravate the burden of equipment, lower the working efficiency. So the compensation interval is expected as long as possible within the scope of allowable errors. In order to solve problem of unacceptable compensation error in the fierce rainfall period within long interval, an ameliorative-algorithm has been presented. The ameliorative-algorithm reduces the compensation errors by add an ameliorative parameter based on the basic- algorithm. The ameliorative parameter is calculated by the corresponding fitting results of cumulative distribution of rain attenuation variety rate. The simulation experiments results shows that: the compensation error of thunderstorm or typhoon has reduced to around 1dB from 2 to 3dB within the 30 seconds interval by using the ameliorative-algorithm. It means ameliorative-algorithm can improve the compensation effect markedly.Sum up the results gotten by the experiments in one word: with the appropriate forecast time, compensation interval and ameliorative parameter, average forecast algorithm (ameliorative-algorithm) is a effective algorithm of power control compensation method, which can meet the requirement of compensation error in VSAT systems. The laws of power control compensation in different conditions, which gain from the experiments, can offer fundamental consult to the study of power control compensation method in the future. |
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