| To realize the real seamless cover information superhighway, satellite communication system becomes the Essential part for its unique superiority, and shows incomparable excellencies. The Ka band (30/20GHz) satellite communication system, with the virtue of more broad frequency band, the realization of more narrow wave and so on, will become the developing mainstream of next generation of satellite communication.With the increase in the demand for a larger capacity of the satellite communications channel, and the ever-growing congestion of the tasks in C and Ku band, there is a constant want of a satellite communication system which can be applied at a higher frequency band with a broader bandwidth and a frequency spectrum of higher efficiency. The traditional C and Ku band satellite communication system can no longer meet the developing needs of various tasks through broadband satellite communications. Ka band satellite communication system, therefore, for all its advantages, eg. an offer of a large bandwidth (3.5GHz), a huge capacity of communications, a narrow wave , a reasonably small terminal size and a possible capacity for more satellites on the orbit plane, will be an inexorable trend in the future satellite communications.However, the Ka band satellite communication system is now confronted with a great challenge - it is easily affected by meteorological factors. Rainfalls, twinkles, as well as atmospheric absorption can all lead to the deterioration of Ka band field-to-air link channel quality, where rain attenuation is the most important restricted factor. The survey data shows that the rain attenuation of uplink of 30GHz will be over 20 dB at some region in more than 0.1% time period. So technique of equalizing rain attenuation in Ka band is one of the most important researches.First, this paper introduced the characters of Ka band satellite communication, produces mechanism and influence of the rain attenuation, ITU-R rain attenuation estimation model. Then it discussed rain attenuation compensation methods and introduced the principles of these rain attenuation compensation methods, and then analyzed their characteristics. The methods include the technology of dividing and collecting including position dividing, frequency dividing, velocity dividing, the technology of power control including open loop power control, close loop power control, feed back power control, adaptive technique including adaptive TDMA, adaptive CDMA, adaptive coding, adaptive modulation etc. These techniques can reinforce each other and works well. Last, it introduced how to compensate the rain attenuation of Ka band satellite communication system with power optimization, which means dynamic power optimization. It is the main part of this paper.Based on the rain data of 65 typical sits in China, and the character of China terrain, this paper adopted model ITU-R. Rain attenuation on up link 20GHz and down link 30GHz was calculated. Combined with terrain and climate distribute status in China, we analyzed Ka band rain attenuation in China, and summarized the terrain characters of Ka band satellite communication system in China: (1) With a vast territory, the distribution of the rain attenuation is uneven. The terrain difference is significant. (2) The distribution of Ka band rain attenuation in China is zonal from contour map of Ka band rain attenuation in China. (3) The area with great rain attenuation is the southeast of our country, and the area with little rain attenuation is the northwest of our country.One of the main limitations of Ka-band satellite communication is the great influence by rain attenuation. So a dynamic countermove must be adopted to guarantee the usefulness of satellite link channels. And the method of dynamic multi-beam optimized power control is a very efficient one.As the first step, based on concrete terrain status and the analysis of rain attenuation characters in China, this research adopt immobile satellite which orbit position is 92°E and down link frequency is 20GHz to do simulation. To get the best percent that the percent of the areas which can be compensated is occupying the whole number of beams in China, dividing the whole country into 9 beams covering areas, by the virtue of multi-beam, the power of satellite transmitter was distributed which can reduce the rain attenuation of each . At the second step it introduces multi-beam dynamic power optimization methods which are for rain attenuation compensation in Ka band satellite communication system. At first, the data is converted( the data of Ku band is converted to be that of Ka band), then the dynamic multi-beam power optimization criterion is established. The Ka band downlink frequency of our country is 20GHz. Depending on the same value curve of rain attenuation of satellite, whose location is at 92 E, according to the designed program flow chart and the relation between the number of beams and the power of each beam in multi-beam systems, considering the real-time weather ,each beam power is allotted dynamically and optimally to compensate the rain attenuation more efficiently, under the condition of the total power constant. At last ,it's simulated with the experimental data. And this method is proved effective.The effect of dynamic multi-optimized power control is obvious by analyzing the result of compensation: the area which can be compensated is bigger after using the method of dynamic optimized power control, and with the augment of rain attenuation, the superiority is more obvious. Then we reach the conclusion that multi-beam dynamic power optimization methods are efficient in reducing rain attenuation.Otherwise, it's not enough to compensate the rain attenuation with the multi-beam dynamic power optimization methods in Ka band muti-beam satellite communication system. This method just make the use of the power resource is more effective. In some area where the attenuation is very serious, if the power of the satellite transmitter is allocate optimized, other rain attenuation compensation methods should be use together, for example: power control, adaptive CDMA, adaptive TDMA and etc. With these methods used together, the rain attenuation in Ka band satellite communication system can be compensated more effectively, so it can ensure that the usability of the communication link when it's raining.We calculate and analyze Ka band rain attenuation of our country in this paper. It can be used as the important reference of Ka band satellite communication system. The method of dynamic optimized power control can be considered in designing multi- satellite communication system. |
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