Research And Application Of Digital MMDS Television System

China’s cable TV transmits television signals to the large and medium-sized cities mainly through fiber optic and coaxial cable. While it is unrealistic to use large-scale of cable laying in the vast rural areas, especially in remote mountainous areas of complex terrain. The MMDS technology comes up to solve the above problems. MMDS, multi-channel microwave distribution system, can directly sent digital TV signals to the user via wireless wave, which not only helps to solve the problem of track laying, but also provides users with more high quality, affordable digital TV programs and effectively meets the needs of users in this area.This article firstly describes the cable TV technology in the United States, Japan, Britain and other countries respectively, then makes comparisons of the cable transmission standards of these countries. The writer explains the channel coding and transmission system of DVB-S, DVB-C, the DVB-T and puts great emphasis on the analysis of DVB-C and DVB-S programs.,at the same time he describes the standard of the MPEG-2and the key technology of MPEG-2video encoding.Secondly, the article compares the transmission distance, the laying of trunk, construction, network maintenance and operational costs of different forms of transmission. And then it makes researches on the overall framework of the MMDS transmission system, including front-end system, the transmitter system, the composition and working principle of the receiver system. By comparative phase-locked circuit method, precision synchronization method and the multiplier circuit law, we decide to adopt the multiplier circuit to design the local oscillator.And it makes studies on the reflection, refraction, diffraction, fading phenomena and the line-of-sight transmission attenuation, the S/N signal to noise ratio, the decline of the reserves of MMDS signals, which comes up in the communication process.On the basis of the MMDS system, this paper focuses on the systematic engineering design of the users’viewing problems in the vast rural areas of Taiyuan City. And it turns up to build a total emission front-end,228Temple Hill transmitting station and the Dongshan relay station. According to real situation of Taiyuan City, it adopts microwave point-to-point transmission and cable transmission to send the signal from the total transmitting station to the228transmitting stations and the Dongshan relay station respectively, and employs the vertical and horizontal two different polarization. The systems engineering design includes the transmitting station location, source selection, system equipment back-up selection, front-end device laying and the transmitting station equipment parameter selection. In addition, it calculates the noise power, the minimum received power, free space loss and the power of the transmitter. This paper makes studies and discussions on the same frequency interference problems. To test the actual operating results of the detection system, more than20villages are choused to carry out the signal test. By analyzing the data collected, the writer concludes that the system designed in this paper can effectively avoid the same frequency interference on the users’ receiving end. On this basis, it can not only help the areas which are covered by the system come up to the expected indicators, but also help the signal quality of the receiving end to achieve the standard of a error-free four image quality.