Studies On The Receiving Channel Models For Mobile Digital TV In Complex Metropolis Environment

The power of an analog TV signal is always measured as visual peak power, that is, the power level reaches while the synchronizing pulses are being transmitted. For analogy stations, the F(50, 50) curves predict minimum field strength for 50% of the locations and 50% of the time. The minimum signal power for demodulating analogy TV is based on the sum of the noise factor at the end of receiving antenna and carrier-to-noise ratio (C/N) of the receivers. (Noise factor is 7dB recommended by Federal Communications Commission (FCC)in UHF band.) The output signal of digital TV transmitters resembles a Gaussian noise signal. The only way to define the power of a digital TV signal is a root mean square value, which has been applied to define the power of radio noise. In the U.S., F (50, 90) defines digital television service; Chinese professional standard GY/T 237-2008 defines a mobile digital TV service with location availability of 99%. Increasing the location and time availability percentages requires additional signal margin. On the transportation courses, the noise factor at the end of receiving antenna is big; and noise components (WGN and impulsive interference) vary as well. The C/N and C/I (carrier-to- interference ratio) of digital television terrestrial broadcasting (DTTB) receivers are different from those of the analogy TV receivers. The law of path gain of DTTB on the transportation courses is also different from that of the analogy TV. Therefore, there occurs ocassional failure when these statistical curves for analog TV coverage are applied to predict service margin of mobile digital TV. However, the study on the characteristics of noise and impulsive interference in DTTB channel and signal path gain on transportation courses are few. Therefore, this paper focuses on models of noise, impulsive interference and signal path gain of mobile digital TV channel on urban transportation course in UHF band.The measurement system, procedure and surveying data for average noise power of a DTTB channel in UHF band on public transportation courses were introduced. The application of genetic algorithm regression analysis to analyze the cumulative distribution curves of the noise factors in digital channel was presented. As a result, not only three-component finite mixture distribution of three-parameter Weibull cumulative distribution functions of noise factors but also the K times standard deviation of the noise factors'upper decile under three typical communication environments were obtained. A coverage model of minimun signal power for decoding based on the noise factors'upper decile plus C/I (assumed impulsive interference channel) was suggested, in contrast with that based on the noise factors'50 deciles plus C/N (assumed AWGN channel). Through the trials of mobile reception of digital television, the results showed that the acceptable percentage of the model based on interference statistical distributions was about 86 %, and that based on AWGN noise is about 40%.The measurement system, procedure and surveying data for field strength of a DTTB channel in UHF band on public transportation courses were introduced. Three-dimension multi-ray tracing models based on physical optics (PO), geometrical optics (GO) and geometrical theories of diffraction (GTD) and especially the uniform theory of diffraction (UTD) were presented to analyze transmission characteristics of a mobile digital TV on urban complex communication environment. A 3-dimensional two-ray tracing model revealed the relations between the formation, range, and components of the shade on the road under viaduct and the varying level of antenna and the horizontal. Taken the four environmental factors (direct projection, ground reflection, transmission as well as bridge corner diffraction )into consideration, a six-path ray tracing model including the high-order transmission wave for viaduct deck with complex dielectric constant and the diffraction wave from the viaduct corner was created. Fading condition in LOS region and shadow region under viaduct was illustrated by means of calculating propagation path gain of different polarization waves through different paths. Having compared the range of path gain in the two regions above, it was revealed that path gain model in shaded region could cover that in LOS region. Having compared the test data with Okumura, Chinese professional standard GY/T 237 and the six-ray tracing model, this paper has come to the following conclusions. A two-ray model (corrected) could predict the path gain of the courses on viaduct. An Okumura curve (corrected) could predict the path gain of the course in the river. A six-ray tracing model in addition to an over ten dB error correction factor to compensate for obstacle loss of roadside buildings could predict the path gain of the courses under viaduct accurately.