Again, your trying to compare an analog CW transmission wiht a digital transmission. many issues come into play.
In te analog system, the coding used is the one invented in the 1920's way before they had DSP processors, so the coding they used was a simple analog waveform, basically a sine wave, or continuos wave system (CW). the power needed to transmit this goes wiht the type of waveform and link budget you allocate.
In a more modern digital system, you replace the need for a giant analog signal to be very clean all teh way to the screen by imposing coding gain. What this means is that you improve the signal to noise ration of hte basic signal (or in the digital case, the Bit Error rate BER) by introducing ways to improve the error of the signal. The things that do this is the basic transmission scheme (PSK, QPSK, QAM, ODFM, etc. what these do is use a radio signal which performs better over the digital link. then you follow this with digital coding gain. By using such things as interleavers ( toremove burst errors) and Viterbi decoders ( to remove random erros) you can get a gain in the signal of more than 4 db (or in laymans terms more than 3 times) the signal. Then add to this ways of transmitting more effciently over the same assigned frequency by using CDMA, GSM, TDMA, OFDMA, etc and you get another 4-5 dB.
In the end, it is not the carefully placed antennas that do all of it, it is the improvment in processing power for both radio design and digital DSP design which lower the power. The dB of coding gain affects the transmission power of the antenna system almost directly. So if you had 9 dB of overall coding and radio gain, you would be able to reduce th transmitted power by 10^3 power, or 1000 times. If you assume that regular analog TV systems have as much as 10,000 Watts in a single antenna, you can reduce this to 10W. If you then use a 20W transmitter like the cellular guys do, you can do better than your 45 mile radius old analog system.
In the old analog system they took 6 MHZ of bandwidth to transmit one TV signal. Mediaflo is putting 6-12 channels on the same bandwidth.
Hope this helps. |
