The radio channel is quite
different from the wired channel. First, the radio channel has a distinct
time-change characteristic. The radio channel is exposed to the air, so it is
vulnerable to the interferences in the
air. The signal is influenced by various interferences, multi-path fading and
shadow fading, so the error bit ratio is rather high. To solve the problems
mentioned above, a series of forward and backward(uplink & downlink)
transmission techniques are applied. The original subscriber data or signaling
data are transformed before being carried by the radio waves. And at the other
end of the transmission, a reverse transforming will be done. This can provide
necessary protection to the transmitting signal. The transformation methods
roughly include the channel coding/decoding, interleaving/de-interleaving,
burst formatting, encryption/decryption, and modulation/demodulation. For the
voice, to pass an analog-to-digital converter is actually a
sampling process in the rate of 8KHz,after quantification each 125μs contains
13bit of code stream; then speech coding is performed with every 20ms as a segment and the code
transmission rate is reduced to 13Kbit/s, which becomes 22.8Kbit/s after the
channel coding; then the voice becomes a code stream at 33.8kbit/s after code
interleaving, encryption and burst formatting and is transmitted finally. The
processing at the terminal is just the reverse of the above procedures.
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