Typical digital communication system
The figure is the block diagram of a typical digital communication system. The message to be sent may be from an analog source (e.g., voice) or from a digital source
(e.g., computer data). The analog-to-digital (A/D) converter samples and quantizes
the analog signal and represents the samples in digital form (bit 1 or 0). The source
encoder accepts the digital signal and encodes it into a shorter digital signal. This is
called source encoding, which reduces the redundancy hence the transmission speed.
This in turn reduces the bandwidth requirement of the system. The channel encoder
accepts the output digital signal of the source encoder and encodes it into a longer
digital signal. Redundancy is deliberately added into the coded digital signal so that
some of the errors caused by the noise or interference during transmission through
the channel can be corrected at the receiver. Most often the transmission is in a high frequency passband; the modulator thus impresses the encoded digital symbols onto
a carrier. Sometimes the transmission is in baseband; the modulator is a baseband
modulator, also called formator, which formats the encoded digital symbols into a
waveform suitable for transmission. Usually there is a power amplifier following
the modulator. For high-frequency transmission, modulation and demodulation are
usually performed in the intermediate frequency (IF). If this is the case, a frequency
up-convertor is inserted between the modulator and the power amplifier. If the IF is
too low compared with the carrier frequency, several stages of carrier frequency conversions are needed. For wireless systems an antenna is the final stage of the transmitter. The transmission medium is usually called the channel, where noise adds to the signal and fading and attenuation effects appear as a complex multiplicative factor on the signal. The term noise here is a wide-sense term, which includes all kinds of random electrical disturbance from outside or from within the system. The channel also usually has a limited frequency bandwidth so that it can be viewed as a filter.
In the receiver, virtually the reverse signal processing happens. First the received
weak signal is amplified (and down-converted if needed) and demodulated. Then
the added redundancy is taken away by the channel decoder and the source decoder
recovers the signal to its original form before being sent to the user. A digital-to-analog (D/A) converter is needed for analog signals.
The block diagram in the figure is just a typical system configuration. A real
system configuration could be more complicated. For a multiuser system, a multiplexing stage is inserted before the modulator. For a multistation system, a multiple
access control stage is inserted before the transmitter. Other features like frequency
spread and encryption can also be added into the system. A real system could be
simpler too. Source coding and channel coding may not be needed in a simple system. In fact, only the modulator, channel, demodulator, and amplifiers are essential in all communication systems (with antennas for wireless systems).