Applications of Subband and Wavelet Transform in Digital Communications
Agostino Abbate, Casimer M. DeCusatis, Pankaj Kumar Das · Birkhäuser Boston eBooks · 2002
Present-day communications systems are quite complex. In the early days of communication, one just needed a modulator for signal to be transmitted, a radio-frequency (r.f.) amplifier, and the antenna. On the receiving side, one needed a receiving antenna, a preamplifier, and a demodulator. Because of the wireless communication which involves networking, the complexity is steadily growing and it will continue to do so to accommodate wireless access to the Internet and multimedia. Figure 8.1.1 shows the block diagram of a digital communication system. Source encoding, also called data compression, removes the redundancy of the source by representing of the data using the smallest number of binary digits [Pro95]. In many cases, specially for secure Internet traffic, for an example, one uses encryption so that unauthorized detection of signal can be minimized or avoided. In contrast to the source coding which removes redundancy, channel coding introduces some redundancy in a predetermined fashion to improve the fidelity of the signal output. The channel has noise and interference, which causes error in the output. By introducing an n-bit codeword for a k-bit of information when n>k, the bit error rate (BER) can be drastically reduced. The digital modulator maps the binary output of the channel encoder to electrical signal waveforms. The modulator could be binary or M-ary, as also amplitude, phase, or frequency modulated or a combination of them.