10. DSP in Communications

What we have learnt so far is how to convert an analog signal into a digital signal and to process it using digital filters. The field of digital signal processing has fully matured and has found applications in diverse fields. In this chapter, we will concentrate on the application of DSP in one particular field, namely, the field of digital communications. Radio, telephony, and video are a few of the areas that are completely enveloped by modern digital and wireless communications. Radio broadcast started with analog communications. It used analog modulation techniques such as amplitude modulation (AM) and frequency modulation (FM) to transmit the message signal using radio frequencies (RF). These modulation schemes use the message signal to modulate a carrier signal in its amplitude (AM) or in its instantaneous frequency (FM) before transmission. Later digital modulation methods were introduced to serve the same purpose as the analog counterparts. Digital modulation plays an important role in modern wireless communications. The art of making very large-scale integrated (VLSI) circuits has evolved tremendously. This has enabled the design and fabrication of application-specific integrated circuits (ASIC), which in turn enables the implementation of complex digital techniques in achieving communications successfully as well as lowering the cost. Digital communication systems have many advantages over the analog counterparts. For instance, digital communications has greater immunity to noise. It is also robust to channel impairments. Another advantage is that many different data can be multiplexed and transmitted over a single channel. The various data may include voice, video, and other data, for instance. Since binary digits (bits) are used in digital communications, there is greater security in the transmitted data. This is not feasible in analog communications. Even if there are errors in the received data, they can be detected and corrected by employing what is known as the channel coding, in which extra bits are added to the data bits. In analog communications, the noise in the channel will distort the message signal and is, therefore, impossible to recover the original signal. Even though digital modulation as such occupies a higher bandwidth than analog modulation, source coding or data compression is used to reduce the message bandwidth to start with. Digital communication link performance can be improved by using encryption and channel equalization techniques. Moreover, field programmable gate arrays (FPGA) enable the implementation of digital modulation and demodulation functions purely in software. This has an enormous implication because many handheld devices can perform a variety of functions well in real time using software. These features are certainly a no-no in analog communications.