By Tam Do, Altera Corp.February 13, 2008 -- dspdesignline.com
A digital communication system shares many similar building blocks that comprise a digital TV transmission system design. These key building blocks begin with channel coding and modulation techniques. It is these similarities that make it easier to take existing design blocks from one system and modify them for use in another one.Digital Communication Basics
All types of transmission, whether it is data, voice, or video, must struggle to overcome the limited spectrum allocation or to achieve maximum bandwidth efficiency. The other challenge is how reduce or handle errors that are caused by noise and fading during the course of the transmission.
The basic signal processing for any digital TV or wireless application is baseband source coding/formatting, channel coding, modulation, multiplexing, signal spreading/scrambling, and timing synchronization (see Figure 1). Figure 1. The Basic Building Block for a Digital Modulator
Source coding and formatting ensure that the digital signals are digitalized with the correct sampling rate, which prevents aliasing and assures they are evenly spread out throughout the transmission spectrum bandwidth in case there are constant ones or zeros. The most common encoded waveforms are Nonreturn-to-zero (NRZ), Return-to-zero (RZ), Phase encoded, and multilevel binary. Long sequence of constant 1s or 0s create discrete spectrum lines in the RF spectrum, this in turn can cause the receiver to go out of lock. Scrambling techniques that use delay gates and exclusive-OR gates can prevent this condition. Therefore the scramblers "whiten" the data by producing data streams that contain bits in sequences in which the state of each bit is independent of adjacent bits.
Channel coding involves digital waveform coding and adding redundant data bits as overheads to battle against noise and signal degradation. M-ary signaling, orthogonal, and antipodal signals are examples of waveform coding that make the receiver's detection process simpler and less prone to errors. Block and convolutional coding are typically used to create forward error correction (FEC) coding.
FEC can achieve the desired bit error rate (BER) with a significantly lower signal-to-noise ratio transmit signal, allowing a low-level, high-bit-rate signal to be received in a higher noise environment than would otherwise be impossible. The improvement in the value of this threshold against noise is referred to as coding gain. For example, if the threshold value of a particular modulation has an SNR of 18 dB without FEC and 15 dB with FEC, the coding gain is 3 dB. Since more bits are transmitted with FEC, coding gain is achieved at the expense of increased channel bandwidth.
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