FEC processing
FEC employed in DTV channel encoding consists of three layers:
● Outer coding
● Interleaving
● Inner coding – for satellite and terrestrial broadcasting only
Outer coding employs the Reed–Solomon code RS (204, 188); this adds 16 bytes to the transport packet, making a total of 188 +16 =204 bytes (Figure 8.3). It can correct up to eight erroneous bytes in any single trans- port packet. If the error is higher than 8 bytes, the packet will be marked erroneous and uncorrectable. A code rate of 1/2 is normally used, and this has to be set at the receiver to ensure the signals are properly decoded.
Reed–Solomon coding does not provide correction for error bursts, i.e. errors in adjacent bits, hence the need for an interleaving stage. Interleaving ensures that adjacent bits are separated before transmission. If the transmission medium introduces lengthy bursts of errors, they are broken down at the receiving end by the de-interleaver before reaching the outer decoder. For a fuller description of the interleaving technique, refer to Appendix A3.
The inner layer employs convolutional coding to ensure powerful error correction capabilities at the receiving end. Such error correction capabilities are essential for satellite and terrestrial DTV broadcasting where the medium of transmission is ‘error-prone’. This layer is not necessary for a QEF medium, such as cable in which there is less than one uncorrected error event per hour of transmission.
Both inner and outer coding involves the addition of redundancy bits which make code words longer. Long code words increase the bit rate,
which in turn increases the potential for errors. These problems may be avoided by shortening the code word, a process known as puncturing the code. Puncturing operates by selectively and periodically removing certain coded bits from each code word according to a regular pattern known to the receiver. At the receiver, dummy bits are reinserted to replace the omitted ones, but they are marked as erasures, i.e. bits with zero confidence in their accuracy. Consider a code rate of 1/2 that is punctured by remov- ing 1 bit in 4, a puncturing ratio of 1/4. The mother code rate of 1/2 pro- duces 2 coded bits for every 1 uncoded bit and thus 4 coded bits for every 2 uncoded bits. If 1 bit in 4 is punctured, then only 3 coded bits are trans- mitted for every 2 uncoded bits, which is equivalent to a code rate of 2/3. In fact, this is exactly how a 2/3 rate is generated.
Puncturing a code word increases its code rate as the number of redun- dant bits is reduced. Punctured codes are obviously less powerful than the original unpunctured mother code. However, there is an acceptable trade-off between performance and code rate as the degree of puncturing increases.
Modulation
The final reduction in the bit rate is provided by the use of advanced modulation techniques. Simple frequency modulation in which logic 0 and logic 1 are represented by two different frequencies is highly inefficient in terms of bit rate and bandwidth requirements. Three types of modulation are used in DVB:
● Differential quadrature phase shift keying (DQPSK) for DVB satellite (DVB-S)
● Quadrature amplitude modulation (QAM) for cable
● Orthogonal frequency division multiplexing (OFDM) for DVB terrestrial (DVB-T)