ical binary digits, and direct detection, as used in our discussion of bit error rate. The coding Forward Error Correction gain tells us by how much we have to increase the energy of the pulses incident on the detector in order to achieve the same bit error rate improvement that results from applying the code. Therefore, it is calculated as a ratio of pulse energies and is normally expressed in decibels. The coding gain can also be expressed in terms of the received Q value, which is speci- fied in equation 6.21 and related to the BER in equation 6.22. If we define Quncoded as the Q value before application of FEC, and Qcoded as the value afterwards, the coding gain is given in decibels as follows (Gallion, 2002; Kumar et al., 2002): ncoding (6.39) The achievable gain is dependent on the details of the particular FEC that is used. It is dif- ficult to generalize but typical values lie in the range of 2 to 8 dB. FEC Types A key challenge for specialists in FEC is to achieve the highest coding gain with the lowest overhead. They have two choices of error-correcting codes: block and convolutional. Block codes are more suitable for use in high-bitrate optical communications systems that require postcoding BER values below 10-1 because their overheads are lower. In a block code (m bytes (in which 1 byte = 8 bits) are added to k information-bearing bytes; m is the "codeword length." The most widely used class of FEC in optical communications is the type of block code called known as "Reed-Solomon" (RS). This is what the FEC literature terms a "nonbinary" subclass of "Bose-Chaudhuri-Hocquenghem" (BCH) codes. (The details are not important in the present context, and the reader is referred elsewhere for an explanation [e.g., Wicker & Bhargava, 1994; Guruswami & Sadan, 1999].) RS codes have been very successfully applied in subsea optical communications and are becoming widespread in terrestrial systems. They have also recently been incorporated into the G-PON standard for fiber access networks (ITU-T G.984.6). The most c