TABLE OF CONTENTS In Chapters 4-6, we discussed the BER performance of several OFDM systems in frequency selective fading channels, where we did not take into consideration any channel coding schemes. In this sense, we can call them "uncoded OFDM systems." Frequency selective fading gives a distortion to the channel frequency response, so when a signal is sent through a frequency selective fading channel, some subcarriers experience high attenuation whereas others experience low attenuation. Although there are indeed subcarriers with low attenuation and no errors, the average BER of an uncoded OFDM system is determined by the worse BERs in subcarriers with high attenuation, so its BER performance is very close to that of an uncoded single carrier system in a frequency nonselective fading channel. We can imagine that if we employ a channel coding scheme over subcarriers, we can improve the BER by means of a frequency diversity effect.
In this chapter, we discuss "the channel coding effect" on the BER performance of an OFDM system in frequency selective fading channels. First, Section 7.2 outlines a convolutional encoding/Viterbi decoding scheme. After explaining the role of interleaving in fading channels, Sections 7.3 and 7.4 show the principles of symbol interleaving and bit interleaving schemes in detail, with much emphasis on how to calculate the path metric in each scheme. Then, Section 7.5 discusses the BER of the interleaved coded OFDM schemes and compares the effect of symbol interleaving with that of bit interleaving. Finally, Section 7.6 draws conclusions.
