FEC coding with a rate adaptive spread spectrum OFDM system

This paper investigates the use of a spread spectrum OFDM modulation to provide a variable rate communications link that is tolerant of multipath fading. The system adaptively varies the amount of spreading in response to changing channel conditions in an attempt to optimize throughput while maintaining acceptable error rate performance. System performance results for AWGN and Rayleigh fading channels, both frequency selective and frequency non-selective, show that a desired BER performance can be maintained. A trade-off analysis between forward error correction (FEC) coding and spreading for a fixed bandwidth expansion factor is performed. For convolutional coding the results show that a low-rate code provides better performance while, for a Reed-Solomon code, a coding rate of approximately 1/2 is best.