On symbol collisions in FH-OFDMA

A frequency-hopping orthogonal frequency-division multiple access (FH-OFDMA) system is considered. Each user transmits by hopping randomly from one cluster of subcarriers to the next. The subcarrier clusters may be fixed groups, or they may be formed randomly at each hopping instant. It is shown analytically that the expected number of collisions per symbol is independent of the hopping strategy. Bit error rate computations via simulations in a frequency-selective fading channel indicate that space-frequency coding reduces the expected number of symbols involved in a cluster collision when the subcarriers are hopped independently.