Performance of an OFDM-CDMA HF modem

Transmission in the high frequency (HF) band is considered not to be reliable, due to channel's highly dispersive characteristics, frequency selectivity, Doppler spread and multipath propagation. Briefly, it can be stated that all inconveniences of a wireless channel are present, making this channel a very hostile environment. Some innovations used in presented HF modem design are described. Orthogonal multicarrier modulation, specifically, orthogonal frequency division multiplex (OFDM), was applied, and utilization of code division multiple access (CDMA) leads to good use of channel's frequency diversity. A convenient receiver design for this type of modulation is presented as well. With Reed-Solomon channel coding possible modem's useful data rates are approximately 2400 and 3600 bps. Long interleaving and channel coding was avoided in order to achieve interactive transmission of digital voice. This, however, leaves an additional possibility of modem's performance enhancement if interactivity is not the primary goal.