Advanced OFDM-CDMA HF modem with self-interference cancellation

Performance of a modem designed for HF communications is presented in this contribution. The aim was to attain the real time communication for interactive voice transmission. Therefore, powerful coding for error protection and long interleaving was not permitted. Frequency selective HF channel was converted into a set of frequency flat channels with multicarrier modulation, orthogonal frequency division multiplex (OFDM). Spread spectrum technique (SST) was applied to improve modem\´s performance. This technique takes advantage of frequency diversity, as one symbol is transmitted (spreaded) over larger number of subcarriers (N), and more symbols (N) are transmitted simultaneously. Use of SS techniques permits us to perform advanced signal processing in receiver and further enhance modem\´s performance. We applied multi user detector (MUD) in receiver, denoting as "virtual users" the symbols transmitted simultaneously with different spreading codes. For data transmission, when delay is not restricted, coding and interleaving can be incorporated, improving drastically the performance.