Spread Spectrum OFDM Modem for HF Voice-Band Link over Fading Channels

Abstract

This paper deals with the application of spread spectrum techniques to MultiCarrier HF modems. OFDM modems are a special option for the transmission over the high dispersive HF medium in front of single carrier modems. Our present contribution is now focused on the application of Spread Spectrum techniques to improve its fading robustness. The transmission bandwidth is the standard voice band channel in the HF band where the spread spectrum characteristic means that one symbol is transmitted over N times the spectrum in the classical N-OFDM scheme. The motivation of this idea is the observation of the OFDM modems behaviour over fading channels with deep nulls showing a quite different performance between carriers: those in the vicinity of deep nulls provide a much higher Bit Error Rate (BER) than others far away of the nulls. Our proposal intends to spread different symbols over all the subcarriers simultaneously in order to homogenize performances. This approach, performed by orthogonal codes, shows an evident link with MC-CDMA techniques. In addition, Multi User Detection (MUD) strategies as Interference Cancellation (IC) with coherent detection are also applied in order to improve its performance.