Channel estimation for long distance HF communications based on OFDM pilot symbols

Although HF communications have mostly been used for voice purposes, since the growing of digital systems, they are becoming a true alternative to satellite communications when talking about long distance and low bit rate demanding applications. The choice is very attractive since the equipment costs are dramatically reduced and it can overcome the targeting problem of GEO satellites from the poles. Ionospheric propagation suffers from serious impairments, i.e. long delay spreading, high attenuation, frequency dependent signal-to-noise ratio and high level of interference. In this paper, an OFDM (Orthogonal Frequency Division Multiplexing) based channel estimation is presented in order to cope with low SNR HF links. It has been tested out in a long distance (12700 Km) HF link that has been established between the Spanish Antarctic Base in Livingston Island and the Ebre Observatory (Spain).