| Last fifteen years improvements in HF digital communications have made ionospheric communications a true alternative to low bit rate, long distance communications, mostly in polar caps where alignment with geostationary satellites is not always possible. Our research group is developing a project whose goal lays in the measure of ionospheric channel characteristics as delay spread and doppler spread in order to implement a new radio modem specially designed for skywave communications from Antarctica. Our previous research efforts have been focused on using PN sequences to evaluate the 12.700 km link from the Spanish Antarctic Base Juan Carlos I (SAB) to Spain, which a minimum of five hops, crossing the equatorial belt. It has been shown that ionospheric channel from Antarctica is a slow fading multipath channel with delay spread ranging from 1.5 to 6 ms and doppler bandwidths from 0.4 to 2 Hz. Therefore, a multicarrier modulation system would be a good choice when dealing with these channel characteristics. As a first step towards a fully multicarrier communications system, a channel estimation based on OFDM pilot symbols has been carried out.
Since SAB has strict power consumption restrictions, especially during austral winter when the base is uninhabited and served only by solar and wind power, low consumption becomes a critical point.
In this paper, two multicarrier transmission schemes have been proposed and compared based on channel measured transfer functions and noise plus interference records, conventional OFDM modulation has been compared with a frequency hopping multicarrier transmission in order to avoid the degradation caused by wide and narrow band interferences. Due to the low bit rate available, special attention has been paid to pilot pattern design, in order to reduce the data rate as less as possible. Comparison of quality and throughput in real transmissions from Antarctica, as well as evolution of BER in front of interferences will be studied. |