Last fifteen years improvements in HF digital systems have made ionospheric communications a true alternative to low bit rate, long distance links, especially in the polar caps where alignment with geostationary satellites is not always possible. Our previous research efforts were focused on using pseudo-noise (PN) sequences and Orthogonal Frequency Division Multiplexing (OFDM) pilot symbols to evaluate the 7900 miles link from the Spanish Antarctic Base Juan Carlos I to Spain, crossing the equatorial belt. In this paper we face the problem of designing the OFDM physical layer. Two multicarrier transmission schemes are proposed and compared based on channel measured transfer functions and noise plus interference records. Special attention is paid to pilot pattern design in order to maximize the system performance while assuring high power and bandwidth efficiency. The quality and throughput in real transmissions from the Antarctica, as well as the evolution of BER in front of interferences, are studied.