Multicarrier modulation proposal for long distance HF data links. 
P. Bergada, J.L. Pijoan, M. Deumal, R. Aquilue 
 
Skywave ionospheric communication from remote points with poor satellite 
visibility can be a real alternative to satellite communication, especially for 
low bit rate demands. Particularly, data from remote sensors placed on the 
Antarctica stations could benefit from HF communication and establish a low 
cost, low power, long distance data link with far countries. With this point in 
mind, the GRECO (Electromagnetic and COmunications Research Group) is studying 
the physical communication layer of very long distance HF links. The main goal 
is to create a data link between the Antarctic Spanish Base placed in Livingston 
Island (62.6S,60.4W) and Observatory de l’Ebre in Spain (40.8N,0.5E). This 
research group has previous experience in HF data links and exhaustive channel 
measurements have been done during the last four Antarctic summer seasons in 
order to have a deep knowledge of the characteristics and impairments, as 
availability, SNR, delay and Doppler spread, of the aforementioned communication 
channel. Apart from the channel characteristics, some constraints must be taken into 
account: i) Due to power restrictions in the Antarctic Base, the power of the 
transmitted signal is strongly limited. ii) At the reception point very low SNR 
are expected and the whole system must be very robust as no retransmission 
request can be implemented. iii) A proper transmission schedule must be thought 
up in order to take full advantage of  the reduced communication availability.     
This paper undertakes a study of various parts of a physical communication layer 
based on OFDM (Orthogonal Frequency-Division Multiplexing). The work is based on 
computer simulations and real data gathered on the above mentioned HF radio 
link. The OFDM symbol has to be accurately designed for a given channel and 
system limitations: i) The length of the OFDM symbol has been deeply studied as 
it might be lower bounded by the channel delay spread to prevent from inter 
symbol interference and upper bounded by the channel coherence time to guarantee 
a constant channel during a whole symbol. ii) The symbol bandwidth is 
constrained by Shannon theorem and since SNR at the receiver is very low, the 
number of carriers per symbol, which determines the symbol bandwidth, is 
therefore limited as well. iii) Peak to average power ratio (PAPR) is a well 
known problem of OFDM systems mostly if non linear amplifiers are used or 
transmitting power constrains the system. Proper PAPR reduction techniques, such 
as soft decision techniques, have been tested in order to enhance the mean 
transmitted power and improve SNR at reception.