## 4 Data Transmission
### OFDM proposal
During the 2006-2007 Antarctic survey several experiments were carried out to test the effects on the distortion introduced by the transmitter's power amplifier in the overall system performance. BPSK-modulated OFDM signals with an average power 1 dB, 5 dB and 10 dB below the input saturation power of the amplifier were transmitted. As it can be observed in Figure 7, employing larger input back off's (IBO's) in the transmitter's power amplifier results in poorer BER performance, even though less nonlinear distortion is introduced. The reason is that the BER degradation caused by a nonlinearity when BPSK mapping is used is very small, even if low IBO is used. As a result, it is much convenient to place the signal close to the saturation point of the amplifier to increase the SNR at the receiver. It is worth mentioning that large modulation schemes (16-QAM or 64-QAM) are much more sensitive to the nonlinear distortion and thus one should also consider the nonlinearty effects in case of employing a 16-QAM or 64-QAM modulated OFDM system. However, this is not the case for long-haul skywave communications. Interferences from the primary and secondary services.Interferences in the HF band are usually slow variant and have a bandwidth similar to that of our skywave communication system. As a result, if a primary or a secondary service operating at almost the same carrier frequency is present at the receiver site, the long-haul skywave communication system will be highly interfered and no communication will be possible. To avoid this one can employ a hopping pattern in the frequency domain so that the interference level is reduced. Figure 8: BER simulation performance of an OFDM system under the worst, the average and no interference conditions. The performance of a FH-OFDM with different hopping rates operating at the worst interference conditions is also shown. During the 2005-2006 Antarctic campaign, noise plus interferences at the receiver site were recorded and used to test system performance under real situations. Figure 8 shows the performance of an OFDM system in a AWGN channel when such interfering signals are present. Three type of interfering conditions are evaluated: worst interference conditions (OFDM WstNPI), average interference conditions (OFDM AvgNPI) and no interference (OFDM AWGN). The performance of an OFDM system employing frequency hopes of 1 kHz every 1, 7 and 31 OFDM symbols in the worst interference conditions is also shown. As it can be observed, interferences cause a serious impact on the system performance. But it can be reduced by introducing frequency hopping. For instance, as it can be observed an OFDM system employing frequency hopes of 1 kHz every 31 symbols operating in the worst interference conditions has almost the same performance as a conventional OFDM system operating at average interference conditions. When comparing the different hopping rates, we observe that a better performance is achieved for slow frequency hopes. The reason is that when the hopping rate increases, less pilots are used to interpolate the channel in each block, yielding to a larger channel interpolation error and, thus, poorer system performance.