## 4 Data Transmission
### Tested Modulations  
When transmitting data through a long-haul HF link, some important issues have to be taken into account. First of all, the best carrier frequency has to be chosen for the specific season and time of the day. Next, depending on the required bit rate, either a narrowband or wideband modulation scheme has to be designed. Our work is focused on wideband modulations specifically adapted to very low SNR scenarios. We have followed a similar evolution as the mobile communications schemes experienced from the second generation to the fourth generation systems. The first proposal was a modified version of the classical Direct Sequence Spread Spectrum (DS-SS) scheme [16]. When using DS-SS techniques, spectrum is spread over a wider bandwidth, proportional to a pseudo noise (PN) sequence length \(L\), being a robust modulation for rejecting high power narrowband interfering signals. The main drawback of the DS-SS proposal is the low spectral efficiency achieved when obtaining a good Bit Error Rate (BER) performance, since the bit rate available for a given bandwidth \(BW\) is about \(BW\)/\(L\). However, there is no need for an accurate channel estimation and overhead synchronization, so it is a useful proposal for long-haul HF links. We have  
Figure 8: The transmitting antenna in Livingston Island. tested several bandwidths from 250 Hz to 20 KHz, as well as some PN sequence lengths from 31 to 127 chips [32]. After testing the basic DS-SS performance, we proposed a DS-SS signaling based physical layer able to achieve higher data rates while maintaining a good performance in low SNR communications [33]. DS-SS signaling consists on mapping \(m\) data bits into one of the \(M=2^{m}\) spreading codes. At the receiver, the transmitted bits are obtained by correlating the received time-frequency synchronized signal with the \(M\) spreading codes. Maximum correlation value determines the most probable transmitted spreading code, _i.e._, the most probable transmitted group of \(m\) bits. The implementation of \(M\) parallel correlators is feasible because of the low bandwidth of the baseband signal. In the wireless communications world, the first spread spectrum standards have been replaced by several multicarrier standards. Orthogonal Frequency Division Multiplexing (OFDM) schemes emerged with Digital Video Broadcasting (DVB-T) and xDSL, and have been selected for the latest versions of the WiFi standards and \(\mathrm{LTE}\). OFDM splits the input stream into several low bit rate subcarriers in such a way that the frequency selective channel becomes a flat channel for each subcarrier, making the equalization very simple. OFDM achieves higher throughputs in a fading channel than DS-SS [32], so it was the next proposal we tested in the Antarctic link. However, two important issues have to be carefully addressed. First, the frequency stability of the clocks in order to reduce the inter-carrier interference. Second, the non-constant envelope of the signal implies that some peaks arise above the mean value. This is the so-called Peak to Average Power Ratio (PAPR), which reduces the RF amplifier efficiency, since the mean power value of the amplifier has to be set to a lower value to avoid saturation and intermodulation products. In order to design the OFDM physical layer, we tested the influence of several key parameters in the overall performance. Those parameters are summarized in Table 1.  
Finally, we have also tested the Single Carrier modulation with Frequency Domain Equalization (SC-FDE) in order to explore the channel capacity without using spread spectrum techniques. The SC-FDE is a specific type of SC-FDMA (Single Carrier Frequency Division Multiple Access) for a single user, which is a near constant envelope version of OFDM that allows us to benefit from the maximum power of the amplifier [34].