# Physical Layer Definition for a Long-Haul HF Antarctica to Spain Radio Link
## 6 Conclusions  
In this paper, the physical layer for a long-haul HF ionospheric link is presented. After years of sounding and conducting modulation tests during antarctic campaigns, the frequencies that show the best availability for each hour have been selected to transmit data in optimum conditions, taking into account the narrowband and wideband sounding, and the delay and Doppler spread measured in the wideband sounding. This part of the study led us to separate the channel performance into daytime and nighttime, depending on the frequency of large availability in every moment. After the frequency selection, and assuming the different performance of the day and the night, spread spectrum techniques, multi-carrier and single carrier techniques were evaluated in terms of throughput and BER. This study allowed us to choose the best modulation for day and nighttime. We identified four different situations where the HF modem may have to perform: (i) narrowband sounding; (ii) wideband sounding; (iii) daytime data transmission (HRM); and (iv) nighttime data transmission (HTM). For each of the situations the frame design has been detailed, given the previously studied results. The HRM reaches throughputs of 267 kbits/h, while the HTM reaches more than 1 Mbit/h. Assuming that the data files are kbytes, both configurations satisfy the requirements. Retransmission is always assumed the method to guarantee a good reception, as the channel is very hostile and the communication is simplex. Despite that, in the future, several channel-coding algorithms will be tested in order to improve the BER at the receiver, specially designed for HRM and HTM modes. The presented definition of the physical layer allows us to conduct tests and measures with four frame structures, closing the radiomodem design and test. Far from closing the research in long haul ionospheric propagation, we do consider to widen the tests using other modulation techniques not considered in this study, such as frequency hopping or FSK modulation schemes, in order to compare their robustness and throughput with the present physical layer results. With this physical layer definition, the research group finishes the design of the HF modem for the 12,700 km link from Antarctica to Spain, serving both the transmission of the Antarctic sensor data of the ASJI and the measurement of data itself as sensors for this hostile channel. The long-haul HF radio link can become an important part of the Antarctic observation systems as a remote sensing method, and serve as an alternative to satellite in real-time transmitting data collected with other environmental sensors [33]. This research has been supported by the Spanish Government Projects CTM2010-21312-C03, CTM2009-13843-C02, CGL2006-12437-C02 and REN2003-08376-C02, and by 2014-URL-Trac-018, 2014-URL-Trac-039 and 2015-URL-Proj-041, supported by Universitat Ramon Llull. In addition to the authors of this paper, the following people have been part of the research groups of these projects: [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]. Special thanks to the three Miquels: [PERSON] (\(\dagger\)), [PERSON] and [PERSON], for their support in the channel analysis. [PERSON] has been a researcher in the project leading the design of the physical layer, by means of the study of the channel performance, the synchronization and the Spread Spectrum modulations performance. [PERSON] is in charge of the narrowband sounding, and works on the definition of the physical layer with multi-carrier modulations. [PERSON] has participated in the definition of the proposal of the physical layer, and has written and reviewed the paper. [PERSON] was the principal investigator during the entire project, and in this paper was in charge of the system description, together with [PERSON].