**4. Physical layer**

HF communications are often designed to operate with high-power amplifiers at the transmitter side, with typical values from 1 to 10 kW. When thinking about remote sensors supplied by batteries or solar panels, the transmitted power has to be much lower. For long-range transmissions between the Antarctic stations and Europe, a power value less than 150 W is desirable, while a value less than 30 W is expected for unattended remote sensors installed around the stations.

In a low-power scenario, the modulation has to be extremely robust with respect to noise and interference. Several replicas of the transmitted signal arrive to the receiver due to the reflection at the different layers of the ionosphere. As the layers are in constant motion, the HF channel behaves as a slow-frequency selective channel, so as the mobile channel for wireless communications. Hence, some of the latest techniques applied to the world of mobile telephony can be adapted to HF communications.

For the long-range link (about 12,760 km) with oblique incidence, the SNR at the receiver is extremely poor, often with negative values. In that situation, we defined two different modulation schemes: (i) the robust mode, for SNRs negative or close to zero and (ii) the fast mode, when the SNR is positive, and the bit rate can increase significantly.

In the robust mode, a modulation based on direct-sequence spread spectrum (DS-SS) was designed. The effective net data rate is very low (hundreds of bps), but the data can be demodulated under high levels of noise and interference. In the fast mode, the single carrier-frequency domain equalization (SC-FDE) modulation was used, since it can handle higher data rates (up to 3 kbps) if the different subcarriers can be received at a positive SNR.

For the NVIS link with vertical incidence, the situation is much more suitable. Although the transmitted power is lower, the received signal is higher while the level of interference and noise coming from the vertical direction is reduced significantly. With a SNR between 10 and 20 dB with an available bandwidth of 3 kHz, a narrowband phase shift keying (PSK) or frequency shift keying (FSK) is proposed, achieving bit rates of tens of kbps that ensure another range of applications, such as messaging, e-mail, and digital voice transmission.

When a node is working in an asynchronous mode, the receiver has to be waiting for an incoming signal continuously. A robust signal detector with low false alarm probability has to be developed, with strict requirements of energy consumption. The following subsections deal with the different modulations and the signal detector in detail.

**31**

**Table 4.**

**Table 3.**

**Constellation Tb** 

**(ms)**

*Cumulative SC-FDE results for PSK constellation. Best results in bold.*

*Advanced HF Communications for Remote Sensors in Antarctica*

In the oblique transmission from Antarctica, two modulations have been designed for the two reception modes [13]: (i) a DS signaling [15] was designed for

The spread spectrum tests were performed using a DS-SS signaling modulation. It consists of the use of a whole family of PN sequences [26] and associating each of them to a symbol. The information about the transmission will be contained in the own sequence by means of the use of a codebook. The receiver also has the codebook available and uses it as a dictionary to obtain the transmitted information. The PN sequences used for these tests were gold sequences [29], taking advantage of

The results for the cumulative density function of the bit error rate (BER) results for two thresholds of 5 and 10% are shown in **Table 3**. They present bit rates around hundreds of bits per second—enough for data to be transmitted and with a reasonable quality assuming that we are facing the most hostile time zone of the channel, the daytime. We found out that the best possible combination is a Gold PN sequence family 2047 length, using a bandwidth of 16.6 kHz

The first tests using SC-FDE were conducted in [16], with the aim of minimizing the problems generated by the peak-to-average power ratio (PAPR) and interchannel interference (ICI) of the previous works using OFDM [17]. The single carrier-frequency domain equalization has been designed with several parameter modifications in comparison with [17], such as variable bandwidth, different length of blocks, and

**PN length Ts (ms) BW (kHz) Bit rate (bps) BER (5%) BER (10%) 123 16.6 89 0.73 0.81** 123 8.3 81 0.66 0.79 127 4 73 0.64 0.78 127 2 65 0.53 0.67

*Cumulative density function of the BER results for two thresholds of 5 and 10%. Best BER values in bold.*

PSK 10 296.3 0.74 0.54 **PSK 30 358.2 0.90 0.56 PSK 50 373.8 0.93 0.57** PSK 70 381.0 0.95 0.39

**Bit rate (bps) Spectral efficiency (bps/Hz) BER (>0.05)**

high robustness mode and (ii) a SC-FDE for high throughput mode [16].

*DOI: http://dx.doi.org/10.5772/intechopen.81108*

**4.1 Modulations for the long-range link**

their low cross-correlation in each family.

and a final bit rate of 89 bps.

*4.1.2 SC-FDE*

*4.1.1 DS signaling*
