*4.1.1 DS signaling*

*Antarctica - A Key to Global Change*

**4. Physical layer**

**Table 2.**

communications.

increase significantly.

can be received at a positive SNR.

messaging, e-mail, and digital voice transmission.

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

**Parameters Wave Mean Variance Mean Variance** Doppler spread (Hz) Ordinary 0.681 0.094 0.378 0.199

Doppler shift (Hz) Ordinary −0.088 0.554 −0.025 0.208

Multipath spread (μs) Ordinary 710.71 2.83 496.01 2.02

**October 24, 2012 March 5, 2013**

Extraordinary 0.123 0.095 0.081 0.061

Extraordinary −0.073 0.478 −0.222 0.177

Extraordinary 921.41 4.46 712.47 3.19

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

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

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

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

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 detec-

expected for unattended remote sensors installed around the stations.

*NVIS sounding results from data of the ionosonde at Ebre Observatory.*

**30**

tor in detail.

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 their low cross-correlation in each family.

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 and a final bit rate of 89 bps.
