*4.1.2 SC-FDE*

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


**Table 3.**

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


**Table 4.**

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

several constellations. The reader is referred to [14] for the results of the extensive tests in SC-FDE, and a synthesis of the best results in terms of the cumulative distribution function (CDF) for a BER greater than 0.05% for a bandwidth of 400 Hz can be found in **Table 4**.

From the results presented in **Table 4**, the best possible configuration in terms of bit rate and cumulative density function of BER is the proposal with a bit time of 50 ms, in close competence with the proposal of 30 ms. The PSK using 50 ms was finally chosen because the ratio between the bit rate and the spectral efficiency against the BER was slightly higher than the same metric for 30 ms. That configuration was finally proposed to implement the high throughput mode of the oblique Antarctic transmission system.

#### **4.2 Modulations for NVIS**

The transmission system using NVIS has to be power efficient in order to attain the requirements of the battery. The tests performed are designed to optimize the required power for bit rates lower than 3 kbps, which cover the major part of remote sensing applications. The tests compare the performance of simple modulations, that is, 2-FSK, 2-PSK, 4-FSK, and 4-QAM (quadrature amplitude modulation), to find out the best modulation to be used with low transmission power, from 0.3 to 24 W. The duration for each test is adjusted so as the same amount of bits is sent for each modulation. The occupied bandwidth is 2.3 kHz to be consistent with the most common HF standards [30]. The results presented are derived from a 4-month survey between February and May 2018. The tests were performed between our premises in Barcelona (41°24′33.62″ N, 2°7′48.82″ E) and a field laboratory in Cambrils (41°4′57.22″ N, 1°4′4.61″ E) placed 96 km away. The frequency was fixed to 4.5 MHz after a detailed analysis of the ionograms from the Ebre Observatory in Roquetes, situated 80 km from Cambrils. We have only performed the transmissions during the day, so the system only uses one frequency. **Figure 9** shows the cumulative density function of the BER for a transmission power of 0.7 and 24 W. For low power transmission (0.7 W), PSK outperforms the rest of the modulations, presenting a BER less than 10<sup>−</sup><sup>3</sup> the 55% of the time and a BER less than 10<sup>−</sup><sup>2</sup> the 80% of the time. For high power (24 W), 2-FSK, 2-PSK, and 4-PSK behave in the same way, while 4-FSK has a much poorer performance. This is because FSK needs a higher bandwidth in order to keep the carrier spacing, and we have made all the tests under the same conditions.

**33**

complexity reasons.

*Advanced HF Communications for Remote Sensors in Antarctica*

In every telecommunication system, a precise time and frequency synchronization is a key issue in order to receive and demodulate the signal in the best conditions. The classical approach to time synchronization uses a PN sequence, finding the starting point of the frame by simple correlation. In practice, the clock differences between transmitter and receiver as well as the Doppler introduced by the channel may cause frequency shifts up to ±50 Hz. It follows that the received signal is rotated in phase and, therefore, hampers the correlation. An initial frequency synchronization in narrowband has to be done first. Hence, a tone of 600 Hz (with respect to the carrier) is added to detect which global frequency appears during the signal transmission. A tone of 600 Hz is often masked by the huge levels of noise and interference that are typical in the HF band. Therefore, we need a way to detect in a robust way that the signal is present with a low probability of false alarm. A known sequence of appearance of the 600 Hz tone is added at the beginning of the frame. Once the frequency shift is corrected, next step is synchronization. As the low-cost hardware is limited in speed, memory capacity, and programmable space of the FPGA, the design of the PN sequence is based on achieving correlation with the use of the smallest possible size, in the fastest way and requiring the minimum memory. A PN m-sequence of order 6 (64 chips) and 11 kHz of bandwidth was

selected. The final header structure can be seen in **Figure 10**.

The development of a wide area network of sensors around the Spanish Antarctic Station (or SAS) needs not only a robust physical layer, but also a robust protocol able to provide reliability, security, and tolerance to latency. In fact, we can see a remote sensor in Antarctica as a particular case of the Internet of Things paradigm. In this context, it is not wise to extend the traditional networking infrastructure based on routers to these networks for cost, efficiency, and protocol

The presented work deals with the issues of utmost importance to achieve quality of service-aware (QoS-aware) communication in wireless and wired sensor networks based on standard communication protocols for the sensor networks around the SAS. The network consists of a system of distributed sensor nodes that interact among them and with infrastructure depending on applications in order to acquire, process, transfer, and provide information extracted from the physical world [31]. Those sensor nodes can be located anywhere and form an ad hoc network, which does not require a communication infrastructure. Sensor nodes are small enough to guarantee pervasiveness in the

**5. Protocols for sensor networks in Antarctica**

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

**4.3 Signal synchronization**

*Synchronization header for the NVIS frame.*

**Figure 10.**

**Figure 9.** *CDF of the BER for an NVIS link with a TX power of 0.7 W (a) and 24 W (b).*

#### *Advanced HF Communications for Remote Sensors in Antarctica DOI: http://dx.doi.org/10.5772/intechopen.81108*

**Figure 10.**

*Antarctica - A Key to Global Change*

Antarctic transmission system.

lations, presenting a BER less than 10<sup>−</sup><sup>3</sup>

the tests under the same conditions.

*CDF of the BER for an NVIS link with a TX power of 0.7 W (a) and 24 W (b).*

**4.2 Modulations for NVIS**

be found in **Table 4**.

several constellations. The reader is referred to [14] for the results of the extensive tests in SC-FDE, and a synthesis of the best results in terms of the cumulative distribution function (CDF) for a BER greater than 0.05% for a bandwidth of 400 Hz can

From the results presented in **Table 4**, the best possible configuration in terms of bit rate and cumulative density function of BER is the proposal with a bit time of 50 ms, in close competence with the proposal of 30 ms. The PSK using 50 ms was finally chosen because the ratio between the bit rate and the spectral efficiency against the BER was slightly higher than the same metric for 30 ms. That configuration was finally proposed to implement the high throughput mode of the oblique

The transmission system using NVIS has to be power efficient in order to attain the requirements of the battery. The tests performed are designed to optimize the required power for bit rates lower than 3 kbps, which cover the major part of remote sensing applications. The tests compare the performance of simple modulations, that is, 2-FSK, 2-PSK, 4-FSK, and 4-QAM (quadrature amplitude modulation), to find out the best modulation to be used with low transmission power, from 0.3 to 24 W. The duration for each test is adjusted so as the same amount of bits is sent for each modulation. The occupied bandwidth is 2.3 kHz to be consistent with the most common HF standards [30]. The results presented are derived from a 4-month survey between February and May 2018. The tests were performed between our premises in Barcelona (41°24′33.62″ N, 2°7′48.82″ E) and a field laboratory in Cambrils (41°4′57.22″ N, 1°4′4.61″ E) placed 96 km away. The frequency was fixed to 4.5 MHz after a detailed analysis of the ionograms from the Ebre Observatory in Roquetes, situated 80 km from Cambrils. We have only performed the transmissions during the day, so the system only uses one frequency. **Figure 9** shows the cumulative density function of the BER for a transmission power of 0.7 and 24 W. For low power transmission (0.7 W), PSK outperforms the rest of the modu-

 the 80% of the time. For high power (24 W), 2-FSK, 2-PSK, and 4-PSK behave in the same way, while 4-FSK has a much poorer performance. This is because FSK needs a higher bandwidth in order to keep the carrier spacing, and we have made all

the 55% of the time and a BER less than

**32**

**Figure 9.**

10<sup>−</sup><sup>2</sup>

*Synchronization header for the NVIS frame.*

### **4.3 Signal synchronization**

In every telecommunication system, a precise time and frequency synchronization is a key issue in order to receive and demodulate the signal in the best conditions. The classical approach to time synchronization uses a PN sequence, finding the starting point of the frame by simple correlation. In practice, the clock differences between transmitter and receiver as well as the Doppler introduced by the channel may cause frequency shifts up to ±50 Hz. It follows that the received signal is rotated in phase and, therefore, hampers the correlation. An initial frequency synchronization in narrowband has to be done first. Hence, a tone of 600 Hz (with respect to the carrier) is added to detect which global frequency appears during the signal transmission. A tone of 600 Hz is often masked by the huge levels of noise and interference that are typical in the HF band. Therefore, we need a way to detect in a robust way that the signal is present with a low probability of false alarm. A known sequence of appearance of the 600 Hz tone is added at the beginning of the frame. Once the frequency shift is corrected, next step is synchronization. As the low-cost hardware is limited in speed, memory capacity, and programmable space of the FPGA, the design of the PN sequence is based on achieving correlation with the use of the smallest possible size, in the fastest way and requiring the minimum memory. A PN m-sequence of order 6 (64 chips) and 11 kHz of bandwidth was selected. The final header structure can be seen in **Figure 10**.

## **5. Protocols for sensor networks in Antarctica**

The development of a wide area network of sensors around the Spanish Antarctic Station (or SAS) needs not only a robust physical layer, but also a robust protocol able to provide reliability, security, and tolerance to latency. In fact, we can see a remote sensor in Antarctica as a particular case of the Internet of Things paradigm. In this context, it is not wise to extend the traditional networking infrastructure based on routers to these networks for cost, efficiency, and protocol complexity reasons.

The presented work deals with the issues of utmost importance to achieve quality of service-aware (QoS-aware) communication in wireless and wired sensor networks based on standard communication protocols for the sensor networks around the SAS. The network consists of a system of distributed sensor nodes that interact among them and with infrastructure depending on applications in order to acquire, process, transfer, and provide information extracted from the physical world [31]. Those sensor nodes can be located anywhere and form an ad hoc network, which does not require a communication infrastructure. Sensor nodes are small enough to guarantee pervasiveness in the

Antarctic environment and may be able to observe a certain phenomenon, measure its physical properties, quantify this observation, and finally, transmit gathered data. Sensor nodes could also have processing and routing capabilities using either a wireless or a wired medium. In this environment, sensor networks must dynamically provide the necessary QoS depending on the type of information transmitted by sensor nodes in a multihop topology, and then, the information should be transmitted to central station through NVIS by implementing a delay tolerant network.

As the network is composed of an extensive mesh of spread nodes, they must be located in the same link layer domain to communicate among themselves. Therefore, they will use link layer mechanisms instead of network layer techniques such as IP networks or routing protocols. Consequently, communications become faster and time response turns tighter.

Each type of data may require specific requirements, for example, a critical alarm may demand strict real-time requirements while monitoring reports may not need latency requirements. In order to face these demands, network architecture must deal with several QoS profiles and it should allow discriminating and/or enforcing specific traffic differentiation.

Taking all the above into account, the ICT requirements identified for the system are as follows:


Due to the large scenario in which the research project is going to be deployed, different technologies will be needed in order to cover all the areas. Some technologies based on IEEE 802.15.4 are presented as wireless communication candidate technologies that work within mesh networks and they are useful for Antarctic local area network coverage. The result has to be able to support large, geographically diverse networks with minimal infrastructure, with potential millions of fixed endpoints. In the upper layers, there may be technologies such as Zigbee or 6LoWPAN.

When working at Layer2 (second layer of the open system interconnection protocol stack), the communication between two different technology domains (IEEE 802.15.4 and NVIS) involves a gateway, enabling the communication between two separate IEEE 802.15.4 domains across a NVIS domain. A Layer2 routing multihop algorithm capable of working over the obtained topology database is needed in complex network topologies. The multihop algorithm is in charge of determining the neighbors to reach a destination, and the communication with that destination will be requested from the link layer. It is important to bear in mind that the information used by the multihop algorithm can be filtered by the topology control algorithm (valid/nonvalid neighbors).

**35**

education.

*Advanced HF Communications for Remote Sensors in Antarctica*

In this chapter, we have reviewed all the recent activities around the application of HF communications for the research community in Antarctica. The long-range transequatorial link aims to communicate the Antarctic station directly to the home country as an alternative to satellite communications for low bit rate applications. For a transmission power up to 250 W, two different transmission modes have been developed, the robust mode and the high throughput mode. The robust mode, which uses spread spectrum modulation, is suited for extreme channel conditions and achieves 85 bps for a bandwidth of 16 kHz for the spread signal. The high throughput mode, which uses multicarrier modulation and achieves 370 bps for a bandwidth of 400 Hz, is suited for good channel conditions. Although these bit rates are low, they are enough for most of the current sensors installed around the

The NVIS link can provide coverage in a surface of approximately 200–250 km radius without the need of line of sight. The main goal of the proposed system is to extend the influence area of the Antarctic stations with the deployment of a wide-area sensor network. When the sensors are distributed in distant zones, it is a hard work to collect the data regularly, and the data are often accessed once or twice a year. With the NVIS solution, all the researchers may get a report of the sensor data in the SAS every day, with no need of direct vision between the sensor and the SAS. The NVIS node has an internal memory that stores the data until the ionosphere conditions allow the transmission. The nodes are intended to be battery powered so the transmission power is kept to a minimum (below 10 W). For NVIS links, the bit rate ranges from 2.3 to 4.6 kbps, depending on channel conditions. On this basis, digital voice and low-resolution images can be sent apart from data from

In addition to the use in the Antarctica or any other remote places, NVIS communications have a straightforward application in case of natural disasters, terrorist acts, and communications for developing countries. During a natural disaster or terrorist attack, all the conventional communication systems such as GPRS, 3G, and 4G can be seriously damaged and the communication systems will stop working properly. Our proposed NVIS system may help sanitary assistants, firefighters, police, and other emergency services to communicate during the event of a disaster. In that case, the ease in putting this system up and not needing direct vision

On the other hand, some parts of the world do not have any communication infrastructures, either because they are uninhabited areas or simply because people cannot afford the price of a conventional communication system. In places where there is no any telecom operator, the communication can only be made via HF and satellite. The NVIS system, based on a low-cost platform, allows the population of developing countries to have access to primary services, such as e-health and

Finally, there is a great deal of applications, which can use the proposed communication protocol architecture. They can be classified in detection (e.g., detection of temperatures exceeding a particular threshold, of unauthorized access), tracking (e.g., the tracking of workers in dangerous work environments), and monitoring

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

Antarctic stations.

**6. Conclusions and other applications**

most of the sensors available on the market.

between the nodes would be a good solution to save lives.

(e.g., monitoring of inhospitable environments).
