Summary & Concluding Remarks

85 95 105 115 125 135 145 155 165

Distance (m) -90

-85 -80 -75 -70 -65 -60 -55 -50 -45 -40 -35

RSSI (dBm)

1 45m : analytical (two-ray) 2.45m : analytical (two-ray) 1.45m : measured (boxplot) 2.45m : measured (boxplot) 1.45m : median of measurements 2 45m : median of measurements

Figure 3.6: Two-ray model RSSI estimates for two antenna heights (dotted lines) and field mea-surements at selected distances (per distance, two side-by-side boxplots [one per height], slightly offset to true distance and color-faded for readability; outliers shown as crosses); medians of mea-surements are connected (solid line).

and cable attenuation. A thorough revision of the methodology is necessary for future experimen-tal campaigns to minimize such sources of error. Nevertheless, this was beyond the exploratory nature of the current measurements. Second, although the two-ray model is able to capture the impact of one of the main path loss components in near-surface communications (the reflected ray), this is not the only propagation effect taking place. Other phenomena, such as additionally reflected rays due to multipath, scattering, diffraction, among others, can also contribute to varia-tions in the measured received power, and may explain, e.g. why the signal loss obtained for the 2.45m height around 95m was not of the magnitude predicted by the model.

3.4 Summary & Concluding Remarks

In this chapter, we have presented initial modeling and characterization studies for shore-to-shore overwater links affected by tides. We have provided analytical results for two antenna heights and polarization showing revealing dynamics due to the impact of shifts on the antenna heights. Then, since we advocate for the use of the two-ray propagation model to predict those dynamics, we have also provided empirical measurements in this direction. Overall, the experimental results showed reasonable agreement with the model in terms of path loss trends, yet also showed a mismatch due to additional propagation phenomena. We argue these initial results are a useful input to further improve the modeling of large-scale dynamics in overwater RF links affected by tides, showing evidence of the applicability of the two-ray model in limited distance/height configurations. In the

next two chapters, we capitalize on some of these results by providing novel link design methods to effectively improve link quality (e.g. received power) in both shore-to-shore and shore-to-vessel communication scenarios; our end goal.

As future work, that is not necessarily related to the direction of the next two chapters, we see two concrete opportunities. First, to further dig into the potential horizontal polarization benefits from an experimental point of view, i.e. looking for opportunities in which this typically less practical configuration (when using a monopole) may have applicability. Similarly, further explore the potential benefits of horizontal polarization as part of polarization diversity configuration, both, theoretically and empirically. Second, to perform a more complete sensitivity analysis of different parameters (e.g. distance, heights, tidal range) influencing path loss in the two-ray model, while looking into details such as number (and depth) of the nulls, break point distance, and the number of horizontal shifts incurred. Also, by considering related concepts such as the Fresnel Zone.

Finally, an additional opportunity related to diversity and the impact of different antenna heights is to formally extend these notions to the domain of multiple-input and multiple-output (MIMO) systems. While a similar modeling direction was taken in [28] for vehicle-to-vehicle communication channels, maritime communication and sea-level dynamics not only related to tides offer clear challenges and opportunities for MIMO systems, especially from the perspective of diversity on counteracting fading, and/or self-interference patterns. We argue this is a promising modeling research line too which may improve the understanding of the vessel-to-shore overwater channel, e.g. by complementing empirical related studies such as [72].

Chapter 4

Tidal-Informed 2-Ray-Model-Based Link Design and Positioning

In this chapter, we address the detrimental impact of tides and surface reflections on RF links of short to medium-range distances for bothshore-to-shore (S2S) and shore-to-vessel(S2V) com-munication. We propose novel link-design strategies aiming at improving path loss performance during the whole tidal cycle and/or at a specific point of the tide. Particularly, our approach capital-izes on the geometrical basis of the two-ray model to contribute with tailored antenna heights and positioning-based methods for stationary (e.g. buoys) and/or mobile (e.g. AUVs) communication nodes. Experimental results with WiFi technology operating in the 2.4 GHz RF band clearly vali-date our link quality model. Analytical results with varying parameter configurations evidence the importance of our research directions. Notably, in experiments with a mobile AUV operating at the surface, our approach showed to outperform the common practice of placing onshore antennas at the largest possible height and/or surface nodes at a short but arbitrary distance.

Most material included in this chapter is derived from the following scientific publications:

■ M. G. Gaitán, P. Santos, L. Pinto and L. Almeida, "Optimal Antenna-Height Design for Im-proved Capacity on Over-Water Radio Links Affected by Tides", in IEEE OCEANS 2020 [5].

■ M. G. Gaitán, P. d’Orey, P. Santos, M. Ribeiro, L. Pinto, L. Almeida and J. Sousa, "Wireless Radio Link Design to Improve Near-Shore Communication with Surface Nodes on Tidal Waters", in IEEE OCEANS 2021 [4].

4.1 Problem Overview

Wireless RF links deployed over water environments (e.g., rivers, lakes, or harbors) are known to be affected by the conductive properties of the water surface, strengthening signal reflections and increasing interference effects [37;38]. Additionally, recurrent natural phenomena such as tides (or waves) cause shifts in the water level that, in turn, intensify self-interference patterns and/or cause additional propagation impairments, e.g.tidal fading. In particular, this latter effect

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– resulting from the impact of tides on the link quality – becomes noticeable when at least one of the communication terminals does not keep a fixed height w.r.t. the water level. While prior methods have been proposed to counteract this detrimental issue [67;98], conventional studies have mostly focused on long-range communication, typically exhibiting propagation conditions different than those for short/medium-range communication scenarios; our main target.

In this chapter, we go beyond conventional studies by proposing methods to mitigate tidal fading in both S2S and S2V scenarios showing few or all the following characteristics:

1. Short/medium range distances. Long-range communication links (e.g. > 1km) often fall after the so-calledbreak pointdistance, i.e. the distance point at which (according to the 2-ray model) RF propagation offers a monotonically decreasing trend. The short/medium range region instead, offers a challenging non-linear behavior leading to deep fades (or nulls) which can be shifted or further aggravated by the influence of tides.

2. Low antenna-heights (Onshore). Typically, remotely-controlled marine robotics (S2V) and IoT environmental monitoring systems (S2S or S2V) rely on onshore stations that use antennas at a low height (e.g. 1m to 5m). This implies water level variations due to tides (e.g. 0.5m or 1.5m) can be in the order of magnitude of the antenna heights, which may lead to big shifts on the propagation conditions experienced between high tide and low tide.

3. Very low antenna-heights(On vessel). Small vessels such as AUVs or USVs often include external antennas of very low height (e.g. 17.5cm for an AUV [135]) with lengths compara-ble to the signal wavelength (e.g. 12.5cm for WiFi@2.4 GHz). Despite being often ignored, tides represent an extra challenge for this cm-level situation that may exacerbate existing propagation effects and/or lead to still unexplored propagation conditions.

In order to take into account these specific observations, we propose here a set of novel tidal-informed and two-ray-based link design strategies to improve communication in S2S and S2V scenarios, considering stationary (e.g. buoys) and/or mobile (e.g. ASVs) communication nodes.

Organization. The remainder of this Chapter is organized as follows. Section4.2formulates our antenna-height design problem for stationary nodes that minimizes tidal fading when taking into account the whole tidal cycle. Section4.3presents a method for short-term positioning of mobile nodes in order to minimize path losses at a specific point of the tide. Section4.4 offer extensive analytical results for the problem/solution in Section4.2, while Section4.5 present experimental results related to Section4.3. Section4.6provide concluding remarks.