Conference Paper

Slow Frequency Hopping for Mitigating Tidal Fading on Rural Long Distance Over-Water Wireless Links

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Abstract

We consider the scenario where long-distance point-to-point wireless links are established over tidal estuaries to bring Internet access to rural communities living in coastal villages. Using real signal strength measurements from links in the Tegola network testbed in northwest Scotland, we highlight the significance of the tidal fading problem, which we reason using a simple geometric model and simulation using the well-known Pathloss radio link planning tool. We propose slow frequency hopping as a cost-effective, software- only alternative to the commonly used space- diversity approach for mitigating tidal fading, and demonstrate its effectiveness using Pathloss simulation.

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... Destructive interference causes the so-called nulls that could move over time according to the tide [11]. Similarly, tidal fading [12], i.e., the recurrent impact of tides on the mean path loss experienced in a link, has been shown to be tremendously detrimental at specific combinations of link distance and antenna height [13]. These two phenomena are predicted by the well-known tworay propagation model [6]. ...
... These two phenomena are predicted by the well-known tworay propagation model [6]. Conventional methods to counteract tidal related issues often rely on classical diversity techniques, e.g., space-diversity [14] or frequency-diversity [13], thus generally requiring additional communication resources (e.g., a second receiver hardware), which may not always be available or feasible. To address such limitation, a technique was recently proposed in [15], which leverages the basis of the two-ray model to find an optimal antenna height design that mitigates average path loss for shore-to-shore links operating over tidal waters. ...
... Yet, more traditional works in the literature are typically focused on the long-range distances [27]- [29], and rarely consider the case of onshore antennas close to the water surface and at near-shore. Moreover, the impact of tides is often ignored in floating nodes, despite being identified as one of the most detrimental issues in shore-to-shore over-water links [7], [8], [12], [13], [30]. All in all, the gap between the existing literature and the impact of tides on the over-water channel is crystal clear, limited to a few initial works [11], [15], [30]- [32] that try to model and predict the trends on the received signal strength through the two-ray propagation model. ...
Conference Paper
Full-text available
Wireless radio links deployed over aquatic areas (e.g., sea, estuaries or harbors) are affected by the conductive properties of the water surface, strengthening signal reflections and increasing interference effects. Recurrent natural phenomena such as tides or waves cause shifts in the water level that, in turn, change the interference patterns and cause varying impairments to propagation over water surfaces. In this work, we aim at mitigating the detrimental impact of tides on the quality of a line-of-sight over-water link between an onshore station and a surface node, targeting mission data transfer scenarios. We consider different types of surface nodes, namely, autonomous underwater vehicles, unmanned surface vehicles and buoys, and we use WiFi technology in both 2.4 GHz and 5 GHz frequency bands. We propose two methods for link distance/height design: (i) identifying a proper Tx-Rx distance for improved link quality at each point of the tidal cycle; (ii) defining the height/distance that minimizes the path loss averaged during the whole tidal cycle. Experimental results clearly show the validity of our link quality model and the interest of method (i). Analytical results confirm method (ii) and show that it outperforms, in both frequency bands, the common practice of placing onshore antennas at the largest possible height and/or surface nodes at a short but arbitrary distance.
... The flat and conductive properties 1 https://aquamon.di.fc.ul.pt/ of the water medium make RF signal reflections stronger and this can lead to extremely severe destructive interference (often referred to as deep fading). The natural water movements (e.g., tides, waves) add extra propagation effects (both path loss and fading), thus increasing design complexity [6]- [8]. ...
... Moreover, the case of links of short-to-medium-range distances (∼100-500m) with antennas installed at a few meters above surface (∼1-5m) is still a borderline scenario [19] with very few efforts fully dedicated to study the impact of tides on wireless links [13], [14]. The conventional methods and guidelines for link design (e.g., [20], [21]), as well as other recent approaches (e.g., [8], [22], [23]), are often optimized for kilometric link distances and/or for much larger antenna heights, and thus, do not show straightforward applicability on this particular setting. In addition, the fact that near-surface antenna heights are within the magnitude order of the tidal range, makes these overall circumstances fairly unique; thus reducing the amount of related/comparable work. ...
... The impact of tides and surface reflections on the receive signal strength of over-water links can be well-described by the geometry of the two-ray model [8], [13], [14]. This model takes the resulting signal strength on the receiver side as the vectorial sum of two copies of the same transmitted signal arriving at the receiver from two different paths: (1) a direct line-of-sight (LoS) path between the transmitter and the receiver, and (2) an indirect path reflected from the surface. ...
Conference Paper
Full-text available
Modern observation systems can be composed by heterogeneous entities (e.g., buoys, USVs, UAVs, onshore sensors, etc.) that rely on dependable communications for coordination and data collection, often provided by over-water radio-frequency (RF) links. In tide-affected water bodies, RF links at a fixed height from the shore can experience the so-called tidal fading, a cyclic time-varying tide-induced interference. To mitigate it, the classical space-diversity reception technique (i.e., the use of two or more receiver antennas positioned at different heights) is often applied, commonly combined with the consideration of having one of the antennas at the largest possible height. Yet, this approach does not always ensure the best performance. In this work, we focus on static over-water links of short-to-medium-range distances that use antennas installed at a few meters above surface. We leverage the geometrical basis of the two-ray propagation model to investigate the optimal single-antenna height design that minimizes overall average path losses over a given tidal range. We then extend this analysis to incorporate a second receiver antenna and identify its optimal antenna height. Analytical results show that our method considerably outperforms the more classical approach, thus enabling superior (average) link capacities.
... Tidal fading is the main source of link quality degradation for these links, and it can severely affect network connectivity. Such correlation is clearly shown by the graph in Figure 5.20(a), from (Macmillan et al., 2010), which shows the signal strength data collected on the link between Site I and Site C: the above curve corresponds to the measured SNR and its two-hour average, and the dashed line shows the tide height data recorded at a nearby location for the same period. ...
... In Appendix F, we present seven management use cases which we took into con-14 An alternative approach of using Stix to mitigate tidal fading is to adopt the slow frequency hopping technique presented in Section 7.1 and described in detail in (Macmillan et al., 2010). In this case, a workflow running on StixAgent builds a two-ray propagation model of each link and slowly varies the transmission frequency to reduce interference. ...
... The next natural question is to explore possible remedies to overcome such signal degradations. By experimenting on the Tegola network,Macmillan et al. (2010) analyse various possible approaches. The first and most obvious is to exploit antenna diversity, by having multiple antennas in-Comparison of measured signal strength values with the values predicted by a simple two-ray reflection model taking into account water level variations over a 45 hour period. ...
Thesis
Broadband speeds, Internet literacy and digital technologies have been steadily evolving over the last decade. Broadband infrastructure has become a key asset in today’s society, enabling innovation, driving economic efficiency and stimulating cultural inclusion. However, populations living in remote and rural communities are unable to take advantage of these trends. Globally, a significant part of the world population is still deprived of basic access to the Internet. Broadband Wireless Access (BWA) networks are regarded as a viable solution for providing Internet access to populations living in rural regions. In recent years, Wireless Internet Service Providers (WISPs) and community organizations around the world proved that rural BWA networks can be an effective strategy and a profitable business. This research began by deploying a BWA network testbed, which also provides Internet access to several remote communities in the harsh environment of the Scottish Highlands and Islands. The experience of deploying and operating this network pointed out three unresolved research challenges that need to be addressed to ease the path towards widespread deployment of rural BWA networks, thereby bridging the rural-urban broadband divide. Below, our research contributions are outlined with respect to these challenges. Firstly, an effective planning paradigm for deploying BWA networks is proposed: incremental planning. Incremental planning allows to anticipate return of investment and to overcome the limited network infrastructure (e.g., backhaul fibre links) in rural areas. I have developed a software tool called IncrEase and underlying network planning algorithms to consider a varied set of operational metrics to guide the operator in identifying the regions that would benefit the most from a network upgrade, automatically suggesting the best long-term strategy to the network administrator. Second, we recognize that rural and community networks present additional issues for network management. As the Internet uplink is often the most expensive part of the operational expenses for such deployments, it is desirable to minimize overhead for network management. Also, unreliable connectivity between the network operation centre and the network being managed can render traditional centralized management approaches ineffective. Finally, the number of skilled personnel available to maintain such networks is limited. I have developed a distributed network management platform called Stix for BWA networks, to make it easy to manage such networks for rural/community deployments and WISPs alike while keeping the network management infrastructure scalable and flexible. Our approach is based on the notions of goal-oriented and in-network management: administrators graphically specify network management activities as workflows, which are run in the network on a distributed set of agents that cooperate in executing those workflows and storing management information. The Stix system was implemented on low-cost and small form-factor embedded boards and shown to have a low memory footprint. Third, the research focus moves to the problem of assessing broadband coverage and quality in a given geographic region. The outcome is BSense, a flexible framework that combines data provided by ISPs with measurements gathered by distributed software agents. The result is a census (presented as maps and tables) of the coverage and quality of broadband connections available in the region of interest. Such information can be exploited by ISPs to drive their growth, and by regulators and policy makers to get the true picture of broadband availability in the region and make informed decisions. In exchange for installing the multi-platform measurement software (that runs in the background) on their computers, users can get statistics about their Internet connection and those in their neighbourhood. Finally, the lessons learned through this research are summarised. The outcome is a set of suggestions about how the deployment and operation of rural BWA networks, including our own testbed, can be made more efficient by using the proper tools. The software systems presented in this thesis have been evaluated in lab settings and in real networks, and are available as open-source software.
... When designing and deploying such systems, the antennas of the static nodes are often placed at a fixed height with respect to the ground surface [2], at the highest possible point, in order to achieve the best link quality. However, this design option fails to account for key path loss and fading propagation phenomena that may degrade the received signal [3]. ...
... The two-ray propagation model [4] is one of the most established and convenient literature methods to describe that phenomenon, treating it as a geometrical problem dependent of the link distance and antenna height, among other parameters. The problem gets further aggravated at coastal areas for shore-toshore and shore-to-vessel links, as tides impose a variation of the geometry of the reflection over time [3], [5]. In the literature, numerous works have explored the fundamentals of this model to explain the propagation of radio signal in other over-water conditions [6] but for long-range links that use much higher antennas. ...
Conference Paper
Full-text available
In the design of shore-to-shore and shore-to-vessel wireless links, the impact of the ray reflected on the surface is often neglected. It adds that, in some coastal areas, the geometry of the reflection changes over time due to tides. When choosing an antenna height for an inshore node, often the largest possible height is used, but this approach can lead to signal degradation. The two-ray model is the most fundamental path loss model to account for the contribution of the reflected ray. We carried out experimental measurements at the shores of a freshwater body to verify that the two-ray model can predict the major trends of the path loss experienced by a 2.4 GHz over-water wireless link. We focus on short-to-medium distance links, with antennas installed a few meters above surface. We observed considerable consistency between measurements and model estimates, leading us to conclude that the two-ray model may bring benefits when applied to the network design of over-water links affected by tidal variations, which is our end-goal.
... The tidal cycle has an impact only on the reflected path, thus modifying the received signal strength due to the dynamic interaction between the two rays. This phenomenon, known as tidal fading [5] is a significant and barely explored problem deemed as one of the most detrimental issues affecting overwater links [7]. ...
... Early works, such as the one in [4], have already reported a clear dependence of received signal strength and water level cycles, but the analysis of such a situation has been brought again to the forefront only recently. The work in [5] has proposed a novel mitigation technique that exploits the frequency dependence of the two-ray model in the form of a channel-hopping approach. This work, when compared with other hardware-redundant strategies, is shown as a cost-effective alternative to overcome the impact of tides on long-range overwater links. ...
Conference Paper
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This work explores the impact of antenna heights and polarization on overwater links during the cycle of tidal variations. We focus our attention on links of short-to-medium-range distances with antenna heights near-to-the-water-surface. The typical use-case for such a scenario is an overwater, water quality monitoring wireless sensor network. The radio propagation is simulated using a featured two-ray model that considers the relative permittivity of the water surface and the antenna polarization. The results show that the performance of overwater links may be better with lower antennas than higher antennas as well as with one polarization or the other, intuitively, during part of the tidal cycle.
... The influence of tides on the link quality is particularly noticeable near the shore and when one of the terminals does not keep a fixed height w.r.t. the water surface. The so-called tidal fading, i.e., the recurrent impact of tides on the mean path losses experienced in a link, is shown to be tremendously detrimental at specific combinations of link distance and antenna height [3], [4] as predicted by the tworay propagation model [5]. Conventional methods to counteract tidal fading often rely on classical diversity techniques, e.g., space-diversity or frequency-diversity , thus generally requiring additional communication resources (e.g., a second receiver hardware), which may not always be available or feasible. ...
Conference Paper
Full-text available
Wireless radio links deployed in aquatic areas (e.g., sea, rivers, lakes, estuaries) are affected by the conductive properties of the water surface, strengthening signal reflections and increasing destructive interference. Recurrent natural phenomena (e.g. tides or waves) cause shifts in water levels further impairing propagation over water surfaces. In this work, we aim to mitigate the detrimental impact of tides on link quality by providing tailored link distance/height-design regions that minimize average path losses. We focus on line-of-sight (LOS) over-water links between onshore stations and different types of surface nodes, namely AUVs, buoys or USVs, using 2.4 GHz and 5 GHz frequency bands. Analytical results targeting mission data transfer scenarios demonstrate that the proposed method outperforms, in both frequency bands, the common practice of placing (i) onshore antennas at the largest possible height and/or (ii) surface nodes at a short but arbitrary distance from the shore. A longer version of this summary was presented at IEEE/MTS OCEANS 2021.
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The prospect scenario for wireless communications and networking technologies in aquatic environments is nowadays promising. The growing interest around this subject in the last decades has recently been accelerated due to the more powerful capabilities of a number of sensing, control and communication devices. Moored, fixed, drifting, and vehicular nodes form now a rich ecosystem of autonomous embedded systems potentially connected in a multi-hop (and over-water) fashion, which demand innovative solutions to satisfy the ever-increasing requirements of reliability, bandwidth, latency and cost. The efforts in this direction, mostly as a result of the push from the Internet-of-Thing (IoT) and related communication paradigms, are now at an early stage, and thus still pose significant technical and research challenges from the perspective of communication and networking for applications involving real-time and/or multimedia network traffic. In this research, we focus on the communication and networking aspects of over-water multi-hop networks aiming at support real-time and/or multimedia (audio/video) traffic using IEEE 802.11 (WiFi) commodity technologies. Special attention is devoted to the impact of cyclic water-level variations (such as tides and waves) on the overall network performance, and how an integrated approach to (i) network design, (ii) protocol adaptation and (iii) routing can contribute to mitigating such an issue.
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Essentials of Radio Wave Propagation, chapter 4 Reflection, scatter and penetration
  • C Haslett
C Haslett. Essentials of Radio Wave Propagation, chapter 4 Reflection, scatter and penetration, pages 95–98. Cambridge University Press, 2008.