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Slow Frequency Hopping for Mitigating Tidal Fading on Rural Long Distance Over-Water Wireless Links
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.
... Despite some aspects of this phenomenon having been effectively described by the tworay model (e.g. [23]- [27]), further investigations on diverse environmental settings (e.g., estuaries and wetlands), using emerging communication technologies (e.g., LoRa) and/or incorporating precise tidal modeling methods, are still scarce. ...
... Traditionally, research reporting and/or mitigating tidal fading have typically focused on kilometric RF links [10], [11], [13], [23], [24], [28], often using antennas installed at several meters above surface. This is in contrast with the current trend in IoT-driven application scenarios (e.g., water-quality monitoring, flooding prevention, etc.), which often require (shorter) links at near-shore with antennas relatively close to the surface [29], [30]. ...
... This section revisits the two-ray propagation model from the perspective of shore-to-shore (S2S) and shore-to-vessel (S2V) RF communication links subject to variations in the water surface level. A similar analysis has been presented before in the literature [23]- [27], but using a simplified version of the two-ray model, and without considering the impact of the intertidal zone. Thus, this section also discusses the geometrical aspects determining if the reflection point (from the two-ray model viewpoint) falls within the intertidal zone, and how this situation may influence the modeling approach. ...
Low-power wide-area networks are extending beyond the conventional terrestrial domain. Coastal zones, rivers, wetlands, among others, are nowadays common deployment settings for Internet-of-Things nodes where communication technologies such as LoRa are becoming popular. In this article, we investigate large-scale fading dynamics of LoRa line-of-sight links deployed over an estuary with characteristic intertidal zones, considering both
shore-to-shore
and
shore-to-vessel
communications. We propose a novel methodology for path loss prediction which captures i) spatial, ii) temporal and iii) physical features of the RF signal interaction with the environmental dynamics, integrating those features into the two-ray propagation model. To this purpose, we resort to precise hydrodynamic modeling of the estuary, including the specific terrain profile (
bathymetry
) at the reflection point. These aspects are key to accounting for a reflecting surface of varying altitude and permittivity as a function of the tide. Experimental measurements using LoRa devices operating in the 868 MHz band show major trends on the received signal power in agreement with the methodology’s predictions.
... 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. ...
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. ...
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. ...
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. ...
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. ...
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.
... Overwater (shore-based) communications is impaired by a number of dynamic factors, such as tides [13] or waves [11]. Tides lead to variable antenna heights causing varying interference patterns between direct and (water) reflected paths resulting in considerable signal variations at the receiver. ...
Unmanned vehicles used in ocean science, defense operations and commercial activities collect large amounts of data that is further processed onshore. For real-time information exchange, the wireless link between the unmanned vehicle and onshore devices must be reliable. In this work, we empirically evaluate a WiFi link between an autonomous underwater vehicle on the surface and an onshore device under real-world conditions. This work allowed i) collecting a large-scale realistic dataset and ii) identifying major factors impairing communication in such scenarios. The TX-RX antenna alignment, the operation mode (manual vs automatic) and varying reflecting surface induced by AUV mobility lead to sudden changes (e.g. nulls) in the received signal strength that can be larger than 20 dB. This study provides useful insights to the design of robust vessel-to-shore short-range communications.
... 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. ...
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.
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.
From the perspective of several measurement campaigns in the offshore environment, it has been reported that the sea surface reflections are the main source of fading. We present a novel solution to this problem, by investigating the analytical implications of the propagation model which best fits the offshore channel characteristics. We also present a novel and yet simple implementation of receiver diversity which can mitigate the fading caused by sea surface reflections and ensure that the link is always steady even under extreme turbulent conditions.
Despite the increasing number of WiFi-based Long Distance (WiLD) network deployments, there is a lack of understanding of how WiLD networks perform in practice. In this paper, we perform a systematic study to investigate the commonly cited sources of packet loss induced by the wireless channel and by the 802.11 MAC protocol. The channel induced losses that we study are external WiFi, non-WiFi and multipath interference. The protocol induced losses that we study are protocol timeouts and the breakdown of CSMA over WiLD links. Our results are based on measurements performed on two real-world WiLD deployments and a wireless channel emulator. The two deployments allow us to compare measurements across rural and urban settings. The channel emulator allows us to study each source of packet loss in isolation in a controlled environment. Based on our experiments we observe that the presence of external WiFi interference leads to significant amount of packet loss in WiLD links. In addition to identifying the sources of packet loss, we analyze the loss variability across time. We also explore the solution space and propose a range of MAC and network layer adaptation algorithms to mitigate the channel and protocol induced losses. The key lessons from this study were also used in the design of a TDMA based MAC protocol for high performance long distance multihop wireless networks [12].
If you need to maximize efficiency in wireless network planning, this quick reference guide to an increasingly complex and difficult topic is for you. Using real-world case studies, practical problems and minimum mathematics, the author explains simply and clearly how to predict signal strengths in a variety of situations. More sophisticated methods, which form the basis of software tools for both network planning and spectrum management, are also described. This will be an invaluable resource for network planners, hardware designers, spectrum managers, senior technical managers and policy makers.
This report describes a computer method for predicting long-term median transmission loss over irregular terrain. The method is applicable for radio frequencies above 20 MHz and may be used either with detailed terrain profiles for actual paths or with profiles that are representative of median terrain characteristics for a given area. Estimates of variability in time and with location, and a method for computing service probability, are included.
Are you fully up-to-speed on today's modern spectrum management tools? As regulators move away from traditional spectrum management methods, introduce spectrum trading and consider opening up more spectrum to commons, do you understand the implications of these developments for your own networks? This is the first book to describe and evaluate modern spectrum management tools. Expert authors offer you unique insights into the technical, economic and management issues involved. Auctions, administrative pricing, trading, property rights and spectrum commons are all explained. A series of real-world case studies from around the world is used to highlight the strengths and weaknesses of the various approaches adopted by different regulators, and valuable lessons are drawn from these. This concise and authoritative resource is a must-have for telecom regulators, network planners, designers and technical managers at mobile and fixed operators and broadcasters, and academics involved in the technology and economics of radio spectrum.
Several research efforts as well as deployments have chosen IEEE 802.11 as a low-cost, long-distance access technology to bridge the digital divide. In this paper, we consider the important issue of planning such networks to the minimize system cost. This is a non-trivial task since it involves several sets of variables: the net- work topology, tower heights, antenna types to be used and their orientations, and radio transmit powers. The task is further com- plicated due to the presence of network performance constraints, and the inter-dependence among the variables. Our first contribu- tion in this paper is the formulation of this problem in terms of the variables, constraints and the optimization criterion. Our second contribution is in identifying the dependencies among the variables and breaking-down the problem into four tractable sub-parts. In this process, we extensively use domain knowledge to strike a bal- ance between tractability and practicality. We have evaluated the proposed algorithms using random in- put sets as well as real-life instances with success. We have been able to show detailed planning of network topology, required tower heights, antenna types, and transmit powers for the Ashwini project, a long distance WiFi network under deployment in Andhra Pradesh, India, In this case, we are able to achieve within 2% additional cost of a lower bound estimate.
Many rural and remote communities around the world see them- selves on the wrong side of the digital divide. In particular, there is evidence to suggest that there is a growing digital divide between urban and rural areas in terms of broadband Internet access with people living in rural areas having fewer choices and pay higher prices for slower speeds. This is true even in developed countries. Motivated by the above observations, there has been an increasing interest in deploying and researching low cost rural wireless net- works with active community participation. This paper presents an overview of our efforts in this direction in deploying a rural WiFi- based long distance mesh network testbed in the Scottish Highlands and Islands. We highlight the unique aspects of our testbed that differentiate it from other existing rural wireless testbeds. We also outline some of the research issues that are currently being investi- gated in this project.
Capacity improvement is one of the principal challenges in wireless networking. We present a link-layer protocol called Slotted Seeded Channel Hopping, or SSCH, that increases the capacity of an IEEE 802.11 network by utilizing frequency diversity. SSCH can be implemented in software over an IEEE 802.11-compliant wireless card. Each node using SSCH switches across channels in such a manner that nodes desiring to communicate overlap, while disjoint communications mostly do not overlap, and hence do not interfere with each other. To achieve this, SSCH uses a novel scheme for distributed rendezvous and synchronization. Simulation results show that SSCH significantly increases network capacity in several multi-hop and single-hop wireless networking scenarios.
This paper was published as Radio Science, 2007, 42, RS4019. It is available from http://www.agu.org/pubs/crossref/2007/2006RS003616.shtml. Copyright 2007 American Geophysical Union. Doi: 10.1029/2006RS003616 Measurements of signal strength are reported for a study of UHF propagation on three transhorizon sea paths in the British Channel Islands. Enhancements of up to 30 dB from the mean occur for periods of hours or days, especially in the summer, and constitute about 5% of the overall data. The probability distribution of received power is tabulated for the three paths and various antenna heights, and is compared with predictions of signal strength using ITU-R Recommendation P.1546-2. The difference between median and upper decile in the data is much less than predicted, whereas the difference between upper decile and the upper percentile is much greater than predicted.
Outdoor community mesh networks based on 802.11 have seen tremendous growth in the recent past. The current understanding is that wireless link performance in these settings in inherently unpredictable, due to multipath delay spread. Consequently, researchers have focused on developing intelligent routing techniques to achieve the best possible performance. In this paper, we are specifically interested in mesh networks in rural locations. We first present detailed measurements to show that the PHY layer in these settings is indeed stable and predictable. There is a strong correlation between the error rate and the received signal strength. We show that interference, and not multipath fading, is the primary cause of unpredictable performance. This is in sharp contrast with current widespread knowledge from prior studies. Furthermore, we corroborate our view with a fresh analysis of data presented in these prior studies. Based on our results, we argue that outdoor rural mesh networks can indeed be built with the link abstraction being valid. This has several design implications, and opens up a fresh perspective on a wide range of technical issues in this domain.
High-quality 2.4 GHz wireless LAN over long-range oversea paths was installed using a space diversity technique. The installed network consists of five wireless links, including a long-range 11.3 km link. To lower the adverse affect of fluctuation in the tidal level, two wireless LAN devices with different antenna heights were installed at one side of the longest link. The antenna heights were suitably determined on the basis of link profile and gained data at the deployment of the equipment. The measured performance indicates that the link with space diversity achieves 100% operating rate in a measured period of time, while the link without space diversity achieves an approximately 90% operating rate in the same time. This method effectively improves the quality of conventional wireless LAN systems.
This paper summarizes the results obtained from tests extending from 1943 to 1946 which were made to determine the meteorological factors controlling the propagation of centimeter waves. Oversea paths of 60 and 200 miles off the West Coast of Great Britain were used for the experiments. Continuous records of radio field strengths and frequent measurements of the meteorological conditions along the paths were made. The correlation between the radio results and the various meteorological parameters is studied in the light of current theories of microwave propagation.
The fluctuations of radio signals at microwave frequencies on overwater paths are explained on the basis of a periodic rise and fall of the water level. From this study, it is seen that the variations in the radio signal strength will contain the frequency of the water-level cycles and also the second and third harmonics of the water-level cycles. This same model predicts that the cross-correlation function of the fluctuations of the radio signal at two vertically-spaced antennas will drop from unity to zero as the separation distance is changed from zero to one-half of a lobe width of a height-gain interference pattern. Although the model assumes reflection from a plane surface, the results of the study successfully explain most of the features of the observed fluctuations of the radio signals on two overwater paths.
Microwave propagation experiments were performed on two separate line-of-sight paths across the Bay of Fundy on the east coast of Canada. The main objective was to investigate the mechanisms of anomalous propagation observed on such oversea paths. An interferometer which employs a wide frequency sweep of 1 GHz (from 9.5 to 10.5 GHz) was used. The parameters studied include the amplitude, angle-of-arrival (AOA), and relative delay time of the received rays. The behavior of these parameters during quiet and disturbed periods are compared and discussed in detail. Statistical results on the ray parameters and on the received signal level are also presented. Explanations for the various experimental observations are offered; these were arrived at by using a ray tracing method combined with refractivity profile modeling. It was found that the observed propagation anomalies result from interactions between specular reflection and atmospheric layer formations.
The results of line-of-sight over-sea propagation measurements over 37.2- and 35.5-km paths near Tokyo, Japan, are presented. Measurements of the fading (0.25, 3.99, 4.15, 6.72, and 18.0 GHz) of the cross polarization distortion (linear polarized wave at 4.15 GHz and circular polarized wave at 6.72 GHz) of the space diversity effect at 4.15 GHz, of the height-gain patterns at 6.72 GHz, and of the frequency-sweep patterns from 18 GHz to 22 GHz are included. The following statistical results are presented: the frequency, seasonal, and path dependence of fading; the effect of different fading occurrence mechanisms, such as K-type, duct-type, and distorted raindrops, for the cross polarization distortion; the diversity improvement factor; the effective reflection coefficient of sea surface; and the path-length difference between direct and sea reflected waves.
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.