Beyond-Line-Of-Sight Communications with Ducting Layer

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MILITARY COMMUNICATIONS

Beyond-Line-of-Sight Communications with Ducting Layer

Ergin Dinc and Ozgur B. Akan

ABSTRACT cussed later [1]. Experimental studies also show that atmospheric ducts are nearly permanent in Near-surface wave propagation at microwave the coastal and maritime environments due to frequencies, especially 2 GHz and above, shows high evaporation rates. As a result, the ducting significant dependence on atmospheric ducts layer communication becomes the dominant that are the layer in which rapid decrease in the propagation mechanism at the lower tropo- refractive index occurs. The propagating signals sphere, especially between 2 and 20 GHz. Since in the atmospheric ducts are trapped between the transmitter and receiver antennas should be the ducting layer and the sea surface, so that the located within the ducting layer and this mecha- power of the propagating signals do not spread nism is effective at the lower troposphere, naval isotropically through the atmosphere. As a result, communications can especially utilize b-LoS these signals have low path loss and can travel communications with the ducting layer. over the horizon. Since atmospheric ducts are Ducting layer studies mainly focus on refrac- nearly permanent in maritime and coastal envi- tivity estimation techniques and radar path loss ronments, ducting layer communication is a calculations [1, 2]. However, there are a few promising method for b-LoS communications studies that focus on the utilization of the atmo- especially in naval communications. To this end, spheric ducts as a communication medium. To we overview the characteristics and the channel this end, [3] provides a high-data-rate employ- modeling approaches for ducting layer commu- ment of the evaporation ducts for b-LoS sensor nications by outlining possible open research networks, where continuous satellite communi- areas. In addition, we review the possible utiliza- cation is expensive and high data rate cellular tion of the ducting layer in network-centric oper- communication is unavailable. In [3], the ducting ations to empower decision making for the b-LoS layer is utilized to monitor a reef site in the operations. Great Barrier Reef of Australia. According to their results, the atmospheric ducts can connect INTRODUCTION a 78 km link at 10.5 GHz with 10 Mb/s 80 percent of the time. Based on the available experi- Atmospheric ducts that are caused by rapid mental results and our reviews, in particular, decrease in the refractive index of the lower state-of-the-art military systems can connect dis- atmosphere have tremendous effects on the tances up to 500–1000 km with the ducting layer. near-surface wave propagation. In the presence Modern naval b-LoS communications can uti- of atmospheric ducts, the signals below the lize either high frequency (HF) radio systems, atmospheric duct are trapped between the sea which are band-limited, to support high data surface and the duct. Therefore, the signals do rates; satellites, which are prone to hostile jam- not spread isotropically through the atmosphere. ming and have high transmission delays; or relay Instead, the signals spread mostly in the ducting nodes, which can be exposed to hostile attacks. layer, which is the area between the sea surface There is a significant need for a direct transmis- and the duct (Fig. 1). As a result, the spreading sion channel between b-LoS units to empower loss decreases considerably compared to the strategical and tactical decisions in network-cen- standard atmosphere, and there is a significant tric operations (NCO). To this end, the ducting amount of increase in the received signal power. layer is a promising communication medium for In this way, the trapped signals can travel over b-LoS applications because atmospheric ducts the horizon, making the ducting layer convenient can provide a direct communication channel for beyond-line-of-sight (b-LoS) communica- Navy–Navy and Navy–low flying units (especially tions. unmanned aerial vehicles, UAVs) thanks to their The formation of atmospheric ducts entirely low spreading loss. In this way, a naval ship can depends on the refractivity of the lower atmo- relay information coming from LoS sources to a sphere, which is determined by atmospheric b-LoS unit within the ducting layer. Thus, the parameters such as wind, pressure, temperature, ducting layer can provide a b-LoS link between The authors are with Koc and, most important, humidity, the key and sites with low transmission delays and low proba- University. essential factor in duct formation, as further dis- bility of detection and interception [4].

REFRACTIVE INDEX, REFRACTIVITY, AND MODIFIED REFRACTIVITY Tropospheric radio refractive index (n) relies on weather conditions: wind, temperature, pressure, and humidity at the lower atmosphere. Refrac- tive index changes slightly with height. It varies between 1.000250 and 1.000400. Therefore, instead of the tropospheric radio refractive index, refractivity, which is the scaled version of Sea the tropospheric radio refractive index, is used, (a) and the refractivity (N) is defined as [5] N = (n – 1) × 106 N-units, (1)

Ducting layer where n is the atmospheric refractive index. Equation 1 is an idealized refractivity expres- sion in which the Earth surface is assumed to be flat, but to be more realistic the Earth’s curvature and the variation of height are considered in the modified refractivity (M) [5], Sea ⎛ h⎞ 6 MN=+⎜ ⎟ × 10 , (b) ⎝ a⎠ (2) =+Nh157 × M-units, Figure 1. Signal spreading in a) the standard atmosphere; b) the atmospheric duct. where h is the height above sea level in kilometers, and a is the radius of the Earth in kilometers. Tropospheric wave propagation depends on The main contribution of this work is an the rate of change in the modified refractivity overview of the characteristics of ducting layer b- that is associated with the refractivity gradient LoS communications and the possible utilization (∂M/∂h). There are four different refractive con- of the ducting layer in b-LoS naval communica- ditions according to the refractivity gradient, as tion networks. To this end, we review the chan- shown in Fig. 2. In the sub-refraction condition, nel modeling techniques and software tools that the signal is bent away from the surface. The can be used for the ducting layer. We present a standard refractivity condition is associated with preliminary analysis to show that ducting layer the wave propagation in the standard atmo- can achieve lower path-loss up to 20 dB at 500 sphere. The super-refraction condition causes km compared to the free space. In addition, we the signal to propagate downward at a rate that make a linear regression fitting for the path loss is larger than the standard condition and smaller curve to find the path loss exponent of the chan- than the curvature of the Earth. nel. To the best of our knowledge, there is no The last and most important one is the trap- study that considers the path loss exponent for ping condition in which the signal is refracted the ducting layer. Therefore, this study provides back to the Earth’s surface. In this way, the both the characteristics of the ducting layer and refracted rays are trapped between surface or the possible application areas by outlining the sea and ducting layer. The trapping condition is open research issues in the field. associated with a negative modified refractivity The remaining information in this article is gradient and duct height is given by the height organized as follows. First, we provide back- where the gradient of the modified refractivity ground information about refractivity and differ- changes from negative to positive, see Fig. 3. ent types of atmospheric ducts. In addition, the The refractivity gradient and the duct height effects of ducts on wireless communication are are the most important parameters for modeling discussed. After that, the channel modeling atmospheric ducts. The characteristics of atmo- approaches and software tools are reviewed, and spheric ducts can be determined via the atmo- a preliminary analysis investigating the path loss spheric parameters by using radiosonde or bulk exponent of the ducting layer is presented. Next, atmospheric measurements. In addition, indirect open research issues on ducting layer channel methods are also available such as refractivity modeling are also pointed out. Lastly, the possi- from clutter techniques which estimates the ble application areas of ducting layer b-LoS refractivity of the lower atmosphere from the communications are pointed out. clutter signal returns [2].

FORMATION AND CHARACTERISTICS OF DUCTS PROPERTIES OF The duct formation is an atmospheric phe- TMOSPHERIC UCTS nomenon. Humidity, air-sea temperature, pres- A D sure and wind play significant roles in the duct The formation and the effects of atmospheric formation. Among all the atmospheric condi- ducts entirely depend on the refractivity of the tions, humidity is the key and essential factor in lower atmosphere. This section provides back- the process. As a result, duct formation is more ground on refractivity, the formation of ducts, probable in regions with high evaporation rates and effects of ducts on wireless communications. such as equatorial and tropical regions.

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The rapid humidity decrease in the few ∂ ∂ meters of the lower troposphere causes the duct Sub-refraction M/ z > 157 formation. In addition, air-sea temperature is the most important contributing factor to the Standard 157 ≥ ∂M/∂z > 78 process. When the air is warmer than the Earth or sea surface, the temperature inversion occurs, Super-refraction 78 ≥ ∂M/∂z > 0 and it enhances the duct formation. Tempera- Ducting layer ture inversion is caused by advection of dry and warm air mass or cooling of land when the air is Trapping 0 ≥ ∂M/∂z still warm at nights. In addition, negative air-sea temperature difference can prevent the duct formation. This weather condition is referred to as unstable. Factors such as rough terrain, high winds, and cold, stormy, rainy, and cloudy conditions also have destructive effects on duct forma- Figure 2. Rays under various refractive conditions. tion [1]. According to their physical formation process, there are three main types of atmospheric EFFECTS OF DUCTS ON WAVE PROPAGATION ducts. Besides the path loss enhancements, atmospheric ducts also introduce some challenges in b-LoS Evaporation Duct — Humidity difference in wireless communications. The main challenges the air-sea boundary causes the formation of introduced by atmospheric ducts and the tech- evaporation ducts. High evaporation rates are niques to overcome these challenges are out- essential for evaporation ducts. Evaporation duct lined below. is the most common type of duct formation, and it presents up to 90 percent of the time in equa- Inter-Symbol Interference — Atmospheric torial and tropical regions. The M profile of the ducts trap the propagating signals at the lower evaporation duct is modeled with the logarithmic atmosphere, causing signals to follow different curve in Fig. 3a [6]. Evaporation duct height is paths. The trapped signals reach the receiver mostly between 10–20 m, and it rarely reaches with different delay spreads and phase compo- up to 40 m. nents. Therefore, these trapped signals can cre- Evaporation ducts are the most promising ate destructive/constructive interference at the duct formation with the potential to be utilized receiver. The receiver should optimally combine in the wireless communication applications these components by using adaptive antenna thanks to their high availability rates. Since techniques to prevent interference. To this end, evaporation ducts are nearly permanent in optimal transmit and receive beam patterns coastal and maritime environments, b-LoS com- should be designed according to the characteris- munications with evaporation ducts can be tics of the atmospheric ducts; these techniques employed specifically for naval communications. provide both diversity and array gain along with interference suppression as recommended in [4]. Surface-Based Duct — Surface-based ducts are mostly caused by advection of warm and dry Unexpected Interference — Up to now, we air over the ocean or land, which creates humidi- have considered both the transmitter and receiver ty and temperature inversion. Surface-based being located within the duct. On the other hand, ducts can be modeled with a trilinear curve in the effects of ducts are prominent even if one of which the negative sloped part represents the the antennas stays within the duct, such as in satel- trapping layer illustrated in Fig. 3b [2], and the lite communications and air-to-ground links in duct height can reach hundreds of meters. Sur- maritime and coastal environments. When satellite face-based ducts occur up to 40 percent of the signals have low elevation angles, these signals can time depending on the season and location; be trapped by atmospheric ducts, and the trapped these statistics can be found in [7]. signals may be received by the receiver with differ- Elevated Duct — The presence of marine lay- ent delay spreads and angles of arrival (AOAs). ers results in the formation of elevated ducts. As a result, the receiver may misinterpret the The physical mechanism that creates elevated AOA of an incoming signal that is refracted by the ducts is similar to that for surface-based ducts. ducting layer, and the surveillance system can be However, in the elevated ducts the lower bound- adversely affected by this effect. ary of the trapping layer is above the surface. The trapped signals in ducts follow longer Elevated ducts occur at relatively greater heights paths and reach the receiver with higher delays, as of 600 up to 3000 m, and can also be modeled experimentally measured in [8]. Therefore, even in with the trilinear curve in Fig. 3c [2]. In particu- LoS ranges, this effect can create multipath fading lar, air-to-ground links are prone to the trapping and change the pattern of the interference in effect of elevated ducts because trapped signals naval systems. As a result, orthogonal frequency- follow longer paths than expected, and can reach division multiplexing (OFDM) can be promising the receiver with higher delay spreads and differ- for the ducting layer if the channel creates high ent angles than expected. Therefore, elevated delay spreads that make the coherence bandwidth ducts have the potential to create unexpected much lower than the bandwidth of the signal. interference in the air-to-ground links. The occurrence rate of the elevated ducts can be up Sea Surface Roughness — The trapped sig- to 50 percent according to season and location; nals in the ducting layer are in contact with the these statistics can also be found in [7]. sea surface. Especially under strong winds, open

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nique to model the ducting layer wave propaga- a. Evaporation duct b. Surface-based duct c. Elevated duct tion with the capabilities of including both complex boundary conditions and refractivity variations of the lower atmosphere [10]. The existing numerical methods can provide fast and accurate results for PE. For these reasons, PE

Height Height Height methods can be used especially for estimating the large-scale path loss in the ducting layer under different refractivity conditions, polariza- Trapping layer Trapping Trapping layer Trapping layer Trapping tion, and surface types. Modified refractivity M Modified refractivity M Modified refractivity M The remainder of this section includes the numerical methods to solve PE, available soft- Figure 3. Modified refractivity models for duct types. ware tools, and preliminary simulation results to show the signal level enhancement due to the atmospheric duct formation. sea surfaces form rough surface boundaries. As a result, the reflection from a rough sea surface Numerical Methods for PE — PE can be makes signals follow different paths; even some solved by numerical operations. There are of the transmitted signals are backscattered. three popular numerical methods for solving Therefore, sea surface roughness causes a PE: split step Fourier (SSF), finite difference decrease in the received power strength, and (FD), and finite element (FE) [11]. The choice modeling the rough surface boundary is an between these methods strongly depends on important challenge in ducting layer propagation the scenario and conditions. The SSF method modeling. There are some experimental studies takes advantage of fast Fourier transform that investigate the effects of rough sea surfaces (FFT) algorithms. Therefore, the implementa- on the wave propagation within ducts [9], but tion of PE with SSF is computationally effi- these experimental works do not provide enough cient compared to other methods, and the data to model extreme surface conditions, and technique can yield accurate and stable solu- there are no more than a few theoretical models tions. Second, FD provides the highest resolu- for sea surface roughness in the presence of tion in modeling the boundary conditions by atmospheric ducts. To this end, this topic utilizing the Crank-Nicholson finite difference requires both experimental and theoretical scheme. Lastly, FE provides better modeling of research attention. the fast variations in the atmospheric conditions and flexibility to model the complex Aerosols and Weather Conditions — The boundary conditions. A good review of these presence of aerosols such as bacteria, pollen, and techniques can be found in [11]. dust are highly probable in the lower atmosphere. The presence of such aerosols in the communica- Wave Propagation Tools — Wave propagation layer may create scattering and reflection, tion tools are available that can be used to which cause radar echoes and path loss increase model the ducting layer. The most important at the receiver side. In addition, hydro-meteors in and widely used one is AREPS, developed by the lower atmosphere, especially rain, can create the Atmospheric Propagation Branch at the significant path loss due to its reflective effects Space and Naval Warfare Systems Center, San between 10 and 20 GHz. To this end, rain and Diego, California [12]. AREPS is a hybrid wave aerosol attenuation in the ducting layer is a propagation tool that combines ray optics and promising open research field as well. PE with SSF methods. It is capable of taking the refractivity index as input from various sources like satellite, meteorological, and bulk CHANNEL MODELING TECHNIQUES measurements. In addition, PETOOL, a free FOR TMOSPHERIC UCTS tool available online, can model both forward A D and backward scattered waves under different Ducting layer wave propagation is complex due terrain and refractivity conditions [13]. Since to the trapping effect because the trapped sig- PETOOL also considers backward scattered nals are in contact with both the rough sea sur- waves, unlike AREPS, it can be used to analyze face and the duct layer. In addition to these regions with terrain blocks between the trans- factors, the ducting layer wave propagation mitter and the receiver such as islands. depends on multiple channel parameters such as PETOOL is calibrated using ray-based methods antenna heights, duct height and type, carrier and AREPS as described in [13]. Since frequency, and polarization. In this section, we PETOOL can give reliable results like AREPS review the channel model approaches to model- and is a free tool, researchers may prefer ing ducting layer communication by outlining PETOOL to model the wave propagation in the open research fields. More important, we pre- ducting layer. Furthermore, TEMPER, devel- sent preliminary simulation results for analyzing oped by the John Hopkins University Applied the path loss exponent to indicate the received Physics Laboratory, is especially preferred for signal level improvements: R&D applications such as high fidelity radar design and 3D wave modeling applications. PARABOLIC EQUATION METHODS TEMPER is solely based on the PE method A parabolic equation (PE) method utilizes [14]. Lastly, Wireless InSite, developed by REM- parabolic approximation to Helmholtz wave COM, is a hybrid model and utilizes the fre- equation. This technique is the dominant tech- quency distributed (FD) time domain method in

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20 170 2.4 GHz Free space 18 5 GHz With duct 10.5 GHz 160 Regression line 16 15 GHz 20 GHz 150 14 140 12

10 130

8 Path loss (dB) Path 120

Receiver height (m) 6 110 4 100 2

0 90 -210-220 -200 -190 -180 -170 -160 -150 -140 -130 103 104 105 Path loss (dB) Range (m) (a) (b)

Figure 4. Path loss variations with respect to receiver height and range in 13 m ideal evaporation duct: a) receiver height vs. path loss; b) path loss vs. range.

order to solve PE. Although this tool is not than 2 (free space) because the signal energy is widely used in ducting layer wave propagation not spread through the atmosphere. Instead, it is modeling, it is a promising modeling tool for concentrated within the ducting layer. The sim- projects that require modeling of complex ple linear regression analysis with Eq. 3 yields a boundary conditions. path loss exponent of 1.2087 for an ideal duct and a flat sea surface (black line in Fig. 4b). We Path Loss Analysis for Evaporation Duct — expect that the path loss exponent will be higher In this section, we present our preliminary analy- for real-world conditions with range-dependent sis to show the received power enhancement refractivity conditions and rough sea surfaces. with atmospheric ducts. Since evaporation ducts Although several works are available to model are more probable compared to other types of the amount of path loss within the atmospheric ducts, we analyze an ideal 13 m evaporation duct ducts, there are no studies that provide the sta- modeled with a logarithmic curve as in [6]. We tistical modeling of the path loss exponent and utilize PETOOL to solve PE for our preliminary shadow fading with respect to the atmospheric large-scale path loss analysis. conditions, sea surface roughness, duct height, Large-scale path loss in wireless communica- antenna heights, and carrier frequency in duct- tions can be represented with the generic formu- ing layer communication. In addition, the effects la of of the polarization and correlation between antennas are open research fields that can be PL = A + 10glog d/d0 + Xs,(3)solved by PE simulations.

AY PTICS ETHODS where g is the path-loss exponent, d0 is the b- R O M LoS range, A is the path loss at d0, and Xs is the Ducting layer wave propagation modeling is shadow fading. dominated by PE because ray analyses introduce Ducting conditions show significant depen- significant errors at high ranges and are much dence on the carrier frequency because the slower. However, ray optics methods are still trapping condition becomes dominant above a used in hybrid models in order to provide fast certain frequency [5]. To this end, we simulate and reliable estimates of the behavior of the the wave propagation within a 13 m evapora- wave-front and propagation angles [10]. tion duct for different carrier frequencies (the Ray analyses utilize the solution for the transmitter is 4 m above sea level). Figure 4a Eikonel equation under nonstandard refractivity represents the receiver height vs. path loss at 80 conditions, and they can give the approximate km distance. As noticed from this figure, 10.5 behavior of the delay spreads and AOA of the GHz can reach the lowest path loss values; sim- multipath components. Since PE is inefficient in ilar experimentally validated results can be modeling these parameters, which are essential found in [3]. Therefore, the signal level to channel modeling, ray optics methods should enhancements become highest in this high fre- be used in ducting layer studies. There are some quency range. experimental studies that focus on AOA in the To show the signal level enhancement with presence of the atmospheric ducts [15]. In [15], respect to the free space, we present path loss they also estimate AOA for the ducting layer by vs. range at 10.5 GHz for the transmitter and using both ray optics and PE methods. However, receiver that are located 4 m above sea level. As the distribution of delay spreads and AOAs in in Fig. 4b, path loss in the ducting layer is signif- the ducting layer is not well studied, and requires icantly lower than in free space. Therefore, we significant experimental and theoretical research can expect the path loss exponent to be smaller efforts. In addition, angles of departure (AODs)

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munications is a promising alternative because the communication at the lower troposphere is less prone to be affected by hostile jamming; also, utilization of the ducting layer eliminates Radar hole the need for an air unit as a relay node. The ducting layer is expected to employ nearly 10 Altitude GHz as in the previous section, and with the Ducting layer error signal level enhancements, will be able to provide the high data rates that modern military applications require. In NCO, the connectivity between units is critical to empower strategical and tactical decisions. With the employment of atmospheric ducts, there will be a direct communication channel between navy-navy and navy-low flying Sea units (especially UAVs). Therefore, the employment of ad hoc networking with the b-LoS ducting layer can provide end-to-end connectivity in naval warfare. For example, a naval ship can Figure 6. Wave propagation with a ducting layer. relay information coming from a LoS source to a b-LoS naval ship. To this end, ad hoc b-LoS networks with atmospheric ducts should be should also be studied for the ducting layer. designed, and the distribution of data rates and Since only signals with low grazing angles can be relay delays should be derived for the ducting trapped by the ducting layer, most of the trans- layer b-LoS systems. Since there are only a few mitted power can spread through the atmo- works using atmospheric ducts as a communica- sphere instead of the ducting layer if the tion medium, ad hoc networking with the duct- beamwidth of the antenna exceeds a certain ing layer is a promising open research field to value, which depends on the antenna placement improve naval NCOs. and atmospheric conditions. To prevent signal spreading through the atmosphere instead of the RADAR APPLICATIONS ducting layer, the maximum beamwidth that can Radar coverage is strongly affected by ducts in be trapped by the atmospheric ducts should be coastal and maritime environments [2]. Without derived with respect to transmitter antenna considering the effects of ducts, the coverage can height, duct height, and carrier frequency by uti- be miscalculated, and miscalculations impose lizing the ray optics methods. unwanted problems like radar holes and altitude errors, as shown in Fig. 5. Since the military radar applications require precise and accurate APPLICATION AREAS OF estimations, the effects of ducts should be taken UCTING AYER into account in coverage estimations. Therefore, D L ducting layer studies also contribute to the radar B-LOS COMMUNICATION coverage analysis as well.

This section provides answers on how the usage WIRELESS SENSOR NETWORKS of atmospheric ducts as a propagation medium Ducting layer b-LoS communications can be can improve current state-of-the-art naval com- used to monitor civil or military sensor networks munication links. To this end, we review the pos- in maritime and coastal areas as in [3] because sible application areas of ducting layer b-LoS the implementation of satellite or cellular com- communications by focusing on military naval munication are expensive in large maritime applications. areas. As a result, the ducting layer can provide low-cost high data rate monitoring for maritime MILITARY APPLICATIONS wireless sensor networks. In current naval b-LoS communications, the transmitter and receiver can use high frequen- ONCLUSION cy (HF) radio systems, or satellites or air units C as a relay node to deliver data packages. How- Ducting layer b-LoS communication is a promis- ever, HF radio systems are band-limited by ing communication method, especially for the nature and inadequate to provide the high data military naval applications to empower tactical rates modern military communications require. decisions in NCOs. Since the ducting layer b- The other often used option is satellite com- LoS communications show strong dependence munications, which is expensive and introduces on the refractive index, which is highly affected high communication delays. In addition, satel- by the atmospheric conditions, well developed lite communication has security problems in channel models are required to determine the modern warfare because it can be strongly characteristics of the channel. To this end, this affected by hostile jamming. Furthermore, article reviews the channel modeling approaches using an air unit as a relay node is dangerous by outlining the open research areas and possi- because the unit can be exposed to hostile ble application areas of ducting layer b-LoS sys- attacks, and the replacement of a relay node tems. More important, this work also provides can waste a considerable amount of mission the path loss exponent for an ideal evaporation time [4]. To this end, ducting layer b-LoS com- duct.

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REFERENCES [13] O. Ozgun et al., “PETOOL: MATLAB-Based One-Way and Two-Way Split-Step Parabolic Equation Tool for Radiowave [1] C. Yardim, Statistical Estimation and Tracking of Refrac- Propagation over Variable Terrain,” Computer Physics Com- Since the ducting tivity from Radar Clutter, Ph.D. dissertation, Univ. CA mun., vol. 182, issue 12, Dec. 2011, pp. 2638–54. layer b-LoS commu- San Diego, Mar. 2007. [14] G. D. Dockery et al., “An Overview of Recent Advances [2] H. V. Hitney et al., “Tropospheric Radio Propagation for the TEMPER Radar Propagation Model,” Proc. IEEE nications show Assessment,” Proc. IEEE, vol. 73, no. 2, Feb. 1985, pp. Radar Conf., Apr. 2007, pp. 896–905. 265–83. [15] R. Akbarpour and A. R. Webster, “Ray-Tracing and strong dependence [3] G. S. Woods et al., “High-Capacity, Long-Range, Over Parabolic Equation Methods in the Modeling of a Tro- Ocean Microwave Link Using the Evaporation Duct,” IEEE J. pospheric Microwave Link,” IEEE Trans. Antennas Prop- on the refractive Oceanic Eng., vol. 34, no. 3, July 2009, pp. 323–30. agation, vol. 53, no. 11, Nov. 2005, pp. 3785–91. index, which is highly [4] M. J. Luddy, J. H. Winters, and A. Lackpour, “Beyond Line- of-Sight Communications with Smart Antennas (BLoSSA),” BIOGRAPHIES affected by the 2011; http://www.atl.lmco.com/papers/1978.pdf. [5] B. R. Bean and E. J. Dutton, Radio Meteorology, Dover, ERGIN DINC [S’12] ([email protected]) received his B.Sc. degree atmospheric condi- 1966. in electrical and electronics engineering from Bogazici Uni- [6] R. A. Paulus, “Evaporation Duct Effects on Sea Clutter,” versity, Istanbul, Turkey, in 2012. He is currently a research tions, well-developed IEEE Trans. Antennas Propagation, vol. 38, no. 11, Nov. assistant at the Next-Generation and Wireless Communica- channel models are 1990, pp. 1765–71. tions Laboratory (NWCL) and pursuing his Ph.D. degree at [7] ITU-R Rec. P. 453-10, “The Radio Refractive Index: Its the Electrical and Electronics Engineering Department, Koc required to deter- Formula and Refractivity Data,” Feb. 2012. University, Istanbul, Turkey. His current research interests [8] Y. S. Meng and Y.-H. Lee, “Measurements and Charac- include communication theory, and beyond-line-of-sight mine the characteris- terizations of Air-to-Ground Channel over Sea Surface communications with troposcatter and atmospheric ducts. at C-Band With Low Airborne Altitudes,” IEEE Trans. tics of the channel. Vehic. Tech., vol. 60, no. 4, May 2011, pp. 1943–48. OZGUR B. AKAN [M’00-SM’07] ([email protected]) received his [9] K. Anderson et al., “The RED Experiment: An Assess- Ph.D. degree in electrical and computer engineering from ment of Boundary Layer Effects in a Trade Winds the Broadband and Wireless Networking Laboratory, Regime on Microwave and Infrared Propagation over School of Electrical and Computer Engineering, Georgia the Sea,” Bull. Amer. Meteor. Soc., vol. 85, Sept. 2004, Institute of Technology in 2004. He is currently a full pro- pp. 1355–65. fessor with the Department of Electrical and Electronics [10] M. Levy, “Parabolic Equation Methods for Electromag- Engineering, Koc University, and director of the NWCL. His netic Wave Propagation,” IEE, 2000. current research interests are in wireless communications, [11] I. Sirkova, “Brief Review on PE Method Application to nano-scale and molecular communications, and informa- Propagation Channel Modeling in Sea Environment” tion theory. He is an Associate Editor of IEEE Transactions Central Euro. J. Eng., vol. 2, issue 1, 2012, pp. 19–38. on Communications, IEEE Transactions on Vehicular Tech- [12] User’s Manual for Advanced Refractive Effects Predic- nology, the International Journal of Communication Sys- tion System (AREPS), Space and Naval Warfare Systems tems (Wiley), the Nano Communication Networks Journal Center, San Diego, CA, 2004, pp. 1–7. (Elsevier), and European Transactions on Technology.

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