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Beyond-Line-Of-Sight Communications with Ducting Layer DINC_LAYOUT_Layout 9/25/14 1:01 PM Page 37 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 per- cent 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]. IEEE Communications Magazine • October 2014 0163-6804/14/$25.00 © 2014 IEEE 37 DINC_LAYOUT_Layout 9/25/14 1:01 PM Page 38 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 curva- ture 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 atmospher- ic duct. where h is the height above sea level in kilome- ters, 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. 38 IEEE Communications Magazine • October 2014 DINC_LAYOUT_Layout 9/25/14 1:01 PM Page 39 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.
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