Received July 9, 2020, accepted July 30, 2020, date of publication August 4, 2020, date of current version August 14, 2020.

Digital Object Identifier 10.1109/ACCESS.2020.3014119

Clustered Jamming in Aerial HetNets WithDecoupled AccessMOHAMMAD ARIF 1, SHURJEEL WYNE 1, (Senior Member, IEEE),KEIVAN NAVAIE 2, (Senior Member, IEEE),MUHAMMAD SAJID HAROON 3, (Graduate Student Member, IEEE),AND SADIA QURESHI41Department of Electrical and Computer Engineering, COMSATS University Islamabad (CUI), Islamabad 45550, Pakistan2School of Computing and Communications, Lancaster University, Lancaster LA1 4WA, U.K.3Telecommunications and Networking (TeleCoN) Research Laboratory, GIK Institute of Engineering Sciences and Technology, Topi 23640, Pakistan4School of Data and Electrical Engineering, University of Technology Sydney, Sydney, NSW 2007, Australia

Corresponding author: Mohammad Arif (mohammadarif911@gmail.com)

This work was supported in part by the EU-funded project ATOM-690750 under Grant H2020-MSCA-RISE-2015, and in part by the U.K.Engineering and Physical Sciences Research Council (EPSRC) under Grant EP/S009620/1.

ABSTRACT The tremendous increase in wireless connectivity demand will result in the degradation ofthe service quality and the scarcity of network capacity and coverage in the beyond 5th generation era.To ensure reliable connectivity and enhance the network’s performance, the evolution of heterogeneousnetworks (HetNets) must incorporate aerial platforms in addition to traditional terrestrial base stations. Theperformance of Aerial-HetNets (A-HetNets) is largely dependent on the users’ association. The conventionaluser-association scheme based on downlink received power provides sub-optimal performance for theedge users. For this reason, decoupled user-association along with the reverse frequency allocation (RFA)strategy has been employed in A-HetNets. The performance of A-HetNets is also affected if wide-bandjammers (WBJs) are present in the vicinity and impose jamming interference. In this paper, a two-tierA-HetNet with RFA and decoupled access is analyzed in the presence of jamming interference. The obtainedresults show that for a signal-to-interference ratio threshold of −20 dBm, the percentage decrease in thecoverage probability of the decoupled access due to WBJ activity is up to 7.4%, 13.5%, and 19.7%, forthe average number of WBJs equal to 2, 4, and 6, respectively. The performance of the decoupled accessin A-HetNets is further decreased by increasing the transmit power of the WBJs while it is increased byincreasing the radius of the WBJ’s cluster.

INDEX TERMS Downlink and uplink decoupling, heterogeneous networks, unmanned aerial vehicles,wide-band jammers.

I. INTRODUCTIONThe evolution of 5th generation (5G) of wireless networkscan increase the data demands, service types, and scale ofwireless networks. It is estimated that the mobile-user andthe machine-type subscribers would reach 17.1bn and 97bn,respectively, by the year 2030 [1]. Such massive increase willlead to the degradation in the quality of service (QoS) andscarcity of network capacity in the beyond 5G era [2]. Thus,to design networks in a cost-efficient manner, the evolutionof base station (BS) coverage cells from macro- to femto-and pico-cells and from terrestrial-fixed to aerial-vehicular is

The associate editor coordinating the review of this manuscript and

approving it for publication was Donatella Darsena .

indispensable. The aerial networks can use network resourcesmore efficiently by creating favorable channel conditionsor by extending the network’s coverage for the target userequipment (UE) to meet the data demands for events, suchas concerts, sports, disasters, and accidents.

A. MOTIVATION AND RELATED WORKTomaintain the QoS in ultra-high data demand environments,a promising solution is to deploy unmanned aerial vehicles(UAVs) [3]–[5]. The UAV-based BSs can provide the line-of-sight link to the target UE in aerial heterogeneous net-works (A-HetNets). The appropriate deployment of UAVsencompasses numerous characterizations depending on theirweight, trasnmit power, and size [6], [7]. The UAVs with the

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

functionalities such as low transmit power, less height, lessendurance capability, and limited battery are usually consid-ered as low-altitude platforms (LAPs). While, the UAVs withextended transmission power, higher altitude, and extensiveendurance capability are considered as high-altitude plat-forms (HAPs).

The performance of HetNets is largely affected, if jam-mers are present in the vicinity and disrupt the legitimatetransmission [8]–[10]. This performance is more vulnerableto jamming, if aerial BSs are present in the network. Thetarget locations for jamming in A-HetNets are organizations,military-bases, air-ports, etc., where the jamming signal istypically introduced using wide-band jammer (WBJ). Themain goal of the WBJs is to jam the legitimate uplink (UL)communication with unwanted energy (i.e., jamming inter-ference) as effectively as possible. The transmission powerof the WBJs is limited due to its wide-band nature, therefore,several WBJs (mostly present in clusters) are necessary to belocated in the vicinity of the target [11].

The distribution of the WBJs in a cluster is consideredmore effective than the non-clustered distribution because ithelps the jammers to obtain targets’ network parameters suchas frequency, location, and transmission power [11], [12].Since the jammers are present in clusters, therefore, theycan be modeled using a Matern cluster process (MCP). Theauthors in [13]–[16], modeled the A-HetNets using an MCPand showed that the design and performance gains of the net-work can be efficiently estimated by modeling the clusteringprocess with an MCP.

Besides, the distribution of WBJs, user-association withthe UAV also plays an important role in characterizingA-HetNets. Traditionally, the performance of A-HetNets isanalyzed with the assumption that the target user is con-nected in downlink (DL) and UL with the same HAP byconsidering DL received power. However, this performanceis sub-optimal for the edge users (e.g., users located at theboundary of a certain cell) which are associated to the HAPwhile, exhibiting a favorable channel condition to the LAP.Therefore, it is important to consider the user-associationby exploiting the joint DL and UL access. To address thisissue, decoupled access for A-HetNets is proposed, wherethe target UE is allowed to associate with the multiple UAVsbased on joint DL and UL association [17]. In the decoupledaccsss for A-HetNets, the target user connects to HAP inDL while to LAP in UL. Furthermore, in [18]–[20], theauthors showed that in HetNets, the edge users can leveragebetter gains in terms of coverage probability, spectral- andenergy-efficiency, if decoupled access is allowed. Neverthe-less, in [20], the spectral efficiency performance for the tra-ditional cellular networks under decoupled access in terms ofFox H-function is analyzed.1 Therein, decoupled access fora multi-tier HetNet has shown improvement in performancegains over the the non-decoupled access.

1Fox H-function enables the analysis of the complex and infinite integrals.The Fox H-function is explained in terms of its implementation in [21].

An effective resource allocation strategy is reverse fre-quency allocation (RFA), whereby the HAP and LAP canuse frequency resources in a reverse manner for DL andUL to attain better performance gains. The authors in [22],[23] showed that a multi-tier HetNet employing RFA strategyperforms better than amulti-tier HetNet without RFA in termsof coverage. In this paper, we use RFA along with the decou-pled access to improve networks’ coverage performance byabating the inter-tier interference of multiple UAVs and thejamming interference of the WBJs.

Without decoupling, [24] analyzed a multi-tier A-HetNetwhich is similar to the traditional cellular HetNet. Therein,the HAPs and the LAPs are modeled using an indepen-dent and homogeneous Poisson point process (HPPP) andderived the analytical expression of the outage probabilitywithout considering RFA and decoupled access. Whereas,in [11], [25], the authors analyzed the coverage probabilityin conjunction with RFA, jammers, and decoupled access.This performance is limited due to the assumption that thesame pathloss exponent is considered across all the tiers of amulti-tier cellular HetNet. Furthermore, the authors in [11]assumed that the jammers are always present in a singlecircular cluster around the target UE which is not a realisticapproach as the jammers can be located in multiple-clustersand with different centers.

Our investigation is different from the recent works in thefollowing.

• The authors in [24] present the analysis of a two-tierA-HetNet without considering the decoupled access.Further, their work lacks the analysis of RFA strategyin the presence of jammers.

• The authors in [17] investigate the performance of thedecoupled access for a two-tier A-HetNet. However,their work lacks the joint analysis of RFA and jammers.

• The works in [11], [12], [25] analyzes HetNets byassuming the same pathloss exponents across the multi-ple tiers of a HetNet which is unrealistic and an oversim-plified approach. In contrast, we employ amulti pathlossexponent model for multiple tiers of A-HetNets.

• The work in [25] investigates RFA, WBJs, and decou-pled access for a conventional cellular HetNet. However,we employ RFA, WBJs and decoupled access for inves-tigating a multi-tier A-HetNet.

B. MAIN CONTRIBUTIONSIn this paper, a multi-tier A-HetNet is analyzed by employingdecoupled access in conjunction with RFA and WBJs. TheWBJs are assumed to be present in multiple-clusters in theconsidered network and the decoupled access is employed byconsidering different pathloss exponents across a multi-tierA-HetNet. The summary of novel contributions is as follow:

• Investigating the UL coverage performance for the edgeusers in the presence of RFA and decoupled access.

• Mitigating the effect of WBJs and RFA on the perfor-mance of multi-tier A-HetNets with decoupled access.

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

• The analytical expression of the coverage probability inconjunction with RFA, WBJs, and decoupled access isderived for multi-tier A-HetNets.

• The performance of multi-tier A-HetNets is analyzedagainst network parameters, such as the transmit powerof the LAP-associated UE, the transmit power of theWBJs, the radius of the WBJs cluster, and the transmitpower of the LAPs.

The validity of the proposed analytical framework is estab-lished by conducting extensive simulations. The conductedanalysis of the decoupled access with RFA stressed to WBJsin A-HetNets indicates a significant performance improve-ment when compared to the non-decoupled access in termsof coverage.

C. PAPER ORGANIZATIONThe rest of this paper is organized in the following. Section IIdescribes the system model for a two-tier A-HetNet in con-junction with RFA and WBJs, Section III describes the pre-liminaries related to target’s association, target’s distancedistribution, andWBJ’s interference, Section IV describes thecoverage performance, Section V describes results and theirdiscussion, and Section VI concludes this paper.

II. SYSTEM MODELIn this paper, we enhance the system model adopted in ourprevious work in [17] by employing RFA in the presenceof WBJs to investigate the performance of the decoupledA-HetNets in UL.

A. INFRASTRUCTURE DEPLOYMENTConsider a two-tier A-HetNet, comprising of HAPs and LAPsas shown in Fig. 1. The HAPs and LAPs are distributed usingan independent HPPP, i.e.,8H and8L , with densities λH andλL , and heights hH and hL , respectively.

B. JAMMERS DEPLOYMENTThe transmit power of the WBJs is very low due to itswide-band nature. Since, in this paper, we focus on clus-tered jamming due to low transmit power levels, therefore,we assumed jamming due toWBJs. TheWBJs are distributedusing an MCP 8J with a density of λJ . The MCP constits ofparent and child nodes. The parent nodes are modeled usingan independent HPPP with density λj. Whereas, the childnodes are independently and uniformly distributed in a clusterwith radius rj centered around each of the parent nodes. Theparent nodes are excluded from the point process while thenumber of child nodes is a Poisson random variable. Theprobability density function (PDF) of the distance betweenthe cluster center and the child node located at the location awithin a cluster of radius rj is given as [26]

fm(a) =1

πr2j, ‖ a ‖≤ rj, (1)

where ‖ a ‖ is the Euclidean distance between the clustercenter and the child node. The child nodes are considered

FIGURE 1. System model for a two-tier A-HetNet.

analogous to the WBJs and thus, can be distributed accordingto an MCP [13], [27]. Dissimilar to [11], where authorsconsidered a worst case scenario in which WBJs are onlypresent around the target UE; we model WBJs according toan MCP such that the WBJs are uniformly-distributed withina cluster. Furthermore, the density of WBJs in A-HetNets isgiven as λJ = λjc̄, where the average number of WBJs percluster is c̄.

C. TARGET DEPLOYMENTThe users are distributed using an independent HPPP 8Uwith a density of λU . The UE whose UL legitimate com-munication is disrupted by the WBJs attacks is consideredas a target UE. We deploy the target UE at the origin ofthe coordinate system. According to the Slivnyak’s Theorem[28], placing a single point at the origin xo(0, 0, 0) of thecoordinate systemwill not change the distribution of the pointprocess.

D. BANDWIDTH ALLOCATIONWe employ frequency reuse in a reverse manner forA-HetNets as shown in Fig. 2. Using RFA [22], [23], thewhole range of frequencies in the DL of HAPs is madeavailable in the UL of LAPs which helps to obtain a bettercoverage performance.

In RFA, we categorize frequencies associated to HAPsand LAPs as F∗ and F∗∗, respectively. Each frequencydescribes two sub-bands based on their use-regions, i.e,.HAP- and LAP-enabled regions. For instance, the sub-bands

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

FIGURE 2. Reverse frequency allocation in A-HetNets.

of HAP-enabled regions are FC∗ and FO∗ while the sub-bandsof LAP-enabled regions are FC∗∗ and F

O∗∗. The HAP-enabled

regions are those regions, where the target UE receives maxi-mum transmit power from theHAP in the DLwhile in the UL,maximum transmit power is obtained by the same HAP fromthe target UE. Whereas, the LAP-enabled regions are thoseregions, where the target UE receives maximum transmitpower from the LAP in the DL while in the UL, maximumtransmit power is obtained by the same LAP from the targetUE. The frequencies are further categorized based on theirDL and UL access. For instance, FC

∗,DL , FC∗∗,DL , F

O∗,DL , and

FO∗∗,DL describe frequencies in the DL, while, FC

∗,UL , FC∗∗,UL ,

FO∗,UL , and F

O∗∗,UL describe frequencies in the UL.

E. RECEIVED SIGNAL STRENGTHThe transmit power of the UAV and the UE associated withthe tier, k is given as Pk and Qk , respectively, where k ∈{L,H}, while the power transmitted by the clustered jammersis given as Pj. The average transmit power obtained from the

UAV connected to tier, k at the target UE is E{SDLk

}= Pk ‖

Xk ‖−αk , where E{.} is the statistical expectation, αk > 2is the pathloss exponent for the UAV associated to tier, k,and ‖ Xk ‖ is the Euclidean distance between the originand the UAV associated to tier, k. The power received fromthe target UE at the UAV connected to tier, k is given asE{SULk

}= Qk ‖ Xk ‖−αk . We assume interference limited

A-HetNets such that the interference power dominates thenoise power.

We assume small scale Rayleigh fading gain between thek-th tier UAV and the UEs as gk ∼ exp(1). Furthermore,the channel is assumed to be static such that the UEs, UAVs,and WBJs are stationary for a particular environment. How-ever, our model is valid for UAV-mobility transmissions. Fordetecting the transmitted signal successfully, τ is the signal-to-interference-ratio (SIR) threshold. Similar to [29], there isa single UE per UAV which acts as a dominant interferer inUL, such that the density of UEs is very high λU ≥ λH +λL .

TheULSIR at the LAP by considering RFA in the presenceof WBJs is expressed as

SINRULL ,

QLgL ‖ XL ‖−αL

IUL8L,AOH

+ IDL8H ,ACL

+ I8J

, (2)

where IUL8L,AOH

=∑

iε8IL,AOH

QLgi ‖ Xi − XL ‖−αL is the UL

interference from the UEs located outside the HAP-enabledregion AOH with the interference process 8L,AOH

, IDL8H ,ACL

=∑vε8I

H ,ACL

PHgv ‖ Xv − XH ‖−αH describes the DL inter-

ference of the HAPs from the LAP-enabled region ACL withinterference process 8H ,ACL

, and I8J =∑

lε8JPjgl ‖ Xl −

XL ‖−αL describes the interference from the WBJs withinterference process 8J .Similarly, the UL SIR at the HAP by considering RFA in

the presence of WBJs is expressed as

SINRULH ,

QHgH ‖ XH ‖−αH

IUL8H ,AOH

+ IDL8L,ACL

+ I8J

, (3)

where IUL8H ,AOL

=∑

iε8IH ,AOL

QHgi ‖ Xi − XH ‖−αH is the UL

interference from the UEs located outside the HAP-enabledregion AOH with the interference process 8H ,AOH

, IDL8L,ACH

=∑vε8I

L,ACH

PLgv ‖ Xv − XL ‖−αL describes the DL inter-

ference of the LAPs from the LAP-enabled region ACL withinterference process 8L,ACL

, and I8J =∑

lε8JPjgl ‖ Xl −

XH ‖−αH describes the interference from the WBJs withinterference process 8J .

III. PRELIMINARIESHere, we present an overview for the UL and DL target’sassociation criteria, the decoupled access, the distance dis-tributions to the serving UAVs, and the interference at theserving UAV.

A. TARGET ASSOCIATIONIn DL, the target UE is connected to the k-th tier UAV, Tk ,based on maximum received signal strength in DL from thattier. Then, the location of location of the associated tier, Tk isgiven as [30]

Tk = arg maxkε(H ,L)Pk ‖ Xk ‖−αk .

While, in UL, the target UE is associated with the k-th tierUAV, Tk based on maximum received signal strength fromthat target UE at the k-th tier UAV. Then, the location oflocation of the associated tier, Tk is given as [30]

Tk = arg maxkε(H ,L)Qk ‖ Xk ‖−αk .

The connection of the target UE with the UAV of tier, kdefines that there isn’t any other UAV located within a sphereof radius Xk , such that P{Xk > x} = e−πλkx

2[28]. Further,

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

FIGURE 3. Decoupled access for the target user in A-HetNets [17].

the PDF distance between the target user and the serving UAVof tier, k is expressed as

fXk (x) = 2πλkxe−πλkx2, x ≥ 0. (4)

B. DECOUPLED ACCESSThe connection of the target UE in DL with the HAPand in UL with the LAP will be considered as a decou-pled access in the analysis to follow (see, Fig. 3). TheUEs located in the regions that take part in the decou-pled access are considered as ‘‘decoupled-enabled regions′′.In practical scenarios, the UEs located in the vicinity ofcell-edge boundary facilitate decoupled access. Furthermore,the authors in [17], [20] showed that the decoupled accessleverages better performance gains for the decoupled-enabledregions, therefore, we derive the association probability of theUEs located in the decoupled-enabled regions. The decou-pled access for the decoupled-enabled regions is expressedas [17]

AD =αH

2αL

H1,11,1

(√πλH )

αH

αL

(QHQL

) 1αL

√πλL

∣∣∣∣∣∣∣∣(0,

12

)(0,αH

2αL

)

−H1,11,1

(√πλH )αH /αL

(PHPL

)1/αL

√πλL

∣∣∣∣∣∣(0,

12

)(0,αH

2αL

) .(5)

The proof of (5) is given in the Appendix (A).

C. DISTANCE DISTRIBUTION TO THE UAVsHere, the distance distribution between the target UE and theservingUAV is derived. Since, the decoupled-enabled regionsleverage better performance gains with the decoupled access,therefore, we focus to derive the distribution of the target UElocated in the decoupled-enabled regions and the target user[20], [30]. For the target UE located in the decoupled-enabled

regions, its PDF distance to the serving LAP is expressedas [17]

f (D)XL (x) =

exp

−πλH(PHPL

) 2αH x

2αLαH

− exp

−πλH(QHQL

) 2αH x

2αLαH

fXL (x)

AD. (6)

Similarly, the PDF distance to the serving HAP is givenas [17]

f (D)XH (x) =

exp

−πλL(PLPH

) 2αL x

2αHαL

− exp

−πλL(QLQH

) 2αL x

2αHαL

fXH (x)

AD. (7)

D. INTERFERENCE CHARACTERIZATIONHere, the interference at the serving UAV is analyzed and theLaplace transform of interference of LAPs, HAPs, UEs, andWBJs is derived.

1) RFA-BASED INTERFERENCEHere, we derive the Laplace transform of the interferenceat the target UE based on the RFA scheme. In the decou-pled access, the intended receiver in UL is the LAP while,in the non-decoupled access, the intended receiver is the HAP.Therefore, the Laplace transform of the interference for boththe decoupled and the non-decoupled access under RFA is aunique expression and needs to be analyzed separately.

The Laplace transform of the interference for boththe decoupled and the non-decoupled access can besub-categorized into the Laplace transform of the UL andthe Laplace transform of the DL interference. The Laplacetransform of the UL interference for the decoupled accessand by excluding the area of the HAP-enabled region isderived as

LIUL8L,AOH

(s)

a= EIUL8

L,AOH

{exp(−IUL8L,AOH

s)}

b= EIUL8

L,AOH,gi{exp(−

τxαL

QL

∑iε8I

L,AOH

QLgir−αLi )}

c= EIUL8

L,AOH

∏

iε8IL,AOH

Egi{exp(−xαLgir

−αLi )

}142222 VOLUME 8, 2020

M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

d= EIUL8

L,AOH

∏

iε8IL,AOH

1

1+ τ( rix

)−αL

e= exp

−2πλL∫ r2

r1

ridri

1+( riτ 1/αL x

)αL

f= exp

−πλLτ 2/αL x2∫ ( r2

τ 1/αL x

)2

( r1τ 1/αL x

)2

dw1+ wαL/2

g= exp

{2πλLτ r

2−αL2

x−αL (αL−2)2F1(1, 1−

2αL; 2−

2αL;−τ

(xr2

)αL)

−2πλLτ r

2−αL1

x−αL (αL−2)2F1(1, 1−

2αL; 2−

2αL;−τ

(xr1

)αL)

}.

(8)

Here, a= is obtained using the Laplace transform definition,

where s =τxαL

QL, b= is obtained by substituting s and IUL8

L,AOH

,

c= follows by simple mathematical manipulations, d= followsby employing the Laplace transform of the interference w.r.t.gi,

e= follows by the probability generation functional (PGFL)

of HPPP [31], where r1 and r2 is the radius of ACH and AO

H ,

respectively,f= follows by substituting w =

( riτ 1/αL x

)2in e=,

g= follows by solving the integration, where 2F1(−,−;−;−)is a Hypergeometric function [32].

Similarly, following the same steps, the Laplace transformof the interference in UL from outside the HAP-enabledregion and with the non-decoupled access can be derived as

LIUL8H ,AOH

(s)

= exp

{2πλH τ r

2−αH2

x−αH (αH−2)2F1(1, 1−

2αH; 2−

2αH;−τ

(xr2

)αH)

−2πλH τ r

2−αH1

x−αH (αH−2)2F1(1, 1−

2αH; 2−

2αH;−τ

(xr1

)αH)

}.

(9)

Furthermore, the DL interference in terms of the Laplacetransform of the interference for the LAP-enabled region andwith the decoupled access can be derived as

LIDL8H ,ACL

(s)

= exp

{2πζHL λH τ z

2−αH1

x−αH (αH − 2) 2F1(1, 1−2αH; 2−

2αH;

−ζHL τ

(xz1

)αH)−

2πζHL λH τ z2−αH0

x−αH (αH − 2) 2F1(1, 1−2αH; 2

−2αH;−ζHL τ

(xz0

)αH)}.

(10)

Here, z0 and z1 are the lower and the upper limits for the

LAP-enabled region. ζHL =PHQL

corresponds to the ratio of

the transmit power of theHAP to the transmit power of theUEassociated with the LAP, where the subscript in ζHL representsthe DL power of the UAV and the subscript in ζHL representsthe UL transmit power of the UE connected to the servingUAV.

Similarly, following the same procedure, the DL interfer-ence in terms of the Laplace transform of the interference forthe LAP-enabled region and with the non-decoupled accesscan be derived as

LIDL8L,ACL

(s)

= exp

{2πζ LHλLτ z

2−αL1

x−αL (αL−2)2F1(1, 1−

2αL;2−

2αL;−ζ LHτ

(xz1

)αL)

−2πζ LHλLτ z

2−αL0

x−αL (αL−2)2F1(1, 1−

2αL;2−

2αL;−ζ LH τ

(xz0

)αL)

}.

(11)

2) CLUSTER-BASED INTERFERENCEHere, the interference from the WBJs is characterized forthe k-th tier UAV in terms of the Laplace transform of the

interference. Consider υ(x) = LI8j (s), where s =xαk τ

Qk, then

using [26], [33], the interference of the k-th tier UAV in ULis expressed by the PGFL, G(υ) and the conditional PGFL,℘(υ) and is expressed as

℘(υ) = E {5xε8υ(x)}

= G(υ)× f, (12)

where

f , exp

{−Pjx2τ 2/αk2π c̄

Qkαkr2jcsc

(2παk

)}. (13)

The PGFL is solved for the pathloss exponent αk > 2. Thus,for the MCP distributed jammers, G(υ) is given as

G(υ)

= exp

{−λjπ

Pjτ 2/αk x2

Qk

∫∞

0

(1−exp

{−c̄

1+tαk/2

})dt

}.

(14)

Then, substituting (13) and (14) in (12) and assuming 1k =PjQk

, the interference of theWBJs is obtained as the following.

LI8j (s)

= exp

{−2π c̄1kx2τ 2/αk

αkr2jcsc

(2παk

)−λjπ1kτ

2/αk x2∫∞

0

(1− exp

{−c̄

1+ tαk/2

})dt}.

(15)

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

IV. COVERAGE PROBABILITY ANALYSISThe probability that the serving UAV in the presence ofWBJsreceives an SIR larger than the SIR threshold is considered ascoverage probability in UL. The coverage probability in ULfor the UAV associated to tier, k is given as

C , EXk{P{SIRUL

Xk > τ }}. (16)

The coverage probability of the decoupled access in UL is theprobability that the LAP in the presence of WBJs receives anSIR larger than the SIR threshold. In conjunction with theRFA, WBJs, and decoupled access, the coverage probabilityin UL is derived as [11], [23]

CD ,∫ r2

r1P{SIRUL

XL > τ }f (D)XL (x)dx

a=

∫ r2

r1P

QLgL ‖ XL ‖−αL

IUL8L,AOH

+ IDL8H ,ACL

+ I8J

> τ

f (D)XL (x)dx

b=

∫ r2

r1E{gL >

τxαL

QL

(IUL8

L,AOH

+ IDL8H ,ACL

+ I8J

)}× f (D)XL (x)dx

c=

∫ r2

r1E{exp

(−sIUL8

L,AOH

)}E{exp

(−sIDL8

H ,ACL

)}×E

{exp

(−sI8J

)}f (D)XL (x)dx

d=

∫ r2

r1LIUL8

L,AOH

(s)LIDL8H ,ACL

(s)LI8j (s) f(D)XL (x)dx, (17)

where a= follows by substituting (2), b

= follows by simplemathematical manipulations, c

= follows by substituting s =τxαL

QLand exploiting the exponentially distributed gains due

to Rayleigh fading, and d= follows by the definition of Laplace

transform of interference. Finally, the UL coverage probabil-ity of the target UE with RFA, WBJs, and decoupled access

is obtained by substituting (8), (10), (15), and (6) in (17) andis expressed in (20), as shown at the bottom of the page.

The probability that the HAP in the presence of WBJsreceives an SIR larger than the SIR threshold is considered asthe non-decoupled UL coverage probability. In conjunctionwith RFA,WBJs, and non-decoupled access, theUL coverageprobability is derived as

CND , EXH{P{SIRUL

XH > τ }}. (18)

Similar to (17), the UL coverage probability with RFA andWBJs for the non-decoupled access can be expressed as

CND=

∫ r2

r1LIUL8

H ,AOH

(s)LIDL8L,ACL

(s)LI8j (s) f(D)XL (x)dx, (19)

The final expression of the UL coverage probability in con-junction with RFA, WBJs, and non-decoupled access can beobtained by substituting the values of (9), (11), (15), and (7)in (19) and is expressed in (21), as shown at the bottom of thepage.

V. RESULTS AND DISCUSSIONHere, a two-tier A-HetNet is analyzed by employing RFA inthe presence of WBJs. The results are obtained by 100, 000independent Monte-Carlo trials using the simulation param-eters listed in Table 1.The coverage probability of the decoupled access (DUDe-

labeled curve) is better than the non-decoupled access(labeled as Non-DUDe) as shown in Fig. 4. This is due to thefact that the distance-dependent pathloss between the LAPand the target user is smaller than the distance-dependentpathloss between the HAP and the target user which resultsin higher UL SIR and the coverage probability. It is observedthat the coverage probability decreases with the increase inthe number of WBJs because each WBJ introduces jamminginterference which compromises legitimate UL communica-tion. The approximate percentage decrease in the coverage

CD

=

∫ r2

r1exp

(πτxαL

[λL

αL/2−1r2−αL2 2F1(1, 1−

2αL; 2−

2αL;−τ

(xr2

)αL)−

λL

αL/2−1r2−αL1 2F1(1, 1−

2αL; 2−

2αL;−τ

(xr1

)αL)

+λH

αH/2−1ζHL z

2−αH1 2F1(1, 1−

2αH; 2−

2αH;−ζHL τ

(xz1

)αH)−

λH

αH/2−1ζHL z

2−αH0 2F1(1, 1−

2αH; 2−

2αH;−ζHL τ

(xz0

)αH)]

−x21Lτ2/αL 2π c̄

αLr2jcsc

(2παL

)−λjπτ

2/αL1Lx2∫∞

0

(1−exp

{−c̄

1+ tαL/2

})dt

)f (D)XL (x)xdx. (20)

CND

=

∫ r2

r1exp

(πτxαH

[2λHαH−2

r2−αH2 2F1(1, 1−2αH; 2−

2αH;−τ

(xr2

)αH)−

2λHαH−2

r2−αH1 2F1(1, 1−2αH; 2−

2αH;−τ

(xr1

)αH)

+2λL

αL−12ζ LH z

2−αL1 2F1(1, 1−

2αL; 2−

2αL;−ζ LH τ

(xz1

)αL)−

2λLαL−2

ζ LH z2−αL0 2F1(1, 1−

2αL; 2−

2αL;−ζ LH τ

(xz0

)αL)]

−x21H τ2/αH 2π c̄

αH r2jcsc

(2παH

)−λjπτ

2/αH1Hx2∫∞

0

(1−exp

{−c̄

1+ tαH /2

})dt

)f (D)XH (x)xdx. (21)

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

TABLE 1. Simulation parameters for the considered A-HetNets.

FIGURE 4. UL coverage probability against τ , for different number ofWBJs.

probability of the decoupled access due to WBJ activity forthe SIR threshold of −20 dBm is equal to 7.4%, 13.5%, and19.7%, for the average number of WBJs equal to 2, 4, and 6,respectively.

The coverage probability increases by increasing the trans-mit power of the UE associated with the LAP as shown inFig. 5. This may be due to the fact that more users are addedin the decoupled-enabled regions. Furthermore, a significantincrease in the coverage probability of the decoupled access isobserved for the WBJs activity. For instance, by consideringthe transmit power of the user associated with the LAP equalto 20 dBm and with the average number of WBJs of 2, 4,and 6, the coverage probability of the decoupled access inUL is equal to 0.5, 0.325, and 0.225, respectively, while,by considering the transmit power of the user associated withthe LAP equal to 30 dBm, the coverage probability of the

FIGURE 5. UL coverage probability against QL, for different number ofWBJs.

decoupled access increases up to 0.75, 0.7, and 0.65, for theaverage number of WBJs equal to 2, 4, and 6, respectively.For the increase in the transmit power of UE associated withthe LAP from 20 dBm to 30 dBm and for the average numberof WBJs equal to 2, 4, and 6, the percentage increase inthe coverage probability of the decoupled access is equal to33.3%, 53.5%, and 65.3%, respectively.

FIGURE 6. UL coverage probability against Pj , for different number ofWBJs.

The coverage probability decreases by increasing the trans-mit power of the WBJs as shown in Fig. 6. This is because ofthe fact that eachWBJ increases its transmit power to increasethe jamming interference in the network. This results in theoverall reduction of the UL SIR and the UL coverage proba-bility. For instance, the coverage probability of the decoupledaccess by considering the transmit power of the WBJs equalto 5 dBm and with the average number of WBJs equal to 2,4, and 6 is equal to 0.8, 0.79, and 0.78, respectively, while,by considering the transmit power of the WBJs equal to10 dBm, the coverage probability of the decoupled accessdecreases up to 0.75, 0.7, and 0.65, for the average number of

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M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

FIGURE 7. UL coverage probability against rj , for different number ofWBJs.

FIGURE 8. UL coverage probability against PL, for different number ofWBJs.

WBJs equal to 2, 4, and 6, respectively. Thus, the percentagedecrease in the coverage probability of the decoupled accessfor the increase in the transmit power of WBJs from 5 dBmto 10 dBm and for the average number of WBJs equal to 2, 4,and 6, is equal to 6.25%, 11.3%, and 16.6%, respectively.

The coverage probability increases by increasing the radiusof the cluster of WBJs as shown in Fig. 7. This is because ofthe fact that the area (for the distribution) of the WBJs withina circular disc increases by increasing the cluster radius whichlowers the cumulative jamming interference at the target UEand results in the higher coverage probability in UL. Forinstance, the coverage probability of the decoupled access byconsidering the radius of the circular disc equal to 100 m andwith the average number ofWBJs equal to 2, 4, and 6 is equalto 0.75, 0.7, and 0.65, respectively, while, by consideringthe radius of the circular disc equal to 200 m, the coverageprobability of the decoupled access increases up to 0.8, 0.79,and 0.78, for the average number ofWBJs equal to 2, 4, and 6,respectively. Thus, for the increase in the radius of the circulardisc from 100 m to 200 m and for the average number ofWBJs equal to 2, 4, and 6, the percentage increase in the

coverage probability of the decoupled access is equal to 6.6%,12.8%, and 20%, respectively.

The coverage probability decreases by increasing the trans-mit power of the LAP as shown in Fig. 8. This is due tothe fact that by increasing the transmit power of the LAPmore UEs prefer coupled association with the LAP at thecost of a decrease in the decoupled association which resultsin decreasing the coverage probability. Furthermore, the cov-erage probability of the decoupled A-HetNets decreases byincreasing the average number ofWBJs due to the increase inthe jamming interference. For the transmit power of the LAPequal to 20 dBm, the approximate decrease in the percentageof the coverage probability of the decoupled access due toWBJ activity is equal to 6.6%, 13.3%, and 20%, for theaverage number of WBJs equal to 2, 4, and 6, respectively.

VI. CONCLUSIONThe performance of a multi-tier A-HetNet is largely depen-dent on the association of the target user with the aerialplatforms. This performance is further exacerbated by thejamming interference due to the presence of theWBJs. In thispaper, the performance of DL and UL decoupled accessalong with RFA and WBJs is analyzed for the A-HetNets.The analytical expression of the coverage probability withthe decoupled and the non-decoupled access is derived.The obtained results showed that the performance of thedecoupled A-HetNets along with RFA is better than thenon-decoupled access. However, this performance is dis-rupted, if WBJs are present in the vicinity. Furthermore, theperformance of the decoupled access in A-HetNets improvesby increasing the transmit power of the target user and theradius of the WBJ’s cluster while degrades by increasing theaverage number of WBJs per cluster and the transmit powerof the WBJs.

APPENDIX APROOF OF (6)

Proof: The decoupled access for the target UE locatedin the decoupled-enabled regions is derived as [17]

AD = P{PHX

−αHH > PLX

−αLL ; QHX

−αHH ≤ QLX

−αLL

}.

Since PH > PL , the joint event is expressed asPLPH

X−αLL <

X−αHH ≤QLQH

X−αLL . Then, the association probability of the

target UE is given as

AD

= P

XH <(PHPL

) 1αH X

αL

αHL

−P

XH <(QHQL

) 1αH X

αL

αHL

142226 VOLUME 8, 2020

M. Arif et al.: Clustered Jamming in Aerial HetNets With Decoupled Access

Hm1,n1v1,w1

(γ ) , Hm1,n1v1,w1

[γ

∣∣∣∣ (a1,A1) , . . . , (av1 ,Av1)(b1,B1) , . . . ,(bw1 ,Bw1

)] = 12πι

∮C

5m1j=10(bj + Bjs)5

m1j=10(1− aj + Ajs)

5v1j=n1+1

0(aj + Ajs)5w1j=m1+1

0(1− bj + Bjs)γ−sdγ. (22)

a=

∫∞

0

1− exp

−πλH (PH/PL)2/αH x2αLαH

−

1−exp

−πλH (QH/QL)2/αH x2αLαH

fXL (x)dx

b= 1− 2πλL∫

∞

0

αH

2αLH1,0

0,1

[(√πλH )

αH

αL

(PHPL

) 1αL x

∣∣∣∣ (−,−)(0,αH

2αL

)]

.12H1,0

0,1

[√πλLx

∣∣∣∣ (−,−)(0,

12

)]x

dx

−

(1− 2πλL

∫∞

0

(αH

2αLH1,0

0,1

×

(√πλH )

αH

αL

(QHQL

) 1αL x

∣∣∣∣ (−,−)(0,αH

2αL

).12H1,0

0,1

[√πλLx

∣∣∣∣ (−,−)(0,

12

)]x

dx

,where a

= follows using (4), while b= follows by defining Fox

H-function in (22), as shown at the top of the page, withvalues of m1 = 1, n1 = 0, v1 = 0, and w1 = 1, and using

(2.9.4) of [34]. In (22), 0 (t) =∞∫0st−1e−sds, γ is a complex

number except zero. Furthermore, 1 6 m1 6 w1, 0 6 n1 6v1,Aj > 0,Bj > 0, C is a complex contour, and aj, bj arecomplex numbers. Finally, (5) follows by (2.8.4) of [34].

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MOHAMMAD ARIF received the B.S. degreein electrical engineering from the University ofEngineering and Technology, Peshawar, Pakistan,in 2012, and theM.S. degree in electrical engineer-ing from COMSATS University Islamabad (CUI),Islamabad, Pakistan, in 2014, where he is currentlypursuing the Ph.D. degree with the Department ofElectrical and Computer Engineering.

His research interests include aerial and terres-trial heterogeneous cellular networks, dual con-

nectivity, uplink and downlink interference management, reverse frequencyallocation, indoor localization, signal processing, and channel coding.

SHURJEEL WYNE (Senior Member, IEEE)received the Ph.D. degree from Lund University,Sweden, in 2009.

From 2009 to 2010, he was a PostdoctoralResearch Fellow with the High-Speed WirelessCenter, Lund University. Since 2010, he has beenwith the Department of Electrical and Com-puter Engineering, COMSATS University Islam-abad (CUI), Islamabad, Pakistan, where he iscurrently an Associate Professor. His research

interests include wireless channel characterization, multi-antenna systems,cooperative communications, physical layer security, and vehicular commu-nications. He was a co-recipient of the Best Paper Award of the Antennasand Propagation Track at the IEEE VTC2013-Spring.

KEIVAN NAVAIE (Senior Member, IEEE) is cur-rently with the School of Computing and Commu-nications, Lancaster University, U.K. His researchinterests include provisioning dependable connec-tivity and positioning to intelligent cyber-physicalsystems. He is a Fellow of the IET, a Senior Fellowof the HEA, and a Chartered Engineer in U.K.He currently serves on the Editorial Board of theIEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS,the IEEE COMMUNICATIONS LETTERS, and the IEEE

COMMUNICATIONS SURVEYS AND TUTORIAL.

MUHAMMAD SAJID HAROON (Graduate Stu-dent Member, IEEE) received the B.Sc. degree inelectronics engineering from International IslamicUniversity Islamabad, Pakistan, in 2007, the M.S.degree in electrical engineering from the COM-SATS Institute of Information Technology, Attock,Pakistan, in 2013. He is currently pursuing thePh.D. degreewith theGhulam IshaqKhan Instituteof Engineering Science and Technology, Swabi,Pakistan, with a focus on interferencemanagement

in next-generation cellular networks using tools from stochastic geometry.

SADIA QURESHI received the M.S. degree inelectrical engineering from the University of Engi-neering and Technology, Peshawar, Pakistan. Sheis currently pursuing the Ph.D. degree with theSchool of Electrical andData Engineering, Univer-sity of Technology Sydney. She is also a full-timeResearch Scholar with the School of Electricaland Data Engineering, University of TechnologySydney. Her research interests include heteroge-neous networks, signal processing, wireless sensor

networks, software-defined networking, and optical networks.

142228 VOLUME 8, 2020