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A Holistic Investigation on Terahertz Propagationand Channel Modeling Toward Vertical

Heterogeneous NetworksKursat Tekbıyık, Student Member, IEEE, Ali Rıza Ekti, Member, IEEE,

Gunes Karabulut Kurt, Senior Member, IEEE, Ali Gorcin, Senior Member, IEEE,Halim Yanikomeroglu, Fellow, IEEE

Abstract—User-centric and low latency communications canbe enabled not only by small cells but also through ubiqui-tous connectivity. Recently, the vertical heterogeneous network(V-HetNet) architecture is proposed to backhaul/fronthaul alarge number of small cells. Like an orchestra, the V-HetNetis a polyphony of different communication ensembles, includinggeostationary orbit (GEO), and low-earth orbit (LEO) satellites(e.g., CubeSats), and networked flying platforms (NFPs) alongwith terrestrial communication links. In this study, we proposethe Terahertz (THz) communications to enable the elements ofV-HetNets to function in harmony. As THz links offer a largebandwidth, leading to ultra-high data rates, it is suitable forbackhauling and fronthauling small cells. Furthermore, THzcommunications can support numerous applications from inter-satellite links to in-vivo nanonetworks. However, to savor thisharmony, we need accurate channel models. In this paper,the insights obtained through our measurement campaigns arehighlighted, to reveal the true potential of THz communicationsin V-HetNets.

I. INTRODUCTION

Flexible wireless communication architectures includ-ing small cells, cell-free designs, heterogeneous networks(HetNets), and multi-band connectivity are widely aspired bythe telecommunication industry for the user-centric systemdesigns toward 6G and beyond to provide the required qual-ity of service (QoS) levels among users [1]. Even thoughfree-space optics (FSO) and mmWave technology have beendiscussed over the years, they are not capable of makingthis dream possible on their own. Terahertz (THz) wirelesscommunication will be a critical enabler for 6G since itproposes a solution for the unlicensed band scarcity below 100GHz and it can be employed by not only macro-scale networks(e.g., inter-satellite links) but also by nano-scale networks(e.g., nanonetworks) [2]. The small cell concept has not been

K. Tekbıyık and G.K. Kurt are with the Department of Electronics andCommunications Engineering, Istanbul Technical University, Istanbul, Turkey,e-mails: {tekbiyik, gkurt}@itu.edu.tr

K. Tekbıyık, A.R. Ekti, and A. Gorcin are with Informatics and InformationSecurity Research Center (BILGEM), TUBITAK, Kocaeli, Turkey, e-mails:{kursat.tekbiyik, aliriza.ekti, ali.gorcin}@tubitak.gov.tr

A.R. Ekti is with the Department of Electrical–Electronics Engineering,Balıkesir University, Balıkesir, Turkey, e-mail: arekti@balikesir.edu.tr

A. Gorcin is with the Department of Electronics and Communica-tions Engineering, Yıldız Technical University, Istanbul, Turkey, e-mail:agorcin@yildiz.edu.tr

H. Yanikomeroglu is with the Department of Systems and Computer Engi-neering, Carleton University, Ottawa, Canada, e-mail: halim@sce.carleton.ca

fully deployed yet because of the backhaul and fronthaul cost.It is thought that along with the development of non-terrestrialnetworks, the pervasive connectivity can be provided, as wellas the backhaul and fronthaul required by small cell andcell-free topologies [3]. This 3D-network structure, depictedin Fig. 1, is termed as the vertical heterogeneous network(V-HetNet). As THz communications can offer an enormousbandwidth enabling ultra-high data rates, it stands out amongthe competing solutions that are likely to be used in V-HetNets.Although the transceiver costs for THz communication are cur-rently high, a cost reduction is expected with the developmentsin semiconductor technologies.

This study presents an overview of the THz-enabledV-HetNets framework. Below, we explain the motivation be-hind the THz communications in V-HetNet and address chal-lenges. By interpreting THz channel behavior with measure-ment results, tips are given on how the system design and re-quirements will be affected by channel behavior in V-HetNets.Finally, we pinpoint the open issues for the realization of THz-enabled V-HetNets by considering the measurement results.

II. WHY TERAHERTZ BAND?

Besides being a candidate technology for 6G, THz com-munications imply a remarkable potential for the future ofwireless communications as detailed below.

A. Utilizing the not Fully-explored Frequencies

There is a need to push the carrier frequency beyond themmWave band to overcome the spectrum scarcity below 100GHz. The THz band allows wireless communication throughtens of GHz bandwidth. Even though the path loss increaseswith frequency, the antenna gain is related to the square ofthe operating frequency. Therefore, high gain antennas cancope with the extreme path loss in the THz band. As known,antennas with high operating frequency have relatively narrowbeams compared to lower frequencies; hence, THz antennascan be utilized in point-to-point links to backhaul small-cells.Moreover, since the antenna size decreases with frequency, itis possible to place a massive number of antennas on smallsurfaces. For example, it is possible to cover 1 mm2 areawith four antennas at 300 GHz. Beyond the massive-multiple-input multiple-output (mMIMO), namely ultra-massive-MIMO

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Fig. 1. The THz-enabled V-HetNet framework including GEO, and LEO satellites, and NFPs along with terrestrial communication links. Furthermore,THz networks will be a crucial enabler for the nanonetworks due to antenna size, while enabling the operation of HetNets in harmony to support e-healthapplications.

(UM-MIMO) [4], can be employed to leverage the com-munication distance as well as the data rate enhancement.Although today’s technology is in scarcity for high power THztransmitters, it is expected that signal generators with highpower can be created in the near future considering studies ongraphene and InGaAs mHEMT MMICs.

B. Revolutionary Network Design Towards 6G

1) Cell-free Networking: It is widely acknowledged thatan architectural revolution is needed in 6G and beyond [5].Tiny cells and a cell-free architecture are demanded to caterthe data-hungry end-users. In this regard, the high path loss,which is the main drawback of the THz band, can be turnedto account for the cell-free design. In other words, the THzband is the region with the highest frequency reuse factor.Thus, it can be concluded that the THz band is ideally suitedfor the cell-free networking. Not only backhaul from accesspoints (APs) to core network but also links between AP anduser equipments can utilize THz waves to build up a cell-free network and satisfy the high data rate for backhaul. Tosupport cell-free networking, a user-centric approach can beadopted for the spatial multiplexing by using UM-MIMO.Additionally, distributed APs are required fronthauling to thecentral processing unit. Therefore, cell-free networking canutilize the THz waves to avoid fiber construction to every AP.

2) Vertical Heterogeneous Networks: Recently, since aerialAPs such as unmanned aerial vehicles (UAVs), NFPs, and

high-altitude platform stations (HAPSs) attract considerableattention, there has been an unprecedented vertical directionalexpansion in the wireless network. Furthermore, it seems thatdata flow between satellites and earth will increase more thanever in the coming years [6]. Although a dense satellite net-work has started to be established with the Starlink project andthe Keppler project, the studies in this area are still nascent.FSO and mmWave technologies have been proposed for theinter-satellite and satellite-to-X links. However, mmWave canonly serve through a total of 9 GHz bandwidth of the availableunlicensed bands. On the other hand, as CubeSats are designedfor LEO operation, they need to move with a velocity of28 × 103 kph to keep their orbits. This velocity necessitatesfast beam steering; hence, FSO with a mechanical gimbaledbeam steering mechanism is susceptible to track the movingtarget. However, one face of 1U CubeSat can be theoreticallycovered by 104 antennas operating at 300 GHz. Furthermore,to the extent allowed by the flight dynamics, the bottom ofthe NFP’s wings can be covered with hundreds of antenna.Therefore, THz enables to fast track a moving receiver withthe help of electronic steering.

THz mMIMO setups can be grouped to serve a differentnumber of users according to demand as illustrated in Fig. 2.These enable a flexible communication network that can bedynamically shaped in conformity with the need [7]. Forexample, the capacity demand possibly exceeds the targetdemand when event times or stadiums serve as an emergency

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Fig. 2. THz networks can employ ultra-massive antenna arrays; hence, itcan be turned into an advantage by dynamically adjustable beam pattern andmultiple spot flexibility.

assembly area. Here, THz-aided HAPSs can be used as a flex-ible communication structure to meet the increasing demand.Given the effort of each new generation to make the networkmore flexible, THz communications has the potential to be acrucial enabler for 6G and beyond.

3) Multi-connectivity: Ultra-reliable and low-latency com-munications in 5G require a high QoS due to stringentrequirements on availability and latency. Diversity and networkavailability increase as the user communicates with morethan one AP to improve QoS. We infer that 6G and beyondsystems can employ different technologies to enhance theQoS levels. For example, in the THz band, different sub-bands/windows can be used for communication at the sametime, or complementary technologies such as mmWave, FSO,and THz can be used simultaneously.

C. Applications from Planetary-scale to Nano-scale

In the V-HetNet framework toward 6G and beyond, it isenvisioned that the world, which was previously connected byground and undersea fiberoptic cables, is now connected over awide space network. The near future communication technol-ogy pushes the limits of science-fiction. With the developmentof e-health technologies, billions of nanodevices and sensorsneed to be connected to the medical center over a unified andseamless network to enable low-latency communications. It isenvisaged that using the THz band in V-HetNets will be usefulto meet the latency and data rate required by e-health andhaptic applications due to extremely short THz pulses on theorder of femtoseconds [8]. For example, the THz band pavesthe way for unprecedented applications initially suffering fromthe size, weight, and power (SWaP) constraints. The size ofnanomachines can only be satisfied by the THz antennas.As illustrated in Fig. 1, THz waves can be employed in theharmony of nanonetworks, body area network (BAN), andwireless local area network (WLAN). For instance, nanoma-

chines communicate with each other and sensors with THzwaves. BAN can utilize either IEEE 802.15.62 or mmWave(e.g., WiGig). The backhaul of AP can be a directional THzbeam.

D. Economic Feasibility

To achieve resilient and reliable connectivity, mmWave andTHz band connection are needed as described in Section II.Furthermore, the mmWave technology market in the telecom-munication industry, which is anticipated to reach $10.92billion by 2026 with a growing compound annual growth rateof 36.3% per year, along with the corresponding industrialecosystem, makes it attractive for the network operators andsemiconductor companies [9]. Therefore, adopting mmWaveand THz band communication will stimulate the economyalong with supporting Tbps data rates, novel usage scenarios,and applications. Thus, network operators exhibit particularinterest in the proper adoption of the THz band communi-cation, which can further reduce the total cost of ownership,capital expenditures, and operational expenditures. Deployinga macro base station in a crowded and populated locationsuch as downtown and providing seamless high-speed datarate connectivity to the remote islands and rural/suburban areascan be quite challenging and also very expensive due to theharsh geographical conditions. Thus, extending the wirelessconnectivity with the help of existing infrastructure to theareas beyond the reach of the fiberline by using the THz bandis anticipated to reduce the total cost. First, no additional orsignificant upgrades are needed for the backhaul traffic by thenetwork operators since already existing infrastructure willbe used for the traffic offloading which will ease the trafficcongestion problem. Second, as for the software and hardwareof the radio access network, there is no need for extrememodifications. Furthermore, the recent advances in the semi-conductor technology enable to design at THz frequencies;thus, antenna and chip can be integrated into the same packageto reduce the production cost.

III. OPERATIONAL CHALLENGES

Attractive properties of THz wireless communication canmake it possible to build up the wireless communicationnetwork of the future. Nevertheless, there are many challengesto overcome. The state-of-the-art semiconductor technologycannot generate high power THz waves. However, somepromising transceiver designs have been proposed. It seemsthat a high output power transceiver will be designed by dis-covering the essence of graphene and complementary metal-oxide semiconductor (CMOS). Although the transmit powerof CMOS power amplifiers is about 1-10 mW, they are inline with SWaP constraints [10]. Fortunately, it is possible todesign high gain antennas in this spectra to recover the signalpower.

In V-HetNets, we do not expect THz waves to suffer asevere loss in inter-satellite communication. The main problemis the operating temperature of the satellites. The temperaturefluctuates concerning the position of Earth and Sun. Satellitesalready have a thermal control system (TCS) to maintain stable

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operation against changes in operating temperature. However,it may require to review the design of TCS. Recently, NASA-JPL has pushed receiver design limits toward CubeSats. THz-enabled satellite communication in the near future is not piein the sky.

Since HAPSs fly above the troposphere, which 99% of thewater vapor in the atmosphere is at this layer, no severe pathloss is expected between HAPSs and LEO satellites. But, therelatively high speed between HAPSs and satellites, and thesharp beam of THz antennas require stringent tracking oftransmitter and receiver. The electronic steering functionalityof UM-MIMO can provide fast and accurate tracking with thecost of large antenna arrays.

A high attenuation rate is observed between HAPSs andground stations since water vapor absorbs most of the powerof the THz wave, and the region where the water vapor ismost dense in the atmosphere is up to 20 km above the Earth.The atmospheric attenuation coefficient at 300 GHz is around3 dB/km and 30 dB/km for very light and dense humiditylevels, respectively. The attenuation coefficients imply that theTHz wave is more advantageous than FSO in terms of pathloss [3, 11].

Another challenge lies in UAV communication. UAVs withhigh mobility vibrate slightly, even when they are suspendedin the air, due to flight aerodynamics. This poses a seriouschallenge to THz communication, which has a very narrowantenna beam. The loss due to misalignment between THzantennas is known to be severe [12]. The beam search maytake a long time after the antenna alignment deteriorates. Atthis point, we think that it would be appropriate to make THzcommunication with wider beams using beam broadening.Also, the phased array can be adaptively controlled with thehelp of a Kalman filter and gyroscope on the UAV; however,this can cause energy scarcity for battery-dependent UAVs.

As high data rate THz pulses have very short-times; multi-path components may not overlap with the first-path compo-nent. However, the propellers’ periodic behavior may give riseto rotor modulation. As the wavelength is much smaller thanpropellers, a simple blockage model can be utilized to analyzethe effect of the rotor modulation.

There are additional challenges that may occur in each layerof V-HetNet. These include beam steering, high-performancecomputation, and low-complexity tracking. Also, ultra-largeantenna array configuration, mutual coupling among anten-nas, and correlation among sub-channels should be jointlyconsidered in the design of UM-MIMO [2]. One of themain challenges is to estimate the propagation channel forUM-MIMO THz communication. Since it is expected to useUM-MIMO systems with thousands of antennas, novel intel-ligently designed channel acquisition algorithms are needed.Deep learning (DL)-based channel estimation techniques canbe utilized ubiquitously. The sparse nature of THz wirelesschannels allows using compressed sensing so that the com-plexity of acquisition methods can be reduced.

IV. OVERVIEW OF MEASUREMENT STUDIES

A. Channel Modeling Methodologies

Wireless channel modeling is mainly categorized undertwo approaches: deterministic and stochastic methods. Thedeterministic methods are propagation medium-specific. Theyrequire the model of the environment in-depth with the ma-terial characteristics and dimensions. Although an accuratechannel model is obtained for a given environment, thecomputation complexity is extremely high, and it exponen-tially increases with the dimensions of the environment. Ray-tracing is the most used deterministic technique for channelmodeling. Ray-tracing models the scattering, diffraction, andreflection effects in with geometric optics. Finite-differencetime-domain, method of moments (MoM), and finite elementmethod (FEM) are some of the other deterministic channelmodeling techniques. Since MoM and FEM are working in thefrequency domain, these methods do not seem to be the bestapproach as they require excessively complex computations inchannel modeling for a huge THz band, but geometric opticsthat become more accurate due to the stronger corpuscularbehavior in the THz band.

Unlike deterministic modeling, stochastic channel modelingaims to create a channel model independent of the environmentas much as possible. For this purpose, a statistical averageof many measurements is used to create the channel model.For the stochastic channel modeling, measurements are usuallycarried out with a vector network analyzer (VNA) or channelsounder. The main difference between these devices is thedomain where they perform operations. Channel sounder com-putes the channel impulse response by using an uncorrelatedm-sequence, but VNA composes many narrowband measure-ments to create channel transfer function. VNA performsmore accurate channel modeling due to calibration for eachnarrowband and encountering low noise, but the measurementtime is long [13]. Another difference is that VNA-assistedmeasurement does not require extra clock synchronization dueto two ports in the same device, whereas channel sounderrequires a strict external clock to sync two distinct ports.

Due to the presence of the colossal bandwidth, multipathcomponents can be distinguished. The nature of the THz bandleads to severe frequency selectivity. As a result, conventionalsmall scale flat fading models are not appropriate, and thebroadening effect cannot be ignored.

B. Measurement Setup and Results

In this section, we detail the THz measurement campaignsand results. Firstly, VNA-assisted measurements are carriedout to observe THz channel behavior with high spectralresolution. Afterward, pulse-based measurements for short-range THz communication will be addressed since on-offkeying (OOK) is seriously envisaged for use in THz wirelesscommunications [8]. The VNA-assisted measurement provideshigh spectral resolution at the cost of long measurement timewhile the pulse-based method lacks high resolution, but itallows taking measurements in a short time.

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TX Extender Module RX Extender Module

WR-03 Waveguides

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Fig. 3. The measurement setup consists of VNA equipped with THz extendermodules with WR-03 waveguide, and extender controller.

1) VNA-assisted Channel Modeling: An experimental mea-surement setup is constructed in the anechoic chamber withdimensions of 7m× 3m× 4m as seen in Fig. 3. To eliminatethe specular effects, the perimeter of the system is coveredwith absorbent material. The measurement setup consists of aVNA, WR-03 waveguided extension modules, and an extendercontroller. WR-03 extension modules expand the VNA, whoseupper limit is 67 GHz, to THz band by multiplying RF inputsignal with 18. In cases where an extender module is used,it should be preferred to use as few frequency multipliers aspossible. This is observed due to the increasing number ofstriking, phase distortion, and intermodulation product effectincreases. A single multiplier, if possible, is preferred to avoidmentioned distortions. The extender module employed in thissetup includes single frequency multiplier.

Driving VNA with the input signal in between 13.34 GHzand 16.67 GHz results in THz waves with carrier frequenciesin between 240 GHz and 300 GHz. To protect the phase andamplitude stability, instead of using all the band provided bythe extender, measurement is taken in the band between 240and 300 GHz by not using the edge points of the band [12].The IF signals downconverted at transmitter and receiver areused to find channel transfer function by comparing them toacquire scattering parameters (i.e., S21). VNA-assisted channelmodeling provides high-frequency resolution by taking mea-surements. 4096 frequency points are employed within theoperation band demarked by the VNA. As a result, 14.648MHz spectral resolution is provided by the measurement setup.

By using measurement results, the frequency dependencyof the path loss is revealed in [12]. The measurement re-sults indicate similar channel behavior at varying distances.However, the loss in some frequency intervals is higher thanin others. For example, there is a noticeable reduction inthe power of the received signal between 270 GHz and 290GHz. As stated above, antenna alignment is crucial for THzcommunication because of the relatively narrow beamwidth.It is shown that the pointing error in antenna alignment givesrise to a sharp fall in the received power. Since the receivedpower decreases exponentially, it is reasonable to model thisdecay as an exponential distribution. The pointing errorsbetween receiver and transmitter are pointed as challenges inSection III. Thus, how pointing error degrades the receivedpower is investigated by tilting the antennas. As seen in

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Fig. 4, the antenna misalignment causes a decrease in thereceived peak power. The power decrease concerning thepeak power corresponding to the line-of-sight (LOS) pathis approximately 2.3 dB and 13 dB for 10◦ and 20◦ tilts,respectively. These results clearly demonstrate the need forprecise alignment between the transceivers. Similar to [11], thehumidity has no significant effect on the received signal power.In the light of measurement results, it is proved that THz-enabled V-HetNets strictly need an accurate resolution in thebeam steering as well as beam broadening before fine-tuning.However, considering that the eavesdroppers cannot be alignedwith the transmit antenna due to the narrow beam, power lossdue to misalignment increases communication security.

2) Pulse-based Channel Modeling: As OOK modulationis mainly considered for THz communication, specifically innanonetworks, we prepare a measurement campaign to analyzethe channel behavior for a pulse train [14]. The measurementsetup detailed above is used without a change except forusing a signal generator and a spectrum analyzer rather thanVNA. As two ports are on distinct devices, the synchronizationis ensured by connecting the reference clock of the signalgenerator to the external clock input of the spectrum analyzervia a cable. The band between 275 GHz and 325 GHz ismeasured by feeding the extender module with a pulse traingenerated by the signal generator. The width of the pulse islimited with a minimum of 140 ns due to signal generatorcapability. At the receiver side, the pulse train is recorderduring 1ms by taking 100, 000 I/Q samples. The study focuseson path loss relations with frequency and distance. In thisregard, Fig. 5 denotes both frequency and distance related pathloss. The same pattern through the band is observed for eachrange.

Due to a frequency dependency, a linear path loss modelmay not be adequate in such a wide band. We denote thatthe path loss obeys the two-slope model [14]. It is theevidence that for THz the distance is another determinant

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along with the frequency. By employing the two-slope model,the normalized received power is illustrated in Fig. 6 for theunit power transmitter at 300 GHz. These results imply that thedistance-dependent channel characteristics require distance-aware medium access control (MAC) to control physical layerparameters such as modulation order and coding rate.

The high frequency reuse factor due to high path lossenables the design of small cell networks and cell-free ar-chitectures. The measurement results point that the mostchallenging part of THz-enabled V-HetNets is the link betweenHAPSs and terrestrial APs as the main path loss arises throughthe long distance, where the majority of water molecules in theatmosphere are found and the antenna alignment is compellingover the long-distance. Because of the frequency selectivity,the THz band can be divided into sub-bands; thus, usingdifferent sub-bands can support the multiple connections inthe V-HetNets.

V. OPEN ISSUES AND RESEARCH DIRECTIONS

Owing to the fact that THz is one of the key enablers ofV-HetNets, in this early stage of the technology, there are manyopen issues to be addressed to make it viable.

a) Cell-free Networking: To construct the user-centricnetwork of the future, it is required to satisfy a decentralizedseamless connection. Therefore, promising tools such as radiostripes, metasurfaces (e.g., reconfigurable intelligent surfaces),and holographic beamforming are needed to meet THz wire-less communication. Moreover, DL can be employed forthe efficient operation of cell-free networks (e.g., controllingtransmitter power in UM-MIMO).

b) Software-defined and Self Organizing Networks: Thesoftware-defined network (SDN) and self-organizing network(SON) pave the way for not only ubiquitous connection butalso low-cost network operation. SDNs and SONs requiredynamic MAC, spectrum sharing, and spectrum switching,DL, and deep reinforcement learning (DRL) enable to improve

Fig. 6. The normalized received power considering the two-slope path lossmodel with unit power transmitter.

the elasticity of networks by considering feedback from themedium to find optimal network policy owing to the fact thathundreds of billions of devices will be connected. DRL andblockchain are promising techniques to meet this demand [15]by utilizing intelligent spectrum sharing protocols. Moreover,federated learning allows to distribute the computation costover the thousands of the connected devices, and thereforethe central processing unit may not be needed. Multi-bandantennas and RF chains are also required to support spectrumswitching.

c) Channel Modeling and Estimation: In the absence ofthe LOS link, the communication may be carried out througha non-line-of-sight link; hence, reflection and scattering ofTHz waves must be investigated in the V-HetNet framework.Few studies model the spatial correlation in UM-MIMO chan-nels. Low-complexity methods with compressed sensing byutilizing the sparsity of THz channels can be investigated.Also, it is expected that DL-based approaches will be use-ful for understanding channel characteristics. For instance,generative adversarial networks are able to generate infinitelymany channel states, so we can model the channel estimationmethodology, which is due to the effect of fast changes inchannel characteristics. It is noted that the weather conditionssuch as rain, fog, and clouds have crucial impacts on thechannel characteristics. Therefore, they should be consideredin the models. Although the channel modeling document byIEEE 802.15.3d Task Group points to some weather impactson the channel, it needs further investigation. Furthermore, forthe feasibility of the operations of HAPSs, thorough airbornechannel modelling activities are needed.

d) Multiple Access Techniques: The severe frequencyselectivity plays an important role when choosing multipleaccess technology. For example, code-division multiple accesssuffers from high-cost equalizer to recover the received signal.However, we believe that subcarrier-based access technologies,like orthogonal frequency division multiple access, are suitableto employ in the THz networks. Yet, it is seen that peak-to-average power ratio reduction methods are required owing

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to an excessive number of nodes. Non-orthogonal multipleaccess (NOMA), proposed to increase spectral efficiency, islikely to be used in THz systems, but there is a trade-offbetween the additional complexity that NOMA would bringand the increase in efficiency. Due to mMIMO and pencil-sharp beams, the beam division multiple access is highlywelcome in THz communications. As in IEEE 802.11ad,single-carrier frequency division multiple access can be usedfor THz communications.

e) Beam Alignment: It seems that utilizing UM-MIMOin user equipment is formidable owing to the high com-putational complexity of beamforming algorithms and spacelimitations for antennas. Hence, a situation-aware MAC layeris required, which allows adaptively utilization of THz andmmWave. Further, intelligent surfaces can be key enablers forTHz user equipment because the computations are made onthe medium rather than user equipment.

VI. SUMMARY

THz-enabled V-HetNets provide not only ubiquitous con-nection but also relatively low-cost dense networks. As pro-jected, different wireless communication technologies, includ-ing FSO, mmWave, and THz, will be orchestrated for 6G andbeyond; however, THz technologies will be the maestro due towidespread application strategies. Howbeit, novel approachesconsidering channel behavior should be adopted from satelliteto in-vivo THz communications.

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BIOGRAPHIES

KURAT TEKBIYIK [StM’19] (tekbiyik@itu.edu.tr) is pursuing his Ph.D.degree in Telecommunication Engineering at Istanbul Technical Universityand is also researcher at TUBITAK BILGEM.

ALI RIZA EKTI received Ph.D. degree in Electrical Engineering fromDepartment of Electrical Engineering and Computer Science at Texas A&MUniversity in 2015. He is currently an assistant professor at BalikesirUniversity and also senior researcher at TUBITAK BILGEM.

GUNE KARABULUT KURT [StM’00, M’06, SM’15] (gkurt@itu.edu.tr)received the Ph.D. degree in electrical engineering from the Universityof Ottawa, Ottawa, ON, Canada, in 2006. Since 2010, she has been withIstanbul Technical University. She is serving as an Associate TechnicalEditor of IEEE Communications Magazine.

ALI GORCIN received his Ph.D. degree in University of South Florida.He is currently assistant professor at Yildiz Technical University in Istanbuland also manager at BILGEM BTE.

HALIM YANIKOMEROGLU [F] (halim@sce.carleton.ca) is a full pro-fessor in the Department of Systems and Computer Engineering at CarletonUniversity, Ottawa, Canada. His research interests cover many aspects of5G/5G+ wireless networks. His collaborative research with industry hasresulted in 37 granted patents. He is a Fellow of the Engineering Institute ofCanada and the Canadian Academy of Engineering, and he is a DistinguishedSpeaker for IEEE Communications Society and IEEE Vehicular TechnologySociety.