Terahertz Band:Lighting up Next-Generation Wireless Communications

Hongqi Zhang,Lu Zhang,Xianbin Yu,2,*

1 College of Information Science and Electronic Engineering,Zhejiang University,Hangzhou 310027,China

2 Zhejiang Lab,Hangzhou 310000,China

Abstract:With the explosion of wireless data rates,the terahertz(THz)band(0.1-10 THz)is envisioned as a promising candidate to break the existing bandwidth bottleneck and satisfy the ever-increasing capacity demand.The THz wireless communications feature a number of attractive properties,such as potential terabit-per-second capacity and high energy efficiency.In this paper,an overview on the state-of-the-art THz communications is studied,with a special focus on key technologies of THz transceivers and THz communication systems.The recent progress on both electronic and photonic THz transmitters are presented,and then the THz receivers operating in direct-and heterodyne reception modes are individually surveyed.Based on the THz transceiver schemes,three kinds of THz wireless communication systems are reviewed,including solid-state electronic systems,photonics-assisted systems and all-photonics systems.The prospective key enabling technologies,corresponding challenges and research directions for lighting up high-speed THz communication systems are discussed as well.

Keywords:terahertz communication;6th-generation;terahertz photonics;terahertz transceivers;terahertz waves

I.INTRODUCTION

Over the past decades,wireless data traffic has been witnessing ongoing tremendously increasing.According to the Cisco report[1],both mobile and wireless data rates are predicted to increase threefold from 2018 to 2023.In the light of Edholm’s law of bandwidth[2],data rates of wireless communication networks are in fact doubled every 18 months,and the wireless data rates will soon approach the same level with the wired communication systems by 2030[2],with a prediction that the capacity demand for wireless connections will exceed 100 Gbit/s within next coming five to ten years.Following this trend,up to Tbit/s wireless data rate is expected to come true in the coming decades[3].Such an exponential increase in the network capacity is hard to be achieved by current microwave based wireless communications.

To satisfy the huge capacity demand,it is vital to explore the new frequency bands such as millimeter wave,infrared,visible light,and so on.With respect to the millimeter wave band,it provides comparatively larger bandwidth than microwave band,while it is still difficult to satisfy the consumers’increasing demand,particularly the wireless data rates well beyond 100 Gbit/s.In a system operating the infrared wave band,the ambient light sources and the moon/sun light usually deteriorate the reliability of wireless transmission link,which is the major obstacles to limit its implementation.In terms of the visible light wave,the communication performance is also dramatically degraded owing to the influence of ambient light[4].

Except the aforementioned frequency regions,the terahertz(THz)band(from 0.1 THz to 10 THz),which locates between the infrared radiation and microwave as shown in Figure 1,has recently received an increasing attention thanks to its large bandwidth available.As the Shannon theorem says,the channel capacity is proportional to the bandwidth,and hence the THz band is expected to support tens or even hundreds of Gbit/s data rates,eventually Tbit/s.In recent years,with the rapid progress in fabricating semiconductor components,some devices can handle practical output power and gain,which indeed accelerate the development of THz communications.On the other hand,attributed to the extremely large bandwidth and wide deployment of optical fibers,photonics-assisted THz wireless links is also coming into the central stage.In this context,the evolutions of both electronic and photonic technologies facilitate the development of THz wireless communication.

Figure 1.The location of THz waves in the electromagnetic spectrum.

The intention of this review paper is to provide a brief overview on the evolution of THz communication.In section II,the transmitters for THz wireless communication systems are reviewed,including solidstate electronic and photonic technology.In addition,the receivers that operate in direct reception or heterodyne reception modes are summarized in Section III.In Section IV,three classes of THz communication systems are surveyed in details,including solid-state electronics system,photonics-assisted system and allphotonics system.In Section V,the key enabling technologies and corresponding challenges are presented,including THz integration chips,modulators,THz antennas and amplifiers.Furthermore,in Section VI,some promising application scenarios lighted up by the THz wireless communication system are envisioned and described.Finally,the conclusions are given in Section VII.

II.TRANSMITTERS OF THZ COMMUNICATION SYSTEMS

In this section,THz transmitter schemes based on the solid-state electronics and photonics technologies are introduced.

2.1 Solid-State Electronics Technology

So far,two types of transmitting structures including THz mixers and diodes based on the solid-state electronics technology have been widely used,as shown in Figure 2.In the structure of Figure 2(a),an analogue signal from a synthesizer is first multiplied into the THz frequency band by a frequency multiplier.Then,a data signal mixes the THz signal in a THz mixer.After the amplification by THz an amplifier,the THz signal is radiated into the free space via a THz antenna.The synthesizer,multiplier,mixer and antenna can be realized by silicon-based(Si-based)and III-V compound semiconductor-based platforms.Figure 2(b)presents a direct modulation scheme by directly modulating the signal onto THz carriers oscillating from a THz diode through a bias tee.Similarly,THz radiation is implemented by a THz antenna after a THz amplifier.

2.1.1 Si-based Transmitters

Si-based transmitters are mainly supported by complementary-metal-oxide-semiconductor(CMOS)and silicon-germanium(SiGe)technologies.Benefit from the advantages of high integration,low-cost and compactness,Si-based schemes have attracted an increasing number of attentions.For example,a 240 GHz quadrature phase shift keying(QPSK)transmitter fabricated by 65 nm CMOS technology has been demonstrated in[5].Based on 40 nm CMOS process,a transmitter operating at 300 GHz is also realized[6].Afterwards,the authors in[7]present a transmitter operating at 390 GHz based on 28 nm CMOS process.Besides,a 240 GHz chipset integrating both transmitter and receiver has been also demonstrated by using 130 nm SiGe technology[8].The key progress on Sibased THz transmitters is listed in Table 1.The table is organized in terms of process nodes,which contains process technology,operation frequency,output power and power consumption.The operation frequency of Si-based transmitters is usually below 500 GHz and the output power is typically less than 0 dBm.In addition,due to the limitations of fmax in the Si-based technologies,the output power goes down as the frequency increases.

Table 1.The key progress of THz transmitters based on silicon technology.

Figure 2.The schematic of transmitter structures in THz communication based on(a)THz mixers and(b)diodes.

The Si-based transmitters have been ubiquitously used in building THz wireless communication links.For instance,the authors in[12]demonstrate a 300 GHz wireless link with 20 Gbit/s by using 16-ary quadrature amplitude modulation format(16-QAM).Moreover,wireless transmission of 48 Gbit/s,and 56 Gbit/s using 300 GHz CMOS transmitter has been also achieved in[13]and[14].

2.1.2 III-V Compound Semiconductor-based Transmitters

In recent years,the transmitters based on III-V compound semiconductors such as indium phosphide(InP)and gallium arsenide(GaAs)have been also intensively studied as THz sources.For instance,a D-band transmitter[15]and a 300 GHz transmitter[16]have been designed via 250 nm InP process.In terms of the GaAs materials,the authors in[17]successfully realize a 300 GHz transmitter by using 35 nm GaAs process.The key progress on THz transmitters based on III-V compound semiconductor technology are surveyed and shown in Table 2.The table is also organized with respect to process technology,operation frequency and output power.THz transmitters with output power of-4 dBm-10 dBm have been realized based on III-V compound semiconductors.The operation frequency is usually below 300 GHz.

The THz communication systems employing III-V compound semiconductor-based transmitters have received ubiquitous attentions as well in recent years.For example,data rates of 20 Gbit/s operating at 300 GHz based on 70 nm InP process has been demonstrated in[20].Wireless transmission of 50 Gbit/s at 300 GHz by using advanced InP technologies has been also reported[23].Later on,by employing high gain parabolic antennas,the authors in[21]demonstrate a data rate of 64 Gbit/s over a 850 m wireless link.

2.1.3 Diodes-based Transmitters

THz diodes including resonant tunneling diode(RTD)and Gunn diode,have been widely employed as THz signal sources[24].Among them,the RTDs are ubiquitously used for THz communication.

The operation principle of RTDs is based on resonant tunneling effect,which is firstly observed by Esaki and his colleagues in 1974[25].Since the transition time to a tunnel through a thin layer is very short,a RTD is well qualified for ultrahigh-speed electronic devices,even in the THz frequency region.In[26],a 16-element RTD oscillator array with output power of 28μW operating at 290 GHz is realized.After-wards,the RTD oscillators operating at 1 THz and 1.1 THz have been also achieved[27,28].The oscillation frequency of RTDs are further increased to 1.55 THz by decreasing the length of slot-antenna[29]and 1.92 THz by lowering the conduction loss[30].The oscillation frequency of RTDs with corresponding output power are summarized in Figure 3.

Table 2.The key progress of THz transmitters based on III-V compound semiconductor.

Figure 3.Progress of RTD in relation to oscillation frequency and output power[26-37].

RTD oscillators possesses quantities of strengths,such as compact and low power consumption.Furthermore,the output signal of the RTDs is easily modulated via a bias voltage[31].These features are suitable for THz wireless communication.For instance,data rates of 11 Gbit/s with wireless distance of 10 cm is presented based on an RTD[38],and then data rates of 34 Gbit/s at the 500 GHz band by employing an RTD THz oscillator is demonstrated[39].

The advantages and disadvantages of different solidstate electronics THz transmitters are summarized in Table 3.THz devices based on the III-V compound semiconductor technology exhibit larger output power in comparison with that based on silicon technology.Nevertheless,attributed to the low-cost and high integration,Si-based technology remains its competitive advantage.Currently,the operation frequency of THz transmitters based on silicon and III-V compound semiconductor is less than 500 GHz,while RTDs will be a good choice for the frequency beyond 500 GHz.The oscillation frequency of currently reported RTDs ranges from tens of GHz to around 2 THz.However,low output power is the main obstacle to limit their applications in communications.Indeed,plenty of research progress have been witnessed in the domain of solid-state electronics.The inherent bandwidth-limiting and complicated circuit design still limit their applications in ultrafast THz wireless communications.

2.2 Photonics Technology

The performance of semiconductor devices deteriorates as the operation frequency increases,which is the major obstacle for high-speed THz wireless communications.However,the photonics technology,which has been ubiquitous studied in recent years,possesses ultrabroad bandwidth for extremely high-speed applications,especially used as a THz source.

As we know,photonics-based technology features quantities of advantages.First,it offers intrinsic high tunability and available modulation bandwidth[40].Besides,driving THz transmitters remotely through low loss optical fibers is another merit provided by photonics.Third,in comparison to electronics-based solutions,the schemes of employing multi-carrier,flexible carrier switching and various complex modulation formats can be easily implemented by photonics[41].Therefore,photonics technology can not only improve the capacity of THz wireless communication systems,but also bridges the wireless linkswith fiber-optic links seamlessly.Furthermore,by employing long optical fibers,the functionalities of signal modulation and THz emission can be separated,which provides great flexibility in next generation networks[42].

Table 3.The comparison of different solid-state electronics THz transmitters.

With regards to the photonic transmitters,the photomixer-based method is commonly used.Figure 4 presents a configuration of transmitters based on photo-mixing.By heterodyne detection of two optical beams,a THz signal is generated.Here,one optical beam is usually employed for baseband signal modulation,and the other acts as an optical beat beam,and then the combined beams are superimposed to a photodiode for THz signal generation.

Figure 4.Schematic configuration of transmitters based on photo-mixing.

2.2.1 Uni-Traveling Carrier Photodiode(UTC-PD)

In 1997,a novel ultrafast photodiode named unitraveling-carrier photodiode(UTC-PD)was invented[43].In the conventional positive-intrinsic-negative photodiodes(PIN-PDs),both electrons and holes have an effect on the response current.Consequently,the overall performance is determined by holes with low velocity.However,as for the UTC-PDs,only electrons are employed as active carriers which significantly improves the saturation output current[44].In terms of the evolution of the UTC-PDs,significant contributions have been made by the Nippon Telegraph and Telephone Public Corporation(NTT)of Japan.In 1998,a UTC-PD which possesses a 150 GHz bandwidth was reported[45].Later on,an enhanced UTCPD with 310 GHz bandwidth was fabricated in 2000[46]and in 2003 an emission power exceeding 20 mW at 100 GHz was obtained[47].By integrating UTCPDs with slot antennas,the emission power of 28μW operating at 700 GHz was realized in 2017[48].Except the NTT institution,the authors from the university college London have also achieved a UTC-PD with 148μW output power operating at 457 GHz[49].

The UTC-PD features high output saturation current,fast response time and high-level integration,which make it a promising candidate for performing THz transmitters.Recently,UTC-PDs have been widely used in establishing photonics-assisted wireless communication systems.During the 2008 Beijing Olympic Games,for example,the NTT developed a photonics-aided THz wireless communication system to deliver the full-HD broadcast signals at 120 GHz[50].In addition,a multichannel THz wireless link of 100 Gbit/s is reported by employing a UTC-PD as transmitter[51].Wireless transmission of 100 Gbit/s operating at 350 GHz[52],and 50 Gbit/s operating at 300 GHz[53]have been successfully demonstrated by using a UTC-PD as the emitter.

2.2.2 Quantum Cascade Lasers(QCL)

The principle quantum cascade laser(QCL)is the electron transition between conduction bands in semiconductor quantum wells and phonon-assisted resonance tunneling.A revolutionary progress occurred in the semiconductor lasers domain is the invention of the first mid-infrared QCL at Bell Labs in 1994[54].Eight years later,another breakthrough arose in the THz field is the realization of THz QCL[55].Since then,quantities of potential applications based on THz QCL,such as spectroscopy,sensing and communications,have been in the fast track of development[56].Up to date,the QCLs possessing peak power of 56 mW in pulsed mode have been obtained[57].In[58],the authors report a 4.4 THz QCL with peak power of 248 mW in pulsed operation.

The QCL possesses a wide range of significant advantages,such as short carrier lifetime,fast intrinsic response,high output power[59]and high-level integration,high sensitivity[60],which exhibits a great potential in high-speed THz communication links.Wireless transmission of 2 m at 4.1 THz[61],and 2.4 m at 3.9 THz[62]are demonstrated by employing a QCL as the transmitter.However,the operation of QCLs usually requires a low temperature,which therefore sets a major obstacle for its practical application in the THz communications[63].

The advantages and disadvantages of different photonics THz transmitters are summarized in Table 4.Firstly,the output power of UTC-PDs is relatively low(usually less than few mW),which constrains the achievable transmission distance in a THz wireless communication system.A QCL supports not only a higher carrier frequency operation(up to several THz),but also simple direct modulation formats(such as OOK).However,the requirement of low temperature environment keep it beyond scope of practical applications in the coming years.Furthermore,the majority of solid-state electronics transmitters operate in the lower THz band(usually less than 1 THz).When the frequency exceeds 1 THz or even rises to 10 THz,photonics-based transmitters such as photo-mixer devices UTC-PD and QCLs are expected to play a dominant role.

III.RECEIVERS OF THZ COMMUNICATION SYSTEMS

Regarding the receivers for THz wireless communication systems,direct reception and heterodyne reception schemes are typically adopted.Figure 5(a)is a schematic diagram of direct reception which can be implemented by either a Schottky barrier diode(SBD)or a quantum well photodetector(QWP).The schematic diagram of heterodyne reception based on electronics with a subharmonic mixer or a Schottky mixer are shown in Figure 5(b).Besides these,the heterodyne reception based on photonics is shown in Figure 5(c),which involves a photoconductive antenna or a photodiode.

Figure 5.Schematic of the receivers in the THz communication systems(a)direct reception based on electronics,(b)heterodyne reception based on electronics and(c)heterodyne reception based on photonics[64].

3.1 Direct Reception

3.1.1 Schottky Barrier Diode(SBD)

Schottky barrier diodes(SBDs)are named by their inventor,Dr.Schottky,and they are basically made by using metal-semiconductor(M-S)contact property.Since the current transportation of M-S contacts mainly relies on majority carriers(electrons with high electron mobility),and the M-S junctions can be accurately manufactured on the submicron scale.Therefore,the SBDs can be used in both sub-millimeter wave and THz wave bands.In[65],an SBD receiver integrated with a log-spiral antenna with a sensitivity of 20 V/W at 100 GHz is demonstrated.Besides,theauthors present an InP-based SBD operating at 300 GHz which shows a sensitivity of 1000 V/W[66].In[67],the performance of an SBD is further improved to a responsivity of 4000 V/W at 100 GHz.

Table 4.The comparison of different photonics THz transmitters.

The SBD holds numbers of advantages.First of all,it offers a short response time and high responsivity[68].Secondly,it inherently exhibits small reverse recovery time[69]and low forward voltage[70].In addition,it functions direct envelope detection without a local oscillator(LO)signal[71].Its potential as a promising detector candidate for THz wireless communication has been arisen.For instance,wireless transmission of 1 Gbps operating at 300 GHz based on a SBD has been demonstrated[65].Moreover,the authors present a 240 GHz wireless transmission links of 5 Gbit/s with short reach[66],and data rates of 24 Gbit/s operating at 300 GHz has been also conducted[72].

3.1.2 Quantum Well Photodetector(QWP)

A quantum well infrared photodetector(QWIP),which has become a mature technology covering the 3-20μm wavelength region,was mainly used for midinfrared spectroscopy[73].With the rapid evolution of diverse THz applications,a THz quantum well photodetector(QWP)exhibits its enabling value.The operation principle of a THz QWP is usually based on the transition between sub-bands,where the bounded electrons in the QWP will transform into the continuous state when the QWP is illuminated by THz signals.Then,the photocurrent is generated in the QWP,and the THz signals can be recovered by measuring the photocurrent[74,75].For instance,a wideband THz QWP with response bandwidth of about 20 THz is realized in[76].

A THz QWP possesses a large amount of advantages.Firstly,The THz QWP typically has high detection sensitivity.Secondly,owing to the mature process technologies,the QWPs are suitable for integration[76].Significant potentials have been shown for the receivers of THz communication.In[77],the authors demonstrate a 3.8 THz wireless transmission link by using QWP as the receiver.Afterwards,a wireless transmission link of 2.2 m operating at 4.13 THz is demonstrated,in which a spectrally-matched QWP is used as the receiver[78].Subsequently,the authors in[79]present a fast detection system by using a THz QCL and QWP,which confirms again that the THz QWP is a promising candidate for high-speed transmission systems.

3.2 Heterodyne Reception

3.2.1 Subharmonic Mixer(SHM)

An electronic mixer is widely used as the receiver for heterodyne reception of wireless communication signals,which shifts a received high frequency signal to the low frequency band for signal processing.However,as the frequency raises into the THz band,the requirements for a LO signal are getting increasingly critical,particularly a fundamental mixer.Alternatively,a SHM mainly uses the nonlinearity of a diode to obtain the n-th(2,4,6...)harmonics of the LO and then mixes with the received signal,eventually outputs a corresponding intermediate frequency(IF)signal,which in turn significantly decreases the frequency of the LO signal.In 1974,the first sub-harmonic mixer was realized by M.Cohn and his colleagues[80].In 2005,a subharmonic mixer whose conversion gain is 23 dB at 60 GHz based on SiGe technology was realized[81].Besides,the authors in[82]demonstrated a SHM with 4.5 dB conversion loss at 73.5 GHz for E-band wireless communication in 2010.Afterwards,a receiver at frequency around 245 GHz based on 130 nm SiGe technology was reported in 2013[83].In 2017,by using the 130 nm SiGe technology,the authors in[84]demonstrated a wideband SHM,featuring a high conversion gain and good linearity above 100 GHz.The table 5 summarizes the performance in terms of process technology,operation frequency,conversion gain,LO power and power consumption of the SHMs.The majority of reported SHMs operates below 300 GHz and the power consumption is less than 100 mW.

In comparison with a fundamental mixer,the SHMs requires only a fractional LO signal[84]while a good receiver sensitivity,which therefore exhibits great potential for THz wireless communications.For instance,a 280 GHz THz wireless link with 100 Gbit/s data rate employing a SHM receiver has been demonstrated[85].Furthermore,the authors present wireless transmission of 100 Gbit/s operating at 300 GHz band using a SHM as the heterodyne receiver[86].

3.2.2 Photoconductive Antenna(PCA)

A photoconductive antenna(PCA)is an electronic switch based on semiconductor or insulator whose conductivity increases when it is exposed to a light.The PCAs can operate not only in the pulsed mode but also in the continuous wave mode[63].The schematic representation of detecting THz waves with a PCA operating in the pulsed and continuous wave mode is shown in Figure 6.With regards to the pulsed mode,in the early 1980s,the emission and detection of a pulsed THz-wave system was first implemented based on a PCA by the THz pioneers,D.Auston[91-94]and D.Grischkowsky[95].As shown in Figure 6(a),when a PCA is illuminated by a femtosecond laser and the photon energy of the laser is higher than the energy gap of the material,a large amount of electron-hole pairs will be generated in the PCA.Under the bias of an external THz electrical field,these electron-hole pairs are accelerated,and a transient photocurrent is formed.The THz signal can then be recovered by measuring the photocurrent.Up to date,the PCAs in the pulsed mode have been widely used in THz systems,with a special emphasis in the THz time domain spectroscopy systems(THz-TDS).

Figure 6.Schematic representation of THz wave detection with a PCA operating in the(a)pulsed mode and(b)continuous wave mode.

With respect to the continuous wave mode,in the early 1990s,a continuous THz wave generation via the PCA used as a photo-mixer was reported[96,97].As shown in Figure 6(b),two optical carriers from laser 1 and laser 2,are coupled and act as a photonics LO for the PCA.Then THz wave mixed with photonics LO to generate IF signal.After the amplification of the transimpedance amplifier(TIA),the IF signal is sampled by the oscilloscope.In this context,the unique advantages make it a promising candidate for THz wireless communications.In 2017,the first THz wireless transmission system using optoelectronic down-conversion based on a PCA receiver was reported[98].Two years later,a multiple-channel wireless transmission of 58 m with data rates of 30 Gbit/s was demonstrated[99].Compared to conventional electronic receivers with limited bandwidth,the optoelectronic reception approach based on the PCAs also exhibits great potential to build up a new class of ultrabroadband THz communication systems,as the PCA features an extremely large response bandwidth.

In summary,the direct reception can be easily achieved by using diode detector or QWP,while the heterodyne detection so far can provide higher sensitivity and wider bandwidth by mixing the receiving signals and a LO at the receiver(such as SHM and PCA).In addition,direct reception usually relies on the non-linearity of device transmission to rectify the THz radiated power to a direct current(DC)value,which consumes less power and fits for large-scalearray integration[100].However,owing to the LO and received signal mixing,heterodyne reception consumes more power in general,which need a larger circuit footprint.

Table 5.Progress of subharmonic mixers.

IV.SYSTEMS IMPLEMENTATION OF THZ WIRELESS COMMUNICATIONS

In this section,some system implementations of THz wireless communication are presented.Based on the different kinds of transmitters and receivers employed,we classify them into three:solid-state electronic systems,photonics-assisted systems and all-photonics system.

4.1 Solid-State Electronic THz Communication Systems

As we discussed before,THz wireless communication systems based on the solid-state electronics possess quantities of advantages such as high integration,high flexibility and high resolution.These features indeed meet the expectations of future generation communication technologies,and hence many solid-state electronic THz communication systems are demonstrated so far.For instance,a THz communication link operating at 625 GHz with data rates of 2.5 Gbit/s using all-electronic components by the Bell Labs has been demonstrated in 2011[101].By using the SBDs,a 140 GHz wireless link of 10 Gbit/s with transmission distance of 1.5 km has been reported[102].Besides that,12.8 Gbit/s 16-QAM modulation through 20 m wireless distance has been also achieved[103].Furthermore,the authors in[104]report a wireless transmission of 60 Gbit/s over 10 m based on the superheterodyne approach.As an example,this system will be introduced in details as below,for a purpose of better understanding the solid-state electronics systems.

The experiment setup of the super-heterodyne THz communication system from Stuttgart of Germany is shown in Figure 7.In this system,a baseband signal from a signal generator and a LO signal is fed into a X-band mixer to generate an IF signal.Subsequently,the signal is used to drive a 300 GHz mixer,whose LO signal originates from two multiplier modules with a multiplication factor of 12 and 3,respectively.After 10 m free space link,the signal is received by a 300 GHz mixer which is symmetrical as the transmitting mixer.Afterwards,the output IF signal mixed with LO signal in the X-band mixer to generate baseband signal.Finally,the baseband signal is sampled by the oscilloscope[104].

4.2 Photonics-assisted THz Communication Systems

For the applications which need wideband and fast instant wireless access,bandwidth-limited solid-state electrics may not be a wise choice.However,photonics-aided technique combining the electronics and photonics exhibits significant advantageous potential to provide a feasible solution.In fact,it has been widely used and progressively developed in the field of THz wireless communications.For example,wireless transmission of exceeding 1 km with 10 Gbit/s has been demonstrated[105].The authors in[106]present a 110 m wireless transmission link with 100 Gbit/s by using a SBD direct receiver.Furthermore,by using more spectrally efficient modulation formats,such as 64-QAM modulation with probabilistic shaping,a 132 Gbit/s photonics-aided wireless link at 450 GHz is obtained[107].

Figure 7.Experiment setup of the THz communication system using a super heterodyne approach from Stuttgart of Germany[104].

Most recently,a record data rate of up to 600 Gbit/s is experimentally demonstrated by multiplexing antennas and benefiting from advanced DSP algorithms[108].In addition,we also report a 26.8 m wireless transmission link with data rates of 119.1 Gbit/s featuring both high speed and long distance in 2020[109].This representative THz-over-Fiber system is shown as follows.As shown in Figure 8,a light beam from a laser is launched into an optical modulator to perform the modulation of baseband signals.After the amplification via an optical amplifier,the signal transmits to a base station.Then,an optical filter is employed to suppress noise.Subsequently,the output signal from a polarizer is coupled with another laser beam,and then superimposes to a photodiode for generating THz signals.Afterwards,the THz signals from photodiode are radiated into the free space towards the receiver via a pair of high gain antennas.At the receiver,the LO signal mixed with the THz signal to generate an IF signal by using a Schottky subharmonic electronic mixer,which is finally sampled by an oscilloscope[109].

4.3 All-Photonics THz Communication Systems

In addition to two classes of THz communication systems above,all-photonics systems also began to raise the interest of several research groups.As early as in 2005,a wireless audio transmission link employing a pair of PCAs as THz transmitter and receiver is first reported[110].A data rate of 20 Mbps is achieved with an on-off-keying(OOK)modulation at 3.27 THz using a QCL[111].Besides these,a data rates of 30 Gbit/s with wireless transmission of 58 m is successfully demonstrated by using a PCA as a photonic heterodyne receiver[99].The schematic diagram of this THz system link from KIT of Germany is illustrated in Figure 9.

As shown in Figure 9,the light beam from a laser is fed into an optical modulator for baseband signal modulation.The modulated optical signal coupled with a LO light,and then superimposes to a photodiode to generate a THz signal.By using a horn antenna,the THz signal is then radiated into a line-of-sight space link.Within the wireless link,a wireless repeater is performed,where two cascaded THz amplifiers are used to boost the received signals for the second wireless propagation towards the PCA.Similarly,two optical carriers are coupled and act as a photonic LO for the PCA at the receiver side.Afterwards,the THz signal mixed with photonic LO in the PCA operating in the continuous wave mode to generate an IF signal,which is stored by the oscilloscope for further processing[99].

Figure 8.Schematic of photonic-assisted THz-over-Fiber system from Zhejiang University of China[109].

Figure 9.Experimental setup of THz wireless links from Karlsruhe Institute of Technology of Germany[99].

Comparing the aforementioned three types of THz communication systems,some observations can be drawn as below.Solid-state electronic systems generally possess lower transmission rates due to the bandwidth-limitation,however longer wireless distance.The photonics-assisted systems can support larger capacities,particularly with the data rates exceeding 100 Gbit/s.Therefore,significant potentials have been witnessed in high-speed THz communications,and the photonics-assisted systems indeed attract more attentions in recent years.An all-photonics THz system usually has a relatively larger operation bandwidth,however lower transmission rates at the current research stage,due to narrow bandwidth limitation of transimpedance amplifiers which provide very high gain.From the practical viewpoint,integrated solid-state electronics-based systems are more miniaturized and compact,while photonics-assisted and all-photonics systems are still on their way to accelerate the system-on-a-chip integration,and they are recognized as a promising solution to provide very high data rates for future communications.

Development progress on THz wireless communication systems in recent years is summarized in Table 6.The table is arranged in the order of electronics offline system,electronics real-time system,photonics offline system and photonics real-time system,which contains data rate,operation frequency,distance,modulation format and output power of THz wave.The typical THz output power ranges from-20 dBm to 13.5 dBm with regards to the electronic systems,and is usually less than 0 dBm when it comes to the photonics systems,which is mainly restrained by low conversion efficiency of optical-to-THz(O/T)converters.

V.KEY TECHNOLOGIES AND CORRESPONDING CHALLENGES OF THZ COMMUNICATIONS

In this section,several key technologies and corresponding challenges of THz communication systems are discussed.The THz integrated chips and modulators are firstly described.Subsequently,we discuss THz antennas and amplifiers,which can be beneficially employed,to the best of our knowledge,to alternatively improve the link budget.

5.1 THz Integrated Chips

Over the past ten years,significant developments have been witnessed in the field of THz integrated technology including electronic and photonic-aided systems.With regards to electronic systems,integrated transmitter chips with output power in the range of 0μW-10 mW has been realized based on semiconductor technologies including Si-based and III-V compound semiconductor-based approaches.In addition,the receivers such as the SHMs can also be fabricated based on above technologies.Both of them have been discussed above.

In terms of the photonic-aided systems,the majority of photonic systems typically consist of a lot of discrete optical components,which causes the issues on compactness,robustness,energy efficiency,and so on.As a consequence,photonic systems are still below the needs of practical applications,and more efforts are really appreciated.In this context,the heterogeneous integration of electronic devices and photonic devices by employing current fabrication process are expected to achieve the THz transceivers featuring low-cost and compact[42].For instance,based on the silicon photonic platform,Harter and his colleagues demonstrate a novel approach employing a plasmonic internal-photoemission detector(PIPED)to generate and detect THz waves.The plasmonic internal-photoemission detector can be integrated to the chip level,which strongly boost the integration of photonics communication systems[124].By employing a silicon photonics transmitter,the first photonicsassisted THz communication system featuring both high integration level and mass production capability is reported and data rates of 10 Gbit/s real-time wireless transmission is successfully demonstrated[125].In addition,a heterogeneously integrated transmitter based on Si-InP platform including tunable lasers,an optical modulator,optical amplifiers,pin photodiode is also demonstrated,which facilitates the practical application of photonics-assisted THz communication links[126].Photonics integration is currently in a fast development track as it indeed provides numbers of advantages,such as eliminating the fiber coupling loss among the different components,and hence reduced energy consumption[127].From the prospective viewpoint,the revolutionary technical breakthroughs in photonic integration is expected to make‘THzsystem-on-a-chip’come into true.As a benefit,THz photonic wireless communications is foreseen to be dramatically accelerated,and to be brought closer to our reality for ultrafast wireless accommodation scenarios in the near future.

However,there are still some challenges in THz integrated chips for photonic communication systems.First,realizing high conversion efficiency in opticalto-THz and THz-to-optical converters is very challenging,which directly affects the obtainable signalto-noise ratio(SNR)for communications.In addition,as the frequency increases to the THz band,the size of device is getting smaller and smaller.Therefore,developing the cost-effective packaging technology of components is another challenge to be addressed[128].

5.2 Modulators

The modulators which are used to modulate baseband signal into carrier wave play a vital role in THz communication systems.In recent years,the emergence of novel two-dimensional(2D)materials such as graphene provides a new opportunity for THz

and optical modulators.The graphene materials have attracted increasingly attention owing to its excellent properties including extraordinary carrier transport properties[129],high carrier mobility and tunable conductivity[130],high modulation speed and depth[131].Graphene has been recognized as a promising material candidate for modulators.

Table 6.PROGRESS OF THZ WIRELESS COMMUNICATION SYSTEMS.

With regards to the electronics systems,THz modulators are indispensable.By using the transmission of metallic ring apertures on a single layer of graphene,the author in[132]achieve a 50% modulation depth in 2014.Besides,by using the electrically gatecontrollable properties of graphene,a phase modulator with phase shift of 32.2 degree has been realized[133].As for the photonics systems,optical modulators are of vital importance as well.In 2012,the first optical modulator employing double-layer graphene structures operating at 1 GHz with high modulation depth has been designed[134].In 2018,a graphene-silicon phase modulator integrated in a Mach-Zehnder interferometer has been demonstrated,which operates in the GHz regime[135].

Unfortunately,although the modulators evolve rapidly in recent years,the current modulation bandwidth is still beyond our expectation,which is actually one of the major obstacles limiting the achievable capacity of THz wireless communication systems[136].Besides,there are some other challenges remaining to be addressed,with regards to energy consumption,insertion loss,etc[137].

5.3 THz Antennas

For THz wireless communication systems,THz antennas are indispensable devices to transmit and detect THz waves.THz antennas have many available types.In addition to the photoconductive antenna mentioned earlier,there are also dielectric lens planar antennas,horn antenna,Cassegrain reflector antennas and lens antennas,and so on.However,as the operation frequency increases,the size of THz antenna becomes smaller and smaller.Consequently,a variety of challenges faced in the THz antennas have arisen.The key challenging aspects are related to the inadequate resonant frequency,low fabrication accuracy and relatively high loss[138].Another challenge of THz antenna is how to make it radiate effectively[139].

As mentioned above,THz wave suffer from a relatively strong attenuation in the atmosphere especially in rain or haze weather.In the future,wireless pointto-point THz wireless links within small distances are ubiquitously used to support a high-speed wireless communication.Therefore,realizing the THz antennas with high gain and high directivity to make up for high path loss is a crucial issue.For overcoming the limited effective area and small gain of single THz antenna,large scale phased-array antennas with small footprints can also provide high gain to compensate the free space path loss.By adjusting the phases and amplitudes of the phased-array antenna and concentrating the energy in pre-concerted directions,will significantly improve the link budget[136].Therefore,a THz beam with high-gain and high-directivity will be obtained,which will significantly improve the connectivity and data capacity of wireless communication links.

However,large-scale phase array antennas also face some challenges.Owing to the small size of THz antennas,highly precise process technology is essential to assemble antennas in the THz region,which is however hard to be achieved based on current process technologies.In addition,effective reduction of the mutual interference caused by different THz antennas in the array is also an open issue[138].

5.4 THz Amplifiers

As we know,one of the most important obstacles for THz being practical in wireless communication systems is the high path loss.THz amplifiers are recognized as a powerful way to generate high power THz signal.Based on InP process,a 0.48 THz amplifier with a 11.7 dB gain is achieved[140].The author in report the first packaged low noise THz amplifiers operating at 850 GHz,the noise figure and gain of which are 11.1 dB and 13.6 dB,respectively[141].Besides,a 2-stage THz amplifier operating at 260 GHz based on 130 nm SiGe technology is reported,the output power of which is 0 dBm[142].Although the gain of reported THz amplifiers evolves rapidly,the saturation output power is still restrained(usually below 0 dBm).In principle,the parasitic capacitances in the transistors limit the size of the amplifiers,which in turn restrains the achievable saturation power.In future,great efforts must be devoted to realizing THz amplifiers with high saturation output power,which is beneficial to compensate the high free space path loss.

Figure 10.THz band for establishing space-air-ground communication links.

VI.VISIONS OF THZ COMMUNICATIONS

In this section,we will discuss some promising research directions that will further facilitate the evolution and vision of THz wireless communication systems based on our knowledge,such as space communication link,THz secure communication and THz wireless access and backhaul networks.

6.1 Terahertz for Space Communication Links

As we all know,THz wireless propagation on the ground suffers from high atmospheric attenuation due to the absorption of the molecules of water and oxygen,which consequently shortens the wireless coverage and achievable data rates severely.However,the moisture effect is trivial,the attenuation is significantly reduced in space with heights above 16 km,which can certainly benefit from THz communications with huge bandwidth and acceptable difficulty in precise beam tracking.Compared with the conventional band spacecraft systems using highly complex bulky radio architectures for high data rate transmission,a THz wireless link has presented its potential of providing low complexity and high-speed communication for space applications[143].

In addition,large-scale phase-array antennas featuring high gain,fast scanning ability and automatic alignment can be applied in satellite communication links to enhance the connectivity of space links.In the future,an increasing number of orbit satellites will be deployed.THz communication is believed to play an essentially important role in building a broadband high-speed information networks for satelliteto-satellite,satellite-to-ground,which serves as the key enabler for the realization of integrated space-airground networks as shown in Figure 10.

6.2 Terahertz for Secure Communications

Due to the openness of the wireless transmission medium,wireless communication is vulnerable to signal interception[144].The THz band holds some characteristics suitable for secure communication.Firstly,high atmospheric attenuation makes the propagation distance short.Secondly,compared to microwave communications,it possesses highly directional and narrow beams.Thirdly,it exhibits less scattering of radiation in comparison with infrared radiation.Besides,it provides large bandwidth for spread spectrum techniques which facilitate the realization of anti-interference and low detection systems[145].

In recent years,scenarios for secure communication including unmanned autonomous vehicles and military applications have attracted increasing attentions.

Figure 11.Envisioned future wireless access and backhaul networks scenarios utilizing the THz wave.

As for the unmanned autonomous vehicles,for the aim to guarantee the safety of vehicle traffic,a reliable communication link is essential.In terms of the military applications,the accuracy and real-time characteristics of information are crucial.Therefore,deploying a secure THz wireless link to ensure the military information can instantly and accurately transmitted to the command office is of vital importance.

However,THz beams are not absolutely secure.The author in[146]demonstrated that the signals transmission can be successful eavesdropped even at high frequencies such as millimeter or THz wave.The eavesdropper can,for example,place a smaller passive object in the path of wireless transmission,and the scattering of the signal will be successfully eavesdropped.To alleviate the eavesdropping mechanism,a technique to characterize the backscatter of the channel is presented.Besides,the author emphasizes that in order to apply security to THz wireless communication links,great efforts are supposed to devoted in the study of new physical layer structures and transceiver designs.

6.3 Terahertz for Wireless Access and Backhaul Networks

As we describe before,wireless access and backhaul networks with large capacity is of course a straightforward promising application for THz wireless communications.Figure 11 illustrates the wireless access and backhaul networks scenarios of using the THz wave.Owing to the surge of mobile users,wireless access and backhaul networks with data rates of up to hundreds of Gbit/s are of the essence in the near future.Considering the high-capacity and the mass deployment in existing optical fiber links,THz photonic wireless technology offers an effective solution.In such a scenario,the THz wireless communication links serves as a combiner between the wireless networks and optical fiber networks.

Again,high atmosphere attenuation of THz wave is still the major obstacle for implementation of largearea radio communications.More critically,limited by the currently obtainable emission power of THz transmitters,the range of THz communication is estimated as several kilometers[147].To overcome this situation,a THz point-to-point wireless links with short reach to support high-speed access and backhaul is so far predictable.In addition,with the explosion of mobile users,how to establish stable connection between terminals and THz links makes a difference.Although the user’s relative position varied with time,the moving speed is much slower than data transmission rate.Hence,the system seems to be static during the period of data transmission[148].In the future,quantities of base stations,even more than the number of mobile users,are supposed to be deployed for enhancing the connectivity and high-speed wireless transmission.

VII.CONCLUSIONS

In this paper,a comprehensive literature review on the evolution of THz communications has been provided,with respects to the transmitters,receivers as well as key technologies enabling THz communication systems.To begin with,the transmitters based on both electronics and photonics technologies are reviewed.Between them,electronics-based transmitters possess higher emission power,while photonics-based transmitters are expected to play a dominant role in the frequency band beyond 1 THz.At the receiver side,direct detection and heterodyne receivers are commonly employed in supporting THz communication systems.By comparing these two reception modes,the heterodyne detection exhibits better sensitivity and wider bandwidth,while direct detection is more costeffective.Among three kinds of THz wireless communication systems:solid-state electronic systems,photonics-assisted systems and all-photonic systems,photonic systems show more preponderance in delivering higher data rates while electronic systems are superior in term of the wireless distance.From the sustainable development viewpoint,THz integration chips,modulators,THz antennas and amplifiers are selectively discussed as four key enabling technologies and corresponding challenges for future THz communication systems.Benefit from new two-dimensional materials such as graphene,novel broadband THz modulators and high gain antennas are of great interests to accelerate the applications of THz communications,such as space communication links,THz secure communications,THz wireless access and backhaul networks.In summary,driven by the rapid progress on developing new THz electronics and photonics technologies,we believe hundreds of Gbit/s even Tbit/s wireless data transmission will be lighted up in foreseeable future.

ACKNOWLEDGEMENT

This work is supported by the National Key Research and Development Program of China(2020YFB1805700,2018YFB1801500&2018YFB2201700),in part by the Natural National Science Foundation of China under Grant 61771424,the Natural Science Foundation of Zhejiang Province under Grant LZ18F010001 and Zhejiang Lab(no.2020LC0AD01).