IEICE TRANS. ELECTRON., VOL.E98–C, NO.12 DECEMBER 20151071

INVITED PAPER Special Section on Terahertz Waves Coming to the Real World

High-Speed Coherent Transmission Using Advanced Photonics inTerahertz Bands

Atsushi KANNO†a), Pham TIEN DAT†, Norihiko SEKINE†, Iwao HOSAKO†, Tetsuya KAWANISHI†,††,Yuki YOSHIDA†††, Members, and Ken’ichi KITAYAMA†††, Fellow

SUMMARY A terahertz-wave communication system directly con-nected to an optical fiber network is promising for application to futuremobile backhaul and fronthaul links. The possible broad bandwidth inthe terahertz band is useful for high-speed signal transmission as well asradio-space encapsulation to the high-frequency carrier. In both cases, thelow-latency feature becomes important to enhance the throughput in mobilecommunication and is realized by waveform transport technology withoutany digital-signal-processing-based media conversion. A highly precise op-tical frequency comb signal generated by optical modulation and the vectorsignal demodulation technology adopted from advanced optical fiber com-munication technologies help perform modulation and demodulation withimpairment compensation at just the edges of the link. Terahertz wave,radio over fiber, waveform transport, coherent detection, multilevel modu-lation, radio on radio.key words: terahertz wave, radio over fiber, waveform transport, coherentdetection, multi-level modulation, radio on radio

1. Introduction

Connectivity to the Internet is now indispensable for access-ing information and knowledge, and thus, for enhancing thequality of human life. Traditionally, a high-speed connec-tion has been established by a wireline connection: a cop-per telephone line and an optical fiber. From the viewpointof advanced optical fiber communication technologies, thespeed of the link in a single channel has achieved 100 Gb/sfor metro and core networks, and now, 400-Gb/s/ch opticaltransport technology is being developed for next-generationhigh-capacity networks [1]–[3]. Additionally, in an accessnetwork, optical fiber cables are deployed to the home:a fiber-to-the-home (FTTH) scheme under a passive opti-cal network configuration. Thus, home users can have awireline connection to the Internet with a speed of up to1 Gb/s [4], [5]. However, as wireline-based communica-tion limits service areas, and thus, limits ubiquitous con-nectivity to the network, mobile communication based onadvanced radio technology is being dramatically developedto enhance connectivity. Now, Long Term Evolution (LTE)technology and its advanced form (LTE-advanced) service

Manuscript received July 15, 2015.Manuscript revised August 16, 2015.†The authors are with the National Institute of Information and

Communications Technology, Koganei-shi, 184–8795 Japan.††The author is also with the Faculty of Science and Engineer-

ing, Waseda University, Tokyo, 169–8555 Japan.†††The authors are with the Graduate School of Engineering,

Osaka University, Suita-shi, 565–0871 Japan.a) E-mail: kanno@nict.go.jp

DOI: 10.1587/transele.E98.C.1071

users with a connection speed on the order of 100 Mb/s; thespeed achieved is comparable to the speed of conventionalFTTH [6], [7]. For next-generation mobile technology be-yond LTE-advanced, so-called 5G, the aim is a connectionspeed of 10 Gb/s to subscribers; the speed will be compa-rable or faster than that for the FTTH [8]–[10]. To supportsuch high-speed communication, a base transceiver station(BTS) for mobile communication should be connected toan optical fiber network to support its high-capacity mobiletraffic as a backhaul link or even a fronthaul link. How-ever, in 5G, the coverage for each BTS becomes smallerthan that for LTE because of the enhancement in the con-nection throughput. In this scenario, a large number of BTSswill be installed in the field; many optical fiber cables willbe deployed. However, as the cables cannot be deployedsomewhere owing to geographical and cost issues, wirelessconnection technology to the BTSs as mobile backhaul andfronthaul links is strongly desired. Actually, the wirelessmobile backhaul has been already evaluated by millimeter-wave radio communication technology with a capacity onthe order of 1 Gb/s; however, it is not sufficient for evennear future LTE-advanced [11].

Terahertz-wave radio communication technology ispromising for the realization of a high-speed backhaul andfronthaul connection by a wireless feature owing to itshigh carrier frequency and its broadness of available band-widths [12], [13]. Moreover, 5G backhaul and fronthaullinks require an ultrahigh-speed feature and a low trans-mission latency compared to the conventional fronthaul us-ing digitized communication technology, such as a 10-Gb/scommon public radio interface (CPRI) and an open radioequipment interface (ORI) [14], [15]. The broad bandwidthof the terahertz radio meets the demands, and it is possibleto provide a high-speed link >10 Gb/s when the availablebandwidth is greater than 10 GHz. However, as the atmo-spheric attenuation coefficient in the band is estimated to be3 dB/km under standard atmospheric conditions without anyrain, the possible transmission distance could be shorter thanthe radio link by conventional microwave- and millimeter-wave radio [16], [17]. In addition, the seamless convergencebetween the optical and radio networks is demanded to en-hance the network functions such as virtualization and opti-mization of the network resources in future network config-urations (Fig. 1) [18].

Radio over fiber (RoF) technology can meet the de-mands for realizing feeder technology of the terahertz signal

Copyright c© 2015 The Institute of Electronics, Information and Communication Engineers

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Fig. 1 Concept of a terahertz backhaul and fronthaul directly connected to an optical fiber networkfor future mobile networks.

to a suitable area and seamless connection to the optical net-work [19]–[21]. Optical-to-electrical (O/E) and electrical-to-optical (E/O) conversion technologies provide direct con-version between the optical and terahertz radio signals with-out any processing latency, and its signal speed is muchfaster than 10 Gb/s [22]–[25]. IIn addition, advanced opticaldigital coherent detection can compensate for transmissionimpairments of the signal in the optical domain, even in theterahertz domain; a waveform of the signal is transportedover the fiber and the air between a transmitter (Tx) and areceiver (Rx) [26]. No conventional media conversion bydigital signal processing (DSP) is required in the O/E andE/O conversions; thus, the possible latency of the transmis-sion could be reduced.

In this paper, we propose and demonstrate high-speedterahertz signal transmission by advanced photonics tech-nology such as digital optical coherent detection and op-tical frequency comb (OFC) generation. An OFC gener-ator (OFCG) configured with optical modulation providesa terahertz-capable optical two-tone signal with low phase-noise characteristics. In Sect. 2, the details of optical signalgeneration for terahertz radio are presented. DSP-assistedcoherent detection can receive a 40-Gb/s-class vector signalsuch as quadrature phase-shift keying (QPSK) generated byan optical subharmonic IQ mixer (Sect. 3). For a reductionin the latency and encapsulation technology of conventionalmicrowave services, a terahertz radio employing radio onradio (RoR) technology is evaluated in Sect. 4. Finally, atransmission distance evaluation of the terahertz radio is dis-cussed in Sect. 5.

2. Optical Terahertz Signal Generation

For seamless conversion from the optical signal to the ter-ahertz signal, optical signal generation is key with a highfrequency stability in order to follow radio regulations. Inaddition, it is also important to maintain compatibility in thesignal generator, especially in the port configurations, withexisting modulators designed for conventional microwave

Fig. 2 Configuration of an optical subharmonic (IQ) mixer.

and millimeter waves. In the following sections, we discussthe modulator configuration by photonics and the enhance-ment in the signal-to-noise ratio (SNR).

2.1 Configuration of an Optical Subharmonic Mixer

To maintain compatibility of connection port configurationwith conventional modulators, we propose the optical sub-harmonic mixer (SHM) configuration shown in Fig. 2 [27],[28]. This consists of an OFCG, an optical filter bank(OFB), an optical modulator, and a high-speed photomixer(PM) based on a unitraveling-carrier photodiode [29], [30].The OFCG generates an optical frequency comb signal witha bandwidth broader than the required frequency in the ter-ahertz band. It should be noted that the generated OFC di-rectly affects the quality of the terahertz signal, such as thefrequency stability and phase-noise characteristic. From thispoint of view, a modulator-based frequency comb generatoris applicable because this OFCG has a frequency separa-tion provided by a microwave synthesizer driving the mod-ulator; that is, the stability could be comparable to that ofthe synthesizer with an order of 1 ppm. In this paper, fora proof-of-concept demonstration, we demonstrate a recir-culating frequency shifter in an amplified optical fiber loopfor OFC signal generation (Fig. 3) [31], [32]. An optical fre-quency shifter, which is based on an optical single-sideband(SSB) suppressed-carrier (SSB-SC) modulator connected toa local oscillator (LO), changes an optical frequency ofthe input optical signal with the frequency of the LO sig-nal [33]. In the amplified loop, this frequency-shifting pro-cess is performed many times by an erbium-doped fiber am-plifier (EDFA) compensating for the insertion loss of the fre-

KANNO et al.: HIGH-SPEED COHERENT TRANSMISSION USING ADVANCED PHOTONICS IN TERAHERTZ BANDS1073

Fig. 3 (a) Block diagram of a recirculating frequency shifter, and (b) itsgenerated optical frequency comb signal.

quency shifter; thus, the resulting OFC signal is output fromthe optical coupler. Figure 3 (b) shows an OFC signal gener-ated with the LO for the frequency shifter operated at a fre-quency of 10 GHz using a laser operating at a wavelength of1548.3 nm. Clear comb separation with a broad bandwidthof 5 nm, which corresponds to approximately 600 GHz, isobserved. This bandwidth is limited by an optical bandpassfilter (OBPF), which has an optical pass-bandwidth of 5 nm,located in the fiber loop, which suppresses unwanted comblines and amplifies spontaneous emission (ASE) noise. Asthe wavelength increases, the noise level also gradually in-creases according to the ASE noise generated by the EDFA.This might cause a degradation in a resulting SNR of the ter-ahertz signal. The SNR enhancement method is described inthe next subsection.

In the optical SHM, an OFB splits the OFC signal intotwo optical components with the frequency separation ofthe required terahertz carrier. An arrayed waveguide grat-ing structure is useful for this purpose as the filter bank. Thesplit signals are used for modulation and an optical refer-ence. An optical modulator is utilized for modulation; forsimple intensity modulation (IM), a simple Mach–Zehnderinterferometer-type optical modulator is applicable. An op-tical phase modulator and optical in-phase/quadrature (IQ)modulator are also applicable for phase and vector modula-tion with advanced modulation formats, respectively. Themodulated signal and optical reference are combined by anoptical coupler; then, a PM performs frequency downcon-version by the heterodyne process to generate the modu-lated terahertz signal [34]. In the optical SHM or subhar-monic IQ mixer (SHIQM), the configuration of the inputand output ports (LO input, data input, and modulated sig-nal output) is just the same as a conventional modulator usedin microwave communication systems. Therefore, the opti-cal SHM/SHIQM could be easily interchanged from con-ventional systems.

Fig. 4 Block diagram of an SNR enhancement system using an injection-locked laser.

Fig. 5 (a) Observed optical spectra and (b) phase noise spectra with(black) and without (gray) injection locking. The phase noise spectra forthe LO signals for an OFCG and SHM LO signal in a receiver are shownfor reference (dashed).

2.2 Signal-to-Noise Ratio Enhancement by InjectionLocking

An enhancement in the SNR of an optical two-tone signalgenerated by the OFCG described above can directly in-crease the SNR of the resulting terahertz signal. There aresome techniques to increase the SNR: stimulated Brillouinscattering (SBS) in the optical fiber and injection-locking toa free-running laser [35], [36]. SBS is a possible techniquefor regenerating the signal by the nonlinear effect in the op-tical fiber. In general, a narrow bandwidth of several tensof megahertz in SBS provides a high suppression ratio ofunwanted components; a high SNR could be obtained. Onthe contrary, an injection-locked laser also regenerates thehigh-SNR optical signal locked to a seed signal with a lowcomplexity. In this paper, the enhancement in the SNR bythe injection locking technique with a seed OFC signal isevaluated.

Figure 4 shows the configuration of the setup for sig-nal regeneration by injection locking. After the filter bank,the optical component picked from the OFC signal is inputinto a free-running Fabry–Perot laser diode (FPLD) passedthrough an EDFA, an optical circulator, and a polarization

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controller (PC). The OBPF suppresses the unwanted lon-gitudinal modes of the regenerated optical signal by theinjection-locked FPLD set after the circulator; then, the sig-nal is combined with an optical reference from the OFB. Theoptical two-tone signals with and without the injection lock-ing technique are shown in Fig. 5 (a). As compared with theseed signal, the optical SNR is increased by at least 10 dB.In addition, there is no degradation in the SSB phase noiseshown in Fig. 5 (b). These results show that the SNR en-hancement technique using an injection-locked laser is ca-pable of terahertz signal generation.

3. High-Capacity Transmission Using Baseband Mod-ulation

For the wireless backhaul, the connection links between theBTSs, and/or the entrance network and the BTSs, high-speed terahertz signal generation and its transmission are re-quired with a capacity greater than 10 Gb/s. In this scenario,baseband modulation by the optical SHIQM with assistanceof DSP can provide a signal with a high spectral efficiencyfor transmission over the fiber as well as air [37].

3.1 Concept of DSP-Aided Coherent Detection

Figure 6 shows the concept of DSP-aided coherent transmis-sion based on RoF technology. In the Tx (RoF Tx), a “radio-friendly” signal is synthesized by the optical SHIQM for di-rect radiation to air. Generally, the signal radiated to air asa radio should be consistent with the quality limited by ra-dio regulations. On the other hand, available active devicessuch as an amplifier have a bandwidth of 30–60 GHz in 300-GHz band, which corresponds to a fractional bandwidth of10–20% in general. As the available bandwidth for the com-munication system is limited by these devices in each chan-nel, a spectrally efficient signal should be synthesized for100-Gb/s-class signal transmission. In this scenario, a vec-tor modulator (optical SHIQM) is employed for terahertzsignal generation. In the O/E conversion by the PM, the ter-ahertz signal is radiated and transmitted over the air. At theRx, a heterodyne-detector-based coherent frequency down-conversion block reproduces an intermediate frequency (IF)component from the received terahertz signal because of thelimited bandwidth of the analog-to-digital converter (ADC).After capturing the signal by the ADC, DSP proceeds withcarrier recovery including frequency downconversion to thebaseband, phase noise compensation, equalizing, and de-modulation, which is similar to the DSP implemented foradvanced optical digital coherent detection. Therefore, DSPfor the optical system can be adopted for the terahertz re-ceiver with small modifications. In this system, the DSPcompensates for the transmission impairments over the fiberas well as those over air. In addition, a waveform includingthe modulation format, symbol rate, etc., is kept in the entirelink. Therefore, the waveform is directly transported fromthe Tx to the Rx in any transmission media.

It should be noted that coherent detection based on het-

Fig. 6 Block diagram of DSP-aided coherent terahertz signal transmis-sion by an optical SHIQM.

Fig. 7 Experimental setup for DSP-aided coherent QPSK signal trans-mission at 325 GHz.

erodyning can enhance the sensitivity of the receiver usinga high-power LO signal; the sensitivity is key for terahertzradio communication because of its low transmitted powerdue to transmission attenuation. However, the frequencydifference between the irradiated carrier and a free-runningLO for the heterodyne detector produces intensity and phasefluctuations in the regenerated IF signals. Thus, the DSP forcompensating for this carrier frequency offset (CFO) shouldbe implemented in the receiver.

3.2 Experimental Setup

The experimental setup for high-capacity transmission withhigh-gain antennas is shown in Fig. 7. The optical SHIQMoperates at 25 GHz as an LO generating a radio-friendly op-tical signal at a carrier frequency of 325 GHz with a QPSKsignal whose symbol rate is 20–32 Gbaud. The seed signalsfor QPSK generation are provided by a two-channel pulsepattern generator with a pseudorandom bit stream (PRBS).The generated optical signal is boosted by an EDFA, fol-lowed by an OBPF, for suppression of amplified sponta-neous emission noise. The amplifier optimizes the opticalpower level for input into a PM. The PM converts the op-tical signal (with a frequency separation of 325 GHz) to a325-GHz terahertz-wave signal. An antenna with a gain of46 dBi radiates the signal to air, and the antenna at the re-ceiver collects the transmitted signal. It should be noted thatan offset parabolic antenna made from aluminum is used asthe antenna for both the Tx and Rx [18]. An electrical SHM,operated at a 12.5-GHz LO signal, performs heterodyne fre-quency down-conversion from 325 GHz to 25 GHz. An IFamplifier optimizes the signal level for input into an ADCwith a sampling speed of 160 Gs/s. A digital storage oscil-loscope is used as the ADC, and offline DSP compensatesfor transmission impairments and demodulation.

3.3 Demonstration

The observed bit error rates (BERs) are shown in Fig. 8.The BERs for 5-m and 10-m transmissions are similar at20 Gbaud and within the forward error correction (FEC)

KANNO et al.: HIGH-SPEED COHERENT TRANSMISSION USING ADVANCED PHOTONICS IN TERAHERTZ BANDS1075

Fig. 8 Observed bit error rates for various transmission distances andsymbol rates. The constellation maps are also shown. The dashed lineindicates the FEC limit of the BER of 2 × 10−3.

limit of the BER of 2× 10−3 using a 7% FEC overhead [38].On the other hand, the under-15-m transmission with 20-Gbaud QPSK has some differences compared to the 10-mtransmission. This is because a Fraunhofer distance be-tween the Fresnel and Fraunhofer zones, which correspondsto the border between the near and far fields, is described by2D2/λ, where D and λ denote the aperture of the antennaand the wavelength of the radio signal, respectively. In thisstudy, D and λwere approximately 8 cm and 1 mm; thus, thedistance is estimated as 12 m [18]. This is the reason whythe behavior of the BER for 10 m and 15 m seems quite dif-ferent. The observed penalty between the two ranges is esti-mated at 8 dB. The penalty between the 5-m and 15-m trans-missions, estimated using the Friis propagation equation, isapproximately 4 dB. The difference between the estimatedand observed values is possibly due to a misalignment of theantenna direction. Constellation maps of the received QPSKsignal are also shown in Fig. 8. A clear symbol separationis exhibited during 20-Gbaud operation. It should be notedthat the expected radio power at 300 GHz is approximatelyless than −13 dBm when the photocurrent is 5 mA owing tothe specification sheet of the photomixer. By using a high-gain antenna pair, 20-Gbaud QPSK radio transmission canbe performed, even if the output power is less than 100μW.

4. Low-Latency Transmission by Radio on Radio

For application to the mobile fronthaul, which is a connec-tion between a signal processing unit and the remote ra-dio heads in the BTS, the transmission latency is key forthe mobile network. Particularly in next-generation mo-bile networks, a centralized radio access network config-uration will be implemented on the basis of a centralizedsignal processing unit connected to a large number of re-mote radio heads by the fronthaul links. In this scenario,the fronthaul technology will be important. Generally, theradio access network is defined as the range between mod-ulation/demodulation and the user equipment (UE) includ-ing the transmission line. Thus, the latency in the fronthaulshould be reduced in order to increase the throughput of

mobile communication. However, conventional fronthaultechnology is based on digitized signal transmission by anon–off keying (OOK); thus, an unavoidable processing la-tency in the signal processing unit and remote radio headsexists for signal conversion between the OOK signal andthe highly spectrum-efficient signal in the mobile network.In the optical domain, an analog RoF link has ultimately lowinvasiveness from/to existing radio services; thus, it is easyto realize by deploying fiber. To apply a similar radio-signalencapsulation scheme to the other radio carriers, the radiosignal can be delivered to the desired location by a wirelessnetwork. This RoR system is capable of avoiding the radiointerference between the delivered signal and the existingradio signals in the microwave bands, even if the signal istransmitted over air [39]–[41].

4.1 Concept of Radio on Terahertz over Fiber System

An RoR in the terahertz band has a great advantage of theavailable bandwidth [42]. In conventional RoR, a carrierfrequency is assumed in the millimeter-wave band, such as60 GHz with a 9-GHz license-free bandwidth. On the otherhand, the possible bandwidth will be greater than 10 GHzin the terahertz band; thus, the number of encapsulationsof microwave services will increase. To generate the RoRsignal in the terahertz band by photonics, an optical SHMis useful for analog modulation by the intensity modulationscheme (Fig. 9). The generated signal, the radio on tera-hertz over fiber (RToF) signal, is capable of transmissionover the fiber and air by terahertz radio. In decapsulation ofthe microwave signal, a simple envelope detector in the ter-ahertz band as well as a terahertz mixer can regenerate themicrowave signal without any DSP in the terahertz-to-radio(T/R) converter, i.e., a manner similar to the DSP-less me-dia conversion described in the sections above. Finally, themicrowave radio is transported via an optical fiber and tera-hertz radio to a UE. In this configuration, the RToF systemis applicable to the mobile fronthaul as well as radio sig-nal delivery to subscribers. To enhance the sensitivity of theT/R converter, a mixer with a free-running LO is applied forcoherent detection. It should be noted that there are issueswith the CFO compensation by DSP described in the sectionabove. A self-homodyne detector in the terahertz band canbe also applied with a high sensitivity and low phase-noisecharacteristics [43].

Particularly in radio interference, a terahertz radiomight not affect existing microwave/millimeter-wave ser-vices and the possible existence of other terahertz radio ser-vices. This is because the terahertz radio link should bebased on a beam-like link by high-gain antennas owing to itslarge atmospheric attenuation. Moreover, as the transmis-sion distance is limited for the same reason, an optimizedcell/link design can mitigate interference.

The RoR described in this paper only focuses on encap-sulation and transport of the radio space in the microwaveband to desired locations by the terahertz radio and opticalfibers. For future millimeter-wave communication systems

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Fig. 9 Conceptual diagram of a radio on an RoF system. The corresponding schematic spectra arealso shown.

Fig. 10 Block diagram of the experimental setup for radio on a terahertzover fiber system with an LTE signal.

such as 5G, analog IF signal transport by the RToF techniqueis also applicable. In that configuration, the subcarrier mul-tiplexing technique in the electrical domain can aggregatemany IF signals for transport to remote radio heads withoutany DSP in the media conversion.

4.2 Experimental Setup

Figure 10 shows the experimental setup for RToF signaltransmission, which consists of four blocks: an opticalSHM, a PM as an optical-to-terahertz converter, a Schottkybarrier diode (SBD) as the envelope detector in a T/R con-verter, and a vector signal analyzer (VSA) as a microwavesignal analyzer. In this study, we used an LTE down-linksignal at a center frequency of 2.1 GHz with a bandwidthof 20 MHz as the signal under test. The microwave signaland an LO signal at a frequency of 12.5 GHz are input intoan optical SHM based on an optical IM; then, the RToF sig-nal is transmitted over a single-mode optical fiber (SMF). Atthe remote site, an EDFA with an OBPF optimizes the op-tical power level of transmitted signal, and finally, the PMconverts from the RToF signal to a 300-GHz terahertz sig-nal. After terahertz transmission over air, the SBD directlyconverts from the received terahertz signal to the microwaveradio, which acts as the T/R converter for decapsulation in aremote radio head. The converted microwave signal is am-plified by a microwave amplifier; then, the signal is radiatedto the UE. In this proof-of-concept demonstration, the VSAwith offline DSP directly connected to the remote radio headperforms the evaluation of the signal before radiation to theUE.

Fig. 11 Demodulated constellation maps for QPSK, 16-QAM, and 64-QAM LTE signals over the RToF link.

Fig. 12 Observed EVMs for various modulation formats over the 10-kmSMF and 5-m air RToF link. Dashed lines denote the required EVM limitfor each modulation format [44].

4.3 Demonstration

Figure 11 shows the obtained constellation maps of the re-ceived signal for 5-m free-space transmission after transmis-sion over the 10-km SMF. The symbols on the constellationmap are clearly concentrated at the ideal point; this impliesthat there is no significant nonlinear distortion in the system.The modulation formats are QPSK, 16-ary quadrature am-plitude modulation (QAM), and 64-ary QAM with an errorvector magnitude (EVM) of approximately 5%.

The capacities of radio services are determined bythe bandwidth and modulation format. Figure 12 showsthe EVM characteristics for various modulation formats of

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QPSK, 16 QAM, and 64 QAM, whose capacities with a20-MHz bandwidth at a 3/4 coding rate are 19.2, 38.3, and57.5 Mb/s, respectively. All of the observed EVMs exhibitthe same behavior at the same transmission distances (5 m).This indicates that the transmitted radio power produces theSNR. Under the high-power transmission condition, for ex-ample, at a photocurrent of 5 mA, all of the formats havean EVM within the limit of 17.5% for QPSK, 12.5% for 16QAM, and 8% for 64 QAM, whose EVMs are regulated atthe output of the BTS [44].

5. Transmission Distance Evaluation

The possible transmission distance is a key characteristic ofwireless systems for application to the backhaul and fron-thaul links. Actually, in the LTE-advanced standard, anintersite distance (ISD), which is the distance between theBTSs, is defined to be approximately 250 m in an urbanmicrocell configuration [45]. In future 5G, the ISD will besmaller; therefore, the transmission distance required by theterahertz radio should be greater than 250 m.

Figure 13 shows the estimated received power at theterahertz Rx in the terahertz band as a function of the trans-mission distance. Assuming an effective isotropic radiopower (EIRP) of 30 dBm, which is realized by the terahertzsignal power of −16 dBm and the antenna gain of 46 dBi,which are the conditions described in Sect. 3, the possibletransmission distance will be less than 10 m. It should benoted that the signal power is estimated using the photocur-rent from the specification sheet of the PM, and the transmit-ted radio power at the input of the antenna is assumed to be−22 dBm, corresponding to a photocurrent of approximately2.5 mA when the observed BER is 2 × 10−3. The estimateddistance agrees well with the results shown in 8. The trans-mission loss is estimated with the free-space path loss andatmospheric attenuation with a coefficient of 3 dB/km. Onthe contrary, the transmission distance would be extended to500 m by an EIRP of 70 dBm. Such a high EIRP transceiver

Fig. 13 Evaluation of the possible transmission distance with an EIRPof 70 dBm. Black solid, gray solid, gray dashed, light-gray thick-solid, andlight-gray two-dotted lines denote the estimated power under conditions ofmedium fog (0.05 g/m3), thick fog (0.5 g/m3), rainfall of 10 mm/h, and rainfall of 50 mm/h, respectively. The corresponding sensitivity and estimatedradio power with an EIRP of 30 dBm are also shown as the dotted andblack-dash-dotted lines, respectively, as a reference.

is realized by the implementation of a transmitter power am-plifier in the terahertz band. In these proof-of-concept ex-periments, there are no terahertz amplifiers installed in theTx and Rx. For the power amplifier, semiconductor-basedand vacuum-tube-based amplifiers are being developed withan output power of 10 mW and beyond 1 W in the terahertzband [46], [47]. Thus, this EIRP will be applicable to a realtransceiver configuration.

For the EIRP of 70 dBm under medium- and thick-fogconditions, the possible transmission distance achieved isapproximately 400 m. These medium- and thick-fog condi-tions have water vapor densities of 0.05 g/m3 and 0.5 g/m3,and the resulting additional losses to atmospheric attenu-ation are approximately 0.7 dB/km and 8 dB/km, respec-tively; the estimated visibilities under these fog conditionsare 300 m and 50 m [48]. The drastic change in the atmo-spheric attenuation of 7 dB cannot affect the possible trans-mission distance for a distance less than 1 km; thus, thedominant loss in the terahertz band could be caused by thefree-space path loss.

Under rainy conditions, a large loss due to the rain-fall reduces the transmission distance in 300-GHz bandrather than fog. Actually, in heavy shower (rainfall of10 mm/h) and heavy rain (50 mm/h) conditions, the spe-cific attenuation coefficients are calculated to be 6.9 dB/kmand 19.1 dB/km, respectively [49]. Even under heavy rainconditions, the possible distance achieved is approximately300 m, which is comparable to the ISD of the current ur-ban microcell configuration described above. Therefore, thecombination of a high-gain antenna and a high-power am-plifier is capable for the fronthaul and backhaul links usingterahertz radio.

6. Conclusion

Terahertz signal transmission by advanced photonics tech-nology is demonstrated by the OFCG-based optical SHM.The optical SHM/SHIQM are configured with structuressimilar to conventional modulators in microwave- andmillimeter-wave communication. The SNR of the OFC sig-nal could be enhanced by the injection locking techniqueto increase the resulting SNR of the terahertz signal. DSP-aided coherent transmission and RToF-based radio signaldelivery are demonstrated using optical SHMs. The esti-mated transmission distance will achieve 300 m, even un-der heavy rain conditions. The middle-range transmissioncapability is capable of application to high-speed mobilebackhaul and fronthaul links. Terahertz communication willbe key to the realization of future mobile networks with athroughput greater than 10-Gb/s.

Acknowledgments

The authors would like to thank Dr. Katsumi Fujii of the Ap-plied Electromagnetic Research Institute, National Instituteof Information and Communications Technology, Japan, foraiding in the usage of a large-scale anechoic chamber. A.

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Kanno, P.T. Dat, N. Sekine, I. Hosako, and T. Kawanishiwish to thank the Ministry of Internal Affairs and Com-munications, Japan, for financial support through the pro-gram “Research and Development to Expand Radio Fre-quency Resources.” Y. Yoshida and K. Kitayama would liketo thank the National Institute of Information and Commu-nications Technology (NICT) for financial support throughthe R&D program, “Agile deployment capability of highlyresilient optical and wireless seamless communication sys-tems” (2012–2015).

References

[1] P. Winzer, “Beyond 100G Ethernet,” IEEE Commun. Mag., vol.48,no.7, pp.26–30, 2010.

[2] Ed. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High SpectralDensity Optical Communication Technologies, pp.11–48, Springer,2010.

[3] M. Birk, P. Gerard, R. Curto, L.E. Nelson, X. Zhou, P. Magill,T.J. Schmidt, C. Malouin, B. Zhang, E. Ibragimov, S. Khatana, M.Glavanovic, R. Lofland, R. Marcoccia, R. Saunders, G. Nicholl,M. Nowell, and F. Forghieri, “Real-time single-carrier coherent100 Gb/s PM-QPSK field trial,” IEEE J. Lightw. Technol., vol.29,no.4, pp.417–425, 2011.

[4] H. Shinohara, “Broadband access in Japan: rapidly growing FTTHmarket,” IEEE Commun. Mag., vol.43, no.9, pp.72–78, 2005.

[5] Rec. ITU-T G.984, “Gigabit-capable Passive Optical Networks,”March 2008.

[6] Rec. ITU-R M.1457, “Detailed specifications of the terrestrial radiointerfaces of International Mobile Telecommunications-2000 (IMT-2000),” Feb. 2013.

[7] Rec. ITU-R M.2012, “Detailed specifications of the terrestrial radiointerfaces of International Mobile Telecommunications Advanced(IMT-Advanced),” Feb. 2014.

[8] A. Osseiran, F. Boccardi, V. Braun, K. Kusume, P. Marsch, M.Maternia, O. Queseth, M. Schellmann, H. Schotten, H. Taoka, H.Tullberg, M.A. Uusitalo, B. Timus, and M. Fallgren, “Scenarios for5G mobile and wireless communications: the vision of the METISproject,” IEEE Comm. Mag., vol.52, no.5, pp.26–35, 2014.

[9] J. Wells, “Faster than fiber: The future of multi-G/s wireless,” IEEEMicrow. Mag., vol.10, no.3, pp.104–112, 2009.

[10] J.G. Andrews, S. Buzzi, W. Choi, S.V. Hanly, A. Lozano, A.C.Soong, and J.C. Zhang, “What will 5G be?” IEEE J. Sel. AreasCommun., vol.32, no.6, pp.1065–1082, 2014.

[11] M. Davis and A. Makleff, “Innovations and new techniques for 4Gmobile backhaul,” Mobile Backhaul Conf. at Int. Cell. Telecommun.Internet Assoc. (CTIA) Wireless 2012, New Orleans, USA, May2012.

[12] C. Jastrow, K. Munter, R. Piesiewicz, T. Kurner, M. Koch, and T.Kleine-Ostmann, “300 GHz Transmision System,” Electron. Lett.,vol.44, pp.213–214, 2008.

[13] H.-J. Song and T. Nagatsuma, “Present and Future of TerahertzCommunications,” IEEE Trans. Terahertz Sci. Tech., vol.1, no.1,pp.256–263, 2011.

[14] CPRI Specification V6.0, “Common Public Radio Interface (CPRI);Interface Specification,” Aug. 30, 2013.

[15] European Telecommunications Standards Institute, “Open radioequipment interface (ORI); requirements for open radio equipmentinterface (ORI) (Release 4),” ETSI GS ORI 001 V4.1.1, Oct. 2014.

[16] Recommendation ITU-R P.676-5, “Attenuation of atmosphericgases,” 2001.

[17] H. Hoshina, T. Seta, T. Iwamoto, I. Hosako, C. Otani, and Y. Kasai,“Precise measurement of pressure broadening parameters for watervapor with a terahertz time-domain spectrometer,” J. Quant. Spec-trosc. Radiat. Transfer, vol.109, no.12–13, pp.2303–2314, 2008.

[18] A. Kanno, T. Kuri, I. Morohashi, I. Hosako, T. Kawanishi, Y.Yoshida, and K. Kitayama, “Coherent terahertz wireless signaltransmission using advanced optical fiber communication technol-ogy,” J. Infrared Milli. Terahz Waves, vol.36, no.2, pp.180–197,2015.

[19] H. Al-Raweshidy and S. Komaki, Radio Over Fiber Technologiesfor Mobile Communications Networks, Aetech House, 2002.

[20] A.J. Seeds and K.J. Williams, “Microwave Photonics,” IEEE J.Lightw. Technol., vol.24, no.12, pp.4628–4641, 2006.

[21] J. Capmany and D. Novak, “Microwave photonics combines twoworlds,” Nature Photonics, vol.1, no.6, pp.319–330, 2007.

[22] H.-J. Song, K. Ajito, A. Wakatsuki, Y. Muramoto, N. Kukutsu,Y. Kado, and T. Nagatsuma, “Terahertz Wireless CommunicationLink at 300 GHz,” IEEE Intl. Topic. Meeting Microw. Photon.(MWP2010), WE3-2, Montreal, pp.42–45, Oct. 2010.

[23] A. Kanno, T. Kuri, I. Hosako, T. Kawanishi, Y. Yasumura, Y.Yoshida, and K. Kitayama, “Optical and millimeter-wave radioseamless MIMO transmission based on radio over fiber technology,”Optics Express, vol.20, no.28, pp.29395–29403, 2012.

[24] S. Koenig, D. Lopez-Diaz, J. Antes, F. Boes, R. Henneberger, A.Leuther, A. Tessmann, R. Schmogrow, D. Hillerkuss, R. Palmer,T. Zwick, C. Koos, W. Freude, O. Ambacher, J. Leuthold, and I.Kallfass, “Wireless sub-THz communication system with high datarate,” Nature Photonics, vol.7, no.12, pp.977–981, 2013.

[25] S. Horiguchi, K. Arakawa, Y. Minamikata, and T. Nagatsuma, “Er-ror-free 30–50 Gbps Wireless Transmission at 300 GHz,” in Proc.2013 Asia-Pacific Microw. Conf., pp.660–662, 2013.

[26] A. Kanno, P.T. Dat, T. Kuri, I. Hosako, T. Kawanishi, Y. Yoshida,and K. Kitayana, “Waveform over fiber: DSP-aided coherent fiber-wireless transmission using millimeter and terahertz waves,” Proc.SPIE, vol.9387, doi:10.1117/12.2077056, pp.1–11, 2015.

[27] A. Kanno, I. Morohashi, T. Kuri, I. Hosako, T. Kawanishi, Y.Yasumura, Y. Yoshida, and K. Kitayama, “16-Gbaud QPSK Ra-dio Transmission Using Optical Frequency Comb with Recirculat-ing Frequency Shifter for 300-GHz RoF Signal,” IEEE Intl. Topic.Meet. Microw. Photon., Noordwijk, Netherland, S7.6, pp.298–301,Sept. 2012.

[28] N. Sekine, A. Kanno, T. Kuri, I. Morohashi, A. Kasamatsu, I.Hosako, T. Kawanishi, Y. Yoshida, and K. Kitayama, “30-Gbps-class terahertz transmission using optical sub-harmonic IQ mixerfor backhaul/fronthaul directly connected to optical networks,” doc.:IEEE 802.15-13-0653-00-0thz, Nov. 2013.

[29] T. Ishibashi and H. Ito, “Uni-traveling-carrier photodiodes,” Tech.Dig. Ultrafast Electron. Optoelectronics, pp.83–87, Lake Tahoe,USA, 1997.

[30] H. Ito, T. Furuta, S. Kodama, and T. Ishibashi, “InP/InGaAs uni-travelling-carrier photodiode with 310 GHz bandwidth,” Electron.Lett., vol.36, no.21, pp.1809–1810, 2000.

[31] T. Kawanish, T. Sakamoto, S. Shinada, and M. Izutsu, “Opti-cal frequency comb generator using optical fiber loops with sin-gle-sideband modulation,” IEICE Electron. Express, vol.1, no.8,pp.217–221, 2004.

[32] A. Kanno and T. Kawanishi, “Phase noise analysis of an optical fre-quency comb using single side-band suppressed carrier modulationin an amplified optical fiber loop,” IEICE Electron. Express, vol.9,no.18, pp.1473–1478, 2012.

[33] T. Kawanishi, T. Sakamoto, and M. Izutsu, “High-Speed Control ofLightwave Amplitude, Phase, and Frequency by Use of Electroop-tic Effect,” J. Sel. Top. Quantum Electron., vol.13, no.1, pp.79–91,2007.

[34] T. Kuri, Y. Omiya, T. Kawanishi, S. Hara, and K. Kitayama, “Opticaltransmitter and receiver of 24-GHz ultra-wideband signal by directphotonic conversion techniques,” IEEE Intl. Topic. Meeting Microw.Photon. (MWP2006), Grenoble, France, W3-3, pp.1–4, Oct. 2006.

[35] T. Schneider, M. Junker, and D. Hannover, “Generation of millime-tre-wave signals by stimulated Brillouin scattering for radio over fi-bre systems,” Electron. Lett., vol.40, no.23, pp.1500–1502, 2004.

KANNO et al.: HIGH-SPEED COHERENT TRANSMISSION USING ADVANCED PHOTONICS IN TERAHERTZ BANDS1079

[36] A. Kanno and T. Kawanishi, “Frequency-stabilized terahertz signalgeneration using an optical frequency comb and an injection-lock-ing technique,” Intl. Top. Meet. Microw. Photon., Sapporo, Japan,TuEC-2, pp.150–152, Oct. 2014.

[37] A. Kanno, T. Kuri, I. Morohashi, I. Hosako, T. Kawanishi, Y.Yoshida, and K. Kitayama, “20-Gbaud QPSK Coherent RadioTransmission at 325 GHz with High-Gain Antennas,” 39th Intl.Conf. Infrared Milli. Terahz. Waves, Arizona, USA, M3/B-3.6,pp.1–2, Sept. 2014.

[38] Rec. ITU-T G.975.1, “Forward Error Correction for High Bit RateDWDM Submarine Systems,” Feb. 2004.

[39] K. Tsukamoto, T. Higashino, and S. Komaki, “Software definableradio networks for the ubiquitous networks,” XXXIIIth GA of URSI,vol.1, pp.205–208, New Dehli, India, Oct. 2005.

[40] J. Bartelt and G. Fwttweis, “Radio-Over-Radio: I/Q-Stream Back-hauling for Cloud-Based Networks via Millimeter Wave Links,”IEEE GLOBECOM Wkshps on First Intl. Workshop on Cloud-Pro-cessing in Heterogeneous Mobile Commun. Netw., Atlanta, USA,pp.772–777, Dec. 2013.

[41] P.T. Dat, A. Kanno, K. Inagaki, and T. Kawanishi, “High-capac-ity wireless backhaul network using seamless convergence of ra-dio-over-fiber and 90-GHz millimeter-wave,” J. Lightw. Technol.,vol.32, no.20, pp.3910–3923, 2014.

[42] A. Kanno, P.T. Dat, I. Hosako, T. Kawanishi, and H. Ogawa, “Ra-dio-on-terahertz over fiber system for future mobile fronthauling,”in Proc. Globecom 2014 – Symp. Sel. Area Commun. Access Netw.Sys., pp.2218–2222, 2014.

[43] T.D. Pham, A. Kanno, and T. Kawanishi, “High-speed and low-la-tency fronthaul system for heterogeneous wireless networks usingseamless fiber–millimeter-wave,” IEEE Intl. Conf. Commun., Lon-don, UK, SAC07-ANS-02, pp.994–999, June 2015.

[44] 3GPP TS36.104 V10.9.0 Releasee 10, “LTE; Evolved Universal Ter-restrial Radio Access (E-UTRA); Base Station (BS) radio transmis-sion and reception,” Feb. 2013.

[45] ITU-R Rep. M.2135-1, “Guidelines for evaluation of radio interfacetechnologies for IMT-Advanced,” Aug. 2009.

[46] D. Pukala, L. Samoska, T. Gaier, A. Fung, X.B. Mei, W. Yoshida,J. Lee, J. Uyeda, P.H. Liu, W.R. Deal, V. Radisic, and R. Lai, “Sub-millimeter-Wave InP MMIC Amplifiers From 300–345 GHz,” IEEEMicrow. Wireless Components Lett., vol.18, no.1, pp.61–63, 2008.

[47] J.C. Tucek, M.A. Basten, D.A. Gallagher, and K.E. Kreischer, “220GHz power amplifier development at Northrop Grumman,” in Proc.IEEE Intl. Vacuum Electron. Conf., pp.553–554, 2012.

[48] Rec. ITU-R P.840-6, “Attenuation due to clouds and fog,” Sept.2013.

[49] Rec. ITU-R P.838-3, “Specific attenuation model for rain for use inprediction methods,” March 2005.

Atsushi Kanno received B.S., M.S., andPh.D. degree in science from the University ofTsukuba, Japan, in 1999, 2001, and 2005, re-spectively. In 2005, he was with the VentureBusiness Laboratory of the Institute of Scienceand Engineering, University of Tsukuba, wherehe was engaged in research on electron spin dy-namics in semiconductor quantum dot structuresusing the optical-polarization-sensitive Kerr ef-fect measurement technique. In 2006, he joinedthe National Institute of Information and Com-

munications Technology Japan. From 2006 to 2007, he was also the mem-ber of the CREST-JST project entitled “Creation of Novel Functional De-vices Using Nanoscale Spatial Structures of the Radiation Field.” He isworking on microwave/millimeter-wave/terahertz photonics, ultrafast op-tical communication systems, lithium niobate optical modulators, and thestudy of ultrafast phenomena in semiconductor optical devices. He is amember of the Institute of Electronics, Information and CommunicationEngineers (IEICE), the Japan Society of Applied Physics (JSAP), and theInstitute of Electrical and Electronic Engineers (IEEE).

Pham Tien Dat received the B.Eng. (Hons.)degree in electronics and telecommunicationengineering from Posts and Telecommunica-tions Institute of Technology, Vietnam, in 2003,and the M.Sc. and Ph.D. degrees in science ofglobal information and telecommunication stud-ies from Waseda University, Japan, in 2008 and2011, respectively. He worked as a Researcherat Research Institute of Posts and Telecommu-nications, Vietnam from 2003 to 2006. In 2011,he joined the National Institute of Information

and Communications Technology, Japan. His research interests are in thefield of microwave/millimeter-wave photonics, radio over fiber and opti-cal wireless systems. Dr. Pham is a member the Institute of Electrical andElectronic Engineers (IEEE).

Norihiko Sekine received the B.S., M.S.,and Ph.D. degrees in electronic engineeringfrom the University of Tokyo, Tokyo, Japan, in1994, 1996, and 1999, respectively. After expe-rience in industry, he joined the National Insti-tute of Information and Communications Tech-nology (NICT), Tokyo. He is currently a Re-search Manager with NICT and his research in-terests include the physical properties of semi-conductor nanostructures in the terahertz regimeand their application to terahertz devices and

systems. Dr. Sekine is a member of the IEEE Photonics Society (IPS),the Institute of Electronics, Information and Communication Engineering(IEICE) of Japan, and the Japanese Society of Applied Physics (JSAP).

1080IEICE TRANS. ELECTRON., VOL.E98–C, NO.12 DECEMBER 2015

Iwao Hosako received the B.S., M.S., andPh.D. degrees from the University of Tokyo,Japan, in 1988, 1990, and 1993, respectively.After two years with NKK Corp’s ULSI Lab-oratory from 1993 to 1994, he joined Commu-nications Research Laboratory (former name ofNICT). He is currently Associate Director Gen-eral of Advanced ICT Research Institute at theNational Institute of Information and Commu-nications Technology (NICT), Tokyo. His re-search during 1995–1998 focused on cryogenic

readout circuits with emphasis on ultralow 1/f-noise transistors for a far-infrared detector system. His research interests include nearly all aspectsof terahertz-device technologies such as semiconductor emitters, detectors,and optical thin films. He is serving on several research committees forterahertz technologies.

Tetsuya Kawanishi received the B.E., M.E.,and Ph.D. degrees in electronics from KyotoUniversity, Kyoto, Japan, in 1992, 1994, and1997, respectively. From 1994 to 1995, hewas with the Production Engineering Labora-tory, Matsushita Electric Industrial (Panasonic)Company, Ltd. In 1997, he was with Ven-ture Business Laboratory of Kyoto University,where he was engaged in research on electro-magnetic scattering and on near-field op- tics.He joined the Communications Research Lab-

oratory, Ministry of Posts and Telecommunications (now the National In-stitute of Information and Communications Technology, NICT), Koganei,Tokyo, Japan, in 1998. In 2004, he was a Visiting Scholar with the Depart-ment of Electrical and Computer Engineering, University of California, SanDiego, USA. In 2015, he joined Waseda University as a Professor of De-partment of Electronic and Physical Systems, and is working on high-speedoptical modulators and on RF photonics. Dr. Kawanishi received the URSIYoung Scientists Award in 1999, an award for young scientists in the fieldof science and technology in 2006, from ministry of Education, Culture,Sports, Science, and Technology, Japan. He is a Fellow of the Institute ofElectrical and Electronic Engineers (IEEE).

Yuki Yoshida received the B. E., M.Inf., and Ph.D. degrees from Kyoto University,Kyoto, Japan, 2004, 2006, and 2009, respec-tively. Since 2009, he has been an assistantprofessor in the Department of Electrical, Elec-tronic and Information Engineering, GraduateSchool of Engineering, Osaka University. Hisresearch interests include digital signal process-ing for communications systems.

Ken’ichi Kitayama received the B.E., M.E.,and Dr. Eng. degrees in communication engi-neering from Osaka University, Osaka, Japan,in 1974, 1976, and 1981, respectively. In1976 he joined the NTT Electrical Communi-cation Laboratory in 1976. In 1982–1983, hespent a year as a Research Fellow at the Uni-versity of California, Berkeley. In 1995, hejoined the Communications Research Labora-tory (Presently, National Institute of Informa-tion and Communications Technology, NICT),

Tokyo. Since 1999, he has been the Professor of the Department of Elec-trical, Electronic and Information Engineering, Graduate School of Engi-neering, Osaka University. His research interests are in photonic networks,optical signal processings, optical code division multiple access (OCDMA)systems, and radio-over-fiber systems. He has published over 240 papersin refereed journals and holds more than 30 patents.