Bauwelinck, Johan

Bohn, Thomas

Breyne, Laurens

Demeester, Piet

Li, Haolin

Torfs, Guy

Van Kerrebrouck, Joris

English

Ghent University Academic Bibliography

Real-time All-Digital Radio-over-Fiber LTETransmissionJoris Van Kerrebrouck∗, Laurens Breyne∗, Haolin Li∗, Johan Bauwelinck∗,Guy Torfs∗, Piet Demeester∗ and Thomas Bohn†∗Department of Information Technology (INTEC) – IDLabGhent University – imecTechnologiepark 15, B-9052 Zwijnaarde, Belgium† Nokia Bell LabsLorenzstrasse 10, 70435 Stuttgart, GermanyEmail: joris.vankerrebrouck@ugent.beAbstract—Radio-over-fiber is a key-enabling-technology fornext generation wireless networks. To support an increasingnumber of antennas, a CloudRAN approach that maximallyaggregates base station functionality of the different antennasites is the most economically feasible. To support light weightantenna units, analog radio-over-fiber is most promising. Tomitigate linearity constraints of optical modulators, an all-digitaltransmitter can be used. In this paper, LTE transmission isdemonstrated over 200 m OM4 multimode fiber using a VCSELand a resonant transimpedance amplifier. The VCSEL is directlymodulated with a high-speed 2-level signal generated in real-timeon an FPGA. After reception of this signal, the adjacent channelpower ratio was measured to be -45.6 dB. This is within the LTEspecifications, proving this is a cost and power effective solutionfor next generation wireless networks.I. INTRODUCTIONRadio-over-fiber (RoF) is seen as one of the key-enablingtechnologies of future wireless networks [1]–[3]. To accommo-date the high throughput envisioned in 5G systems, differentsolutions are explored. The most straightforward ones followthe scaling path by reducing the cell size of the network. Moredisruptive approaches envision large-scale antenna systemsto form a Massive multiple-input-multiple-output (MIMO)system. In the end, these antenna systems can even be dis-tributed in its environment [4]. Beyond-5G approaches suchas proposed in [5] even go further and target thousands ofantennas interconnected by fiber infrastructure.To interconnect all these antennas, the CloudRAN approachis economically the most viable. Here, all base station func-tions are centralized such that they can be efficiently sharedover different antenna sites, interconnected with a fiber-basednetwork. However, with the increasing amount of antennas, thecost of the remote antenna units (RAUs) should be reduced.This is a demand that is incompatible with digital RoFapproaches such as Common Public Radio Interface (CPRI),which in essence transmits digital I/Q samples to an RAU. Atthe RAU, the optical signal is received, reconstructed using aDAC, up-converted using a locally synthesized RF carrier andamplified.Another approach consists of directly modulating the RFsignal on an optical carrier, such that an analog RoF link isobtained. In this way, the RF signal can be easily reconstructedat the antenna site using a linear transimpedance amplifier(TIA), for example a resonant TIA as described in [6]. Aroundthe RF frequency, a photodiode is conjugately matched to alow noise amplifier (LNA). As such maximum power can beextracted from the photodector. One downside of this approachis the requirement of linear optical modulation. Comparedto traditional non-return-to-zero transmission, the linearityrequirements of wireless RF signals are tremendous and theykeep rising every standard generation. As a result, expensiveand costly optical modulators such as Mach-Zehnder Modu-lators (MZM) or highly linear directly modulated distributedfeedback (DFB) lasers [7] are required.To mitigate the linearity requirement, different authors [8],[9] explored how low-cost, less linear modulators, such asvertical cavity lasers (VCSELs), can still be used to trans-mit RF signals. Delta-Sigma modulation of the RF signaloversamples the signal and quantizes the signal to a limitedamount of quantization levels (typically 2, resulting in a single-bit stream). The quantizer is placed in a feedback system,which is configured to shape the quantization noise outsidethe frequency band of interest. As such very high signal-to-noise ratios can be obtained. Implementation of very high-speed delta-sigma modulators is however not trivial, as a result,most demonstrators rely on off-line generated bitstreams whichare transmitted using a pulse pattern generator.To allow real-time modulation of an all-digital RF trans-mitter, [10] combines delta-sigma modulation and pulsewidthmodulation. In this paper, we present a real-time all-digitalradio-over-fiber link consisting of a digital transmitter whichmodulates a VCSEL using the aforementioned algorithm andan analog RoF receiver. Thanks to the very-high speed in-terfaces available in modern FPGAs, a 27.5Gbps bitstreamis generated containing an LTE signal at a central carrierfrequency of 3.44 GHz. The output of the FPGA is directlyconnected to an 850 nm multimode VCSEL and transmittedover 200 m OM4 multimode fiber. A resonant TIA is employedat the receiving side. It is demonstrated that thanks to thecombination of oversampling and noise shaping, a VCSELcan be used to transmit LTE signals over this link, providinga cheap and power efficient interconnection system for nextgeneration wireless networks.This paper is organized as follows: section II describes theused transmission system expanding both on the digital signalgeneration as well as on the resonant TIA used to receive thesignal. Section III describes the transmission measurementsand the paper is concluded in section IV.II. SYSTEM DESCRIPTIONTo combine the benefits of digital RoF and analog RoF, wepropose the system depicted in Fig. 1. The system consists ofa digital modulator driving a VCSEL and a resonant opticalreceiver. The digital modulator takes digitally available I/Qsamples and modulates them on a 3.44 GHz central carrierfrequency which is outputted using a 2-level pulse-width mod-ulated (PWM) data stream of 27.5 Gbps. The modulator shapesthe quantization noise originating from the limited amplitudelevels in such a way that it falls outside the frequency bandof interest. As a result, the analog signal can be recoveredby simply bandpass filtering the signal. Additionally, due tothe digital nature of this modulated signal, cheap non-linearoptical transmitters can be used. In our system, a VCSEL isused. At the receive site, a photodiode is power matched toa low-noise amplifier at the frequency of interest forming aresonant TIA, as such, the required bandpass filter is directlyintegrated providing already partly the required out-of-bandnoise suppression. In the following paragraphs, the differentcomponents will be described in more detail.Fig. 1. Block diagram of the proposed system.A. Single-bit all-digital modulatorThere are mainly three categories of all-digital RF signalmodulators. The first is pulse width modulation (PWM). Theinstantaneous amplitude and phase of an RF signal are mappedto a pulse width and a pulse position in a pulse train. Giventhe limited timing resolution of these systems, it is difficult toachieve a high signal-to-noise ratio (SNR) and meet the verystringent adjacent channel power ratio (ACPR) requirements oftypical mobile communication standards. A second approachuses Delta-Sigma (∆Σ) modulation [11], [12]. Here, the signalis quantized in a closed loop system. This allows to feedbackthe quantization noise and shapes it in such a way that insidethe frequency band of interest a very high SNR can beobtained. The obtainable SNR relates to the order of the noiseshaping filter and to the oversampling rate with respect to thesignal bandwidth of the modulator. Excellent ACPRs can beobtained using this technique, however, the spectral efficiencyis quite low and ∆Σ modulators are difficult to implementat very high sampling rates. A possible solution to decreasethe sample rate is parallelizing ∆Σ modulators. But due to thenon linearity in the ∆Σ loop, typical loop unrolling techniquesused in digital signal processing can no longer be exploited. Athird option is explored in [10], here a hybrid implementationcombining the advantages of PWM and ∆Σ modulation isproposed, i.e. high coding efficiency and a good ACPR. In[10], the authors observe that the general method for encodinga complex baseband signal to a binary pulse sequence is tomap the carrier frequency (fc) to the pulse repetition rate,the phase to the pulse position and the amplitude to thepulse width. Fig. 2 shows a single pulse with starting positionDa = Dγ − 12D and end position Db = Dγ + 12D. Thecomplex amplitude (qm) of the carrier signal associated withthis pulse can be calculated as follows:qm =∫ DbDae−j2pifctdt (1)=1pisin(piD)e−2pijDγ (2)Due to the finite resolution of the PWM signal (in ourcase one eight of the bit period of the ∆Σ modulator), onlya discrete set of complex baseband signals can be directlymapped to the carrier frequency. This set can be used as outputsignals of a complex quantizer.Fig. 2. Pulse position of a PWM signal.Using this quantizer, a complex delta-sigma modulator canbe constructed. The deviation from these points, which is thequantization noise, can be fed back to the input of the systemsuch that it can be shaped outside the frequency band ofinterest. Fig. 3 shows the block diagram of the used modulator.The implemented modulator is first order withH(z) =z−11− z−1 .QuantizerIQ to Pulse Mapperz-1++-x(t) y(t)Fig. 3. Block diagram of the hybrid ∆Σ Pulsewidth modulator, x(t) is amultilevel complex digital baseband signal, y(t) is a 2-level coded RF signals.After ∆Σ modulator, the constellation points are thenmapped to a pulse train according to their definition in (2).This process is completely implemented on an Virtex-7 FPGAand using one of the high-speed serial outputs high-speedPWM signal of 27.5 Gbps can be generated with an amplitudeof 600 mVpp. The FPGA directly drives a VI Systems V50-850M, 850 nm, multimode VCSEL. The measured electro-optical bandwidth of this device is well above 15 GHz fordriving currents exceeding 4 mA. As a result, little bandwidthdegradation will be seen when driving the VCSEL with aPWM stream of 27.5 Gbps while non-linearities of the VCSELwill be mitigated due to the single-bit modulation. The DCcharacteristics of the VCSEL are shown in Fig. 4. The VCSELshould be biased sufficiently high to avoid sub-thresholdoperation. At higher modulation currents, gain compressionwill occur. However, thanks to the single-bit modulation, thiswon’t degrade the electrical to optical conversion.0 1 2 3 4 5 6 7 8 9 10 11 12Biasing current [mA]0123456Output power [mW]00.511.522.53Voltage [V]PVFig. 4. Optical output power and forward voltage of the VCSEL as a functionof driving current. The threshold current is 0.6 mA while the threshold voltageis 1.7 V.B. Resonant TIAThe signal is received using a resonant TIA built using aGaAs 5 GHz PIN photodetector with a responsivity of 0.4 A/Wat 850 nm and a low noise amplifier. This photodetectoris matched to 50Ω around 3.44 GHz. The resulting inputreflection coefficient is shown in Fig. 5. Next the output of thematched photodetector is connected to a low noise amplifier(Mini-circuits, PMA3-83LN+) having 19 dB gain at 3.44 GHzand an output referred 1 dB-compression point of 18 dBm.To ensure sufficient out of band suppression an additionalbandpass filter is added behind the amplifier. The completereceiver has a bandwidth of 120 MHz.The total link, measured from the electrical input of theVCSEL, over the matched photodector, amplifier and bandpassfilter, has a link budget of 4 dB at 3.44 GHz.III. MEASUREMENTSThe goal of the experiment is to demonstrate real-time LTEtransmission at 3.44 GHz over fiber using cheap optical com-ponents such as VCSELs and multimode fiber. The photographof the optical setup is shown in Fig. 6.The 5 MHz LTE test pattern used in this paper consistsof 25 resource blocks, each having 12 subcarriers that aremodulated with a QPSK signal, is generated. This signal3 3.2 3.4 3.6 3.8 4Frequency [GHz]-16-14-12-10-8-6-4-20mag(S11) [dB]Fig. 5. Input reflection coefficient of matched photodetector.is modulated on a single-bit stream of 27.5 Gbps with anamplitude of 600 mVpp. To maximize the coding efficiency,the crest factor is maximally reduced by clipping the signal,given the constraint that the error-vector-magnitude (EVM)stays within the LTE specifications. The resulting modulatedstream leaving the FPGA has an in-band power of -11.25 dBm,a crest-factor of 5.7 dB and an EVM of 15.16 %. The measuredACPR is -49.25 dB which is 5 dB above the -44.20 dBspecified by the standard.Next, the VCSEL is biased with a current of 3 mA, yieldingan optical output power of 1.4 dBm. The FPGA outputis connected via a bias-tee to the modulation input of theVCSEL. The optical output is directly connecting the resonantTIA which delivers an electrical output power of -7.25 dBm.The EVM is measured equal to the electrical measurementsbut the averaged ACPR is reduced to -46.70 dB as shown inFig. 7.Fig. 6. Optical measurement setup.Following, 200 m of OM4 multimode fiber is added betweenVCSEL and PD. The 200 m OM4 multimode fiber has anattenuation of 2.5 dB/km and consists of 2 pieces of 100 m.As such 2 additional fiber connections with each an attenuationof 0.4 dB are added, summing up to a total loss of 1.3 dB. Tocompensate for the loss of the fiber, the biasing of the VCSELis increased to 4 mA yielding an optical output power of3 dBm. Hence, the incident optical power at the photodetectoris 1.7 dB. An attenuation of 2 dB in the optical domain will,due to the quadratic characteristic of the PD, be translatedto 4 dB of attenuation of the electrical link budget. Thisis also reflected in the measurements shown in Fig. 8: thereceived electrical power is -11.80 dBm. The measured EVMis 15.27 % and the average ACPR is -45.6 dB, which is stillwithin the standard’s specifications. The influence of the fiberis limited and the experiment shows that multimode fiber issuitable candidate for short-reach applications.Channel Bandwidth Spacing Lower Upper LimitTX 4.515 MHz -7.37 dBmAdjacent 4.515 MHz 5 MHz -47.43 dB -45.96 dB -44.2 dBAlternate 4.515 MHz 10 MHz -53.62 dB -51.97 dB -44.2 dBFig. 7. Adjacent channel power ratio measurement of LTE all-digital radio-over-fiber transmission back-to-back.Channel Bandwidth Spacing Lower Upper LimitTX 4.515 MHz -11.80 dBmAdjacent 4.515 MHz 5 MHz -46.23 dB -44.97 dB -44.2 dBAlternate 4.515 MHz 10 MHz -51.38 dB -49.72 dB -44.2 dBFig. 8. Adjacent channel power ratio measurement of LTE all-digital radio-over-fiber transmission over 200 m OM4 multimode fiber.IV. CONCLUSIONIn this paper, we have demonstrated real-time all-digitalradio-over-fiber transmission of LTE signals. Digital I/Q sam-ples of an LTE signal are modulated on a 3.44 GHz carrierusing a combination of delta-sigma modulation and pulsewidthmodulation. As a result, both high ACPR and coding efficiencywere obtained. Thanks to the high-speed interfaces availablein modern FPGAs, single-bit modulation could be used todirectly drive a VCSEL mitigating its non-linear characteristic.At the receive side, a resonant TIA was used to maximizepower extraction around 3.44 GHz. The demonstration veri-fies that LTE transmission is possible using low-cost opticalcomponents with a very lightweight remote antenna unit. Thereceived signal undergoes minimal degradation with respectto EVM and still fulfills the adjacent channel power speci-fications imposed by the standard. As such, this techniquesdemonstrates its viability for large antenna systems to supportfuture cellular networks.ACKNOWLEDGMENTThis work was supported by the European Commission inthe framework of the H2020-ICT-2014-2 project Flex5Gware(Grant agreement no. 671563). Piet Demeester thanks the ERCfor his advanced grant 695495 ATTO: A new concept for ultra-high capacity wireless networks.REFERENCES[1] P. T. Dat, A. Kanno, and T. Kawanishi, “Radio-on-radio-over-fiber:efficient fronthauling for small cells and moving cells,” IEEE Trans.Wireless Commun., vol. 22, no. 5, pp. 67–75, Oct. 2015.[2] F. Ponzini and L. Giorgi, “5G radio over fiber for small-cells,” in2016 18th International Conference on Transparent Optical Networks(ICTON), Jul. 2016, pp. 1–1.[3] B. Lannoo, A. Dixit, D. Colle, J. Bauwelinck, B. Dhoedt, B. Jooris,I. Moerman, M. Pickavet, H. Rogier, P. Simoens, G. Torfs, D. V.Ginste, and P. Demeester, “Radio-over-fibre for ultra-small 5G cells,” in2015 17th International Conference on Transparent Optical Networks(ICTON), Jul. 2015, pp. 1–4.[4] E. Bjo¨rnson, M. Matthaiou, A. Pitarokoilis, and E. G. 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Real-time all-digital radio-over-fiber LTE transmission

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