1860 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 20, OCTOBER 15, 2012

Multilevel Modulations for Gigabit Accessin a Simple Millimeter-Wave

Radio-Over-Fiber LinkA. H. M. Razibul Islam, Student Member, IEEE, Masuduzzaman Bakaul, Member, IEEE,

and Ampalavanapillai Nirmalathas, Senior Member, IEEE

Abstract— A spectrally efficient and simplified full-duplexmillimeter-wave (mm-wave) radio-over-fiber (RoF) link isproposed and experimentally demonstrated. For downlink, amultilevel amplitude-shift-keying (M-ASK) modulation, such as4-ASK, for a total data rate of 5 Gb/s, is applied on two uncor-related optical carriers at the central station (CS) for remoteheterodyning at the photodetector. The mm-wave signal gener-ated through the photodetection process is then self-homodynedat the mm-wave receiver to recover the desired 4-ASK data.For uplink, an RF self-homodyning technique is applied againto retrieve the 4-ASK 5-Gb/s data at the base station beforebeing transported to the CS as a simpler baseband-over-fibertransmission. While the actual generation, transport, and datarecovery remain simplified for the proposed mm-wave RoF link,M-ASK implementation in such a system provides a spectrallyefficient and simplified scheme which supports higher data rates.

Index Terms— Gigabit broadband access, microwavephotonics, millimeter-wave radio-over-fiber, optical heterodyning,optical-wireless system integration.

I. INTRODUCTION

MULTI-GIGABIT broadband wireless access (BWA)technologies such as wireless metropolitan area

networks (WMAN) at 10–66 GHz, wireless personal areanetworks (WPAN) at 60 GHz and E bands operating at 70, 80and 90 GHz are gaining particular interests now-a-days due totheir ability to provide high data rates for the ever-increasingbandwidth demands at consumer-levels [1]. Commonly termedas millimeter-wave (mm-wave) frequencies, such spectral-congestion-free domain is expected to offer aggregate channelbandwidths of 10 GHz or higher [2]. Higher atmosphericgain at these wireless frequencies however reduces the cellsize to pico- or femto cells (10–100 meters) and thus requiresa large number of base stations (BS) for operational coverageof a certain geographic area. MM-wave radio-over-fiber

Manuscript received June 4, 2012; revised August 23, 2012; acceptedSeptember 2, 2012. Date of publication September 6, 2012; date of currentversion September 26, 2012.

A. H. M. R. Islam and M. Bakaul are with National ICT AustraliaLtd. (NICTA), Department of Electrical and Electronic Engineering,University of Melbourne, Melbourne VIC-3010, Australia (e-mail:aislam@ee.unimelb.edu.au; bakaulm@unimelb.edu.au).

A. Nirmalathas is with the Department of Electrical and ElectronicEngineering, University of Melbourne, Melbourne VIC-3010, Australia(e-mail: nirmalat@unimelb.edu.au).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2012.2217485

(RoF) creates a potential solution in this regard where datamodulated mm-wave signal is transported and distributed tothe remote BSs through low loss optical media and laterrecovered at the mm-wave receiver [3]. As such hybrid opticaland wireless approaches are destined to support customers ataccess domain, simplified and efficient mm-wave generation,transport and data recovery are pivotal for their commercialdeployments.

Several techniques for downlink have been proposedover the years to address such challenges including thephase/frequency locked correlated optical heterodyning [4], [5]and coherent spectral generations using various modulationformats [6], [7]. However, most of such techniques needcomplex locking mechanisms and/or high-speed modulators,local oscillators (LO) and associated electro-optic and RFcircuitry. For uplink, signal translational approach [8], [9]generally requires high-speed LO and associated RF deviceswhich decreases the performance-to-cost ratio of RoF sys-tems. Recently, we proposed [10] a combination of unlockedoptical heterodyning and RF self-homodyning which avoidsany phase/frequency locking and most of the high-speeddevice requirements in such systems. Although spectrally-inefficient, simple ASK modulation was chosen in this case. Inthis letter, to enable efficient utilization of signal bandwidthto deliver higher data rates, we introduced multilevel ASK(M-ASK) modulation in the proposed simplified RoF sys-tem while keeping the actual generation, transport and datarecovery schemes unchanged. Although, currently there areno mm-wave RoF standards on M-ASK, 4-ASK return-to-zero (RZ) format is chosen and envisaged in our work as itdoubles the data rate [11]. Additionally, reducing the symbolrate through 4-ASK implementations allows cheaper RF andoptoelectronic components at the transmitter and receiverwhile offering reduced hardware complexity due to usingincoherent detection at the receiver [12]. Therefore, a total datarate of 5 Gbps is used in both downlink and uplink directionsfor full-duplex demonstrations.

II. SYSTEM CONFIGURATION AND EXPERIMENTAL SETUP

Figure 1 shows the system configurations and experimentalsetup for both the downlink and uplink demonstrations. Fordownlink experiments, two uncorrelated distributed feedback(DFB) lasers operating at 1549.110 nm and 1549.396 nm

1041–1135/$31.00 © 2012 IEEE

ISLAM et al.: MULTILEVEL MODULATIONS FOR GIGABIT ACCESS IN A SIMPLE mm-WAVE ROF LINK 1861

DFB LD 1

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Fig. 1. System configuration and experimental setup for the full-duplex mm-wave RoF link.

were combined at central station (CS) to create the mm-wavecarrier frequency of 35.75 GHz. Linewidths of the lasers were200 KHz and 5 MHz respectively with relative intensity noiseof −145 dBc/Hz. A Tektronix arbitrary waveform generator(AWG) was used to generate 5 Gbps RZ data in 4-ASKformat with a signal bandwidth of 2.5 GHz. Since the filteringbandwidth requirement for RZ format is 1.5 times the data rate,a LPF of 3.8 GHz bandwidth was utilized after the generateddata. The combined optical carriers were both modulated usinga 2.5 GHz Mach-Zehnder modulator (MZM). A drive voltageof 1 volt (peak to peak) was applied from the AWG to theMZM and the bias voltage of the MZM was 1.8 volt (peakto peak). After the necessary optical amplifications by anerbium doped fiber amplifier (EDFA), an optical bandpassfilter (OBPF) of 4 nm bandwidth was used to remove unwantedamplified spontaneous emission noise. The signal was thentransported over a 25.1 km single mode fiber (SMF) beforeremote heterodyning with a high-speed photodetector (PD).The resulted signal after PD is a combination of phase-noise-affected mm-wave signal at 35.75 GHz and its basebandreplica. Therefore, a combination of Miteq low-noise amplifier(LNA) and a medium power amplifier (MPA) operating atKa band (26.5–40 GHz) was used to boost the modulatedmm-wave signal with around +30 dB amplification. Thisband-limited amplification also suppressed the basebandreplica, as expected. Back to back (BTB) transmission wasmade by using an Anritsu DC-65 GHz RF power dividerwhich split the signal before leading to the Miteq mixer for RFself-homodyning. Another Ka band LNA was utilized beforedriving the signal to the LO port of the mixer to providesufficient power before RF self-homodyning. A variable phaseshifter at Ka band was added before the RF port of the mixerto enable phase-matching of signals in the two split paths.After RF self-homodyning, a LPF of 3.8 GHz bandwidth wasused to recover the baseband data.

For the uplink demonstration, a 35.75 GHz LO at thecustomer unit (CU) is used to up-convert the AWG generated5 Gbps ASK RZ data. Similar to downlink, Ka band LNAand MPAs were then used to amplify the mm-wave signal.At the BS, RF self-homodyning arrangements were madesimilar to downlink to retrieve the baseband 4-ASK data. Therecovered data is then modulated using a DFB laser operating

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Fig. 2. (a)–(e) Optical and RF spectra for downlink at respective locationsa to d of the experimental setup in Fig. 1.

at 1549.15 nm and later detected by a low-speed PD andrecovered at the CS.

III. RESULTS AND DISCUSSION

For the downlink demonstration, relevant optical andelectrical spectra at respective points a to d of Fig. 1 are shownin Fig. 2(a)–(e). The modulated spectra after modulatingthe 4-ASK RZ data are shown in Fig. 2(a). The modulatedmm-wave carrier after the PD with baseband replica is pre-sented in Fig. 2(b). As can be seen, the modulated mm-wavesignal is weak and hence needs to be amplified. Fig. 2(c)presents the amplified signal which filtered the broadbandnoise at Ka band and thus the noise floor was dropped down.RF spectra of the successfully recovered baseband data afterthe RF self-homodyning and LPF are shown in Fig. 2(d). Anenlarged version of the recovered 4-ASK data is also shownin Fig. 2(e).

1862 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 20, OCTOBER 15, 2012

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Fig. 3. (a)–(e) Optical and RF spectra for uplink at respective locationse to i in Fig. 1.

Similarly, for uplink, relevant RF and optical spectra atrespective points at e to i of Fig. 1 is shown in Fig. 3(a)–(e).The up-converted mm-wave signal after the mixing of LOand 4-ASK data is shown in Fig. 3(a). The amplified versionof the modulated mm-wave signal is presented in Fig. 3(b).RF spectra of the signal after the RF self-homodyning andfiltering is shown in Fig. 3(c) with an inset of the retrieveddata at BS. Fig. 3(d) shows the modulated optical spectra atuplink prior to SMF transmission. The detected and filtered4-ASK data is finally presented in Fig. 3(e).

The bit error rate (BER) can be estimated from the signalto noise ratio (SNR) of the lower eye-openings (eye-diagramsare shown in Figs. 5 and 6) using the relations as follows[13]–[15]:

SN R =[R × (

PL1 − PL0

)]2

(σ0 + σ1)2 (1)

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Here, R is the responsivity of the PD, PL1 and PL0 are thepower associated with logic 1 and logic 0. σ 2

0 and σ 21 are the

noise variances of the signal and can be expressed as

σ 20 = PN0 + N FAM P + K T Bn N FR X (3)

σ 21 = PN1 + N FAM P + K T Bn N FR X . (4)

Here, PN0 and PN1 are the total noise of the PD usinga Gaussian approximation including the shot noise variancecomponents associated with PL0 and PL1 respectively. Totalnoise in this case constitutes the thermal noise, shot noiseand relative intensity noise. N FAM P and N FR X are the noisefigure of amplifiers and noise figure of the receiver respec-tively. K T Bn is the thermal noise at the RF receiver where Kis the Boltzmann’s constant, T is the temperature in Kelvin

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Fig. 4. Estimated SNR for the downlink using 4-ASK RZ data.

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Fig. 5. BER curves. (a) and (b) Eye diagrams for the downlink using 4-ASKRZ data.

and Bn is the noise bandwidth. Fig. 4 represents the downlinkSNR for the lower-eye openings at varying received opticalpower. For the error-free reception of data at a received opticalpower of −7 dBm, we found the SNR to be 12.54 dB. Thismethod provides an estimated SNR of the lower-eye based onthe assumption that the noise distribution is Gaussian white.Therefore, the estimated BER from Eq. (2) might not be veryaccurate. Hence, to further investigate the BER, we used a highspeed real-time oscilloscope where transmitted and receiveddata sequences were captured and later compared through off-line error counting to calculate the total BER. Fig. 5 shows theBER curves and eye-diagrams for the downlink experiments.At a BER of 10−9, receiver sensitivity after 25.1 km SMF wasfound to be −8.2 dBm. A negligible power penalty of 0.25 dBwith respect to BTB was found in this case.

Similarly for uplink, BER curves and eye-diagrams areshown in Fig. 6. After the SMF transmissions, a receiversensitivity of −9.3 dBm was found at a BER of 10−9. Thepower penalty in this case was 0.3 dB compared to BTBconditions.

In both the downlink and uplink BER plots, BER resultsdeviate slightly from the expected theoretical linear depen-dence. This could be due to the additional noise from EDFA

ISLAM et al.: MULTILEVEL MODULATIONS FOR GIGABIT ACCESS IN A SIMPLE mm-WAVE ROF LINK 1863

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Fig. 6. BER curves. (a) and (b) Eye diagrams for the uplink using 4-ASKRZ data.

and/or mm-wave RF components which eventually limit theBER floor.

IV. CONCLUSION

A full-duplex mm-wave RoF link using multi-level ASKmodulation is proposed and demonstrated. The demonstratedtechnique exploits a simpler mm-wave RoF solution employ-ing a combination of unlocked optical heterodyning and RFself-homodyning, as proposed in [10]. Introduction of multi-level amplitude modulation in such phase-insensitive-detectionbased systems can successfully realize the benefits of higher-order modulations, such as spectral efficiency and gigabitaccess; in addition to providing simplifications in generation,transport and data recovery processes. Moreover, realization ofspectral efficiency using M-ASK could be a preferable choice,as it avoids the complexities and processing associated withcomplex modulation schemes such as quadrature amplitudemodulation or differential phase-shift-keying.

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