We demonstrate the feasibility of Radio Over Fiber (ROF) transmission of 4 Sub-Carrier Multiplexed (SCM) 64- QAM data streams to a pair of low-complexity Radio Access Units (RAUs), which form part of a virtual MIMO architecture under the control of a Central Unit (CU). The ROF signal is transmitted over low frequency RF carriers and heterodyne detection is used at the RAUs. The 4 coherent optical carriers used for data transmission as well as for heterodyning at the RAUs are generated using a single laser rather than several inevitably non-coherent lasers. Our proposed system transmits duplex data of 480 Mbit/s and 240 Mbit/s in the downlink and uplink directions, respectively at mm-wave frequencies of 25 GHz or 50 GHz

Ghafoor, Salman

Hanzo, Lajos

English

Southampton (e-Prints Soton)

IEEE COMMUNICATIONS LETTERS, ACCEPTED FOR PUBLICATION 1Sub-Carrier-Multiplexed Duplex 64-QAMRadio-over-Fiber Transmission for Distributed AntennasSalman Ghafoor and Lajos HanzoAbstract—We demonstrate the feasibility of Radio Over Fiber(ROF) transmission of 4 Sub-Carrier Multiplexed (SCM) 64-QAM data streams to a pair of low-complexity Radio AccessUnits (RAUs), which form part of a virtual MIMO architectureunder the control of a Central Unit (CU). The ROF signalis transmitted over low frequency RF carriers and heterodynedetection is used at the RAUs. The 4 coherent optical carriersused for data transmission as well as for heterodyning at theRAUs are generated using a single laser rather than severalinevitably non-coherent lasers. Our proposed system transmitsduplex data of 480 Mbit/s and 240 Mbit/s in the downlink anduplink directions, respectively at mm-wave frequencies of 25 GHzor 50 GHz.Index Terms—Radio-over-ﬁber transmission, distributed an-tennas, 64QAM.I. INTRODUCTIONDISTRIBUTED Antenna Systems (DAS) using mm-wavesand sophisticated, yet low-complexity 6 bit/symbol 64-QAM transmissions are proposed for a novel DAS application.DASs provide a high capacity per subscriber by re-using thesame frequency band in all the adjacent cells [1]. In thisscenario, the optical ﬁber backbone linking the Radio AccessUnits (RAUs) plays an important role as a beneﬁt of its highbandwidth, transparency to RF signals and low attenuation.Hence the RAUs are connected to a Central Unit (CU) bytheir optical ﬁber backbone and all the signal processing taskscan be performed at the CU, where the RAU antennas act aselements of a virtual Multiple Input Multiple Output (MIMO)architecture [1], as shown in Fig 1. Consider Link 1, whichconsists of two RAUs connected through two spans of opticalﬁber with a CU. The RAUs transmit RF signals at mm-wave tothe Mobile Stations (MSs), which are connected to a particularRAU depending upon their received signal quality. Trans-mission of mm-waves utilizing remote heterodyne detection(RHD) at the RAU by the use of separate lasers has beenproposed in [2], [3]. However, the separate laser sources usedin [2] and [3] require complex signal processing for achievingcoherence amongst them. RHD is achieved by generatingmultiple coherent sidebands from a single laser in [4] totransmit baseband data at a mm-wave frequency from a singleRAU. However, the technique used in [4] transmits a high-power optical carrier along with the sidebands through theManuscript received August 24, 2011. The associate editor coordinatingthe review of this letter and approving it for publication was O. Dobre.The authors are with the School of ECS, University of Southampton, SO171BJ, United Kingdom (e-mail: {sag1z07, lh}@ecs.soton.ac.uk; http://www-mobile.ecs.soton.ac.uk).The ﬁnancial support of the National University of Science and Technology(NUST), Pakistan, as well as that of the EPSRC, UK under the auspices of theIndia-UK Advanced Technology Centre and the China-UK Bridge is gratefullyacknowledged.Digital Object Identiﬁer 10.1109/LCOMM.2011.101711.111794MSMSλ3 λ2λ3λ4λ3MSMSλ3 λ2 λ1 λ4MSRAU1RAU2MS RAU1RAU2Virtual MIMOProcessingLink 2 Link 1CURAU: Radio Access UnitMS; Mobile StationCU: Central UnitS: SectorS2 S3S1 S4Fig. 1. A Virtual MIMO architectureﬁber, which may distort the vulnerable 6 bit/symbol 64-QAMROF signal due to Cross Phase Modulation (XPM) which isless of a problem for conventional1 bit/symbol On-Off Keying(OOK) [5]. The XPM imposes phase noise on the ROF signal,which is converted to amplitude noise by the ﬁber-induceddispersion [5]. Two different methods of generating multiplecoherent sidebands using cascaded optical modulators and/orhigh frequency local oscillators to generate multiple mm-wavefrequencies were described in [6]. Since cascaded modulatorsare used, the number of components in the system is increasedand both methods of [6] transmitted only uni-directional data.Bidirectional data transmission at two mm-wave frequenciesto two RAUs using a single modulator has been demonstratedin [7]. However, the use of high-frequency local oscillatorsand a modulator coupled with narrow-band optical ﬁlteringmakes the system of [7] less attractive. Furthermore, all themethods mentioned above transmit only 1 bit/symbol. In [8],multi-level modulation formats have been used, but the systemrequires a high frequency local oscillator and a modulator totransmit unidirectional data and only a single RF channel maybe transmitted by a single RAU. Against this background, ournovel contributions are:1- We demonstrate the feasibility of transmitting 4Sub-Carrier Multiplexed (SCM) 6 bit/symbol 64-QAM mm-wave signals in a DAS by incorporatingcoherent optical carriers in the transmitted signal forheterodyne detection at the RAUs.2- Our scheme transmits ROF signals simultaneouslyto two different RAUs in a duplex architecture andat two different mm-wave frequencies, using only asingle laser source located at the CU.The simulation tool used for our performance studyis the commercial software VPITransmissionMaker 8.51089-7798/11$26.00 c ⃝ 2011 IEEE2 IEEE COMMUNICATIONS LETTERS, ACCEPTED FOR PUBLICATION(www.vpiphotonics.com).II. THE PROPOSED VIRTUAL MIMO ARCHITECTUREFig. 2 shows the design of Link 1 seen in Fig 1. FourContinous Wave (CW) optical signals are generated from asingle Laser Diode (LD) using the Optical Carrier Suppression(OCS) technique of [9]. The sinusoidal signal applied to theDual-Drive Mach Zehnder Modulator (DD-MZM) of Fig 2for the generation of multiple sidebands has a frequency of12.5 GHz and an amplitude of 0.7 times 𝑉𝜋,w h e r e𝑉𝜋 isthe voltage required to induce a phase shift of 𝜋 for eachof the DD-MZM arms. A more detailed discussion on theuse of DD-MZM for OCS technique is mentioned in [9]. Theoutput of the DD-MZM which is ampliﬁed using the ErbiumDoped Fiber Ampliﬁer (EDFA) of Fig 2, is fed to a 1x4Arrayed Waveguide Grating (AWG) having a channel spacingof 25 GHz. Each channel of the AWG performs Gaussianﬁltering with a 3 dB bandwidth of 25 GHz centered at theodd-indexed optical side-bands generated during OCS. Theﬁrst-order odd-indexed side-bands are labelled 𝜆2 and 𝜆3,while the second-order odd-indexed side-bands are labelled𝜆1 and 𝜆4,a ss h o w ni nF i g2 .T h eﬁrst-order side-bands 𝜆2and 𝜆3 have a higher Optical Signal to Noise Ratio (OSNR)compared to the second order side-bands 𝜆1 and 𝜆4. Therefore,𝜆1 and 𝜆4 are used to transmit down-link (DL) RF data, whichusually requires lower OSNR compared to the UL RF data,since we can afford the employment of more sophisticatedreceivers at the CU. Single-drive MZMs are used to intensitymodulate the optical carriers 𝜆1 and 𝜆4 with four 64-QAM RFsignals that are SCM, as portrayed in Fig 2. Each RF signalhas a symbol rate of 20 MSymbols/s and the resultant SCMsignals have center frequencies in the range spanning from1 GHz to 1.12 GHz for RAU1 and 1.16 GHz to 1.28 GHzfor RAU2. Hence the total transmission rate conveyed to eachRAU is 480 Mbit/s (4 channels x 20 MSymbols/s x 6 bits persymbol). Our proposed scheme also supports potential RAUsectorization by forwarding each of the four SCM channelsto four separate antennas of the sector S1, S2, S3 and S4,as shown in Fig 1. The unmodulated optical carriers 𝜆2 and𝜆3 are coupled together using an Optical Coupler (OC) andattenuated using an Optical Attenuator (OA), before beingcoupled with the modulated optical carriers 𝜆1 and 𝜆4,a sseen at the output OC of the CU of Fig 2 . The unmodulatedcarriers 𝜆2 and 𝜆3 are attenuated in order to reduce the effectof phase noise due to XPM on the modulated carriers. Thephase noise increases upon increasing the average power ofthe signal, therefore the average power per optical side-bandshould be kept low. The spectral plot of the resultant opticalsignal is shown in the inset of Fig 2. It can be observed fromthe spectral plot that due to the imperfection of OCS andowing to the ﬁnite extinction ratio of the AWG based Gaussianﬁltering, the even-indexed side-bands are still partially presentalong with the odd-indexed side-bands. The composite opticalsignal is then transmitted from the CU through 5 km of SingleMode Fiber (SMF) towards RAU 1 of Fig 1. At RAU 1, thereceived signal is ampliﬁed and Gaussian ﬁltered using the 1x4AWG, which has the same parameters as the AWG employedat the transmitter. The EDFA is placed before the AWG so thatthe Ampliﬁed Spontaneous Emission (ASE) noise generatedby the EDFA is ﬁltered by the AWG. At RAU 1, the datamodulated optical channel 𝜆4 is dropped. The signal 𝜆2 issplit into two parts using a 3 dB Optical Splitter (OS), asseen in Fig 2. The lower branch is modulated with the ULRAU1-CU RF data, while the upper branch is coupled with𝜆4 and detected by a high-speed Photo-Detector (PD). The PDhas a responsivity of 0.8𝐴/𝑊, a dark current value of 2𝑛𝐴and thermal noise of 10×10−12𝐴/𝑊 1/2. At the output of thePD, a 50 GHz mm-wave RF carrier modulated with the RFdata is obtained through the process of heterodyning [2]. Afterthe PD, a Gaussian band-pass Electrical Filter (EF) having a 3dB bandwidth of 4 GHz and a center frequency of 50 GHz isused to remove the unwanted frequencies from the mm-waveRF signal. At this stage, the ampliﬁed output of the EF canbe directly transmitted to the MSs from the RAU’s antenna,as shown in Fig 2. However, in order to characterize thequality of the received mm-wave signal, it is down-convertedto its initial RF frequency using self-mixing [10]. After down-conversion, the resultant RF signal having four SCM channelsenters the 64-QAM demodulator block seen at the top of Fig2, where each of the SCM channels is processed separatelyto obtain Error Vector Magnitude (EVM)1 results. It is worthnoting here that instead of using 𝜆2 for heterodyning and ULRAU1-CU data transmission, we can also use the unmodulatedsignal 𝜆3 which would generate the mm-wave RF signal at 25GHz. Hence our method provides two options for the centerfrequency of the mm-wave RF signal which renders moreﬂexibility in the choice of RF components and services usedat the RAU. The UL data signal at RAU1 is also composedof four SCM channels spanning from 1 GHz to 1.06 GHz.Each SCM channel is a 64-QAM signal having a symbol rateof 10 MSymbol/s. Hence the total data rate transmitted inthe UL direction by a single RAU is 240 Mbits/s. As seenin Fig 2, the ROF signals 𝜆1 and 𝜆3 at RAU1 are coupledtogether and transmitted over 5 km SMF towards RAU2. Thesignal received at RAU2 is ampliﬁed and Optically BandpassFiltered (OBPF) to separate 𝜆1 and 𝜆3. The ROF signal 𝜆3is then optically split into two paths in Fig 2, one of whichis used for heterodyning to generate the mm-wave RF signal,while the other is used for transmitting the UL data receivedat the RAU2. The mm-wave RF signal is then generated andprocessed in the same way as at RAU1. The UL SCM RFsignal transmitted by RAU2 has center frequencies in therange of 1.16 GHz to 1.22 GHz and has the same data rate aswell as modulation format as that of RAU1. After propagatingthrough 5 km SMF, the ROF signal 𝜆3 at RAU2 is combinedwith the UL ROF signal 𝜆2 of RAU1. The combined signalsare received at the CU after they propagate through anotherspan of 5 km of SMF. At the CU, the optical signals areﬁltered and photo-detected to obtain the RF signals. A 64-QAM demodulator is used to obtain EVM results of all theRF signals received from each of the RAUs.III. PERFORMANCE RESULTSIn this section, we present EVM results for both the DL andUL 64-QAM signals. The EVM is plotted against the Signal to1The EVM is a measure of the difference between the expected complex-valued voltage of a demodulated symbol and the actual received symbol [11]GHAFOOR and HANZO: SUB-CARRIER-MULTIPLEXED DUPLEX 64-QAM RADIO-OVER-FIBER TRANSMISSION FOR DISTRIBUTED ANTENNAS 3λ4λ3λ2λ1f2 (   )f2 (   )f1 (  ) Signal64 QAM4 SCM ROFf1 (  ) Signal64 QAM4 SCM ROFf2 (   ) f1 (  ) f2 (   )λ3λ2πave=20 mW Pλ2λ4f1 (  )λ1λ325GHzAWGOC OCSMF5 km5 kmSMFOS OS OCOC25GHzAWGOCSplitterMixerEAEF PDSignal4 SCM ROF64 QAMOADemodulator64 QAMOBPFOBPFSignal64 QAM4 SCM ROFDemodulator64 QAM          &Photo−detectorEDFALink 1Central UnitDD−MZMLDRIN=−140 dB/HzLinewidth=20 MHzDCBiasEDFASMF5 kmSMF5 kmEDFAOANoise Figure=6 dBEAEF PDOCMixerSplitterDemodulator64 QAMOSOANoise Figure=6 dBRAU 2RAU 2RAU 1RAU 1−50 −25 0 25 50−80−60−40−20Frequency (GHz) [Relative to 193.1 THz]Power (dBm)Fig. 2. Schematic of the proposed optical link based virtual MIMO architecture.20 25 30 35 4024681012SNR of Received Signal (dB)EVM (%)  RAU: 1; Ch: 1RAU: 1; Ch: 2RAU: 1; Ch: 3RAU: 1; Ch: 4RAU: 2; Ch: 1RAU: 2; Ch: 2RAU: 2; Ch: 3RAU: 2; Ch: 4(a)20 25 30 35 4024681012SNR of Received Signal (dB)EVM (%)  RAU: 1; Ch: 1RAU: 1; Ch: 2RAU: 1; Ch: 3RAU: 1; Ch: 4RAU: 2; Ch: 1RAU: 2; Ch: 2RAU: 2; Ch: 3RAU: 2; Ch: 4(b)Fig. 3. EVM vs SNR for (a) CU-RAU DL and (b) RAU-CU UL directions.Noise Ratio (SNR) of the received RF signal. The value of theSNR is varied by importing in-band optical noise on the ROFchannel. Fig 3a and 3b show the results for the DL and ULRF signals, respectively. Since we have used higher-orderside-bands for the transmission of DL signals, the maximum SNRachieved for the DL signals is lower than the maximum SNRof UL signals due to the lower OSNR of the DL optical side-bands compared to UL optical side-bands. It can be observedfrom Fig 3a and 3b that all the RF channels of both directionshave EVM values that are close to each other. The EVM valuesachieved for the DL and UL directions are as low as 2.3%and 1.4%, respectively. These results show that our proposedarchitecture is suitable for the transmission of high-bit-rate RFsignals. In order to provide further insight into the results, wehave presented the constellation diagrams in Fig 4 for the thirdSCM channel of RAU2 received in the DL and UL directions.The SNR values chosen for Fig 4 are set to the maximum−8 −4 0 4 8−8−4048RealImaginary(a) RAU: 2(DL), SNR=32.1dB−8 −4 0 4 8−8−4048RealImaginary(b) RAU: 2(UL), SNR=38.2dBFig. 4. Constellation diagrams for DL and UL signals.achievable values for channel 3. It can be observed from Fig4 that the constellation points are conﬁned well within theirdecision boundaries.IV. CONCLUSIONSWe have demonstrated the transmission of 64-QAM, 4-SCMduplex mm-wave data to two RAUs that are located at 5 kmand 10 km from the CU in a virtual MIMO architecture. Ourtechnique uses a single laser source to transmit all the data andalso allows two different radio frequencies to be generated atthe RAU. Good values of EVM are obtained for both the DLand UL signals using components having parameter valuesclose to commercially available components.REFERENCES[1] A. Hekkala, M. Lasanen, I. Harjula, L. C. Vieira, N. J. Gomes, A.Nkansah, S. Bittner, F. Diehm, and V. Kotzsch, “Analysis of and com-pensation for non-ideal ROF links In DAS,” IEEE Wireless Commun.,vol. 17, pp. 52–59, June 2010.[2] T. Kuri and K. Kitayama, “Optical heterodyne detection technique fordensely multiplexed millimter-wave-band radio-on-ﬁber systems,” IEEEJ. Lightwave Technol., vol. 21, pp. 3167–3179, Dec. 2003.[3] X. Zhang, B. Liu, J. Yao, K. Wu, and R. Kashyap, “A novelmillimeter-wave-band radio-over-ﬁber system with dense wavelength-division multiplexing bus architecture,” IEEE J. Lightwave Technol.,vol. 54, pp. 929–937, Feb. 2006.[4] M. F. Huang, J. Yu, Z. Jia, and G. K. Chang, “Simultaneous generationof centralized lightwaves and double/single sideband optical millimeter-wave requiring only low-frequency local oscillator signals for radio-over-ﬁber systems,” IEEE J. Lightwave Technol., vol. 26, pp. 2653–2662,Aug. 2008.[5] W. H. Chen and W. I. Way, “Multichannel single-sideband SCM/DWDMtransmission systems,” IEEE J. Lightwave Technol., vol. 22, pp. 1679–1693, July 2004.4 IEEE COMMUNICATIONS LETTERS, ACCEPTED FOR PUBLICATION[6] J. Yu, G.-K. Chang, Z. Jia, A. Chowdhury, M.-F. Huang, H.-C. Chien,Y.-T. Hsueh, W. Jian, C. Liu, and Z. Dong, “Cost-effective optical mil-limeter technologies and ﬁeld demonstrations for very high throughputwireless-over-ﬁber access systems,” IEEE J. Lightwave Technol., vol. 28,pp. 2376–2397, Aug. 2010.[7] Y.-T. Hsueh, Z. Jia, H.-C. Chien, J. Yu, and G.-K. Chang, “A novelbidirectional 60-GHz radio-over-ﬁber scheme with multiband signalgeneration using a single intensity modulator,” IEEE Photon. Technol.Lett., vol. 21, pp. 1338–1340, Sep. 2009.[8] J. James, P. Shen, A. Nkansah, X. Liang, and N. J. Gomes, “Nonlin-earity and noise effects in multi-level signal millimeter-wave over ﬁbertransmission using single and dual wavelength modulation,” IEEE Trans.Microw. Theory Tech., vol. 58, pp. 3189–3198, Nov. 2010.[9] S. Ghafoor and L. Hanzo, “Reduced dispersion duplex DQPSKradio-over-ﬁber communications using single-laser-based multiple side-bands,” in Proc. IEEE International Conference on Communications,pp. 1–5, June 2011.[10] A. Wiberg, P. Perez-Millen, M. V. Andres, P. A. Andrekson, and P.O. Hedekvist, “Fiber-optic 40-GHz mm-wave link with 2.5-Gb/s datatransmission,” IEEE Photon. Technol. Lett., vol. 17, pp. 1938–1940, Sep.2005.[11] “IEEE 802.11.a Standardization (ISO/IEC 8802-11:1999/Amd1:2000(E),”

A novel bidirectional 60-GHz radio-over-ﬁber scheme with multiband signal generation using a single intensity modulator,”

A novel millimeter-wave-band radio-over-ﬁber system with dense wavelengthdivision multiplexing bus architecture,”

Analysis of and compensation for non-ideal ROF links In

Cost-effective optical millimeter technologies and ﬁeld demonstrations for very high throughput wireless-over-ﬁber access systems,”

Fiber-optic 40-GHz mm-wave link with 2.5-Gb/s data transmission,”

Multichannel single-sideband SCM/DWDM transmission systems,”

Nonlinearity and noise effects in multi-level signal millimeter-wave over ﬁber transmission using single and dual wavelength modulation,”

Optical heterodyne detection technique for densely multiplexed millimter-wave-band radio-on-ﬁber systems,”

Reduced dispersion duplex DQPSK radio-over-ﬁber communications using single-laser-based multiple sidebands,” in

Simultaneous generation of centralized lightwaves and double/single sideband optical millimeterwave requiring only low-frequency local oscillator signals for radioover-ﬁber systems,”

Sub-Carrier-Multiplexed Duplex 64-QAM Radio-Over-Fiber Transmission for Distributed Antennas

Sub-Carrier-Multiplexed Duplex 64-QAM Radio-Over-Fiber Transmission for Distributed Antennas

Abstract

Similar works

Full text

Available Versions

Southampton (e-Prints Soton)