Multiple side-bands are generated from a single laser diode with the aid of optical carrier suppression, which facilitates the transmission of reduced-rate parallel streams mapped to the side-bands. As a benefit, all the signal processing tasks including demultiplexing of a high data rate signal into multiple low data rate signals can be performed at the central unit. The technique can also be useful in reducing the effect of dispersion in case of high frequency Radio Over Fiber (ROF) communications by transmitting a single high data rate signal as multiple low data rate signals using the side-bands generated. We demonstrate the duplex transmission of a bit-rate of 768 Mbit/s over a 50 km fiber using three side-bands in each direction. The three side-band aided system has a BER performance which is close to that of the idealized benchmarker operating in a back-to-back mode

Ghafoor, Salman

Hanzo, Lajos

English

Southampton (e-Prints Soton)

MZMPCDQPSK RF Data f32 DQPSK RF Data, f1 DQPSK RF Data, f1 DQPSK RF Data, fMZMPC2 DQPSK RF Data, fMZMPCDQPSK RF Data f3VbiasDCPave = 20 mWOA PSOAλ4λ5λ6λ2λ1λ3λ4λ5λ6PCMZMPCMZMPCMZMMUX3X1Dual−ArmMZMRF Sinusoidal SignalLDRIN = −140 dB/HzLinewidth = 20 MHzπMUX6X1 50 km SMFDownlinkBER MeasurementUplink50 km SMFMSC−to−BS TransmitterBS−to−MSC TransmitterBS ReceiverNF = 6 dBNF = 6 dBMSC ReceiverBER Measurement1x8AWGGHz2525GHzAWG1x4AWG: Arrayed Waveguide GratingPC: Polarization ControllersOA: Optical Amplifier (EDFA)MUX: MultiplexerLD: Laser DiodePS: Power SplitterNF: Noise FigureFig. 3. Schematic of the proposed bi-directional optical linkapplying a RF signal having a frequency of 12.5 GHz. It canbe observed from Fig 2.b that the output spectrum of the MZMcontains a carrier attenuated by 40 dB, while the even-indexedside-bands are attenuated by 32 dB compared to the ﬁrst-orderodd-indexed side-bands. The frequency spacing between thesupressed carrier and the ﬁrst side-band on both sides of thecarrier as well as that between the consecutive odd-indexedside-bands is twice that of the RF frequency applied. TheOCS technique allows us to obtain six different-wavelengthoptical carriers from a single laser source. Therefore thistechnique is useful in terms of reducing both the cost andcomplexity of a MSC or BS, which would require six different-wavelength laser sources in the absence of the proposedOCS technique. Furthermore, the OCS technique allows usto accurately control the spacing between the side-bands bysimply varying the frequency of the RF signal applied.III. THE PROPOSED BI-DIRECTIONAL LINKThe schematic of our system is shown in Fig 3. The signalgenerated by the laser diode (LD) having the parameters shownin the ﬁgure is passed through a DD-MZM biased using aDC source. A RF sinusoidal signal having a frequency of12.5 GHz is applied to the upper and lower branches of theMZM with the aim of generating multiple side-bands at thedesired frequencies, as seen in Fig 2.b. The signal at theoutput of the MZM consists of higher-order side-bands, againas shown in Fig 2.b. This signal is then ampliﬁed using anErbium Doped Fiber Ampliﬁer (EDFA) having a noise ﬁgureof 6 dB. The output of the EDFA is fed into a 1x8 ArrayedWaveguide Grating (AWG) which has a channel spacing of 25GHz. The AWG has parameters similar to the 25 GHz AWGmanufactured by ANDevices. The transfer function of AWG’sﬁltering exhibits a Gaussian shape having a 3 dB bandwidthof 25 GHz. At the output of the AWG, the six desired odd-indexed side-bands spaced 25 GHz apart are retrieved. Theoptical signals having different central frequencies are labelledby λ1, λ2, λ3, λ4, λ5 and λ6, as shown in Fig 3. The signalsλ1, λ2 and λ3 used for example for the MSC-BS downstreamtransmission, are intensity modulated at the MSC using aMZM by the RF DQPSK signals having central frequenciesof f1, f2 and f3, respectively. After intensity modulation,the resultant ROF signals having optical carriers of λ1, λ2and λ3 are attenuated and multiplexed with the remainingthree signals, namely with λ4, λ5 and λ6, which are thentransmitted without imposing any modulation, using the MUXblock of Fig 3. The signals λ4, λ5 and λ6, which will beused later for upstream transmission at the BSs, are alsoattenuated to suitable levels before being multiplexed at theMSC. Since the EDFAs amplify all the six optical side-bandstogether, it has no individual control over the power of eachside-band. The attenuators are used for adjusting the requiredinput power of each optical side-band. The symbol rate of theDQPSK RF signal is 128 Msymbols/s, hence a data rate of256 MBit/s is supported. The central frequencies f1, f2 andf3 of the RF signals are 1.792 GHz, 2.048 GHz and 2.304GHz respectively. As mentioned in [2] and [12], a dispersiveﬁbre exhibits a frequency dependent transfer function. Therelationship between the received power Pel of the electricalsignal for a ﬁbre having a length of L and a dispersionparameter D is given by [12],Pel ∝ cos[πLcD(felfo)2], (5)where c is the speed of light, fel is the frequency of theelectronic signal and fo is the central frequency of the opticalcarrier. It can be deduced from Equation 2 that the qualityof the received RF signals at the receiver will be affecteddepending upon their central frequencies. Furthermore, sincewe have three unmodulated continous wave signals alongwith the modulated signals travelling in the downstream di-rection, Cross Phase Modulation (XPM) will be encounteredbetween the signals. The XPM imposes phase noise whichis converted to amplitude noise by the dispersive ﬁber [13].The phase noise increases upon increasing the average powerof the signal, therefore the average power per optical side-band should be kept low. In order to minimize the XPM aswell as the distortions produced by Self Phase Modulation(SPM), we have used the optical attenuators shown in Fig33 to reduce the optical power per channel. The power ofeach optical channel in our simulation is reduced to -1 dBmbefore transmission. Following optical multiplexing in Fig 3,the signals are transmitted over a 50 km Standard Single ModeFiber (SSMF) section. It is worth mentioning here that we canextract any of the signals λ1-λ3 at a shorter length of ﬁber todeliver RF signals to BSs at different distances. However, inour investigations we have considered the worst-case scenario,where all the signals propagate along the entire 50 km sectionof SSMF. After travelling through the SSMF, the signal issplit into two paths, one of which goes to the BS receiver,while the other to the BS transmitter. At the BS receiver,a 1x4 AWG having the same channel-spacing and ﬁlteringparameters as the one used at the transmitter, separates thethree modulated side-bands. At the BS transmitter, again a1x4 AWG separates the unmodulated optical side-bands λ4-λ6. In the absence of these unmodulated signals λ4-λ6, theBSs would require separate LDs to transmitt the upstreamsignal back to the MSC’s receiver. The ﬁltered signals λ4-λ6 generated at the output of the 1x4 AWG seen in Fig 3are then intensity modulated by the RF DQPSK signals tobe transmitted from the BS to the MSC, which have centralfrequencies of f3, f2 and f1, respectively. We have consideredthe same data rate and modulation format for the upstreamBS-MSC RF signals as the downstream MSC-BS RF signals.The ROF signals are then optically multiplexed in Fig 3 andtransmitted over another 50 km ﬁber section to the MSC’sreceiver. After propagating through the ﬁber, the signals arereceived at the MSC and detected as seen in Fig 3. In orderto quantify the integrity of the received signals, we have useda RF DQPSK receiver at the MSC. The architecture of theMSC receiver has elements reminiscent of the BS’s receiverand is shown in Fig 4. More explicitly, the MSC receiver’sinput has a 1x4 AWG to extract the required optical signals,say λx. The optical signal is then detected by a PD, whoseparameters are also shown in Fig 4.DQPSKDemodulatorBERCalculationPD EF AGCIQλx−12 10x10 A/Hz1/2Dark Current = 2 nAThermal Noise = Responsivity = 0.8 A/WAWG: Arrayed Waveguide GratingAGC: Automatic Gain ControlPD: Photo DetectorEF: Electrical Filter25GHzAWG1x4InputFig. 4. DQPSK receiver setupThe resultant electronic RF signal at the output of the PDof Fig 4 also contains the upper harmonics of the desiredsignal, which are generated by the non-linear process of photo-detection. As seen in Fig 4, this electronic signal is thenampliﬁed using an electronic Automatic Gain Control (AGC)ampliﬁer. The parameters of the AGC were conﬁgured formaintaining a constant output power for variable input RFpowers. After AGC-aided ampliﬁcation, the signal is passedthrough a ﬁrst order Gaussian Electronic Filter (EF), which isa band-pass ﬁlter having an identical center frequency to thatof the RF signal and a 3 dB bandwidth of ﬁve times the symbolrate. The signal at the output of EF is then demodulated by theDQPSK demodulator of Fig 4, as detailed in [14]. The low-pass ﬁlters of the demodulator seen in ﬁg 4 exhibit a Gaussianshape having a 3 dB bandwidth of 0.85 times the symbol rate.The ON-OFF keyed I and Q electrical bit sequences at theoutput of the DQPSK demodulator are then subjected to BERcalculation in Fig 4 for the I and Q signals separately. TheBER of the DQPSK signal is obtained by averaging the BERsof the I and Q signals [15].IV. PERFORMANCE RESULTS−100 −50 0 50 100−70−60−50−40−30−20−10Frequency (GHz) [Relative to 193.1 THz]Power (dBm)(a)−100 −50 0 50 100−70−60−50−40−30−20Frequency (GHz) [Relative to 193.1 THz]Power (dBm)(b)Fig. 5. (a). Spectrum of the Downstream Input Signal of the Fiber. (b).Spectrum of the Upstream Input Signal of the Fiber.Let us now characterize the proposed architecture’s perfor-mance. Fig 5.a and Fig 5.b show the optical spectrum of theROF signal at the input of the ﬁber in the MSC-BS down-stream and in the BS-MSC upstream directions, respectively.As seen in Fig 5.a, the MSC-BS downstream signal spectrumconsists of six optical side-bands. Three of these carriers areintensity-modulated to generate the ODSB signals, while thesignals λ4-λ6 are transmitted without any modulation. Fig 5.bshows the BS-MSC upstream signal spectrum at the input ofthe ﬁber. It can be observed from Fig 5.a and 5.b that thereis a slightly higher degree of spectral noise at the base of thehigher-order side-bands compared to the ﬁrst-order side-band.The reason for this is the lower powers of the higher-orderside-bands at the output of the DD-MZM, which generatesa higher ASE noise at these wavelengths in the EDFA.The BER performance of the downstream and upstream ischaracterized in Fig 6.a and 6.b, respectively. In Fig 6, we4

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