RESEARCH ARTICLE

Simulative analysis of the influence of continuous wave laserpower on different data formats for a bi-directionalradio over fiber communication system

Satbir Singh & Amarpal Singh & Shashi B. Rana

Received: 7 January 2010 /Accepted: 20 January 2012 /Published online: 17 February 2012# Optical Society of India 2012

Abstract Radio over Fiber (RoF) is being used in-creasingly for the distribution of cellular/radio andCATV signals to extend both coverage and capacitywithin buildings and also in metropolitan areas. In thispaper we have investigated by simulating the influ-ence of Continuous Wave (CW) laser source poweroperating at a center frequency of 1550 nm for RZ andNRZ data formats at different bit rates of 2.5Gb/s,5.0Gb/s and 10Gb/s for SCM (sub carrier multiplex-ing) based RoF system. The simulated results havebeen analyzed in this paper for fixed no. of channels(N070) and constant dispersion level (16.75 ps/nm/km)over a bidirectional single mode optical fiber. Further ithas been investigated in this paper that the NRZ dataformat is comparatively better for RoF communicationsystems over short to medium transmission distances(from 2–12 km) for wireless/mobile communicationthan RZ data format. Maximum Q-factor of 15.94 dB

with minimum bit error rate (less than 10−9 ) at a bit rateof 5.0 GB/s has been achieved for this RoF communi-cation model.

Keywords Bit error rate (BER) . IM-DD . Qualityfactor (Q-factor) . Radio over Fiber (RoF) . SCM .

Remote Antenna Units (RAUs)

Introduction

Radio over Fiber is a hybrid technology designed toenable the efficient and cost effective transport ofwireless signals over optical fibers [1]. In RoF scheme,optical signal is modulated at radio frequencies andtransmitted via the optical fiber [2, 3]. RoF makes itpossible to centralize the RF signal processing func-tions in one shared location (head end), and then to useoptical fiber, which offers low signal loss (0.2 dB/kmfor 1550 nm, and 0.5 dB/km for 1310 nm wave-lengths) to distribute the RF signals to the remoteantenna units (RAUs) [4]. At the RAU, the transmittedRF signal is recovered by direct detection in the PINphoto diode (PD). The signal is then amplified andradiated by an antenna. This method of transportingRF signals over the optical fiber is called IntensityModulation with Direct Detection (IM-DD) and isthe simplest form of the RoF link. There are two waysof modulating the light source :(i) The RF signaldirectly modulates the laser diode’s current. The directmodulation scheme is simple but suffers from a laser –

J Opt (January–March 2012) 41(1):25–32DOI 10.1007/s12596-012-0059-z

S. Singh (*) : S. B. RanaDepartment of Electronics & Communication Engineering,Guru Nanak Dev University,Regional Campus,Gurdaspur, Punjab, Indiae-mail: satbir1_78@yahoo.co.in

S. B. Ranae-mail: shashi_rana12@yahoo.co.in

A. SinghDepartment of Electronics & Communication Engineering,Beant College of Engineering and Technology,Gurdaspur, Punjab, Indiae-mail: s_amarpal@yahoo.com

frequency chirp effect, this chirp effect results in se-vere degradation of the RoF system performance. (ii)To operate the laser in continuous wave (CW) modeand then use an external modulator such as the Mach-Zehnder Modulator (MZM), to modulate the intensityof the light. After transmission through the opticalfiber and direct detection on a photodiode, the photo-current is a replica of the modulating RF signal ap-plied either directly to the laser or to the externalmodulator at the head end or central control station.IM-DD radio over fiber systems use single mode fiberfor distribution of RF signals [5]. The biggest advan-tage of IM-DD is that amplitude modulation (AM) andmulti-level modulation formats such as multi QAM,PSK and DPSK may be transported. Sub-Carrier Mul-tiplexing (SCM) can also be used in Radio over fibersystems [6]. The performance of RoF system dependson following parameters: Method used to generate theoptically modulated RF signal, optical fiber chromaticdispersion, laser and RF power level, nonlinearity dueto an optical power level, bit rate and modulationformat used [7]. A. Kaszubowska et al. investigatedthe system performance for different values of channelspacing and spectral efficiencies for RoF system [8].In the paper he demonstrated a hybrid WDM/SCMbased radio over fiber distribution system employingthe technique of wavelength interleaving using 155Mbps Non Return to Zero (NRZ) data stream. KosukeUegaki et al. investigated convergence of communi-cation and broadcasting which makes use of Radioover Fiber (RoF) network [9]. He focused that inRoF networks, although system costs can be reducedby employing sub-carrier multiplexing (SCM), theinfluence of nonlinear distortion becomes large. G.M. Sil et al. presented a novel BER estimation methodfor sub carrier multiplexed signals using QAM modu-lation of RoF system [10]. David Wake discussed theRoF distribution system for cellular communication[11]. Q. Chang and Yikai Su. proposed and experi-mentally demonstrated a novel RoF system for simul-taneous generation and transmission of RF signalsover a 25 km of single mode fiber with less than1.2 dB power penalties using a dual-parallel Mach-Zehnder modulator and a single-drive Mach-Zehndermodulator [12]. M. Garcia Larrode et al. demonstratedthe feasibility of generating two QAM radio signalssimultaneously at 17.3 GHz and 17.8 GHz after4.4 km of multimode fiber in an optically frequencymultiplication radio over fiber link for wireless multi

standard support at the antenna site [13]. Jianxin Ma etal. investigated the influence of the modulation indexof MZM modulator on radio over fiber link with ASKmillimeter-wave signal [14]. In the present paper wehave comparatively analyzed the impact of CW laserpower over RZ and NRZ data formats for the RoFcommunication system.

System set up for radio over fiber model

RoF technology is proposed as a solution for reducingcost and providing highly reliable communicationservices. The RoF system is very cost-effective be-cause the localization of signal processing in centralstation and it uses a simple base station. Radio overfiber system realizes the transparent transform be-tween RF signal and optical signal. Figure 1 showsthe scheme for the bidirectional RoF system. Thesystem setup shows implementation approach fortransmitting sub carrier multiplexing (SCM)–Ampli-tude Shift Key (ASK) encoded multiple data channels(analog and digital channels) over a common (SMF)optical fiber for uplink and downlink connections.Here, the length of optical fiber taken is 12 km.

Description of radio over fiber communicationmodel

This radio over fiber link is set up by the simulationsoftware Optisystem™. In this simulation model, a RFlocal oscillator having a frequency of 10 GHz has beenused in the transmitter section. RF sinusoidal wave isamplitude modulated by pseudo-random bit sequence(PRBS) data formats (RZ and NRZ) at different bitrates of 2.5Gbit/s, 5.0Gbit/s and 10Gbit/s. A hybridcoupler with 0 dB loss is used. A narrow bandwidthcontinuous wave (CW) laser source with the wave-length of 1550 nm and linewidth of 1 MHz is used inthis communication model. The corresponding fre-quency of CW laser source is 193.1THz for this modelas shown in Fig. 1. The output of the hybrid couplerand laser source is then modulated via a LiNbO3

Mach-Zehnder Modulator (MZM). The extinction ra-tio of MZM is taken as 30 dB. The numbers ofchannels used by carrier generator are 70 at a frequen-cy of 49.35 MHz with a channel spacing of 6 MHz.Here EDFA is tuned at a center frequency of 193.4

26 J Opt (January–March 2012) 41(1):25–32

THz for analysis of BER and Q-factor of this commu-nication model. The transmitted optical signals havebeen sent over single mode optical fiber with length of12 km. An optical circulator is used in this modelwhich collects the light and directs it to a non-reciprocal output port. A bidirectional reflective filter(Gaussian type, and bandwidth of 12 GHz) at thecenter frequency of 193.1 THz and having a reflectionof 99% is used for frequency reuse for uplink connec-tion in this simulative analysis of RoF system.

At the receiver section, the optical signal is detectedby a PIN-photodiode having a responsivity of 0.9 A/Wand then it is amplitude demodulated. The downlinkmicrowave signal is boosted by an electrical amplifier(EA) with a gain of 15 dB and noise power of −60dBm. PIN photodiode (transmitted signal detected bya PD to generate the photocurrent at a base station, thephotocurrent goes through a band pass filter (BPF) andlaunched in a wireless channel in the base station) isused which reconverts the optical pulses into electricalsignals (RF signals radiated with the help of antennasto a specified area), which are analyzed using bit errorrate analyzer and RF spectrum analyzer. For the uplink

connection, the optical spectrum and waveform of theremaining optical carrier used with the help of a bidi-rectional reflective filter with an insertion loss 0 dB.The uplink optical sidebands produce crosstalk when

(Central Control Station)

(Uplink Connection

Base Station for Downlink

with frequency reuse)

(Connection)

RF Local Oscillator (10GHz)

Data Signal

Hybrid Coupler MZM

CW Laser Source

EDFA

SMF

Reflective Filter

PIN-PD

RF Signal (AM demod.)

RF and BER Analyzer I

Optical Analyzer

PIN-PD

RF and BER Analyzer II

I/P Data Signal

Amplitude Modulator

Circulator

Fig. 1 Set up for bidirec-tional Radio over FiberCommunication Model

Table 1 Simulation Parameters for the experiment setup ofRoF system

Parameter Value

Reference Wavelength 1550 nm

Attenuation 0.22 dB/Km

Dispersion 16.75 ps/nm/km

GVD parameters : β2 and β3 −20 ps2/km and 0.08 ps2/km

PMD Coefficient 0.5 ps/(Km)2

Effective Fiber Core Area 80 μm2

EDFA Power 13 dBm

No. of Channels and 70

Channels Frequency 49.25 MHz

Frequency Spacing 6 MHz

Length of optical fiber 12 Km (each in UL and DL)

Line Width 1 MHz

PIN Responsivity 0.9 A/W

J Opt (January–March 2012) 41(1):25–32 27

uplink data is detected at the control station. Thecrosstalk can be reduced with the help of Bessel opti-cal filter having bandwidth of 10 GHz with depth of100 dB. The eye diagrams, BER and Q-factor valuesof the signals have been measured by BER analyzers Iand II at the base station and control station for down-link and uplink connections. Table 1 shows the valuesof various simulation parameters used for the RoFsystem set up.

The optical signals from the laser and the RF oscil-lator are analyzed as:

SLD tð Þ ¼ A:exp j wLD tð Þ þ fLD tð Þð ÞSRF tð Þ ¼ VRF:cos wRF tð Þ þ fRF tð Þð Þ ð1Þ

where ‘A’ and ‘VRF’ define the amplitudes of laserdiode and the RF oscillator, ωLD and ωRF defineangular frequencies of the signals from the laser diode

Fig. 2 Maximum Eye amplitudes at receiver section at 193.1THz with RZ modulation format and at a frequency of 10 GHzfor sine generator over optical distance (SMF) of 12 Km at

different laser power levels (dBm) for a constant dispersion16.75 ps/nm/km for BER analyzer a 2.5 Gbps b 5.0 Gbps c10 Gbps

28 J Opt (January–March 2012) 41(1):25–32

and the RF oscillator. ϕLD(t) and ϕRF(t) are phasenoises of the laser diode and RF oscillator.

Results and discussion

The performance of a RoF system is judged by the biterror rate (BER), which is defined as the probability ofincorrect identification of a bit by the decision circuit ofthe receiver. Most of the RoF systems require BER lessthan 10−9. The BER depends on the signal to noise(SNR) of the current generated at the receiver when aoptical bit stream is converted into the electric domain.The BER is given by the following simple expression:

BER ¼ 0:5 erfc Q=p2ð Þ � exp �Q2=2

� �=Q

p2p ð2Þ

where Q (quality factor) parameter is written as:

Q ¼ I1 � I0ð Þ= σ1 þ σ0ð Þ ð3Þ

where I1 and I0 represent average currents for 1 and 0bits and σ1 and σ0 are the corresponding variances. Thesimulations have been carried out to study and investi-gate the bit error rate and quality factor improvementsfor the radio over fiber system at different power levelsof CW laser source. The input data is modulated withRZ-format over light signal emitted by a CW lasersource by using MZM modulator and an EDFA.

The received RF power is a function of manyfactors such as: differential delay, line width of thelaser source, RF oscillator and bandwidth of the elec-trical filter. The BER increases when we carefullyincrease the laser power (in this case 2 dBm) for alaser line width of 1 MHz. The maximum and mini-mum values of Q-factor for Fig. 2a is 12.98 dB and12.53 dB respectively. The selection of EDFA poweris also very important for RoF system. In this simula-tion model, we have selected EDFA power level that ischosen to be 13 dBm with a gain of 20 dB for betterresults. When bit rate increases from 5 Gbps to10 Gbps, the thickness of the received eye patternspreading under the influence of different laser powersfor a constant dispersion of 16.75 ps/nm/km over anoptical distance of 12 km. Consequently the Q-factorand BER decreases. The minimum achieved value ofBER is less than 10−9 with Q-factor of 13.10 dB asshown in Figs. 2b & c. The another value achieved forQ factor is 11.07 dB again at minimum BER.

As shown in Figs. 3 and 4, the maximum andminimum values of Q-factor obtained at a bit rate of2.5 Gbps and at a bit rate of 10 Gbps are 13 dB and10.57 dB respectively for this simulative model. Whenwe compare the two different parameters like receivedRF power (in dBm) and Q-factor (in dB) with respectto varying laser power (in dBm) over a single modefiber (constant dispersion of 16.75 ps/nm/km andlength of 12 Km) then we observe in Fig. 3 thatlinearization of laser power increases initially and thenit remains constant.

As shown in Fig. 3, the constant level of laser power(in dBm) starts from 0 dBm to 1 dBm for respective bitrates from 2.5 Gbps to 10 Gbps. Similarly Fig. 4 shows

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3-77

-76

-75

-74

-73

-72

-71

-70

-69

laser Power(in dBm)

Rec

eive

d R

F P

ower

Bit rate 2.5GBps

Bit rate 5.0GbpsBit rate 10Gbps

Fig. 3 Received RF Power vs. laser power at different bit ratesfor RoF system

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 310

11

12

13

14

15

16

17

18

laser Power(in dBm)

Q-F

acto

r(dB

)

Bit rate 2.5GBps

Bit rate 5.0GbpsBit rate 10Gbps

Fig. 4 Q-Factor vs. laser power at different bit rates for RoFsystem

J Opt (January–March 2012) 41(1):25–32 29

an increase in the Q-factor w.r.t the varying laser power.From the above eye diagrams (in Fig. 5), the achievedBER and Q-factor are significantly good. The NRZmodulation format is comparatively better for RoFsystem over short to medium transmission distances(from 2 to 12 km). At a bit rate of 5 Gbps, theminimum achieved value of BER is very less than10−9. The corresponding minimum value of BER at

20 Gbps is approximately 10−9. Hence the maximumQ-factor value is 21.63 dB at 5 Gbps. The minimumvalue of Q-factor is 5.69 dB at 20 Gbps. So a gain of15.94 dB in the Q-factor has been obtained by settingan intermediate value of bit rate for NRZ data formatthan RZ data format.

Figures 6 and 7 show a comparative analysis ofvarious parameters for a constant power level of laser

Fig. 5 Received Eye Patterns at 193.1 THz with NRZ modulation format and at a frequency of 10 GHz for sine generator at a constantlaser power level (4dBm) for a constant dispersion 16.75 ps/nm/km a 2.5 Gbps b 5.0 Gbps c 10 Gbps d 20 Gbps

30 J Opt (January–March 2012) 41(1):25–32

source at a constant dispersion of 16.75 ps/nm/km fordifferent optical fiber lengths. Here the plots show thecomparison for three different bit rates 2.5 Gbps,5.0 Gbps and 10 Gbps. The data format used for thesimulative analysis is NRZ. The Q-factor increases aswe increase the bit rate up to 5 Gbps. When we takebit rate of 10 Gbps, the corresponding BER andQ-factor again decreases as the length of optical fiberincreases. The increase in the bit rate will lead to small

distorted eye patterns. For these three bit rates, themaximum achieved values of Q-factor reduce overan optical fiber length of 2 Km to 12 Km. So up tooptical distances from 2Km to 12Km, the resultsobtained are optimal and may be implemented in realtime RoF communication system. The Fig. 7 gives usanalysis of received power levels at different bit ratesand at different optical fiber lengths. The received powerlevels decreases from −60.5 dBm to −69.45dBm, as anoptical distance increases from 2 to 12 Km. Hence overan optical length of 12 km, the respective values ofQ-factors and received powers obtained at the basestation are 10.92 dB, 12 dB, 11.60 dB and −69.45dBm,−69.40 dBm,−69.10 dBm at different bit rates of2.5 Gb/s, 5.0 Gb/s and 10.0 Gb/s.

Conclusion

This paper deals to the study and simulation analysisfor the model of radio over fiber system at differentlaser power levels with fixed no. of channels and forvarying optical fiber lengths. This RoF model is ana-lyzed for a constant dispersion level over an opticaldistance of 12 Km for different modulation formats. Ithas been revealed that when bit rate increases from5 Gbps to 10 Gbps, the thickness of the received eyepattern spreading under the influence of different laserpowers for a constant dispersion over an optical dis-tance of 12 km. The NRZ data format is comparativelybetter than RZ data format for RoF system over shortto medium transmission distances (2 to 12 km) forwireless communication. At a bit rate of 5 Gbps, themaximum achieved value of Q-factor is 21.63 dB. Itsminimum value is 5.69 at 20 Gbps. It is concluded thatthe performance of the radio over fiber system can bemore sensitive to the laser power and laser line widthfor short and medium distances transmission. A gainof 15.94 dB in the Q-factor has been obtained bysetting an intermediate value of bit rate for NRZ dataformat. Hence the power level (in dBm) and linewidth(in MHz) of CW laser source should be carefullychosen for radio over fiber communication system.This type of the simulative RoF model can be usedfor real communication applications such as cableTV/Radio transmission, IP TV and Cellular/Mobiletransmission.

2 3 4 5 6 7 8 9 10 11 1210

12

14

16

18

20

22

Optical Fiber Length(Km)

Q-F

acto

r(dB

)

Bit rate 2.5 Gbps

Bit rate 5 GbpsBit rate 10 Gbps

Fig. 6 Comparison of Q-factors at different bit rates forRoF system

2 3 4 5 6 7 8 9 10 11 12-70

-69

-68

-67

-66

-65

-64

-63

-62

-61

-60

Optical Fiber Length(Km)

Rec

eive

d P

ower

(dB

m)

Bit rate 2.5 Gbps

Bit rate 5 GbpsBit rate 10 Gbps

Fig. 7 Received RF power levels at different bit rates forRoF system

J Opt (January–March 2012) 41(1):25–32 31

References

1. B. Charbonnier, H. Lee Bras, P. Urvoas, Q.T. N’Guyen, M.Huchard, A. Pizzinat, Upcoming perspectives and futurechallenges for RoF, IEEE Letters, 21–23 (2007)

2. T. Kuri, K. Kitayam, A. Stohr, Y. Ogrwa, Fiber-opticmillimeter-wave downlink system using 60 GHz-bandexternal modulation. J. Lightwave Tech. 17(5), 799–806(1999)

3. S.Z. Pinter, X.N. Fernand, Fiber-wireless solution for broad-band multimedia access. IEEE Canadian Review Summer2005, pp. 6–9 (2005)

4. H. Al Raweshidy, Radio over Fiber Technologies forMobile Communication Networks (Artech house, Inc,USA, 2002)

5. M. Fabbri, P. Faccin, Radio over fiber technologies andsystems: new opportunities. 9th International Conferenceon Transparent Optical Networks (ICTON ’07), pp. 230–233 (2007)

6. O.K. Tonguz, H. Jung, Personal communications accessnetworks using subcarrier multiplexed optical links. IEEEJ. Lightwave Tech. 14, 1400–1409 (1996)

7. T.S. Cho, C. Yun, J.-I. Song, Analysis of CNR penalty ofradio over fiber systems including the effects of phase noisefrom laser and RF oscillator. IEEE J. Lightwave Tech. 23(12), 4093–4099 (2005)

8. A. Kaszubowska, L.P. Barry, P. Anandarajah, L. Hu,Characterization of wavelength interleaving in radio-

over-fiber systems employing WDM/SCM. Journal ofOptics Communications, 260, 144–149 (2006)

9. K. Uegaki, K. Kumamoto, K. Yasukawa, K. Inagaki, T.Higashino, K. Tsukamoto, S. Komkj, A novel nonlineardistortion suppression method in RoF systems usingoptical filter. IEEE International Topical Meeting onMicrowave Photonics, pp. 33–36, 3–5 Oct. 2007

10. G.M. Sil, H. Louchet, A. Richter, Efficient BER estimationfor radio over fiber systems. IEEE Conference on OpticalFiber Communication and the National Fiber Optic Engi-neers Conference, 2007(OFC/NFOEC 2007), pp. 1–3, 25–29 March 2007

11. D. Wake, Trends and prospects for radio over Fiber pico-cells, Microwave Photonics, International topical meetingon 5–8 Nov, 2002, W3-1, pp. 21–24.

12. Q. Chang, Y. Su, A radio over fiber system for simultaneousgeneration and transmission of multiband signals, in Proc.Eur. Conf. Optical Communication (ECOC), Berlin, Ger-many, 2007, pp. 091

13. M. Garcia Larrode, A.M.J. Koonen, J.J. Vegas Olmos,I. Tafur Monroy, T.C.W. Schenk, RF bandwidth capacityand SCM in a RoF link employing optical frequency mul-tiplication. 31st European Conference on Optical Commu-nications (ECOC-2005) (CP502), Glasgow, UK, 25–29Sept. 2005

14. J. Ma, J. Yu, C. Yu, X. Xin, H. Huang, Influence of themodulation index of Mach-Zehnder modulator on RoF linkwith ASK millimeter wave signal. Opt. Laser Tech. 41, 11–16. Elsevier (2008)

32 J Opt (January–March 2012) 41(1):25–32