Measurement of optical impairments in OFDMbased radio-over-fiber communication systems

Muhammad Imran Khan,Student Member IEEEDept. of Electrical andComputer EngineeringRyerson UniversityToronto, Canada

email: mkhan@ee.ryerson.ca

Kaamran RaahemifarDept. of Electrical andComputer EngineeringRyerson UniversityToronto, Canada

email: kraahemi@ryerson.ca

AbstractRadio-over-Fiber (RoF) technology allows radio fre-quency transmission directly over the fiber. Optical link employslaser, optical fiber and optical detector to transmit RF signal.RoF has an ability to carry signals for tens of kilometers, butthis transmission is limited by optical impairments such as lasernonlinearity, fiber dispersion, and optical detector noises. In thispaper, we study the effects of multimode fiber core refractiveindex profile, optical detector noises and atmospheric instabilityon the achievable performance of an OFDM based RoF link.The optical-signal-to-noise ratio (OSNR) and cumulative signal-to-noise ratio (SNRc) are used to evaluate the feasibility ofthe proposed scheme for a low-cost, reliable, robust, and highdata rate fiber-wireless link. Thereafter, numerical results aregenerated for the downlink to illustrate the performance of theOFDM based RoF system.

Index Terms-OFDM, radio-over-fiber, optical signal-to-noise ratio, cumulative signal-to-noise ratio, optical fiberdispersion.

I. INTRODUCTION

The Radio-over-Fiber link is a fiber-optic system that usedto transmit RF signals between the base station and the centralbase station. In the RoF link, a laser diode is used to convertthe RF signals to the optical signals that propagate throughfiber-optic cables. At the end of the RoF link, an opticalreceiver that consists of a photodiode converts the opticalsignals back to the RF signals. The optical fiber base wirelessaccess can support the 4th generation (4G). One way is touse the OFDM technique to carry signals through fibre-opticcables between a central base station and a radio access point(RAP). The RAP operates like an extended antenna from thebase station. It transmits and receives the signals of mobilestations (MSs). For the WLAN system, the MS can be alaptop with IEEE 802.11b interface. The integration of theWLAN system is responding to the demands for high datarate applications and reasonable mobility. The employment ofthe RoF link in this architecture allows reduction in cell sizethat increases the frequency reuse, thus improves the spectrumefficiency. The RAP with relatively simple functions is notonly inexpensive, but its compact size can save estate cost.A centralized network can be constructed using RoF links toconnect multiple RAPs to a single central base station (CBS).

The major impairments of the RoF link that limit thesignal quality are the nonlinear distortion and various noisemechanisms. These impairments limit the optical power canbe transmitted through a fiber, thus affect the data rate anddistance that signals can propagate.

A. Laser Nonlinearity

One important nonlinearity that comes from the laser isthe intermodulation distortion (IMD). IMD occurs when thenonlinearity of a device with multiple input frequencies causesundesired output at other frequencies. The laser diode canbe modeled accurately as a third order polynomial withoutmemory as described in [1] and [2].Therefore, for an input signal of c(t), the optical power of

a laser diode neglecting the optical noise can be expressed as:

P (t) = Po[1 + c(t) + 2c2(t) + 3c

3(t)] (1)

where, Po is mean optical power at the photodiode, 2 and 3are the constants that describe the second and third order lasernonlinearity respectively. The second order intermodulationproduct creates DC and double frequency interference com-ponents. The third order intermodulation product creates in-band and triple frequency interference components. The totalpower of the intermodulation product distortion depends onthe quality of the laser diode. A better laser diode has lowernonlinearity constants 2 and 3.

B. Optical Fiber Dispersion

When the light is coupled into a fiber link, depending on therefractive index, the size of the fiber, and the angle of coupling,a number of different modes exists. Each mode in the fibertravels with a different velocity. As a result, pulse broadeningoccurs analogous to the delay spread in the wireless channels[3].For some specific design parameters, only single mode of

light is transmitted through the fiber core. Hence, resultingin single mode fibers, which do not have modal dispersion(or multi-mode pulse broadening). Although single mode fibercomes with high performance and much less pulse spread, itis both expensive and fragile. In this paper, we investigate

2011 21st International Conference on Noise and Fluctuations978-1-4577-0192-4/11/$26.00 2011 IEEE 409

the system performance over multimode fibers as a low costalternative.Multimode fibers are cheap, easy to handle and widely

deployed in the field. There are mainly two important kindsof dispersion mechanisms in fibers: material and modal dis-persion.

C. Optical Noises

1) Quantum Noise: The quantum noise arises from therandom nature of the photons emitted from a laser diode andcollected at a photodiode. The noise power of the shot noiseis proportional to the average received photocurrent Ip =

The optical signal that propagates through fiber from thebase station to the radio access point experiences signal atten-uation, optical noise and nonlinear distortion. These effects onthe signal can be seen from the photocurrent output. At theend of RoF link, the photocurrent output can be expressed as:

iD(t) =1

N

N1Xk=0

Aiak;i(t )exp(j2k (nNg)N

)

cos(!t ) hMMF (t) + nopt(t): (9)Here, we didnt consider the laser nonlinearity. The signal

only experiences fiber attenuation and optical noises, where,nopt(t) is the optical noise. This noise consists of shot noise,RIN noise and thermal noise.

A. Derivations for Optical Link Loss and OSNR

The input RF signal not only undergoes electrical-to-optical-to-electrical (e-o-e) conversion, but also experiences othersystem losses such as optical connector loss, splitter loss, andsplicing loss [7] [8]. These losses will add into signal andreferred to as total fiber loss or Floss;dB .The total loss due to the RoF link, can be defined as,

Floss;dB = Fe=o +Fo=e +Fmisc. Where, Fmisc is loss due toconnectors, splitters, and splices.Therefore, we can defined the total loss as [8];

Floss;dB = 10 log[ZinG2m

] + 10 log[

Fig. 3. OSNR versus modulation index m

OSNR increases slowly with modulation index. Within therange from 0.05 to 0.3, the maximal OSNR is more than 15dB.

Fig. 4. Distance between a MS and the RAP

Fig. 4 illustrates the relationship between the system per-formance and the distance between an MS and the RAPin a different way. Fig. 5 shows the different curves withvarious lengths of RoF link with an optimal used. It isvery interesting that for small index , the cumulative SNRimproves inversely with the distance. While at large indexvalue, the performance is almost independent of distancebetween mobile station and the radio access point. The reasonis that the nonlinear distortion is much larger than the noisein the air interface.

Fig. 5. Distance between a MS and the RAP

Fig. 5 illustrate the same trend as Fig. 4 but has differentindex . When is small, the performance deteriorates withthe distance. When is large, the performance is independentof the distance.

Fig. 6. Cumulative SNR versus distance

Fig. 6 illustrates the relationship between the system per-formance in terms of the cumulative SNR and the distancebetween a mobile station and the radio access point. Adownward trend in the performance versus the distance isobserved. This means that the optical noise is dominant butalso the nonlinear distortion played an important role in RoFlink. However, when the RoF is operated at the optimal value,the cumulative SNR is better than the rest even for a 10 kmRoF link.

V. CONCLUSIONIn this paper, the radio-over-fiber architecture is investigated

and then the fiber based radio access mechanisms for the airinterface and the ROF link are evaluated. In the numericalresults, the performance of proposed system is evaluatedin terms of cumulative SNR and OSNR using various sys-tem parameters. The system performance is limited by thenonlinear distortion of RoF link when the cumulative RMSoptical modulation index is large. Large means large RFpower modulates the laser. When is larger than the optimalrange, excessive nonlinear distortion deteriorates the systemperformance. The results have shown that as long as the fiberlength is reasonable, multimode fibers can be used for ROFsystems. The analysis will be useful in designing OFDM basedRoF communication system for system engineers. Also, ouranalysis helps in finding better interrelationship between thefiber length, the radio cell size and signal to noise ratio.

REFERENCES[1] J. Daly, Fiber Optic Intermodulation Distortion, IEEE Transactions on

Communications, vol. 30, no. 8, pp. 1954 - 1958, August 1982.[2] K. Stubkjaer and M. Danielsen, Nonlinearities of GaAlAs lasers

Harmonic distortion, IEEE Journal of Quantum Electronics, vol. 16,no. 5, pp. 531 - 537, May 1980.

[3] Gerd Keiser, Optical Fiber Communications, Boston, MA : McGraw-Hill, Edition 5th, 2000.

[4] Takanori Okoshi, Optical Fibers, Academic Press, San Diego, CA,USA, Edition 2nd, 1982.

[5] D. Gloge and E. A. J. Marcatili, Multimode theory of graded-corefibers, Journal of Bell Systems Technology, pp. 1563 - 1578, November1973.

[6] William Shieh, and Ivan Djordjevic, OFDM for Optical Communica-tions, Elsevier Academic Press, San Diego, CA, USA, Edition 1st, 2010.

[7] Microwave Fiber Optics Group, A System Designers Guide to RF andMicrowave Fiber Optics, Ortel Corporation, 1999.

[8] X. N. Fernando and A. Anpalagan, On the design of optical fiber basedwireless access systems, Proceedings of International Conference onCommunication, pp. 3550 - 3555, 2004.412