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PERFORMANCE EVALUATION &

OPTIMIZATION OF ERROR FREE

WDM RADIO OVER FIBER LINK

KUMARI KALPNA

Department of Instrumentation & Control Engineering, College of Engineering & ManagementKapurthala, Punjab-144601, India

BINDIYA JAIN

Department of Electronics & Communication Engineering, DAV institute of Engineering & TechnologyJalandhar, Punjab, India

Abstract:

In this paper, we introduce WDM radio over fiber (RoF), which is one of enabling technologies for 3G

and beyond. To minimize the problem of high attenuation, power loss and to improve the efficiency of

frequency resuse, the Performance Evaluation of WDM radio over fiber transmission system (RoF),

based on various performance measures such as Q-factor, eye opening, BER and jitter has been made at

different data rates.

Keywords: WDM RoF System; BER; Q factor; Eye Opening; Timing jitter.

1. Introduction

Nowadays communications target to transmit a variety of services. Those are classical telephony, facsimiletransmission, but also the Internet traffic, data transmission, radio and television broadcasting etc.Consequently, various transmission media are used as metal and fiber cables, and microwave, millimeter wave,and optical free space communication links. However, owing to top performance of contemporary optical fibers

there is a tendency to exploit optics as far as possible. Thus fibers are used not only for digital voice or Internettraffic transmission, but also for expanding Radio-over-Fiber transmission applications that exploit the opticalcarrier wave amplitude modulation with a microwave carrier, including analogue cable television transmission.The next generation of access networks is rushing the needs for the convergence of wired and wireless servicesto offer end users greater choice, convenience, and variety in an efficient way. This scenario will require thesimultaneous delivery of voice, data, and video services with mobility feature to serve the fixed and mobile

users in a unified networking platform. In other words, new telecom systems require high-transmissionbandwidths and long haul with reliable mobility [1]. Radio over Fiber (RoF) application has attracted muchattention recently because of the increasing demand for capacity/coverage and the benefits it offers in terms of

low-cost base station deployment in macrocellular system. RoF systems are now being used extensively forenhanced cellular coverage inside buildings such as office blocks, shopping malls and airport terminal. RoF isfundamentally an analog transmission system because it distributes the radio waveform, directly at the radio

carrier frequency, from a central unit to a Radio Access Point (RAP) [2].Wireless communications is entering a new phase where the focus is shifting from voice to multimedia

services. Present consumers are no longer interested in the underlying technology; they simply need reliable andcost effective communication systems that can support anytime, anywhere, any media they want. Furthermore,new wireless subscribers are signing up at an increasing rate demanding more capacity while the radio spectrumis limited [3]. Second generation (2G) mobile communication systems based on digital signal processingtechniques has been very successful for decades. It leads to the development of third generation mobile systems.Third generation (3G) mobile communication systems, wideband code-division multiple access (WCDMA), and

CDMA2000, were initially proposed and designed to be a high performance and high bandwidth system to carryhigh data rate services, systems that can support various service types and various users demand [4] .To satisfy

this increasing demand, the high capacity of optical networks should be integrated with the flexibility of radionetworks. The deployment of optical fiber technology in wireless networks provides great potential forincreasing the capacity without largely occupying additional radio spectrum [5]. RoF system consists of a

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Central Station (CS) and Base Station (BS) connected by an optical fiber link or network. Modulated radiosignals have to be available at the input end of the RoF system, which subsequently transported them over adistance in the form of optical signals.

The RoF basic concept is to distribute the radio-frequency (RF) signals by optical transmission to radio

access points (RAPs) so that the RAPs are not required to perform complicated functionalities such asmodulation, coding, up/down conversion and multiplexing. RoF systems can provide specialized coverage of

wireless services by using an extended optical backbone. These systems are suitable for variety applications,such as in-building coverage, outdoor cellular systems, and broadband fixed and mobile wireless access. Theyare entirely transparent to the system frequency, protocol, and bit rate. This characteristic makes them extremelyinteresting for the convergence of optical and mobile systems.RoF technology has been investigated by many

Research Groups in the last years. However, the great majority of works published in literature are based onsimulations and/or experiments carried out in laboratories.

RoF technology is a technology by which microwave (electrical) signals are distributed by means ofoptical components and techniques [7]. RoF systems depicted in Fig. 1 are used to transport microwave signals.Radio over Fiber (RoF) application has attracted much attention recently because of the increasing demand forcapacity/coverage and the benefits it offers in terms of low-cost base station deployment in macrocellularsystem. RoF systems are now being used extensively for enhanced cellular coverage inside buildings such asoffice blocks, shopping malls and airport terminal. RoF is fundamentally an analog transmission system because

it distributes the radio waveform, directly at the radio carrier frequency, from a central unit to a Radio Access

Point (RAP). Note that although this transmission system is analog, the radio system itself may be digital suchas GSM. Mainstream optical fiber technology is digital. Telecommunication networks use synchronous digitalhierarchy transmission technology in their cores. Fiber-based data networks such as fiber distributed datainterface and gigabit Ethernet all use digital transmission.Fiber transmission links to base stations in mobilecommunications systems are digital. Digital optical fiber transmission links are therefore ubiquitous in

telecommunications and data communications, constituting a high volume market worth billions of dollarsworldwide.

Fig 1: WDM ROF Transmission system

The electrical signal may be baseband data, modulated IF, or the actual modulated RF signal to bedistributed is used to modulate the optical source. The resulting optical signal is then carried over the optical

fiber link to the remote station where the data is converted back into electrical form by the photo detector.

2. Benefits of ROF systems:

Advantages and benefits of the RoF technology include the following:

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2.1 Low Attenuation LossElectrical distribution of high frequency microwave signals either in free space or through transmission

lines is problematic and costly. In free space, losses due to absorption and reflection increase with frequency.Use of optical fibers, offer much lower losses. These losses are much lower than those encountered in free space

propagation and copper wire transmission of high frequency microwaves. Therefore, by transmittingmicrowaves in the optical form, transmission distances are increased several folds and the required transmission

powers reduced greatly [5].

2.2 Large Bandwidth

Optical fibers offer enormous bandwidth. There are three main transmission windows, which offer low

attenuation, namely the nm 850, nm 1310 and nm 1550 wavelengths. The high optical bandwidth enables highspeed signal processing that may be more difficult or impossible to do in electronic systems [5].

2.3 Immunity to Radio Frequency Interference

Immunity to electromagnetic interference is a very attractive property of optical fiber communications,especially for microwave transmission. This is so because signals are transmitted in the form of light through thefiber [5].

2.4 Easy Installation and Maintenance

In RoF systems, complex and expensive equipment is kept at the CSs, thereby making remote basestations simpler. For instance, most RoF techniques eliminate the need for a local oscillator and relatedequipment at the Remote Station (RS) [5].

2.5 Reduced Power ConsumptionReduced power consumption is a consequence of having simple RSs with reduced equipment. Most of

the complex equipment is kept at the central SC [5].

3. Performance Measures:

The right choice of the performance evaluation criteria for the characterization of optical transmissionlinks represents one of the key issues for an effective design of future long-haul optical systems [8]. The

evaluation criteria should provide a precise determination and separation of dominant system limitations,making them crucial for the suppression of propagation disturbances and a performance improvement. The most

widely used performance measures for performance evaluation are the Q-factor, BER and jitter, eye opening[10].

3.1 Q-factor

Q-factor represents the signal-to-noise ratio at the receiver decision circuit in voltage or current unit.

3.2 BER

The BER can be estimated from following Equation. The BER gives the upper limit for the signalbecause some degradation occurs at the receiver end [8].

BER =

2

22

exp

)2

(2

1

Q

QQ

erfc

3.3 Eye opening

Considering only samples at the optimum sampling instant, it is the difference between the minimumvalue of the samples decided as logical 1 and the maximum value of the samples decided as logical 0.

3.4 Jitter

Jitter value is evaluated as the standard deviation of the position of the maximum of the received signal

referred to the bit frame.

4. System Description and Results:

In order to compare the transmission performances of various fibers figure shows the simulation modelof the system. Fig.2 indicates a simulation model of an optical communication system at 10 Gb/s around 1550

nm central wavelength .The simulation has been carried out using a commercial simulation package OptSim.Simulation is done for 10 km length of various fibers like standard single-mode (SM) fiber, Dispersioncompensation fiber, teralight fiber etc. Standard SM fiber has loss 0.2 dB/km and dispersion 16 ps/nm/km at

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reference frequency. It has zero dispersion at 1391.53354633 nm wavelength, fiber average beat length 5m. CWLorentzian Laser used was having center emission wavelength 1550 nm, CW power 1mWand FWHM linewidth10MHz as main characteristics was used as the optical source. Amplitude dual-arm Mach Zehnder modulator isused here to modulate the optical signal of desired format having the following parameters: excess loss 0 dB,

offset voltage corresponding to the phase retardation in the absence of any (on both arms) electric field 0.5 V,extinction ratio 20 dB, chirp factor 0 and average power reduction due to modulation 3 dB.

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Fig 2: WDM RoF Transmission link considered for simulation.

Datasource

101010

DPSKModulator

ElectroAbsorption

Modulator

CW Lorentzian

laser

DPSKDemodulator

Data Channel1

[IF1]

Optical

Splitter

Datasource

101010

DPSKModulator

ElectroAbsorption

Modulator

CW Lorentzian

laser

Opticalamplifier

Optical

amplifier

Fiber link 2

at 10 Km

Fiber link 1 at

1Km

Optical

Combiner

SOAMZI

OpticalFilter 2

OpticalFilter 1

Photodiode

Electrical

Filter

ElectricalSignal

Multiplier

DPSK

Demodulator

BER estimator

Eye diagram

analyzer

Q Factor

Jitter

Data Channel2

[IF2]

BER estimator

Eye diagram

analyzer

Q Factor

Photodiode

Electrical signal

multiplier

Central Station

(CS)

Remote Node

(RN)

Remote Antenna Station (RAS)

Cells/Microcells/Picocells

Section B

Section A

Electricalfilter

Jitter

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5. Results:

Table 5.1 & 5. 2: Comparative study of the performance metric indices of WDM RoF System at various

data rates.

# At Section A:

S.No. Parameters Q value [dB] Jitter [ns] BER Eye opening [a.u.]

Data rate[Gbps]

1. 1.5 9.686512 0.0312685 0.00114414 3.39938e-0072. 2.0 10.206633 0.0318283 0.000618246 3.17119e-0073. 3.5 10.060838 0.0313109 0.000764695 3.60716e-0074. 4.0 10.371890 0.0316091 0.0004985 4.76325e-0075. 5.5 10.620311 0.0316927 0.000365147 8.48817e-0076. 6.5 9.632132 0.0316949 0.00122341 1.97337e-0087. 7.0 9.722221 0.0313288 0.00112138 6.57325e-0088. 8.5 9.836501 0.0316139 0.000957761 3.62001e-0079. 9.0 10.560282 0.0317983 0.000370819 4.28472e-00710. 10.0 11.000574 0.0319599 0.000199306 4.63927e-007

Table 5.1: Simulation results at different data rates on WDM RoF Remote Antenna Station A.

# At Section B:

Table 5.2: Simulation results at different data rates on WDM RoF Remote Antenna Station B.

A pseudo random sequence length of bits taken one bit per symbol is used to obtain realistic output

values at the receiver. Firstly, to observe the impact of data rate upon system performance, simulation results areobtained for different data rates varying from 1.5Gbps to 10Gbps. It was observed that for the data rate up to10Gbps, Q factor for the system remains nearer to 10dB and also BER at data rate 6.5 Gbps is at minimum value& jitter remains almost nearer to the value of 0.0307 that shows a good performance of WDM RoF system. Aswe increases the data rates further, impact upon the Q factor, jitter, eye opening etc. comes into play. It isinvestigated that system provides optimum results at data rate of 9.5 Gbps (refer Table 5.1 & 5.2). The eyediagrams obtained for the system at various data rates are shown in figures (refer Fig. 3 to 12).

S.No. Parameters Q value [dB] Jitter [ns] BER Eye opening [a.u.]

Datarate[Gbps]

1. 1.5 9.742236 0.0314526 0.00107178 3.27156e-0072. 2.0 9.714900 0.0317134 0.00115153 3.21517e-0073. 3.5 9.605962 0.0308473 0.00125388 1.52769e-0064. 4.0 9.312404 0.0310956 0.00173913 2.29042e-0075. 5.5 8.474493 0.0309313 0.00398179 2.0033e-0076. 6.5 10.209459 0.0317742 0.000661857 6.52316e-0077. 7.0 9.020807 0.0311503 0.00241097 3.52251e-0078. 8.5 9.053125 0.0307204 0.00229256 6.3256e-0079. 9.0 9.018063 0.0308247 0.00263116 3.19057e-00710. 10.0 9.746002 0.0317006 0.00123162 6.26198e-007

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3(a) 3(b)

Fig. 3(a) Eye Diagram at 1.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm. &

3(b) Eye Diagram at 1.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm.

4(a) 4(b)Fig. 4(a) Eye Diagram at 2.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &

4(b) Eye Diagram at 2.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

5(a) 5(b)

Fig. 5 (a) Eye Diagram at 3.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &5 (b) Eye Diagram at 3.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

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6(a) 6(b)Fig. 6 (a) Eye Diagram at 4.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &

6 (b) Eye Diagram at 4.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

7(a) 7(b)

Fig. 7 (a) Eye Diagram at 5.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &7 (b) Eye Diagram at 5.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

8(a) 8(b)

Fig. 8 (a) Eye Diagram at 6.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &8 (b) Eye Diagram at 6.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

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9(a) 9(b)

Fig. 9 (a) Eye Diagram at 7.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &

9 (b) Eye Diagram at 7.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

10(a) 10(b)

Fig. 10 (a) Eye Diagram at 8.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &

10 (b) Eye Diagram at 8.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

11(a) 11(b)

Fig. 11 (a) Eye Diagram at 9.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &

11 (b) Eye Diagram at 9.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm

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12(a) 12(b)

Fig. 12 (a) Eye Diagram at 10.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &

12 (b) Eye Diagram at 10.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm6. Conclusion:

In this paper, we have proposed a novel WDM RoF system and experimentally demonstrated the

simultaneous generation and transmission of the 1.5 to 10 Gbps, 0 to 50-GHz signals over 10-km Standard SMfiber and wavelength of 1550 nm with less power penalties. It is investigated that system provides optimumresults at data rate of 9.5 Gbps The system is of low cost & can be scaled to higher data rates.

7. References:

[1] David J. T. Heatley, Optical Wireless:The Story So Far, IEEE 1998, pp 72-82.[2] Michel Goloubkoff, I Outdoor and Indoor Applications for Broadband Local Loop with Fibre supported mm-wave Radio Systems,

IEEE, 1997,pp 31-34.

[3] Mohammad Shaifur Rahman, Jung Hyun Lee, Youngil Park, and Ki-Doo Kim, Radio over Fiber as a Cost Effective Technology forTransmission of WiMAX Signals World Academy of Science, Engineering and Technology 56 (2009).

[4] Hoon Kim, Radio-over-Fiber Technology for Wireless Communication Services, Samsung Electronics Oct. 13, 2005.[5] Anthony Ngoma, Radio-over-Fiber Technology for Broadband Wireless Communication Systems, 2005[6] W. D. Jemisona, Fiber radio: from links to networks, IEEE, 2001, pp 169-172[7] Kwansoo Lee, Radio over Fiber for Beyond 3G, IEEE, July 2005.[8] Vishal Sharma, Amarpal Singh, Ajay K.Sharma, Simulative investigation of the impact of EDFA and SOA over BER of a single-tone

RoF system Available online at w.w.w.sciencedirect.com in Elsevier Science Direct, International Journal for Light and Electron

Optics, Optik, Germany, 10th Jan., 2009.[9] Hyoung-Jun Kim, and Jong-In Song, Full-Duplex WDM-Based RoF System Using All-Optical SSB Frequency Upconversion and

Wavelength Re-Use Techniques IEEE Transactions on Microwave Theory and Techniques, July 2010.

[10] Vishal Sharma, Amarpal Singh, Ajay K. Sharma, Simulative investigation of nonlinear distortion in single-and two-tone RoF systemsusing direct-and external-modulation techniques Available online at W.w.w.sciencedirect.com in Elsevier Science Direct,

International Journal for Light and Electron Optics, Optik, Germany, 14th Dec., 2008.

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