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2.5 Gb/s hybrid WDM/TDM PON using radioover fiber technique

 ARTICLE  in  OPTIK - INTERNATIONAL JOURNAL FOR LIGHT AND ELECTRON OPTICS · SEPTEMBER 2013

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Universiti Malaysia Perlis

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Optik 124 (2013) 3678–3681

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Optik

 journal homepage: www.elsevier .de/ i j leo

2.5 Gb/s hybrid WDM/TDM PON using radio over fiber technique

Abdullah O. Aldhaibani a,∗, S. Yaakob b, R.Q. Shaddad a, S.M. Idrus a, M.Z. Abdul Kadir b,A.B. Mohammad a,b

a Faculty of Electrical Engineering, Universiti TeknologiMalaysia, Johor81310, Malaysiab Advanced Physical Technology (APT), TelekomMalaysia (TM) RD Sdn Bhd, Lingkaran Teknokrat Timur, 63000 Cyberjaya, Selangor, Malaysia

a r t i c l e i n f o

 Article history:

Received 6 June 2012Accepted 10 November 2012

Keywords:

Radio over fiber

Hybrid WDM/TDM-GPON

EVM

OSNR 

a b s t r a c t

Hybrid gigabit-passive optical network (GPON) is a hybrid passive optical network, where wavelength

division multiplexing (WDM) GPON and time division multiplexing (TDM)-GPON are integrated into asingle passive optical network, reducing cost and increasing the data rate. In this paper, 2.5 Gb/s GPON

downstream link is presented, using the radio over fiber (RoF) technique in GPON network architecture.

It has been done by means of commercial OptiSystem simulation software, where, differential phase shift

keying (DPSK) modulation is proposed by using 2.4 GHz radio frequency. The propagation of radio signals

along a 25 km standard single mode fiber (SMF) investigated. The simulated model can support 32 and

64 users. The analysis was made based on the performance of eye diagram, optical signal to noise ratio

(OSNR), constellation diagram, error vector magnitude (EVM) and received power.

© 2012 Elsevier GmbH. All rights reserved.

1. Introduction

Huge demand for multimedia data, higher data-speeds, suchas high-definition video and an increasing number of users; are

putting pressure on wireless communication systems vendors tooffer higher data rates.Reducingthispressurecouldbe achieved byusinga microcellular system, which means decreasing the cell size,andthus, reducingthe power consumptionof themobilestation[1].However, thenumberof basestations willincrease andthe network

will becomeverycomplex. These conditions drivethe useof opticalfiber as an efficient medium for radio network backhaul. The RoFoffers a combination of high capacity optical fiber and flexibilitywireless networks. This technology is introduced to reduce infra-

structure cost and the complexity of remote antenna units (RAU).Moreover, RoF technology is a candidate to enhance the perfor-mance of wirelesscommunication systems withthe combinationof large bandwidth (BW) and low attenuation characteristics offered

by optical fiber [2].

The sharing of infrastructure and equipment between severalbase stations (BS) through passive optical network (PON) architec-ture is emerging as a low cost solution [2,3]. This makes the PONs

over active deployments dominant and reported worldwide, theGPON standard preferred in America, while Ethernet PON is theelected standard in Asia, with more than 10 million subscribers in

 Japan alone [2,3].

∗ Corresponding author.

E-mail address: zeed aoa@yahoo.com (A.O. Aldhaibani).

The characteristics of GPON technology has been standardized

by International Telecommunication Union-T (ITU-T) in Recom-mendation G.984 series [4]. Combining GPON architecture withROF technology has been done in [5], which demonstrates a lower

cost-efficient and attractive solution for the distribution of 3GBS. Nevertheless, the demands on capacity in access networks isincreasing rapidly due to the huge number of users, making thedistribution of 4th generation (4G) base station good solution inwireless communication systems such as WiFi and Wimax, with

radio frequency 2.4GHz.RoF using analog modulation on gigabit passive optical network

architecture as reported in [6], involved the distribution of eightBSs by using WDM and splitter, with distance from 10 to 100 km.

The results were good only with 10, 20, and 50km for BER. Affidaetal. [7], characterizedthe distribution of IEEE802.11WLANserviceusing RoF technique in GPON network architecture. Good resultswere obtained at the maximum fiber distance of 20km where

lower BER; and higher OSNR values as those specified as stan-

dardswereobserved. In addition MartinBouda et al.haveproposedand demonstrated the cost-effectiveness of a Wavelength-SharedHybrid PON architecture which is a low cost solution as a number

of cells increase.This paper focuses on the downlink part only. It presents the

transmission performance of a downstream link GPON networkwith a 2.5 Gb/s bit rate.

2. Radio-over-fiber (RoF) techniques

RoF presents the distribution of radio frequency (RF) signalsover optical fiber links from a central office to RAU. Radio over

0030-4026/$ – seefrontmatter © 2012 Elsevier GmbH. All rights reserved.

http://dx.doi.org/10.1016/j.ijleo.2012.11.013

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fiber-scheme for both down and up-link paths is demonstratedbased on digitized RF-over-fiber technique. The digitized RF-over-

fiber technique is able to improve spectral efficiency and minimizehardware count, while the performance is limited by inter-channelinterference as a result of spectra aliasing of the regenerated IFsignals which limits the signal-to-noise-ratio [8]. There are many

techniques which are used to transport the RF signal to end-userssuch as; RF-over-fiber, intermediate frequency (IF)-over-fiber, andbaseband-(IF)-over-fiber. Where the first two former schemes areexamples of analog photonic links and the last is a digital photonic

link. The RF signal is modulated at the CO in RF band and combinedwith continuous wave (CW) laser by Mach–Zehnder Modulator(MZM), which converts the signal to optical with direct transmis-sion through the fiber to the BSs. At the BS, the signal is detected

by photo detector (PD); without any need for frequency transla-tion at the remote BS. RF-over-fiber transport has the advantageof realizing simple base-station designs with additional benefitsof centralized control, independence of the air-interface and also

enabling multiwireless band operation. However, one of its majordrawbacks is the requirement for high-optical modulation tech-niques that have the ability to generate speed modulated opticalsignals as well as high-speed photo detectionschemes that directly

convert the modulated optical signals back to signals in the RF

domain, Using the external modulator, the dispersion effect can beminimized; thus, this is preferred most in any RoF system [9,10].

3. Hybrid structure

The need to mitigate complex and costly time-sharing of avail-able bandwidth in TDM based PONs has led to the development of new types of access networks that simplify network operation and

handle the ever growing bandwidth requirements. These accessnetworks are called WDM-PONs, which employ several indepen-dent wavelengths. Each single wavelength can carry data at ratesof up to electronic processing limits of a few Gbps [11]. PON archi-

tecture system includes three important apparatus: optical line

terminals (OLT), optical network units/optical network terminals(ONU/ONT) and optical distribution network (ODN). The transmit-ters at the OLT side generate a single wavelength carrying the data

destined for a particular ONU. In OLT the RF signal is modulatedby a DPSK sequence generator and combined with CW laser atwavelengths starting from 193.1THz to 193.8THz. The frequencyspacing is 100 GHz which defines the type of elements employed

in the network and the cost associated with them. For example, afrequencyspacingof 100GHz makes thetransmissionDense-WDM(DWDM). These wavelengths are then coupled onto a single fiberusing WDM Multiplexer, with specific insertion loss, located inside

the CO. The multiplexed output is, in turn, connected to a primarysingle mode fiber (SMF) of length 25km which terminates on aWDM Demultiplexer as shown in Fig. 1 [12].

The WDMDemultiplexer separates all the wavelengths, accord-

ing to the way they were combined at the OLT side, and feeds each

Fig. 1. WDM-PONs.

Fig. 2. Hybrid WDM/TDM.

one to a power splitter which distributes the signal to four users as

shown in Fig.2. Photodiode detects thesignal andpasses it to DPSKdecoder.

The hybrid WDM/TDM GPON consists of 32 ONUs, as shown inFig. 2. They are separated into WDM groups sharing eight wave-lengths in a WDM mode. Within each group, four ONUs share onewavelength in a TDM mode. The TDM-PON downstream traffic is

handled by broadcasts from the OLT to all connected ONUs.

4. Simulation design

Thissectionbrieflydescribes thesimulationsetup in OptiSystem10.0. Allnecessary parameters arebased on theGPONstandardizedproperties [4]. Fig. 3 shows the hybrid WDM/TDM-PON scheme. In

theCO, downlink a DPSK signalwas generated. Thedownlink chan-

nels are multiplexed/demultiplexed by a 1xN multiplexer (MUX)and a demultiplexer (DMUX), respectively. The multiplexed down-link DPSK signals are sent through the fiber and demultiplexed at

receiver, which connects each wavelength to a splitter with a splitratio of 4. Finally, each splitter connects to four BS. Fig. 3 shows theschematic diagram of the simulation system, at OLT; the electricaldata signal is generated by the pseudo-random bit sequence Gen-

erator (PRBS), with 2.5Gbps bit rate. The data is modulated by adifferential Phase Shift Keying (8DPSK) sequence generator and anM-ary pulse generator producing M-ary signal. The M-ary signal isfed into a quadrature modulator (QM) at 2.4 GHz combined with

A CW laser diode (LD) at frequency 193.1 THz by Mach–ZehnderModulator (MZM) to convert the electrical signal to an optical sig-nal for transport through a 25km SMF. At the ONU in receiver, thesignal is detected by a photodiode, and fed to clock recovery in

order to recover the data stream before it is passed to a quadrature

Fig. 3. Schematic diagram downlink hybrid WDM/TDM.

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 Table 1

General parameters.

Parameters Value

Operation frequency 2.4 GHz

Type of modulation DPSK

Bite rate (2.5 Gbps)

Sequence length (256)

Samples per bit (64)

Fiber length 25 km

Reference wavelength 1550 nm

demodulator. In order for the signal to be discovered by a DPSKsequence decoder and pulsed by RZ pulse generator to get thebinary signal, the QM is connected to an M-ary threshold detec-

tor; to quantize the signal based on a suitable value of thresholdamplitudes. The constellation of signal is display by constellationvisualizer. Eye diagram tools are added to plot the M-ary signal atthe quadrature modulatoroutput of receiver, which consist of PRBS

generator, RZ generator and eye diagram Analyzer.

5. Result and discussion

Hybrid WDM/TDM model has been successfully simulated andanalyzed by a commercial optical system simulator, OptiSystem.

The model has been constructed by following the general parame-ters in Table 1.

A convenient way to measure the performance of the system isby usingan eye diagram forthis simulation, with referenceto Fig.4,

the eye opening is clearly indicating that the system performanceis good.

A constellation diagram is a representation of a signal modu-lated by a digital modulation scheme such as quadratureamplitude

modulation or differential phase-shift keying.In thissimulation,weuse differential phase-shift keying (8DPSK) signal which uses 8 bitsper symbol. The number of bits in each symbolis 3, andthe constel-lation result is given by formula 2n. The signal constellation of the

transmitter is taken from the output of M-ary pulse generator asillustrated in Fig. 5(a). The received signal at the receiver is shown

Fig. 4. The eye diagram at the receiver of the first ONU.

Fig.5. Theconstellation ofthe electricalsignalat the(a) transmitter and(b) receiver.

Fig. 6. OSNR performance based on data rate andlength of fiber.

in Fig. 5(b). It canbe clearly seen that the constellation of theoutputsignal is similar to theinput signal with some amplitudeand phase

errors which will be measured by the EVM.The OSNR performance forthe variedfiber length anddata rates

is shown in Fig.6. It can beseen thatthe OSNRat 0.1 nmbandwidthdisplays a decreasing pattern along the length of the fiber. In cer-

tain optical systems, the maximum value of OSNR, for 2.5 Gbps is40dB and the minimum is 35.2dB; for 1.25Gbps the maximumvalue of OSNR is 41.7dB and minimum value is 36dB, while themaximum value for OSNR at 1Gbps is 44dB and minimum value

is 37.6dB. As shown graphically in Fig. 6, the OSNR is greater oversmall distances and is reduced over large distances. The OSNR isalso reduced fractionally while the data rate increased because of increased noise.

Fig. 7 illustrates thereceived optical versus thefiber length. Thepower is found to be reduced linearly with increasing fiber lengthdueto attenuation. Theoptical power at the transmitter is 0 dBm. Itcan beseen from Fig.7, that, thereceived optical power is−19dBm

along 25km fiber, which means we can increase the number of wavelengths for each splitter to eight or extend the length of fiber.The power is reduced due to attenuation, dispersion, and losseswhich are contributed to by all devices of the network builder.

The EVM is defined as the root-mean-square value of the differ-ence between a collection of the measured and the ideal symbols.

The difference is normalized by the average power per symbol inthe constellation. The valuesof EVMvary accordingto type of mod-

ulation used, such as QPSK, 16 QAM, etc. [14,15]. The followingequation is used to calculate the EVM [13]:

EVM RMS  =

(1/N )

N 

r −1|Sideal,r  − Smeas,r |

2

(1/N )N 

r −1|Sideal,r |2

(1/2)

(1)

Fig. 8 displays the EVM of the downlink signals versus the fiberlength. The results show that the EVM increase a little bit as lengthof fiber increases. The same figure shows that the EVM is increased

when data rate increases. Where at 50km it is 26%, 27%, and 30.3%for 1, 1.25, and 2.5Gbps, respectively.

Fig.9 shows theEVM increasesto −10.3dB as the receive power

reduces to−24dBm [16]. It canbe clearly observedfrom Fig. 10 thatwhen OSNR is increased, the EVM value is reduced.

Fig. 7. The received optical power at thereceiverversus thelength of fiber.

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Fig. 8. EVM versus fiber length (downlink).

Fig. 9. EVM of different receives powers.

Fig. 10. The EVMversus OSNR.

6. Conclusion

The hybrid WDM/TDM GPON using radio over fiber techniquehas been introduced and analyzed. The 2.5Gb/s hybrid GPON withRoF with digital modulation (8DPSK) is implemented. The simula-

tion results show that the Hybrid WDM/TDM GPON with 2.5Gbps,8DPSK and 2.4GHz, gives a good performance for 32–64 users over25km fiber length. The value of EVM is increased as the distanceincreases, reaching to 30.3% at 50km. In contrast, the OSNR is

reduced to35 dBmas the distanceis increasedto 50km. GoodOSNR 

and power budget have been calculated for the proposed PON. TheOSNR is reduced while the number of wavelengths increased as a

result of channel interference. Power receiver reduced to −24dBmat 50km fiber length. The results show that the Hybrid WDM/TDMGPON offers a promising solution for today’s communication tosupport the continuous increase in the number of wireless Internet

users and demands on bandwidth.

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