investigation of dwdm over ocdma system based on parallelly combined ssfbg encoderdecoders

8/22/2019 Investigation of DWDM Over OCDMA System Based on Parallelly Combined SSFBG EncoderDecoders

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Investigation of DWDM over OCDMA System Based on Parallelly

CombinedSSFBG Encoder/Decoders

Huayong Zheng, Biao Chen, Dawei Wang, Xuezhi Hong and Sailing HeCentre for Optical and Electromagnetic Research, Joint Laboratory of Optical Communication,

Zhejiang University, Hangzhou 310058, [email protected]

AbstractWe propose a novel DWDM over OCDMA scheme

with parallelly combined Super-Structured Fiber Bragg

Grating (SSFBG) based encoder/decoders. In our scheme, a

group of SSFBGs with the same code but different central

reflection frequencies are parallelly combined together to

support larger number of DWDM channels. The number of

DWDM channels per code can be increased without increasing

the chip-rate of SSFBG. The performance of a 27-channel (9-

DWDM3-OCDMA) system is investigated by simulation,and error-free transmission is achieved for all 27 channels.

Keywords-OCDMA; DWDM; SSFBG; Channel spacing;

I. INTRODUCTIONThe passive optical network (PON) is a promising

technique for access network. Optical code division

multiplexing access (OCDMA) over wavelength division

multiplexing (WDM) PON is regarded as an attractive

network for the next generation PONs. The OCDMA/WDM

PON provides many advantages, such as asynchronous

operation, protocol transparency, huge user capacity andhigh speed. In the recent demonstrations of this hybrid

system, SSFBG is employed as the OCDMA encoder/decoder [1-4]. As shown in [5], the signal can still be

successfully recovered by the 320G-chip/s SSFBG OCDMA

decoders when encoded signal was filtered by a 100GHz

DWDM filter. Furthermore, a 4-DWDM/2-OCDMA systembased on 127chip, 320G-chip/s SSFBG with the channel

spacing of 100GHz has been experimentally demonstrated

nowadays [6]. In [6], data from a bundle of DWDM

channels are encoded or decoded simultaneously with a

single wide-spectrum encoder or decoder. Compared to the

existing OCDMA over DWDM approaches [1-4], thisapproach, DWDM over OCDMA, is cost-effective because

it employs fewer optical encoder/decoders for the same

capacity. However, in the system above, only five 100GHzspacing DWDM channels per code can be supported with a

single pair of 320G-chip/s SSFBG encoder/decoder. Thenumber of DWDM channel per code is limited by the

achievable chip-rate of SSFBG (640G-chip/s at most as

reported).

In this paper, we propose a novel DWDM over OCDMA

scheme supporting a much larger number of DWDM

channels (still with 100GHz channel spacing), using 127-chip , 320-Gchip/s SSFBG as the OCDMA en/decoder.

Figure 1. Architecture of the proposed DWDM over OCDMA system

with parallelly combined SSFBG encoder/decoders.

A 2.5G-bit/s 9-DWDM/3-OCDMA system has been

simulated with VPI-transmissionmaker and Matlab. BER

performance and eye diagram are illustrated, and the results

verify the error-free transmission of all 27 channels.

II. OPERATION PRINCIPLESThe network architecture is depicted in Fig. 1. Similar to

the scheme in [6], M DWDM channels share the same

OCDMA encoder/decoder in each node, which achieves

higher encoder/decoder efficiency, compared to the

traditional OCDMA over WDM hybrid system [1-4].

Its obvious that the larger M is, the higher the

encoder/decoder efficiency. One straightforward method

would be using higher chip-rate SSFBG encoder/decoder,

since more DWDM channels per code can be covered by

640G-chip/s SSFBG shown in [1] than 320G-chip/s encoder

/decoder in [6]. However, its a challenge to fabricate

SSFBGs with chip rate higher than 640G-chip/s.

In our scheme, instead of using higher chip-rate SSFBG,

a group of parallelly combined SSFBGs of the same codebut with different central reflection wavelengths are used to

support larger number of DWDM channels per code. A

group of encoder/decoders with the same code serve as one

encoder/decoder with extreme broad reflection spectrum.

Fig. 2 shows the reflected optical spectrum of three

parallelly combined 127-chip, 320G-chip/s SSFBGs with

the central reflection frequencies of 192.8THz, 193.1THz

and 193.4THz, i.e. 1554.94nm, 1552.52nm, and 1550.12nm,

respectively. As showed in Fig. 2, nine DWDM channels

are covered by the combined spectrum of three different

978-1-4244-6554-5/11/$26.00 2011 IEEE


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Figure 2. Reflected optical spectrum of three SSFBGs, and spectrum of

the covered DWDM channels and the encoded signal.

encoders. The encoded signals spectrum is also illustrated,

lying below the DWDM channels. Besides the sharp edge of

the filter response, all the channels have almost the samereflected power. For simplicity, group drop filters before the

encoders are not presented in Fig. 1, and will be included in

the simulation setup.

III. SIMULATION SETUP AND RESULTSThe simulation setup of a 2.5G bit/s 9-DWDM/3-OCDMAhybrid system is illustrated in Fig.3. Nine pulse transmitterswith different emission frequencies (from 192.7THz to193.5THz, i.e. 1549.32nm to 1555.75nm, with 100GHzspacing) are employed as the DWDM transmitters. Eachtransmitter contains a modified Wichman-Hill-Generator(powerful PRBS) with the bit rate of 2.5G bit/s, and a

Super-Gaussian shaped optical pulse generator. The pulsewidth of the original signal is 3ps, and is broadened to~10ps after the filtering of the 100GHz DWDM multiplexer.The multiplexed signal is then amplified and split to threegroups of OCDMA encoders. Each group has three encoderswith the same Gold code pattern but different centralreflection frequencies (192.8THz, 193.1THz and 193.4THz).The encoder/decoders are based on temporal phase encodingSSFBG and we calculate the SSFBGs spectral response bynumerically solving the coupled mode equations [7] [8].

Figure 3. Simulation setup of the 2.5G bit/s 9-DWDM/3-OCDMA system.

Since the reflection spectrum of each single encoder is~4nm, there is a strong overlap at the edge of the reflectionspectrum of two adjacent encoders (e.g. encoder 1-1 andencoder 1-2). Therefore, a 300GHz band-pass filter is added

before each encoder to eliminate the interferences fromadjacent encoders. In practical application, all the filters ineach group can be replaced by a single commercial availableDWDM channel band filter. After encoding, the signals of

the three groups pass through different length of fiber todecorrelate the signals. The decorrelated signals are thencoupled together and amplified before feeding into 20kmstandard single mode fiber (SMF) and correspondingamount of dispersion compensation fiber (DCF).

At the receiver, the matched decoders are employed to

decode the signal of every channel. It should be noticed that

after every decoder, three DWDM channels are decoded

simultaneously. In this simulation, we employ a tunable

band-pass filter instead of DWDM DEMUX, to filter out

each channel. A tunable attenuator is added before the

photodiode (with 3dB bandwidth of 1.875GHz).Fig.4 shows the simulated bit error rate (BER) versus

received power at PD for the decoded signals of all ninewavelengths with one of the OCDMA codes (code 3).Apparently, the BER performances of all channels are goodand similar. The difference of received power at BER of1.0e-9

between all the channels is within 1.5dB. The

performance of three channels (central wavelength of2, 5,8, corresponding to the center frequencies of 192.8 THz,193.1 THz, and 193.4 THz) is better than the other channels.This is because the central wavelengths of these threechannels lie exactly on the middle of each SSFBGsreflection spectrum (see Fig.1), which means they havehigher reflectivity. Inset figure of Fig. 4 shows the clearopened eye diagram of the received electrical signal after PDof the channel with central wavelength 2 and code 3.

The BER of all channels decoded by the other twoOCDMA codes are also simulated. Fig. 5 shows the BER

performance of channels (central wavelength of2 and 4)

Figure 4. BER versus received power for nine channels decoded by code

3, inset is the eye diagram of the received electrical signal after PD of the

channel with central wavelenth 2 and code 3.


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decoded by the three codes respectively. From the two

groups of curves, we see that the difference of received

power (at BER of 1.0e-9) inside each group is within 1dB.We also simulated the BER performance of single channel

transmission (only the channel with central wavelength 2 is

active, and encoded by three OCDMA codes). As shown in

Fig. 5, the dashed line indicates the BER curve of the single

channel decoded by code 1. The power penalty of multi-

channel transmission is ~1.5dB. This reveals that thecrosstalk from adjacent channels is trivial, as the main

degradation in this hybrid system is caused by multiple

access interference (MAI) and beat noise [1]. Inset figure in

Fig. 5 shows the eye diagram of decoded optical signal of

the channel with central wavelength 3 and code 2.

The received power at BER of 1.0e-9 of all the 27

channels (9-DWDM3-OCDMA) is illustrated in Fig. 6. As

we can see, the needed received power fluctuates in a region

between -23.5 dBm and -25.5 dBm, which indicates that all

the channels can transmit and encode/decode signals

independently under a uniform optical power level.

Figure 5. BER versus received power for two DWDM channels decoded

by all three OCDMA codes, inset is the eye diagram of the decoded optical

signal of the channel with central wavelength 3 and code 2.

Figure 6. Received power at BER of 1.0e-9 for all the 27 channels.

IV. CONCLUTIONSIn this paper, we propose a novel OCDMA over DWDM

scheme with the WDM channel spacing of 100GHz. The

reflection spectrum of each 127-chip, 320G-chip/s SSFBG

encoder/decoder covers three DWDM channels. And byparallelly connecting a group of them with the same

OCDMA code but different central reflection wavelengths,

nine DWDM channels per code scheme is successfullydemonstrated by simulation. Simulation results of a 2.5G

bit/s 9-DWDM/3-OCDMA hybrid system verify that error-

free transmission of all 27 channels can be achieved.

REFERENCES

[1] Ken-ichi Kitayama, Xu Wang, and Naoya Wada, OCDMA overWDM PON Solution Path to Gigabit Symmetric FTTH, J.LightwaveTechnol, vol.24, no.4, pp.1654-1662, 2006

[2] P.C.Teh, M.Ibsen, J.H.Lee,P.Petropoulos, and D.J.Richardson,Demonstration of a Four-Channel WDM/OCDMA System Using255-Chip 320G-Chip/s Quaternary Phase Coding Gratings, IEEEPhoton. Technol. Lett., vol 14, no2, pp.227-229, 2002

[3] C.Tian, Z.Zhang, M.Ibsen, P.Petropoulos, and D.J.Richardson, A16-Channel Reconfigrable OCDMA/DWDM System UsingContinuous Phase Shift SSFBGs, IEEE Journal on Selected Topicsin Quantun Electronics, vol.13, no.5, pp.1480-1486,2007

[4] Xu Wang, Naoya Wada, etc., Field Trial of 3-WDM10-OCDMA10.71-Gb/s Asynchronous WDM/DPSK-OCDMA Using Hybrid E/DWithout FEC and Optical Thresholding , J.Lightwave Technol, vol25, no.1, pp.207-215, 2007

[5] Dawei Wang, Liang Cheng and Biao Chen, ExperimentalInvestigation of Colorless ONU employing SSFBG inWDM/OCDMA-PON, pp.1-2, ACP, 2009Asia

[6] Daiwei Wang, Biao Chen, Xuezhi Hong, and Sailing He, TheDWDM over OCDMA Approach Based on Superstructured FiberBragg Gratings, IEEE Photon. Technol. Lett. under review

[7] Peh Chiong Teh, Petropoulos, etc., A comparative study ofperformance of seven- and 63-chip optical code-division multiple-

access encoders and decoders based on superstructured fiber Bragggratings, J.Lightwave Technol, vol. 19, no.9, pp.1352-1365, 2001

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