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