OPTICAL DOMAIN MULTIUSER INTERFERENCE ESTIMATION

FOR SPECTRAL PHASE ENCODING OPTICAL FIBRE

CDMA SYSTEMS

Arash Yazdani

1,4

, Morteza Noshad

, Hojat Jaliloghli

Amirhossein Jaliloghli

and Mohammad Noshad

Islamic Azad University of Noor, Noor, Iran

Electrical and Computer Engineering Department, University of Tabriz, Tabriz, Iran

Islamic Azad University of Tabriz, Tabriz, Iran

Telematics Department, Universidad Politecnica de Catalunya, Barcelona, Spain

Electrical Engineering Department, University of Virginia, Charlottesville, U.S.A.

Keywords:

Optical ﬁber code division multiple access, Spectral phase encoding, Optical domain multiuser interference

detection, Temporal multiuser interference estimation, Nonlinear multiuser interference estimation.

Abstract:

In this paper, an effective technique for reducing the multi-user interference (MUI) in spectrally-phase-

encoded optical code division multiple access (SPE-OCDMA) systems has been proposed. In this method

the effect of MUI signal on the main user’s signal has been decreased using multiuser interference detection

(MID). Two structures have been introduced for accomplishing MID and degrading its destructive effect on

the decoded ultra-short light pulse; temporal multiuser interference estimation (TMIE) and nonlinear mul-

tiuser interference estimation (NMIE). Both of these methods utilize two photo-detectors for estimating the

MUI signal. In spite of simple structure of these two receivers, their performance analysis shows an improved

performances comparing to other receivers.

1 INTRODUCTION

A spectrally-phase-encoding optical ﬁbre code divi-

sion multiple access (SPE-OFCDMA) system is a

system in which a phase encoding has been applied

on a coherent ultra-short light pulse with a rectangu-

lar spectral pattern (Salehi et al., 1990). Similar to

the other kind of optical ﬁbre CDMA systems, the

MUI is the main limiting factor on the performance of

the SPE-OFCDMA systems (Salehi et al., 1990). The

MUI effect in these systems appear as an noise-like

signal distributed in the bit period in the vicinity of the

ultra-short optical pulse of main user. But because of

the slow response time of photo-detectors comparing

to the ultra-short pulse duration, a part of the noise-

like signals of other users is also gathered by the de-

tector and degrade the performance of the receiver.

Some solutions such as using time gating (Lee et al.,

2002), second harmonic generation (SHG) (Ni et al.,

2007), self phase modulation (SPM) (Ni et al., 2007),

and two photon absorption (TPA) detectors (Jamshidi

and Salehi, 2007), have been proposed for the ﬁbre

OCMDA systems to alleviate the MUI problem.

Multi-user interference estimation has been pro-

posed for decreasing the undesired interference ef-

fect of the users in multiple access networks (Brandt-

Pearce and Aazhang, 1994). But because all of these

methods was in electrical domain, the estimation pro-

cess takes much time and limits the bit-rate of the sys-

tem. Also it needs to the information of the other

users. In this manuscript we investigate a novel

method for declining the MUI effect on the signal of

the desired user in SPE-OFCDMA systems. In this

approach a interference estimation in optical domain

has been used for this purpose. Actually two photo-

detectors with different integration times are used to

estimate and reduce the MUI destructive effect on de-

sired signal. In the both of the proposed structures for

MID the effect of interference is reduced after making

an estimation of it. In the TMIE structure, an estima-

tion of the interference is made using the temporal

distribution of the MUI signal. Then an estimation

of the interfering signal is made using the output of

second detector and subtracted from the output of the

146

Yazdani A., Noshad M., Jaliloghli H., Jaliloghli A. and Noshad M..

OPTICAL DOMAIN MULTIUSER INTERFERENCE ESTIMATION FOR SPECTRAL PHASE ENCODING OPTICAL FIBRE CDMA SYSTEMS.

DOI: 10.5220/0003526701460149

In Proceedings of the International Conference on Data Communication Networking and Optical Communication System (OPTICS-2011), pages

146-149

ISBN: 978-989-8425-69-0

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

ﬁrst one. In the second structure, NMIE, the inter-

ference has been estimated from the decoded signal

and a nonlinear function of it. The simulation results

shows the performance of the proposed technique.

2 PRINCIPLES OF SPECTRAL

PHASE ENCODING SYSTEMS

In the SPE-OCDMA systems, the bandwidth of the

ultra-short optical pulse of each user has been divided

into N

spectral bins with identical bandwidth and

the phase of each bin has been altered according to

a speciﬁc code sequence of that user. So at the out-

put of the transmitters the optical pulse become as a

noise-like signal. At the detector the conjugate of the

desired transmitter’s code is applied on the received

signal. Thus all of the phase alternations removed

at the decoder and the ultra-short pulse has been re-

vealed. For the signals of the other users the phase

shifts rearranged at the decoder but aren’t removed.

So the signals of the interfering users remain spread

and noise-like. In this study it is considered that users

have equal power at the receiver and each bit stream

from each user is synchronized. It should be noted

that power equality and synchronization are not nec-

essary for the system although we have used them to

simplify the analysis and simulation.

Figure 1: Integration times of the ﬁrst and second photo-

detectors.

The MID receiver can be implemented in two

ways. In the ﬁrst way is to use temporal multiuser

interference estimation (TMIE) to lower the MUI ef-

fect. This receiver consists of two photo-detectors,

one for collecting the main signal in the time duration

of T

and the other for gathering the interference sig-

nal due to the other users in the time duration of T

The two detectors can have an overlap in their inte-

gration time. The integration time of photo-detectors

is depicted in Figure 1. So the output signals of each

Figure 2: Structure of TIME receiver.

detector can be expressed as

−

(t)dt (1)

−

(t)dt (2)

We deﬁne a new output based on the outputs of

the two photo-detectors (Figure 2) as follows

X = X

− rX

. (3)

where r is deﬁned as the ratio of integration times

Figure 3: TIME receiver after a nonlinear optical device.

Figure 4: Structure of NIME receiver.

of the ﬁrst and second detectors, i.e. r = T

. Re-

garding the autocorrelation function of the interfer-

ence signal in SPE-OCDMA systems (Salehi et al.,

1990), the mean of the MAI can be eliminated in X

but its variance will be increased. So the error proba-

bility will be declined.

The TMIE receiver can be used alongside the

other nonlinear approaches such as SHG and SPM as

illustrated in Figure 3. In this case the received signal

passes through a nonlinear media before the detection

OPTICAL DOMAIN MULTIUSER INTERFERENCE ESTIMATION FOR SPECTRAL PHASE ENCODING OPTICAL

FIBRE CDMA SYSTEMS

147

Figure 5: BER versus the number of active users for SPA,

MID and SHG receivers.

and then TMIE by two photodiodes is used for esti-

mating the Interference signal. Similar to the previ-

ous case the mean of the MAI signal can be removed

by selecting an appropriate value for the parameter r.

The other type of the MID receiver is Nonlin-

ear Multiuser Interference Estimation (NMIE) as de-

picted in Figure 4. In this kind of MID receiver two

photo-detectors are used for interference estimation

as before but with the same integration time. The dif-

ference between the photo-detectors is that the ﬁrst

one integrates on the decoded signal but the second

one integrates on the decoded signal passed through

a nonlinear media. The output of the receiver in the

term of the ﬁrst and second photo-detectors outputs

can be expressed as follows

X = f(X

) − X

. (4)

where f(x) is a function for canceling the mean of

the MUI signal at the output which depends on the

function of the nonlinear media. For example for the

receiver with SHG as nonlinear media f(x) is αx

where α is a proper coefﬁcient.

3 PERFORMANCE ANALYSIS

In this section, performance analysis of the proposed

system will be made and compared with that of SHG

and Single Photon Absorption (SPA) codes. MAI has

been considered as the main performance degrading

factor of the SPE-OCDMA system and the shot noise

and thermal noises have been neglected. In our per-

formance analysis, 1Gbps bit rate and 20nm optical

bandwidth have been considered. Also, code-length

has been supposed to be 500. The results are for the

equal probable bit transmitting.

Figure 6: BER versus the number of active users for re-

ceivers using SHG and both SHG and MID.

The bit error rates for three different receivers,

MID, SHG and SPA, are depicted in Figure 5 versus

the number of active users for the 10mW optical peak

power for each user. As can be seen the SHG receiver

has the best performance comparing to the other re-

ceivers. But the interesting part of these results is the

performance of the MID receiver. In MID receiver we

are able to improve the system performance only by

using two photo-detectors and nothing else. Despite

the simple structure of the MID receiver, its BER is

only one order worse than the SHG receiver. Figure

6 shows the performance of a system when both SHG

and MID are used at the receiver. According to these

results the performance of the receiver with both SHG

and MID is a bit better than that of a receiver with

only SHG.

4 CONCLUSIONS

In this paper a novel receiver for SPE-OCDMA sys-

tems has been introduced. The proposed system

works based on the estimation and reducing the inter-

ference signal of the other users. Two different struc-

tures, TMIE and NMIE, have been proposed. But of

these structures utilize two photo-detectors to make

estimation from the MUI signal. The TMIE receiver

has an acceptable performance comparing to the SHG

receiver regarding its uncomplicated structure.

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FIBRE CDMA SYSTEMS

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