Transmission Analysis of Optical Code Division Multiple ... · 745 Transmission Analysis of Optical Code Division Multiple Access (OCDMA) Communication Systems in the Presence of

745

Transmission Analysis of Optical Code Division Multiple Access (OCDMA)

Communication Systems in the Presence of Noise in Local Area Network

Applications

Ahmed Nabih Zaki Rashed1; Mohamed M. Zahra

2; Mohamed Yassin

3;

Ismail A. Abd El-Aziz4; Shreen A.El-Bheiry

5

1 Electronics and Electrical Communications Engineering Department

Faculty of Electronic Engineering, Menouf 32951, Menoufia University, EGYPT

[email protected]

2 Faculty of Engineering, El-ALzhar University, EGYPT

3, 4

Faculty of Engineering & Technology, Banha University, EGYPT

5 Faculty of Specific Education Mass Communication Department, Menoufia University, EGYPT

Abstrak – OCDMA is an essential part of the digital communication system now

days for long haul, high speed networks. The biggest challenge with Optical

CDMA system is to maintain the performance of the system and offer high

bandwidth in case of higher number of users at minimum cost. As the number of

users increase, the input requirements i.e. transmitted power, bit rate etc start

increasing sharply which contribute to the additional cost. It has recently attracted

significant research interest because of the advantages it offers in terms of the

flexibility in the management of the system resources. We have taken into account

the system design parameters are determined such as BER (bit error rate), signal

to noise ratio (SNR), transmission bit rates, and optical received power for

different code lengths. The Optical CDMA systems suffer from the problem of

multiple access interference (MAI).As the number of users increase the BER error

rate degrades because the effect of MAI (multiple access interference) increases.

So, there is a limitation in number of users, as the number of users increase SNR

decrease and probability of error increases.

Key Words – Shot noise; Thermal noise; BER; SNR; Received signal power; Code

length parameter

1 Introduction

The development of fiber optics communication in the last few years has made the optical fiber a

strong candidate for the future of telecommunication system. The optical fiber offers a vast amount of

bandwidth that can be utilized for communication. One of utilizing this is signal multiplexing. Due to

the large bandwidth and the associated high bit rates, the multiplexing process is beyond the

capabilities of pure electronic methods and has to be implemented optically as well. Code division

multiple access (CDMA) is a strong candidate for creating effective multiple methods for the optical

subscriber access network because of its asynchronous access and code multiplexing [1]. CDMA is a

strong candidate for creating effective multiple methods for the optical subscriber access network

because of its asynchronous access and code multiplexing. OCDMA system has attracted increasing

International Journal of Basic and Applied Science,

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Rashed, et. al.

746 Insan Akademika Publications

attention in recent years due to the following advantages: asynchronous access capability, accurate

time of arrival measurements, flexibility of user allocation, ability to support variable bit rate, busty

traffic and security against unauthorized users. Moreover, the OCDMA method is preferable for

multiplexing in the optical domain because it uses broad bandwidths in optical devices for the

electrical CDMA method and the Electrical-to-Optical (E/O) conversion [2].

Optical networks provide higher capacity and reduced costs for new applications such as the internet,

video, multimedia, and advanced digital services. Fortunately, an alternative to optical time division

multiple access (OTDMA) and dense wavelength division multiple access (DWDMA) networking

schemes, optical CDMA communication systems, require neither the time nor the frequency

management systems. Optical CDMA can operate asynchronously, without centralized control, and it

does not suffer from packet collisions. As a result, optical CDMA systems have lower latencies than

OTDMA or DWDMA. Furthermore, since time and frequency (or wavelength) slots do not need to be

allocated to each individual user, significant performance gains can be achieved through multiplexing.

Also, OTDMA and DWDMA systems are limited by hardware because of the slot allocation

requirements. In contrast, CDMA systems are only limited by the tolerated bit error rate relationship to

the number of users, affording the designer a much more flexible network design [1, 2].

Code Division Multiple Access (CDMA) technique was originally investigated in radio frequency

communication systems [3]. This multiplexing technique consists to allow to each subscriber a

specific code word. This code word permits to the transmitter to modulate its data sequences. In order

to satisfy faster and more reliable optical communication system requirements and optimize the huge

optical bandwidth sharing, Optical CDMA presents an attractive solution. The advantages of this

technique are principally the asynchronous users emission and the possibility to emit at any time and

on any wavelength without generating more interference. In Optical CDMA technique, each bit is

divided up into L time periods, called chips. By sending a short optical pulse during some chips

intervals [4], and leaving the others to “0”, an optical signature sequence can be created. For this

particular case, specific optical codes have to be conceived because of the scrambling phase of the

optical channel, which not permit to use bipolar optical signals. An optical code is defined by its length

L (number of chips), its weight w (number of "1" chips), and its multiplexing capacity N (number of

users). In addition, for a useful optical code, the intercorrelation and asynchronous autocorrelation

levels have to be limited. Two important families of optical codes were previously developed: Optical

Orthogonal Codes (OOC) [2] and Prime Sequences (PS) [5].

In the present study, OCDMA scheme has been an increasing interest for fiber optic systems because it

allows multiple users to access the system asynchronously and simultaneously. OCDMA is expected

for further ultrahigh speed and real time computer communications where there is strong demand for

the systems to support several kinds of data with different traffic requirements. We have analyzed the

performance in terms of SNR and BER. We have taken into account several kinds of data (such as

code length parameter, number of active users) with different bit rates.

2 General Block diagram of Tx/Rx OCDMA Communication System

Code division multiple access (CDMA) scheme has been an increasing interest for fiber optic network

because it allows multiple users to access the network asynchronously and simultaneously. Optical

code-division multiple-access (CDMA) is expected for further ultra-high speed and real-time computer

communications where there is strong demand for the systems to support several kinds of data with

different traffic requirements. As shown in Fig. 1 that data are coming into the data conversation unit

which converted the data in electrical form. This converted data is driving the laser driver. This laser

light is passing through the optical fiber. Temperature controller controls the temperature of optical

fiber [6].

Rashed, et. al. International Journal of Basic and Applied Science,

Vol 01, No. 04, April 2013, pp. 745-762

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Fig. 1. Transmitter optical code division multiple access (OCDMA) communication system.

Figure 2 Shows that signals are received in photo detector block, preamplifier amplify signal because

it may be weaken during transmission time. Then it goes through filter it elements some noise. Finally

we get the output from decision circuit [7].

Fig. 2. Receiver optical code division multiple access (OCDMA) communication system.

Recently scientists propose multi code direct detection optical CDMA system is support several kinds

of data in different bit rates coping with a multimedia network. In this system each user is assign a set

of sequence code generated from time shifted version of optical orthogonal codes (OOC) to support

several kinds of such data. In this way we can achieve our expected bit rate [8].

3 Theoretical Model and Analysis

The system SNR can be further expressed as a function of number of active users as follows [9]:


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22

122

22

2 2

16

22

12

1

PsIRPPP

MM

SNR

dl

n

…(1)

Where M is the number of active users, P is the code length, Id is the dark current, s1=RdPreceived is the

signal current, where Rd is the detector resistance, Preceived is the received optical power and σn is the

total noise and can expressed as follows [10]:

222shthn …(2)

Where σth is the thermal noise and can be estimated as the following formula:

BR

TK

L

Bth

42

…(3)

Where KB is the boltzmann's constant (1.38x10-23

J/K), T is the ambient temperature, RL is the load

resistance and B is the transmission bit rate with non-return to zero code (NRZ), which is given by the

following formula [10]:

PTB

7.0

…(4)

Where TP is the signal propagation delay through waveguide fiber which can be given by [11]:

c

LnTP

…(5)

Where L is the fiber length in km, c is the speed of light (3x108 m/sec), and n is the refractive index of

the material based fiber link cable which can be expressed as the following formula:

26

2

25

24

2

23

22

2

211

A

A

A

A

A

An

…(6)

The empirical equation coefficients as a function of ambient temperature and room temperature for

pure silica fiber as: A1S=0.691663, A2S=0.03684043 (T/T0)2, A3S=0.4079426, A4S=0.0116241 (T/T0)

2,

A5S=0.8974749, A6S= 84.76543 (T/T0)2. Where T is ambient temperature in K, and T0 is the room

temperature and is considered as 300 K. For the plastic fiber material, the coefficients of the Sellmeier

equation are given as: A1P= 0.4963, A2P= 0.6965 (T/T0)2, A3P= 0.3223, A4P= 0.718 (T/T0)

2, A5P=

0.1174, and A6P= 9.237. Moreover, the Shot, σth, can be estimated as the following formula [11]:

psh IBe22 …(7)

Where e is the electron charge (1.6x10-19

C), and IP is the photocurrent. Optical orthogonal code

(OOC) is a family of 0, l sequences with good auto and cross correlation properties that are suitable for

CDMA in positive systems [5]. An (P, W, λa, λc) OOC code is sequences of length P, weight W with

autocorrelation constraint of λa [14], and cross-correlation constraint of λc. Here the code length has


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taken to be 10 ≤ Code length, P ≤ 100, 2 ≤ Code weight, W ≤ 8 [15]. Therefore the maximum number

of user supported by OCDMA scheme is given by [12, 13]:

)1(

)1(

WW

PM

…(8)

Bit error rate (BER) performance is simulated with respect to signal to noise ratio (SNR), number of

users M, OOC length P, and threshold level Th. Each user transmits data at rate of B Gbit/sec with On-

Off Keying (OOK) modulated transmission average power of 0.5 W. Where the received power can be

estimated as [16]:

LTreceived

nePP …(9)

Where L is the transmission distance, PT is the transmitter power and is given by:

OOKT PP

WP

…(10)

Where POOK=0.5 Watt is the average power of OOK modulated signal and is given by [17]:

P

WTTSSystemThreshold h5.0

…(11)

Where Th is the threshold value, W is the code weight, and P is the code length. The bit error rate of

OCDMA communication system can be estimated as the following [18, 19]:

,8

exp..

2

SNR

SNRBER

…(12)

Because of the high quality of submarine cable circuits and the ease and convenience of international

Direct Dialing (IDD), which is becoming commonplace, the continued rapid growth in. demand will

likely continue. The cost of underwater cable system, relative to satellite links, depends on traffic and

density. Cables have an advantage over short; paths, especially if traffic is heavy. As distance increases

and/or density, decreases, satellites become economically more attractive. Therefore the estimated total

fiber cost (max. and min.) of submarine fiber cable OCDMA system can be expressed as a function of

number of active users (M), and transmission length (L), can be estimated as the following formula

[20]:

LM

CT

375max

M$ …(13)

LM

CT

282min

M$ …(14)


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4 Simulation Results and Performance Analysis

We have investigated the transmission analysis of OCDMA communication systems in the presence

and absence of noise under the set of the wide range of the operating parameters as shown in Table 1 is

listed below.

Table 1. Proposed operating parameters in OCDMA communication system [3, 5, 8, 13].

Operating parameters Value

Operating optical signal wavelength, λ 1.3 μm-1.55 μm

Threshold value, Th 15

Code length, P 10100

Code weight, W 210

Transmission distance, L 1 km10 km

Load resistance, RL 50 kΩ

Photo current, IP 10 μ A-100 μA

Ambient temperature, T 300 K350 K

Dark current, Id 10 nA

Detector resistance, Rd 50 kΩ

Based on the model equations analysis, assumed set of the operating parameters as listed in the Table 1

above, and based on the series of the figs. (3-23), the following facts are assured:

i) Figs. (3, 4) have assured that number of users increases with increasing code length and

decreasing code weight under the set of the operating parameters considerations.

ii) Fig. 5 has indicated that transmitted signal power increases with increasing code weight and

decreasing code length under the set of the operating parameters considerations.

Fig. 3. Variations of number of users against variations of both code length and weight at the

assumed set of the operating parameters

10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

35

40

45

50

code length - P

Nu

mb

er

of

user

- M

Number of user vs Code length with different weight length

W=2

W=5

W=8


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Fig. 4. Variations of number of users against variations of both code weight and code length at the


Fig. 5. Variations of transmitted signal power against variations of both code length and code weight

at the assumed set of the operating parameters

2 3 4 5 6 7 8 9 100

5

10

15

20

25

30

35

40

45

50

Code Weight -W

Nu

mb

er

of

users

-M

Number of Users vs Code Weight with different code length

P=10

P=30

P=50

P=70

P=100

10 20 30 40 50 60 70 80 90 1000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Code Length - P

Tra

nsm

itte

d P

ow

er

(w

att

)

Transmitted Power vs Code Length

W=2

W=5

W=8


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Fig. 6. Variations of thermal noise against variations of both ambient temperature and operating

optical signal wavelength at the assumed set of the operating parameters.

Fig. 7. Maximum system cost in relation to both code length and code weight with transmission

distance (1 km) at the assumed set of the operating parameters.

300 305 310 315 320 325 330 335 340 345 35013.05

13.1

13.15

13.2

13.25

13.3

13.35

13.4

Temreture (k)

-LOG10( Thermal Noise) vs Tempreture

-LO

G10 (

Th

erm

al N

ois

e)

wavelength=1.3µm

wavelength=1.45µm

wavelength=1.55µm

10 20 30 40 50 60 70 80 90 1000

500

1000

1500

2000

2500

Code Length - P

MA

X. C

ost

(millio

n $

)

MAX.Cost vs Code Length for distance ( 1km)

W=2

W=5

W=8


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Fig. 8. Minimum system cost in relation to both code length and code weight with transmission




10 20 30 40 50 60 70 80 90 1000

200

400

600

800

1000

1200

1400

1600

1800

Code Length - P

MIN

.Co

st

(Millio

n $

)

MIN.Cost vs Code Length with distance ( 1km)

W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 1000

50

100

150

200

250

300

350

400

450

500

Code Length - P

MA

X. C

ost

(millio

n $

)

MAX.Cost vs Code Length for distance ( 5km)

W=2

W=5

W=8


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10 20 30 40 50 60 70 80 90 1000

50

100

150

200

250

300

350

400

Code Length - P

MIN

.Co

st

(Millio

n $

)

MIN.Cost vs P at distance equl 5km

W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 1000

50

100

150

200

250

Code Length - P

MA

X. C

ost

(millio

n $

)

MAX.Cost vs Code Length for distance equal 10 km

W=2

W=5

W=8


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Fig. 13. Variations of shot noise versus variations of operating optical signal wavelength and photo

current at the assumed set of the operating parameters.

10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100

120

140

160

180

200

Code Length - p

MIN

.Co

st

(Millio

n $

)

MIN.Cost vs P at distance equl 10km

W=2

W=5

W=8

1.3 1.35 1.4 1.45 1.5 1.558.2

8.4

8.6

8.8

9

9.2

9.4

9.6

9.8

Wavelength ( µm)

-L

OG

10 (

sh

ot

no

ise)

- LOG10 (shot noise) vs wavelength

Ip=1µA

Ip=5µA

Ip=10µA


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Fig. 14. Received signal power in relation to both code length and code weight for pure silica fiber

with transmission distance (1 km) at the assumed set of the operating parameters.


with transmission distance (5 km) at the assumed set of the operating parameters.

10 20 30 40 50 60 70 80 90 1000

0.005

0.01

0.015

0.02

0.025

Code Length - P

Receiv

ed P

ow

er

(watt)

Received Power vs Code length for pure silica optical fiber -distance (1km )

W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 100-56

-54

-52

-50

-48

-46

-44

-42

-40

-38

Code Length - P

Receiv

ed

Po

wer

(dB

m)


W=2

W=5

W=8


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with transmission distance (10 km) at the assumed set of the operating parameters

Fig. 17. Received signal power in relation to both code length and code weight for plastic optical

fiber with transmission distance (1 km) at the assumed set of the operating parameters

10 20 30 40 50 60 70 80 90 100-124

-122

-120

-118

-116

-114

-112

-110

-108

-106

Code Length - P

Receiv

ed

Po

wer

(dB

m)


W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 1000

0.005

0.01

0.015

0.02

0.025

Code Length - P

Re

ce

ive

d P

ow

er

(dB

m)

Received Power vs Code Length for plastic optical fiber - distance(1km)

W=2

W=5

W=8


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Rashed, et. al.






10 20 30 40 50 60 70 80 90 100-56

-54

-52

-50

-48

-46

-44

-42

-40

-38

Code Length - P

Receiv

ed

Po

wer

(dB

m)

Received Power vs Code Length for plastic optical fiber - distance ( 5km)

W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 100-124

-122

-120

-118

-116

-114

-112

-110

-108

-106

Code Length - P

Receiv

ed

P

ow

er

(dB

m)

Received Power vs Code Length for plastic optical fiber - distance(10km)

W=2

W=5

W=8


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Fig. 20. Signal to noise ratio in relation to both code length and code weight for pure silica fiber at

the assumed set of the operating parameters

Fig. 21. Signal to noise ratio in relation to both code length and code weight for plastic optical fiber

at the assumed set of the operating parameters

10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

Code Length - P

SN

R (

dB

)

SNR vs Code Length for pure silica optical fiber

W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

Code Length -P

SN

R (

dB

)

SNR(dB) vs Code Length for plastic optical fiber

W=2

W=5

W=8


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Fig. 22. Bit error rate in relation to both code length and code weight for pure silica fiber at the


Fig. 23. Bit error rate in relation to both code length and code weight for plastic optical fiber at the


iii) Fig. 6 has proved that thermal noise decreases with increasing operating optical signal

wavelength and decreasing ambient temperature at the assumed set of the operating parameters.

iv) Figs. (7-12) have indicated that maximum and minimum system cost planning based on both

pure silica and plastic fibers decreases with decreasing both code length and code weight under

the assumed set of the operating parameters. It is also observed that system cost planning

decreases with increasing transmission distance for both fibers under sturdy considerations.

v) Fig. 13 has indicated that shot noise decreases with increasing both operating optical signal

wavelength and photo current under the assumed set of the operating parameters.

10 20 30 40 50 60 70 80 90 1001

2

3

4

5

6

7

8

9

10x 10

-9

Code Length - P

Bit

Err

or

Ra

te (

BE

R)

Bit Error Rate vs Code Length for pure silica optical fiber

W=2

W=5

W=8

10 20 30 40 50 60 70 80 90 1001

2

3

4

5

6

7

8

9

x 10-9

Code Length - P

Bit

Err

or

Ra

te (

BE

R)

Bit Error Rate vs Code Length for plastic optical fiber

W=2

W=5

W=8


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vi) Figs. (14-19) have indicated that received signal power based on both pure silica and plastic

fibers decreases with decreasing both code length and code weight under the assumed set of the

operating parameters. It is also observed that received signal power decreases with increasing

transmission distance for both pure silica and plastic fibers under sturdy considerations.

vii) Figs. (20-23) have assured that signal to noise ratio increases and bit error rates decrease for

both pure silica and plastic optical fibers based OCDMA systems with increasing code length

and adjust increasing code weight at five step weight.

5 Conclusion

In a summary, the model has been investigated the transmission analysis of optical code division

multiple access (OCDMA) communication systems in the presence of noise in local area network

applications. It is theoretically found that the increased code length, this results in the increased

number of users at constant code weight at w=5 to get maximum signal to noise ratio and minimum bit

error rates. Thermal and shot noise have the dramatic effects on the OCDMA communication systems.

The suitable choice of code length and code weight have the great effects on the maximum and

minimum system cost planning and received signal power at different transmission distances for both

plastic and pure silica fibers based OCDMA communication systems.

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