performance analysis of a novel coding technique for...

Vol. 6, No. 6 June 2015 ISSN 2079-8407 Journal of Emerging Trends in Computing and Information Sciences

©2009-2015 CIS Journal. All rights reserved.

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299

Performance Analysis of a Novel Coding Technique for SAC-OCDMA 1 Satyasen Panda, 2 Urmila Bhanja

1 Asst. Prof., Department of Electronics Engineering, SIT, Bhubaneswar 2 Dr., Assoc. Prof., Department of Electronics Engineering, IGIT, Sarang

ABSTRACT The optical code division multiple access (OCDMA), the most advanced multiple access technology in communication is gaining popularity because of its asynchronous access capability, faster speed, efficiency, security and unlimited bandwidth. Many codes are developed in spectral amplitude coding optical code division multiple access (SAC-OCDMA) with minimum cross correlation property to reduce the Multiple Access Interface (MAI) and Phase Induced Intensity Noise (PIIN) noises. This paper presents a novel SAC-OCDMA code with minimum cross correlation property that lies between zero to one, to further minimize the Multiple Access Interface (MAI) referred in this work as modified random diagonal code (MRDC), which is found to be more scalable compared to the other existing SAC-OCDMA codes. In this work, the proposed MRDC code is implemented in an optical system using the optisystem software for the spectral amplitude coded optical code-division multiple-access (SAC-OCDMA) scheme. The new code family based on the MRDC code reveals properties of minimum cross-correlation, flexibility in selecting the code parameters and supports a large number of users, combined with high data rate and longer fibre length. Simulation results depict that the optical code division multiple access system based on the proposed MRDC code accommodates maximum number of simultaneous users with higher data rate transmission, lower bit error rates (BER) and longer travelling distance without any signal quality degradation, as compared to the former existing SAC-OCDMA codes. Keywords: Cross Correlation (CC), Optical Code Division Multiple Access (OCDMA), Spectral Amplitude Coding Optical Code Division Multiple Access (SAC-OCDMA), Multiple Access Interface (MAI), Phase Induced Intensity Noise (PIIN), Modified random diagonal codes (MRDC) 1. INTRODUCTION

Currently, optical code division multiple access (OCDMA) technique is gaining popularity because the technique supports asynchronous access networks, dynamic bandwidth assignment and multimedia services [1, 2]. Furthermore, OCDMA systems are employed for local area networks and for access network applications [3–6]. The performance of the OCDMA system is measured by numerous quantitative parameters such as the data rate, bit error rate (BER), number of users, the powers of transmitter and receiver, and the type of codes employed. Significant OCDMA performance parameters such as the types of OCDMA codes and the transmitted data rate play an important role in deciding the numbers of users that are allowed to access a network asynchronously [7].

The OCDMA systems suffer from different noises such as a shot noise, thermal noise, dark current, phase intensity induced noise (PIIN), and multiple-access interference (MAI) arising from other users. Of these noises, the MAI is generally considered as a dominating source of noise [8]. Hence, recent research mostly concentrates on to develop novel OCDMA codes that minimize the MAI. Spectral amplitude coding for the optical code division multiple access (SAC-OCDMA) system offers a good solution that reduces the MAI effect by utilizing codes with a fixed in-phase cross correlation [9]. Lots of codes are developed in the literature for the SAC-OCDMA networks such as an enhanced double weight (EDW) code [10], a modified frequency-hopping (MFH) code [11], a modified quadratic congruence (MQC) code [12], an optical orthogonal (OOC) code [13], a prime code [14], a khazani–Syed (KS) code [15], a random diagonal (RD) code [16], a modified double

weight (MDW) code [17], a Dynamic Cyclic Shift (DCS) code[18] etc. However, the above mentioned codes suffer from several limitations such as the code length is often too long (e.g., for the OOC and the EDW codes), the code construction is limited by the code parameter (e.g., for the MQC, DCS and MFH codes), the cross-correlation increases with increasing weight number (e.g., for the prime code and the RD code). In addition, some of the SAC-OCDMA codes suggested in the literature do not support large numbers of simultaneous users or high data rates [10-17].

To overcome these problems in this work, a novel Modified Random Diagonal Code (MRDC) is suggested. The MRDC is designed on combination of diagonal matrixes. The proposed novel MRDC has several advantages and has the following contributions as mentioned below.

a. Cross-correlation that lies between zeros to one that cancels the MAI.

b. Flexibility in choosing the parameters such as code weight (W) and number of users (K).

c. A simple and flexible design. d. Supports larger number of active users with

higher data rates for longer distances. e. Almost no overlapping occurred for the spectra

characteristics for different users.

This paper is organized as follows. The design and construction of the proposed MRDC is described in section 2. Simulation using the optisystem software and performance analysis of the proposed code is explained in section 3. Finally, conclusions are drawn in section 4.

1 [email protected], 2

[email protected]




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2. THE PROPOSED MRDC DESIGN Modified Random Diagonal Code (MRDC) is a

modified version of Random Diagonal Code (RD) [16, 21].In an OCDMA system, the signature sequence consists of unipolar (0, 1) sequence. Generally, a code is

denoted as (N, W, λ c ) where, N is the code length, W is

the code weight and λ c is the in-phase cross correlation.

The cross correlation is defined as expressed in equation (1) [19].

λ c = 1

N

i iiX Y

(1)

Many OCDMA code strategies are proposed

where, the major concerns of designing the codes are to reduce the MAI, code length and code weight. However, in almost all the existing SAC-OCDMA codes, the code length increases with the increase in the number of users. As a result, the cross correlation becomes more than 1.

To reduce the MAI the cross correlation should be as small as possible. Therefore, in this work care has been taken to design a novel SAC-OCDMA code to reduce the code length, to keep the cross correlation minimum and to keep the auto correlation maximum. So, in this work a novel SAC-OCDMA code named as modified random diagonal code (MRDC) is developed. This code consists of two parts:

a. Data segment b. Code segment

2.1 Data Segment

The data segment consists of a diagonal matrix so it has only one ‘1’ in each column. Hence, the cross correlation in the code segment becomes 0 and weight becomes 1.

For K=2, Data segment = 1 0

0 1

For K=3, Data segment =

1 0 0

0 1 0

0 0 1

2.2 Code Segment The code segment consists of a chess board matrix of two columns and number of rows is equal to the number of users.

For K=2, Code segment = 0 1

1 0

For K=3, Code segment =

0 1

1 0

0 1

The final MRDC code (Z) is designed as below.

Z= D C

Where, D is the Data segment

C is the Code segment

For k=2, the MRDC code becomes

Z=1 0 0 1

0 1 1 0

For K=3,

Z=

1 0 0 0 1

0 1 0 1 0

0 0 1 0 1

In the proposed MRDC code, the length of the

code is (N) = K+W. In this work, ‘K’ represents the number of users and ‘W’ denotes the weight of the code. The Table-1 below shows the comparison between different codes and it can be clearly seen that the proposed MRDC presents advantages over other existing codes [10, 11, 13, 16 and 17].As is observed from the table 1, the code length of the proposed MRDC is the least among other codes for 30 numbers of users. Furthermore, it is also observed that only the proposed MRDC supports 30 users with the weight W = 2 and a cross- correlation

between zero to one ( λ c = 0-1).

Table 1: Comparison of different properties of the

proposed MRDC and the other codes used for the SAC-OCDMA systems [19]

3. THE SIMULATION AND

OBSERVATION The developed MRDC code is validated using an

OCDMA trans-receiver circuit. The receiver in the circuit employs a direct detection receiving technique. This

S.No Code No. of users

Weight Code length

Cross-correlation

1 MD 30 2 60 0 6 RD 30 4 35 Variable 2 OOC 30 4 364 1 3 Prime

Code30 31 961 2

4 EDW 30 3 60 1 5 MDW 30 4 90 1 6 MRDC 30 2 32 1




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OCDMA trans-receiver circuit simulated using opti-system v12.0 software. Figure 1 below presents the block diagram of the circuit and a simulation circuit for 3 users is shown in figure 2 below.

Figure 1: Block diagram of transmitter and receiver used for the proposed MRDC code [1, 2, 3, 7, 12, 15]

Figure 2: Simulation diagram of Tran receiver used for the proposed MRDC code using optisystem software for 3 users

[19].

As shown in the above figures 1 and 2, in the encoder side continuous wave (CW) lasers are used as optical source with the input power is set to 0 dBm and produce pulses with a repetition rate equal to the bit rate

of the system. The code chips for different users are given as input through the user defined bit sequence generators and are converted to equivalent non return to




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zero (NRZ) signals. A Mach-Zehnder modulator modulates the signals from CW lasers and the NRZ pulses. The modulated signals are multiplexed in a WDM multiplexer and then are transmitted in a single mode optical fibre with the natural channel parameters. In the receiver side, first a demultiplexer is used to transmit the data to 3 different receivers. For each receiver, a uniform Fibre Bragg Grating (FBG) is used to filter the received signal corresponding to the transmitted wavelength at the source. A PIN photo diode is used to convert optical data to electrical signal which is then transmitted through a low pass Gaussian filter and regenerators. A visualizer is connected at the end to analyse the received signal. Table 2 below depicts the parameters used in the simulation.

Table 2: Parameters used in the simulation [19] Photo detector quantum efficiency (g) 0.6

Number of users 20

Operating wavelength (λ 0 ) 1550 nm

Electrical bandwidth (B) 311

Data rate ( bR ) 622Mbps-10

Receiver noise temperature ( nT ) 300

Receiver load resistor ( LR ) 1030Ω

The performance of the system is characterizing

by referring to the BER and the eye pattern as shown below.

The average signal to noise ratio (SNR) is calculated as below in equation 2[16, 20].

SNR =

22 2

2

42

WR P

LKTB R

PwEBRL L

(2)

Where, R = Responsively P = Effective power of broad band Source at receiver. W = weight of the code L = code length E = Electronic charge B = Receiver electrical bandwidth K = Boltzmann constant T = Absolute temperature R = Receiver load resistance

Using Gaussian approximation the BER is found

as below in equation 3[16, 20].

BER =1

2 8

SNRerfc (3)

SNR represents the signal to noise ratio and erfc

is the error complimentary function. The eye patterns for different input powers such as -17 dBm, -19 dBm, -21 dBm and -23 dBm are tested for the proposed MRDC as shown in the figures below 3, 4, 5and 6 respectively for 30 users with RZ encoding. The figures depict that with the proposed MRDC, the SAC-OCDMA system gives an acceptable BER of 10-15 when the input power is fixed at -19 dBm and below that the BER deteriorates.

Figure 3: Eye diagram of one MRDC channels at Power input = -17 dBm, 30 user, RZ encoding




303







304

Figure 7: Eye diagram of one MRDC channels 40 user, 80 km, RZ encoding, 622 mbps, power 0 dbm, 1550 nm






305

Figure 10: Eye diagram of one MRDC channels 40 user, 40 km, RZ encoding, 2 Gbps, power 0 dbm, 1550 nm

Figure 11: Eye diagram of one MRDC channels 40 user, 40 km, RZ encoding, 10 Gbps, power 0 dbm, 1550 nm

Table 3: Comparison among BER versus the number of active users for the proposed MRDC and different SAC-OCDMA codes [19].

Types of code Active number of users Order of BER Data rate MFH

30

10-7

622Mbps RD 10-25 MD 10-25

MRDC 10-35 MFH

50

10-4

622Mbps RD 10-11 MD 10-24

MRDC 10-29 MFH

70

10-3

622Mbps RD 10-6 MD 10-13

MRDC 10-21 MFH

90

10-2

622Mbps RD 10-3 MD 10-9

MRDC 10-17




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In the figures 7, 8 and 9 the efficacy of the proposed MRDC is presented. It is observed that the BER for the proposed MRDC at a data rate of 622 Mbps is at an acceptable level upto a distance of 120 km when the numbers of active users are fixed at 40. At 120 km, the BER is observed to be the order of 10-12.

It is also observed from the figure 10 that when the data rate is increased to 2 Gbps the BER is of the order of 10-11 for 40 numbers of active users. Figure 11 depicts that the proposed MRDC fails to give an acceptable level of BER when the data rate increases to 10 Gbps.

Table 3 above presents a comparison of different existing codes such as RD, MFH, MD with the proposed MRDC codes [19]. It is observed from the table that the proposed MRDC gives the best result in terms of BER with the increase in the active number of users at a data rate of 622 Mbps. 4. CONCLUSION

A new code family with minimum cross-correlation for SAC-OCDMA systems is successfully designed, and simulated. The new code family, which called Modified random Diagonal code (MRDC), shows a better system performance as compared to the former SAC-OCDMA codes with the same system complexity [16,19,21]. The MRDC code offers several advantages such as minimum cross-correlation, less BER, more flexibility, and easy code construction as compared to that of other existing SAC-OCDMA codes [16, 19, and 21]. Also, the code performs better in terms of flexibility in choosing the code parameter (e.g., number of users, the code weight and cross correlation) and the number of users can be increased without any increase in the code weight and code complexity [19, 21]. In the absence of multiple access interference, the system shows excellent performance with spectral amplitude coded asynchronous optical CDMA. Unlike the other existing SAC-OCDMA codes, in the proposed MRDC code, the code length does not increase with the increase in the number of users [19].

Finally, simplicity in the code construction and

flexibility in cross-correlation control has made this code a compelling candidate for future OCDMA applications. ACKNOWLEDGEMENT

The authors would like to extend their sincere appreciation to the all India council of technical education (AICTE) for the funding of this research through the research project number. 20/AICTE/RIFD/RPS (POLICY-II) 2/2012-13. REFERENCES [1] Chen Biao, Wang Fu-chang, Hu Jian-dong and He

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