Swati Sharma* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 245 - 247
BER Performance of Zero Vector Combinatorial Code Set For Incoherent SAC-OCDMA Swati Sharma
Keywords- Bit error rate (BER), Optical Code Division Multiple Access (OCDMA), Zero Cross Correlation Code (ZCC), Zero Vector Combinatorial Code (ZVCC), effective power Psr
I. INTRODUCTION
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The number of users is increasing day by day and so is the requirement to accommodate more number of users in the communication network. Many techniques have been researched till now to multiplex more number of users on the same bandwidth like TDMA (Time Division Multiple Access), WDMA (Wavelength Division Multiple Access), etc but they had some limitations like centralized control, latency problem , requirement of expensive lasers etc. thus the focus was shifted to OCDMA (Optical Code Division Multiple Access) which had the advantages of better wavelength utilization, low loss, low interference, high cardinality, high security of data etc, which made OCDMA so popular among the researchers [7, 9].
Optical CDMA has proved to be a very promising class of technology that takes advantages of excess bandwidth in the optical fiber to map low information rate into high rate optical sequences. It is one of the multiple access techniques which allow multiple users to access the same spectral range asynchronously without interfering with one another [5]. In this paper, incoherent system has been considered due to its low component cost and less complexity. Here the performance of the system is evaluated in terms of SNR (Signal to noise ratio) and BER (Bit Error Rate). For any system taken into consideration, BER depends upon several factors like code weight, code length, effective power from each user, and number of users simultaneously accessing the optical channel and
ISSN: 2230-7818
data rates from the users. For typical optical CDMA system the minimum acceptable BER is 10-9. The main factor to be considered is MAI (Multiple access interference), which is responsible for degradation of BER hence the system performance. MAI in a system is induced due to the overlapping of spectra from different users. The other factor is effective power from each user, which highly influences the BER of the system. The key to an effective OCDMA system is the choice of code family in order to obtain a good autocorrelation and almost zero cross correlation properties for encoding the source [6]. This property depicts that the codeword assigned to each user should be easily distinguished from every other sequence in the code set and a shifted version of itself. By using these properties MAI in a system can be reduced significantly.
ES
Abstract— In this paper the performance of an incoherent Spectral Amplitude Coded (SAC) Optical CDMA system has been analyzed using the recently proposed code Zero Cross Correlation Code (ZVCC). The BER of a system depends on various factors like code weight, effective power from each user, code length and the number of active users. A comparative study between ZVCC and the previously proposed Zero Cross Correlation (ZCC) Codes has been carried out. The simulation results showed that ZVCC outperforms ZCC in terms of BER and cardinality.
Er.Preeti
Assistant Professor, ECE Department University Institute of Engineering and Technology Chandigarh, India
T
M.E (Pursuing), Student / ECE University Institute of Engineering and Technology Chandigarh, India ece_sharma@yahoo.com
If MAI is managed, BER will be improved automatically. In intend, ZVCC (zero vector cross correlation) codes have been designed to satisfy this property [1]. ZVCC have zero cross correlation ( Îť c = 0) i.e. a system using ZVCC, theoretically have no two users interfering with each other, hence MAI is negligible. This paper concentrates on two codes, ZVCC and Zero Cross correlation Codes (ZCC). ZCC has already been implemented and simulated and the results showed that it performs better than the previously designed code like MFH (Modified Frequency Hopping), MDW (Modified Double Weight), and OOC (Optical Orthogonal Codes) [2]. No doubt ZCC performs better in terms of BER and SNR but it has limitations as well such as long code length, complex design algorithm. Thus ZVC code sets were proposed which had the advantages of small code length and simple design algorithm [1]. The rest of the paper is organized as follows: Section II contains the system specifications, then in Section III the simulation results are discussed and in the last, Section IV concludes the research work along with the future scope. II. SYSTEM SPECIFICATIONS The system considered for the present work is an incoherent SAC (Spectral Amplitude Coding) OCDMA
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Swati Sharma* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 245 - 247
For a system using ZVCC, BER can be easily calculated using the equation given as: 1 BER Pe erfc SNR 8 2 Where erfc is the error function given as
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w=8
-250
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e u du
1
w=12 w=20
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10 No. of users
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Further, on increasing the effective power from -10dBm to -5dBm there is a significant improvement in BER. The Fig.2 clearly depicts the decrease in BER to an approximate value of 10-300. Moreover with the increase in effective power the number of simultaneous users is also increased. The simulation results showed that the active users were increased from 98 at w=4 & Psr = -10dBm, to 233 at the same code weight and effective power.
PERFORMANCE ANALYSIS
Considering the system parameters described in the previous section, the incoherent SAC-OCDMA system has been evaluated. Further, the effect of various system parameters on BER, have been studied.
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The main focus is on the ZVC code family. The construction of ZVCC is simple and provides the flexibility of choosing the number of simultaneous users and the code weight. The effects of various system parameters on BER have been analyzed and shown in this section.
BER versus no. of simultaneous users at Psr = 0dBm
0
10
w=4 -50
10
w=8
BER versus no. of simultaneous users at Psr = -10dBm
0
10
3
Fig 2: BER versus no. of simultaneous users at effective power, Psr = -5 dBm
x
where, „u‟is the unit step function. III.
-50
w=4
10
2
BER versus no. of simultaneous users at Psr = -5dBm
0
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erfc ( x)
2
10
T
Incoherent SAC optical OCDMA system has been evaluated and analyzed for ZVCC code set in this paper. The performance evaluation of the system has been done using MATLAB 7, in terms of BER. The system parameters considered are: spectral line width of source (∆v) = 3.75 THz, receiver temperature (T r) = 300 K, load resistance (RL) = 1030 ohm, and photodiode quantum efficiency 0.6 [3,8]. The simulated performance is presented using 80 MHz electrical bandwidth at 1550nm central frequency. The bit rate per user is considered as 160 Mbps.
increase the code weight from 4 to 8. It is clearly observed that with the increase in code weight i.e to w=20 the increase in number of users is very less i.e it increase by a figure of 10, hence it would be more advisable to use w =4 for the application in which less number of active users are there.
BER
system. Incoherent system has been considered here due to its simple architecture (as no phase information has to be sent along with the signal) and low cost.
w=12 -100
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w=20
-20
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BER
-150
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-40
BER
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w=20 -100
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Fig.1 BER versus no. of simultaneous users with effective power, Psr = -10dBm.
It is clear from Fig. 1 that BER is far lesser than 10-9 when the number of simultaneous users is 100, if we
ISSN: 2230-7818
Fig 3: BER verses no. of simultaneous users at effective power, Psr =0dBm (ideal case)
Fig. 3 is the ideal scenario, where effective power is 0dBm that means no loss has incurred during the transmission process. This is a theoretical scenario as; it is not possible to have no loss in the system. At Psr = 0dBm, the BER is enhanced to a very high value of 10-300.
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Swati Sharma* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 7, Issue No. 2, 245 - 247
Also, the simulation results of the incoherent SACOCDMA system have been compared to the simulation results of a previously proposed code known as zero cross correlation (ZCC), to find out the best suited code for the system. BER versus no of simultaneous users at Psr = -10dBm
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-50
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-100
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ZCC
Thus it can be concluded that on increasing the effective power the BER can be reduced to a very low value but the usage of power is increased in the system. Moreover with the increase in code weight BER improves slightly, but this means that more number of 1‟s will have to be accommodated in the code sequence, which will result in more power being used in the system. Hence, we can say that there is always a tradeoff between the code weight and power, i.e in an application where less number of users are to be used, there the code weight 4 and effective power -10dBm can be used.
BER
ZVCC
ZVCC for w=4 ZVCC for w=20
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-200
ZCC for w=4 ZCC for w=20
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-250
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No. of users
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It is depicted by the Fig.4 that for the same code weight and the effective power the BER is better for ZVCC. BER versus no of simultaneous users at Psr = -5dBm
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ZCC -50
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-100
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ZVCC
-150
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10
The simulated results of ZVCC and ZCC at 160Mbps over a communication channel at 1550nm central frequency shows a better performance in terms of BER. Also, ZVCC results in better system performance than ZCC in terms of BER in terms of same system specifications.
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Fig. 4: Comparison of ZCC and ZVCC for BER at Psr = -10dBm
BER
T
10
It can be clearly concluded from the simulation results that the BER is reduced significantly to 10-250 with the increase in effective power to -5dBm, and so was the phase induced intensity noise (PIIN). At minimum acceptable BER 10-9, the numbers of users were increased from 98 to 233. Moreover with the increase in code weight from 4 to 20 the BER was further reduced and the number of users increased.
ZVCC for w=4
V. REFERENCES
[1]
Hassan yousif Ahmad, Ibrahima Faye and N. M.Saad, “Zero Vector Combinatorial Code Family for spectral amplitude coding (SACOCDMA)” IJCSNS International journal of computer science and network security, Vol. 8, No. 3, March 2008
[2]
E.I. Babekir, N. M. Saad, N. Elfadel, A. Mohammed, A.A. aziz, M.S.Anuar, S.A. Alijunid and M.K.Abdullah, “Study of optical spectral CDMA zero cross correlation codes,” IJCSNS International journal of computer science and network security, Vol. 7, No. 7, July 2007.
[3]
Z. Wei and H. Ghafouri- Shiraz, “Proposal of a novel code for spectral amplitude- coding optical CDMA systems,” IEEE Photon. Technol, Lett. , vol 14, pp. 414-416, March 2002.
[4]
J.A. Salehi, F.R.K. Chung, and V.K. Wei, “Optical orthogonal codes: Design, analysis, and applications,” IEEE Transactions on Information Theory, vol. 35, pp. 595-605, May 1989.
[5]
J. A. Salehi and C. A. Brackett,” Code division multiple access techniques in optical fiber networks-part II: system performance analysis,” IEEE trans. on commun, vol.37, no.8, aug. 1989.
[6]
Paul R. P. Prucnal, Mario A. Santro,” Spread spectrum fiber optic local area network using optical processing,” IEEE Journal of lightwave technology, Vol. 4, No.5, May 1986.
[7]
Kerim Fouli and Martin Maier, “ OCDMA and Optical Coding : Principles, Applications , and Challenges”,Institut National de la Recherche Scientifique (INRS)
[8]
Ivan B. Djordjevic and Bane Vasic ,”Combinatorial Constructions of Optical Orthogonal Codes for OCDMA Systems”, IEEE June 2004
[9]
Andrew Stok, Edward H. Sargent, “Lightening the local area: Optical code division access and quality of service provisioning,” IEEE Nework, November/December 2000.
ZVCC for w=20
-200
10
ZCC for w=4
-250
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ZCC for w=20
-300
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100
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No. of users
Fig. 5: Comparison of ZCC and ZVCC for BER at Psr = -5dBm
On increasing the effective power the BER is also increased and more number of users can be accommodated. From Fig.4 and Fig 5 it is very clear that ZVCC outperforms ZCC. IV. CONCLUSION Many codes have been studied in recent years with zero cross correlation property [2,4, 1], but the ZCC and ZVCC codes which are studied recently were regarded as the best zero cross correlation codes. Thus these two codes were simulated for the OCDMA system.
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