Faculty of Engineering, El-ALzhar University, EGYPT. Faculty of Engineering & Technology, Banha University, EGYPT

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Transmission Analysis of Optical Code Division Multiple Access (OCDMA) Communication Systems in the Presence of Noise in Local Area Network Applications Ahmed Nabih Zaki Rashed 1 ; Mohamed M. Zahra ; Mohamed
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Transmission Analysis of Optical Code Division Multiple Access (OCDMA) Communication Systems in the Presence of Noise in Local Area Network Applications Ahmed Nabih Zaki Rashed 1 ; Mohamed M. Zahra ; Mohamed Yassin 3 ; Ismail A. Abd El-Aziz 4 ; Shreen A.El-Bheiry 5 1 Electronics and Electrical Communications Engineering Department Faculty of Electronic Engineering, Menouf 3951, Menoufia University, EGYPT 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 745 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 []. 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, ]. 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, 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) [] 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. 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]. 746 Insan Akademika Publications Fig. 1. Transmitter optical code division multiple access (OCDMA) communication system. Figure 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.. 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]: 747 SNR 1 M M 1 16 n P P P R I s P l d 1 (1) Where M is the number of active users, P is the code length, I d is the dark current, s 1 =R d P received is the signal current, where R d is the detector resistance, P received is the received optical power and σ n is the total noise and can expressed as follows [1]: n th sh () Where σ th is the thermal noise and can be estimated as the following formula: 4 K th R B L T B (3) Where K B is the boltzmann's constant (1.38x1-3 J/K), T is the ambient temperature, R L 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 [1]:.7 B T P (4) Where T P is the signal propagation delay through waveguide fiber which can be given by [11]: n L T P c (5) Where L is the fiber length in km, c is the speed of light (3x1 8 m/sec), and n is the refractive index of the material based fiber link cable which can be expressed as the following formula: n A 1 A3 A5 1 A A 4 A 6 (6) The empirical equation coefficients as a function of ambient temperature and room temperature for pure silica fiber as: A 1S = , A S = (T/T ), A 3S =.47946, A 4S = (T/T ), A 5S = , A 6S = (T/T ). Where T is ambient temperature in K, and T is the room temperature and is considered as 3 K. For the plastic fiber material, the coefficients of the Sellmeier equation are given as: A 1P =.4963, A P =.6965 (T/T ), A 3P =.33, A 4P =.718 (T/T ), A 5P =.1174, and A 6P = Moreover, the Shot, σ th, can be estimated as the following formula [11]: e B I sh p (7) Where e is the electron charge (1.6x1-19 C), and I P is the photocurrent. Optical orthogonal code (OOC) is a family of, l sequences with good auto and cross correlation properties that are suitable for CDMA in positive systems [5]. An (P, W, λ a, λ c ) OOC code is sequences of length P, weight W with autocorrelation constraint of λ a [14], and cross-correlation constraint of λ c. Here the code length has 748 Insan Akademika Publications taken to be 1 Code length, P 1, Code weight, W 8 [15]. Therefore the maximum number of user supported by OCDMA scheme is given by [1, 13]: ( P 1) M W( W 1) (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 T h. Each user transmits data at rate of B Gbit/sec with On- Off Keying (OOK) modulated transmission average power of.5 W. Where the received power can be estimated as [16]: P received P T e L n (9) Where L is the transmission distance, P T is the transmitter power and is given by: W P P T P OOK (1) Where P OOK =.5 Watt is the average power of OOK modulated signal and is given by [17]: Threshold System TS.5T h W P (11) Where T h 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]: SNR BER.exp. SNR 8, (1) 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 []: 375 C T max M L 8 C T min M L M$ (13) M$ (14) 749 Number of user - M 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 Operating optical signal wavelength, λ Threshold value, T h Code length, P Code weight, W Transmission distance, L Load resistance, R L Photo current, I P Ambient temperature, T Dark current, I d Detector resistance, R d Value 1.3 μm-1.55 μm km1 km 5 kω 1 μ A-1 μa 3 K35 K 1 na 5 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. (3-3), 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 Number of user vs Code length with different weight length code length - P Fig. 3. Variations of number of users against variations of both code length and weight at the assumed set of the operating parameters 75 Insan Akademika Publications Transmitted Power (watt) Number of users -M Number of Users vs Code Weight with different code length 5 P=1 45 P=3 P=5 4 P=7 35 P= Code Weight -W Fig. 4. Variations of number of users against variations of both code weight and code length at the assumed set of the operating parameters Transmitted Power vs Code Length Fig. 5. Variations of transmitted signal power against variations of both code length and code weight at the assumed set of the operating parameters 751 MAX. Cost (million $) -LOG1 ( Thermal Noise) LOG1( Thermal Noise) vs Tempreture wavelength=1.3µm wavelength=1.45µm wavelength=1.55µm Temreture (k) 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. 5 MAX.Cost vs Code Length for distance ( 1km) 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. 75 Insan Akademika Publications MAX. Cost (million $) MIN.Cost (Million $) MIN.Cost vs Code Length with distance ( 1km) Fig. 8. Minimum system cost in relation to both code length and code weight with transmission distance (1 km) at the assumed set of the operating parameters MAX.Cost vs Code Length for distance ( 5km) Fig. 9. Maximum system cost in relation to both code length and code weight with transmission distance (5 km) at the assumed set of the operating parameters. 753 MAX. Cost (million $) MIN.Cost (Million $) MIN.Cost vs P at distance equl 5km Fig. 1. Minimum system cost in relation to both code length and code weight with transmission distance (5 km) at the assumed set of the operating parameters. 5 MAX.Cost vs Code Length for distance equal 1 km Fig. 11. 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. 754 Insan Akademika Publications -LOG1 (shot noise) MIN.Cost (Million $) MIN.Cost vs P at distance equl 1km Code Length - p Fig. 1. Minimum system cost in relation to both code length and code weight with transmission distance (1 km) at the assumed set of the operating parameters. - LOG1 (shot noise) vs wavelength Ip=1µA Ip=5µA Ip=1µA Wavelength ( µm) Fig. 13. Variations of shot noise versus variations of operating optical signal wavelength and photo current at the assumed set of the operating parameters. 755 Received Power (dbm) Received Power (watt) Received Power vs Code length for pure silica optical fiber -distance (1km ) 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. Received Power vs Code length for pure silica optical fiber -distance (5km ) Fig. 15. Received signal power in relation to both code length and code weight for pure silica fiber with transmission distance (5 km) at the assumed set of the operating parameters. 756 Insan Akademika Publications Received Power (dbm) Received Power (dbm) Received Power vs Code length for pure silica optical fiber -distance (1km ) Fig. 16. 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.5. Received Power vs Code Length for plastic optical fiber - distance(1km) 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 757 Received Power (dbm) Received Power (dbm) Received Power vs Code Length for plastic optical fiber - distance ( 5km) Fig. 18. Received signal power in relation to both code length and code weight for plastic optical fiber with transmission distance (5 km) at the assumed set of the operating parameters Received Power vs Code Length for plastic optical fiber - distance(1km) Fig. 19. 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 758 Insan Akademika Publications SNR (db) SNR (db) 1 9 SNR vs Code Length for pure silica optical fiber Fig.. 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 1 9 SNR(dB) vs Code Length for plastic optical fiber Code Length -P Fig. 1. 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 759 Bit Error Rate (BER) Bit Error Rate (BER) 1 x Bit Error Rate vs Code Length for pure silica optical fiber Fig.. Bit error rate in relation to both code length and code weight for pure silica fiber at the assumed set of the operating parameters 9 x 1-9 Bit Error Rate vs Code Length for plastic optical fiber Fig. 3. Bit error rate in relation to both code length and code weight for plastic optical fiber at the assumed set of the operating parameters 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. (7-1) 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. 76 Insan Akademika Publications vi) vii) 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. Figs. (-3) 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 us
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