IJRIT International Journal of Research in Information Technology, Volume 2, Issue 4, April 2014, Pg: 507- 511

International Journal of Research in Information Technology (IJRIT) www.ijrit.com

ISSN 2001-5569

Optical Code Division Multiple Access System Jyoti Rana1, Er. Ankur Singhal2, Dr. Naresh Kumar3 and D.R. Rana4 1

M.Tech student, Electronics and Communication Engineering, Maharishi Markandeshwar University Mullana Ambala, Haryana, India [email protected] 2

Associate Professor, Electronics and Communication Engineering, Maharishi Markandeshwar University Mullana Ambala, Haryana, India [email protected] 3

4

Post Doctoral Research Fellow, Institute of Telecommunication, University of Aveiro, Portugal [email protected]

Associate Professor, Electronics and Communication Engineering, National Institute of Technology Hamirpur, Himachal Pradesh, India [email protected]

Abstract In this paper various multiple access techniques such as WDMA, TDMA and CDMA have been studied. In these techniques number of users can transmit their data simultaneously. The CDMA technique is the best among these techniques because assigning unique codes to each user with same frequency allocation. With optical domain we can achieve higher data rate of transmission. The main advantages of Optical CDMA system are soft capacity and network flexibility but limiting factors such as dispersion, Relative Intensity Noise, Multiple Access Interference etc. affect the system performance. So for the best performance of the system these limiting factors would be considered and their effect would be mitigated.

Keywords: code division multiple access (CDMA), bit error rate (BER), multiple access interference (MAI), relative intensity noise (RIN).

1. Introduction Optical communication systems are the most recent technology used world wide in the present era. Main advantages of optical fiber communication are high speed, large capacity and high reliability. Future requires high data rate and ultra fast services such as video on demand and TV over IP etc. For this fiber optics is the best transmission medium due to its tremendous bandwidth and extremely low loss. As number of users is increasing day by day, channel allocator need to increase number of channels with the same pace. To provide access to user’s multiplexing concept was used and to save bandwidth and access ability simultaneously to each user multiple access concept utilized. Various multiple access protocols are used for user’s data transmission and the present era is of speed and security prioritized for each user, various access techniques developed such as TDMA, WDMA and CDMA are used. In these techniques CDMA is one of most promising technology which has potential to overcome the existing obstacles with flexible and effective sharing of spectrum and jam resisting.

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 4, April 2014, Pg: 507- 511

2. Multiple Access In multiple access techniques number of user’s can transmit their data simultaneously on a single channel. In optical domain major multiple access techniques are wavelength division multiple access (WDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In WDMA system each user transmits their data at different wavelength. This system suffer from lower inter symbol interference (ISI) but it’s designing and installation cost are high as compared to TDMA system as it requires sharp optical filters. TDMA is asynchronous scheme which provide high capacity links for ultra long haul links with low power consumption. TDMA systems sensitivity to other users is high and high synchronization overhead is required because of burst transmissions and technically more complex. TDMA system also requires strong centralized control to allocate time slots and to manage the network operation. CDMA employs Spread Spectrum technology and a special coding technique to allow multiple users to be multiplexed over the same channel. This technique is asynchronous and operates without centralized control. This technique is flexible and it allows spectral spreading tailored to the most appropriate domain: time, wavelength or combination of both [1].CDMA uses unique spreading codes to spread the baseband data before transmission. The signal is transmitted in a channel which is below noise level. The receiver then uses a correlator to dispread the desired signal which is passed through a narrow band pass filter. Unwanted signals will not be dispread and will not pass through the filter. CDMA system has soft capacity and it permits growing the users beyond the nominal maximum capacity with some quality of service degradation but without extensive upgrades to the infrastructure [2]. In the high bit rate long haul communication links single mode fibers are used due to low propagation loss and large bandwidth availability [3]. Optical CDMA signals do not require bandwidth reservation and provides a fair division of bandwidth among all users. Users can be added to, or dropped from, the network with minimal reconfiguration of other users, allowing for soft capacity on demand and network flexibility. With tunable transmitters and receivers transmissions may easily be configured to broadcast to all other users, multicast to selected users or unicast to a single user [4]. Optical CDMA communication systems offer an interesting approach for local area networks due to their unique attributes of optical processing, asynchronous transmission potential for high information security and the capability for multiple user access [5]. The main advantages of Optical CDMA are such as; optical systems offer an inherent security to each user’s information at the physical transmitting medium level as well as the coding level used for transmission. No need of centralized network control because of the simplicity of the protocol. Optical CDMA systems have lower latencies than TDMA or WDMA [6]. Optical CDMA is a broadcast technology, with all information going to all parts of the network. Effective utilization of bandwidth and high tolerance to noise is assured by Optical CDMA. No need for the strict timing synchronization as OCDMA uses random and simultaneous access protocol.

3. Isseues in OCDMA With number of advantages in Optical CDMA system there are number of factors such as dispersion, RIN, MAI etc. limit/effect system and hence degrade their performance. All these effects arise in the system due to various causes which are discussed as follows:

3.1 Dispersion Non-linearity in fiber is one of the major limiting factors in the performance of the Optical CDMA system. Dispersion is a phenomenon in which the phase velocity of a wave depends on its frequency and media having such property are termed dispersive media. Dispersion in fiber causes spreading of optical pulse and hence degrades system performance because of increase in inter-symbol interference (ISI) and reduced optical peak power [7]. From [8] dispersion in single mode fiber can be given as (1)

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 4, April 2014, Pg: 507- 511

In Eq. (1) is dispersion in SMF, S0 zero dispersion slope, laser central wavelength and λ0 zero dispersion wavelength. From theoretical analysis it was proved that dispersion makes system to suffer from power penalty as well as limit the number of users [9]. Hence for accommodation of large number of users it is necessary to minimize the effect of dispersion on the transmission medium.

3.2 RIN Instability in the power level of optical source lead to generation of noise and that noise is mostly known as Relative Intensity Noise (RIN). Laser intensity reduces signal to noise ratio (SNR) and increases bit error rate (BER) which degrades the system performance. RIN is strongly dependent on type of laser and independent of link length [8]. Laser intensity noise depends on properties of laser, back reflections, and filters used after conversions from optical to electrical or vice versa. From [10] RIN is defined by the signal to noise ratio: (2) is mean square intensity fluctuation of the laser output and is average laser light In Eq. (2) intensity. This noise characterization is very essential to optimize system performance as in a receiver laser intensity fluctuations creates noise that exceed shot noise of photo-detector and hence becomes a limiting factor. Laser relative intensity noise (RIN) has impact on Optical CDMA because of fluctuations of laser light [11].

3.3 MAI Optical CDMA system supports multiple user data transmission simultaneously on a single channel. With this advantage in the receiver section when a desired user’s data is recovered then for that user other users data appear as interference which is most popularly known as multiple access interference (MAI). The interference caused in K user system is given as: (3) In Eq. (3)

is power level of the desired user,

power level of K-1 users which is reduced by a

is power level of noise component. MAI appears due to nonfactor of N, where N is processing gain, and zero cross-correlation between codes [12]. In the case of three user system transmitting at a data rate of 2.5Gbps on a distance of 100km due to MAI effect BER was observed to be 10-8.8 [13]. There are different methods employed for the reduction of multiple access interference (MAI).

4. Solution for OCDMA Problems Research to mitigate the effect of factors aroused in the system is in progress and few solutions already in use according effect arising in the system.

4.1 Dispersion In SMF it is found that dispersion arising in the system is major limiting factor in the performance of the system. To mitigate this effect number of alternates found and among them most popularly used alternates are Dispersion Compensation Fiber (DCF), Dispersion Shifted Fiber (DSF) and Fiber Bragg Grating (FBG). In channel short length DCF placed with the negative dispersion to compensate positive dispersion over the length of SMF. DCF has much narrow core than SMF. According to the placement of DCF in the channel dispersion compensation is categorized in two implementations: pre and post compensation. In pre compensation DCF is placed prior to the SMF while in other it is placed after the SMF. In these schemes pre compensation produces slight improvement in performance over the post compensation as pre compensation decreases signal power faster than post compensation due to higher attenuation of DCF. In

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post compensation signal power falls more slowly in SMF and experience anomalous dispersion while signal power is higher [10]. DSF are characterized by a large negative dispersion. In DSF core profile is manipulated to introduce dispersion in opposite direction from the direction in which dispersion operates. Optical CDMA system use DSF instead of SMF to mitigate effect of dispersion. Fiber Bragg Gratings are spectral filters and they rely on Bragg Effect. For dispersion compensation chirped fiber gratings are used where wavelength varies with position. It is possible to achieve sufficient compensation in short length of fiber for dispersion of a long span of transmission. It is based on the introduction of wavelength specific time delays. By combining such a FBG with a standard optical circulator, highly effective dispersion compensation can be realized.

4.2 RIN Minimization of laser RIN is an important consideration for obtaining optimum performance in the system. To mitigate RIN influence of external noise sources should be reduced and laser design should be optimized so that its susceptibility to external noise and influence of quantum noise are minimized [14].

4.3 MAI In [15] for the reduction of MAI power of wavelength is controlled using in situ method. This method control and change the wavelength s power distribution within Optical CDMA codes during transmission inside of optical fiber in such a manner that MAI formed is adequately suppressed. In [16] dynamic cyclic shift codes were used to reduce the effect of MAI as these codes have property of variable cross correlation. In [17] Optical Hard Limiter (OHL) and Optical Normalizer (ON) are used to mitigate the MAI effect. OHL is an optical threshold element combined with optical feedback to achieve bistability. OHL improves bit error rate of the system but random nature of incoming bits coupled with the masking noises and jitters the optimum threshold voltage cannot be prefixed to a stable constant level so dynamic adjustment required. One of the solutions to decrease multiple access interference is increasing the length of code with the increase in the number of users. ON normalizes optical signal power by attenuating input optical signal to the specified average output power level. This is used to attenuate all input signals to the same average output power regardless of their different average input powers with the advantage of causing light intensity to be within the dynamic range of photosensitive device and hence providing safety to the devices damage from high light intensity.

5. Conclusions Optical CDMA is one of the most upcoming technologies for multiplexed asynchronous user for multiple access techniques and greater scalability than other optical multiplexing technique. It provides flexible quality of service and data access security. OCDMA requires increasing demand for high speed, large capacity communications in optical networks which allow multiple users to share the fiber bandwidth. Multi-access interference (MAI) which arises when there are a large number of users in the system because of the fact that one user data becomes noise for all other users in the channel. Finally it degrades the system performance in the form of bit error rate and Q factor as the number of users increase. This is due to the Multi Access Interference (MAI) coming into effect with increase in number of users. Other degrading factors which have been studied are dispersion and RIN. Thus, effect of dispersion and RIN on Optical CDMA which occurs in the system because of increase in inter-symbol interference, reduced optical peak power and laser intensity reduces the signal to noise ratio and increases bit error rate which degrades the system performance respectively.

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