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This paper describes a performance analysis of an incoherent optical code-division multiple-access scheme based on wavelength/time codes. The system supports 16 users operating at 2.5Gb/s/user while maintaining bit-error rate (BER) <10-11 for the correctly decoded signal. It has been observed that there are two major problems giving rise to performance degradation of the system in terms of number of users and type of code.
In this paper we have studied the optical simulator Encoding/ Decoding for different lengths & gain in terms of Quality factor (Q) and Bit Error Rate (BER) performance. The system supports 16 users while maintaining bit-error rate (BER) < 10- 11 for the correctly decoded signal. Our aim is to design and simulate a Tree Network Topology Optical Code Division Multiple Access System for large number of users using Wavelength-Time code and to analyze the performance of the system based on BER and Eye Diagram under the influence of number of simultaneous users with different received powers.
Index Terms: NRZ, LAN, BER, MAI, WDMA, TDMA
Optical communication systems are a main part of the digital communications in long haul networks, high speed LAN and MAN. The main advantages of the optical fiber communications are the high speed, large capacity and high reliability by the use of the broad bandwidth of the optical fiber. The sharing of spectrum is required to achieve high capacity by simultaneously allocating the available bandwidth to multiple users .
The major synchronous multiple access protocols are Time Division Multiple Access (TDMA), Wavelength Division Multiple Access (WDMA) and Code Division Multiple Access (CDMA). TDMA is an efficient multiple access protocol in networks with heavy traffic demands, but in situations where the channel is sparsely used, TDMA is inefficient. WDMA can be used as a degree of design freedom with respect to routing and wavelength selection. But the fundamental disadvantage in WDMA is that sophisticated hardware such as wavelength-controlled tunable lasers and high quality narrowband tunable filters for each channel is required. CDMA was originally investigated in the context of radio frequency communications systems, and was first applied to the optical domain in the mid-1980s. Using optical fiber together with semiconductor laser transmitter has made it possible to transmit high bit rate data signals with low attenuation. , .
Optical Code Division Multiple Access (OCDMA) combines the large bandwidth of the fiber medium with the flexibility of the CDMA technique to achieve high-speed connectivity. It allows multiple users to access the network asynchronously, ability to support variable bit rate and bursty traffic, privacy and security in transmission, and scalability of the network. In Optical CDMA systems, every channel is identified by a unique pseudo noise key (signature sequence codes), whose bandwidth is much larger than that of the input data , .
The Optical CDMA offers an interesting alternative for ultra high-speed LANs because neither time management nor frequency management of all the nodes is necessary. Optical CDMA can operate asynchronously without centralized control and does not suffer from packet collisions .
Fig 1.1 A schematic diagram of an OCDMA communication system
In OCDMA, every user channel is identified by a unique signature sequence code, so multiplexing gains can be high. In contrast to TDMA and WDMA where the maximum transmission capacity is determined by the total number of time/wavelength slots (i.e. hard-limited), OCDMA allows flexible network design because the bit error rate (BER) depends on the number of active users (i.e. soft-limited) . In order to realize the high data rates over long distances down the SM fiber, techniques must be found to overcome the pulse spreading and reduce power penalty due to dispersion.  The invention of erbium-doped fiber amplifier (EDFA) paved the way for development of high bit rate, all optical ultra long distance which illuminated the need for electronic repeaters. .
Nonlinear effects along with dispersion are the destructive forces for pulse propagation in ultra high-bit-rate optical transmission system which is needed to be taken in to account. .A general Block diagram of optical CDMA multi-user system is shown in Fig. 1.1.
One of the basic properties of the optical fiber is its enormous low-loss bandwidth of several tens of terahertz. 
The primary task of the receiver or decoder in an OCDMA network is to recover the signal in the presence of other interfering signals.
Hence the codes suitable for OCDMA systems should have the following properties: 
A code should be distinguishable from a shifted version of itself.
A code should be distinguishable from a possibly shifted version of all other codes in the set.
In OCDMA each user on the O-CDMA system has been assigned a unique signature sequence. The O-CDMA encoded data is then sent to the N: 1 combiner-fiber-1: N splitter (in a local area network) or a 1: N coupler (in an access network) and broadcast to all nodes .
Antonio J. Mendez et al. showed rigorously that 2-D wavelength/time codes have better SE than one-dimensional (1-D) CDMA/WDM combinations (of the same cardinality). Then, the paper describes a specific set of wavelength/time (W/T) codes and their implementation.. This research shows that OCDMA implementation complexity (e.g., incorporating double hard-limiting and interference estimation) can be avoided by using a guard time in the codes and an optical hard limiter in the receiver.
Shivaleela, E.S. et al.  described the basic principles of a new family of wavelength/time multiple-pulses-per-row (W/T MPR) codes, for incoherent FO-CDMA networks, which have good cardinality, spectral efficiency, and minimal cross correlation values. In addition, an expression for the upper bound on the cardinality of W/T MPR codes is derived. Another feature of the W/T MPR codes is that the aspect ratio can be varied by a tradeoff between wavelength and temporal lengths.
Vincent J. Hernandez et al.  developed an optical code division multiple access (O-CDMA) technology demonstrator (TD) based on two dimensional (2-D) codes. The 2-D codes are derived from folded optimum Golomb rulers, implemented as wavelength/time ë/T codes. With the TD, They demonstrated six asynchronous users at less than 10-11 bit error rate (BER) and up to eight users at 10-8 BER, limited by saturation in the optical preamplifier.
Vincent J.Hernandez et al. discussed a technology demonstrator for an incoherent optical code-division multiple-access scheme based on wavelength/time codes. The system supports 16 users operating at 1.25 Gsymbols/s/user while maintaining bit-error rate (BER) < 10-11 for the correctly decoded signal. Experiments support previous simulations which show that coherent beat noise, occurring between the signal and multiple access interference, ultimately limits system performance.
A.J Mendez et al.  described the design and construction of the matrices; analyzed their performance from a communications viewpoint; described their use as codes for the asynchronous, concurrent communication of multiple users; and analyzed the bit error rate performance based on capturing and modeling a typical network topology and performing a numerical modeling of the system. The matrices can be interpreted (implemented) as space/time (S/T) or wavelength/time (W/T) matrix codes for OCDMA applications.
Sun Shurong et al.  gave a new family of two-dimensional optical orthogonal code (2-D OOC), one-coincidence frequency hop code (OCFHC)/OOC, which employs OCFHC and OOC as wavelength hopping and time-spreading patterns, respectively
QAQZXZ In optical CDMA, multiple Access interference (MAI) is dominant factor compared to photo detector shot noise, dark current and thermal noise that limits number of user as well as data rate of the system. The type of code used is a major factor influencing the performance of any CDMA system. Hence a need was felt to use wavelength-time codes which provide better cardinality and good correlation property.
In this paper we have studied the optical simulator Encoding/ Decoding for different no of users in terms of quality factor (Q) and bit error rate (BER) performance with different attenuation factors & designed and simulated a tree network topology optical code division multiple access system for large number of users using wavelength-time code. We have analyzed the performance of the system based on BER and eye diagram under the influence of number of simultaneous users with different received powers. The paper is divided in to five sections, after introduction in section I, section II discusses simulation set-up, section III gives results & discussions followed by conclusion in section IV & finally references are given in section V.
II Simulation Setup
Fig 1.2 Simulation Setup of OCDMA system
The simulation setup for OCDMA system is shown in figure 1.2.Here we have demonstrated an incoherent OCDMA system based on a wavelength-time spreading coding technique. The two-dimensional W/T code has been redesigned by using 4 wavelengths and 4-time slots in this system. Four channel WDM network has been used for generating the carrier signal. This carrier signal is used to modulate the PRBS data of the user. After modulation an encoder is used for encoding the signal. Four mode-locked lasers have been used to create a WDM multi-frequency light source i.e. carrier signal using OptMUX1. Mode locked laser is used for generating pulses of pulse width of 100ps at a repetition rate equal to data rate of the system. The wavelengths range from 1550 to 1551.2 nm, with 0.4nm wavelength spacing.
The PRBS data generator (PRBS1 to PRBS5) is used to generate random data of 26 pattern length. An electrical NRZ signal generator (ElecGen1 to ElecGen5) is used to convert digital data into electrical signal. A Mach-Zehnder LiNbO3 modulator modulates the multiplexed 4 wavelengths according to the NRZ electrical data.
Fig. 1.3 Spectrum at WDM multiplexer output Fig.1.4 Optical pulses generated by WDM source
Fig. 1.5 NRZ signal data Fig.1.6 Modulated data before encoding for user 1
The modulated signals are distributed to the respective encoders, which have been assigned a unique W/T code respective to each encoder. In an encoder four optical filters and four shift signals are used to produce the encoded bit stream. The optFil is used to filter out one spectral wavelength and then the shiftSig is used to produce a pulse at specified chip. The optMux2 combines four of the displaced pulses to form an encoded signal. The encoders use delay line arrays providing delays in terms of integer multiples of chip times. The placement of the delay line arrays and the amount of each delay are dictated by the specifics of the user signatures. The combiner combines four of the displaced pulses to form an encoded signal. Thus, the eight encoders of the system generate 16 encoded signals for 16 users. The encoded data from all users are multiplexed and then passed through 75 km span of single mode fiber (SMF) followed by a loss compensating optical amplifier. The output signal from a fiber span then passed through splitter (Optsplit1) to split the signal and routed to the user's decoder. The decoder, tuned to the same structure as the corresponding encoder but with negative delays as compared to encoder, providing delays in terms of integer multiples of chip times. The decoded signal finally arrives at optical receiver.
The eye diagram analyzer has been used to take the plot of eye pattern. Bit error rate values for different received power have been taken from BER tester. The system has been redesigned for different users. More than 40 simulations have been run for this analysis.
III RESULTS AND DISCUSSIONS
The performance of an optical CDMA system depends on a number of factors. Some of these are the type of code, data rates, number of simultaneous users (N) and attenuation factor etc. Of the above mentioned parameters, number of users and attenuation are important parameters because BER directly depends upon these parameters. Also, the error performance of any receiver is governed by the Q-factor. Therefore, in the present analysis eye diagram and the bit error rate variation of an optical CDMA receiver for increase in the number of simultaneous users with different attenuation factors.
In the given paper we placed encoders for five users out of 16 for simplicity. The encoded data from all users are multiplexed and then passed through a 75-km span of standard single mode fiber (SMF) followed by a loss compensating optical amplifier. Amplifiers can also be used to compensate for the insertion losses due to encoders, multiplexers, demultiplexers and decoders if needed. The output signal from a fiber span then passed through splitter/demultiplexer and routed to the user's decoder. The decoded signal finally arrives at optical receiver and BER tester. Encoders and decoders respectively use delay and inverse delay line arrays  providing delays in terms of integer multiples of chip times. The placement of the delay-line arrays and the amount of each delay are dictated by the specifics of the user signatures.
The analysis of OCDMA System based on two-dimensional W/T code for different users has been done. BER values and Eye diagrams were captured from BER Tester and Eye diagram analyzer.
The eye diagrams were captured from optical simulation tool OPTSIM using eye diagram analyzer at receiver side and received signal is also analyzed by signal analyzer. The eye diagrams representing the one, two, three, four and five users it can do the same up to sixteen concurrent users are shown below.
The eye diagram and received signal for different number of users is shown below. The correctly received signal justifies the system performance. Figure 1.7 to 1.16 shows eye diagrams for different number of users and corresponding electrical detected signal. At the receiver side the detected signal Q-factor and BER are calculated
Fig 1.7 Eye Diagram for one user, with -12dBm Fig 1.8 Received signal for one user with -12dBm
Received power Received power
Fig 1.9 Eye Diagram for two users with -12 dBm Fig 1.10 Received signal for two users at -12dBm Received power Received power
Fig 1.11 Eye Diagram for three users with -12 dBm Fig 1.12 Received signal for three users at -12 dBm
Received power Received power
Fig 1.13 Eye Diagram for four users with -12 dBm Fig 1.14 Received signal for four users at -12dBm
Received power Received power
Fig 1.15 Eye Diagram for five users with -12 dBm Fig 1.16 Received signal for five users at -12dBm
Received power Received power
. As the BER and the Q factor have a direct relationship, i.e. as the no of users increases the bit error rate and Q factor degrades at the same time. As the number of simultaneous user is increased the error performance of the system shows a deteriorating trend. This can be seen as the number of users increases up to five, the system performance degraded MAI increases in the output.
Fig1.17 Q-factor Vs BER for users at different received powers
Hence, in this paper, a complete analysis of the optical CDMA system has been carried out. BER and Eye diagram analysis for different number of users and at different Received power has been done.
The optical CDMA system has been designed using these W/T matrix code and WDM-type components. A computer simulation was used to assess the propagation of these codes at high data rates, over a long span of fiber. The Optical CDMA system has been designed for 2.5Gbps data rate with different values of received power. A comparative BER and Eye diagram analysis of High Speed OCDMA system for asynchronous concurrent communication of multiple users has been done. The architecture has been proposed for a number of users with different values of received power and different values of Q-factors and BER has been calculated. The results obtained at 2.5Gbps data rate, shows that for different number of users as the received power deceases, Bit Error Rate increases at the same time. From the above results it can be seen that at 2.5 Gbps the received power should not be less than -15dBm for desired BER (<10-11) value. At 2.5Gbps the designed system can work up to 16 users to provide BER (<10-11). As the number of user increases, the amplitude of MAI pulses are more than the desired pulse. Hence correct decoding of data signal is not possible. From the above points, it is found that there exists a trade-off between the received power and the desired BER (<10-11) required.
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