Author Topic: Eight User, 4Gb/s, Spectral Phase-Encoded OCDMA System in time domain for Metrop  (Read 2966 times)

0 Members and 1 Guest are viewing this topic.

IJSER Content Writer

  • Sr. Member
  • ****
  • Posts: 327
  • Karma: +0/-1
    • View Profile
Quote
Author : Savita R.Bhosale Dr. S. L. Nalbalwar and Dr. S.B.Deosarkar
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
Download Full Paper : PDF

Abstract: In optical code division multiple access (OCDMA) system, many users share the same transmission medium by assigning unique pseudo-random optical code (OC) to each user. OCDMA is attractive for next generation broadband access networks due to its features of allowing fully asynchronous transmission with low latency access, soft capacity on demand, protocol transparency, simplified network management as well as increased flexibility of QoS control and enhanced confidentiality in the network. Hence, in this paper, we proposed a technique using spectral phase encodingin time domain for eight users. This technique is proved to be much effective to handle eight users at 4 Gb/s bit rate for Metropolitan area Network (MAN). Results indicate significant improvement in term Beat Error Rate (BER) and very high quality factor in the form of Quality of Service (QoS). In our analysis, we have used Pseudo Orthogonal (PSO) codes. The simulations are carried out using OptSim (RSOFT).

Keywords:  MAI, OCDMA,  OOC, PSO, QoS, BER ,PON,ISD,CD.
 
1  INTRODUCTION
OPTICAL code division multiple access (OCDMA), where users share the same transmission medium by assigning unique pseudo-random optical code (OC), is attractive for next generation broadband access networks due to its features of allowing fully asynchronous transmission with low latency access, soft capacity on demand, protocol transparency, simplified network management as well as increased flexibility of QoS control [1~3]. In addition, since the data are encoded into pseudo-random OCs during transmission, it also has the potential to enhance the confidentiality in the network [4~6]. Figure1. illustrates a basic architecture and working principle of an OCDMA passive optical network (PON) network. In the OCDMA-PON network, the data are encoded into pseudo random OC by the OCDMA encoder at the transmitter and multiple users share the same transmission media by assigning different OCs to different users.

At the receiver, the OCDMA decoder recognizes the OCs by performing matched filtering, where the auto-correlation for target OC produces high level output, while the cross-correlation for undesired OC produces low level output. Finally, the original data can be recovered after electrical thresholding. Recently, coherent OCDMA technique with ultra-short optical pulses is receiving much attention for the overall superior performance over incoherent OCDMA and the development of compact and reliable en/decoders (E/D) [7~12]. In coherent OCDMA, encoding and decoding are performed either in time domain or in spectral domain based on the phase and amplitude of optical field instead of its intensity.
 
Fig.1. Working principle of an OCDMA network

In coherent time spreading (TS) OCDMA, where the encoding/decoding are performed in time domain. In such a system, the encoding is to spread a short optical pulse in time with a phase shift pattern representing a specific OC. The decoding is to perform the convolution to the incoming OC using a decoder, which has an inverse phase shift pattern as the encoder and generates high level auto-correlation and low level cross-correlations.

2 SIMULATION SET-UP
The encoders use delay line arrays providing delays in terms of integer multiples of chip times. The placement of delay line arrays and the amount of each delay and phase shifts are dictated by the specific of the signatures. PSO matrix codes are constructed using a spanning ruler or optimum Golomb ruler is a (0,1) pulse sequence where the distances between any of the pulses is a non repeating integer, hence the distances between nearest neighbors, next nearest neighbors, etc., can be depicted as a difference triangle with unique integer entries. The ruler-to-matrix transformation increases the cardinality (code set size) from one (1) to four(4)and the ISD (=Cardinality/CD) from 1/26 to 4/32 = 1/8.The ISD translates to bit/s/Hz when the codes are associated with a data rate and the code dimension is translated into the bandwidth expansion associated with the codes as follows:


ISD   =((throughput))/((bandwidth required) )

        =((cardinality*data rate))/((1/Tb)(bandwidth expansion))

       =((n*r*R))/((R)(CD))

       =(n*r)/((CD))

The enhanced cardinality and ISD, while preserving the OOC property, are general results of the ruler-to-matrix transformation.

We can convert the PSO matrices to wavelength/time (W/T ) codes by associating the rows of the PSO matrices with wavelength (or frequency) and the columns with time-slots, as shown in Table I. The matricesM1.M32 are numbered 132 in the table, with the corresponding assignment of wavelengths and time-slots. For example, code M1 is (λ1 ; λ1 ; λ3; λ1 ) and M9 is ( λ1,λ4;0;λ7,λ8;0); here the semicolons separate the timeslots in the code. (The codes M1 and M9 are shown in bold numerals.)
We focus on codes like M1 because it shows extensive wavelength reuse, and on codes likeM9 because it shows extensive time-slot reuse. It is the extensive wavelength and time-slot reuse that gives these matrix codes their high cardinality and high potential ISD. Four mode-locked lasers are used to create a dense WDM multi-frequency light source. Pseudo-orthogonal (PSO) matrix codes [3] are popular for OCDMA applications primarily because they retain the correlation advantages of PSO linear sequences while reducing the need for bandwidth expansion. PSO matrix codes also generate a larger code set. An interesting variation is described in [1] where some of the
wavelength/time (W/T) matrix codes can permit extensive wavelength reuse and some can allow extensive time-slot reuse. In this example, an extensive time-slot reuse sequence is used for User 1 (λ1λ3;0;λ2λ4;0).There are four time slots used without any guard-band giving the chip period of 100 ps. Codeset for time spreading is mapped as C1:{0; λ2;0;λ4},C2:{λ1;0;λ3;0}.C8:{λ1; λ2;0;0}.Code set to apply binary phase shift mapped as M1:{ 0;1;0;1;} M2:{1;0;1;0;}.M8:{0;0;1;1;}.(1represents as a π phase shift,0 represents as no phase shift)

TABLE 3
SPE O-CDMA system parameters used for simulation
 
3 PROPOSED SPE O-CDMA SCHEME
1) Lasers (mode locked laser requited to produce 4
     wavelength signal)
2) Encoders consisting of required components like
    fiber delay lines, PRBS, External Modulator,
    multiplexers
3) Multiplexers
4) Optical fiber of 60 km length
5) De multiplexers
6) Decoders corresponding to each encoder
7) Receiver etc
8) BER analyzer
9) Eye Diagram analyzer
10) Signal analyzer

Read More: Click here...