JOURNAL OF INFORMATION AND COMMUNICATION TECHNOLOGIES, VOLUME 2, ISSUE 8, SEPTEMBER 2012 1

© 2012 JICT www.jict.co.uk

The Performance of Optical CDMA Access Network for Point to Multipoint Configuration

using Spectral Detection Technique M.M.Ismail, M.A.Othman, H.A.Sulaiman, M.H.Misran and M.A.Meor Said

Abstract—In this paper, we are proposing a detection scheme known as spectral direct detection technique implemented with Fiber Bragg Grating (FBG) act as encoder/decoder. This FBG based is used to encode and decode the spectral amplitude coding namely modified double weight (MDW) code in Optical Code Division Multiple Access (OCDMA). This code is used due to its flexibility where its weight can be any even number that greater than two. Moreover, it can maintain the cross-correlation parameter equal to one. The simulation will be carried out using OptiSystem version 7.0 and the performance is characterized through bit error rate (BER) and power received at various bit rate

Index Terms— Fiber Bragg Grating (FBG), Optical Code Division MuiltipleAcess (OCDMA), modified double weight (MDW) and bit error rate (BER) .

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1 INTRODUCTION

Multiple  access  techniques  are  required  to  meet  the  demand   for   high-­‐‑speed,   large-­‐‑capacity   communica-­‐‑tions   in   optical   networks,  which   allow  multiple   users  to   share   the   fiber   bandwidth   [1].Optic   code   division  multiple  access  (OCDMA)  scheme  is  considered  to  be  a  viable   multiple-­‐‑access   technique   for   future   all-­‐‑optical  networks.  This   is  mainly  due   to   the  availability  of  ex-­‐‑cess   bandwidth   in   a   fibre-­‐‑optic   medium   whereby  CDMA   technique   takes   advantage   to   construct   a   re-­‐‑dundant   signalling   scheme   in   order   to   establish   an  asynchronous   and   robust  multi-­‐‑access   system   for   the  number   of   users   [2].Besides,   his   technique   becoming  popular  because   their  advantages   such  as   the   flexibil-­‐‑ity   in   the   allocation   of   channels,   ability   to   operate  asynchronously,   enhanced   privacy   and   increased   ca-­‐‑pacity  in  bursty  networks.  

In  all  types  of  optical  code  division  multiple  access  (OCDMA)   systems,   spectral   amplitude-­‐‑coding   (SAC)  systems   have   received  more   attention   in   recent   years  since  multiuser   interference  can  be  eliminated  when  a  code  with  fixed  in  phase  cross  correlation  is  used  [3,  4].  The   elimination   for   MAI   occurs   at   the   receiver   side  depending  on  what  detection  technique  is  used.  In  op-­‐‑tical  CDMA  systems,   the  detection  process   affects   the  design  of  receiver  and  transmitter.  Basically,   there  are  two   types   of   detection   technique;   coherent   and   inco-­‐‑

herent.  The   coherent  detection   technique   refers   to  de-­‐‑tection  signal  with  knowledge  of  phase  information  of  the  carrier,  while  incoherent  detection  technique  is  re-­‐‑versely  from  coherent  detection  technique.  By  referring  to   incoherent  detection   technique,   this   technique  does  not   need   phase   synchronization.   Therefore,   it  will   re-­‐‑duce  the  hardware’s  complexity  of  the  systems.  Due  to  this   advantage,  we  prefer   to   choose   incoherent  detec-­‐‑tion   technique   rather   than   coherent   detection   tech-­‐‑nique  for  this  paper.  

In  general,   there  are  three  detection  techniques  for  SAC-­‐‑OCDMA   systems;   direct   detection   technique,  complementary   detection   technique   [5]   and   AND-­‐‑subtraction   technique   [6].  At   the   receiver   side,   the   se-­‐‑lection  of  the  detection  technique  depends  on  the  used  code  to  get  the  maximum  system  performance.In  opti-­‐‑cal   systems,   rectangular   filters   are  used   as   the   encod-­‐‑er/decoder   in   front   of   LED   in   order   to   narrow   down  the  linewidth  before  transmission  [7].  In  this  paper,  we  used   Fiber   Bragg   Grating   (FBG)   as   encoder/decoder  because  it  acts  as  a  perfect  filter.   In  FBG  based  encod-­‐‑er/decoder,  the  strain  and  temperature  that  introduced  to   the   grating   have   direct   effect   to   the   Bragg   wave-­‐‑length  [8-­‐‑10].  Thus,  the  value  of  the  Bragg  wavelength  of  the  FBG  will  be  shifted  by  these  two  elements.  Gen-­‐‑erally,   the   existing  Optical  CDMA  coding   scheme   are  using  an  arrayed  of  FBG  as  the  encoder  and  decoder  to  provide  an  all-­‐‑optical  signal  processing  to  the  systems  and  a  balanced  photo  detection  scheme  at  the  receiver  [11].  

———————————————— • M.M.Ismail, M.A.Othman, H.A.Sulaiman, M.H.Misran and M.A.Meor

Said are with Centre for Telecommunication Research and Innovation, Fakulti Kej. Elektronik dan Kej. Komputer, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.

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In   this   paper,   we   employed   spectral   direct   detec-­‐‑tion   technique  because  we  want   to  eliminate   the  mul-­‐‑tiple   access   interference   (MAI).   MAI   is   the   dominant  source  of  deterioration  in  OCDMA  systems;  thus  good  design  of  the  code  sequence  and  detection  scheme  [12-­‐‑13]   is   important   to   reduce   the   effect   of   MAI.   At   the  same  time  we  used  FBG  as  the  replacement  for  filter  as  encoder/decoder.   This   new   approach   is   to   reduce   the  complexity  of  the  systems,  improve  the  BER  and  at  the  same   time   decrease   the   number   of   FBG   as   encod-­‐‑er/decoder.   The   implementation   of   spectral   direct   de-­‐‑tection   technique   is   completely   different   from   AND  subtraction   detection   technique.   Figure   1   shows   the  implementation  of  spectral  direct  detection  technique.  

   

Figure  1:  Spectral  Direct  Detection  Technique  [14]  

2   EXPERIMENTAL  SETUP  

This   system   is   designed   and   simulated,   using  OptiSystem  Version  7.0  which  is  widely  used  for  opti-­‐‑cal   communication   system   simulations.   One   of   the  simulation  setup  is  shown  in  figure  2.  Each  chip  has  a  spectral  width   of   0.4nm.PIN  photodiodes   are   used   to  convert   the   optical   signals   into   electrical   signals.   The  nonlinear  effects  were  activated  according  to  the  typi-­‐‑cal  industry  values  to  simulate  the  real  environment  as  close   as   possible.   Table   1   show   the   parameter   values  used  for  the  simulation  of  the  OCDMA  system.  

 

   

Figure  2:  Layout  Design  of  OCDMA  using  Spectral  Direct  Detection  Technique  

 

 Parameter   Value  

Length  of  fibre   0  to  100km  Broad  band  source  power   10dBm  

Bit  rate   500  to  3000Mbps  Wavelength   1550nm  

Chromatic  dispersion  co-­‐‑efficient  

18ps/nm.km  

Dark  current   5nA  Polarization  mode  disper-­‐‑

sion  coefficient  0.5ps/km  

Attenuation   0.25dB/km  Cut  off  frequency   0.75  x  bit  rate  

 Table  1:  Simulation  Parameter  

 

5   RESULTS  AND  DISCUSSIONS  

Performance   of   the   system   is   checked   on   the   basis   of  BER,  fiber  length,  and  received  power.  Figure-­‐‑3  shows,  the  effect  of  transmission  length  on  BER.  For  this  pur-­‐‑pose   fibre   length   is   varied   from   10   to   100   Km,   other  parameters  like  source  power  (10dbm)  and  bitrate  (622  Mbps)  are  kept  constant.      

   

 Figure  3:  BER  vs  Transmission  Length  

 This  graph  shows  that  an  increase  in  fibre  length  caus-­‐‑es   increment   in  dispersion  of   the   input  signal,  so  BER  also  increases  with  distance.            Figure  4  shows  effect  of  Bit  rate  on  Received  power,  for   this   purpose   bit   rate   is   varied   from   500   to   3000  Mb/s  and  other  parameter   like   source  power   (10dbm)  and  Fiber  length  (10km)  are  kept  constant.        

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Figure  4:  Received  Power  vs  Bit  Rate    This   graph   shows   that   when   bit   rate   increases   pulse  width   decreases,   pulse   become   more   sensitive   to   the  dispersion,   dispersion   losses   increases   results   in   the  loss  of  power  so  received  power  decreases.            Figure  5  shows  the  effect  of  bit  rate  on  BER,  for  this  purpose  bit  rate  varied  from  100  to  1000  Mb/s  and  oth-­‐‑er  parameter  like  source  power  (10dbm)  and    Fiber  length  (10km)  kept  constant.      

   

Figure  5:  BER  vs  Bit  Rate    This   graph   shows   that   when   bit   rate   increases   then  BER  of  the  system  increase.  

7 CONCLUSION In conclusion, we successfully simulate the OCDMA system by using Optisystem software. Generally, per-formance of the OCDMA system decreases as the bit rate increases. This is due to effect of attenuation and dispersion in the fibre. In this paper, we have used spectral direct detection technique. Theoretically, for this detection scheme, the performance of OCDMA is better compared to AND subtraction detection tech-nique. This is because effect of MAI and PIN has been eliminated.

ACKNOWLEDGMENT The authors wish to thank Universiti Teknikal Malaysia

Melaka for financing this project and also to this journal. This work was supported in part by a grant from Univer-siti Teknikal Malaysia Short-Term Grant 2011.

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