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									Role of Optical CDMA in Access
            Sen Zhang
      EECS 864    Spring 2003

    Three multiple access approaches:
•   TDMA
     – Assign different time slots to different user to share the same
       physical medium
•   WDMA
     – Assign different wavelengths or frequency to different user
•   CDMA
     – Assign different codeword or signature sequence to different
       user to sha2re the same physical medium

    Items used in the Optical CDMA
•   Tb : Signal bit period
•   Each bit is divided up into n time periods, called chips.
•   Tc : period of chip, Tb =n Tc
•   Codeword or signature sequence :address codeword multiplied with
    signal bit sequences to uniquely distinct different user
•   n: length of codeword
•   w: Hamming weight: total number of illuminated chips in the address
•   MAI (Multiple Access Interference) : the crosscorrelation between
    different users sharing a common fiber channel.
•   E.g. Two optical codewords with length n=32 and weight w=4

                                                       Reference [2]
    A Typical Optical CDMA System
•  For a local access network, the
   N*N star is replaced by a 1*N
• For a Rx of node i, the received
   information can be expressed as
   following. Where Si(t) is the
   signature code assigned to node
   i, Ui(t) is the information sent to
   node i.
• We hope MAI is as small as
   possible and autocorrelation of
                                                     Reference [2]
   signature code is as large as
  Tb                                Tb                            Tb
  ∫ Si (t)×∑U j(t)×S j(t) =Ui (t)× ∫ Si (t)×Si (t) + ∑U j(t)× ∫ Si (t)×S j (t)
  0          j
                                     14 244 j≠i
                                       4       3                   0
                                                                  14 244   3
                                       Autocorrel                    MAI         4
           A CDMA-PON System (e.g.)
•   This is a proposed CDMA-PON
    System in ref [5].
•   Both forward link and reverse link
    use PN ⊕ Walsh code
•   PN code is used for timing
•   Walsh code for channel identification
•   A protocol control channel is used to
    compensate for the different lengths
    among channels for the reverse link.
•   Receivers at OLT check the condition
    of synchronization and power from
    each ONU and send this information
    back to each ONU through the
    protocol channel
•   Each ONU align its timing according
    to the protocol channel
   Issues in Optical CDMA network
• Near-far effect
   – There is no near-far effect in forward link (from OLT at
     center office to ONUs at homes)
   – In reverse link, some user’s information may suffer
     larger attenuation, thus its signal may not be extracted
     from the MAI noise at center office.
• Dispersion compensation and nonlinear effects combat
   – Ultra-short optical pulse is sensitive to dispersion and nonlinear

   Issues in Optical CDMA network
• Adding/dropping of a user will influence the QoS of other
  active users
• Design of Codeword or signature sequence is most
  important in determining the performance of the Optical
  CDMA network
   – Unipolar character of codeword makes it difficult to
      achieve real Optical Orthogonal (cross-correlation is 0)
   – Signature sequences determine the:
       • Number of active users because of MAI
       • Spectral efficiency
   Designing of Optical Codewords
• For any codeword the non-shifted autocorrelation (or
  Hamming Weight) should be made as large as possible.
• For any codeword the shifted autocorrelation must be
  much less than the Hamming Weight.
• The cross-correlation between any pair of codewords must
  be small.
• The size of the set of all allowed codewords (or cardinality
  of the code) must meet or exceed the number of network
  members to be supported.
• Two kinds of codewords : Optical Orthogonal Codes
  (OOCs) & Higer-dimensional Optical Orthogonal
     Optical Orthogonal Codes (e.g.)
•   Commonly encoder sends
    codeword for bit ‘1’ and all-
    zero sequence for bit ‘0’
•   To maximize the number of
    users, the length of OOC n
    should be large and weight w
    should be small
     – Energy per encoded bit is low
       and may compromise the
       overall power budget
     – Very high speed electrical and
       optical equipments needed to
       generate ultra-short optical
                                        Reference [4]

    Higher-Dimensional Optical Codes
     – Every signature sequence is reusable and is sent simultaneously at different
     – Requires a centralized network controller to distribute wavelength channels
       uniformly to the transmitting nodes. So it’s not wavelength efficient.
•   Multiple-wavelength OOCs (MWOOCs)
     – Spread the optical orthogonal codes in both the temporal and wavelength domains
     – MWOOCs can support a much larger number of users with lower probability.
     – You need a tunable single wavelength laser or a bank of fixed-wave-length lasers
       or a broadband source spectrally sliced to provide all the required wavelengths
     – Equal weight for all codewords
•   Multiple-weitght two-dimensional codes
     – Variation of MWOOCs. It can have different weight for codewords. It’s useful for
       providing different QoS levels at the physical layer.

Multiple-wavelength Optical CDMA
•   An active researching area in
    Optical CDMA
•   A single pulse per row in the code
•   When each column of the code
    matrix (chip slot) is limited to one
    pulse, the transmitted code
    waveform is always on and appears
    as a form of wavelength-hopped
•   Each column of the code matrix can
                                           Reference [1]
    also have no pulse

MWOOCs (e.g.)

 Reference [3]
E.g. MWOOCs (Cont.)

    Reference [3]

    QoS Implementation in an Optical
           CDMA network
•   A centralized network control to dynamically allocate new codes to users and monitor
    the CDMA encoders.
•   A distributed network control can be allowed if using an advanced code generation
    algorithm. In each node of the network, the level of MAI of the network is monitored
    and code selection logic will direct the encoder to encode the data source with the
    appropriate signature code.
•   Code selection logic
     –   Multiple codewords to priority service
     –   Different length of codeword to services
     –   Different weight of codeword to services

                      Structure of dynamic encoder from reference [1]

• Fairness: All users share the optical bandwidth in a fair
  way. The active users can be much larger than the number
  of wavelengths.
• Flexibility: codewords give the designers the easy way to
  find a balance between the BER performance and the
• TDMA requires centralized control to allocate time slots
  and maintain synchronous operation
• WDMA in LANs requires a significant amount of dynamic
  coordination between nodes
• Optical CDMA can operate asynchronously, without
  centralized control, and doesn’t suffer from packet
• Optical CDMA provide differentiated service at
  the physical layer, unlike TDMA or WDMA
  where QoS is usually handled in the higher OSI
  levels such as the data link or network layer
• Security: codewords give out an advantage of
  inherent security. It’s nearly impossible to find out
  the codeword used without any back ground

    Technological Barriers on the Road
             of Realization
•   Cumulative shot and beat noise are the major physical channel
    impairments that limit the performance of Optical CDMA.
     – The shot noise builds as the square root of the received optical power,
       proportional to the number of users.
     – The Optical beat noise can be canceled through clever control of the
       optical phase coherence
•   The combination of optical CDMA with FEC (Forward error
    correction) may dramatically improve the performance.
     – It’s expensive and impractical to implement FEC in electrical domain at
       very high speed.
     – Implementing FEC encoding/decoding using optical processing is
       desirable but relys on the realization of those optical processing such as
       addition, multiplexing, demultiplexing, Fourier transforms etc.

 Technological Barriers on the Road
          of Realization
• Cost barriers
   – Encoding and Decoding hardware usually use fiber Bragg gratings
     (FBG) , Arrayed waveguide Grating (AWG) or Multiple
     Frequency Laser (MFL). Those equipment are expensive.
   – Broadband light source is expensive.
   – The promise of integration
• Perception Barriers
   – The “inefficient’ use of spectrum by O-CDMA is really an attempt
     to perform processing functions in the optical, rather than
     electrical, domain.

[1] The role of optical CDMA in Access Networks, Andrew Stock et.
    IEEE Communications Magazine Sep. 2002
[2] Lighting the Local Area: Optical Code-Division Multple Access and
    Quality of Service Provisioning, Andrew Stok et. IEEE Network,
    Nov./Dec. 2000
[3] Strategies for realizing Optical CDMA for dense, high-speed, long
    span, optical network applications, Antonio J. Mendez et. Journal of
    lightwave technology, vol. 18, No.12 Dec. 2000
[4] Comparison of OTDMA and Synchronous OCDMA with Optical and
    Electrical Decoding, Robert Fritsch, Joachim Speidel
[5] A symmetric-Structure CDMA-PON System and its implementation,
    Byung-gu Ahn et. IEEE photonics technology letters, vol. 14. NO. 9
    Sep, 2002


• 1. Could u give an example of MWOOC
  codewords? Explain their autocorrelation
  and crosscorrelation.
• 2. Is there near-far effect in forward link or
  reverse link? Why?


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