Orthogonal Frequency Division Multiple Access by hyk52398

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									Orthogonal Frequency Division
       Multiple Access
         (OFDMA)


      Khaja Mohammad Shazzad
Outline

1.   Background
        Multiple Access (MA) Methods
2.   Orthogonal Frequency Division Multiplexing
     (OFDM) Based Multiple Access (OFDMA)
        Orthogonality Principle
        OFDM
        OFDM-FDMA
3.   Advantages and Disadvantages of OFDMA
4.   Conclusion
                            2
Multiple Access (MA)

   General wireless cellular systems are multi-users
    systems
   Radio resource are limited
    –   Limited Bandwidth
    –   Limited number of channels
   The radio resource must be shared among multiple
    users
   Multiple Access Control (MAC) needed
    –   Contention-based
    –   Non-contention-based
Contention-based
Multiple Access(MA)

   Contention-based
    –   Each terminal transmits in a decentralized way
    –   No central controller (Base stations or access points)
    –   Example:
            ALOHA
            Carrier Sensing Multiple Access (CSMA)
        Standard:
    –   GSM [l] uses the slotted ALOHA in the terminal’s initial
        access process
    –   IEEE 802.11 uses CSMA/CA based contention access
        scheme

                                 4
Non-contention-based
Multiple Access (MA)

    A logic controller (BS or AP) is needed to
     coordinate the transmissions of all the terminals
    The controller informs each device when and on
     which channel it can transmit
    Collisions can be avoided entirely
    Two Subdivisions
    1. Non-channelization
    2. Channelization
Non-channelization
Non-contention-based MA

   Terminals transmit sequentially using the same
    channel
   Example:
    –   Polling based medium access
   Standard:
    –   IEEE 802.15(WPAN)
    –   IEEE 802.11(WLAN)
Channelization
Non-contention-based MA

   Terminals transmit simultaneously using different
    channels
   Most commonly used protocols in cellular systems
   Example:
     – 1. Time Division Multiple Access (TDMA)
     – 2. Code Division Multiple Access( CDMA)
     – 1. Frequency Division Multiple Access (FDMA)
   Standard
     – 1. GSM (TDMA)
     – 2. IS-95 (CDMA)
     – 3. American Mobile Phone System, AMPS (FDMA)
Time Division Multiple Access
(TDMA)

   GSM
    –   Time slot 0.577 ms
    –   Frame 4.6 ms
    –   8 time slots per frame
    –   Frequency band 20 KHz




                             8
Code Division Multiple Access
(CDMA)

   IS-95
    –   Orthogonal Walsh codes
    –   64 codes (channels)
    –   One pilot channel
    –   Seven paging channels
    –   55 traffic channels
    –   Each carrier 1.25 MHz
Frequency Division Multiple Access
(FDMA)

   American Mobile Phone
    System (AMPS)
    –   Total Bandwidth 25 MHz
    –   Each Channel 30 KHz
Orthogonal Frequency Division
Multiplexing (OFDM) Based Multiple
Access (OFDMA)

 Orthogonality Principle
 OFDM
 OFDM-FDMA
Orthogonality Principle

   Vector space
    –   A, B and C vectors in
        space are orthogonal to
        each other                     B
    –   A.B=B.C=C.A=0
    –   (A+B+C).A=(mod A)^2
    –   (A+B+C).B=(mod B)^2
                                           A
    –   (A+B+C).C=(mod C)^2
                                   C



                              12
Orthogonality Principle cont..

   Real Function space

     f1 (t )  A sin( wt )
     f 2 (t )  B cos(wt )
      T
          f1 (t ) f 2 (t ) dt  0
     f m (t )  M sin( mwt )
     f n (t )  N cos(nwt )
      T
          f m (t ) f n (t ) dt  0
Orthogonality Principle cont..

                                m.n Ν
                                T
                                 sin(mwt)sin(nwt)dt  0 where m  n
                                0
                                T
                                 sin(mwt)cos(nwt)dt  0
 f (t )  sin(wt ) sin(2wt )    0

                               Here mw and nw are called
                               m-th and n-th harmonics of
                               w respectively
Orthogonal Frequency Division
Multiplexing(OFDM)

   It is a special kind of FDM
   The spacing between carriers are such that
    they are orthogonal to one another
   Therefore no need of guard band between
    carriers.
   One example makes the thing clear



                      15
Example of OFDM

   Lets we have following information bits
    –   1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …
   Just converts the serials bits to parallel bits
           C1           C2            C3            C4

            1            1            -1            -1
            1            1             1            -1
            1            -1           -1            -1
           -1            1            -1            -1
           -1            1             1            -1
           -1            -1            1            1
 Example of OFDM cont..

Modulate each column with corresponding sub-carrier using BPSK




  Modulated signal for C1          Modulated signal for C2




  Modulated signal for C3          Modulated signal for C4
Example of OFDM cont..

   Final OFDM Signal = Sum of all signal
                                             V(t)
    V (t )   I (t ) sin( 2nt )
            N 1

                   n
            n 0




               Generated OFDM signal, V(t)
OFDM-FDMA (OFDMA)

   Each terminal occupies
    a subset of sub-carriers
   Subset is called an        user4
    OFDMA traffic channel      user3
   Each traffic channel is
                               user2
    assigned exclusively to
    one user at any time       user1
OFDM-FDMA (OFDMA)

   The IEEE 802.16e/ WiMax use OFDMA as
    Multiple access technique
    –   Bandwidth options 1.25, 5, 10, or 20 MHz
    –   Entire bandwidth divided into 128, 512, 1024 or
        2048 sub carriers
    –   20 MHz bandwidth with 2048 sub carriers has 9.8
        KHz spacing between sub carriers
OFDM-FDMA (System View)
Advantages of OFDMA

   Multi-user Diversity
    –   broadband signals experience frequency selective
        fading
    –   OFDMA allows different users to transmit over
        different portions of the broadband spectrum (traffic
        channel)
    –   Different users perceive different channel qualities,
        a deep faded channel for one user may still be
        favorable to others

                            22
Advantages of OFDMA cont..




                     Multi-user Diversity
Advantages of OFDMA cont..

   Efficient use of Spectrum

                                4/3 Hz per symbol




                                6/5 Hz per symbol
Advantages of OFDMA cont..

   Receiver Simplicity
     – It eliminates the intra-cell interference avoiding
        CDMA type of multi-user detection
     – Orthogonality of code destroyed by selective
        fading
     – Only FFT processor is required
   Bit Error Rate performance is better only in Fading
    environment
Disadvantages of OFDMA
                                                       2
   Peak to average power              x(t )
    ratio (PAPR)                PAPR 
                                        P        avg




The large amplitude variation increases in-band noise and
increases the BER when the signal has to go through amplifier
nonlinearities.
Disadvantages of OFDMA cont..

   Synchronization
     – Tight Synchronization between users are
       required for FFT in receiver
     – Pilot signals are used for synchronizations
   Co-channel interference
    –   Dealing with this is more complex in OFDM than in CDMA
    –   Dynamic channel allocation with advanced
        coordination among adjacent base stations

                             27
Research issues and Conclusion

   Future works
    –   Peak-to-average power reduction in OFDM
    –   Timing and Frequency Synchronization
    –   Efficient digital signal processing Implementation of OFDM
    –   Multiple input/Multiple output (MIMO) OFDM
   Conclusion
    –   Different variations of OFDMA are proposed and
        have different pros and cons


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Thank You

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