Chapter 5 TDMA FDMA CDMA OFDMA Reading Assignment Solution

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Text of Chapter 5 TDMA FDMA CDMA OFDMA Reading Assignment Solution

  • Orthogonal Frequency Division Multiple Access (OFDMA)

    Zeeshan Asim


  • 2


    1. Background

    Multiple Access (MA) Methods

    2. Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)

    Orthogonality Principle



    3. Advantages and Disadvantages of OFDMA

    4. Conclusion

  • 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


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    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 terminals

    initial access process IEEE 802.11 uses CSMA/CA based contention access


  • 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)


    1. GSM (TDMA) 2. IS-95 (CDMA) 3. American Mobile Phone System, AMPS


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    Time Division Multiple Access (TDMA)


    Time slot 0.577 ms

    Frame 4.6 ms

    8 time slots per frame

    Frequency band 20 KHz

  • Code Division Multiple Access (CDMA)


    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

  • Objectives

    What is OFDM

    How OFDM works

    Types of OFDM

    Differences from other multiplexing techniques


  • What is OFDM


    The first OFDM schemes presented in 1966

    Development of OFDM over years

  • A


  • How OFDM works

    It distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other

    than their own.

  • Types of OFDM






  • C-OFDM

    Coded OFDM

    Digital Audio Broadcasting (DAB)

    Digital Video Broadcasting (DVB-T)

    COFDM offers real benefit in the presence of isolated narrow-band interfering signals

  • Multiple Input, Multiple Output OFDM (MIMO-OFDM)

    Developed by Iospan Wireless

    Uses multiple antennas to transmit and receive radio signals

    Spatial multiplexing

  • V-OFDM

    Vector OFDM

    Developed by CISCO

    Increases subscriber coverage

    Lowers the cost of provisioning and deploying infrastructure

    Employs both frequency and spatial diversity

    Creates a robust processing technique for multi-path fading and narrow band interference

  • Subscriber

    Base Station


  • W-OFDM

    Wideband OFDM

    Invented by Wi-LAN

    Large spacing between carriers

    Advantages: - Optimal performance against Multi-path

    - Less sensitive to carrier offset

    -Optimal power efficiency of the transmitter amplifier

    - More immune against fading

  • Flash-OFDM

    Fast-hopped OFDM

    Wide-band spread-spectrum technology

    Avoids the compromises inherent in other mobile data systems

    Capability to work around interfering signals

  • Differences from other multiplexing techniques

    OFDM versus WDM

    -more flexible

    -higher bandwidth efficiency

  • Differences from other multiplexing techniques

    OFDM versus TDM

    -number of carriers



    -capacity/efficiency advantages

    -complexity and cost issues

  • Applications

    Digital Audio and Video Broadcasting

    Asymmetric Digital Subscriber Line (ADSL)

    Wireless Networking

    Power-line Technology


    Each terminal occupies a subset of sub-carriers

    Subset is called an OFDMA traffic channel

    Each traffic channel is assigned exclusively to one user at any time






    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)

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    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

  • 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

    Peak to average power ratio (PAPR)





    The large amplitude variation increases in-band noise and

    increases the BER when the signal has to go through amplifier


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    Disadvantages of OFDMA cont..


    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

  • Thank You Questions