Wimax ofdma
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  • 1. Basics for WI-MAX Binoy Tomy NexGTech Wireless R & D [email_address] +91 9952170483
  • 2. INTRO
    • WiMAX air interface is based on OFDM/OFDMA physical layer (PHY).
    • 3. To understand how OFDM and OFDMA work, lets start with its "mother" namely FDM (Frequency Division Multiplexing).
  • 4. OFDMA
    • Like OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub-carriers are divided into groups of sub-carriers.
    • 5. Each group is named a sub-channel.
    • 6. The sub-carriers that form a sub-channel need not be adjacent.
    • 7. In the downlink, a sub-channel may be intended for different receivers. In the uplink, a transmitter may be assigned one or more sub-channels.
  • 8. Sub-channelization
    • Sub-channelization defines sub-channels that can be allocated to subscriber stations (SSs) depending on their channel conditions and data requirements.
    • 9. Using sub-channelization, within the same time slot a Mobile WiMAX Base Station (BS) can allocate more transmit power to user devices (SSs) with lower SNR (Signal-to-Noise Ratio), and less power to user devices with higher SNR.
  • 10.
    • Sub-channelization in the up-link can save a user device transmit power because it can concentrate power only on certain sub-channel(s) allocated to it.
    • 11. This power-saving feature is particularly useful for battery-powered user devices, the likely case in Mobile WiMAX.
  • 12. Cyclic Prefix
    • To avoid ISI and ICI
    • 13. A copy of last part of the OFDM symbol is prepended to the T.Xed symbols and removed at the receiver
  • 14.
    • Data subcarriers for data transmission.
    • 15. Pilot subcarriers for various estimation and synchronization purposes.
    • 16. Null subcarriers for no transmission at all, used for guard bands and DC carriers.
    Type of OFDMA Subcarrier
  • 17. Advantages
    • Effective against ISI and multipath fading
    • 18. Can adjust modulation and coding for each carriers
    • 19. Better spectral efficiency
    • 20. Low complexity modulation that can easily be implemented using IFFT/FFT
  • 21. SOFDMA
    • SOFDMA (S-OFDMA) adds scalability to OFDMA.
    • 22. Scales the FFT size.
    • 23. Smaller FFT size is given to lower bandwidth channels, while larger FFT size to wider channels.
    • 24. OFDMA mode used in IEEE 802.16e.
    • 25. Bandwidth : 1.25 MHz to 20 MHz.
    • 26. Vary the number of subcarriers and control transmission rates and the number of individual users simultaneously.
  • 27. PRINCIPLE
    • Subcarrier Spacing.
    • 28. Number of Subcarrier scales with Bandwidth.
    • 29. Based on the concept of Subchannel, the smallest unit of Bandwidth allocation is fixed and independent of Bandwidth.
    • 30. Capacity of each individual subchannel remains constant.
  • 31. Features
      1, Advanced Modulation & Coding. 2, Hybrid Automatic Repeat Request. 3, High-efficiency uplink subchannels structures. 4, Multiple input Multiple output diversity. 5, Inter operability.
  • 32.  
  • 33. PARAMETERS