Wimax ofdma
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Wimax ofdma



basic for wimax study

basic for wimax study



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Wimax ofdma Wimax ofdma Presentation Transcript

  • Basics for WI-MAX Binoy Tomy NexGTech Wireless R & D [email_address] +91 9952170483
    • WiMAX air interface is based on OFDM/OFDMA physical layer (PHY).
    • To understand how OFDM and OFDMA work, lets start with its "mother" namely FDM (Frequency Division Multiplexing).
    • Like OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub-carriers are divided into groups of sub-carriers.
    • Each group is named a sub-channel.
    • The sub-carriers that form a sub-channel need not be adjacent.
    • 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.
  • Sub-channelization
    • Sub-channelization defines sub-channels that can be allocated to subscriber stations (SSs) depending on their channel conditions and data requirements.
    • 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.
    • 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.
    • This power-saving feature is particularly useful for battery-powered user devices, the likely case in Mobile WiMAX.
  • Cyclic Prefix
    • To avoid ISI and ICI
    • A copy of last part of the OFDM symbol is prepended to the T.Xed symbols and removed at the receiver
    • Data subcarriers for data transmission.
    • Pilot subcarriers for various estimation and synchronization purposes.
    • Null subcarriers for no transmission at all, used for guard bands and DC carriers.
    Type of OFDMA Subcarrier
  • Advantages
    • Effective against ISI and multipath fading
    • Can adjust modulation and coding for each carriers
    • Better spectral efficiency
    • Low complexity modulation that can easily be implemented using IFFT/FFT
    • SOFDMA (S-OFDMA) adds scalability to OFDMA.
    • Scales the FFT size.
    • Smaller FFT size is given to lower bandwidth channels, while larger FFT size to wider channels.
    • OFDMA mode used in IEEE 802.16e.
    • Bandwidth : 1.25 MHz to 20 MHz.
    • Vary the number of subcarriers and control transmission rates and the number of individual users simultaneously.
    • Subcarrier Spacing.
    • Number of Subcarrier scales with Bandwidth.
    • Based on the concept of Subchannel, the smallest unit of Bandwidth allocation is fixed and independent of Bandwidth.
    • Capacity of each individual subchannel remains constant.
  • 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.