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 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.
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
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.
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.