Basics for WI-MAX
Binoy TomyNexGTech Wireless R & [email protected]+91
9952170483
INTRO
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).
OFDMA
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
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.
PRINCIPLE
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.
PARAMETERS
![Topology control and Interference aware Resource ... · for Quality of Service (QoS) and the use of Scalable OFDMA, and is sometimes called “Mobile WiMAX” [4]. The 802.16 standard](https://img.pdfslide.us/doc/110x75/601d591238bc4e5ec7346bf9/topology-control-and-interference-aware-resource-for-quality-of-service-qos.jpg)
