Physical Layer (PHY)

What is WiMAX?

WiMAX = Worldwide Interoperability for Microwave Access

Think of it as WiFi’s long-range cousin:

• WiFi: ~100m range, best effort, unlicensed spectrum
• WiMAX: several km range, QoS support, licensed spectrum

WiMAX was designed for metropolitan area networks: city-wide coverage from a single base station.

IEEE 802.16e

The 802.16e standard added mobility support to WiMAX.

Key features:

• Mobile users (not just fixed)
• Handover between base stations
• Power saving modes

This made WiMAX a competitor to cellular (3G/4G), though LTE eventually won.

OFDMA-Based PHY

WiMAX uses OFDMA for its physical layer, the same technology later adopted by LTE.

How it works:

• Spectrum divided into many subcarriers
• Subcarriers grouped into subchannels
• Different subchannels assigned to different users

WiMAX was one of the first mobile broadband systems to use OFDMA.

Scalable Parameters

WiMAX PHY is scalable, adapting to available bandwidth:

Subcarrier spacing stays constant. More bandwidth = more subcarriers, not wider subcarriers.

Frame Structure

WiMAX transmission is organized into frames (typically 5ms).

Each frame contains:

• Downlink (DL) subframe: base station → users
• Uplink (UL) subframe: users → base station

The frame is a 2D grid:

• Horizontal axis: Time (OFDM symbols)
• Vertical axis: Frequency (subchannels)

Frame Components

Preamble:

• First symbol of the frame
• Used for synchronization and channel estimation

FCH (Frame Control Header):

• Contains DL-MAP location info
• Tells users where to find their data

DL-MAP / UL-MAP:

• Maps that tell each user which time-frequency slots are theirs
• DL-MAP: describes downlink allocations
• UL-MAP: describes uplink allocations

Bursts:

• Actual user data
• Each burst = one user’s allocation
• Can use different modulation (QPSK, 16-QAM, 64-QAM)

Subchannelization Modes

WiMAX offers different ways to group subcarriers into subchannels:

PUSC (Partial Usage of Subchannels):

• Subcarriers distributed across the band
• Provides frequency diversity
• Good for mobile users

FUSC (Full Usage of Subchannels):

• All subcarriers used
• Maximum diversity
• Downlink only

AMC (Adaptive Modulation and Coding):

• Adjacent subcarriers grouped together
• Allows channel-aware scheduling
• Good for stationary users with strong signals

PUSC is most common. It handles mobility well by spreading each user’s data across the spectrum.

Adaptive Modulation

WiMAX adapts modulation based on channel conditions:

Better signal = higher modulation = more throughput. The system constantly adapts.

Duplexing

WiMAX supports both:

TDD (Time Division Duplex):

• DL and UL share the same frequency
• Take turns in time
• More flexible for asymmetric traffic
• Most common in WiMAX

FDD (Frequency Division Duplex):

• DL and UL use different frequencies
• Simultaneous transmission
• Requires paired spectrum

MIMO Support

WiMAX 802.16e supports MIMO:

• 2x2 and 4x4 configurations
• Spatial multiplexing for throughput
• Space-time coding for reliability

MIMO in WiMAX works the same way as in WiFi and LTE: multiple antennas, multiple streams.

Summary