802.16/WiMAX

EECS 228a, Spring 2006Shyam Parekh

References• IEEE 802.16-2004 (802.16REVd)• IEEE 802.16-2005 (802.16e)• Intel’s Whitepapers, 2004 (

http://www.intel.com/technology/itj/2004/volume08issue03/)• “IEEE Standard 802.16: A Technical Overview of the

WirelessMAN Air Interface for Broadband Wireless Access,” C. Eklund et al., IEEE Communication Magazine, June 2002

• “Broadband Wireless Access with 802.16/WiMax: Current Performance Benchmarks and Future Potential,” A. Ghosh et al., IEEE Communication Magazine, Feb 2005

• “Wireless Communication Standards: A Study of IEEE 802.11, 802.15, and 802.16,” T. Cooklev, 2004

Electromagnetic Spectrum

Source: LBL

802.16 Standards History

802.16a(Jan 2003)

• Extension for 2-11 GHz: Targeted for non-line-of-sight, Point-to-Multi-Point applications like “last mile” broadband access

802.16(Dec 2001)

• Original fixed wireless broadband air Interface for 10 – 66 GHz: Line-of-sight only, Point-to-Multi-Point applications

802.16c(2002)

802.16 AmendmentWiMAX System Profiles

10 - 66 GHz

802.16REVd (802.16-2004)

(Oct 2004)

• Adds WiMAX System Profiles and Errata for 2-11 GHz

802.16e(802.16-2005)

(Dec 2005)

• MAC/PHY Enhancements to support subscribers moving at vehicular speeds

• First standard based on proprietary implementations of DOCSIS/HFC architecture in wireless domain

Applications of 802.16 Standards

802.16 Network Architecture

802.16 Network Architecture (2)

Scope of 802.16 Standards

Physical Layer SummaryDesignation Applicability MAC Duplexing

WirelessMAN-SC 10-66 GHz Licensed Basic TDD, FDD, HFDD

WirelessMAN-SC 2-11 GHz Licensed Basic, (ARQ), (STC), (AAS)

TDD, FDD

WirelessMAN-OFDM

2-11 GHz Licensed Basic, (ARQ), (STC), (AAS)

TDD, FDD

2-11 GHz License-exempt

Basic, (ARQ), (STC), (DFS), (MSH), (AAS)

TDD

WirelessMAN-OFDMA

2-11 GHz Licensed Basic, (ARQ), (STC), (AAS)

TDD, FDD

2-11 GHz License-exempt

Basic, (ARQ), (STC), (DFS), (MSH), (AAS)

TDD

Channel Characteristics

• 10-66 GHz– Very weak multipath components (LOS is

required)– Rain attenuation is a major issue– Single-carrier PHY

• 2-11 GHz– Multipath– NLOS– Single and multi-carrier PHYs

Wireless Performance(as of 2003)

Source: S. Viswanathan, Intel

OFDMA Subchannels

• A subset of subcarriers is grouped together to form a subchannel• A transmitter is assigned one or more subchannels in DL direction (16 subchannels are supported in UL in OFDM PHY)• Subchannels provide interference averaging benefits for aggressive frequency reuse systems

OFDM Basics

Orthogonal Subcarriers

Cyclic Prefix in Frequency Domain Cyclic Prefix in Time Domain

Equalizers are avoided in OFDM

time

Cyclic Prefix Useful Symbol Time

time

Note: All signals & multipath over a useful symbol time are from the same symbol & add constructively (no ISI)

Note: dashed lines represent multipath

Narrow bandwidth long symbol times all significant multipaths arrive within a symbol time minimizing ISI no equalization low complexity

Tx Signal

Rx Signal

Source: Lucent

Tradeoffs of FFT size

• The FFT size determines the number of sub-carriers in the specified bandwidth

• Larger FFT sizes lead to narrower subcarriers and smaller inter-subcarrier spacing–More susceptibility to ICI, particularly in high Doppler (Note: Doppler shift for 125 km/hr for operation at 3.5

GHz is v/λ = 35 m/sec/0.086 m = 408 Hz)–Narrower subcarriers lead to longer symbol times

less susceptibility to delay spread

• Smaller FFT sizes the opposite is true

Source: Lucent

OFDMA Scalability

• Supports s wide range of frame sizes (2-20 ms)Source: Intel “Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN”

Time Division Duplexing (TDD)

General Downlink Frame Structure

• Downlink Interval Usage Code (DIUC) indicates burst profile

General Uplink Frame Structure

• Uplink Interval Usage Code (UIUC) indicates burst profile

OFDMA TDD Frame Structure

• DL-MAP and UL-MAP indicate the current frame structure• BS periodically broadcasts Downlink Channel Descriptor (DCD) and Uplink Channel Descriptor (UCD) messages to indicate burst profiles (modulation and FEC schemes)

Frame Structure – Another View

Network Entry Process

SDU and PDU

Connections

• 802.16/WiMAX is connection oriented

• For each direction, a connection identified with a 16 bit CID

• Each CID is associated with a Service Flow ID (SFID) that determines the QoS parameters for that CID

PDU Transmission

Source: R. Marks (NIST) IEEE Presentation

QoS Mechanism

Generic MAC Frame

Generic MAC Header

Generic Bandwidth Request

Management Messages

• Management messages are broadcast or sent on three CIDs in each direction: Basic, Primary, and Secondary– Uplink Channel Descriptor– Downlink Channel Descriptor– UL-MAP– DL-MAP– DSA-REQ– DSA-RSP

Key Management Messages (1)

Key Management Messages (2)

Scheduling Types and QoSScheduling Type Parameters

Unsolicited Grant Service (UGS)

Max Sustained Traffic Rate, Maximum Latency,

Tolerated Jitter

Real-Time Polling Service (rtPS)

Max Sustained Traffic Rate, Min Reserved Traffic Rate, Committed Burst Size, Maximum Latency, etc.

Non-real-time Polling Service (nrtPS)

Committed Information Rate, Maximum Information Rate

Best Effort (BE) Maximum Information Rate

• Extended rtPS was introduced in 802.16e that combines UGS and rtPS: This has periodic unsolicited grants, but the grant size can be changed by request

Scheduling Classes

Advanced 802.16 Features

• Multiple Input and Multiple Output (MIMO)– MIMO channel capacity is given by

C = B log2 det(I + SNR.HH*T/N) where H is MxN channel matrix with M and N are

receive and transmit antennas, resp.

• Hybrid-ARQ– For faster ARQ, combines error correction and

detection and makes use of previously received versions of a frame

• Adaptive Antenna System (AAS)– Enables directed beams between BS and SSs

WiBro (Wireless Broadband)

• WiBro is an early large-scale deployment of 802.16 in South Korea (Dec 2005)

• Demonstrates 802.16 performance as compared to 3G/4G cellular alternatives

• 3 operators have been licensed by the government (each spending ~$1B)

WiMAX Opportunities

• There is a work opportunity to create/enhance 802.16/WiMAX network level simulation– Contact sparekh@lucent.com

• Technical contributions characterizing 802.16 performance and network capacity are much needed