ECE537/6 #1Spring 2009© 2000-2009, Richard A. Stanley

ECE537 Advanced and High Performance Networks

6: WiMAX and More

Professor Richard A. Stanley, P.E.

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Overview of Tonight’s Class

• Student presentations/discussions on wireless network extensions

• Review of last time

• Issues in mobile networking implementations

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Last time…

• Wireless networking is growing rapidly in importance

• There are many “special” considerations for wireless networking

• Unlike most wired networking, physical layer effects play a large in proper design of a network and its protocols

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Summary

• Wireless networking adds many demands to both the design of network physical layer elements and to protocols

• Increasing demand for wireless networking will likely stretch our ability to provide robust networking that compares favorably with wired systems

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WiMAX Introduction

• Goal: Provide high-speed Internet access to home and business subscribers, without wires.

• Base stations (BS) can handle thousands of subscriber stations (SS)

• Access control prevents collisions.• Supports

– Legacy voice systems– Voice over IP– TCP/IP– Applications with different QoS requirements

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Some Possible Uses

• Connecting Wi-Fi hotspots to the Internet• Wireless alternative to cable and DSL for

“last mile” broadband access• Providing telecommunications services • Source of network connectivity as part of a

business continuity plan or other survivability scheme

• Providing portable connectivity

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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

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Applications of 802.16 Standards

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802.16 Network Architecture

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802.16 Network Architecture (2)

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Scope of 802.16 Standards

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IEEE 802.16 Protocol Standards (1)

Standard Description Status

802.16-2001 Fixed Broadband Wireless Access (10–63 GHz) Superseded

802.16.2-2001 Recommended practice for coexistence Superseded

802.16c-2002 System profiles for 10–63 GHz Superseded

802.16a-2003 Physical layer and MAC definitions for 2–11 GHz Superseded

P802.16b License-exempt frequencies (Project withdrawn) Withdrawn

P802.16d Maintenance and System profiles for 2–11 GHz(Project merged into 802.16-2004)

Merged

802.16-2004 Air Interface for Fixed Broadband Wireless Access System(rollup of 802.16-2001, 802.16a, 802.16c and P802.16d)

Superseded

P802.16.2a Coexistence with 2–11 GHz and 23.5–43.5 GHz(Project merged into 802.16.2-2004)

Merged

802.16.2-2004 Recommended practice for coexistence(Maintenance and rollup of 802.16.2-2001 and P802.16.2a)

Current

802.16f-2005 Management Information Base (MIB) for 802.16-2004 Superseded

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IEEE 802.16 Protocol Standards (2)

Standard Description Status

802.16-2004/Cor 1-2005

Corrections for fixed operations (co-published with 802.16e-2005) Superseded

802.16e-2005 Mobile Broadband Wireless Access System Superseded

802.16k-2007 Bridging of 802.16 (an amendment to IEEE 802.1D) Current

802.16g-2007 Management Plane Procedures and Services Superseded

P802.16i Mobile Management Information Base (Project merged into 802.16-2009) Merged

802.16-2009 Air Interface for Fixed and Mobile Broadband Wireless Access System(rollup of 802.16-2004, 802.16-2004/Cor 1, 802.16e, 802.16f, 802.16g and P802.16i)

Current

802.16j-2009 Multihop relay Current

P802.16h Improved Coexistence Mechanisms for License-Exempt Operation In progress

P802.16m Advanced Air Interface with data rates of 100 Mbit/s mobile & 1 Gbit/s fixed In progress

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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

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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 (sometimes)– Single and multi-carrier PHYs

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Wireless Performance(as of 2003)

Source: S. Viswanathan, Intel

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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

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OFDM Basics

Orthogonal Subcarriers

Cyclic Prefix in Frequency DomainCyclic Prefix in Time Domain

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Equalizers 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

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FFT Size Tradeoffs

• 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 trueSource: Lucent

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OFDMA Scalability

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

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Physical layer

• ”Burst single-carrier” modulation

• Allows use of directional antennas

• Allows use of two different duplexing schemes:– Frequency Division Duplexing (FDD)– Time Division Duplexing (TDD)

• Support for both full and half duplex stations

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Half- and Full-Duplex

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Time Division Duplexing (TDD)

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Frequency Division Duplexing (FDD)

• Each transmitter / receiver pair operates on a separate frequency

• Advantages?

• Disadvantages?

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TDMA• Using both

– TDM (Time Division Multiplexing) and– TDMA (Time Division Multiple Access)

• What is the difference?

TDM TDMA

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Physical layer

• Adaptive Data Burst Profiles– Transmission parameters (e.g. modulation and FEC

settings) can be modified on a frame-by-frame basis for each SS.

– Profiles are identified by ”Interval Usage Code” (Downlink IUC and Uplink IUC)

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General Downlink Frame Structure

• Downlink Interval Usage Code (DIUC) indicates burst profile

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TDD Downlink subframe

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FDD burst framing

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FDD Downlink subframe

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General Uplink Frame Structure

• Uplink Interval Usage Code (UIUC) indicates burst profile

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Uplink periods

• Initial Maintenance opportunities– Ranging– To determine network delay and to request power or profile

changes.– Collisions may occur in this interval

• Request opportunities– SSs request bandwith in response to polling from BS.– Collisions may occur in this interval aswell.

• Data grants period– SSs transmit data bursts in the intervals granted by the BS.– Transition gaps between data intervals for synchronization

purposes.

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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)

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Frame Structure – Another View

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Media Access Control (MAC)

• Connection orienteded– Connection ID (CID), Service Flows(FS)

• Channel access:– UL-MAP

– Defines uplink channel access– Defines uplink data burst profiles

– DL-MAP– Defines downlink data burst profiles

– UL-MAP and DL-MAP are both transmitted in the beginning of each downlink subframe (FDD and TDD).

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Network Entry Process

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Mac and Physical Layers

• The MAC is comprised of three sublayers. • The Service Specific Convergence Sublayer (CS) provides

any transformation or mapping of external network data, received through the CS service access point (SAP), into MAC SDUs received by the MAC Common Part Sublayer (MAC CPS) through the MAC SAP.

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SDU and PDU

SDU: service delivery unitPDU: packet delivery unit

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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

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PDU Transmission

Source: R. Marks (NIST) IEEE Presentation

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QoS Mechanism

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Generic MAC Frame

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Generic MAC Header

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Generic Bandwidth Request

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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

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Key Management Messages (1)

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Key Management Messages (2)

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The QoS Object Model

Scheduling type

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Bandwidth request and allocation (1)

• SSs may request bw in 3 ways:– Use the ”contention request opportunities” interval upon

being polled by the BS (multicast or broadcast poll).

– Send a standalone MAC message called ”BW request” in an allready granted slot.

– Piggyback a BW request message on a data packet.

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Bandwidth request and allocation (2)

• BS grants/allocates bandwidth in one of two modes:• Grant Per Subscriber Station (GPSS)• Grant Per Connection (GPC)

• Decision based on requested bw and QoS requirements vs available resources.

• Grants are realized through the UL-MAP.

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Unicast Polling1. BS allocates space for the SS in

the uplink subframe.

2. SS uses the allocated space to send a bw request.

3. BS allocates the requested space for the SS (if available).

4. SS uses allocated space to send data.

Poll(UL-MAP)

RequestAlloc(UL-MAP)

Data

BS SS

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4 Types of Scheduling Service

• Unsolicited Grant Service (UGS)– Real-time, periodic fixed size packets (e.g. T1 or VoIP)– Restrictions on bw requests (Poll-Me bit)– Slip Indicator (SI)

• Real-Time Polling Service (rtPS)– Real-time, periodic variable sizes packets (e.g MPEG)– BS issues periodic unicast polls.– Cannot use contention requests, but piggybacking is ok.

• Non-Real-Time Polling Service (nrtPS)– Variable sized packets with loose delay requirements (e.g. FTP)– BS issues unicast polls regularly (not necessarily periodic).– Can also use contention requests and piggybacking.

• Best Effort Service– Never polled individually– Can use contention requests and piggybacking

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Scheduling Types and QoS

Scheduling 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

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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 being

receive and transmit antennas, respectively

• 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

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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)

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WiMAX Opportunities

• There is a work opportunity to create/enhance 802.16/WiMAX network level simulation

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

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Other Wireless Network Opportunities

• Is there anything magical about 802.11 or 802.16?

• Could we use wireless techniques to extend, say, Ethernet, without such protocols?

• Why would this be good / bad?

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Homework

• Examine the 802.16 protocol set. How could you apply it to extend the range or usability of an existing network? What problems must be dealt with? What manner of training or other preparation must users have? Would 802.11 be more appropriate for this application? Is this a “one size fits all” solution? Why or why not? Prepare a paper of approximately 1100 words describing your findings.

• Be prepared to discuss your findings with the class for 5-10 minutes next week. You may use slides if you desire.

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Disclaimer

• Parts of the lecture slides contain original work of B. Jornar and Shyam Parekh and remain copyrighted materials by the original owner(s). The slides are intended for the sole purpose of instruction in computer networks at Worcester Polytechnic Institute.