WiMAX (Worldwide Interoperability for Microwave Access) Notes

Introduction to WiMAX

• This material is for SDSU students only and should not be circulated without the instructor's consent.

Content Overview

• Introduction to WiMAX
• MAC and Protocol Stack
• QoS Provision Mechanism

WiMAX Access Network Architecture

• WiMAX stands for Worldwide Interoperability for Microwave Access.
• Key components include Subscriber Stations (SS) and Base Stations (BS).

WiMAX Access Architecture

• Operates as a Wireless Metropolitan Area Network (Wireless MAN).
• Supports various customers: SOHO, multi-tenant, residential.
• Utilizes a core network.
• May involve repeaters to extend coverage.

WiMAX Backhaul

• Employs various backhaul technologies.
• Point-to-Point and Point-to-Multipoint configurations.
• Connects Customer Premises (Home, Business, HOTSPOT) to
  • Internet
  • Telco Core Network
  • Private (Fiber) Network

Evolution Towards Mobile Communication

• Evolution from IEEE 802.16-2004 (fixed) to IEEE 802.16e (mobile).
• Supports nomadicity/pedestrian mobility and full mobility.
• Illustrates the progression from fixed access to full mobility.

Nationwide WiMAX Coverage

• Top 10 Best WiMax-Covered Cities (according to Business Insider):
  • Baltimore
  • Portland, Ore
  • Houston
  • Atlanta
  • Philadelphia
  • Las Vegas
  • Chicago
  • Seattle
  • Charlotte, Greensboro, and Raleigh
  • Hawaii (Oahu, Maui, and Lanai)

What is WiMAX?

• WiMAX is associated with the IEEE 802.16/a/d/e standards.
• Standards are issued by the IEEE 802.16 subgroup.

IEEE 802.16 Introduction

• IEEE 802.16 (2001)
  • Air Interface for Fixed Broadband Wireless Access System MAC and PHY Specifications for 10 – 66 GHZ (LoS).
  • One PHY: Single Carrier.
  • Connection-oriented, TDM/TDMA MAC, QoS, Privacy.
• IEEE 802.16a (January 2003)
  • Amendment to 802.16, MAC Modifications and Additional PHY Specifications for 2 – 11 GHz (NLoS).
  • Three PHYs: OFDM, OFDMA, Single Carrier.
  • Additional MAC functions: OFDM and OFDMA PHY support, Mesh topology support, ARQ
• IEEE 802.16d (July 2004)
  • Combines both IEEE 802.16 and 802.16a
  • Some modifications to the MAC and PHY.
• IEEE 802.16e (2005)
  • Amendment to 802.16-2004
  • MAC Modifications for limited mobility - handoff.

IEEE 802.16 Introduction - Coverage and Speed

• Coverage range up to $50km$ and speeds up to $70Mbps$ (shared among users).

IEEE 802.16 Introduction - Different Flavors of WiMAX

• 802.16a: Fixed Outdoor Applications (E1/T1 service for enterprises, Backhaul for Hotspots, Limited residential Broadband access).
• 802.16REVd: Fixed Outdoor Applications (Indoor Broadband access for residential users such as High-Speed Internet, VoIP).
• 802.16e: Limited Mobility Applications (Portable Broadband access for consumers, Always Best Connected).

IEEE 802.16 MAC Highlights

• High speed, long distance, QoS provision.
• Coverage range up to $50km$. The 802.16 MAC protocol is connection-oriented.
• Supports Point-to-Multipoint architecture.
• Upon entering the network, each Subscriber Station (SS) creates one or more connections to/from the Base Station (BS).
• The MAC layer schedules air-link resources and provides Quality of Service (QoS) differentiation.

802.16 MAC Protocol Stack

• Service Convergence Sub-layer (ATM, IP):
  • Mapping upper layer service to and from 802.16 MAC connections.
  • Primary task: classify service data unit (SDU) to proper MAC connections.
  • Payload header suppression & reconstruction.
• Core MAC Common Part Sub-layer:
  • Provides Medium Access Control functionalities.
  • Point-to-multipoint (Multicast like).
  • Addressing uses 16bits CID (Connection ID), not 48bits MAC Address.
  • Perform link adaptation (Radio Link Control).
  • Handle network entry and leaving.
• Privacy Sub-layer:
  • Based on Private Key Management (PKM).
  • Providing security, encryption, and decryption of MAC PDU payload data.

802.16 MAC-PHY Protocol Stack

• Transmission convergence sublayer: transforms variable length MAC protocol data units (PDUs) into fixed length FEC blocks.

Addressing and Management Connections

• MAC address is used as device identification; addressing uses CID (Connection ID).
• On entering the network, SS is assigned 3 management connections:
  • Basic connection: transfer short, time-critical msg (ex. radio link control msg).
  • Primary management connection: transfer long, delay-tolerant msg (authentication, traffic connection setup).
  • Secondary management connection: transfer standard msg (DHCP, TFTP, SNMP).

MAC PDU Format

• Fixed Length MAC header, Variable length payload, and optional CRC.
• Two types of headers: Generic Header, Bandwidth Request Header (no payload).
• The generic header transmits data or MAC messages.
• The BR header is used by the SS to request more bandwidth on the UL.
• The maximum length of the MAC PDU is 2048 bytes, including header, payload, and CRC.

Generic Header Fields

• HT: Header Type (0 generic, 1 bw req)
• EC: Encryption Ctrl (0 no enc, 1 enc)
• Type: subheader or payload type
• CI: CRC indicator (0 no CRC)
• EKS: Encryption Key Sequence
• LEN: PDU length, including header and CRC
• HCS: Header Check Sequence

Transmission of MAC PDU

• Incoming SDUs (Service Data Unit) from a higher layer (ATM, IP) are formatted (Fragmentation or Packing).
• Fragmentation/Packing efficiently uses bandwidth.
• Concatenation is used to transmit many PDUs in one burst.
• Encryption only applies to Payload.

PHY Support and Frame Structure

• The frame is divided into DL and UL subframes.
• The DL-MAP, UL-MAP, DL Channel Descriptor (DCD), UL Channel Descriptor (UCD), and other broadcast messages are sent at the beginning of these first bursts.
• The DL-MAP and UL-MAP, DCD and UCD completely describe the contents of the DL and UL subframes, specifying the SS's, subchannels, coding, and modulation.

TDD Frame Structure

• Time division duplex (TDD), sometimes referred to as TDM.

QoS and Schedule Service

• 802.16 applies Quality of Service to treat different applications differently.
• Applications include: Email, FTP, Video game, Movie, Web browsing, etc., with different bandwidth requirements.

QoS Analogy: Similar to Mail System

• Urgent Mail: USPS express next day, pay more, fast delivery, delivery guaranteed, signature required.
• Regular mail: regular air mail, not urgent, pay less, slow delivery, no delivery guarantee, no signature requirement.

QoS Schedule Service Types

• UGS: Unsolicited Grant Services
  • For CBR or CBR-like services (e.g., T1/E1, G.711 without silence suppression, CBR).
  • BS scheduler offers fixed size UL BW grants on a real-time periodic basis.
  • The SS does not need to send any explicit UL BW req.
• rtPS: Real-time Polling Services
  • For rt-VBR-like services (e.g., MPEG video).
  • BS scheduler offers real-time, periodic, UL BW request opportunities.
  • The SS uses the offered UL BW req opportunity to specify the desired UL BW grant.
  • The SS cannot use contention-based BW req.
• nrtPS: non-real-time polling services
  • For nrt-VBR-like services, such as bandwidth-intensive file transfer – FTP, HTTP.
  • The BS scheduler shall provide timely (on the order of a second or less) UL BW request opportunities.
  • The SS can use contention-based BW req opportunities to send BW req.
• BE: Best Effort
  • For best-effort traffic (e.g., SMTP).
  • The SS uses the contention-based BW request opportunities.

Miscellaneous Issues - Contention Based UL Slots

• Two types of Contention-based UL slots:
  • Initial Ranging: Used for new SS to join the system; requires a long preamble.
  • BW Request: Used for sending BW req; short preamble.
• Collision Detection and Resolution:
  • Detection: SS does not get the expected response in a given time.
  • Resolution: a truncated binary exponential backoff window.

Where Contention Exists

• Contention slots are used for initial ranging and bandwidth requests.

Conclusion

• WiMAX features: High speed, long distance, QoS provision.
• Employs TDD for Downlink and Uplink management.
• Utilizes scheduling rather than contention.
• WiMAX has QoS, while WiFi typically does not.