1. WiMAX for Broadband Wireless Access
By:
Karim M. El Defrawy
ICS
UCI-2005

2. Outline What is WiMAX 802.16 Introduction 802.16 MAC Highlights
Reference Model
MAC Convergence Sub-Layer (CS)
MAC Common Part Sub-Layer (CPS)
MAC Privacy Sub-Layer (PS)
Questions

3. What is WiMAX?
Worldwide Interoperability for Microwave Access (WiMAX) is the common name associated to the IEEE a/REVd/e standards.
These standards are issued by the IEEE subgroup that originally covered the Wireless Local Loop technologies with radio spectrum from 10 to 66 GHz.

4. 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 a (January 2003)
Amendment to , 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 d (July 2004)
Combines both IEEE and a
Some modifications to the MAC and PHY
IEEE e (2005?)
Amendment to
MAC Modifications for limited mobility

5. IEEE Introduction
Coverage range up to 50km and speeds up to 70Mbps(shared among users).

6. IEEE Introduction
Source: WiMAX, making ubiquitous high-speed data services a reality, White Paper, Alcatel.

7. IEEE MAC -- Highlights
WirelessMAN: Point-to-Multipoint and optional mesh topology
Connection-oriented
Multiple Access: DL TDM & TDMA, UL TDMA;UL OFDMA & TDMA, DL OFDMA & TDMA (Optional)
PHY considerations that affect the MAC
Duplex: TDD, FDD, FDX FDD BS and SS, HDX FDD SS
Adaptive burst profiles (Modulation and FEC) on both DL and UL
Protocol-independent core (ATM, IP, Ethernet)
Flexible QoS offering (CBR, rt-VBR, nrt-VBR, BE)
Strong security support

8. Reference Model

9. Adaptive PHY
Source: Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless, Technical White Paper, Intel.

10. Adaptive Burst Profiles
Burst profile: Modulation and FEC
On DL, multiple SSs can associate the same DL burst
On UL, SS transmits in an given time slot with a specific burst
Dynamically assigned according to link conditions
Burst by burst
Trade-off capacity vs. robustness in real time

11. Duplex Scheme Support
The duplex scheme is Usually specified by regulatory bodies, e.g., FCC
Time-Division Duplex (TDD)
Downlink & Uplink time share the same RF channel
Dynamic asymmetry
does not transmit & receive simultaneously (low cost)
Frequency-Division Duplex (FDD)
Downlink & Uplink on separate RF channels
Full Duplexing (FDX): can Tx and Rx simultaneously;
Half-duplexing (HDX) SSs supported (low cost)

12. IEEE 802.16 MAC – OFDM PHY TDD Frame Structure

13. IEEE 802.16 MAC – OFDM PHY FDD Frame Structure

14. FDD MAPs Time Relevance
DL MAP
UL MAP
DL MAP
UL MAP
DOWNLINK
UPLINK
frame
Broadcast
Half Duplex T
erminal #1
Full Duplex Capable User
Half Duplex T
erminal #2

15. IEEE 802.16 MAC addressing and Identifiers
SS has 48-bit IEEE MAC address
BS has 48-bit base station ID
Not a MAC address
24-bit operator indicator
16-bit connection ID (CID)
32-bit service flow ID (SFID)
16-bit security association ID (SAID)

16. IEEE 802.16 MAC – Convergence Sub-Layer (CS)
ATM Convergence Sub-Layer:
Support for VP/VC switched connections
Support for end-to-end signaling of dynamically created connections
ATM header suppression
Full QoS support
Packet Convergence Sub-Layer:
Initial support for Ethernet, VLAN, IPv4, and IPv6
Payload header suppression

17. IEEE 802.16 MAC -- CS – Packet Convergence Sub-Layer
Functions:
Classification: mapping the higher layer PDUs (Protocol Data Units) into appropriate MAC connections
Payload header suppression (optional)
MAC SDU (Service Data Unit), i.e, CS PDU, formatting

18. IEEE 802.16 MAC -- CPS – MAC PDU Format

19. IEEE 802.16 MAC -- CPS -- Three Types of MAC PDUs
Data MAC PDUs
HT = 0
Payloads are MAC SDUs/segments, i.e., data from upper layer (CS PDUs)
Transmitted on data connections
Management MAC PDUs
HT =0
Payloads are MAC management messages or IP packets encapsulated in MAC CS PDUs
Transmitted on management connections
BW Req. MAC PDUs
HT =1; and no payload, i.e., just a Header

20. IEEE 802.16 MAC -- CPS – Data Packet Encapsulations

21. IEEE 802.16 MAC – CPS -- MAC Management Connections
Each SS has 3 management connections in each direction:
Basic Connection:
short and time-urgent MAC management messages
MAC mgmt messages as MAC PDU payloads
Primary Management connection:
longer and more delay tolerant MAC mgmt messages
Secondary Management Connection:
Standard based mgmt messages, e.g., DHCP, SNMP, …etc
IP packets based CS PDU as MAC PDU payload

22. IEEE 802.16 MAC – CPS – MAC Management Messages
MAC mgmt message format:
MAC mgmt msg can be sent on: Basic connections; Primary mgmt connection; Broadcast connection; and initial ranging connections
41 MAC mgmt msgs specified in
The TLV (type/length/value) encoding scheme is used in MAC mgmt msg, e.g., in UCD msg for UL burst profiles,
(type=1, length=1, value=1)  QPSK modulation
(type=1, length=1, value=2)  16QAM modulation
(type=1, length=1, value=3)  64QAM modulation

23. IEEE 802.16 MAC – CPS – MAC PDU Transmission
MAC PDUs are transmitted in PHY Bursts
The PHY burst can contain multiple FEC blocks
MAC PDUs may span FEC block boundaries
Concatenation
Packing
Segmentation
Sub-headers

24. IEEE 802.16 MAC – CPS – MAC PDU Concatenation

25. IEEE 802.16 MAC – CPS – MAC PDU Fragmentation

26. IEEE 802.16 MAC – CPS – MAC PDU Packing

27. IEEE 802.16 MAC – CPS QoS Three components of 802.16 QoS Service Flow
Service flow QoS scheduling
Dynamic service establishment
Two-phase activation model (admit first, then activate)
Service Flow
A unidirectional MAC-layer transport service characterized by a set of QoS parameters, e.g., latency, jitter, and throughput assurances
Identified by a 32-bit SFID (Service Flow ID)
Three types of service flows
Provisioned: controlled by network management system
Admitted: the required resources reserved by BS, but not active
Active: the required resources committed by the BS

28. IEEE 802.16 MAC – CPS – Uplink Service Classes
UGS: Unsolicited Grant Services
rtPS: Real-time Polling Services
nrtPS: Non-real-time Polling Services
BE: Best Effort

29. IEEE 802.16 MAC – CPS – Uplink Services: UGS
UGS: Unsolicited Grant Services
For CBR or CBR-like services, e.g., T1/E1.
The 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.

30. IEEE 802.16 MAC – CPS – Uplink Services: rtPS
rtPS: Real-time Polling Services
For rt-VBR-like services, e.g., MPEG video.
The 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.

31. IEEE 802.16 MAC – CPS – Uplink Services: nrtPS
nrtPS: non-real-time polling services
For nrt-VBR-like services, such as, bandwidth-intensive file transfer.
The BS scheduler shall provide timely (on a order of a second or less) UL BW request opportunities.
The SS can use contention-based BW req. opportunities to send BW req.

32. IEEE 802.16 MAC – CPS – Uplink Services: BE
BE: Best Effort
For best-effort traffic, e.g., HTTP, SMTP.
The SS uses the contention-based BW request opportunities.

33. IEEE 802.16 MAC – CPS – Bandwidth Grant
BW grants are per Subscriber Station:
Allows real-time reaction to QoS need, i.e., SS may re-distribute bandwidth among its connections, maintaining QoS and service-level agreements
Lower overhead, i.e., less UL-MAP entries compare to grant per connection
Off- loading base station’s work
Requires intelligent subscriber station to redistribute the allocated BW among connections

34. IEEE 802.16 MAC – CPS – BW Request/Grant Mechanisms
Implicit requests (UGS): No actual requests
BW request messages, i.e., BW req. header
Sends in either a contention-based BW req. slot or a regular UL allocation for the SS;he special B
Requests up to 32 KB with a single message Request
Incremental or aggregate, as indicated by MAC header–
Piggybacked request (for non-UGS services only)
Presented in Grant Management (GM) sub-header in a data MAC PDU of the same UL connection
is always incremental
Up to 32 KB per request for the CID
Poll-Me bit
Presented in the GM sub-header on a UGS connection
request a bandwidth req. opportunity for non-UGS services

35. IEEE 802.16 MAC – CPS -- Contention UL Access
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

36. IEEE 802.16 MAC – CPS UL Sub-Frame Structure
Source:

37. IEEE MAC – CPS – Ranging
Ranging is a process of acquiring the correct timing offset, and PHY parameters, such as, Tx power level, frequency offset, etc. so that the SS can communicate with the BS correctly.
BS performs measurements and feedback.
SS performs necessary adjustments.
Two types of Ranging:
Initial ranging: for a new SS to join the system
Periodic ranging (also called maintenance ranging): dynamically maintain a good RF link.

38. IEEE 802.16 MAC – CPS – Automatic Repeat reQuest (ARQ)
A Layer-2 sliding-window based flow control mechanism.
Per connection basis.
Only effective to non-real-time applications.
Uses a 11-bit sequence number field.
Uses CRC-32 checksum of MAC PDU to check data errors.
Maintain the same fragmentation structure for Retransmission.
Optional.

39. IEEE 802.16 MAC – Privacy Sub-layer (PS)
Two Major Functions:
Secures over-the-air transmissions
Protects from theft of service
Two component protocols:
Data encryption protocol
A client/server model based Key management protocol (Privacy Key Management, or PKM)

40. IEEE 802.16 MAC – PS -- Security Associations
A set of privacy information, e.g., encryption keys, used encryption algorithm
Three types of Security Associations (SAs)
Primary SA: established during initial registration
Static SA: provisioned within the BS
Dynamic SA: dynamically created on the fly
Identified by a 16-bit SAID
Connections are mapped to SAs

41. IEEE 802.16 MAC – PS -- Multi-level Keys and Their Usage
Public Key
Contained in X.509 digital certificate
Issued by SS manufacturers
Used to encrypt AK
Authorization Key (AK)
Provided by BS to SS at authorization
Used to derive KEK
Key Encryption Key (KEK)
Derived from AK
Used to encrypt TEK
Traffic Encryption Key (TEK)
Provided by BS to SS at key exchange
Used to encrypt traffic data payload

42. IEEE 802.16 MAC – PS -- Data Encryption
Use DES (Data Encryption Standard) in CBC (Cipher Block Chaining) mode with IV (Initialization Vector).
CBC IV is calculated from
IV parameter in TEK keying info; and
PHY synchronization field in DL-MAP.
Only MAC PDU payload (including sub-headers) is encrypted.
MAC PDU headers are unencrypted.
Management messages are unencrypted.

43. IEEE 802.16 MAC – one big item is out of scope
Scheduler

44. Questions ??

45. References
IEEE
Alcatel White Paper: WiMAX, making ubiquitous high-speed data services a reality
Intel White Paper: Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless
WiMAX Forum:

46. IEEE 802.16 MAC – commonly used terms
BS – Base Station
SS – Subscriber Station, (i.e., CPE)
DL – Downlink, i.e. from BS to SS
UL – Uplink, i.e. from SS to BS
FDD – Frequency Division Duplex
TDD – Time Division Duplex
TDMA – Time Division Multiple Access
TDM – Time Division Multiplexing
OFDM – Orthogonal Frequency Division Multiplexing
OFDMA - Orthogonal Frequency Division Multiple Access
QoS – Quality of Service