WiMAX (Worldwide Interoperability for Microwave Access) uses the IEEE 802.16 standards for broadband wireless access over long distances. It supports both point-to-multipoint and mesh topologies with a connection-oriented MAC layer that provides quality of service. The physical layer uses OFDM, OFDMA, or single carrier modulation and the MAC layer supports various services classes for different types of traffic.

1. WiMAX for Broadband Wireless Access

2. What is WiMAX?
 Worldwide Interoperability for
Microwave Access (WiMAX) is the
common name associated to the IEEE
802.16a/REVd/e standards.
 These standards are issued by the IEEE
802.16 subgroup that originally covered
the Wireless Local Loop technologies
with radio spectrum from 10 to 66 GHz.

3. 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

4. IEEE 802.16 -- Introduction
Coverage range up to 50km and speeds up to
70Mbps(shared among users).

5. IEEE 802.16 -- Introduction
Source: WiMAX, making ubiquitous high-speed data services a reality, White Paper, Alcatel.

6. IEEE 802.16 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

7. Reference Model

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

9. 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

10. 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)

11. IEEE 802.16 MAC
– OFDM PHY TDD Frame Structure
DL Subframe
Frame n-1
pre.
Time
Adaptive
Frame n Frame n+1
UL subframe
FCH
DL
burst 1
DL
burst n
UL
MAP
Broadcast Conrol msgs
... UL burst 1 UL burst m
DL
MAP
DCD
opt.
UCD
opt.
...
DL
burst 2
UL TDMA
DL TDM
pre. pre.

12. IEEE 802.16 MAC
– OFDM PHY FDD Frame Structure
DL Subframe
Frame n-1
pre.
Time
Broadcast
Control Msgs
Frame n Frame n+1
UL subframe
FCH
DL
burst 1
DL
burst k
...
DL TDMA
UL burst 1 UL burst m
DL
burst 2
DL
burst n
DL
burst k+1
...
DL TDM
...
UL TDMA
DL
MAP
UL
MAP
DCD
opt.
UCD
opt.
pre.
pre.
UL MAP for next
MAC frame UL
bursts
pre. pre.

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

14. 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)

15. 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
 Full QoS support

16. 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
Packet PDU
(e.g., IP packet, Ethernet Packet)
PHSI
MAC SDU = CS PDU
Payload Header Suppression Index
Optional, Depending on upper layer
protocol

17. IEEE 802.16 MAC -- CPS
– MAC PDU Format
CRC
(optional)
MAC PDU payload (optional)
Generic MAC
Header
(6 bytes)
LEN
msb
(3)
H
T
CID msb (8)
LEN lsb (8)
Generic MAC Header Format
(Header Type (HT) = 0)
BW Req. Header Format
(Header Type (HT) =1)
msb lsb
E
C
Type (6 bits)
rs
v
C
I
EKS
(2)
rs
v
HCS (8)
CID lsb (8)
BW Req.
msb (8)
H
T
CID msb (8)
BWS Req. lsb (8)
E
C
Type (6 bits)
HCS (8)
CID lsb (8)

18. 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

19. IEEE 802.16 MAC -- CPS
– Data Packet Encapsulations
P
H
SI
MAC PDU
Ethernet Packet
Ethernet Packet
Packet PDU
(e.g., Ethernet)
CS PDU
(i.e., MAC SDU)
HT
FEC block 1
CRC
MAC PDU Payload
OFDM
symbol
1
PHY Burst
(e.g., TDMA burst)
Preamble
OFDM
symbol
2
OFDM
symbol
n
......
FEC
FEC Block 2 FEC block m
......
FEC Block 3

20. 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
 MAC mgmt messages as MAC PDU payloads
 Secondary Management Connection:
 Standard based mgmt messages, e.g., DHCP,
SNMP, …etc
 IP packets based CS PDU as MAC PDU payload

21. IEEE 802.16 MAC – CPS
– MAC Management Messages
 MAC mgmt message format:
MAC mgmt msg payload
mgmt
msg
HD
8 bits
• MAC mgmt msg can be sent on: Basic connections; Primary
mgmt connection; Broadcast connection; and initial ranging
connections
• 41 MAC mgmt msgs specified in 802.16
• 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

22. 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

23. IEEE 802.16 MAC – CPS
– MAC PDU Concatenation
MAC PDU 2
HT
FEC block 1
CRC
MAC PDU Payload
OFDM
symbol
1
PHY Burst
(e.g., TDMA burst)
Preamble
OFDM
symbol
2
OFDM
symbol
n
......
FEC
FEC Block 2 FEC block m
......
FEC Block 3
MAC PDU 1
HT CRC
MAC PDU Payload ......
MAC PDU k
HT CRC
MAC PDU
Payload
Multiple MAC PDUs are concatenated into the same PHY burst

24. IEEE 802.16 MAC – CPS
– MAC PDU Fragmentation
FEC block
1
OFDM
symbol
1
PHY Burst
Pre.
MAC SDU
OFDM
symbol
n1
......
FEC FEC Block
m1
......
MAC SDU
seg-1
HT CRC
MAC PDU Payload
HT CRC
MAC PDU
Payload
A MAC SDU can be fragmented into multiple segments, each
segment is encapsulated into one MAC PDU
FEC block
1
OFDM
symbol
1
PHY Burst
Pre.
OFDM
symbol
n2
......
FEC Block
m2
......
HT CRC
MAC PDU
Payload
MAC SDU
seg-2
MAC SDU
seg-3
F
S
H
F
S
H
Fragmentation
Sub-Header
(8 bits)
F
S
H

25. IEEE 802.16 MAC – CPS
– MAC PDU Packing
MAC
SDU 1
Fixed size MSDUs, e.g., ATM
Cells, on the same connection
HT CRC
MAC PDU Payload
HT CRC
Packing with fixed size MAC SDUs (no packing sub-header is needed)
......
PSH
MAC
SDU 2
MAC
SDU k
Packing with variable size MAC SDUs (Packing Sub-Heade is neeeded)
PSH ...... PSH
MAC SDU or
seg. 1 MAC SDU or seg 2
MAC SDU or
seg n
Variable size
MSDUs or MSDU
segments, e.g.,
IP packets, on
the same
connection
Packing
Sub-Heder
(16 bits)

26. IEEE 802.16 MAC – CPS
QoS
 Three components of 802.16 QoS
 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

27. 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

28. 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.

29. 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.

30. 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.

31. 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.

32. 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

33. 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

34. 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

35. IEEE 802.16 MAC – CPS
UL Sub-Frame Structure
Source: http://www.cygnuscom.com/pdf/WP_PN_Article.pdf

36. IEEE 802.16 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.

37. 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.

38. 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)

39. 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

40. 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

41. 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.

42. References
 IEEE802.16-2004
 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: www.wimaxforum.com
 http://en.wikipedia.org/wiki/WiMax

43. 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