Fundamentals of Wimax

IEEE Standard 802.16:
A Technical Overview of the Mobile
WiMAX Air Interface and Beyond

Eyal Verbin

Contents

1. Overview of WiMAX • Quality of Service
• Background on IEEE 802.16 and WiMAX • Scheduling
• Salient Features of WiMAX • Adaptive Modulation and Coding
2. Physical Layer • Security
• Network Entry Procedures
• The Broadband Wireless Channel
• Power saving Modes
• OFDM Principles
• Mobility Management
• Channel Coding
• Hybrid-ARQ 4. WiMAX Network Architecture
• OFDM Symbol Structure • Network Reference Model
• Frame Structure • Protocol Layering
• Fractional Frequency Reuse • IP Address Assignment
• Transmit Diversity and MIMO • Authentication and Security Architecture
• Ranging • Quality of Service Architecture
• Power Control • Mobility Management
• Channel Quality Measurements • Paging
3. Medium Access Control Layer
• Convergence Sublayer
• MAC PDU Construction and Transmission
• Bandwidth Request and Allocation
• ARQ

Background on IEEE 802.16 and WiMAX

 Air interface is based on IEEE 802.16-2009
 IEEE 802.16 was formed in 1998 to develop LOS point to multipoint for operation in the 10GHz –
66GHz band
 The original 802.16 standard was based on single carrier
 Many of the MAC concepts were adopted from the cable modem DOCSIS
 In December 2005 IEEE 802.16e-2005 was approved as a standard for mobile wireless system,
which forms the basis for Mobile WiMAX and adopts multi carrier technology
 WiMAX forum used IEEE work to develop interoperable standard
 For practical reasons a smaller set of design choices (profiles) were selected
 System profile defines the subset of mandatory and optional PHY and MAC features
 WiMAX forum also defines higher layers networking specifications

Salient Features of WiMAX (1)

 OFDM based physical layer
 Enables good resistance to multipath and allows operation in NLOS conditions
 High peak data rates
 Typically, using 10MHz spectrum using TDD scheme with 3:1 DL/UL split, the peak PHY data rate is
about 25Mbps (DL) and 7Mbps (UL)
 Scalable bandwidth
 FFT size may scale from 128 bit to 1024 bit FFT allowing channel bandwidths of 1.25MHz to
10MHz.
 Adaptive modulation and coding
 WiMAX supports a number of modulation and channel coding schemes and allows the scheme to be
changed on a per user and per frame basis
 Link layer retransmission
 Auto retransmission requests (ARQ) are supported on top of physical layer error correction schemes
to enable reliable data transmission
 Orthogonal frequency division multiple access (OFDMA)
 Different users can be allocated with different subsets of the OFDM tones

Salient Features of WiMAX (2)

 Flexible and dynamic per user resource allocation
 DL and UL resources and transmission schemes are controlled by the scheduler in the base station.
 Advance antenna techniques
 Beamforming, space time coding and spatial multiplexing may be used to improve system capacity
and spectral efficiency
 Quality of service support
 Connection oriented architecture to support variety of applications, each with its own characteristics.
 Robust security
 Strong encryption using Advance Encryption Standard (AES) and flexible authentication architecture
based on Extensible Authentication Protocol (EAP)
 Support for mobility
 Secure seamless handover for full mobility applications and various power saving mechanisms
 IP based architecture
 Network architecture is based on an all IP platform. All end to end services are delivered over an IP
architecture

The Broadband Wireless Channel (1)

 The main challenge of broadband wireless system is the multipath
propagation
 Fast Fading: different reflection arrive at the receiver with different phases. The
combined effect can be constructive or destructive, which causes very large
observed difference in amplitude of the receive signal
 Different symbols arrive at different time to the receiver, resulting in Inter Symbol
Interference (ISI)
 Different approached for mitigation of fading:
 Spread spectrum and rake receivers
 Equalization
 Multicarrier transmission

The Broadband Wireless Channel (2)

Open Loop MIMO in WiMAX (2)

 Spatial Multiplexing
 Used to increase system capacity by exploiting the
dispersive nature of the wireless channel
 System capacity grows linearly with Min{NTx, NRx}
 Spatial Multiplexing (MIMO Matrix B)
 Multiple data streams are transmitted at the same time
and in the same frequency from different BS antennas
 Mandates multiple receive antennas at the MS
 Assuming channels are uncorrelated, receiver can
retrieve the data using decoding algorithm known as
VBLAST
 Collaborative Spatial Multiplexing (CSM)
and in the same frequency from different MS
 Assuming channels are uncorrelated, BS can retrieve the
data using the same Matrix B technique

OFDM Principles (1)

 Multicarrier transmission
 Dividing high bit rate data stream into several parallel lower bit rate streams (subcarriers)
 Minimize intersymbol interference (ISI) by making the symbol time substantial larger
than the channel delay spread
 OFDM is a spectrally efficient version of multicarrier scheme
 Subcarriers are orthogonal, so that guard bands between subcarriers is not required
 Created using inverse discrete Fourier transform (IDFT)
 To completely eliminate ISI, guard intervals are inserted between consecutive
OFDM symbols
 The duration of the guard interval is a tradeoff between the delay spread that can be
handled and the power loss associated with it.
 Size of FFT is chosen as a balance between protection against multipath, Doppler
shift and design complexity.

OFDM Principles (2)

 Advantages
 Robustness to channel delay spread
 Reduced computational complexity
 Exploitation of frequency diversity
 Coding and interleaving the information across the subcarriers
 Provides a flexible multiple access scheme
 Resources are allocated in a frequency-time grid
 Robustness against narrowband interference
 Suitable for coherent demodulation using pilot based channel estimation
 Drawbacks
 High peak to average ratio that causes non linearities and clipping distortion
 Can be mitigated using digital pre-distortion techniques
 Sensitivity to phase noise and frequency dispersion
 Requires accurate frequency synchronization

Channel Coding

Subcarrier Antenna #0
Mapping
IFFT D/A
and Pilot
Insertion
From MAC
Space
Channel Symbol
Randomizer Interleaver Time
Encoder Mapping
Encoder

Subcarrier Antenna #1
Mapping
IFFT D/A
and Pilot
Insertion

Channel Coding
 Randomizer
 Improves FEC performance and synchronization capabilities
 Channel Encoder
 Convolution Code (CC)
 Used for encoding of Frame Control Header (FCH)
 Convolution Turbo Code (CTC)
 Used for all transport and management connections
 Repetition Code
 Further increase signal margin over the modulation and FEC mechanisms
 Applies only to QPSK modulation

 Interleaver
 Improves FEC performance by ensuring that adjacent coded bits are mapped onto non
adjacent subcarriers (frequency diversity) and that adjacent bits are alternately mapped
to less and more significant bits of modulation constellation
 Symbol Mapping
 QPSK
 16QAM
 64QAM (optional for UL)

Hybrid ARQ (1)

 HARQ is an optional part of the PHY and can be enabled on a per connection basis.
 HARQ renders performance improvements due to SNR gain and time diversity
achieved by combining previously erroneously decoded sub packets and
retransmitted sub packet.
 Based on N ‘Stop and Wait’ mechanism
 Transmitter waits for ACK/NACK before transmitting again
 Multiple HARQ processes (channels) may be activated per connection to increase the rate
 Operates at the FEC block level and combines PHY and MAC (Hybrid)
 The FEC encoder is responsible for generating HARQ sub packets.
 The sub packets are combined by the receiver FEC decoder as part of the decoding process.
 The receiver combines the newly received burst with the formerly received bursts to enhance decoding performance.
 Based on 16 bit CRC, the receiver replies with an ACK if the sub packet decoding
succeeded and with a NACK if the decoding failed.

Hybrid ARQ (2)

 ACK/NACK signaling
 DL: Dedicated PHY layer ACK/NACK UL channel
 Feedback is synchronized with the transmission, i.e. receiver provides feedback in a fixed delay
relative to the transmission (default is one frame)
 UL: ARQ ACK message.
 Feedback is implicitly indicated through the UL allocation
 Feedback is unsynchronized, i.e. receiver may provide feedback any time following the HARQ
transmission
 In order delivery
 Due to the N ‘Stop and Wait’ scheme, out of order delivery of HARQ packets is possible.
 Since some applications are sensitive to the delivery order, e.g. TCP, there is an option to
guarantee in order delivery by using PDU SN subheaders.

Symbol Structure

Frequency Domain
Representation

 Mobile WiMAX Profile includes
support of 512 and 1024
FFT, depending on channel BW
 512FFT: 3.5MHz, 5MHz
 1024FFT: 7MHz, 8.75MHz, 10MHz

 The guard interval used to prevent ISI
is a cyclic prefix. This structure is
needed to prevent Inter Carrier
Interference (ICI)
Time Domain Representation

OFDM Symbol Parameters

 Primitive parameter definitions
 BW: Nominal channel bandwidth (e.g. 10MHz)
 Nused : Number of used subcarriers (e.g. 840 for 10MHz)
 Ndata: Number of data subcarriers (e.g. 720 for 10MHz)
 n: Over sampling factor (e.g. 28/25 for 10MHz)
 CP: Cyclic prefix, i.e. Tg/Tu (1/8)
 Derived parameter definitions
 NFFT : Smallest power of two greater than Nused (e.g. 1024 for 10MHz)
 Sampling Frequency Fs = nBW: (e.g. 11.2 MHz for 10MHz)
 Subcarrier spacing ∆f=Fs/NFFT: (e.g. 10.9 KHz for 10MHz)
 Useful symbol time Tu = 1/∆f: (e.g. 91.4 Sec 10MHz)
 CP time Tg = CP∙Tu: (e.g. 11.4 Sec for 10MHz)
 OFDMA symbol time Ts = Tg + Tu: (e.g. 102.9 Sec for 10MHz)

OFDM Spectral Efficiency

 Data Rate
R  N data  bm  cr / Ts

R N data bm cr n
 Spectral Efficiency Efficiency  
BW (1  CP ) N FFT

5
 DL Example (10 MHz, 64QAM 5/6) 35Mbps  720  6  /102.9
6
 Spectral efficiency = 3.5 bit/sec/Hz

OFDM Symbol Structure: Terminology

 Slot: Smallest allocation unit in
the time-frequency domain.
Consists of a single subchannel
and of one to three OFDM
symbols. Contains 48 data
subcarriers
 Data Region: A contiguous
allocation of slots in the time-
frequency domain
 Subchannel Group: A single set
of contiguous logical
subchannels. Each logical
subchannel is mapped to a set
of physical subcarriers
 Segment: One or more
subchannel groups that are
controlled by a single instance
of BS MAC

Symbol Structure & Permutation

 Permutation: The mapping of physical subcarriers to logical subchannels
 Permutation Zone: A set of OFDM symbols over which the same permutation is used.
A frame may contain one or more permutation zones
 Two categories of permutations:
 Distributed Permutation: Draws subcarriers pseudo randomly to form subchannel.
Provides frequency diversity and inter cell interference averaging. Includes two
permutations:
 Contiguous Permutation: Groups a block of contiguous subcarriers to form a
subchannel. Enables multi user diversity by choosing the subchannel with the best
frequency response.
 In general, distributed permutation perform well in mobile applications, while
contiguous permutation are well suited for fixed or low mobility environments.

DL Partial Use of Subcarriers (PUSC) Symbol Structure

 Used subcarriers are split into clusters of fourteen contiguous subcarriers.
 Clusters are mapped to six major groups as a function of Cell ID and DL Permutation Base
parameters
 Three segments are created from the groups
 Logical subchannels are created from a permutation of cluster pairs such that each group is
made up of clusters that are distributed throughout the subcarriers space

 Slot is one subchannel by two OFDM symbols. It contains 48 data subcarriers and eight pilot
subcarriers

DL PUSC Symbol Structure

Parameter 1024 FFT 512 FFT

DC subcarriers 1 1

Guard subcarriers 183 91

Data subcarriers 720 360

Pilot subcarriers 120 60

Subcarriers per cluster 14 14

Clusters 60 30

Data subcarriers per slot 48 48

Subchannels 30 15

UL PUSC Symbol Structure

 Subcarriers are split into groups of four consecutive physical subcarriers over three
OFDM symbols. Each group is termed a tile
 Six tiles generate a subchannel. Tiles are mapped to logical subchannels based on UL
Permutation Base parameter
 Slot is one subchannel by three OFDM symbols. It is comprised of 48 data
subcarriers and 24 pilot subcarriers in 3 OFDM symbols
 Pilot density is higher than DL since no preamble is available on the UL

OFDMA PHY: UL PUSC Symbol Structure

Parameter 1024 FFT 512 FFT

DC subcarriers 1 1

Guard subcarriers 183 103

Used subcarriers 840 408

Tiles 210 102

Subcarriers per tile 4 4

Data subcarriers per slot 48 48

Subchannels 35 17

Tiles per subchannels 6 6

Frame Structure (Time Division Duplex)
 IEEE 802.16e PHY supports both FDD and TDD. Mobile WiMAX profiles currently
available for TDD only
 Each frame is divided into DL and UL sub frames separated by Transmit To receive Gap
(TTG) and Receive to Transmit Gap (RTG)
 Profiles define a finite set of possible DL/UL splits (UL varies between 25% and 45% of
the frame)
 Frame duration: 5msec
 Subframe may be divided into multiple zones on OFDM symbol boundaries. Each Zone
is characterized by a specific permutation mode and multiple antenna scheme

Preambles & Pilots
 The first symbol in the DL transmission used for synchronization and channel
estimation.
 Preamble subcarriers are boosted BPSK modulated with a specific PN code
 To generate the preamble the PHY uses a series of 114 binary PN sequences. The
sequence to be used is determined by the segment number and the Cell ID. It is
mapped to every third subcarrier except the DC carrier.
 Enables MS to obtain signal measurements and extract Cell ID for multiple co-
channel cells with a single reception of preamble
 No preambles are available on the UL (except for AAS zone). Channel estimation on
the UL is derived from the pilots

DL Subframe (1)
 Multiplexing: OFDMA
 Preamble Time

DL Burst #8 DL Burst #12

 First symbol of the DL subframe
FCH
DL Burst #2
DL Burst #9

Frequency
DL MAP DL Burst #1
(Cont’d) (UL MAP)

 Used for time and frequency DL MAP
DL Burst #3
DL Burst #10
DL Burst #13

DL Burst #11

synchronization, initial channel
estimation, noise and interference DL Burst #14

Preamble
estimation
 Carries BS information (Cell ID and Not Allocated DL Burst #15

segment)
 Frame Control Header (FCH) DL Burst #16

 Transmitted with QPSK ½ and
Zone #1: PUSC 1/3 SISO Zone #2: PUSC 1/3 MIMO Zone #3: PUSC All MIMO

repetition of four and occupies the first
four subchannels of the segment
 Indicates used subchannel groups (PUSC zone)
 FEC scheme for the MAPS

 MAPS are transmitted at QPSK ½ with
FEC and repetition as indicated by FCH
 Indicates MAP length

DL Subframe (2)

 DL MAP and UL MAP are broadcast
messages carrying information elements (IE) Time

DL Burst #8 DL Burst #12

 IE defines the DL and UL bursts FCH
DL Burst #2
DL Burst #9

Frequency
 The scope of the DL MAP is the current frame
DL MAP DL Burst #1
(Cont’d) (UL MAP)
DL Burst #10
DL MAP DL Burst #13

 The scope of the UL MAP is the next frame DL Burst #3
DL Burst #11

 Standard DL IE includes:
 Connection Identifier (CID)
DL Burst #14

Preamble
 Downlink Interval Usage Code (DIUC), which
defines the MCS and the FEC used for the Not Allocated DL Burst #15

burst
 Repetition coding indication DL Burst #16

 Burst boundaries
 Symbol offset (start of burst in time domain)
 Subchannel offset (start of burst in frequency domain) Zone #1: PUSC 1/3 SISO Zone #2: PUSC 1/3 MIMO Zone #3: PUSC All MIMO

 Number of symbols (burst duration in time domain)
 Number of subchannels (burst duration in frequency
domain)
 Boosting (power boosting for the burst +6 dB to -
12 dB to provide DL power control)

UL Subframe

 Multiple Access: OFDMA
Time

 No Preambles
3 Symbols 3 Symbols

Perio

 Standard UL IE includes:
Initial dic

Frequency
UL Burst #1
6 SC Ranging/HO Rang
Ranging ing/
BWR

CQICH 12 SC

 Connection Identifier (CID) 6 SC ACK

UL Burst #2

 Uplink Interval Usage Code
 Duration (in OFDMA slots) UL Burst #3

 Repetition coding indication
 Dedicated Control Zones
Not Allocated Not Allocated

 UL Ranging Noise Burst 10 SC

 Dedicated UL ranging subchannel
 Used for BW requests as well Zone #1 Zone #2
Segmented PUSC Un-Segmented PUSC

 Quality Information Channel
 UL CQICH is allocated for the MS to feedback
channel state information
 UL ACK Channel
 Allocated to feedback DL HARQ acknowledgement

Fractional Frequency Reuse (1)
F1
 Frequency reuse is defined as (C N S): F3

 C - number of BS in the reuse cluster F1 F2

 N - number of the channels (or channel group) F3

F2 F1
 S - number of the sectors of each BS F3

 Examples of classical frequency reuse schemes: (1x3x3) F2

 Reuse 3: Marked as (1 3 3) and requires 3
frequency assignment F1

 Reuse 1: Marked as (1 1 3) and requires one F1

frequency assignment F1 F1

F1
 Segmentation F1 F1

 PUSC symbol structure enables division of the F1

subcarriers into three segments and allows a reuse 3 (1x1x3) F1

scheme with a single channel assignment
 Reuse 1 scheme has higher capacity at the center F1
of the cell but is susceptible to interference at the F1
{Seg. 0}

cell edge. F1
{Seg. 2}
F1
{Seg. 0} {Seg. 1}
F1
 Reuse 3 scheme has lower capacity but provides a {Seg. 2}
F1 F1
more reliable link at the cell edge {Seg. 1}
F1
{Seg. 0}

{Seg. 2}
F1
(1x3x3)
{Seg. 1}

Fractional Frequency Reuse (2)

 Fractional Frequency Reuse (FFR): By exploiting the frequency – time grid structure
of the OFDM frame it is possible to combine Reuse 1 and Reuse 3
 FFR can be implemented in both time and frequency domain
 Time domain FFR
 Subframe is divided into two zones
 R3 zone in which a single segment is allocated and subcarriers are boosted
by 5dB
 R1 zone in which all subcarriers are allocated
 The zones boundary is static across the whole coverage area
 Users are allocated dynamically to one of the zones based on their CINR reports

Frequency Reuse Parameters Selection
 Cell ID
 Each three sector BS is assigned with Cell ID (range: 0..31)
 Should be unique among neighbors
 Each sector in the BS is assigned with unique segment (range: 0..2)
 The preamble index is calculated as 32*Segment + Cell ID
 DL Permutation Base
 Used to randomize pilot modulation and subcarrier permutation
 If R1 is used, DL Permutation Base should be set to a unique value among neighbors (range: 0..31)
 UL Permutation Base
 Used to randomize pilot modulation and subcarrier permutation
 If R1 is used, UL Permutation Base should be set to a unique value among neighbors (range: 0..127)
 If R1 is not used
 UL Permutation Base for neighbor BS with the same FA should be set with an offset of 35 (e.g. 0, 35,
70, 115)
 UL Permutation Base the three sectors in the same BS should be set to the same value (to maintain
orthogonality)

Multiple Antenna Techniques
 Open Loop MIMO (IO-MIMO)
 Channel State Information (CSI) is not available at the
transmitter
 Space Time Block Coding (STBC) – Matrix A
 Spatial Multiplexing – Matrix B
 Collaborative UL MIMO (CSM)
 Closed Loop MIMO (IO-BF)
 CSI is required at the transmitter, through feedback
channels or reciprocity in TDD
 Beamforming techniques

Open Loop MIMO (1)

 Diversity
 Improves probability of the receiver to overcome
fades.
 Diversity order (d) = NTx x NRx
 BER is proportional to CINR-d
 Maximum Receive Ratio Combining (MRC)
 Multiple receive paths are combined coherently
 Space Time Block Code (STBC or Matrix A)
 A single data stream is replicated and
transmitted over two antennas
 Redundant data is encoded using a
mathematical algorithms known as STBC.
 Receiver may combine this with MRC to
increase diversity order

Open Loop MIMO (2)

 Spatial Multiplexing
 Used to increase system capacity by exploiting the
dispersive nature of the wireless channel
 System capacity grows linearly with Min{NTx, NRx}
 Spatial Multiplexing (MIMO Matrix B)
and in the same frequency from different BS antennas
 Mandates multiple receive antennas at the MS
 Assuming channels are uncorrelated, receiver can
retrieve the data using decoding algorithm known as
VBLAST
 Collaborative Spatial Multiplexing (CSM)
and in the same frequency from different MS
 Assuming channels are uncorrelated, BS can retrieve the
data using the same Matrix B technique

Closed Loop MIMO
 Beamforming
 Leverage arrays of transmit and receive antennas to control
the directionality and shape of the radiation pattern.
 Channel information is communicated from the MS to the
BS using Uplink Sounding. Based on CSI, the BS utilizes
signal processing techniques to calculate weights to be
assigned to each transmitter controlling the phase and
relative amplitude of the signal
 Can be used for interference cancellation.
 Can be used for both coverage and capacity enhancements

Dynamic Selection of MIMO Mode

 Adaptive Mode Selection
 Dynamic adaptation algorithms are required to
optimize system performance and select the
appropriate mode based on DL SNR and
channel conditions

Ranging
 Ranging is an UL PHY procedure that maintains
the quality of the radio link communication
between BS and MS.
 BS estimates CINR, time of arrival and frequency
error of MS transmission and provides power,
timing and frequency adjustment commands
 Initial and periodic ranging procedures are defined
 Both regular transmission and contention
transmission can be used
 Contention transmission is done in special UL
regions using ranging (CDMA code)
 Codes are created using PRBS generator and are
BPSK modulated
 Each MS randomly chooses one ranging code from
a bank of specified binary codes.
 256 distinct codes are available and are divided by
configuration into four groups:
 IR codes
 PR codes
 BR codes
 HO codes
 Since codes are orthogonal, BS can process multiple
codes transmitted simultaneously by different MS

Power Control (1)
 Power control mechanisms are supported in the UL to maintain the quality of the
link. Basic requirements of the power control mechanism are:
 Power control is designed to support fluctuations of 30dB/sec
 BS accounts for the effect of various bust profiles on amplifier saturation while issuing
power control commands
 MS reports maximum transmission power for each modulation
 MS maintains the same transmitted power spectral density (PSD), regardless of the
number of assigned subchannels. Therefore, transmission power level is proportionally
decrease or increased with the subchannel assignment without specific power control
messages
 The requirements calls for a complex link adaptation algorithm that makes a
joint decision regarding MCS, resource allocation and power adjustment
 MS reports available power headroom periodically and on a per demand basis

Power Control (2)
 Closed Loop Power Control
 MS adjust its PSD based on BS commands only.
 BS command may be explicit or implicit (by modifying the MCS)

 Open Loop Power Control
 MS adjust its PSD independently, based on changes in the DL signal level according
the following formula

P(dBm)= L+C⁄N+NI – 10log10(R)+Offset_SSperSS+Offset_BSperSS
 L: Estimated propagation loss
 C/N: Carrier to noise for the burst profile in the current transmission
 NI: Estimated average power level of noise an interference
 R: repetition rate
 Offset SS per SS: Correction factor employed by the SS (set to zero for passive mode)
 Offset BS per SS: Correction factor employed by the BS

 Closed loop power control may be combined with open loop as an outer mechanism,
using the ‘Offset BS per SS’ parameter

Channel Quality Measurements
 MS provides BS with feedback on the quality of the DL signal. This feedback
drives the link adaptation algorithm. Reported metrics include:
 Received Signal Level (RSSI)
 Carrier to Interference and Noise Ratio (CINR)
 Based on preamble for R3 and R1 frequency reuse schemes
 Based on pilots in specific zone
 Preferred MIMO mode
 Feedback can be carried over the Channel Quality Indication Channel (CQICH) in a special UL region
or over MAC control message

Throughput Calculation Example
1. Calculate number of OFDM symbols in frame
 47 symbols for 10MHz channel
2. Determine DL/UL split based on profile
 26/21
3. Deduce one symbol from DL subframe for preamble
4. Deduce overhead
 DL: 4 symbols for the MAPs
 UL 3 symbols for ranging, HARQ feedback and CQICH zones
5. Calculate number of slots available for data
 DL: PUSC 30 x (20/2)=300
 UL: PUSC 35 x (18/3)=210
6. Determine burst profile and MIMO mode
 DL: 64QAM 5/6 Matrix B
 UL: 16QAM 1/2
7. Calculate bits per frame
 DL: 300 x 48 x 6 x (5/6) x 2=144,000
 UL: 210 x 48 x 4 x (1/2)=20,160)
8. Calculate bits per second by dividing by frame duration
 DL: 28.8Mbps
 UL: 4Mbps

Part II
Medium Access Control Layer

MAC Functions

 Segment or concatenate service data units (SDU) received from higher layers
into the MAC protocol data unit (PDU)
 Select the appropriate burst profile and power level to be used for
transmission (link adaptation)
 Retransmission of MAC PDU (ARQ)
 Provide QoS control and priority handling of MAC PDU associated with
different data and signaling bearers (Packet Scheduling)
 Schedule MAC PDU over PHY resources (frame building)
 Mobility management (handover)
 Security and key management
 Provide power saving modes (Idle/Sleep)

MAC: Protocol Layers

Network

Network Interface
Received SDU’s
MAC-CS
Con #1 Con #2 Con #n

MAC-CPS
Fragmentation

Radio
Link
Resource
Maintenance
BW Request Control
Scheduler
ARQ AMC
Manager

Security
Data Encryption

PHY and RF
Link Quality
ACK PHY module Feedback
Feedback (e.g. CINR)

UL ACK channel DL burst Ranging channel CQICH channel

Convergence Sublayer (CS)

 Convergence sublayer is an adaptation layer that masks the higher layer protocol
and its requirements from the MAC layer
 Several convergence sublayers are supported
 IPv4/IPv6 with and without ROHC
 802.3 (Ethernet)
 802.1/Q VLAN
Upper Layer Entity (e.g. bridge, router) Upper Layer Entity (e.g. bridge, router)
 IPv4/IPv6 over 802.3
SDU
 IPv4/IPv6 over 802.1/Q VLAN
SAP SAP

CID 1

CID 2
Classification Reconstruction
text (e.g. undo PHS)
text

CID n

{SDU, CID,...} {SDU, CID,...}

SAP SAP
802.16 MAC CPS 802.16 MAC CPS

Convergence Sublayer Functions

 Classification
 WiMAX MAC is connection oriented. Each unidirectional logical connection between MS and BS is
identified by a Connection Identifier (CID). Connection can carry user plane data and control plane
information
 CS performs many-to-one mapping between higher layer applications and a specific connection.
Applications with different QoS requirements are mapped to different connections.
 The mapping is performed on the basis of the header fields of the higher layer protocol, e.g. VLAN,
IP source address.
 Classification may be performed at the BS or at the ASN-GW

 Packet Header Suppression (PHS):
 Repetitive portion of the packet header may be suppressed by the transmitter and restored by the
receiver
 Improves efficiency of the network, especially for applications with small packet size (e.g. VoIP)
 PHS rules at the transmitter and the receiver are synchronized during service flow initiation and
modification
 PHS may be performed at the BS or at the ASN-GW
 Robust Header Compression (ROHC) is an alternative to PHS, which is transparent to the MAC
operation. Defined by RFC 3095, ROHC compress the IP, UDP, RTP and TCP headers of IP packets
(can compress 60 bytes of overhead into 3 bytes)

MAC PDU Construction and Transmission

 SDU arriving from higher layer are assembled to create MAC PDU.
 Depending on the size of allocation, multiple SDU can be packed on a single
PDU, or a single SDU can be fragmented over multiple PDUs.
 Multiple MAC PDUs intended for the same receiver can be concatenated onto a
single transmission burst

SDU 1 ARQ Block SDU 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Fragment 1 Fragment 2 Fragment 1 Fragment 2

Header Fragment 1 Header Fragment 2 Fragment 1 Header Fragment 2

PDU 1 PDU 2 PDU 3

DL/UL Burst

ARQ

 For application sensitive to packet error (TCP), ARQ can be used on top of
HARQ to eliminate residual error rate.
 ARQ can be enabled on a per connection basis.
 For ARQ-enabled connection, SDU is first partitioned into fixed length ARQ
blocks and a block sequence number (BSN) is assigned to each block.
 The length of the ARQ blocks and the ARQ window size (number of blocks managed by the
transmitter and receiver at an given time) are set during connection establishment.
 Once SDU is partitioned into ARQ blocks, the partition remains in effect until all the blocks have
been received and acknowledged by the receiver
 ARQ enable connection are limited in throughput by Block Size x Window Size / ACK Latency
 For ARQ enabled connection, fragmentation and packing subheader contains the
BSN of the first ARQ block following the subheader.
 Receiver feedback (ACK) can be sent as a stand alone MAC PDU or piggybacked
on the payload of a regular MAC PDU
 ARQ feedback can be selective or accumulative

MAC PDU Structure (1)

 Each MAC PDU consists of a header which may followed by a payload and a
cyclic redundancy check (CRC)

MSB

LSB
Generic MAC Payload: SDU’s & Subheaders CRC
Header (Optional) (Optional)
6 bytes 0-2038 bytes 4 bytes

 Generic MAC Header (GMH) is used for carrying user plane data and MAC
control messages
 HT: Header type (HT = 0 for GMH)

HT=0 (1)

Rsv (1)

Rsv (1)
EC (1)
 EC: Encryption control

CI (1)
EKS LEN
Type (6)
 Type: Indicates subheaders included in the payload (2) MSB (3)
 CI: CRC indicator
 EKS: Encryption key sequence LEN LSB (8) CID MSB (8)
 LEN: Length of MAC PDU in bytes
 CID: Connection ID associated with the PDU
 HCS: Header check sequence CID LSB (8) HCS (8)

MAC PDU Structure (2)

 Signaling MAC header is defined used for the UL
(this header is not followed by payload)
 Signaling header type I
 BW request header (aggregate/incremental)
 BW request and UL TX power report header
 BW request and CINR report header
 CQICH allocation request header
 PHY channel report header (DIUC, TX power, TX power
headroom)
 BW request and UL sleep control header
 SN report header (ARQ)
 Signaling header type II
 Used for MS feedback report
 14 feedback permutations are defined: CINR, TX power,
DIUC, AMC band indication bitmap, MIMO feedback, etc.

Bandwidth Request and Allocation

 All decisions related to DL resource allocation to various MS are made by the BS on a
per CID basis. BS schedules MAC PDUs based on the connection QoS requirements.
The allocation is indicated in the DL MAP.
 MS requests UL BW in bytes on a per connection basis by using either stand alone
BW requests or piggybacking BW requests on generic MAC PDU.
 BW request can be incremental or aggregate
 UL grants are done on a per MS basis and indicated in the UL MAP. MS UL scheduler
distribute the granted allocation among its various connections.
 BS supports BW polling, whereby dedicated (unicast polling) or shared (multicast
polling) UL resources are provided to the MS to make BW requests.
 Multicast polling is based on contention mechanism, in which MS sends a randomly selected code in a
dedicated UL region.
 Contention is resolved using an exponential backoff window mechanism

Quality of Service

 Each service flow is associated with QoS parameters: maximum traffic rate,
guaranteed traffic rate, maximum latency and Priority. MAC layer is responsible
to ensure QoS requirements subject to loading conditions.
 Each service flow is mapped to a certain transport connection with its own QoS
parameters. Transport connections may be Unicast, Multicast or Broadcast
 Two Management connections are established for each MS to reflect different
levels of QoS requirements
 Basic management connection: Used to transfer short, time-critical MAC and radio control messages
 Primary management connection: Used to transfer longer, more delay-tolerant messages such as
authentication and connection setup

QoS Architecture

Data Packet
Classification Scheduler
(SDU)

Classification Service Flow Attributes Scheduler

•IP Protocol •Maximum traffic rate •Select PDU based on SF
•Source/Dest IP Address •Minimum reserved traffic rate attributes and subject to
•ToS •Latency available resources
•Source/Dest MAC Address •Priority
•VLAN •Grant/polling interval

Service Flows: Three Phase Activation

 SF defined in BS/MS
 QoS parameters known to BS/MS. Usually defined by
Provisioned higher layer entity
 SFID assigned
 Traffic disabled

 Transient stage
 QoS parameters are a subset of the provisioned
Admitted set, following BS admission control
 Resources are allocated
 CID assigned
 Traffic disabled

 Traffic enabled
Active

Data Services & Scheduling Types
 Five scheduling services used to collect BW requirements from MS’s:
 Unsolicited Grant Service (UGS)
 Real time applications generating fixed rate data
 Provides fixed size grants on periodic basis and does not need the MS to explicitly request BW.
 Extended Real Time Polling Service (ertPS)
 Real time applications with variable rate, guaranteed rate and latency, e.g. VoIP with silence
suppression
 Similar to UGS, but allows dynamic adaptation of grant size based on MS feedback
 Real Time Polling Service (rtPS)
 Real time applications generating variable rate data
 BS provides unicast polling opportunities for the MS to request BW
 Non Real Time Polling Service (nrtPS)
 Delay tolerant applications with guaranteed data rate
 Similar to nrtPS, except that MS is allowed to use contention BW requests in addition to the polling
 Best Effort (BE)
 Applications with no rate or delay requirements
 Based on contention based polling opportunities

Scheduling Algorithms

 The scheduler prioritizes the backlogged SDUs in the DL and the pending BWR in the UL.
Prioritization is done on a per SF basis based on the various attributes associated with the
service flow.
 Scheduler target: Maximize system capacity subject to service requirements of each flow.
Scheduling procedure is outside the scope of the WiMAX standard and has been left to the
equipment manufacturers to implement. It has a profound impact on the overall capacity and
performance of the system, thus it serves as a key differentiator among vendors.
 Classical scheduling algorithm
 Strict Priority (SP) SFi = argmax(iPi)
 Proportional Fairness (PF) SFi = argmin(iri /Ri)
 Adaptive PFS takes into account link condition (spectral efficiency) in order to maximize
system capacity
 APFS metric SFi = argmin(iwiri /Ri)
 Combination of different algorithms is possible, e.g. SP for the guaranteed rate and APFS for
the excess bandwidth

Adaptive Modulation and Coding Algorithms (1)

 WiMAX supports dynamic adaptation of modulation and coding scheme as well as MIMO
mode on a per connection and per frame basis.
 Link adaption algorithms aim to maximize spectral efficiency while maintaining link quality
metric (typically target packet error rate)
 DL adaptation
 Input:
 DL CINR feedback from the MS based on DL preamble and/or DL pilots
 Preferred MIMO mode based on channel conditions as perceived by the MS
 HARQ error rate based on MS feedback received on the HARQ ACK UL channel
 Output:
 MCS
 MIMO Mode (Matrix A/Matrix B)
 Zone (e.g. R1 zone or R3 zone)

Adaptive Modulation and Coding Algorithms (2)

 UL adaptation
 Input:
 UL CINR as measured by the BS PHY
 MS transmission power headroom as reported by the MS
 HARQ error rate as indicated by BS PHY
 Output:
 MCS
 Power adjustment
 Maximum number of subchannels that may be allocated
 MIMO mode
 Two modes of operation are supported: The first selects a solution that maximize the spectral efficiency (highest order
possible MCS) and the second selects a solution that maximizes the user throughput, i.e. the spectral efficiency multiplied
by the maximum number of subchannels

Security

 Security architecture of mobile WiMAX support the following requirements:
 Privacy: Provide protection from eavesdropping as the user data traverse the network
 Data integrity: Ensure the user data and control messages are protected from being modified
while in transit
 Authentication: A mechanism to ensure that a given user/device is the one it claims to be.
Conversely, the user/device should be able to verify the authenticity of the network that it is
connecting to (mutual authentication)
 Authorization: Mechanism to verify that a given user is authorized to receive a particular
service
 Access control: Ensure that only authorized users are allowed to get access to the offered
services

Public Key Infrastructure (PKI)

 On way to enable secure symmetric key encryption is to establish a shared secret
between transmitter and receiver.
 Asymmetric key encryption is a solution to the key distribution problem.
 Based on a public key and a private key that are generated simultaneously using the same algorithm,
RSA
 Ciphertext that is encrypted with one key can be decrypted by the other key
 Public key infrastructure can be used for variety of security applications:
 Authentication (see example in next slide)
 Shared secret key distribution
 Message integrity
 Digital certificates

PKI – Mutual Authentication

User A User B

Send (Random Number A, My Name) encrypted with public key of B

Send (Random Number A, Random Number B, Session Key) encrypted with public key of A

Send (Random Number B) encrypted with session key

Begin transferring data encrypted with session key

Authentication and Access Control

 In general, access control system has three elements:
 Supplicant: an entity that desired to get access
 Authenticator: an entity that controls the access gate
 Authentication server: an entity that decides whether the supplicant should be admitted
 Extensible Authentication Protocol (EAP)
 A simple encapsulation protocol that can run on any L2 protocol
 Based on a set of negotiated messages that are exchanged between the supplicant and the
authentication server
 EAP includes a number of EAP methods, which define the rules for authenticating a user and/or a
device and the set of credentials.
 EAP Transport Layer Security (TLS) defines a certificate based strong mutual authentication.
 In WiMAX, EAP runs from the MS to the BS over PKMv2 (Privacy Key Management) security
protocol. The BS relays the authentication protocol to the authenticator in the ASN-GW. From the
authenticator to the authentication server, EAP is carried over RADIUS or DIAMETER.

Encryption

 Mobile WiMAX encryption is based on Advanced Encryption Standard (AES)
which is a symmetric key encryption system.
 AES algorithm operates on a 128 bit block size of data. The encryption key size
in the case of WiMAX is 128 bits long.
 The AES Traffic Encryption Key (TEK) is also AES encrypted using the Key
Encryption Key (KEK)
 The KEK is a derivative of the Authorization Key (AK) which is a shared
secret between the MS and the BS.
 Cipher based MAC (CMAC) is used as the mandatory mode for message
authentication
 AES data encryption provides a built in data authentication capability
 AES encryption adds 12 bytes of overhead.

Network Entry

Frequency Scanning Authentication

DL & UL Synchronization Registration

Initial Ranging Service Provisioning

Negotiate Basic Capabilities

Network Entry: Frequency Scanning

• MS scans frequency bands in search for the DL Frequency Scanning Authentication

preamble DL & UL Synchronization Registration

• Scanning is performed on a predefined list of
frequencies Initial Ranging Service Provisioning

• MS selects best carrier frequency base on signal Negotiate Basic Capabilities

strength or CINR
• MS scans for all preamble indexes in the selected
carrier (114 indexes) and selects the best based on
RSSI or CINR

Network Entry: Downlink and Uplink Acquisition

• BS regularly broadcasts control messages:

– Downlink Channel Descriptor (DCD) DL & UL Synchronization Registration

– Uplink Channel Descriptor (UCD)
– DL-MAP Initial Ranging Service Provisioning

– UL MAP

• MS acquires DL once valid DCD and DL-MAP are decoded
– To make a valid DCD and DL-MAP BSID and NAI should match MS configuration and
DCD and DL MAP should indicate the same DCD change counter
– To maintain DL SYNC MS should periodically receive DL-MAP and DCD

• MS acquires UL once valid UCD and UL-MAP are decoded
– To make a valid UCD and UL-MAP UCD and UL MAP should indicate the same UCD
change counter
– To maintain UL SYNC MS should periodically receive UL-MAP and UCD

Network Entry: Ranging

• Ranging is required to align BS and MS in terms of

power, frequency and timing DL & UL Synchronization Registration

• BS measure MS offsets from the UL transmission and Initial Ranging Service Provisioning

provides appropriate adjustments

MS

CDM BS
( IR C A
ode)

BS measures arrival time and
signal power and determines
-RSP e) required adjustments
RNG Continu
t,
djus tmen
(A
MS makes adjustments CDM
( IR C A
ode)

-RSP
RNG ess)
(Su cc IE
ation
A Alloc
CDM

R
(MS NG-REQ
MAC
A ddr
ess)

-RSP D)
RNG imary CI
d Pr
ic an
(Bas

Network Entry: Negotiation of Basic Capabilities

• Basic capabilities include supported modulations, FEC,

MIMO modes, HARQ, Privacy, etc. DL & UL Synchronization Registration


MS BS Negotiate Basic Capabilities

SBC-R
EQ

SP
S BC-R

Network Entry: Authentication
• Based on PKMv2 which uses EAP as the underlying Frequency Scanning Authentication

authentication mechanism
Authenticator DL & UL Synchronization Registration

MS BS AAA Server
(ASN)
SBC-REQ
MS Status Update
SBC-RSP Initial Ranging Service Provisioning

EAP Request/Identity


EAP Response/Identity
(my ID, e.g. MS MAC address)

EAP Request/EAP TLS
(TLS Start)

EAP Response/EAP TLS
(TLS Client Hello)

EAP Request/EAP TLS
(TLS Server Hello, TLS Certificate) EAP over RADIUS

(TLS Certificate)

EAP Request/EAP TLS
(TLS Finished)

EAP Success
MSK, PMK, AK MSK Established
Established MSK
PMK, AK
Established
AK Transferred to BS
SA-TEK Challenge

SA-TEK Request

SA-TEK Response

Key Request

Key Reply

Network Entry: Registration

• Registration capabilities include management mode, IP

version supported, ARQ support, supported CS, etc. DL & UL Synchronization Registration



MS BS

REG-R
EQ

-RSP
REG

Network Entry: Service Provisioning

• Creation of service flows can be initiated by either the
MS or the BS DL & UL Synchronization Registration


MS BS Negotiate Basic Capabilities

-REQ
DSA

DSA-R
SP

-A CK
DSA

Power Saving Modes

 Power saving modes enable the MS to conserve its battery resources – a critical
feature required for handheld devices.
 Two power saving modes are defined:
 Sleep Mode
 Idle Mode

Sleep Mode

 Sleep Mode is a state in which an MS conducts pre-negotiated periods of
absence from the Serving BS air interface. These periods are characterized
by the unavailability of the MS, as observed from the Serving BS, to DL or
UL traffic. Sleep Mode is intended to minimize MS power usage.
 Power Saving class may be activated per connection basis. Activation of
certain Power Saving Class means starting sleep/listening windows
sequence associated with this class. There are three types of Power Saving
Classes, which differ by their parameter sets, procedures of
activation/deactivation and policies of MS availability for data
transmission.

Idle (Paging) Mode

 Idle Mode is a mechanism that allows MS to become periodically available
for DL broadcast traffic messaging without registration at specific BS.
 Idle Mode benefits MS by removing the active requirement for Handovers
and all normal operation requirements. By restricting MS activity to
scanning at discrete intervals, Idle Mode allows the MS to conserve power
and operational resources.
 Idle Mode helps the network and BS to conserve resources by eliminating
the need to perform any link maintenance activity and handover related
procedures for MS in idle mode.

Idle Mode: Theory of Operation (1)

 The BS are divided into logical groups called paging groups. A BS may
be a member of one or more paging groups.
 MS in idle mode periodically monitors DL broadcast to determine the
paging group of its current location. When MS detects that it has moved
to a new paging group it performs location update, in which it informs
the network its new location.
 In case of pending DL traffic, the network needs to page the MS only in
all BS belonging to the current paging group of the MS

Idle Mode: Theory of Operation (2)

 On a periodic basis, the MS shall scan and synchronize on the DL for the
preferred BS in order to decode any BS broadcast paging message
 A BS Broadcast Paging message is an MS notification message indicating
either the presence of DL traffic pending, through the BS or some network
entity, for the specified MS or to poll the MS and request a location update
without requiring a full network entry.
 During idle mode MS can be in one of two states: paging-unavailable or
paging-listen interval.
 Paging-unavailable: MS is not available for paging and can power down or scan for
neighbouring BS.
 Paging-listen interval: MS listens to DCD and DL MAP of the serving BS to
determine when the broadcast paging message is scheduled
 Paging broadcast message can indicate pending DL traffic and instruct the MS to
perform network re-entry, request MS to perform location update or indicate to the
MS to return to paging unavailable state.

Mobility Management

 Handover: The migration of the MS from the air interface of one BS to the air
interface of another BS, while maintaining connection
 Network topology advertisement: BS broadcasts information about the network
topology using the MOB_NBR-ADV message:
 The message provides channel information for neighbouring base stations, which is
normally provided by each BS own DCD/UCD message. The BS obtains that information
over the backbone.
 MS scanning of neighbour BS: A BS may allocate time intervals to MS for the
purpose of monitoring and measuring the radio conditions of neighbouring BS. The
time during which the MS scans for available BS will be referred to as a scanning
interval.
 Handover may be MS initiated (typically in order to improve link quality) or BS
initiated (typically to perform load balancing)

Handover Process

 Scanning and target cell selection
 Based on certain triggers (e.g. CINR of target BS falls below 20dB, MS scans link quality of neighbouring BS
and select a suitable target BS.
 Handover Initiation
 MS initiated using MOB_MSHO-REQ
 BS initiated using MOB_BSHO-REQ

 Network re-entry with target BS
 Target BS DL SYNC and acquisition of DL/UL channel parameters
 Using information from NBR-ADV, this process can be shortened
 Initial ranging or Handover ranging
 MS RNG-REQ includes serving BS ID and target BS ID
 If the Target BS had previously received HO notification from Serving BS over the backbone then Target BS
may place a non-contention based Initial Ranging opportunity
 Negotiate Basic Capabilities, Authorization, etc.
 Handover optimization: target BS may request MS data from backbone to accelerate network entry. This data
may be used by the target BS to skip certain NE steps.
 Termination of context with previous BS

Handover Messaging - Example
MS Serving BS Target BS ASN-GW

Operational

V
BR-AD
MOB_N

MOB_S
CN-REQ

CN-RSP
MOB_S

Scanning & Association
Association Coordination
RNG-REQ

RNG-RSP

MOB_M
SHO-RE
Q

P
SHO-RS
MOB_B

MOB_H
O-IND

Obtain MS operational
parameters
Network re-entry

Operational

General Design Principles of the Architecture

 Functional decomposition: Required features are decomposed into functional
entities. The architecture shall specify open and well defined reference points
between the functional entities.
 Deployment modularity and flexibility: The architecture shall support a broad
range of deployment options. It shall scale from the simple case of a single
operator with a single base station to a large scale deployment by multiple
operators with roaming agreements
 Support of variety of usage models: Architecture shall support fixed, nomadic,
portable and mobile usage models. Both Ethernet and IP services shall be
supported.
 Decoupling of access and connectivity services: The architecture shall allow
decoupling of the access network from the IP connectivity network and services
 Support for a variety of business models: The architecture shall allow for logical
separation between the network access provider (NAP), the network service
provider (NSP) and the application service provider (ASP)
 Extensive use of IETF protocols: Network layer procedures and protocols used
across the reference points shall be based on appropriate IETF RFCs.

Access Service Network (ASN) Functions

 Access Service Network (ASN): Owned by the NAP and includes a complete set of
network functions needed to provide radio access to a WiMAX subscriber:
 WiMAX L2 connectivity with the MS
 Network discovery and selection of the WiMAX subscriber’s preferred NSP
 AAA proxy: transfer of device and/or user credentials to selected NSP AAA and temporary
storage of user profiles.
 Relay functionality for establishing IP connectivity between MS and CSN
 Mobility related functions, such as handover, location management and paging within the
ASN, including support for mobile IP
 ASN comprises network elements such as one or more Base Stations and one or more
ASN Gateways.
 BS is defined as representing one sector with one frequency assignment implementing
the R1 interface. BS functions include scheduling, service flow management,
admission control, tunnelling toward the ASN-GW, DHCP proxy, authentication
relaying, user plane encryption
 ASN-GW functions include ASN location management and paging, temporary
caching of subscriber profiles and keying material, authenticator, service flow
authorization and user plane routing

Connectivity Service Network (CSN) Functions

 Connectivity Service Network (CSN): A set of network functions that provide IP
connectivity services to the WiMAX subscribers. CSN provides the following functions:
 IP address allocation to the MS for user sessions
 AAA proxy or server for user and/or device authentication, authorization and accounting
 Policy and access control based on user subscription profiles
 Subscriber billing and inter-operator settlement
 Inter-CSN tunnelling for roaming
 Inter-ASN mobility and mobile IP home agent functionality
 Connectivity infrastructure for services such as Internet access, VPN and IP multimedia
 CSN comprises network elements such as routers, AAA proxy/servers and subscribers
database.

Protocol Layering

 Control plane is based on UDP/IP
 Data plane is based on GRE tunnelling within the ASN and IP in IP tunnelling
between ASN and CSN
 WiMAX architecture is designed to support both IP packets and Ethernet
packets, using IP-CS and ETH-CS, respectively.
 Within the ASN packets can be either routed or bridged

Protocol Layer Architecture: IP-CS

 Example presents a routed ASN. For bridged ASN, the shaded layers (GRE, IP)
would be replaced by Ethernet layer

Protocol Layer Architecture: Ethernet-CS

 Example presents a routed ASN. For bridged ASN, the shaded layers (GRE, IP)
would not be needed

GRE Tunneling

 Generic Routing Encapsulation (GRE) may be used as
tunnelling mechanism across R4 or R6.
 Allows for tunnelling of IP packets, Ethernet frames
or WiMAX specific payload
 DSCP in the Encapsulation IP Header specifies the
QoS Class. Note that it MAY differ from the DSCP in
the Encapsulated Payload.
 Source and Destination IP Addresses specify the
tunnel end points.
 The meaning of the GRE Key value is defined by the
node that allocates the Key value. GRE Key can
indicate one of the following: Specific connection, in
case classification is done by ASN-GW or Specific
MS, in case classification is done by BS
 The Sequence Number may be used for
synchronization of Data Delivery during HO.

Network Discovery and Selection

 In the general case, it is assumed that MS operates in an environment in which multiple
access networks are available and multiple service providers are offering services over
those networks. Mobile WiMAX specifies a process for network discovery and selection
 NAP discovery
 MS detects available NAPs in a wireless coverage area based on
information broadcasted by BS (Operator ID). Operator ID is
assigned by IEEE
 NSP discovery
 MS discovers available NSPs associated with the discovered NAPs
based on information either broadcasted by the BS using System
Identity Information message (SII-ADV) or unicasted to the MS
(SBC-RSP). NSP ID is assigned by IEEE
 NSP enumeration and selection
 MS selects preferred NSP based on dynamic information obtain
through the air interface and configuration information. Selection
may be automatic or manual.
 ASN attachment
 MS indicates its NSP selection by attaching to an ASN associated
with the selected NSP, and by providing its identity and home NSP
domain in the form of NAI
 The ASN uses the realm portion of the NAI to determine the next
AAA hop to where the MS’s AAA packets should be routed.

IP Address Assignment (1)

 Network Architecture supports either Mobile IP or Simple IP
 Mobile IP requires Home Agent
 Simple IP reduces scope of network and does not support mobility

 Mobile IP is used to provide CSN Anchored Mobility
 CSN Anchored Mobility Management or Macro mobility is when the MS changes to a new
anchor Foreign Agent
 Mobile IP allows an MS to communicate with other nodes after changing its point of
attachment to the network
For example, handover between BS on separate ASN-GW, or inter-technology handover
 Mobile IP is achieved by allocating an MS both a Home Address (HoA) and a Care-of
Address (CoA)
 Two forms of Mobile IP are defined; Proxy Mobile IP (PMIP) and Client Mobile IP (CMIP)
 CMIP is required to enable Inter-technology handover

IP Address Assignment (2)

 Dynamic Host Control Protocol (DHCP) is used as the primary mechanism to
allocate IP address to the MS
 The network architecture provides flexibility in allocating IP addresses to MS
 ASN-GW provides a DHCP Proxy Server
 Mobile IP or Simple IP
 Home Agent can be configured with local pool of Mobile IP Addresses
 Mobile IP only
 ASN-GW can be configured with local pool of IP addresses
 Simple IP only
 AAA Server can allocate IP addresses using IP Address Manager
 Mobile IP or Simple IP
 Simple IP
 IP address is either assigned from local address pool, or retrieved as RADIUS attributes from
AAA Server
 The ASN-GW DHCP proxy is used to transfer IP address information to MS

Authentication and Security Architecture

 Designed to support all IEEE 802.16 security services using EAP based AAA
framework.
 Supports both user and device authentication
 Supported EAP methods: EAP-TLS and EAP-TTLS
 In addition, AAA framework is used for service flow authorization, QoS policy
control and secure mobility management
 AAA framework basic steps:
 MS sends a request to the network access server (NAS) function in the
ASN
 NAS forwards the request to the service provider AAA server (NAS acts as
an AAA client on behalf of the user)
 AAA server evaluates the request and returns an appropriate response to
the NAS
 NAS sets up a service and notifies the MS

ASN Security Architecture

 Authenticator (ASN-GW or BS)
 Communicates with the AAA server using RADIUS/DIAMETER
 Authentication Relay (BS)
 Functional entity that relays EAP packets to the authenticator via an authentication relay protocol
 Key Distributor (ASN-GW or BS)
 Functional entity that holds the keys (MSK and PMK) generated during the EAP exchange
 The MSK is sent to the Key Distributor from the home AAA server, and the PMK is derived
locally from the MSK.
 Derives AK and creates AKID for an <MS, BS> pair and distributes the AK and its context to the
Key Receiver in a BS via an AK Transfer protocol
 Key Receiver (BS)
 Holds the AK and responsible for generation of IEEE 802.16e specified keys from AK

Authentication Protocols

 PKMv2 is used to perform over-the-air user/device authentication. PKMv2 transfers EAP over the
IEEE 802.16 air interface between MS and BS in ASN.
 Depending on the Authenticator location in the ASN, a BS may forward EAP messages over
authentication relay protocol (e.g. over R6 reference point) to Authenticator.
 The AAA client on the Authenticator encapsulates the EAP in AAA protocol packets and forwards
them via one or more AAA proxies to the AAA Server in the CSN of the home NSP

Authentication Procedure

 Initial network entry and
Authenticator
MS BS AAA Server
negotiation (ASN)

 Exchange of EAP messages Network Entry
Link Activation
 Establishment of the shared entity
EAP Request/Id
master session key (MSK)
EAP Response/
Identity
 Generation of authentication
EAP over RADIUS
key (AK)
ement
MSK and EMSK Establish MSK
 Transfer of authentication
key PMK derivation from MSK

AK derivation from MSK
 Transfer of security
AK
associations
SA-TEK Challenge
 Generation and transfer of SA-TEK Request

traffic encryption keys SA-TEK Response
(TEK) Key Request

 Service flow creation Key Reply

Quality of Service Architecture
 Architecture designed to support static and
dynamic service flow provisioning
 Home Policy Function (PF)
 Contains policy database of the home NSP and evaluates
service requests against these policies. Requests may come
from the SFA or from the AF

 Application Function (AF)
 An entity that can initiate service flow creation on behalf of a
user, e.g. SIP proxy client

 AAA server
 Holds users QoS profile and associated policy rules
 Option 1: The information is downloaded to the SFA during NE
as part of the authentication and authorization procedure
 Option 2: AAA server can provision the PF with subscriber
related information and the PF shall determine how incoming
SF are handled

 Service Flow Authorization (SFA)
 Evaluates SF request against user QoS profile (in case AAA
information was downloaded to SFA)

 Service Flow Management (SFM)
 Responsible for creation, admission, activation, modification
and deletion of SF

Service Flow Creation (Static)

 Example assumes users
associated policies were
downloaded to the SFA
from the AAA
 Based on Resource
Reservation
Request/Response

ASN Gateway: Mobility Function

 Handover may be MS initiated (typically for link
quality maintenance) or ASN initiated (typically for HA
load balancing)
 ASN anchored mobility – anchored Foreign Agent
R3
(FA) unchanged R3
ASN- ASN-
 No impact on IP level GW1 GW2

 Data Path function (DPF): responsible for setting up and R4
managing bearer paths needed for data packet transmission.
 Handover function (HO): responsible for making HO decisions

R6
and performing the signalling procedures related to HO

R6
R6
 Context function: responsible for exchange of state information
among network elements impacted by HO

 CSN anchored mobility – anchored FA changed BS1 BS2 BS3
R8
 Involves mobility across different IP subnets and therefore
requires IP layer mobility management


R1
Two types of Mobile IP implementations are defined

R1

R1
 Client MIP – based on mobile IP client at the MS
 Proxy MIP – ASN-GW implements the mobile IP client
on behalf of the MS. PMIP is transparent to the MS.

Handover Procedures
MS Initiated – preparation phase

Handover Procedures
MS Initiated – action phase
Serving/
Anchor ASN-
MS Serving BS Target Target BS’s Authenticator
GW
ASN-GW

MOB_HO-IND
HO_cnf
HO_cnf
HO_Ack
HO_Ack

Context_Req
Context_Req
Context_Rpt
Context_Rpt

Path_Prereg_Req
Path_Prereg_Req
Path_Prereg_Rsp
Path_Prereg_Rsp
Path_Prereg_Ack
Path_Prereg_Ack

RNG-REQ
Path_Reg_Req
Path_Reg_Req
Path_Reg_Rsp
Path_Reg_Rsp
RNG-RSP

CMAC_Key_Count_Update
CMAC_Key_Count_Update
CMAC_Key_Count_Update_Ack
CMAC_Key_Count_Update_Ack

Path_Dereg_Req
Path_Dereg_Req
Path_Dereg_Rsp
Path_Dereg_Rsp
Path_Dereg_Ack
Path_Dereg_Ack

HO_Complete
HO_Complete

Paging and Idle Mode Operation

 Paging is the method used to alert an idle MS about incoming message.
 Paging architecture is based on three functional entities
 Paging Controller (PC)
 Administrates activities of idle mode MS
 Typically located at the ASN-GW
 Paging Agent (PA)
 BS functional entity that handles interaction between PC and air interface related paging functionalities
 One or more PA can form a Paging Group (PG), which is managed by the network operator. PA may
belong to more than one PG
 Location Register (LR)
 A database containing information on idle mode MS (e.g. PGID, paging cycle, paging offset, SF
information)

Fundamentals of Wimax

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