IEEE Standard 802.16:

A Technical Overview of the Mobile

WiMAX Air Interface and Beyond

Eyal Verbin

Contents

1. Overview of WiMAX

Background on IEEE 802.16 and WiMAX

Salient Features of WiMAX

2. Physical Layer

The Broadband Wireless Channel

OFDM Principles

Channel Coding

Hybrid-ARQ

OFDM Symbol Structure

Frame Structure

Fractional Frequency Reuse

Transmit Diversity and MIMO

Ranging

Power Control

Channel Quality Measurements

3. Medium Access Control Layer

Convergence Sublayer

MAC PDU Construction and Transmission

Bandwidth Request and Allocation

ARQ

Quality of Service

Scheduling

Adaptive Modulation and Coding

Security

Network Entry Procedures

Power saving Modes

Mobility Management

4. WiMAX Network Architecture

Network Reference Model

Protocol Layering

IP Address Assignment

Authentication and Security Architecture

Quality of Service Architecture

Mobility Management

Paging

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

Part IWiMAX Physical Layer

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)

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

Channel

Encoder

Subcarrier

Mapping

and Pilot

Insertion

Space

Time

Encoder

Symbol

MappingInterleaver

IFFT

IFFT

D/A

D/A

Randomizer

Subcarrier

Mapping

and Pilot

Insertion

Antenna #0

Antenna #1

From MAC

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 CodeFurther 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.

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

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

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)

Frequency Domain

Representation

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)

s/NFFT: (e.g. 10.9 KHz for 10MHz)

Useful symbol time Tu (e.g. 91.4 Sec 10MHz)

CP time Tg = u: (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

Spectral Efficiency

DL Example (10 MHz, 64QAM 5/6)

Spectral efficiency = 3.5 bit/sec/Hz

(1 )

data m r

FFT

N b c nREfficiency

BW CP N

/data m r sR N b c T

535 720 6 /102.9

6Mbps

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

First symbol of the DL subframe

Used for time and frequency synchronization, initial channel estimation, noise and interference estimation

Carries BS information (Cell ID and segment)

Frame Control Header (FCH)

Transmitted with QPSK and

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 with

FEC and repetition as indicated by FCH

Indicates MAP length

Pre

am

ble

FCH

DL MAP

DL MAP

( )

DL Burst #2

DL Burst #3

DL Burst #1

(UL MAP)

DL Burst #8

DL Burst #9

DL Burst #10

DL Burst #13

DL Burst #11

DL Burst #12

DL Burst #14

Time

Fre

qu

en

cy

Not Allocated

Zone #1: PUSC 1/3 SISO Zone #2: PUSC 1/3 MIMO

DL Burst #15

DL Burst #16

Zone #3: PUSC All MIMO

DL Subframe (2)

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

IE defines the DL and UL bursts

The scope of the DL MAP is the current frame

The scope of the UL MAP is the next frame

Standard DL IE includes:Connection Identifier (CID)

Downlink Interval Usage Code (DIUC), which defines the MCS and the FEC used for the burst

Repetition coding indication

Burst boundariesSymbol offset (start of burst in time domain)

Subchannel offset (start of burst in frequency domain)

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)

Pre

am

ble

FCH

DL MAP

DL MAP

( )

DL Burst #2

DL Burst #3

DL Burst #1

(UL MAP)

DL Burst #8

DL Burst #9

DL Burst #10

DL Burst #13

DL Burst #11

DL Burst #12

DL Burst #14

Time

Fre

qu

en

cy

Not Allocated

Zone #1: PUSC 1/3 SISO Zone #2: PUSC 1/3 MIMO

DL Burst #15

DL Burst #16

Zone #3: PUSC All MIMO

UL Subframe

Multiple Access: OFDMA

No Preambles

Standard UL IE includes:

Connection Identifier (CID)

Uplink Interval Usage Code

Duration (in OFDMA slots)

Repetition coding indication

Dedicated Control Zones

UL Ranging

Dedicated UL ranging subchannel

Used for BW requests as well

Quality Information Channel

UL CQICH is allocated for the MS to feedback

channel state information

UL ACK Channel

Allocated to feedback DL HARQ acknowledgement

Time

Fre

qu

en

cy

Initial

Ranging/HO

Ranging

Perio

dic

Rang

ing/

BWR

ACK

UL Burst #1

UL Burst #2

UL Burst #3

CQICH

6 SC

6 SC

Noise Burst 10 SC

12 SC

3 Symbols 3 Symbols

Not AllocatedNot Allocated

Zone #1

Segmented PUSC

Zone #2

Un-Segmented PUSC

Fractional Frequency Reuse (1)

Frequency reuse is defined as (C N S):

C - number of BS in the reuse cluster

N - number of the channels (or channel group)

S - number of the sectors of each BS

Examples of classical frequency reuse schemes:

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

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

Segmentation

PUSC symbol structure enables division of the subcarriers into three segments and allows a reuse 3 scheme with a single channel assignment

Reuse 1 scheme has higher capacity at the center of the cell but is susceptible to interference at the cell edge.

Reuse 3 scheme has lower capacity but provides a more reliable link at the cell edge

F1

F2

F3

F1

F2

F3

F1

F2

F3

(1x3x3)

F1

F1

F1

F1

F1

F1

F1

F1

F1

(1x1x3)

F1

{Seg. 0}

F1

{Seg. 1}

F1

{Seg. 2}F1

{Seg. 0}

F1

{Seg. 1}

F1

{Seg. 2}F1

{Seg. 0}

F1

{Seg. 1}

F1

{Seg. 2}

(1x3x3)

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

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 (1)

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)

Multiple data streams are transmitted at the same time and in the same frequency from different MS

Assuming channels are uncorrelated, BS can retrieve the data using the same Matrix B technique

Open Loop MIMO (2)

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

Closed Loop MIMO

Adaptive Mode Selection

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

Dynamic Selection of MIMO Mode

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

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,

P(dBm)= L+C N+NI 10log10(R)+Offset_SSperSS+Offset_BSperSS

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 IIMedium 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

Fragmentation

SchedulerARQ

Manager

Link

Maintenance

Data Encryption

ACK

FeedbackPHY module

Link Quality

Feedback

(e.g. CINR)

Radio

Resource

Control

Con #1 Con #2 Con #n

Network Interface

MAC-CS

MAC-CPS

Security

PHY and RF

UL ACK channel DL burst Ranging channel CQICH channel

BW Request

AMC

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

IPv4/IPv6 over 802.3

IPv4/IPv6 over 802.1/Q VLAN

text

Upper Layer Entity (e.g. bridge, router) Upper Layer Entity (e.g. bridge, router)

802.16 MAC CPS

Classification

CID 1

CID 2

CID n

SAP

SAP

SDU

{SDU, CID,...}

802.16 MAC CPS

text

Reconstruction

(e.g. undo PHS)

SAP

SAP

{SDU, CID,...}

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

1 171615141312111098765432

Header Fragment 1 Header Fragment 2 Fragment 1 Header Fragment 2

DL/UL Burst

SDU 1 SDU 2

Fragment 1 Fragment 2 Fragment 1 Fragment 2

ARQ Block

PDU 3PDU 2PDU 1

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)

Generic MAC Header (GMH) is used for carrying user plane data and MAC

control messages

HT: Header type (HT = 0 for GMH)

EC: Encryption control

Type: Indicates subheaders included in the payload

CI: CRC indicator

EKS: Encryption key sequence

LEN: Length of MAC PDU in bytes

CID: Connection ID associated with the PDU

HCS: Header check sequence

Generic MAC

Header

6 bytes

Payload: & Subheaders

(Optional)

0-2038 bytes

CRC

(Optional)

4 bytes

MS

B

LS

B

CID LSB (8) HCS (8)

LEN LSB (8) CID MSB (8)

LEN

MSB (3)Type (6)

HT

=0 (

1)

EKS

(2)EC

(1)

Rsv

(1)

CI

(1)

Rsv

(1)

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

(SDU)Classification Scheduler

Classification

IP Protocol

Source/Dest IP Address

ToS

Source/Dest MAC Address

VLAN

Service Flow Attributes

Maximum traffic rate

Minimum reserved traffic rate

Latency

Priority

Grant/polling interval

Scheduler

Select PDU based on SF

attributes and subject to

available resources

Service Flows: Three Phase Activation

SF defined in BS/MS

QoS parameters known to BS/MS. Usually defined by higher layer entity

SFID assigned

Traffic disabled

Transient stage

QoS parameters are a subset of the provisioned set,

following BS admission control

Resources are allocated

CID assigned

Traffic disabled

Traffic enabled

Provisioned

Admitted

Active

Data Services & Scheduling Types

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((1+ wi)ri /Ri)

The weight i is inversely proportional to the link quality

The parameter can be controlled by the operator in order to balance between absolute fairness and maximization of capacity

Combination of different algorithms is possible, e.g. SP for the guaranteed rate and APFS for the excess bandwidth

Adaptive PFS

Absolute fairness: each SF

receives equal BW

Lower system capacity

Link quality awareness: SF

with better link quality are

preferred

Higher spectral efficiency

8 SF with equal BW requirements and different channel conditions

Adaptive Modulation and Coding Algorithms

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) in case FFR is used

DL Adaptation

Phase I (current) Algorithm

Select MCSA if MS reported CINR margin(fixed, global) > Threshold(MCSA) and no higher order MCS

meets this requirement

Select Matrix B if MS reported CINR margin(fixed, global) > Matrix B Threshold AND MS reported

Matrix B as its preferred MIMO mode. Otherwise, select Matrix A

Phase II Algorithm

Adds HARQ error rate feedback into consideration, by adjusting both the MCS and the margins in case

HARQ error rate goes outside a certain window

This approach makes the system much less sensitive to the configured CINR thresholds

Select MCSA if MS reported CINR margin(dynamic, per MS) > Threshold(MCSA) and no higher order

MCS meets this requirement

Select Matrix B if MS reported CINR margin(dynamic, per MS) > Matrix B Threshold AND MS

reported Matrix B as its preferred MIMO mode. Otherwise, select Matrix A

If HARQ error rate falls below a HARQ Error Low threshold, decrease margin and increase MCS by one

step (e.g. From 16QAM to 16QAM ) or based on CINR, whichever provides better spectral

efficiency

If HARQ error rate rises above HARQ Error High threshold, increase margin and decrease MCS by one

step or base on CINR, whichever provides better link budget

UL Adaptation (1)

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

UL Adaptation (2)

For each MS with each UL CINR measurement, for each supported MCS calculate

required power adjustment, expected power headroom and maximum possible

number of subchannels for the MS, where

The required power adjustment is based on the difference between measured CINR and the CINR

threshold of the specific MCS, including margins

The expected power headroom is the difference between MS reported maximum power per MCS and the

MS transmission power following the required adjustment

Expected power headroom is updated by the BS based on periodic power headroom reports from the

MS

Maximum number of subchannels per MCS is calculated as N = Floor(10^(Power Headroom/10)/24)

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:

In Spectral Efficiency Mode: From the list of MCS for which the calculated number of subchannels is not

less then the minimum configuration (typically 2) Select MCSi = argmaxi(bi)

In User Throughput Mode: Select MCSi = argmaxi(biNi)

UL Adaptation - Example

Assumptions:

CINR thresholds are 2, 5, 8 and 11 dB for QPSK , QPSK , 16QAM and 16QAM , respectively.

CINR margin 4dB

MS maximum TX power 25dBm and 23dBm for QPSK and 16QAM, respectively

MS current transmission power 3dBm per subcarrier (PSD)

MS measured UL CINR 8dB

Minimum subchannels per user: 1

Required power offset is -2dB, +1dB, +4dB and +7dB for QPSK , QPSK , 16QAM and 16QAM , respectively

Expected power headroom following adjustment is 24dB, 21dB, 16dB and 13dB for QPSK , QPSK , 16QAM and 16QAM , respectively

Maximum number of subchannels is 10, 5, 1 and 0 for QPSK , QPSK , 16QAM and 16QAM , respectively

In spectral efficiency mode the selected MCS will be 16QAM with power correction of +4dB and a single allocated subchannel

In user throughput mode the selected MCS will be QPSK with power correction of -2dB and maximum of 10 allocated 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

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

User B

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

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

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency Scanning

Network Entry: Frequency Scanning

MS scans frequency bands in search for the DL preamble

Scanning is performed on a predefined list of frequencies

MS selects best carrier frequency base on signal strength or CINR

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

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency Scanning

Network Entry: Downlink and Uplink Acquisition

BS regularly broadcasts control messages:

Downlink Channel Descriptor (DCD)

Uplink Channel Descriptor (UCD)

DL-MAP

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

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency Scanning

Network Entry: Ranging

Ranging is required to align BS and MS in terms of power, frequency and timing

BS measure MS offsets from the UL transmission and provides appropriate adjustments

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency Scanning

CDMA(IR Code)

MS

BS

RNG-RSP

(Adjustm

ent, Con

tinue)

BS measures arrival time and

signal power and determines

required adjustments

MS makes adjustments

CDMA A

llocation

IE

CDMA(IR Code)

RNG-RSP

(Success

)

RNG-REQ(MS MAC Address)

RNG-RSP

(Basic an

d Primary

CID)

Network Entry: Negotiation of Basic Capabilities

Basic capabilities include supported modulations, FEC, MIMO modes, HARQ, Privacy, etc.

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency Scanning

MS

SBC-RSP

BS

SBC-REQ

Network Entry: Authentication

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency ScanningBased on PKMv2 which uses EAP as the underlying authentication mechanism

MS BS

EAP Request/Identity

Authenticator

(ASN)AAA Server

MS Status Update

EAP Response/Identity

(my ID, e.g. MS MAC address)

MSK

AK Transferred to BS

SA-TEK Challenge

SA-TEK Request

SA-TEK Response

Key Request

Key Reply

SBC-REQ

SBC-RSP

EAP Request/EAP TLS

(TLS Start)

EAP Response/EAP TLS

(TLS Client Hello)

EAP Request/EAP TLS

(TLS Server Hello, TLS Certificate)

EAP Response/EAP TLS

(TLS Certificate)

EAP Request/EAP TLS

(TLS Finished)

EAP Response/EAP TLS

EAP Success

MSK EstablishedMSK, PMK, AK

Established

PMK, AK

Established

EAP over RADIUS

Network Entry: Registration

Registration capabilities include management mode, IP version supported, ARQ support, supported CS, etc.

MS

REG-RSP

BS

REG-REQ

DL & UL Synchronization

Initial Ranging

Negotiate Basic Capabilities

Authentication

Registration

Service Provisioning

Frequency Scanning

Network Entry: Service Provisioning

Creation of service flows can be initiated by either the MS or the BS