Public Use MOTOROLA and the Stylized M Logo are registered in the US Patent & Trademark Office. All other product or service names are the property of

1Public UseMOTOROLA and the Stylized M Logo are registered in the US Patent & Trademark Office. All other product or service names are the property of their respective owners. © Motorola, Inc. 2005

WiMAX: IEEE802.16 Standards / Protocol

Presented byDr. Sim Moh Lim

[email protected]


Agenda

• Part 2: WiMAX: IEEE802.16 Standards / Protocol (2h)– Concept of OFDM– OFDM System– Concept of OFDMA– Physical layer– MAC layer


Basic Features

Concept of OFDM


Orthogonal Frequency Division Multiplexing (OFDM)

• Frequency Division Multiplexing (FDM) technique that divides the channel into multiple orthogonal subchannels– Input data stream is divided into several substreams of a lower

data rate (increased symbol duration) – each substream is modulated and simultaneously transmitted on

a separate subchannel with carrier orthogonal to each other

OFDM is more spectral efficient as compared to FDM (allows more transmission channels)

FDM OFDM

Source: CSCE 4520/5520 Fall 2006, Shori Fukatsu


Orthogonality Concept• Cross-correlation:

– <x(t)y(t)> = 1/T∫T x(t)y(t)dt – Correlator: a device that does multiplying and

the integration

• Auto-correlation: – <x(t)x(t-d)> = 1/T∫T x(t)x(t-d)dt

• If x(t) and y(t) are orthogonal over T

– <x(t)y(t)> = 1/T∫T x(t)y(t)dt = 0

– 1/T∫T x(t)x(t)dt = 1/T∫T y(t)y(t)dt = C, constant


Application of Orthoganality• Send signal, s(t) = a.x(t) + b.y(t)

– a and b are the messages; x and y are the carriers.

• At the desired “receiver” of message a and carrier x(t),we can use a correlator to recover message:– < s(t)x(t)>

= < [a.x(t)+b.y(t)]x(t) > = a<x(t)x(t)> + b<y(t)x(t)> = a.C + b.0= a (constant C can be set to 1)

• Example: Chebyshev polynomial of the first kind, Jacobi polynomial, Legendre polynomial, and cosines.


OFDM Basics• Each subcarrier can be used to carry

one complex QAM symbol, di of

duration T

– The send signal is given by:

s(t) =Re{∑i di exp(j2Π(fc+ iΔf)t)} for

0 < t < Tb

– Its complex baseband is:

s(t) =∑i di exp(j2Π it/T)

which is the Inverse Fourier Transform (IFT) of N symbols.

– N is frequently called the FFT size

– The discrete time equivalent is the inverse discrete Fourier Transform

(IDFT) : s[n] =∑i di exp(j2Πin/N)

FrequencyΔf

• Note that the subcarriers are orthogonal to each other• However, shape will be distorted (becomes non-

orthogonal) by– (i) Frequency Offset– (ii) Fading

Source: www.iec.org/online/ tutorials/ofdm/topic04.html


Condition for Orthogonality in OFDM• To maintain orthogonality (no interference between subcarriers):

– 1/Tb∫Tb cos(2Πfct)cos(2Π(fc+Δf)t)dt = 0

• 1/Tb∫Tb cos(2Πfct)cos(2Π(fc )t)dt = 1/2

– Which can be simplified to: 1/Tb = Δf where

• Δf = sub-carrier spacing

• Tb = symbol duration

• If N-point IDFT (or DFT) is used

– Total bandwidth (in Hz) , BW = NΔf

– Ts = Tb + Tg = symbol duration after Cyclic Prefix addition

Time

Tb

Physical interpretation of orthogonality requirement:all sin-waves used must have integer number of cycles within Tb


OFDM and Multipath delay Multipath causes

inter-carrier interference (ICI) Delayed subcarrier #2 signal has no complete integer

number of cycles within 1 OFDM symbol (integration time)

inter-symbol interference (ISI) The phase transitions in the delayed path are causing

problem

ICI Solid line: first arriving pathDashed line: delayed path

Tb = integration time

ISISource: Book on OFDM by R. Prasad


OFDM and Multipath delay through the operation of Cyclic Prefix insertion/

extraction Interference from previous transmitted blocks is

eliminated provided the delay spread is much smaller than Tg

Source: Book on OFDM by R. Prasad

Cyclic Prefix in Time Domain

Source: EECS 228a, Shyam Parekh


OFDM System


FEC IFFT

DAC

LinearPA

add cyclic extension

bits

fc

Complex baseband OFDM symbol, s(t)

Pulse shaper &

Generic OFDM Transmitter

Serial toParallel

• From earlier analysis, each subcarrier can be used to carry one complex QAM symbol, di of duration T

– The corresponding complex baseband of the OFDM signal is:

s(t) =∑ di exp(j2Π it/T)

which is the Inverse Fourier Transform (IFT) of N symbols: can be implemented efficiently using IFFT

[d1 d2 d3 … dN]T


AGC

fc

VCO

Sampler FFTError

Slot &TimingSync.

Generic OFDM Receiver

RecoveryP/S and

Detection


Baseband of OFDM System• Baseband portion of OFDM system

• y[i] = x[i] © h[i] where © = circular convolution– h[i] represents the channel impulse response

• Y[k] = X[k] . H[k] where H[k] is the channel freq response• kHkYkX̂ 1

h[i]

x[i]

y[i]

X[k]

Y[k]

Remarks: i is time domain index, k is frequency domain index


Add

Cyclic

Prefix

Serial/

Parallel

]0,[nX

]1,[nX

]1,[ NnX

Parallel/

SerialIFFT

]0,[nx

]1,[nx

]1,[ Nnx

OFDM Transmitter

• S/P acts as Time/Frequency mapper

• IFFT generates the required Time domain waveform

• Cyclic Prefix acts like guard interval and makes “equalization” easy (FFT-cyclic convolution vs channel-linear convolution)

1

0

2],[

1],[

N

k

N

kij

eknXN

inx

X[n,k] x[n,i]


OFDM Receiver

• Cyclic Prefix is discarded

1

0

2],[

1],[

N

i

N

ikj

einyN

knY

FFT

]0,[nY

]1,[nY

]1,[ NnY

Parallel/

Serial

Serial/

Parallel

Remove

Cyclic

Prefix

]0,[ny

]1,[ny

]1,[ Nny

• FFT generates the required Frequency Domain signal

• P/S acts like a Frequency/Time Mapper

Y[n,k]y[n,i]


OFDM Basics

• If the Cyclic Prefix > Max. Delay Spread, then the received signal after FFT, at the kth tone for the nth OFDM block can be expressed as – Y[n,k] = X[n,k] H[n,k] + W[n,k]where– W[n,k] is the additive noise– H[n,k] is the channel frequency response

• Estimated X[n,k] = Y[n,k]H-1[n,k] = X[n,k] + W[n,k] H-1[n,k]


OFDMA


Orthogonal Frequency Division Multiple Access

(OFDMA)• Orthogonal Frequency Division Multiple Access

(OFDMA) is a multi-user version of the popular OFDM digital modulation scheme.

• A subset of subcarriers is grouped together to form a subchannel

• Multiple access is achieved in OFDMA by dynamically assigning subsets of subchannels to individual users.

• This allows simultaneous low data rate transmission from several users.

• WirelessMAN-OFDMA is based on scalable OFDMA (SOFDMA) – Support scalable channel bandwidths from 1.25 to 20 MHz


OFDM vs OFDMA

• Only one user can transmit during an OFDM symbol

• Sub-channelization enables several users to transmit at the same time

OFDM OFDMA


OFDMA Subchannels A subscriber can be assigned one or more

subchannels Subchannels provide interference averaging

benefits for aggressive frequency reuse systems

Source: IEEE Tutorial


OFDMA Features

• adaptive user-to-subcarrier assignment – Based on feedback information about the channel conditions

• possible to achieve even better system spectral efficiency.– If the assignment is done sufficiently fast, this further

improves the OFDM robustness to fast fading and narrow-band cochannel interference

• support differentiated Quality of Service (QoS), i.e. to control the data rate and error probability individually for each user.– Different number of sub-carriers can be assigned to different

users


Multiuser diversity: adaptive user-to-subcarrier

• Each MS/subscriber faces a different fading channel; hence, radio resource management can use multiuser diversity to maximize system throughput.– Allocate logical channel

numbers (subcarriers) and symbol numbers based on the channel strength

Source: Jungnam Yun and Mohsen Kavehrad, PHY/MAC CROSS-LAYER ISSUES IN MOBILE WiMAX


WiMax Protocol Overview


Scope of 802 standards

• Physical (Layer 1)– Concern physical interface and the rules by

which bits are passed from one to another.• Data Link (Layer 2)

– Provides means of activating, maintaining and deactivating a reliable point-to-point link

Medium

IEEE 802 model

Source: IEEE

Source: Internet


ATMtransport

ATMtransport

IPtransport

IPtransport

Service Specific ConvergenceSublayer (CS)


IEEE 802.16 protocol layering

MAC Common Part Sublayer(MAC CPS)


Security sublayerSecurity sublayer

Physical Layer (PHY)Physical Layer (PHY)

MA

C

Like typical IEEE 802 standards, IEEE 802.16 specifies the Medium Access Control (MAC) [Layer 2] and PHY [Layer 1] layers of the wireless transmission system.


ATMtransport

ATMtransport

IPtransport

IPtransport



IEEE 802.16 Physical layer





MA

C

•IEEE 802.16 offers 5 PHY options

Designation Applicability

WirelessMAN-SC 10 -66 GHz

WirelessMAN-SCa

Below 11GHz

Licensed bands

WirelessMAN-OFDM

Below 11GHz

Licensed bands

WirelessMAN-OFDMA

Below 11GHz

Licensed bands

WirelessHUMAN Below 11GHz

Licensed-exempt bands

Source: S-72.3240 WMAN


ATMtransport

ATMtransport

IPtransport

IPtransport



IEEE 802.16 MAC layer





MA

C

The IEEE 802.16 MAC layer consists of three sublayers.


ATMtransport

ATMtransport

IPtransport

IPtransport







MA

C

CS maps data (ATM cells or IP packets) to a certain unidirectional connection identified by the Connection Identifier (CID) and associated with a certain QoS.

CS adapts higher layer protocols to MAC CPS.

May also offer payload header suppression.



ATMtransport

ATMtransport

IPtransport

IPtransport







MA

C

MAC CPS provides the core MAC functionality:

• System access

• Bandwidth allocation

• Connection control

Note: QoS control is applied dynamically to every connection individually.



ATMtransport

ATMtransport

IPtransport

IPtransport







MA

C

The privacy sublayer provides authentication, key management and encryption.



Summary of MAC


Phy & MAC processing

PHY

PHY Baseband Processing

RFFront-end

Bit level

symbol level

A/D or D/A

Lower MAC (LMAC):•H-ARQ,•Ranging (Access),•Scheduling, Framing, Control, Signaling,•QoS, •MBS, •Security

Lower MAC (LMAC):•H-ARQ,•Ranging (Access),•Scheduling, Framing, Control, Signaling,•QoS, •MBS, •Security

Upper MAC (UMAC):•ARQ,•Handoff,•Idle mode protocol,•Sleep mode protocol, •MBS,•Session/ Connection management,•RRM/RLC,•QoS

Upper MAC (UMAC):•ARQ,•Handoff,•Idle mode protocol,•Sleep mode protocol, •MBS,•Session/ Connection management,•RRM/RLC,•QoS

DSP / ARC processor

Mix/ Analog/ RF IC

General processor, eg. ARM926

Upper layers

DDC/ DUC


Base /Subscriber station block diagram

RISC Engine:UMAC and

general control

RISC Engine:UMAC and

general control

DSP Engine:LMAC and

PHY

DSP Engine:LMAC and

PHY

ADC/ DAC

RF IC

Peripheral controllerRadio control and measurement

DMA controller

Ethernet controller

TDM controller

External processor controller

Memory controllerExternal memory

External processor

I/O devices, etc

Baseband ICOSC, PA, LNA, Amp ctr

Power, etc

Power supply


Thank Thank YouYou


WiMax Physical Layer


PHY Layer Features of IEEE 802.16-2004

Feature Benefit

256 point FFT OFDM waveform

Built in support for addressing multi-path in outdoor LOS and NLOS environments.

Adaptive Modulation and variable error correction encoding per RF burst

Ensures a robust RF link while maximizing the number of bits/second for each subscriber unit.

TDD and FDD support

Addresses varying worldwide regulations when one or both may be allowed


PHY Layer Features of IEEE 802.16-2004 (Continued)

Feature Benefit

Flexible Channel Sizes (Can be an integer multiple of 1.25 MHz, 1.5 MHz, and 1.75 MHz with a maximum of 28 MHz.

Provides the flexibility to operate in many different frequency bands with varying channel requirements around the world.

Designed to support adaptive antenna systems (AAS).

Smart antennas can suppress interference and increase system gain. They are becoming important to BWA deployment as their costs come down.


WiMax Data Link Layer


MAC Layer Features of IEEE 802.16-2004

Feature Benefit

TDM/TDMA Scheduled Uplink/Downlink frames.

Efficient bandwidth usage

Scalable from 1 to hundreds/ thousands of subscribers

Allows cost effective deployments by supporting enough subscribers to deliver a robust business case

Connection-oriented • Per Connection QoS

• Faster packet routing and forwarding


MAC Layer Features of IEEE 802.16-2004 (Continued)

Feature Benefit

QoS • Low latency for delay sensitive services

• Optimal transport for video, Data prioritization

ARQ • Improves end-to-end performance by hiding RF

layer induced errors from upper layer protocols

Adaptive Modulation

• Enables highest data rates allowed by channel

conditions, improving system capacity

Security and Encryption

• Protects user privacy

Automatic Power Control

• Minimizes self interference

Documents

Public Use MOTOROLA and the Stylized M Logo are registered in the US Patent & Trademark Office. All other product or service names are the property of