2 WiMAX Principle

7/30/2019 2 WiMAX Principle

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WiMAX Principle

Speaker:

ZTE Corporat ion


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Contents

WiMAX Proto co l Stack

MAC Layer

PHY Layer


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IEEE802.16 Protocol Reference Model


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Sublayer Function


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Contents


MAC Layer

PHY Layer


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MAC Layer

MAC Distinct Feature: On connection basis

MAC includes 3 sublayers

Convergence Sublayer

CS provides any transformation or mapping of external network

data

through the CS service access point (SAP)

into MAC SDUs (which received by common part sublayer)

Common part Sublayer

CPS provides the core MAC functionality of system access,

bandwidth allocation, connection establishment, connection

maintenance, and handover etc

Security Sublayer

SS provides authentication, secure key exchange, and encryption.


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MAC Layer Connection

MAC layer is connection oriented

Data and Messages are transmitted by connection between MS and

BS

Connection is directional: UL and DL 16bit CID (Connection Identifier) is utilized to manage connection

Basic Connection

Primary Management Connection

Secondary Management Connection

PHYPHY

MAC

Connection1

MAC

Connect ion1

Connect ion N

Connect ion2


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Contents


MAC Layer

PHY Layer

OFDM

OFDMA

Permutat ions

TDD Frame

Channel Coding and Modu lat ions


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Multi-path in Time and Frequency

Time domain: Impulse response

Frequency domain: Frequency response


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Broadband Fading Parameters

Delay Spread

The time that elapses between the first arriving path and the last (non-negligible) path

Coherence Bandwidth

The range of frequencies where the channel remain relatively flat. It isinversely proportional to the delay spread.

Doppler Spread

Is the measure of the spectral broadening caused by the time rate ofchange of a mobile radio channel.

Coherence Time

The time duration where the channel response remains the same. It isinversely proportional to the Doppler spread

Angular Speed

The statistical distribution of the angle of the arriving energy

Coherence Distance

The maximum spatial separation over which the channel responseremains constant. It is inversely proportional to the angular speed.

Channel respo nseis how the system reacts when presented with a brief signal input


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Single Carrier / Multi-Carrier System

Single carrier system

When the data rate of the system ishigher, the effect of delay spread of

the mobile radio channel becomes

worse.

When the Inter-Symb ol Interference

(ISI) of the system becomes serious,

the performance limits of

communication system are reached.

Multi-carrier system

The increased symbol durationimproves the robustness of

system against delay spread.


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FDM vs OFDM

FDM OFDM

FDM

Whole band is divided into non-overlap sub-bands, guardbands arenecessary

Data are map into the sub-bands.

At receiver, distinguish sub-bands by a group of filters.

Having poor frequency efficiency

OFDM

Whole band is divided into overlap subcarriers, subcarriers are orthogonalto each other, guardbands are not necessary

Data are map into the subcarriers.

Frequency efficiency was improved for allowing overlap in spectrum.


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Principle of OFDM

The basic principle of OFDM is to split a high-rate data

stream into a number oflower rate streams

IFFT FFT

IFFT (Inverse Fast Fourier Transform )

enables a large number of sub-carriers

with low complexity.


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OFDM Block Diagram

L carriers

Frequency Domain

Time Domain Channel Frequency Response

Equalization


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OFDM-Cyclic Prefix

Cyclic Prefix (CP)

Multi-path will cause ISI ISI is crosstalk between different symbol, which is very common in

wireless communication

CP

The CP duration is longer than the channel delay spread

Can completely eliminate ISI

Support multi CP configurations

1/4, 1/8,1/16,1/32, flexible to different scenarios


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OFDM Advantage and Disadvantage

Advantages

Very easy and efficient in dealing with multi-path fading Robust again narrow-band interference

High spectrum efficiency

Disadvantages

Sensitive to frequency offset and phase noise

Peak-to-average problem reduces the power efficiency of RF

amplifier at the transmitter


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Contents


MAC Layer

PHY Layer

OFDM

OFDMA

Permutat ions

TDD Frame

Channel Coding and Modulations


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OFDM & OFDMA

OFDM

All carriers are transmitted in parallel

Only one user is supported at the same time

OFDMA

Divides the carrier space into many groups

Many users can be supported at the same time


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OFDMA Time Domain

Each subcarriers time duration is referred to as the useful symbol

time Tb.

A copy of the last Tgof the useful symbol period, termed CP, is used

to collect multipath, while maintaining the orthogonality of the tones

(subcarriers).


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Frequency Domain(1)--Subcarriers

Data sub-carriers for data transmission Pilot sub-carriers for estimation and synchronization purposes

Null sub-carriers for no transmission; used for guard bands and

DC carriers

Active (data and pilot) sub-carriers are grouped into subsets of

sub-carriers called sub-channels.


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Frequency Domain(1)--Subchannel

Active (data and pilot) sub-carriers are grouped into subsets of sub-

carriers called sub-channels.

The WiMAX OFDMA PHY supports sub-channelization in both DL and

UL.

The minimum frequency-time resource unit of sub-channelization is one

slot, which is equal to 48 data tones (sub-carriers).

Two types of sub-carrier permutations for sub-channelization

Diversity: mobile applications

Contiguous: fixed portable, low mobility


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OFDMA Symbol Parameters (1)

Primitive parameters characterize the OFDMA symbol:

BWThis is the nominal channel bandwidth.

NusedNumber of used subcarriers.

n Sampling factor. This parameter, in conjunction with BW and

Nuseddetermines the subcarrier spacing, and the useful symbol time.

GThis is the ratio of CP time to useful time. Values may be 1/4,

1/8, 1/16, 1/32

Derived parameters:

NFFT Smallest power of two greater than Nused

Sampling Frequency: Fs = floor(nBW 8000) 8000

Subcarrier spacing:f=Fs/ NFFT Useful symbol time: Tb = 1 f

CP Time: Tg= G Tb

OFDM Symbol Time: Ts = Tb + Tg

Sampling time: TbNFFT


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OFDMA Symbol Parameters (2)


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Contents


MAC Layer

PHY Layer

OFDM

OFDMA

Permutat ions

TDD Frame



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OFDMA Permutations

Different ways of grouping subcarriers (tones) into channels in

802.16 are called Permutations

Three main permutations:

FUSC: Full Usage of Subchannels (downlink only)

Achieves best frequency diversity by spreading tones over entireband

PUSC: Partial Usage of Subchannels (uplink & downlink)

Groups tones into tiles/clusters to enable fractional frequency re-use

Still has distribution of tones across band for each subchannel

AMC (or Band AMC): Adaptive Modulation and Coding (UL & DL)

a.k.a. Adjacent Subcarrier Permutation

Uses adjacent tones for each subchannel for use with

beamforming>Minimum channel use is a slotand contains 48 tone-symbols


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DL- FUSC (1)

All data tones are used to create sub-channels

One sub-channel consists of 48 data tones

Pilots are allocated first

Constant set pilot and variable set pilot (variable set pilot

may change for each symbol allowing more accurate

channel estimation. Remaining data tones are mapped into various sub-channels.

Sub-carrier

Symbol n

Sub-channel 1 Sub-channel 2Constant Set Pilot Variable Set Pilot


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DL- FUSC (2)

Total subcarriers = 1024

Guard subcarriers = 173

DC subcarrier = 1

Pilot subcarriers = 82

VariableSet#0 = 36

ConstSet#0 = 6

VariableSet#1 = 35

ConstSet#1 = 5

Data subcarriers = 768


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DL-PUSC (1)

14 sub-carriers over 2 symbols are grouped into a cluster. In a cluster (28 tones), 24 are assigned for data and 4 are for

pilot

Each cluster is assigned a random number using a

pseudorandom numbering scheme, and the total clusters are

grouped into 6 (Group 1 Group 6) One sub-channel is formed by 2 clusters from the same group

DL-PUSC can be used for segmentation of BS, where G1, G3,

G5 are assigned to the 1st, 2nd, 3rd segments respectively.

G2, G4, G6 may be assigned to any segment as required.

A Segment is a subdivision of the set of available OFDMA subchannels. One segment is

used for deploying a single instance of the MAC


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DL-PUSC (2)

Frequency

Time

Subchannel

Clusters

Data

subcarriers

Pilot

subcarriers

Data

subcarriers

Pilot

subcarriers

(Group 1) (Group 2) (Group 6) (Group 1)

G1G2

G3

G4

G5

G6


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DL-PUSC (3)

Total subcarriers = 1024

Guard subcarriers = 183

DC subcarrier = 1

Number of clusters = 60

Number of subchannels = 30


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UL-PUSC(1)

Sub-carriers are grouped into tiles (4 sub-carriers x 3 symbols).

In a tile (12 tones), 8 are assigned for data and 4 are for pilot.

Each tile is assigned a random number using a pseudorandom

numbering scheme, and the total tiles are grouped into 6.

One sub-channel is formed by 6 tiles from the same group.

UL-PUSC can be used with segmentation to allow tighterfrequency reuse patterns.


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UL-PUSC(2)

Frequency

Time

Subchannel

Tiles

Data

subcarriers

Pilot

subcarriers

(Group 1) (Group 2) (Group 6) (Group 1)

G1G2

G3

G4

G5

G6


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UL-PUSC(3)

Total subcarriers = 1024 Guard subcarriers = 183

DC subcarrier = 1

Number of tiles = 210

Number of subchannels = 35


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Band AMC(1)

All sub-carriers constituting a sub-channel are adjacent to each

other. 9 adjacent sub-carriers are grouped into a bin.

In a bin, 8 are assigned for data and 1 is for pilot.

4 adjacent bins in the frequency domain make up a band.

One sub-channel is formed by 6 contiguous bins within the

same band. (i.e. 1 bin over 6 symbols, 2 bins over 3 symbols,or 3 bins over 2 symbols)

Band AMC allows easier exploitation of multiuser diversity.

Multiuser diversity provides significant improvement in

overall system capacity and throughput, since a sub-

channel at any given time is allocated to the user with the

highest SNR/capacity in that sub-channel.


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Band AMC(2)

Data

subcarriers1x6 AMC

subchannel

Frequency

Time

2x3 AMC

Subchannel

Bin

3x2 AMC

Subchannel

Pilot

subcarriers

Band

BinBinBin Bin


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Slot

A slot in the OFDMA PHY requires both a time and subchannel

dimension for completeness and is the minimum possible data

allocation unit.

For DL FUSC, one slot is one subchannel by one OFDMA

symbol.

For DL PUSC, one slot is one subchannel by two OFDMA

symbols.

For UL PUSC, one slot is one subchannel by three OFDMA

symbols.

For the adjacent subcarrier permutation, one slot is one

subchannel by two, three, or six OFDMA symbols.


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OFDMA Data Mapping (DL-PUSC)


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OFDMA Data Mapping (UL-PUSC)


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Contents


MAC Layer

PHY Layer

OFDM

OFDMA

Permutat ions

TDD Frame



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2006, ZTE Corporation. All rights reserved.

Why TDD

Initial release of Mobile WiMAX is TDD

For interference issues, TDD requires system-wide

synchronization, but is the preferred duplexing mode:

TDD enables adjustment of the downlink/uplink ratio to

efficiently support asymmetric downlink/uplink traffic

TDD assures channel reciprocity for better support of link

adaptation, MIMO and other closed loop advanced antenna

technologies.

TDD only requires a single channel providing greater flexibility

for adaptation to varied global spectrum allocations.

Transceiver designs for TDD implementations are less complex

and therefore less expensive.


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TDD Frame


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Preamble for

Synchronization

Frame Control Header(FCH) for

Frame configuration information

DL-MAP / UL-MAP for sub-channel allocation and othercontrol information

UL Ranging for MS to perform closed-

loop time, frequency, power

adjustment, and bandwidth requests

UL CQICH for the

MS to feedback

channel state

information

UL ACK for theMS to feedbackDL HARQacknowledge

TDD Frame Control Information


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Permutation Zones

Permutation Zone is a number of contiguous OFDMA symbols, in the DL or the UL,

that use the same permutation formula.

The DL subframe or the UL subframe may contain more than one permutation zone


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OFDM & OFDMA Summary

Multi-carrierdigital communication modulation techniques

Orthogonal frequency to eliminate the ICI; Overlapped spectrum to improve frequency efficiency

With CP

To eliminate ISI in multi-path transmission;

Power and data rate loss is worthwhile

IFFT and FFT to realize orthogonal modulation and

demodulation

Asymmetric DL/UL data rate with different sub-carrier

allocation

Adopting Dynamic sub channel allocation to use higher SNR

channel, to overcome frequency selective fading


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Contents WiMAX Proto co l Stack

MAC Layer

PHY Layer

OFDM

OFDMA

Permutat ions

TDD Frame



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2 WiMAX Principle