22-05-0005-00-0000-ofdma-tutorial-ieee802-22-jan-05.ppt

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1Runcom Technologies Ltd.Submission Eli Sofer, Runcom

January 2005 Doc.: IEEE802.22-05-0005r0

Slide1

Tutorial on Multi Access OFDM (OFDMA) Technology

IEEE P802.22 Wireless RANs Date: 2005-01-04

Name Company Address Phone email

Eli Sofer RuncomTechnologies

2 Hachoma St., 75655Rishon Lezion, Israel

+972 3 9528440 [email protected]

Yossi Segal RuncomTechnologies

2, achoma St. 75655Rishon Lezion, Israel

+972 3 952 8440 [email protected]

Authors:

Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or

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creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution;

and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and

accepts that this contribution may be made public by IEEE 802.22.

Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf

including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patentholder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working

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Carl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a

draft standard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at

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>

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mailto:[email protected]









Slide2

Abstract

The contribution presents a tutorial on Multi Access OFDM (OFDMA) technology which has been

endorsed in leading standards such as- ETSI DVB-RCT and IEEE802.16a,d and 16e. Essential

parameters of UpLink and DownLink and simulation results are presented. System capabilities

and advantages are also discussed. The tutorial could offer an insight and understanding of

OFDMA technology to be considered as a candidate for WRAN system





Slide3

Tutorial on

Multi Access OFDM (OFDMA)

Technology

Eli Sofer

Runcom Technologies Ltd





Slide4

Contents

• OFDMA System Architecture

•Illustrated Example

• OFDMA System Properties

• Coverage and Capacity


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Slide5

OFDMA System

Architecture




Slide6

• Duplexing Technique

FDD/TDD

• Multiple Access Method

TDMA/OFDMA

OFDM Symbols allocated by TDMASub-Carriers within an OFDM Symbol allocated by OFDMA

• Diversity

Frequency, Time, Code (CPE and BS), Space TimeCoding, Antenna Array




Slide7

FDD (Frequency Division Duplexing ) Uses One Frequency

for the DownLink, and a Second Frequency for theUpLink.

TDD (time Division Duplexing) Uses the same frequencyfor the Downlink and the Uplink.

In any configuration the access method is OFDMA/TDMA .

F2 - Frequency band

UpLink

F1 - Frequency band

DownLink

FDD

F1 - Frequency band

UpLink

F1 - Frequency band

DownLink

TDD

Duplexing - Principles




Slide8

Using OFDMA/TDMA, Sub Channels are allocated inthe Frequency Domain, and OFDM Symbols allocatedin the Time Domain.

OFDMA-TDMA Principles

TDMA

TDMA\OFDMA

t

N

m


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Submission Eli Sofer, Runcom


Slide9

DownLink OFDMA Symbol

Total Frequency Band

Guard Band Guard Band

Symbol PilotsSub-Channel Data Carriers





Slide10

• Burst Structure is defined from one Sub-channel in theFrequency domain and n OFDMA time symbols in thetime domain, each burst consists of N data modulatedcarriers.

• Adaptive Modulation and Coding per Sub-Channel inthe Down-Link

• Forward APC controlling (+6dB) – (-6dB) digital gainon the transmitted Sub-Channel

• Supporting optional Space Time Coding employing

Alamouti STC.

• Supporting optional Adaptive Array.

DownLink Specification





Slide11

• FFT size : 2048

• Guard Intervals : ¼, 1/8, 1/16, 1/32

• Coding Mandatory: concatenated RS GF(256) andConvolutional coding (k=7,G1=171,G2=133, keepingoverall coding rate to = ½, ¾

• Coding Optional: Convolutional Turbo Code (CTC),Turbo Product Code (TPC) with coding rates close to= ½, ¾

• QPSK, 16QAM, 64QAM modulation

• Modulo 4, Pilot based Symbol Structure.• 32 Sub-Channels of 48 data carriers each

Example- DownLink Specification





Slide12

Downlink Pilot and Data Carriers Allocation Scheme

0 12

symbol

indexn

n+1

L=0

L=2

carrier index

Variable Location Pilot Fixed-location Pilot DataAllocation Key:

n+2 L=1

n+3 L=3

n+4 L=0

time

24 N used

-1

0 6 18 N used

-1

0 3 15 N used

-1

0 9 21 N used

-1

0N

used -1

30

27

12 24





Slide13

Space Time Coding

Tx

diversity

encoder

IFFT DACFilter RF

IFFT DACFilter RF

Subcarrier

modulation

IFFT input

packing

Tx

Rx

RF DAC Filter FFT Diversity

Combiner

Sub-

channel

demod.

Log-

Likelihood

ratios

Decoder





Slide14

UpLink OFDMA Symbol

Total Frequency Band


Data Carriers

Sub-Channel #1

Pilots Carriers

Sub-Channel #1

Data Carriers

Sub-Channel #x

Pilots Carriers

Sub-Channel #1





Slide15

• Burst Structure is defined from one Sub-channel in theFrequency domain and 3 OFDMA time symbols in the

time domain, each burst consists of 144 data

modulated carriers.

• Adaptive Modulation and Coding per User in theUpLink

• User Can be allocated 1 up to 32 Sub-Channels

• 2 Sub-Channels are used as the Ranging Sub-Channels

for User Ranging and fast Band-Width Request.

Example of UpLink Specification





Slide16

• FFT size : 2048

• Guard Intervals : ¼, 1/8, 1/16, 1/32• Coding Mandatory: concatenated RS GF(256) and

Convolutional coding (k=7,G1=171,G2=133, keepingoverall coding rate to = ½, ¾

• Coding Optional: Convolutional Turbo Code (CTC),Turbo Product Code (TPC) with coding rates close to= ½, ¾

• QPSK, 16QAM, 64QAM modulation

• Modulo 13, Pilot based Sub-Channel Structure.• 32 Sub-Channels of 53 carriers each, 5 carriers used

as pilots, 48 carriers used for data

Example of UpLink Specification





Slide17

Example for UpLink Sub-Channel Pilot and

Data Carriers Allocation Scheme

0 13 26 27 40 52

26 28 42150 2 52

symbol

indexn

n+1

L=0

L=2

frequency

Variable Location Pilot Fixed-location Pilot DataAllocation Key:

26 30 44170 4 52

n+2 L=4

26 4936220 9 52

n+11 L=9

26 38 51240 11 52

n+12 L=11

0 13 26 27 40 52

n+13 L=0

time





Slide18

Frequency band

1 2 3 30 31 32

block 1

1

2

3

each group contains

53 carriers

•All usable carriers are divided into 32 carrier groupsnamed basic group, each main group contains 53 basic

groups.

Using Special Permutations for carrier allocation

J 2005 D IEEE802 22 0 000 0





Slide19

User #1

Total Frequency band

User #2


0 5 521 222 10 1

User 1 = 0,5,2,10,4,20,8,17,16,11,9, 22 ,18,21,13,19,3,15,6,7,12,14,1

User 2 = 2,10,4,20,8,17,16,11,9,22,18, 21 ,13,19,3,15,6,7,12,14,1,0,5

• Carriers are allocated by a basic series and it’s cyclic permutations

for example:• Basic Series:

0,5,2,10,4,20,8,17,16,11,9,22,18,21,13,19,3,15,6,7,12,14,1

• After two cyclic permutations we get:

2,10,4,20,8,17,16,11,9,22,18,21,13,19,3,15,6,7,12,14,1,0,5


J 2005 D IEEE802 22 05 0005 0





Slide20

• The Carriers of each Sub-Channel are spread all over theusable frequency for best frequency diversity

• The allocation by permutation gives an excellent Reuse

factor - almost 1.

• The allocation by permutation give an excellent

interference spreading and averaging.


January 2005 Doc : IEEE802 22 05 0005r0





Slide21

• The CDMA like synchronization is achieved byallocating several of the usable Sub-Channels for theRanging process, the logic unit they consist is called aRanging Sub-Channel.

• Onto the Ranging Sub-Channel users modulate a Pseudo

Noise (PN) sequence using BPSK modulation• The Base Station detects the different sequences and uses

the CIR that he derives from the sequences for:

– Time and power synchronization

– Decide on the user modulation and coding

Using CDMA like modulation for Ranging






Slide22

• Aloha vs. CDMA BW request (32 codes) – CDMA efficiency is better by a factor of six

– CDMA latency is better by a factor of four

DVB-RCT MAC Performance

0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

Collision expectation value

S u c e s s f u l B W r

e q u e s t s p e r s l o t






Slide23

Illustrated Example

January 2005 Doc : IEEE802 22-05-0005r0





Slide24

• Subscriber Units at the Current OFDMA Symbol = 3

• Sub-Channels Allocated to Subscriber-Unit #1 = 12



• Number Of New Subscriber-Units Requesting Services = 3

All Subscriber-Units Suffer Different Multi-Paths and

different Attenuation's

Example

January 2005 Doc : IEEE802 22-05-0005r0




January 2005 Doc.: IEEE802.22 05 0005r0

Slide25

• Constellation at the Base Station

Example





January 2005 Doc.: IEEE802.22 05 0005r0

Slide26

• Users Separation

Example





y

Slide27

• User Estimation

-2 -1.5 -1 -0.5 0 0 .5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Constellat ion to Est iam te

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Est imated vec

Example - Results





y

Slide28

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Constellation to Estiamte

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Estimated vec

• User Estimation

Results





Slide29

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Constellation to Estiamte

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Estimated vec

• User Estimation

Results





Slide30

0 20 40 60 80 100 120 1400

50

100

150

200

250

300Despreading on All Users

• Finding New Subscriber-Units Requesting Services,

Using the Ranging Pilots (CDMA/OFDM Techniques)

Results





Slide31

OFDMA System -

Properties





Slide32

InterferenceUser SubCarriers

Allocation

SubCarriers

Interference

SubCarriers

Nulled

SubCarriers

Total Frequency band

• Narrowband Interference Rejection

– Easy to Avoid/Reject Narrowband Dominant Interference . – Less Interfered Part of the Carrier Can Still Be Used .

Interference Rejection/Avoidance





Slide33

• Using shaping on the signal peaks

• Limiting the PAPR to a constant value by

vector reduction

PAPR Reduction





Slide34

OFDM

Single Carrier

Scheme

4 MHz

-80

Frequency(MHz)

dB

• Rectangular Spectrum Shape (Brick Wall)

• Small Frequency Guard band

Spectrum Properties





Slide35

Spectrum Properties





Slide36

In OFDM, channel impairment are solved in the

same way Group Delays are solved, by Channel

estimation

Group Delay





Slide37

Phase Noise Effect onOFDM

Phase Noise Effect on

S.C

Phase Noise Effects





Slide38

• Timing Sensitivity

Low timing sensitivity is needed, and simple phase and channel

estimators solve timing problems.

• Frequency Sensitivity

solved by locking onto the Base-Station transmission and deriving

the Subscriber Unit’s clocks from it.

• Equalization

No Equalizers are needed, channel impairment and timing

problems are both solved with simple phase and channel

estimators





Slide39

System Coverage and

Capacity





Slide40

By allocating different Sub-Channels to different sectors

we can reach reuse factor of 1 with up to 12 sectors(changing the polarity enhances the performance)

HorizontalSub-hannels Set 1

F1

Vertical

Sub-hannel

s Set 1

F1 V e r t i c a l

S u b - h a n n e l

s S e t 2

F 1

H o r i z o n t a l

S u b - h a n n

e l

s S e t 2

F 1

Using Reuse Factor of 1





Slide41

• Use modulations with various Bit/Hz capabilities asAdaptive N-QAM.

• Use Adaptive FEC (Convolutional & Reed-Solomon or Turbo code)

• Maximal frequency reuse between cells/sectors

(close to 1).

• Maximum sectors allocation.

• The use of statistical Multiplexing and concentration.

• Adaptive Carrier Allocations.

• Adaptive Power Control

Capacity


C




Slide42

OFDM Cells

(64 mode)

OFDMA Cell

(2k mode)

64QAM users

16QAM users

QPSK users

Coverage





Slide43

Coverage - Simulations


Coverage Simulations




Slide44


• Multi Sector Coverage, 3 Sectors, 3 Frequencies, achieves2.8Bits/s/Hz/Cell, 22.5Mbps/Sector


Coverage Simulations




Slide45


• Multi Sector Coverage, 6 Sectors, 6 Frequencies, achieves2.8Bits/s/Hz/Cell, 22.5Mbps/Sector


OFDMA Advantages Summary (1




Slide46

• Averaging interference's from neighboring cells, by using different

basic carrier permutations between users in different cells.• Interference’s within the cell are averaged by using allocation with

cyclic permutations.

• Enables orthogonality in the uplink by synchronizing users in time

and frequency.

• Enables Multipath mitigation without using Equalizers and training

sequences.

• Enables Single Frequency Network coverage, where coverage

problem exists and gives excellent coverage.

OFDMA Advantages- Summary (1






Slide47

OFDMA Advantages - Summary (2

• Enables spatial diversity by using antenna diversity at the Base

Station and possible at the Subscriber Unit.• Enables adaptive modulation for every user QPSK, 16QAM,

64QAM and 256QAM.

• Enables adaptive carrier allocation in multiplication of 23 carriers

= nX23 carriers up to 1587 carriers (all data carriers).• Offers Frequency diversity by spreading the carriers all over the

used spectrum.

• Offers Time diversity by optional interleaving of carrier groups in

time.






Slide48


• Using the cell capacity to the utmost by adaptively using

the highest modulation a user can use, this is allowed bythe gain added when less carriers are allocated (up to18dB gain for 23 carrier allocation instead of 1587carriers), therefore gaining in overall cell capacity.

• The power gain can be translated to distance - 3 times thedistance for R 4 and 8 time for R 2 for LOS conditions.

• Enabling the usage of Indoor Omni Directional antennasfor the users.

• MAC complexity is the same as for TDMA systems.






Slide49


• Allocating carrier by OFDMA/TDMA strategy.

• Minimal delay per OFDMA symbol of 300µsec.

• Using Small burst per user of about 100 symbols for

better statistical multiplexing and smaller jitter.

• User symbol is several times longer then for TDMAsystems.

• Using the FEC to the outmost by error detection of

disturbed frequencies.

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