22-05-0005-00-0000_ofdma_tutorial_ieee802-22_jan 05

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

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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


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Slide3

Tutorial on

Multi Access OFDM (OFDMA)

Technology

Eli Sofer

Runcom Technologies Ltd


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Slide4

Contents

OFDMA System Architecture

Illustrated Example

OFDMA System Properties

Coverage and Capacity


5/495Runcom Technologies Ltd.Submission Eli Sofer, Runcom


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 frequency forthe Downlink and the Uplink.

In any configuration the access method is OFDMA/TDMA .

F 2 - F r e q u e n c y b a n d

U p L i n k


D o w n L i n k

F D D


U p L i n k


D o w n L i n k

T D D

Duplexing - Principles




Slide8

Using OFDMA/TDMA, Sub Channels are allocated in theFrequency Domain, and OFDM Symbols allocated inthe Time Domain.

OFDMA-TDMA Principles

TDMA

TDMA\OFDMA

t

N

m


9/499Runcom Technologies Ltd.

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 ofN 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 : , 1/8, 1/16, 1/32

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

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

QPSK, 16QAM, 64QAM modulation

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

Example- DownLink Specification


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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 Nused

-1

0 6 18 Nused

-1

0 3 15 Nused

-1

0 9 21 Nused

-1

0N

used -1

30

27

12 24


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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


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Slide14

UpLink OFDMA Symbol

Total Frequency Band

Guard Band Guard Band

Data Carriers

Sub-Channel #1

Pilots Carriers

Sub-Channel #1

Data Carriers

Sub-Channel #x

Pilots Carriers

Sub-Channel #1


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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


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Slide16

FFT size : 2048

Guard Intervals : , 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 = ,

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

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


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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


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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 05 0005 0


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Slide19

U s e r # 1

T o t a l F r e q u e n c y b a n d

U s e r # 2

G u a r d B a n d G u a r d B a n d

0 5 52 1 2 22 1 0 1

U s e r 1 = 0 ,5 ,2 ,1 0 ,4 ,2 0 ,8 ,1 7 ,1 6 ,1 1 ,9 , 2 2 ,1 8 , 2 1 , 1 3 , 1 9 , 3 , 1 5 , 6 , 7 , 1 2 , 1 4 , 1

U s e r 2 = 2 ,1 0 ,4 ,2 0 ,8 ,1 7 ,1 6 ,1 1 ,9 ,2 2 ,1 8 , 2 1 ,1 3 , 1 9 , 3 , 1 5 , 6 , 7 , 1 2 , 1 4 , 1 , 0 , 5

Carriers are allocated by a basic series and its 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


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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


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Slide21

The CDMA like synchronization is achieved by allocatingseveral of the usable Sub-Channels for the Ranging

process, the logic unit they consist is called a RangingSub-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



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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

SucessfulBWrequestsperslot



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Slide23

Illustrated Example

January 2005 Doc : IEEE802 22-05-0005r0


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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


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Slide25

Constellation at the Base Station

Example



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Slide26

Users Separation

Example



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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

C ons te l la t ion t o E s t 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

E s tim ate d vec

Example - Results



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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



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29Runcom Technologies Ltd.Submission Eli Sofer, RuncomSlide29

-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



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0 20 40 60 80 100 120 1400

50

100

150

200

250

300DespreadingonAll Users

Finding New Subscriber-Units Requesting Services, Using

the Ranging Pilots (CDMA/OFDM Techniques)

Results



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OFDMA System -

Properties



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I n t e r fe r e n c eU s e r S u b C a r r ie r s

A l lo c a t io n

S u b C a r r ie r s

I n t e r f e r e n c e


N u l l e d


T o t a l F r e q u e n c y b a n d

Narrowband Interference Rejection

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

Interference Rejection/Avoidance



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Using shaping on the signal peaks

Limiting the PAPR to a constant value by

vector reduction

PAPR Reduction



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O F D M

S i n g l e C a r r i e r

S c h e m e

4 M H z

- 8 0

F r e q u e n c y( M H z )

d B

Rectangular Spectrum Shape (Brick Wall)

Small Frequency Guard band

Spectrum Properties



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Spectrum Properties



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In OFDM, channel impairment are solved in the same

way Group Delays are solved, by Channel

estimation

Group Delay



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Phase Noise Effect onOFDM

Phase Noise Effect on

S.C

Phase Noise Effects



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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 Units clocks from it.

Equalization

No Equalizers are needed, channel impairment and timing problems

are both solved with simple phase and channel estimators



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System Coverage and

Capacity



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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-hannel

s Set 1

F1

Vertical

Sub-hannel

s Set 1

F1Vertical

Sub-hannel

s

Set2

F1

Horizontal

Sub-hann

el

s

Set2

F1

Using Reuse Factor of 1



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

Use Adaptive FEC (Convolutional & Reed-Solomon orTurbo 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


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OFDM Cells

(64 mode)

OFDMA Cell

(2k mode)

64QAM users

16QAM users

QPSK users

Coverage



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Coverage - Simulations


Coverage Simulations


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


Coverage Simulations


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


OFDMA Advantages Summary (1


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Averaging interference's from neighboring cells, by using different

basic carrier permutations between users in different cells. Interferences 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




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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.




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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 to 18dBgain for 23 carrier allocation instead of 1587 carriers),therefore gaining in overall cell capacity.

The power gain can be translated to distance - 3 times thedistance for R4 and 8 time for R2 for LOS conditions.

Enabling the usage of Indoor Omni Directional antennasfor the users.

MAC complexity is the same as for TDMA systems.




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Allocating carrier by OFDMA/TDMA strategy.

Minimal delay per OFDMA symbol of 300sec.

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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