22-05-0005-01-0000_OFDMA_Tutorial_IEEE802 22_Jan 05 r1

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1Runcom Technologies Ltd.Submission Eli Sofer, Runcom

January 2005 Doc.: IEEE802.22-05-0005r1

Slide1

Tutorial on Multi Access OFDM (OFDMA) TechnologyIEEE P802.22 Wireless RANs Date: 2005-01-04

Name Company Address Phone emailEli Sofer Runcom

Technologies2 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 organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in thecreation of an IEEE Standards publication; to copyright in the IEEEs name any IEEE Standards publication even though it may include portions of this contribution; andat the IEEEs sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and acceptsthat 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 Group of

patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that thedraft publication will be approved for publication. Please notify the Chair 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 draftstandard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at [email protected] .>
http://standards.ieee.org/guides/bylaws/sb-bylaws.pdfhttp://standards.ieee.org/guides/bylaws/sb-bylaws.pdfmailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://standards.ieee.org/guides/bylaws/sb-bylaws.pdf


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Slide2

AbstractThe 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 capabilitiesand 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 onMulti 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


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Slide5

OFDMA SystemArchitecture




Slide6

Duplexing Technique

FDD/TDD

Multiple Access Method TDMA/OFDMAOFDM Symbols allocated by TDMASub-Carriers within an OFDM Symbol allocated by OFDMA

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




Slide7

FDD (Frequency Division Duplexing ) Uses One Frequencyfor the DownLink, and a Second Frequency for theUpLink.

TDD (time Division Duplexing) Uses the same frequency for the 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




Slide9

Combining TDMA and FDMA TDMA/FDMA operation = OFDMA Frequency sub-channels are composed of multiple, non-

adjacent carriers

TX#4TX#3

TX#2

TX#6

TX#5

TX#1

Time

F

r e q u e n c y

b i n

2468

101214161820

2224262830

00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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Submission Eli Sofer, Runcom


Slide10

Multipath The challenge The RF signal travels through multiple paths The RF channel is characterized by a delay profile Problematic when delay spread is comparable to

symbol duration


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Submission Eli Sofer, Runcom


Slide11

Multipath The solution OFDM modulates in parallel multiple narrow band sub-

carriers Multipath duration becomes short relative to symbol

duration

Pilot and guard sub-carriers are also insertedData carriers

Total Frequency band

Pilot Carriers

Frequency guard Band Frequency Guard Band


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Slide12

Multipath The solution

The sub-carriers are converted by IFFT to a timedomain signal A guard interval (cyclic prefix) is added to collect

multipath

A long guard interval (GI) reduces efficiency butenhances multipath handling capability OFDM/OFDMA enable handling peak delay spread as

large as the GI

GI

t

OFDM symbol


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Slide13

DownLink OFDMA Symbol

Total Frequency Band

Guard Band Guard Band

Symbol PilotsSub-Channel Data Carriers


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Slide15

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

Example- DownLink Specification


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Slide16

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

0 N used -1

30

27

12 24


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Slide17

Space Time Coding

Txdiversityencoder

IFFT DACFilter RF

IFFT DACFilter RF

Subcarrier modulation

IFFT inputpacking

Tx

Rx

RF DAC Filter FFT DiversityCombiner

Sub-channeldemod.

Log-Likelihood

ratiosDecoder


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Slide18

UpLink OFDMA Symbol

Total Frequency BandGuard Band Guard Band

Data CarriersSub-Channel #1

Pilots CarriersSub-Channel #1

Data CarriersSub-Channel #x

Pilots CarriersSub-Channel #1


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Slide19

Burst Structure is defined from one Sub-channel in theFrequency domain and 3 OFDMA time symbols in thetime domain, each burst consists of 144 data modulatedcarriers.

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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Slide20

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

J 2005 D IEEE802 22 05 0005 1


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Slide21

Example for UpLink Sub-Channel Pilot and Data Carriers Allocation Scheme

0 13 26 27 40 52

26 28 42150 2 52

symbolindex

n

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

J 2005 D IEEE802 22 05 0005 1


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Slide22

Frequency band

1 2 3 30 31 32

block 1

1

2

3each group contains

53 carriers

All usable carriers are divided into 32 carrier groupsnamed basic group, each basic group contains 53 sub-carriers

Using Special Permutations for carrier allocation

January 2005 Doc : IEEE802 22 05 0005r1


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Slide23

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 ,1U 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


January 2005 Doc : IEEE802 22 05 0005r1


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Slide24

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 excellentinterference spreading and averaging.


January 2005 Doc : IEEE802 22-05-0005r1


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January 2005 Doc.: IEEE802.22 05 0005r1

Slide25

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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January 2005 Doc.: IEEE802.22 05 0005r1

Slide26

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


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y

Slide28

Subscriber Units at the Current OFDMA Symbol = 3 Sub-Channels Allocated to Subscriber-Unit #1 = 12 Sub-Channels Allocated to Subscriber-Unit #2 = 9 Sub-Channels Allocated to Subscriber-Unit #3 = 6

Number Of New Subscriber-Units Requesting Services = 3

All Subscriber-Units Suffer Different Multi-Paths anddifferent Attenuation's

Example



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29Runcom Technologies Ltd.Submission Eli Sofer, RuncomSlide29

Constellation at the Base Station Example



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Users Separation

Example



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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 o n s t e l l a t io n t o E s t i am 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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-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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-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

S u b C a r r ie r s

N u lle dS u b C a r r ie r s

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 sameway Group Delays are solved, by Channelestimation

Group Delay



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Phase Noise Effect onOFDM

Phase Noise Effect onS.C

Phase Noise Effects



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Timing Sensitivity Low timing sensitivity is needed, and simple phase and channelestimators solve timing problems.

Frequency Sensitivity solved by locking onto the Base-Station transmission and derivingthe Subscriber Units clocks from it.

Equalization No Equalizers are needed, channel impairment and timing problemsare 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 1F1

VerticalSub-hannel

s Set 1F1

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



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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 (closeto 1).

Maximum sectors allocation. The use of statistical Multiplexing and concentration. Adaptive Carrier Allocations. Adaptive Power Control

Capacity


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l


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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 theused spectrum.

Offers Time diversity by optional interleaving of carrier groups intime.




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Using the cell capacity to the utmost by adaptively usingthe 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 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.




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Allocating carrier by OFDMA/TDMA strategy. Minimal delay per OFDMA symbol of 300 sec. 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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