Tutorial on OFDMA

1Runcom Technologies Ltd.Submission Eli Sofer, Runcom

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

Slide 1

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

Name Company Address Phone email Eli Sofer Runcom

Technologies 2 Hachoma St., 75655 Rishon Lezion, Israel

+972 3 9528440 [email protected]

Yossi Segal Runcom Technologies

2, achoma St. 75655 Rishon Lezion, Israel

+972 3 952 8440 [email protected]

Authors:

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Slide 2

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



Slide 3

Tutorial onMulti Access OFDM (OFDMA)

Technology

Eli Sofer

Runcom Technologies Ltd



Slide 4

Contents

• OFDMA System Architecture

• Illustrated Example

• OFDMA System Properties

• Coverage and Capacity



Slide 5

OFDMA System Architecture



Slide 6

• Duplexing Technique FDD/TDD

• Multiple Access Method TDMA/OFDMA

OFDM Symbols allocated by TDMA Sub-Carriers within an OFDM Symbol allocated by OFDMA

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



Slide 7

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

TDD (time Division Duplexing) Uses the same frequency for 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



Slide 8

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

OFDMA-TDMA Principles

TDMA

TDMA\OFDMA

t

N

m



Slide 9

DownLink OFDMA Symbol

Total Frequency BandGuard Band Guard Band

Symbol PilotsSub-Channel Data Carriers



Slide 10

• Burst Structure is defined from one Sub-channel in the Frequency domain and n OFDMA time symbols in the time domain, each burst consists of N data modulated carriers.

• Adaptive Modulation and Coding per Sub-Channel in the Down-Link

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

• Supporting optional Space Time Coding employing Alamouti STC.

• Supporting optional Adaptive Array.

DownLink Specification



Slide 11

• 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, keeping overall 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



Slide 12

Downlink Pilot and Data Carriers Allocation Scheme

0 12

symbolindex

n

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

0 Nused -1

30

27

12 24



Slide 13

Space Time Coding

Txdiversityencoder

IFFT DACFilter RF

IFFT DACFilter RF

Subcarrier modulation

IFFT input packing

Tx

Rx

RF DAC Filter FFT Diversity Combiner

Sub-channel demod.

Log-Likelihood

ratiosDecoder



Slide 14

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



Slide 15

• Burst Structure is defined from one Sub-channel in the Frequency 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 the UpLink

• 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



Slide 16

• 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, keeping overall 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



Slide 17

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



Slide 18

Frequency band

1 2 3 30 31 32

block 1

12

3each group contains

53 carriers

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

Using Special Permutations for carrier allocation



Slide 19

User #1

Total Frequency band

User #2

Guard Band Guard Band

0 5 521 222 10 1

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




Slide 20

• The Carriers of each Sub-Channel are spread all over the usable 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.




Slide 21

• The CDMA like synchronization is achieved by allocating several of the usable Sub-Channels for the Ranging process, the logic unit they consist is called a Ranging 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



Slide 22

• 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

Su

ce

ssfu

l B

W r

eq

ue

sts

pe

r slo

t



Slide 23

Illustrated Example



Slide 24

• 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 and different Attenuation's

Example



Slide 25

• Constellation at the Base Station Example



Slide 26

• Users Separation Example



Slide 27

• 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

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

Example - Results



Slide 28

-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


-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



Slide 29

-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


-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



Slide 30

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



Slide 31

OFDMA System - Properties



Slide 32

InterferenceUser SubCarriers

Allocation

SubCarriers

Interference

SubCarriers

NulledSubCarriers

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



Slide 33

• Using shaping on the signal peaks

• Limiting the PAPR to a constant value by vector reduction

PAPR Reduction



Slide 34

OFDM

Single CarrierScheme

4 MHz

-80

Frequency(MHz)

dB

• Rectangular Spectrum Shape (Brick Wall)• Small Frequency Guard band

Spectrum Properties



Slide 35

Spectrum Properties



Slide 36

In OFDM, channel impairment are solved in the same way Group Delays are solved, by Channel estimation

Group Delay



Slide 37

Phase Noise Effect on OFDM

Phase Noise Effect on

S.C

Phase Noise Effects



Slide 38

• 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.

• EqualizationNo Equalizers are needed, channel impairment and timing problems are both solved with simple phase and channel estimators



Slide 39

System Coverage and Capacity



Slide 40

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

ertic

alS

ub-h

anne

ls

Set

2F

1

Hor

izon

tal

Sub

-han

nel

s S

et 2

F1

Using Reuse Factor of 1



Slide 41

Use modulations with various Bit/Hz capabilities as Adaptive 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



Slide 42

OFDM Cells(64 mode)

OFDMA Cell(2k mode)

64QAM users

16QAM users

QPSK users

Coverage



Slide 43

Coverage - Simulations



Slide 44


Multi Sector Coverage, 3 Sectors, 3 Frequencies, achieves 2.8Bits/s/Hz/Cell, 22.5Mbps/Sector



Slide 45


Multi Sector Coverage, 6 Sectors, 6 Frequencies, achieves 2.8Bits/s/Hz/Cell, 22.5Mbps/Sector



Slide 46

• 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)



Slide 47

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.



Slide 48


• Using the cell capacity to the utmost by adaptively using the highest modulation a user can use, this is allowed by the gain added when less carriers are allocated (up to 18dB gain for 23 carrier allocation instead of 1587 carriers), therefore gaining in overall cell capacity.

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

• Enabling the usage of Indoor Omni Directional antennas for the users.

• MAC complexity is the same as for TDMA systems.



Slide 49


• 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 TDMA

systems.• Using the FEC to the outmost by error detection of

disturbed frequencies.

Tutorial on OFDMA

Documents

concatenated

phase noise

turbo product

multi access

5mbpssectorsubmission

submission

convolutional

multi sector