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MODIFIED PILOT MODIFIED PILOT STRUCTURE FOR STRUCTURE FOR WIMAX PUSC WIMAX PUSC PERMUTATION SCHEMEPERMUTATION SCHEME

By: Khalid ElWazeer and Mohamed Khairy2

OutlineOutlineWiMAX PUSC current pilot

structureStudied methods in channel

estimationThe proposed modified pilot

structureSimulation results for both the

original and the modified casesConclusion

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WiMAX PUSC Pilot WiMAX PUSC Pilot StructureStructureIn PUSC permutation scheme, the

OFDM symbol is divided into clusters

Each cluster is composed of 14 adjacent subcarriers as shown:

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Studied Estimation Studied Estimation TechniquesTechniques1D interpolation on single OFDM symbol.2D cascaded time/frequency interpolation.2D simplification to the cascaded

time/frequency interpolation.2D MMSE robust filtering based on two

OFDM Symbols.2D MMSE robust filtering based on three

OFDM symbols.Cascaded one dimensional time/frequency

MMSE robust filtering.Cascaded time interpolation/frequency

filtering.Cascaded frequency interpolation/time

filtering.5

MMSE Filtering IllustrationMMSE Filtering Illustration

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Cascaded Cascaded Filtering/Interpolation Filtering/Interpolation IllustrationIllustration

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Comparison between studied Comparison between studied methods for original pilot methods for original pilot structurestructure

0 5 10 15 20 25 30 35 4010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

SNR

BE

R

6 Taps, 20usec, 40 Hz "Dual" || Fix SNR, (Comparison) - 100F*44S WiMAX

Casc. FiltersCasc. InterpInterpT + FilterFFilterT + InterpFPer3D Filters2D Fltering2D Simple Interp.

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Comparison – cont.Comparison – cont.

0 5 10 15 20 25 30 35 40-80

-70

-60

-50

-40

-30

-20

-10

SNR

MS

E

6 Taps, 20usec, 40 Hz "Dual" || Fix SNR, (Comparison) - 100F*44S WiMAX

Casc. FiltersCasc. InterpInterpT + FilterFFilterT + InterpF3D Filtering2D 16 Taps2D Simple Interp.

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Pilot Structure Pilot Structure ModificationModificationFrom the previous figures, a BER

floor exists for high SNRs.This floor is mainly due to the

edges of the symbol.The errors occur at the edges

because the number of pilots at these edges is insufficient.

If enough pilots exist at the edges of the symbol, this floor disappears.

Our contribution is mainly for the clusters at the left most and the right most edges of the symbol.

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Pilot Structure Modification – Pilot Structure Modification – cont.cont.One extra pilot is added in each

side of the OFDM symbol, such that after time filtering/interpolation, two successive pilots always exist at both edges.

These two pilots are mainly to guard the left-most and right most edges of the symbol, and make them less error prone.

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Proposal IllustrationProposal Illustration

Right Most Cluster

Left Most Cluster12

Results for Modified PilotsResults for Modified Pilots

0 5 10 15 20 25 30 35 4010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

SNR

BE

R

6 Taps, 20usec, 40 Hz "Dual", Fix SNR, Modified Pilots || (Comparison) - 100F*44S WiMAX

Casc. FiltersPerCasc. InterpInterpT + FilterFFilterT + InterpF3D Filters (5f,6f,5f)2D Filtering2D Simple Interp.

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Results for Modified Pilots – Results for Modified Pilots – cont.cont.

0 5 10 15 20 25 30 35 40-90

-80

-70

-60

-50

-40

-30

-20

-10

SNR

MS

E

6 Taps, 20usec, 40 Hz "Dual", Fix SNR, Modified Pilots || (Comparison) - 100F*44S WiMAX

Casc. FiltersCasc. InterpInterpT + FilterFFilterT + InterpF3D Filtering2D Filtering2D Simple Interp.

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ConclusionConclusionThe modified pilot structure

removes the BER error floorThis modification is very

important for future systems with high data rates, in which large constellations (256 QAM for ex.) are used with high SNRs.

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