Incremental Pilots in OFDM

Parichaya Shrivastava

What will we discuss?• Brief overview of OFDM

– Why use OFDM?– How does it work?

• Pilots – Why do we need them?

• Incremental Pilots- Concept- What am I trying to do?

• Higher spectral efficiency• Lower multi-path distortion

Why use OFDM?

Higher spectral efficiency

Example of an OFDM modem

Multipath problem

‘A’ Multipath solution

‘The’ Multipath solution

Example of an OFDM modem

OFDM challanges

• Synchronization requirements are much more strict :– Requires more complex algorithms for time /

frequency synch

• Peak-to-average power ratio– Large signal peaks require higher power

amplifiers– Amplifier cost grows nonlinearly with required

power

Pilot based OFDM system model

Main equation:

Channel estimation equation

Frequency domain channel estimation

Pilot allocation 1

Pilot allocation 2

Why Incremental Pilots?

• Some channels (e.g. where there is more frequency selective fading), increased number of pilots will help.

• More pilots- Interpolation error small -> Better channel

estimation -> Better BER

Channel encoder

010 010001Data sub-

carriers

Pilot subcarrie

rs

IFFT CPS/P

16-QAM

Pilot PN Sequence

BPSK

P/S

DAC

Transmitter with channel encoder

Incremental pilots trade-off

Better CEBetter BER

Less redundancy: Worse BERLess redundancy: Worse BER

Loss in redundancy acceptable

Loss in redundancy. Crude figure

Work to do

• See the performance under different channel conditions

• Different coding schemes (Convolutional, Turbo, LDPC)

• Complexity of implementing higher coding rates for different coding schemes

References:1. Eric Phillip Lawrey, “Adaptive Techniques for multiuser OFDM” Phd Thesis. Online

http://www.skydsp.com/resources/OFDM_thesis_lawrey.PDF2. Marina Mondi, “Orthogonal Frequency Division Multiplexing/Modulation : OFDM”. Online.3. Keithley Corporation, “An Introduction to Orthogonal Frequency Division Multiplex

Technology”. Online http://www.ieee.li/pdf/viewgraphs/introduction_orthogonal_frequency_division_multiplex.pdf

4. Muhammad Saad Akram, “Pilot-based Channel Estimation in OFDM Systems” . Online http://www2.imm.dtu.dk/pubdb/views/edoc_download.php/5405/pdf/imm5405.pdf

5. Sung-Yoon Jung and Sunghwan Kim, “Channel Estimation and LDPC Code Puncturing Schemes Based on Incremental Pilots for OFDM”. ETRI Journal, Volume 32, Number 4, August 2010

Thank you!