incremental pilots in ofdm parichaya shrivastava
TRANSCRIPT
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!