Transcript Incremental Pilots in OFDM

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?
Why use OFDM?
• Higher spectral efficiency
• Lower multi-path distortion
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
Transmitter with channel encoder
010
010001
16QA
M
Channel
encoder
Data subcarriers
DAC
S/
P
IFFT
Pilot PN Sequence
BPSK
Pilot
subcarrie
rs
CP
P/
S
Incremental pilots trade-off
Loss in
redundancy
acceptable
Better CE
Better BER
Less redundancy:
Worse BER
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.PDF
2. 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.p
df
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!