1. Indira Rajagopal Joydeep Acharya Madhavi V Ratnagiri Sumathi Gopal
Implementation and Performance Evaluation of an OFDM Modem With Variations in Cyclic Prefix Length and Channel Coding for Different Channels
Indira Rajagopal
Joydeep Acharya
Madhavi V Ratnagiri
Sumathi Gopal
Course: Communication Theory (ECE 545); Rutgers University
Professor: Dr. Predrag Spasojevic

2. Topics Introduction to OFDM Description of OFDM system simulated
Channel Simulation
Results
Conclusions
References
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OFDM Performance

3. Introduction to OFDM Orthogonal Frequency Division Multiplexing;
Part of xDSL, IEEE a standards
Improves Data rates, such as 56Mbps in IEEE a
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OFDM Performance

4. Introduction to OFDM Resilience to frequency selective channels
In real life we encounter channels which exhibit variation of gain and phase with frequency. A wideband signal is hence likely to get distorted differently at different frequencies. Hence if we split the wideband signal into multiple narrow band signals, the channel can be treated as constant over those individual narrow bands, and the
Courtesy : “Multicarrier Primer”
Resilience to frequency selective channels
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OFDM Performance

5. OFDM Transmitter Bits per OFDM symbol = (IFFT_Size/2) * log2(M) S/P
Channel
Coding
Inter
Leaving
S/P
Modulation (M)
IFFT
Cyclic
Prefix
Mapping
from
size
N/2 to N
Source
Data To
Modulation, IFFT is the crux of OFDM. The incoming bitsteam is split into separate subchannels, each correesponding to a ortho. Subcarrier.
Why IFFT: instead of using multiple analog ortho. Subcarriers, same effect is achieved digitally using IFFT/FFT.
Bits per OFDM symbol = (IFFT_Size/2) * log2(M)
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OFDM Performance

6. Channel Coding BCH Code k=16; t = 3; Convolutional code;
Inter
Leaving
S/P
Modulation
IFFT
P/S
Cyclic
Prefix
Mapping
from
size
N/2 to N
Source
Data To
BCH Code
n = 31;
k=16;
t = 3;
Convolutional code;
Rate = ½ ; K=7
Courtesy: a std.
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OFDM Performance

7. Interleaving Read out b1 b4 b7 b10 b2 b5 b8 b11 Fill in b3 b6 b9 b12
Channel
Coding
Inter
Leaving
S/P
Modulation
IFFT
P/S
Cyclic
Prefix
Mapping
from
size
N/2 to N
Source
Data To
Read out b1 b4 b7
b10 b2 b5 b8
b11
Fill in b3 b6 b9
b12
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OFDM Performance

8. Interleaving b1 b4 b7 b10 Read out b2 b5 b8 b11 Fill in b3 b6 b9 b12
Channel
Coding
Inter
Leaving
S/P
Modulation
IFFT
P/S
Cyclic
Prefix
Mapping
from
size
N/2 to N
Source
Data To b1 b4 b7
b10
Read out b1 b4 b7
b10 b2 b5 b8
b11 b3 b6 b9
b12 b2 b5 b8
b11
Fill in b3 b6 b9
b12
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OFDM Performance

9. Modulation Schemes Used
Channel
Coding
Inter
Leaving
S/P
Modulation
IFFT
P/S
Cyclic
Prefix
Mapping
from
size
N/2 to N
Source
Data To
QPSK (4-QAM)
16-QAM
64-QAM
We have employed QPSK… in our system
higher order mod schemes are used to achieve higher data rtes.
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OFDM Performance

10. N/2 to N Mapper and IFFT X*n-k = Xk where k : 1,… n/2 IFFT Block
Channel
Coding
Inter
Leaving
S/P
Modulation
IFFT
P/S
Cyclic
Prefix
Source
Data To
Mapping
from
size
N/2 to N
X*n-k = Xk where k : 1,… n/2
Mapping done so that the input seq to IFFT is conjugsate-symmetric so that we have real valued ifft output.
IFFT
Block
Real Valued
samples N
N/2
Mapper
Re(XN/2) X1
XN/2-1
Im(XN/2)
X*N/2-1
X*1 X2
XN/2
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OFDM Performance

11. Adding cyclic prefix x(n) * h(n) = X(k)H(k) S/P Modulation Mapping
from
size
N/2 to N
IFFT
P/S
Channel
Coding
Source
Data
Inter
Leaving
Modulation
Cyclic
Prefix To
Channel
Modulation
Modulation
x(n) * h(n) = X(k)H(k)
Original N Samples
Time
Added Prefix
Cyclic prefix length
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OFDM Performance

12. Channel Models AWGN: Rayleigh Flat fading:
r = s + n
Rayleigh Flat fading:
r = ρs + n where ρ is Rayleigh distributed
Rayleigh Frequency Selective:
r = h*s + n where h is channel impulse response
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OFDM Performance

13. OFDM Receiver De- Coding De- Inter Demod FFT P/S Remove Cyclic Prefix
Channel
De-
Coding
De- Inter
Leaving
Demod
FFT
P/S
Remove
Cyclic
Prefix
Mapping
from
size
N to N/2
From
Received
Bits
Thres-
holding
S/P
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OFDM Performance

14. Thresholding S/P FFT Mapping from size N to N/2 Thres- holding P/S
Demod
Remove
Cyclic
Prefix
From
Channel
Demod
De- Inter
Leaving
Channel
De-
Coding
Received
Bits
Demod
Demod
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OFDM Performance

15. Thresholding based on decision regions for 16-QAM
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OFDM Performance

16. De-Interleaving Read out b1 b2 b3 b4 b5 b6 Fill in b7 b8 b9 b10 b11
S/P
Mapping
from
size
N to N/2
FFT
Thres-
holding
P/S
Demod
From
Channel
Remove
Cyclic
Prefix
Demod
De- Inter
Leaving
Channel
De-
Coding
Received
Bits
Demod
Demod
Read out b1 b2 b3 b1 b2 b3 b4 b5 b6 b7 b8 b9
b10
b11
b12 b4 b5 b6
Fill in b7 b8 b9
b10
b11
b12
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OFDM Performance

17. Topics Introduction to OFDM Description of OFDM system Channel Models
Results
Conclusions
References
04/23/2003
OFDM Performance

18. Received Signal Constellation for 16-QAM in Presence of AWGN
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OFDM Performance

19. Bit Error Rate for AWGN for various IFFT sizes
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OFDM Performance

20. Bit Error Rate for AWGN for various Modulation Schemes
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OFDM Performance

21. Bit Error Rate for AWGN with and without Coding
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OFDM Performance

22. OFDM spectrum
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OFDM Performance

23. Coded and uncoded BER for flat fading Rayleigh channel
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OFDM Performance

24. Effect of varying cyclic prefix in a flat fading channel
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OFDM Performance

25. Conclusions
We were able to demonstrate the performance of an OFDM system in AWGN channel and a Rayleigh channel with flat fading
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OFDM Performance

26. Further work
Obtain BER plots of frequency selective channel with and without coding
Demonstrate change in BER with change in length of cyclic prefix, for a frequency selective channel
Analyze any tradeoffs between coding and length of cyclic prefix
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OFDM Performance

27. References IEEE 802.11a standard
J. M. Cioffi, “A multicarrier primer,” ANSI T1E1.4 Committee Contribution, Nov
Rappaport, T.S., “Wireless Communications, Principles and Practice”, Second Edition Prentice Hall.
Sklar Bernard, “Digital Communications” , Pearson Education Asia, Second Edition, 2001.
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OFDM Performance

28. It was FUN! Thank You for your patience
Acknowledgements
WINLAB students and faculty:
Predrag, Leo, Praveen, Hithesh, Ahmed, Lang, Rueheng, Jassi, Amith, and many more.
Zoran Kostic at Thomson Inc.
It was FUN! Thank You for your patience
04/23/2003
OFDM Performance