Implementation and Performance Evaluation of an OFDM Modem With Variations in Cyclic Prefix Length and Channel Coding for Different Channels

1
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

3
Introduction to OFDM

• Orthogonal Frequency Division Multiplexing
• Part of xDSL, IEEE 802.11a standards
• Improves Data rates, such as 56Mbps in IEEE
  802.11a

4
Introduction to OFDM
Courtesy Multicarrier Primer

• Resilience to frequency selective channels

5
OFDM Transmitter
Bits per OFDM symbol (IFFT_Size/2) log2(M)
6
Channel Coding
BCH Code n 31 k16 t 3 Convolutional code Rate ½ K7

Courtesy 802.11a std.
7
Interleaving
Read out
b4
b10



b7
b1
b2
b5
b8
b11
Fill in
b3
b9
b6
b12
8
Interleaving
b4
b10
Read out



b1
b7
b1
b4
b7
b2
b10
b5
b11
b8
b9
b3
b6
b12
b2
b5
b8
b11
Fill in
b3
b9
b6
b12
9
Modulation Schemes Used

• QPSK (4-QAM)
• 16-QAM
• 64-QAM

10
N/2 to N Mapper and IFFT
Xn-k Xk where k 1, n/2
11
Adding cyclic prefix
P/S
S/P
IFFT
Mapping from size N/2 to N
Modulation
Channel Coding
Inter Leaving
Cyclic Prefix
Modulation
To Channel
Source Data
Modulation
Modulation
x(n) h(n) X(k)H(k)
Original N Samples
Time
Cyclic prefix length
Added Prefix
12
Channel Models

• AWGN
• r s n
• Rayleigh Flat fading
• r ?s n where ? is Rayleigh distributed
• Rayleigh Frequency Selective
• r hs n where h is channel impulse
  response

13
OFDM Receiver
14
Thresholding
FFT
P/S
Mapping from size N to N/2
Thres- holding
S/P
Demod
Remove Cyclic Prefix
Channel De- Coding
De- Inter Leaving
Demod
From Channel
Received Bits
Demod
Demod
15
Thresholding based on decision regions for 16-QAM




16
De-Interleaving
FFT
P/S
Mapping from size N to N/2
Thres- holding
S/P
Demod
Remove Cyclic Prefix
Channel De- Coding
De- Inter Leaving
Demod
From Channel
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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Topics

• Introduction to OFDM
• Description of OFDM system
• Channel Models
• Results
• Conclusions
• References

18
Received Signal Constellation for 16-QAM in
Presence of AWGN
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Bit Error Rate for AWGN for various IFFT sizes
20
Bit Error Rate for AWGN for various Modulation
Schemes
21
Bit Error Rate for AWGN with and without Coding
22
OFDM spectrum
23
Coded and uncoded BER for flat fading Rayleigh
channel
24
Effect of varying cyclic prefix in a flat fading
channel
25
Conclusions

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

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

27
References

• IEEE 802.11a standard
• J. M. Cioffi, A multicarrier primer, ANSI
  T1E1.4 Committee Contribution, Nov. 1991.
• Rappaport, T.S., Wireless Communications,
  Principles and Practice, Second Edition Prentice
  Hall.
• Sklar Bernard, Digital Communications , Pearson
  Education Asia, Second Edition, 2001.

28
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