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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

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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

topics
Topics
  • Introduction to OFDM
  • Description of OFDM system simulated
  • Channel Simulation
  • Results
  • Conclusions
  • References

OFDM Performance

introduction to ofdm
Introduction to OFDM
  • Orthogonal Frequency Division Multiplexing;
  • Part of xDSL, IEEE 802.11a standards
  • Improves Data rates, such as 56Mbps in IEEE 802.11a

OFDM Performance

introduction to ofdm4
Introduction to OFDM
  • Resilience to frequency selective channels

Courtesy : “Multicarrier Primer”

OFDM Performance

ofdm transmitter

S/P

S/P

Mapping

from

size

N/2 to N

IFFT

Modulation (M)

Channel

Coding

Inter

Leaving

Cyclic

Prefix

Source

Data

Modulation (M)

To

Channel

Modulation (M)

Modulation (M)

OFDM Transmitter

Bits per OFDM symbol = (IFFT_Size/2) * log2(M)

OFDM Performance

channel coding

P/S

S/P

Mapping

from

size

N/2 to N

IFFT

Modulation

Channel

Coding

Inter

Leaving

Cyclic

Prefix

Modulation

To

Channel

Source

Data

Modulation

Modulation

Channel Coding

Courtesy: 802.11a std.

OFDM Performance

interleaving

P/S

S/P

Mapping

from

size

N/2 to N

IFFT

Modulation

Channel

Coding

Inter

Leaving

Cyclic

Prefix

Modulation

To

Channel

Source

Data

Modulation

Modulation

Interleaving

Read out

b4

b10

b7

b1

b2

b5

b8

b11

Fill

in

b3

b6

b9

b12

OFDM Performance

interleaving8

P/S

S/P

Mapping

from

size

N/2 to N

IFFT

Modulation

Channel

Coding

Inter

Leaving

Cyclic

Prefix

Modulation

To

Channel

Source

Data

Modulation

Modulation

Interleaving

b4

b10

Read out

b1

b7

b1

b4

b7

b10

b2

b5

b8

b11

b3

b6

b9

b12

b2

b5

b8

b11

Fill in

b3

b6

b9

b12

OFDM Performance

modulation schemes used

P/S

S/P

Mapping

from

size

N/2 to N

IFFT

Modulation

Channel

Coding

Inter

Leaving

Cyclic

Prefix

Modulation

To

Channel

Source

Data

Modulation

Modulation

Modulation Schemes Used
  • QPSK (4-QAM)
  • 16-QAM
  • 64-QAM

OFDM Performance

n 2 to n mapper and ifft

P/S

S/P

Mapping

from

size

N/2 to N

IFFT

Modulation

Channel

Coding

Inter

Leaving

Cyclic

Prefix

Modulation

To

Channel

Source

Data

Modulation

Modulation

X1

IFFT

Block

Re(XN/2)

X1

X2

Real Valued

samples

Mapper

XN/2-1

Im(XN/2)

XN/2-1

X*N/2-1

XN/2

X*1

N

N/2

N

N/2 to N Mapper and IFFT

X*n-k = Xk where k : 1,… n/2

OFDM Performance

adding cyclic prefix
Adding cyclic prefix

P/S

S/P

Mapping

from

size

N/2 to N

IFFT

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

Added Prefix

Cyclic prefix length

OFDM Performance

channel models
Channel Models
  • AWGN:

r = s + n

  • Rayleigh Flat fading:

r = ρs + n where ρ is Rayleigh distributed

  • Rayleigh Frequency Selective:

r = h*s + nwhere h is channel impulse response

OFDM Performance

slide13

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

OFDM Receiver

OFDM Performance

slide14

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

OFDM Performance

de interleaving
De-Interleaving

S/P

Mapping

from

size

N to N/2

FFT

Thres-

holding

P/S

Demod

Remove

Cyclic

Prefix

Channel

De-

Coding

From

Channel

Demod

De- Inter

Leaving

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

OFDM Performance

topics17
Topics
  • Introduction to OFDM
  • Description of OFDM system
  • Channel Models
  • Results
  • Conclusions
  • References

OFDM Performance

ofdm spectrum
OFDM spectrum

OFDM Performance

conclusions
Conclusions

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

OFDM Performance

further work
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

OFDM Performance

references
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.

OFDM Performance

acknowledgements
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

OFDM Performance

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