Implementing Adaptive Modulation in a Software-Defined Cognitive Radio

1
Implementing Adaptive Modulation in a
Software-Defined Cognitive Radio

• Brandon Bilinski
• Computer Engineering Senior,
• Clemson University

2
What is a Software-Defined Radio?

• An SDR is a flexible radio where programmable
  hardware is controlled by software.
• SDRs can tune to any frequency band or change to
  any modulation type.
• I am employing adaptive modulation.

3
Previous Implementations

• With the frequency bands getting more and more
  crowded, a solution is needed.
• Software-Defined radios allow for the advent of
  cognitive radios.
• Many cognitive radios sense busy channels and
  change to free channels
• What if there are no free channels available to
  change between?

4
Why adapt modulations?

• If a channel is noisy it is more beneficial to
  change modulations rather than adjust power.
• The following modulation types had constant power
  and bandwidth.

5
First Steps of Adaptive Modulation

• Construct a library of various modulation types
  in the FPGA chip.
• My radio contains 32-ary Orthogonal, BPSK, QPSK,
  and 16-QAM
• All modulation types designed in Simulink using
  the Xilinx FPGA blockset.
• BPSK Binary Phase Shift Keying
• QPSK Quadrature Phase Shift Keying
• QAM Quadrature Amplitude Modulation

6
32-ary Orthogonal Modulation

• This is the most robust modulation scheme used in
  my radio.
• This scheme uses 32 chips to represent 5 bits of
  data, so it is also the slowest.
• Each signal is orthogonal as a result of the
  orthogonality of the Hadamard matrix.

7
What are Hadamard Matrices?

• A Hadamard matrix is a matrix of the form 2(n-1)
  x 2(n-1) where every row is orthogonal.
• If you multiply the Hadamard matrix by the
  transpose of any row, you will get n in the
  appropriate row and 0s elsewhere.

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32-ary Orthogonal Implementation
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32-ary Orthogonal Modulation Cont.

• Transmit two words 00000 and 00001
• Corresponding Hadamard Rows
• 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
  1 1 1 1 1 1 1
• 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
  1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

10
BPSK Modulation

• The transmitter modulates only one bit at a time.
• A 0 is mapped to -1cosine while a 1 is
  mapped to 1cosine.
• The BPSK constellation is mapped on the complex
  plane to the right

11
BPSK Implementation
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BPSK Modulation Cont.

• Transmit the following bits 11110101

13
QPSK Modulation

• QPSK is less robust than BPSK, but it can send
  twice the bits in one transmission.
• QPSK has four possible words 00, 01, 10,
  11 that are modulated using a combination of
  sine and cosine.
• The constellation mapping is once
• again shown on the right

14
QPSK Implementation
15
QPSK Modulation Cont.

• Transmit the following bits 11011000

16
16-QAM Modulation

• d3- 0 cosine is negative
• 1 cosine is positive
• d2- 0 1x cosine
• 1 2x cosine
• d1- 0 sine is negative
• 1 sine is positive
• d0- 0 1x sine
• 1 2x sine

• Unlike BPSK and QPSK, 16-QAM involves changes in
  phase and amplitude.
• In this way, each word can be 4 bits long, which
  allows you to send twice the bits that are
  possible with QPSK.
• This is the toughest scheme that I implement to
  demodulate due to close proximity of points in
  the constellation.

17
16-QAM Implementation
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How do you get environment stats?

• How do you know what the CENR is in the
  environment in order to adapt your modulation
  types?

19
Adapting 32-Orthogonal Modulation

• As previously stated, multiplying an nxn Hadamard
  matrix by a transpose of one row of the matrix
  theoretically results in an nx1 matrix of n and
  all 0s.
• To estimate the signal to noise ratio we can use
  the following equation 1 (second_largest/max_p
  roduct)
• The following example is for 4-ary
  modulation

1 - (.7/3.9) .821
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32-ary Ratio Statistic

• When the ratio gets above .38, then it is
  desirable to switch to BPSK.
• With additional orthogonal schemes included, a
  ratio lower than .38 would signify a switch to
  16-ary orthogonal.

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BPSK Distance Statistic

• If distance is greater than .6, switch to 32-ary
  orthogonal.
• If distance is under .37, switch to QPSK.

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QPSK Distance Statistic

• If distance is greater than .37, switch to BPSK
• If distance is less than .19, switch to 16-QAM

23
16-QAM Distance Statistic

• If the distance is greater than .19, switch down
  to QPSK.

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Future Plans

• Finish 16-QAM demodulator and add timing and
  phase synchronization in all demodulators.
• Implement modulation library in FPGA
• Enable modulation scheme switching by programming
  the DSP chip in the radio.
• Move the radio through different noise
  environments to ensure modulation switching
  occurs where expected.
• Add Forward Error Correcting codes to improve
  robustness of system.

25
Acknowledgements

• Faculty Advisor Dr. Michael Pursley
• Graduate Student Advisors Joel Simoneau
  Tommy Royster
• Dr. Noneaker, Dr. Xu and Josh Lawrence
• Dr. Russell, Dr. Hubing, Dr. Baum, Dr. Hubbard
  and Dr. Fishman.

26
Questions?

• Brandon Bilinski
• Computer Engineering Senior,
• Clemson University