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Implementing Adaptive Modulation in a Software-Defined Cognitive Radio

Implementing Adaptive Modulation in a Software-Defined Cognitive Radio. Brandon Bilinski Computer Engineering Senior, Clemson University. What is a Software-Defined Radio?. An SDR is a flexible radio where programmable hardware is controlled by software.

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Implementing Adaptive Modulation in a Software-Defined Cognitive Radio

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  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.

  8. 32-ary Orthogonal Implementation

  9. 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 -1*cosine while a ‘1’ is mapped to 1*cosine. • The BPSK constellation is mapped on the complex plane to the right:

  11. BPSK Implementation

  12. 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. 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 16-QAM Modulation • 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

  18. 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_product) The following example is for 4-ary modulation: 1 - (.7/3.9) = .821

  20. 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.

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

  22. 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.

  24. 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

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