Passive, Wireless, Orthogonal Frequency Coded SAW Sensors and Tags – Design and System

1. Passive, Wireless, Orthogonal Frequency Coded SAW Sensors and Tags – Design and System D.C. Malocha, D. Puccio, and N. Lobo School of Electrical Engineering & Computer Science University of Central Florida Orlando, Fl 32816-2450 Acknowledgements: Funding has been provided through the NASA STTR grants program with industry partners of MSA and ASRD, and through the NASA Graduate Student Research Program.

2. Orthogonal Frequency Coded (OFC) SAWs • Multiple access operation • Improved range due to enhanced processing gain • Increased sensitivity resulting from reduced compressed pulse time ambiguity • Fractional bandwidth can meet ultra-wideband (UWB) specifications • Inherent security using spread spectrum

3. OFC/PN Background: Spreading and Coding a Signal • Given a signal bit length = τB • To code a bit, divide the bit into N chips, such that, τB = N·τC • For a pseudo noise PN sequence, there is a single carrier frequency but the phase within a chip changes. • For OFC-PN, the chip carrier frequency is a variable and the chip phase is also a variable.

4. Background: Orthogonal Frequency Theory

5. Brief Background • Single frequency tag • OFC tag

6. Single Frequency SAW Tag Example • N single frequency bits implemented with N inline reflectors • Bit locations and phases indicate identification and/or sensor information • Impulse response is series of decaying pulses • All reflectors have same reflection coefficient • Multiple reflections ignored in analysis

7. Single Frequency SAW Tag - Optimal Reflection Coefficient • Optimal chip reflection coefficient maximizes power received from last chip • Optimal chip reflection coefficient defined as • Power received from last chip decreases as chip count is increased given Ropt

8. Schematic of OFC SAW ID Tag Chip length Bit Length Example: OFC Bit – 7chips/bit

9. Approach • Study a methodology to optimize reflective structures for OFC devices • Minimize device insertion loss • Maintain chip orthogonality • Find optimum values for bit length, chip length, and strip reflectivity as a function of device fractional bandwidth • Maintain processing gain • Minimize ISI effects

10. Boundary Conditions for Analysis • Assume only a single in-line grating analysis. • Assumes no weighting within each reflective region which composes a chip. • First order assumptions are made to understand the phenomenon and verified by COM models and simulation. • Multiple parallel tracks can be approached in a similar manner.

11. SAW OFC Reflector Coding • SAW device implementation of the ideal OFC code using a reflective structure assumes that the ideal chip can be accurately reproduced by a reflector • Chip frequency response: Sin(x)/x • Chip time response: • Uniform amplitude of chips for maximum coding, processing gain (PG) and correlation output. • To what degree can all of the above be achieved versus design parameters

12. Intra-chip & Inter-chip Reflector Considerations • Chip reflector uniformity for OFC coding • Intersymbol interference (ISI) – the intra reflections ring longer than • Energy rolloff due to propagation through chips • Processing gain decrease • Coding diversity loss • Orthogonality • Chip processing gain due to reflector response • Frequency & time domain distortion

13. OFC Reflector Bank Uniformity fc=chip frequency determined by orthogonality As fc increases, Nc increases and chip reflectivity increases

14. Response of Reflector Test Structure Under proper conditions, a SAW reflector looks similar to a Sampling function in frequency and a Rect function in time. Reflectivity is a function of the substrate and reflector material, reflector film thickness, substrate coupling coefficient and line-to-width ratio. The reflector width is approximately the chip length. How approximate is it???

15. Simulation of a reflector grating frequency response for 1% reflectivity per strip, and 4 different grating lengths. Ng equals the number of reflective strips in each grating.

16. Plot of magnitude of reflectivity versus the product of the number of strips and reflectivity per strip (Ng.r). For small reflector loss, chip reflectivity, Ng.r,should be large but for reasonable sin(x)/x frequency response, Ng.r product should definitely be less than 2.

17. OFC Adjacent Frequency Reflection • OFC yields reduced reflections between reflectors compared to single frequency PN due to orthogonality • Non-synchronous orthogonal frequencies are partially reflected • The closer the adjacent frequency chips the greater the partial reflection • Must understand non-synchronous reflectivity for all chips

18. Adjacent Frequency Reflection • Assume an RF burst near fo as interrogation signal • Very small reflection of incident adjacent frequency RF burst from weak reflector • Large adjacent frequency reflection from strong reflector • Transmission through the reflector bank can be compromised if chip reflectivity is too large which causes energy rolloff for trailing chips. Small Reflectivity Large Reflectivity

19. Frequency Transmission vs Reflectivity as a Function of Frequency Offset fSAW is the synchronous reflector of interest is a prior asynchronous reflector in bank For 90% transmission, r*Ng<2 • COM simulations used to determine non-synchronous reflector transmission coefficient • Analysis performed for reflector center frequencies 1,2,3 orthogonal frequencies higher and lower than incident wave

20. OFC Adjacent Reflectivity -1st Order Analysis • Several assumptions made to simplify analysis • Use chirp interrogation signal & OFC reflector • All reflectors have equal reflectivity • No propagation loss • Multiple reflections ignored • Only adjacent frequency reflector transmission coefficients considered

21. Adjacent Frequency Reflector Transmission Example Independent of the OFC frequency code sequence, the sum of the adjacent frequency interactions is always equal to Nf-1, but the interactions for a given frequency is code dependent.

22. Total Reflected OFC Power Equations defined to relate several OFC reflector bank parameters • Ptot= total output power • Tadj=adjacent center frequency transmission • Ro=chip reflectivity • r= electrode reflectivity • Ng= # of reflector chip electrodes • Nf= # of frequencies

23. Example Reflected Power Prediction • 10% bandwidth • 2% electrode reflectivity • No repeated frequencies • Predictions compared with COM simulations • Large variations caused by multi-reflection interference Approximate analysis and COM model agree well for Nf<10. Optimum reflected power for 10<Nf<15.

24. Optimal Reflection Coefficient • Reflected power for 5% and 10% fractional bandwidths • Optimal empirically derived relationship for # of frequencies (Nf), strip reflectivity (r) and %BWbit: • White line indicates maximum reflected power • Total reflected power is maximized for R0 ~ 80%

25. Check for Self-Consistence • We now have 2 equations for Nf where the first equation is exact and the second is approximate based on optimization of parameters. • Substitute r=1.4/Ng for 80% reflectivity (%BW=5) and the exact expression for Nf as a function of fractional bandwidths, which yields: • The number of strips in the reflector is given by: Ng=(2*%BWchip)-1 • Solving both sides of the equation yields: • The predictive analysis seems reasonable with the various assumptions made.

26. Reflector Test Structure Time Response How is the time response affected by chip length and reflectivity. How approximate is the time domain reflector compared to a Rect function???

27. Simulation of a SAW grating time response for 1% reflectivity and 4 different grating lengths. Time scale is normalized to reflect the number of wavelengths at center frequency As Ng*r increases: 1. Impulse response length of reflector increases beyond desired chip -ISI 2. Energy leakage beyond desired chip increases- energy loss Ng*r=1 appears to be maximum for minimum ISI

28. Chip Correlation with Synchronous Interrogator Pulse Correlation is greater than ideal, IR length is near ideal and sidelobes are low. Correlation is greater but sidelobes apparent due to intra-chip-reflections

29. Chip Correlation with Adjacent Frequency Asynchronous Interrogator Pulse Near ideal response. Cross correlation shows null at chip center, as expected due to OFC properties. Cross correlation shows reduced null at chip center, and trailing correlation sidelobe distortion.

30. Measured Device Example • fo= 250 MHz • %BW=28%; BW=69 MHz • YZ LiNbO3, k2=.046, r~3.4% • (# frequencies) = (# chips) =7 • # of reflectors at fo = 24 • Ng*r ~ .72 • Chip reflector loss~4dB

31. COM Simulation versus Experimental Results COM predictions Dual delay OFC device having two reflector banks and 7 chips/bank Experimental Measurement

32. General Results and Conclusions • Various OFC chip criteria were investigated to provide guidance in choosing optimal design criteria. • The ISI and pulse correlation distortion appear to be a limiting or controlling factor for maximizing the chip reflectivity and suggests Ng*r<1. • For Ng*r=1, chip reflector loss is approximately 2.5 dB. • Based on reflective power predictions and simulations, the largest number of chip frequencies should be between 10 and 15, with the precise number of frequencies dependent on the bit fractional bandwidth and strip reflectivity.