Waveform synthesis for RFID transmitters
A waveform synthesis technique for radio frequency identification (RFID) transmitters and an RFID system making us of the technique are disclosed. The RFID transmitter in example embodiments synthesizes a continuous transmitter waveform from a symbol alphabet without Nyquist or interpolation filters. High spectral occupancy waveforms are achieved which include the ability to do both linear and nonlinear predistortion with no increase in computational load once the signal set has been adapted to compensate for linear and nonlinear distortion in the transmitter analog circuitry. A polarity generator can be used to impart the required polarity to each waveform. The RFID transmitter can be employed in RFID readers to reduce the computational requirements of the digital signal processor (DSP).
1. A radio frequency identification (RFID) system comprising:
a storage medium comprising a plurality of stored waveforms, each stored waveform representing a respective RFID symbol usable in a sequence of RFID symbols according to an RFID protocol; and
a polarity generator to selectively impart a polarity to each stored waveform based on the RFID protocol and the respective RFID symbol represented by the corresponding one of the stored waveforms to assemble the sequence of RFID symbols.
2. The RFID system of claim 1 further comprising:
an RF source to produce a carrier wave; and
an RF amplifier connected to the RF source and the polarity generator for transmitting the sequence of RFID symbols as output from the polarity generator modulated onto the carrier wave to synthesize an RFID transmitter waveform.
3. The RFID system of claim 2 wherein the stored waveforms include at least one of linear and nonlinear predistortion applied to substantially undistorted waveforms.
4. The RFID system of claim 3 wherein the stored waveforms are represented in Cartesian coordinates for use in a direct conversion transmitter.
5. The RFID system of claim 4 further comprising a modulator connected to the polarity generator, the RF source and the RF amplifier to form the direct conversion transmitter and produce the RFID transmitter waveform as an amplitude modulated RF signal.
6. The RFID system of claim 3 wherein the stored waveforms are represented in polar coordinates for use in a polar modulation transmitter.
7. The RFID system of claim 6 further comprising a phase shifter connected to the polarity generator, the RF source and the RF amplifier to form the polar modulation transmitter and produce the RFID transmitter waveform.
8. The RFID system of claim 3 wherein the undistorted waveforms are products of an optimization procedure minimizing spectral occupancy.
9. The RFID system of claim 8 wherein the optimization procedure comprises quadratic programming using a discrete Fourier transform.
10. The RFID system of claim 3 wherein at least some of the RFID symbols in the sequence of RFID symbols comprise a data-1 having length from about 1.8 to about 1.95 type-A reference interval (tari).
11. The RFID system of claim 3 wherein the carrier wave has a frequency selected from high-frequency, ultra-high-frequency, or microwave frequency bands.
12. A method of synthesizing a radio frequency identification (RFID) transmitter waveform, the method comprising:
repeatedly retrieving, by a processor, a stored waveform from a plurality of stored waveforms, each stored waveform representing a respective RFID symbol usable in a sequence of RFID symbols according to an RFID protocol;
imparting, by the processor, a polarity to each stored waveform, the polarity being selectively set based on the RFID protocol and the respective RFID symbol represented by the stored waveform; and
assembling, by the processor, the sequence of RFID symbols each based on the corresponding one of the stored waveforms and the selectively set polarity.
13. The method of claim 12 wherein the stored waveforms include at least one of linear and nonlinear predistortion applied to substantially undistorted waveforms.
14. The method of claim 13 further comprising modulating the sequence of RFID symbols onto a carrier wave to synthesize an RFID transmitter waveform.
15. The method of claim 14 wherein the stored waveforms are represented in Cartesian coordinates for use in a direct conversion transmitter.
16. The method of claim 14 wherein the stored waveforms are represented in polar coordinates for use in a polar modulation transmitter.
17. The method of claim 16 wherein at least some of the RFID symbols in the sequence of RFID symbols comprises a data-1 having from about 1.8 to about 1.95 type-A reference interval (tari) length.
18. The method of claim 17 wherein the carrier wave has a frequency selected from high-frequency, ultra-high-frequency, or microwave frequency bands.
19. Apparatus comprising:
means for storing a plurality of waveforms;
means for retrieving a stored waveform from the plurality of waveforms, each stored waveform representing a respective radio frequency identification (RFID) symbol usable in a sequence of RFID symbols according to an RFID protocol;
means for imparting a polarity to the stored waveform, the polarity being selectively set based on the RFID protocol and the respective RFID symbol represented by the stored waveform; and
means for assembling the sequence of RFID symbols each based on the corresponding one of the stored waveforms and the selectively set polarity.
20. The apparatus of claim 19 further comprising means for modulating the sequence of RFID symbols onto a carrier wave to synthesize an RFID transmitter waveform.
21. A method of making a radio frequency identification (RFID) system to synthesize a transmitter waveform, the method comprising:
assembling a prototype waveform using sinusoidal rising and falling edges for each respective RFID symbol from among a plurality of RFID symbols according to an RFID protocol;
creating lower and upper bounds for the prototype waveform so that rise and fall times are preserved;
constructing a periodic waveform from the prototype waveform;
executing an optimization algorithm using the periodic waveform to extract a corresponding run time waveform;
similarly extracting the corresponding run time waveform for each respective RFID symbol to obtain and store a plurality of run time waveforms in a storage medium; and
providing a polarity generator to selectively impart a polarity to each run time waveform based on the RFID protocol and the respective RFID symbol represented by the corresponding one of the run time waveforms to assemble a sequence of RFID symbols for the transmitter waveform.
22. The method of claim 21 further comprising applying at least one of linear and nonlinear predistortion to the corresponding run time waveform prior to storing the corresponding run time waveform.
23. The method of claim 22 wherein the optimization algorithm is a quadratic programming algorithm.