Method and apparatus for generating dedicated data channels in backscatter RFID systems
An antenna apparatus for backscattering an incoming radio frequency (RF) signal includes an antenna for backscattering the incoming RF signal in accordance with a reflection coefficient characteristic of the antenna. A variable impedance circuit includes an output electrically connected to the antenna. A low pass delta sigma modulator is coupled to the variable impedance circuit and digitally controls the output of the variable impedance circuit, such that the reflection coefficient of the antenna is adjusted based on the output of the variable impedance circuit.
1. An antenna apparatus for backscattering an incoming radio frequency (RF) signal comprising:
an antenna for backscattering the incoming RF signal in accordance with a reflection coefficient characteristic of the antenna;
a variable impedance circuit having an output electrically connected to the antenna; and
at least one low pass delta sigma (ΔΣ) modulator coupled to the variable impedance circuit and digitally controlling the output of the variable impedance circuit;
wherein the reflection coefficient (Γ) of the antenna is adjusted based on the output of the variable impedance circuit.
2. The antenna apparatus of claim 1 wherein an output of the at least one low pass delta sigma modulator switches the output of the variable impedance circuit between two states to adjust the reflection coefficient.
3. The antenna apparatus of claim 1 wherein an input signal applied to the low pass delta sigma modulator consists of one of a complex modulation signal offset from the incoming radio frequency signal by +/−ω o .
4. The antenna apparatus of claim 3 wherein the complex modulation signal consists of one of a GMSK, QPSK, nPSK, nQAM, and an OFDM signal.
5. The antenna apparatus of claim 1 wherein an output of the low pass delta sigma modulator is one of a return to zero (RTZ) and a non-return to zero (NRZ) type signal.
6. The antenna apparatus of claim 1 further comprising at least a second low pass delta sigma modulator coupled to the variable impedance circuit, wherein the output of the variable impedance circuit is further digitally controlled by the at least a second low pass delta sigma modulator.
7. The antenna apparatus of claim 6 wherein input signals to the first and second low pass delta sigma modulators comprise in-phase (I) and quadrature (Q) signals respectively.
8. The antenna apparatus of claim 7 wherein the I and Q signals are adjusted to compensate for errors that may arise in generating Γ o exp(j0°), Γ o exp(j180°), Γ o exp(j90°), and Γ o exp(j270°).
9. The antenna apparatus of claim 8 wherein the errors are compensated for in a radio frequency identification (RFID) reader device electromagnetically coupled to the antenna.
10. The antenna apparatus of claim 6 wherein a combined output of the first and second said low pass delta sigma modulators switches the output of the variable impedance circuit between four states to adjust the reflection coefficient of the antenna.
11. The antenna apparatus of claim 10 wherein the reflection coefficient comprises four states which are relative from each other by Γ o exp(j0°), Γ o exp(j180°), Γ o exp(j90°), and Γ o exp(j270°).
12. The antenna apparatus of claim 11 wherein the first said low pass delta sigma modulator ((ΔΣ) I ) switches the states between 0 degrees and 180 degrees.
13. The antenna apparatus of claim 12 wherein the second low pass delta sigma modulator ((ΔΣ) Q ) switches the states between 90 degrees and 270 degrees.
14. The antenna apparatus of claim 13 wherein outputs of the first and second low pass delta sigma modulators alternately switch between each other, wherein if (ΔΣ) I generates 0, 180, 180, 0, 0, 180 . . . and (ΔΣ) Q generates 90, 90, 270, 270, . . . , Γ is controlled to adjust by 0, 90, 180, 90, 180, 270, 0, 270, . . . .
15. The antenna apparatus of claim 6 wherein the input signals applied to the low pass delta sigma modulators comprise sine and cosine wave forms offset from a frequency of the incoming RF signal by φ 1 , where ω 1 can be either positive or negative.
16. The antenna apparatus of claim 6 wherein the input signals applied to the low pass delta sigma modulators consists of a complex modulation signal offset from a frequency of the incoming RF signal by one of +ω o , −ω o and zero.
17. The antenna apparatus of claim 16 wherein the complex modulation signal consists of one of a GMSK, nPSK, QPSK, nQAM, and OFDM signal.
18. The antenna apparatus of claim 6 wherein outputs of the low pass delta sigma modulators consist of one of a return to zero (RTZ) and a non-return to zero (NRZ) type signal.
19. The antenna apparatus of claim 1 further comprising at least one filter device at the variable impedance circuit to filter out of band noise output from the at least one low pass delta sigma modulator.
20. The antenna apparatus of claim 1 wherein the antenna apparatus is included a tag terminal of a radio frequency identification (RFID) system including a reader device, wherein the antenna apparatus is activated for backscattering RF signals only when the tag terminal is within a predetermined critical distance of the reader device.
21. The antenna apparatus of claim 1 wherein the antenna comprises part of a tag terminal, the tag terminal electromagnetically coupled to a reader device within a radio frequency identification (RFID) system, the RFID system comprising clocking the low pass delta sigma modulator, generation of the clocking consisting of one of a clock circuit within the tag reader and a clock circuit generated based on a frequency of the incoming RF signal.