TRANSMISSION APPARATUS FOR A WIRELESS DEVICE USING DELTA-SIGMA MODULATION

Abstract

A transmission apparatus for a wireless device, comprising: an antenna for receiving an original signal and for backscattering a modulated signal containing information from the wireless device; a variable impedance coupled to the antenna, the variable impedance having an impedance value; a delta-sigma modulator coupled to the variable impedance for modulating the impedance value, and thereby a backscattering coefficient for the antenna, in accordance with the information to generate the modulated signal; and, a decoder coupled to the delta-sigma modulator for generating the impedance value from the information.

Claims

1. A transmission apparatus for a wireless device, comprising: an antenna for receiving an original signal and for backscattering a modulated signal containing information from the wireless device; a variable impedance coupled to the antenna, the variable impedance having an impedance value; a delta-sigma modulator coupled to the variable impedance for modulating the impedance value, and thereby a backscattering coefficient for the antenna, in accordance with the information to generate the modulated signal; and, a decoder coupled to the delta-sigma modulator for generating the impedance value from the information.

2. (canceled)

3. (canceled)

4. (canceled)

5. (canceled)

6. (canceled)

7. The transmission apparatus of claim 1 wherein the information is an N-bit digital waveform that is applied to the decoder and then to the delta-sigma modulator to produce a control signal for the variable impedance that is related to the N-bit digital waveform.

8. (canceled)

9. (canceled)

10. (canceled)

11. The transmission apparatus of claim 1 wherein the information is a complex modulation signal that is offset in frequency from the original signal.

12. (canceled)

13. (canceled)

14. (canceled)

15. The transmission apparatus of claim 1 wherein the information is a complex modulation signal that alternates between an in-phase signal and a quadrature signal via a control signal, and the variable impedance switches between backscattering coefficients that are 90 degrees offset from each other depending on whether the complex modulation signal is the in-phase signal or the quadrature signal.

16. (canceled)

17. (canceled)

18. (canceled)

19. (canceled)

20. (canceled)

21. (canceled)

22. (canceled)

23. (canceled)

24. The transmission apparatus of claim 1 wherein the information is an N-bit digital waveform that is adjusted to compensate for errors in at least one of the decoder, the delta-sigma modulator, and the variable impedance.

25. The transmission apparatus of claim 1 wherein the variable impedance includes a filter for filtering noise generated by at least one of the decoder and the delta-sigma modulator.

26. (canceled)

27. (canceled)

28. (canceled)

29. The transmission apparatus of claim 1 wherein the original signal is received from an RFID reader that is configured to correct for errors in at least one of the decoder, the delta-sigma modulator, and the variable impedance.

30. (canceled)

31. A transmission apparatus for a wireless device, comprising: an inductor for receiving an original signal and for transmitting by mutual inductance a modulated signal containing information from the wireless device; a variable impedance coupled to the inductor, the variable impedance having an impedance value; a delta-sigma modulator coupled to the variable impedance for modulating the impedance value, and thereby a value of the mutual inductance, in accordance with the information to generate the modulated signal; and, a decoder coupled to the delta-sigma modulator for generating the impedance value from the information.

32. (canceled)

33. (canceled)

34. (canceled)

35. (canceled)

36. The transmission apparatus of claim 31 wherein the information is an N-bit digital waveform that is applied to the decoder and then to the delta-sigma modulator to produce a control signal for the variable impedance that is related to the N-bit digital waveform.

37. (canceled)

38. (canceled)

39. (canceled)

40. The transmission apparatus of claim 31 wherein the information is a complex modulation signal that is offset in frequency from the original signal.

41. (canceled)

42. (canceled)

43. (canceled)

44. The transmission apparatus of claim 31 wherein the information is a complex modulation signal that alternates between an in-phase signal and a quadrature signal via a control signal, and the variable impedance switches between impedance values that are 90 degrees offset from each other depending on whether the complex modulation signal is the in-phase signal or the quadrature signal.

45. (canceled)

46. (canceled)

47. (canceled)

48. (canceled)

49. (canceled)

50. (canceled)

51. (canceled)

52. (canceled)

53. The transmission apparatus of claim 31 wherein the information is an N-bit digital waveform that is adjusted to compensate for errors in at least one of the decoder, the delta-sigma modulator, and the variable impedance.

54. The transmission apparatus of claim 31 wherein the variable impedance includes a filter for filtering noise generated by at least one of the decoder and the delta-sigma modulator.

55. (canceled)

56. (canceled)

57. (canceled)

58. The transmission apparatus of claim 31 wherein the original signal is received from an RFID reader is configured to correct for errors in at least one of the decoder, the delta-sigma modulator, and the variable impedance.

59. (canceled)

60. The transmission apparatus of claim 1 wherein the delta-sigma modulator is one of a low-pass delta-sigma modulator and a band-pass delta-sigma modulator.

61. The transmission apparatus of claim 1 wherein the delta-sigma modulator is a single bit delta-sigma modulator.

62. The transmission apparatus of claim 1 wherein the delta-sigma modulator switches the impedance value between at least two states.

63. The transmission apparatus of claim 31 wherein the delta-sigma modulator is one of a low-pass delta-sigma modulator and a band-pass delta-sigma modulator.

64. The transmission apparatus of claim 31 wherein the delta-sigma modulator is a single bit delta-sigma modulator.

65. The transmission apparatus of claim 31 wherein the delta-sigma modulator switches the impedance value between at least two states.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Features and advantages of the embodiments of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[0018] FIG. 1 is a block diagram illustrating a radio frequency identification (RFID) system in accordance with the prior art;

[0019] FIG. 2 is a block diagram illustrating transmission of energy and data between a reader and a tag in a RFID system in accordance with the prior art;

[0020] FIG. 3 is a block diagram illustrating communications between a reader and multiple tags in an RFID system in accordance with the prior art;

[0021] FIG. 4 is a block diagram illustrating a transmission apparatus for a tag for backscattering ASK and/or on-off keying (“OOK”) signals in accordance with the prior art;

[0022] FIG. 5 is a block diagram illustrating a transmission apparatus for a tag for backscattering PSK signals in accordance with the prior art;

[0023] FIG. 6 is a block diagram illustrating multiple tags communicating back to a reader using the same frequency spectrum in accordance with the prior art;

[0024] FIG. 7A is a block diagram illustrating a transmission apparatus for a wireless device for backscattering signals to a reader based on a digital wave form input in accordance with an embodiment of the invention;

[0025] FIG. 7B is a block diagram illustrating a variable impedance circuit for the transmission apparatus of FIG. 7A in accordance with an embodiment of the invention;

[0026] FIG. 8 is a graph illustrating the relationship between Gamma (Γ) and Z.sub.i in accordance with an embodiment of the invention;

[0027] FIG. 9 is a block diagram illustrating a transmission apparatus with an adder for a wireless device for backscattering arbitrary modulated signals to a reader based on I and Q data input in accordance with an embodiment of the invention;

[0028] FIG. 10A is a block diagram illustrating inductive coupling between a reader and a wireless device in a RFID system in accordance with an embodiment of the invention;

[0029] FIG. 10B is a block diagram illustrating an equivalent circuit for the RFID system of FIG. 10A in accordance with an embodiment of the invention; and,

[0030] FIG. 11 is a block diagram illustrating a transmission apparatus using inductive coupling for a wireless device for transmitting signals to a reader based on a digital waveform input in accordance with an embodiment of the invention.

[0031] It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] In the following description, details are set forth to provide an understanding of the invention. In some instances, certain software, circuits, structures and methods have not been described or shown in detail in order not to obscure the invention. The term “apparatus” is used herein to refer to any machine for processing data, including the systems, devices, and network arrangements described herein. The term “wireless device” is used herein to refer to RFID tags, RFID transponders, cellular telephones, smart phones, portable computers, notebook computers, or similar devices. The present invention may be implemented in any computer programming language provided that the operating system of the data processing system provides the facilities that may support the requirements of the present invention. Any limitations presented would be a result of a particular type of operating system or computer programming language and would not be a limitation of the present invention. The present invention may also be implemented in hardware or in a combination of hardware and software.

[0033] FIG. 7A is a block diagram illustrating a transmission apparatus 800 for a wireless device 130 for backscattering signals to a reader 120 based on a digital wave form input 830 in accordance with an embodiment of the invention. And, FIG. 7B is a block diagram illustrating a variable impedance 810 circuit for the transmission apparatus 800 of FIG. 7A in accordance with an embodiment of the invention. The present invention provides a method and apparatus for generating complex waveforms for passive and semi-passive RFID systems 100. The complex wave forms may generate any type of complex modulation signals such as 8-constellation phase shift keying (“8 PSK), orthogonal frequency-division multiplexing (“OFDM”), or n-constellation quadrature amplitude modulation (“nQAM”). The method and apparatus may also be used to generate frequency channels for each wireless device 130. According to one embodiment, the transmission apparatus (e.g., 800) includes an antenna 133 coupled to a variable impedance 810 having an array of impedances (e.g., the first impedance Z.sub.1 and the second impedance Z.sub.2 in FIG. 7B) that are switched on or off (via the first switch S.sub.1 and the second switch S.sub.2 in FIG. 7B, respectively) via a backscattering decoder 820 and a delta-sigma (ΔΣ) modulator 840 in the wireless device 130. The signal 830 applied to the input of the decoder 820 may consist of any type of digital signal. The transmission apparatus 800 may include a processor 880 for controlling the decoder 820, delta-sigma (ΔΣ) modulator 840, and variable impedance 810, memory 890 for storing information (e.g., digital waveforms 830), and related hardware and software as is known to one of skill in the art.

[0034] FIG. 8 is a graph illustrating the relationship between Gamma (Γ) and Z.sub.i in accordance with an embodiment of the invention. Here, Γ is the reflection coefficient and Z.sub.i is the impedance seen by the antenna 133. The reflection coefficient is directly proportional to the digital wave form 830. According to one embodiment of the invention, for backscattering RF applications, the reflection or backscattering coefficient Gamma (Γ) is given by:

Γ.sub.i=αe.sup.jφ.sup.i

where φ.sub.i is the phase, α is the magnitude of the reflection coefficient, and j is the square root of −1. The back scattering impedance (i.e., the impedance seen by the antenna 133) is then given by:

[00003]$Z_i=\frac{Z_s\left(1+\alpha .Math..Math.e^{j.Math..Math.{\phi }_i}\right)}{\left(1-\alpha .Math..Math.e^{j.Math..Math.{\phi }_i}\right)}$

where Z.sub.s is a constant (typically 50 ohms) and Z.sub.i is the back scattering impedance value.

[0035] Assuming the phase is zero:

[00004]$Z_i=\frac{Z_s\left(1+\alpha \right)}{\left(1-\alpha \right)}$

[0036] If s(t) is a signal (e.g., a sine wave) that is to be sent to the reader 120, it must be directly related to α(t) (e.g., s(t) is directly proportional to α(t)) and thus Γ. This produces an impedance value Z.sub.i that varies with time.

[0037] In this embodiment, the signal s(t) would be backscattered back to the reader 120 by the wireless device 130. In the transmission apparatus 800 shown in FIG. 7A, N-bits 821 are applied to the variable impedance 810 via the delta-sigma (ΔΣ) modulator 840 such that the impedance value Z.sub.i is encoded as shown in FIG. 8. Here, the variable impedance 810 has N states. If there are any errors in the encoding or imperfections in encoding of Z.sub.i, these may be corrected within the reader 120. This is possible if for some time the signal s(t) is known by the reader 120. The reader 120 than may add distortion to the incoming signal to correct for these imperfections.

[0038] Referring again to FIGS. 7A and 7B, the digital wave form information 830 (e.g., N-bit information) is applied to an element such as a decoder 820 that converts a scatter value or reflection coefficient Γ for the information 830 into an impedance value Z.sub.i. This impedance value Z.sub.i (e.g., N-bits 821) is then applied to a delta-sigma (ΔΣ) modulator 840 which controls the switches S.sub.1, S.sub.2 in the variable impedance 810 to switch between respective impedances Z.sub.1, Z.sub.2 based on the output of the delta-sigma (ΔΣ) modulator 840. For example, if the output of the delta-sigma (ΔΣ) modulator 840 is “1”, the impedance value Z.sub.i is set to the value of the second impedance Z.sub.2. If the output of the delta-sigma (ΔΣ) modulator 840 is “0”, the impedance value Z.sub.i is set to the value of the first impedance Z.sub.1. For example, impedance values of 116 ohms for the second impedance Z.sub.2 and 21 ohms for the first impedance Z.sub.1 may correspond to reflection coefficients Γ of 0.4 and −0.4, respectively. If the desired reflection coefficient Γ is zero, the decoder 820 may determine an impedance value Z.sub.i of 50 ohms (as per the graph shown in FIG. 8). The delta-sigma (ΔΣ) modulator 840 now generates an output that produces an average impedance value for the variable impedance 810 of 50 ohms by switching between the 21 and 116 ohms impedances Z.sub.2, Z.sub.1.

[0039] As shown in FIG. 7B, the variable impedance 810 circuit may be made up of an array of impedances Z.sub.1, Z.sub.2 that are switched in and out by respective switches S.sub.1, S.sub.2 depending on the digital decoder 820 and delta-sigma (ΔΣ) modulator 840. Also, the variable impedance 810 may be controlled via an analog signal, that is, after the Gamma to Z.sub.i decoder 820 and delta-sigma (ΔΣ) modulator 840, a digital to analog converter (“DAC”) (not shown) may be added to drive the variable impedance 810.

[0040] The delta-sigma (ΔΣ) modulator 840 may be of variable design. For example, according to one embodiment, the delta-sigma (ΔΣ) modulator 840 may include or be a low-pass delta-sigma (ΔΣ) modulator. According to another embodiment, the delta-sigma (ΔΣ) modulator 840 may include or be a band-pass delta-sigma (ΔΣ) modulator. According to one embodiment, the the delta-sigma (ΔΣ) modulator 840 may be a single bit delta-sigma (ΔΣ) modulator.

[0041] The delta-sigma (ΔΣ) modulator 840 generates an output bit stream that represents the input data 821 from a DC level to some predetermined design bandwidth. Beyond the predetermined design bandwidth, quantized noise of the delta-sigma (ΔΣ) modulator 840 may increase until, at some design cutoff point, the signal may be deemed to have too much quantization noise. According to one embodiment, one or more filters may be included in the variable impedance 810 circuit to filter out-of-band noise output from the delta-sigma (ΔΣ) modulator 840. The variable impedance 810 circuit has an output electrically connected to the antenna 133. The delta-sigma (ΔΣ) modulator 840 is coupled to an input to the variable impedance 810 circuit to digitally control the output of the variable impedance 810 circuit such that the reflection coefficient Γ of the antenna 133 may be adjusted by changing the impedance value Z.sub.i of the variable impedance 810 circuit. According to one embodiment, the output of the delta-sigma (ΔΣ) modulator 840 switches the impedance value Z.sub.i of the variable impedance 810 between at least two states or impedance values Z.sub.i.

[0042] According to one embodiment, the delta-sigma (ΔΣ) modulator 840 may be of any order based on the bandwidth of the signals being applied to it. In addition, the clock applied to the delta-sigma (ΔΣ) modulator 840 may set the over-sampling rate.

[0043] FIG. 9 is a block diagram illustrating a transmission apparatus 1000 with an adder 1050 for a wireless device 130 for backscattering arbitrary modulated signals to a reader 120 based on I and Q data input 1030 in accordance with an embodiment of the invention. According to one embodiment, the digital waveform 830 may be in-phase (“I”) and quadrature (“Q”) data 1030 as shown in FIG. 9. In FIG. 9, a digital signal generator (“DSS”) 1040 may optionally up-convert (or offset) the I and Q data 1030. For example, the DSS 1040 may provide sine (or cosine) and cosine (or sine) signals 1070 that are applied to I and Q data by respective mixers 1071. Alternatively, the DSS 1040 may generate a constant value that is multiplied onto the I and Q data (i.e., the mixers 1071 act as gain elements). The Gamma to Z.sub.i decoder 1020 receives the up-converted (or offset) I and Q data and applies it to the variable impedance 1010 via the delta-sigma (ΔΣ) modulator 1080. The variable impedance 1010 may be made up of an array of impedances that are switched in or out (e.g., a parallel array of impedances with respective switches).

[0044] Summarizing the above, and referring again to FIGS. 7A and 7B, according to one embodiment an antenna 133 is used to backscatter an incoming radio frequency signal coming from a reader 120. The antenna 133 is electrically coupled to an array of impedance devices Z.sub.1, Z.sub.2 connected to switches S.sub.1, S.sub.2. The array of impedance devices (e.g., variable impedance 810) may be digitally controlled by a digital block (e.g., decoder 820) and a delta-sigma (ΔΣ) modulator 840 that are driven by an arbitrary N-bit digital waveform (e.g., 830). The digital block 820 presents an output to the array of impedances 810 via the delta-sigma (ΔΣ) modulator 840 that is related to the N-bit digital waveform 830. A change in the impedance value of the array of impedances 810 backscatters the incoming radio frequency signal thus producing a direct up-converted version of the output of the digital waveform 830 with respect to the incoming radio frequency. The output of the digital block 820 and delta-sigma (ΔΣ) modulator 840 switches the array of impedances 810 between various states, which changes the characteristics of the reflection coefficient Γ. The signal 830 applied to the digital block 820 may take the form of any complex modulation signal, for example, GMSK, nPSK, 8 PSK, nQAM, OFDM, etc., and such signals may be offset from the incoming radio frequency signal by a frequency+/−ω.

[0045] Referring again to FIG. 9, the input 1030 to the digital block 1020 may alternate between in-phase (i.e., I) and quadrature (i.e., Q) signals via a control signal, for example. Also, the array of impedances 1010 may switch between backscattering coefficients that are 90 degrees offset from each other depending on whether the data is I or Q data. For example, if the I signals would produce backscattering coefficients at theta degrees then the Q signals would produce backscattering coefficients that are theta+90 degrees. The control signal may be a clock signal. The signals 1070 applied to the I and Q signals 1030 by the DSS 1040 may take the form of a direct current (“DC”) signal (i.e., no frequency offset) or of sine and cosine waves at a selected frequency (i.e., to give a frequency offset of ω). The I and Q signals applied to the digital block 1020 may be adjusted to compensate for any errors in the impedance array 1010, the delta-sigma (ΔΣ) modulator 1080, or the digital block 1020. The array of impedances 1010 may include some filtering characteristics to filter off some of the digital block's 1020 or delta-sigma (ΔΣ) modulator's 1080 out-of-band noise. And, the reader 120 used to detect the backscattered signal from the wireless device 130 may compensate for any errors generated within the impedance array 1010, the digital block 1020, or the delta-sigma (ΔΣ) modulator 1080.

[0046] FIG. 10A is a block diagram illustrating inductive coupling between a reader 120 and a wireless device 130 in a RFID system 1300 in accordance with an embodiment of the invention. FIG. 10B is a block diagram illustrating an equivalent circuit 1310 for the RFID system 1300 of FIG. 10A in accordance with an embodiment of the invention. And, FIG. 11 is a block diagram illustrating a transmission apparatus 1400 using inductive coupling for a wireless device 130 for transmitting signals to a reader 120 based on a digital waveform input 1430 in accordance with an embodiment of the invention.

[0047] According to one embodiment, communication between the reader 120 and the wireless device 130 may occur by sensing inductive loading changes in the reader 120. Here, the reader 120 communicates with the wireless device 120 via magnetic or inductive coupling. This is shown in FIGS. 10A and 10B. FIGS. 10A and 10B show the basic principle of an inductive coupled RFID system 1300. For inductive coupled systems 1300, the underlying coils are defined by their size. It is known that a coupling system of two coils 1320, 1330 may be represented by an equivalent transformer. The connection between these two coils 1320, 1330 is given by the magnetic field (B) and the underlying value to describe this connection is the mutual inductance (M) and/or the coupling factor (k).

[0048] The law of Biot and Savart is given by:

[00005]$\stackrel{.fwdarw.}{B}=\frac{{\mu }_o.Math.i_1}{4.Math.\pi }.Math.\underset{s}{\int }.Math.\frac{\stackrel{.fwdarw.}{\mathrm{ds}}\times \stackrel{.fwdarw.}{x}}{|\stackrel{.fwdarw.}{x}.Math.{|}^3}$

[0049] This allows the calculation of the magnetic field at every point as a function of the current, i.sub.1, as well as the geometry. Here, μ.sub.o is the permeability, x is the distance, and S describes the integration-path along the coil. Furthermore, the mutual inductance and the coupling factor are given by:

[00006]$M=\underset{A_2}{\int }.Math.\frac{B\left(i_1\right)}{i_1}.Math.{\mathrm{dA}}_2$$k=\frac{M}{\sqrt{L_1.Math.L_2}}$

[0050] In these equations, A.sub.2 describes the area of the second coil and L.sub.1 and L.sub.2 are the inductances of the two coils 1320, 1330. The distance x between the reader-coil 1320 and transponder-coil 1330 also determines the coupling factor. The equivalent model for this coupling is shown in FIG. 10B. The impedance value Z.sub.i as seen by the reader 120 is directly related to the admittances Y1 and Y2. The admittances Y1 and Y2 are either modulated via amplitude (e.g., ASK) or in phase (e.g., PSK). The admittances Y1 and Y2 may also be modulated using multi-phase PSK and multi-amplitude ASK.

[0051] General speaking, the signal received back by the reader 120 is a function of the impedance value changing in the wireless device 130. Once this impedance value changes, the signal seen by the reader 120 is modified and the reader 120 can detect this.

[0052] As in the case of backscattering, as shown in FIG. 11, a variable impedance 1410 may be modified by a decoder 1420 via a delta-sigma (ΔΣ) modulator 1440. Here, L 1405 is the inductance on the wireless device side. As in the case of backscattering, the same methods described above may be used, for example, for: (i) generating I and Q signals; (ii) general mapping from decoding to what the reader sees; and, (iii) if a signal is known by the reader, pre-distorting the signal to produce a corrected signal.

[0053] Summarizing the above, and referring again to FIG. 11, according to one embodiment there is provided a transmission apparatus 1400 for modifying an incoming radio frequency (RF) signal comprising: an inductive element 1405; an array of impedances 1410 controlled by switches and circuits having an output electrically coupled to the inductive element 1405; and, at least one digital block 1420 coupled to the array of impedances 1410 via a delta-sigma (ΔΣ) modulator 1440 for digitally controlling the impedance value Z.sub.i of the array of impedances 1410; wherein the incoming RF signal is modified as the coupled array of impedances 1410 of the inductive element 1405 is adjusted.

[0054] The output of the decoder 1420 and delta-sigma (ΔΣ) modulator 1440 may switch the array of impedances 1410 between various states which modifies the incoming RF signal. The signal 1430 applied to the digital block 1420 may take the form of any complex modulation signal, for example, GMSK, nPSK, 8 PSK, nQAM, OFDM, etc., and such signals may be offset from the incoming radio frequency signal by a frequency+/−ω.

[0055] The input 1430 to the digital block 1420 may alternate between the in-phase (i.e., I) and quadrature (i.e., Q) signals via a control signal, for example. Also, the array of impedances 1410 may modify the incoming RF signal from 0 to 90 degrees offset depending on whether the data is I or Q data. For example, if the I signal would produce an impedance value at theta degrees then the Q signal would produce an impedance value that is theta+90 degrees. The control signal may be a clock signal. The signals (e.g., 1070) applied to the I and Q signals may take the form of a DC signal or of sine and cosine waves at a selected frequency. The I and Q signals applied to the digital block 1420 may be adjusted to compensate for any errors in the impedance array 1410 due to variations in the impedance value in the array. The array of impedances 1410 may have some filtering characteristics to filter off some of the DAC quantized out-of-band noise. And, the reader 120 used to detect the modulated signal may compensate for any errors generated within the impedance array 1410, the digital block 1420, or the delta-sigma (ΔΣ) modulator 1440.

[0056] Thus, according to one embodiment, there is provided a transmission apparatus 800 for a wireless device 130, comprising: an antenna 133 for receiving an original signal and for backscattering a modulated signal containing information 830 from the wireless device 120; a variable impedance 810 coupled to the antenna 133, the variable impedance 810 having an impedance value Z.sub.i; a delta-sigma (ΔΣ) modulator 840 coupled to the variable impedance 810 for modulating the impedance value Z.sub.i, and thereby a backscattering coefficient Γ for the antenna 133, in accordance with the information 830 to generate the modulated signal (e.g., an arbitrary modulated signal); and, a decoder 820 coupled to the delta-sigma modulator 840 for generating the impedance value Z.sub.i from the information 830.

[0057] In the above transmission apparatus 800, the variable impedance 810 may be coupled in series with the antenna 133. The wireless device 130 may be powered by energy 140 from the original signal. The variable impedance 810 may include an array of impedances and respective switches. The decoder 820 may include a backscattering coefficient Γ to impedance value Z.sub.i decoder. The information 830 may be an N-bit digital waveform 830. The N-bit digital waveform 830 may be applied to the decoder 820 and then to a delta-sigma (ΔΣ) modulator 840 to produce a control signal 821 for the variable impedance 810 that is related to the N-bit digital waveform 830. A change in the impedance value Z.sub.i may backscatter the original signal to produce the modulated signal, the modulated signal being a frequency offset (e.g., up-converted) form of the N-bit digital waveform 830. The control signal 821 for the variable impedance 810 may switch an array of impedances within the variable impedance 810 which may change characteristics of the backscattering coefficient Γ of the antenna 133. The information 830 may be a complex modulation signal 1030. The complex modulation signal 1030 may be offset in frequency from the original signal. The complex modulation signal 1030 may be one of a GMSK signal, a nPSK signal, a 8 PSK signal, a nQAM signal, and an OFDM signal. The complex modulation signal 1030 may be represented by I+jQ, where I is an inphase component, Q is a quadrature component, and j is a square root of −1. The complex modulation signal 1030 may alternate between an in-phase signal (I) and a quadrature signal (Q) via a control signal. The variable impedance 810, 1010 may switch between backscattering coefficients that are 90 degrees offset from each other depending on whether the complex modulation signal 1030 is the in-phase signal (I) or the quadrature signal (Q). The control signal may be a clock signal. The transmission apparatus 800, 1000 may further include a digital signal generator 1040. The digital signal generator 1040 may apply a constant value signal to the in-phase signal (I) and the quadrature signal (Q). The digital signal generator 1040 may apply sine and cosine wave signals 1070 to the in-phase signal (I) and the quadrature signal (Q), respectively. The complex modulation signal 1030 may be a sum of an in-phase signal (I) and a quadrature signal (Q). The transmission apparatus 800, 1000 may further include a digital signal generator 1040. The digital signal generator 1040 may apply a constant value signal to the in-phase signal (I) and the quadrature signal (Q). The digital signal generator 1040 may apply sine and cosine wave signals 1070 to the in-phase signal (I) and the quadrature signal (Q), respectively. The N-bit digital waveform 830 may be adjusted to compensate for errors in at least one of the decoder 820, the delta-sigma (ΔΣ) modulator 840, and the variable impedance 810. The variable impedance 810 may include a filter for filtering noise generated by at least one of the decoder 820 and the delta-sigma (ΔΣ) modulator 840. The modulated signal may be an arbitrary signal. The wireless device 120 may be a RFID tag. The original signal may be received from a RFID reader 120. The RFID reader 120 may be configured to correct for errors in at least one of the decoder 820, the delta-sigma (ΔΣ) modulator 840, and the variable impedance 810. The transmission apparatus 800 may further include a processor for controlling the transmission apparatus 800 and memory for storing the information 830. The delta-sigma (ΔΣ) modulator 840 may be one of a low-pass delta-sigma modulator and a band-pass delta-sigma modulator. The delta-sigma (ΔΣ) modulator 840 may be a single bit delta-sigma modulator. And, the delta-sigma (ΔΣ) modulator 840 may switch (S.sub.1, S.sub.2) the impedance value Z.sub.i between at least two states (Z.sub.1, Z.sub.2).

[0058] The above embodiments may contribute to an improved method and apparatus for communications between wireless device 130 and reader 120 in backscattered and inductively coupled radio frequency identification systems and may provide one or more advantages. For example, the wireless devices 130 of the present invention are not limited in the nature of signals that they may backscatter or inductively couple to the reader 120. In addition, the wireless devices 130 of the present invention allow for filtering of these signals. In addition, the delta-sigma (ΔΣ) modulator 840 reduces the number of impedances that need to switch states in order to produce a signal. Furthermore, the delta-sigma (ΔΣ) modulator 840 enables high levels of modulation with as few as only one impedance.

[0059] The embodiments of the invention described above are intended to be exemplary only. Those skilled in this art will understand that various modifications of detail may be made to these embodiments, all of which come within the scope of the invention.