Method and apparatus for signal processing in RFID receivers

• US 20070177694A1
• Filed: 01/17/2006
• Published: 08/02/2007
• Est. Priority Date: 01/17/2006
• Status: Abandoned Application

• Current Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Original Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Inventors: Okunev, Yuri, Powell, Kevin
• Application Number:
  US11/332,220
• Publication Number:
  US 20070177694A1
• Time in Patent Office: Days
• US Class Current: 375/333
• CPC Class Codes: H04L 27/14 Demodulator circuits; Receiver circuits

Abstract

The present invention provides methods and apparatuses for demodulation and decoding of backscattered RFID tag signals, represented by their in-phase and quadrature components at the output of the demodulator in the receiver portion of a reader interrogator. Correlation coefficients for the in-phase and quadrature components of the received signal are calculated over a shifted bit interval. Performing a correlation over a shifted bit interval relative to the real bit interval allows the base-band receiver to involve a two-bit interval in making a decision about each transmitted bit. In contrast, in a conventional decoding algorithm, a single bit interval is involved in the decision-making process. Thus, the current method provides a 3 dB energy gain compared to the conventional method. A single zero-mean reference signal is used to compute correlation coefficients, eliminating constant components of the received signal, and simplifying digital implementation of the base-band receiver. A value of the output data is determined based on the combined correlation coefficients.

Claims

1. A method for decoding an encoded data signal, comprising:

(a) receiving the encoded data signal including a plurality of data symbols, wherein a time period T is a length of a data symbol of the encoded data signal, wherein a current data symbol begins at time t and ends at time (t+T);

(b) correlating an in-phase component of the received signal with a reference signal over a period t−

T/2 to t+T/2 to generate a first in-phase correlation coefficient for the current data symbol;

(c) correlating an in-phase component of the received signal with a reference signal over a period t+T/2 to t+3T/2 to generate a second in-phase correlation coefficient for the current data symbol;

(d) correlating a quadrature component of the received signal with the reference signal over a period t−

T/2 to t+T/2 to generate a first quadrature correlation coefficient for the current data symbol;

(e) correlating a quadrature component of the received signal with the reference signal over a period t+T/2 to t+3T/2 to generate a second quadrature correlation coefficient for the current data symbol;

(f) multiplying the first in-phase correlation coefficient with the second in-phase correlation coefficient to produce an in-phase cross-correlation value;

(g) multiplying the first quadrature correlation coefficient with the second quadrature correlation coefficient to produce a quadrature cross-correlation value;

(h) adding the in-phase cross-correlation value and the quadrature cross-correlation value to produce an integral cross correlation value; and

(i) determining a decoded value for the current data symbol based on the integral cross-correlation value.

2. The method of claim 1, wherein the plurality of data symbols are encoded in the encoded data signal according to FM0 encoding.

3. The method of claim 2, wherein the encoded data signal comprises data from a backscattered signal received from a radio frequency identification (RFID) tag.

4. The method of claim 1, wherein step (b) comprises:

computing the first in-phase correlation coefficient C_I0(n−

1) according to

$C_{I<br><br>\text{\hspace{1em}<br><br>}<br><br>0}<br><br>\left(n-1\right)=\underset{k=\frac{K_{n-1}}{2}+1}{\overset{\frac{K_n}{2}}{\sum <br><br>}}<br><br>{I<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right)}^{*}<br><br>R<br><br>\text{\hspace{1em}<br><br>}<br><br>0<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right),$

where

I(kΔ

t)=k-th sample of the in-phase component of the encoded data signal;

R0(kΔ

t)=k-th sample of a reference signal;

K_j=K=number of samples within a symbol interval, where index j indicates the time interval in which the summation is performed for decoding the n-th symbol; and

Δ

t=T/K.

5. The method of claim 4, wherein step (c) comprises:

computing the second in-phase correlation coefficient C_I0(n) according to

$C_{I<br><br>\text{\hspace{1em}<br><br>}<br><br>0}<br><br>\left(n\right)=\underset{k=\frac{K_n}{2}+1}{\overset{\frac{K_{n+1}}{2}}{\sum <br><br>}}<br><br>{I<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right)}^{*}<br><br>R<br><br>\text{\hspace{1em}<br><br>}<br><br>0<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right).$

6. The method of claim 4, wherein step (f) comprises:

calculating the in-phase cross correlation MI(n) according to

MI(n)=C_I0(n)*C_I0(n−

1).

7. The method of claim 1, wherein step (d) comprises:

computing the first quadrature correlation coefficient C_Q0(n−

1) according to

$C_{Q<br><br>\text{\hspace{1em}<br><br>}<br><br>0}<br><br>\left(n-1\right)=\underset{k=\frac{K_{n-1}}{2}+1}{\overset{\frac{K_n}{2}}{\sum <br><br>}}<br><br>{Q<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right)}^{*}<br><br>R<br><br>\text{\hspace{1em}<br><br>}<br><br>0<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right)$

where

Q(kΔ

t)=k-th sample of the in-phase component of the encoded data signal;

R0(kΔ

t)=k-th sample of a reference signal;

K_j=K=number of samples within a symbol interval, where index j indicates the time interval in which the summation is performed for decoding the n-th symbol; and

Δ

t=T/K.

8. The method of claim 7, wherein step (e) comprises:

computing the second quadrature correlation coefficient C_Q0(n) according to

$C_{Q<br><br>\text{\hspace{1em}<br><br>}<br><br>0}<br><br>\left(n\right)=\underset{k=\frac{K_n}{2}+1}{\overset{\frac{K_{n+1}}{2}}{\sum <br><br>}}<br><br>{Q<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right)}^{*}<br><br>R<br><br>\text{\hspace{1em}<br><br>}<br><br>0<br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right).$

9. The method of claim 7, wherein step (g) comprises:

calculating the quadrature cross correlation MQ(n) according to

MQ(n)=C_Q0(n)*C_Q0(n−

1).

10. The method of claim 1, wherein step (i) comprises:

inverting a sign of the integral cross correlation to produce a determined signed value;

determining the decoded value to be equal to 0 if the determined signed value is negative; and

determining the decoded value to be equal to 1 if the determined signed value is positive.

11. The method of claim 4, wherein the reference signal R0(kΔ

t) is a function equal to the sign of the signal sin(kΔ

tΩ

), where Ω

=2π

/T is the subcarrier frequency.

12. The method of claim 7, wherein the reference signal R0(kΔ

t) is a function equal to the sign of the signal sin(kΔ

tΩ

), where Ω

=2π

/T is the subcarrier frequency.

13. A base-band digital receiver that decodes an encoded data signal, the encoded data signal including a plurality of data symbols, wherein a time period T is a length of a data symbol of the encoded data signal, wherein a current data symbol begins at time t, the receiver comprising:

an in-phase correlator that correlates an in-phase component of the encoded data signal with a reference signal over a period t+T/2 to t+3T/2 to generate an in-phase correlation coefficient for the current data symbol;

a quadrature correlator that correlates a quadrature component of the received signal with the reference signal over the period t+T/2 to t+3T/2 to generate a quadrature correlation coefficient for the current data symbol;

a first delay module that receives the in-phase correlation coefficient associated with the time period t+T/2 to t+3T/2 while outputting a delayed in-phase correlation coefficient associated with a prior time period t−

T/2 to t+T/2;

a second delay module that receives the quadrature correlation coefficient associated with the time period t+T/2 to t+3T/2 while outputting a delayed quadrature correlation coefficient associated with the prior time period t−

T/2 to t+T/2;

a first multiplier that multiplies the in-phase correlation coefficient with the delayed in-phase correlation coefficient to produce an in-phase cross-correlation value;

a second multiplier that multiplies the quadrature correlation coefficient with the delayed quadrature correlation coefficient to produce a quadrature cross-correlation value; and

a decision module that adds the in-phase cross-correlation value and the quadrature cross-correlation value to produce an integral cross-correlation value, and determines a decoded value for the current data symbol based on the integral cross-correlation value.

14. The receiver of claim 13, wherein the plurality of data symbols are encoded in the encoded data signal according to FM0 encoding.

15. The receiver of claim 13, wherein the encoded data signal comprises data from a backscattered signal received from a radio frequency identification (RFID) tag.

16. The receiver of claim 13, wherein the in-phase correlator comprises:

a digital multiplier that receives the in-phase signal component of the encoded data signal, and multiplies the in-phase signal component by the reference signal; and

an in-phase adder-accumulator that receives and accumulates an output of the digital multiplier over all samples within a shifted bit interval spanning from t+T/2 to t+3T/2, to generate the in-phase correlation coefficient.

17. The receiver of claim 13, wherein the quadrature correlator comprises,

a digital multiplier that receives the quadrature signal component of the encoded data signal, and multiplies the quadrature component by the reference signal;

and

a quadrature adder-accumulator that receives and accumulates an output of the digital multiplier over all samples within the shifted bit interval spanning from t+T/2 to t+3T/2, to generate the quadrature correlation coefficient.

18. The receiver of claim 13, wherein the decision module comprises:

an adder that adds the in-phase cross-correlation value and the quadrature cross-correlation value to produce an integral cross-correlation value;

a first logic module that inverts a sign of the integral cross-correlation value to produce a determined signed value; and

a second logic module that determines the decoded value to be equal to 0 if the determined signed value is negative, or to be equal to 1 if the determined signed value is positive.

19. The receiver of claim 13, further comprising:

a demodulator that demodulates the encoded data signal into the in-phase component and the quadrature component.

20. The receiver of claim 13, further comprising:

a template generator module that generates the reference signal, wherein the in-phase correlator and the quadrature correlator each receives the generated reference signal.