Transient analysis system for characterizing RF transmitters by analyzing transmitted RF signals

• US 5,758,277 A
• Filed: 09/19/1996
• Issued: 05/26/1998
• Est. Priority Date: 09/19/1996
• Status: Expired due to Term

• Current Assignee: Authorize Net LLC (Visa, Inc.)
• Original Assignee: Corsair Communications, Inc. (Visa, Inc.)
• Inventors: Hawkes, Kelly Davidson
• Primary Examiner(s): Chin, Tommy P.
• Assistant Examiner(s): VINCENT, DAVID ROBERT
• Application Number:
  US08/716,840
• Time in Patent Office: 614 Days
• Field of Search: 380/23, 380/25, 455/410, 455/425, 455/67.1, 455/411, 324/76.12, 364/728.07, 364/728.03, 379/145
• US Class Current: 455/410
• CPC Class Codes: H04B 17/391 Modelling the propagation channel
• CPC Class Codes: H04W 12/06 Authentication
• CPC Class Codes: H04W 12/126 Anti-theft arrangements, e.g. protection against subscriber identity module [SIM] cloning
• CPC Class Codes: H04W 12/79 Radio fingerprint

Abstract

A transient analysis system that characterizes the transients in RF signals from radio transmitters. Transient characterization permits identification of the transmitter and is useful in cellular telephone fraud prevention systems. The characterization is based on the analysis of the chirp waveform or the complex auto-correlation sequence.

Claims

1. An apparatus for determining the transient characteristics of a radio transmitter that transmits a signal comprising:

receiving means for receiving the transmitted signal and producing therefrom a down-converted signal;

converting means for digitizing the down-converted signal to generate a pre-detection intermediate frequency sequence;

processing means for processing the pre-detection intermediate frequency sequence to extract a chirp waveform;

said processing means including,

transforming means for converting the pre-detection intermediate frequency sequence into a complex pre-detection intermediate frequency sequence;

primary demodulation means for multiplying the complex pre-detection intermediate frequency sequence by a delayed conjugated version of the complex pre-detection intermediate frequency sequence;

filtering means for operating on the output of the primary demodulation means to produce a complex auto-correlation sequence;

secondary demodulation means for multiplying the complex auto-correlation sequence by a delayed conjugated version of the complex auto-correlation sequence; and

arc-tangent means for finding an argument of the output from the secondary demodulation means to form a chirp waveform, and

analysis means for analyzing the chirp waveform to derive one or more parameters that characterize said transmitter.

2. The apparatus of claim 1 that further includes:

determining means for determining a portion of the chirp waveform to analyze.

3. The apparatus of claim 1 wherein the radio transmitter is a cellular telephone.

4. The apparatus of claim 1 wherein the analysis means further includes:

calculation means for calculating a dot product between the chirp waveform and a reference vector to form said one or more parameters.

5. The apparatus of claim 1 that further includes:

comparison means for determining the likelihood that said parameters are typical for said transmitter.

6. An apparatus for determining the transient characteristics of a radio transmitter that transmits an RF signal comprising:

receiving means for receiving the RF signal and producing therefrom a down-converted signal;

converting means for digitizing the down-converted signal to generate a pre-detection intermediate frequency sequence;

processing means for processing the pre-detection intermediate frequency sequence to extract a complex auto-correlation sequence;

said processing means including,

transforming means for converting the pre-detection intermediate frequency sequence into a complex pre-detection intermediate frequency sequence;

demodulation means for multiplying the complex pre-detection intermediate frequency sequence by a delayed conjugated version of the complex pre-detection intermediate frequency sequence; and

filtering means for operating on the output of the demodulation means to form said complex auto-correlation sequence; and

analysis means for analyzing the complex auto-correlation sequence to derive one or more parameters that characterize said transmitter.

7. The apparatus of claim 6 that further includes:

determining means for determining a portion of the complex auto-correlation sequence to analyze.

8. A wireless subscriber authorization system including,

a plurality of wireless subscriber units where each wireless subscriber unit includes an RF transmitter for transmitting a transmitted signal,

a receiver for receiving each transmitted signal and producing therefrom, for each transmitted signal, a down-converted signal,

apparatus for determining the transient characteristics of each RF transmitter comprising:

converting means for digitizing the down-converted signal to generate a pre-detection intermediate frequency sequence;

processing means for processing the pre-detection intermediate frequency sequence to extract a chirp waveform;

said processing means including,

mixing means for down-converting the pre-detection intermediate frequency sequence, z(k), to form a frequency shifted sequence, w(k), as follows;

##EQU16## filtering means for removing negative frequency components of z(k) to form a complex pre-detection intermediate frequency sequence, x(n), as follows;

##EQU17## where D'"'"' is a decimation index and h_i, represents filter coefficients,

primary demodulation means for multiplying the complex pre-detection intermediate frequency sequence, x(n), by a delayed conjugated version, x*(n-1), of the complex pre-detection intermediate frequency sequence to form a primary demodulation output, b(n), as follows;

space="preserve" listing-type="equation">b(n)=x*(n-1)x(n)

decimating means for decimating and filtering said primary demodulation output, b(n), to form a decimated waveform, e(m), as follows;

##EQU18## where K is the filtering length and D is the decimation index, lowpass filter means for lowpass filtering the decimated waveform, e(m), to form the complex auto-correlation sequence, g(m), as follows;

##EQU19## where l_i represent lowpass filter coefficients and L is the filter length,

amplifier means for adjusting the amplitude of the complex auto-correlation sequence, g(m), by a factor of 1/a² to form the adjusted complex auto-correlation sequence, s(m), as follows;

##EQU20## secondary demodulation means for multiplying the adjusted complex auto-correlation sequence, s(m), by a delayed conjugated version, s*(m-1), of the complex auto-correlation sequence to form a secondary demodulation sequence, q(m) as follows;

space="preserve" listing-type="equation">q(m)=s(m)s*(m-1)

arc-tangent means for finding an argument of said secondary demodulation sequence, q(m), to form said processing waveform as a chirp waveform, d(m), as follows;

##EQU21## analysis means for analyzing the chirp waveform to derive one or more parameters that characterize said transmitter;

subscriber authorization means for processing said parameters to determine if each of said wireless subscriber units is authorized.

9. A wireless subscriber authorization system including,

a plurality of wireless subscriber units where each wireless subscriber unit includes an RF transmitter for transmitting a transmitted signal,

a receiver for receiving each transmitted signal and producing therefrom, for each transmitted signal, a down-converted signal,

apparatus for determining the transient characteristics of each RF transmitter comprising:

converting means for digitizing the down-converted signal to generate a pre-detection intermediate frequency sequence;

processing means for processing the pre-detection intermediate frequency sequence to extract a chirp waveform;

analysis means for analyzing the chirp waveform to derive one or more parameters that characterize said transmitter;

said analysis means including,

transient vector means for processing said chirp waveform, d(m), to form transient vectors, D_i, including D₀, D₁, and D₂, alignment vector storage means for storing alignment vectors T_k,

first computational means for computing the dot product, y_ki, of the alignment vectors, T_k, and the transient vectors, D_i,

interpolation means using the dot product, y_ki, to form the filter coefficients h₀, h₁, and h₂,

filter means using the filter coefficients h₀, h₁, and h₂, to filter the transient vectors, D_i, to form a chirp vector, C, as follows;

space="preserve" listing-type="equation">C=D.sub.0 h.sub.0 +D.sub.1 h.sub.1 +D.sub.2 h.sub.2

reference vector storage means for storing reference vectors R_k,

second computational means for computing the dot product of the reference vectors, R_k, and the transient vectors, D_i, to form said parameters as p_i ; and

subscriber authorization means for processing said parameters to determine if each of said wireless subscriber units is authorized.

10. The apparatus of claim 9 wherein said transient vectors, D_i, including D₀, D₁, and D₂, where M is a sample number for the start of the transient, are given as follows:

##EQU22##

11. A wireless subscriber authorization system including,

a plurality of wireless subscriber units where each wireless subscriber unit includes an RF transmitter for transmitting a transmitted signal,

a receiver for receiving each transmitted signal and producing therefrom, for each transmitted signal, a down-converted signal,

apparatus for determining the transient characteristics of each RF transmitter comprising:

converting means for digitizing the down-converted signal to generate a pre-detection intermediate frequency sequence;

processing means for processing the pre-detection intermediate frequency sequence to extract a complex auto-correlation sequence;

said processing means including,

transfomring means for converting the pre-detection intermediate frequency sequence into a complex pre-detection intermediate frequency sequence;

demodulation means for multiplying the complex pre-detection intermediate frequency sequence by a delayed conjugated version of the complex pre-detection intermediate frequency sequence to form a demodulation output;

filtering means for filtering said demodulation output to produce said processing waveform as a complex auto-correlation sequence, and

analysis means for analyzing the complex auto-correlation sequence to derive one or more parameters that characterize said transmitter;

subscriber authorization means for receiving said parameters and for processing said parameters to determine if each of said wireless subscriber units is authorized.

12. In a wireless subscriber authorization system including a plurality of wireless subscriber units where each wireless subscriber unit includes an RF transmitter for transmitting a transmitted signal and including a receiver for receiving each transmitted signal and a processor for producing therefrom, for each transmitted signal, a chirp waveform, an authorization method comprising:

determining the transient characteristics of each RF transmitter by analyzing the chirp waveform to derive one or more parameters that characterize said transmitter;

processing said parameters to determine if each of said wireless subscriber units is authorized, said processing step including,

a mixing step for down-converting the pre-detection intermediate frequency sequence, z(k), to form a frequency shifted sequence, w(k), as follows;

##EQU23## a filtering step for removing negative frequency components of z(k) to form a complex pre-detection intermediate frequency sequence, x(n), as follows;

##EQU24## where D'"'"' is a decimation index and h_i represents filter coefficients, a primary demodulation step for multiplying the complex pre-detection intermediate frequency sequence, x(n), by a delayed conjugated version, x*(n-1l), of the complex pre-detection intermediate frequency sequence to form a primary demodulation output, b(n), as follows;

space="preserve" listing-type="equation">b(n)=x*(n-1)x(n)

a decimating step for decimating and filtering said primary demodulation output, b(n), to form a decimated waveform, e(m), as follows;

##EQU25## where K is the filtering length and D is the decimation index, a lowpass filter step for lowpass filtering the decimated waveform, e(m), to form the complex auto-correlation sequence, g(m), as follows;

##EQU26## where l_i represent lowpass filter coefficients and L is the filter length,

an amplifier step for adjusting the amplitude of the complex auto-correlation sequence, g(m), by a factor of l/a² to form the adjusted complex auto-correlation sequence, s(m), as follows;

##EQU27## a secondary demodulation step for multiplying the adjusted complex auto-correlation sequence, s(m), by a delayed conjugated version, s*(m-1), of the complex auto-correlation sequence to form a secondary demodulation sequence, q(m) as follows;

space="preserve" listing-type="equation">q(m)=s(m)s*(m-1)

an arc-tangent step for finding an argument of said secondary demodulation sequence, q(m), to form said processing waveform as a chirp waveform, d(m), as follows;

##EQU28##