Simultaneous call transmission detection

Abstract

A method of determining the presence of a secondary carrier signal in a time-domain sum-signal including a primary carrier signal is disclosed, the method comprising: transforming the sum-signal into the frequency domain; extracting at least one peak corresponding to a heterodyne tone from the transformed sum-signal; determining the presence of a secondary carrier signal in the sum-signal based on said at least one peak. A corresponding apparatus is also disclosed.

What is claimed is: 
     
         1 . A method of determining the presence of a secondary carrier signal in a time-domain sum-signal including a primary carrier signal, the method comprising:
 transforming the sum-signal into the frequency domain; extracting at least one peak corresponding to a heterodyne tone from the transformed sum-signal;   determining the presence of a secondary carrier signal in the sum-signal based on said at least one peak.   
     
     
         2 . A method according to  claim 1  further comprising identifying a primary carrier signal within the sum-signal. 
     
     
         3 . A method according to  claim 1  or  2  further comprising determining a conjugate of a sideband of the frequency-domain primary carrier signal; and attenuating the primary carrier signal by using said conjugate of the sideband of the primary signal. 
     
     
         4 . A method according to  claim 3  wherein the conjugate of the sideband of the primary signal is subtracted from the opposing frequency sideband of the sum-signal. 
     
     
         5 . A method according to any preceding claim wherein the step of transformation into the frequency domain is performed using a Fourier Transform (FT). 
     
     
         6 . A method according to any preceding claim wherein the step of transformation into the frequency domain is performed using a discrete transform comprising input and output bins. 
     
     
         7 . A method according to  claim 6  wherein the signal is split and mapped onto the input of said transform with a larger size than the signal, wherein in a central part of said transform comprises zero-valued input bins. 
     
     
         8 . A method according to  claim 7  wherein a second half of said signal is mapped onto a first part of the transform input and a first half of the signal is mapped onto a final part of the transform input. 
     
     
         9 . A method according to any of  claims 2  to  8  wherein identifying a primary carrier signal comprises estimating a primary carrier frequency. 
     
     
         10 . A method according to  claim 9  wherein estimating the frequency of a primary carrier comprises determining at least one of the highest magnitude frequency output bin and determining the peak frequency. 
     
     
         11 . A method according to  claim 10  wherein the three highest frequency output bins are determined and a quadratic curve is fitted so as to interpolate the peak frequency. 
     
     
         12 . A method according to  claim 9  or  10  further comprising down-converting the frequency-domain sum signal based on said estimated frequency. 
     
     
         13 . A method according to  claim 12  further comprising phase-rotating the frequency-domain down-converted sum-signal. 
     
     
         14 . A method according to  claim 12  or  13  wherein the down-conversion is performed by convolving the transform output with a window filter. 
     
     
         15 . A method according to  claim 14  wherein the window filter comprises a closed-form cosine function. 
     
     
         16 . A method according to  claim 15  wherein the window filter comprises a Blackman family window, preferably Blackman-Nuttall window. 
     
     
         17 . A method according to  claim 14  wherein the window filter comprises one of Kaiser or Equiripple. 
     
     
         18 . A method according to any preceding claim wherein determining the presence of a secondary transmission comprises performing a symmetry analysis on said peak. 
     
     
         19 . A method of determining the presence of a second carrier signal in a time-domain sum-signal, the method comprising:
 identifying a primary carrier signal within the sum-signal;   attenuating the primary carrier signal from within the sum signal;   extracting at least one peak corresponding to a heterodyne tone;   performing a symmetry analysis on said at least one peak to determine the presence of a secondary transmission.   
     
     
         20 . A method according to  claim 18  or  19  wherein identifying a primary carrier signal comprises estimating a primary carrier frequency. 
     
     
         21 . A method according to  claim 19  or  20  wherein identifying a primary carrier signal comprises estimating the phase of the primary carrier. 
     
     
         22 . A method according to  claim 20  comprising transforming the sum-signal into the frequency domain. 
     
     
         23 . A method according to  claim 22  wherein estimating the frequency of a primary carrier within the sum-signal comprises determining at least one of the highest magnitude frequency output bin and determining the peak frequency. 
     
     
         24 . A method according to  claim 21  comprising transforming the sum-signal into the frequency domain. 
     
     
         25 . A method according to  claim 24  wherein the step of transformation into the frequency domain is performed using a discrete transform comprising input and output bins. 
     
     
         26 . A method according to  claim 25  wherein the phase estimation is performed by determining at least one highest magnitude frequency output bin and determining the phase of the peak frequency component. 
     
     
         27 . A method according to  claim 22  or  25  wherein the three highest frequency output bins are determined and a curve is fitted so as to interpolate the peak frequency. 
     
     
         28 . A method according to  claim 27  wherein the fitted curve is a quadratic curve. 
     
     
         29 . A method according to any of  claims 19  to  28  wherein following down-conversion, the quadrature component of the time-domain sampled signal is transformed into the frequency domain prior to peak extraction. 
     
     
         30 . A method according to any of  claims 19  to  29  wherein the down-converting comprises mixing the sampled signal with a sinusoid with a frequency and phase corresponding to an estimated frequency and phase of the primary carrier. 
     
     
         31 . A method according to  claim 22  or  24  wherein the transformation into the frequency domain is a Fourier Transform (FT), preferably a Fast Fourier Transform (FFT). 
     
     
         32 . A method according to any of  claims 18  to  31  wherein the symmetry analysis comprises:
 determining a measure of the ratio of the magnitude of a peak at a certain frequency above a central frequency with the magnitude of the signal at the corresponding frequency below the central frequency. 
 
     
     
         33 . A method according to  claim 32  comprising comparing said asymmetry ratio to a pre-determined threshold. 
     
     
         34 . A method according to  claim 32  or  33  wherein the magnitude of the peak is used in conjunction with said ratio in determining the presence of a secondary transmission. 
     
     
         35 . A method according to  claim 34  comprising producing a confidence score of a secondary carrier transmission being present based on the peak magnitude and asymmetry ratio. 
     
     
         36 . A method according to any preceding claim wherein the presence of a secondary transmission is determined only if a peak corresponding to the carrier of a primary transmission is present. 
     
     
         37 . A method according to  claim 36  wherein the presence of a secondary transmission is determined only if the magnitude of said primary carrier peak is above a pre-determined threshold. 
     
     
         38 . A method according to  claim 36  or  37  wherein the presence of a secondary transmission is determined only if the width of said primary carrier peak is below a pre-determined frequency threshold. 
     
     
         39 . A method according to any preceding claim wherein following peak extraction, if two peaks are within a minimum frequency separation of one another, the peaks are combined into a single peak prior to determination of the presence of a secondary carrier. 
     
     
         40 . A method according to  claim 39  wherein the minimum frequency separation is between 5 Hz and 50 Hz, preferably between 7 Hz and 15 Hz, preferably approximately 10 Hz. 
     
     
         41 . A method according to  claim 39  or  40  wherein the peak with the lower magnitude is discarded. 
     
     
         42 . A method according to any preceding claim wherein the sum-signal is decimated so as to reduce the bandwidth. 
     
     
         43 . A method according to  claim 42  wherein the frequency domain transform output is gain-transformed so as to compensate for decimator ripple. 
     
     
         44 . A method according to  claim 43  wherein the gain transformation is the reciprocal of the gain due to the magnitude spectrum of the decimator. 
     
     
         45 . A method according to any preceding claim wherein the sum-signal is sampled. 
     
     
         46 . A method according to any preceding claim wherein the sum-signal is sampled in overlapping blocks. 
     
     
         47 . A method according to  claim 46  wherein the sampling consists of the most recent T seconds of the signal and the sampling rate is M times per second, where T*M>1. 
     
     
         48 . A method according to  claim 47  wherein T is between 1 and 4, and M is between 2 and 8. 
     
     
         49 . A method according to  claim 48  wherein T=2 and M=4. 
     
     
         50 . A method according to any preceding claim further comprising:
 estimating a noise floor of the down-converted signal;   subtracting a measure of said noise floor from the signal prior to peak extraction.   
     
     
         51 . A method according to  claim 50  wherein the noise floor estimation comprises performing a moving-average. 
     
     
         52 . A method according to  claim 51  wherein the moving average comprises summing contiguous blocks of D 1  samples and calculating the median across D 2  of said blocks. 
     
     
         53 . A method according to  claim 52  wherein D 1  is approximately equal to D 2 . 
     
     
         54 . A method according to any preceding claim further comprising alerting an operator to the presence of a secondary transmission. 
     
     
         55 . A method according to  claim 54  wherein alerting an operator comprises inserting a tone into an audio output. 
     
     
         56 . A method according to  claim 54  or  55  wherein alerting an operator comprises indicating the presence of a secondary transmission on a user interface. 
     
     
         57 . A method according to any of  claims 54  to  56  wherein alerting an operator comprises indicating the confidence level of the presence of a secondary transmission. 
     
     
         58 . A method of reducing windowing artefacts in a time/frequency transform of a signal:
 applying a windowing function to a signal;   mapping the signal onto the input of an oversampled transform;   wherein a central part of the transform input has zero-valued input bins;   performing a time/frequency transform;   outputting a frequency spectrum of the signal.   
     
     
         59 . A method according to  claim 58  wherein the transform is a Fourier transform 
     
     
         60 . A method according to  claim 58  or  59  wherein a second half of the signal is mapped onto a first part of the transform input and a first half of the signal is mapped onto a final part of the transform input. 
     
     
         61 . A method according to  claim 58  or  60  further comprising the step of determining the presence of a secondary carrier signal from the frequency spectrum of the signal. 
     
     
         62 . A method according to  claim 61  further comprising alerting a user to the presence of a secondary carrier signal. 
     
     
         63 . A method according to any preceding claim wherein the signal comprises voice communication. 
     
     
         64 . A method according to  claim 63  wherein the voice communication is transmitted from an aircraft and intended to be received by an air traffic controller. 
     
     
         65 . An apparatus adapted to carry out a method according to any preceding claim. 
     
     
         66 . An apparatus for determining the presence of a secondary carrier signal in a time-domain sum-signal including a primary carrier signal, the apparatus comprising:
 means for transforming the sum-signal into the frequency domain;   means for extracting at least one peak corresponding to a heterodyne tone from the transformed sum-signal;   means for determining the presence of a secondary carrier signal in the sum-signal based on said at least one peak.   
     
     
         67 . An apparatus for determining the presence of a second carrier signal in a time-domain sum-signal, the method comprising:
 means for identifying a primary carrier signal within the sum-signal;   means for attenuating the primary carrier signal from within the sum signal;   means for extracting at least one peak corresponding to a heterodyne tone;   means for performing a symmetry analysis on said at least one peak to determine the presence of a secondary transmission.   
     
     
         68 . An apparatus for reducing windowing artefacts in a time/frequency transform of a signal:
 means for applying a windowing function to a signal;   means for mapping the signal onto the input of an oversampled transform;   wherein a central part of the transform input has zero-valued input bins;   means for performing a time/frequency transform;   means for outputting a frequency spectrum of the signal.   
     
     
         69 . An apparatus according to any of  claims 65  to  68  comprising a radio. 
     
     
         70 . An apparatus according to  claim 69  wherein the radio is a software-defined radio. 
     
     
         71 . A method substantially as described herein and/or illustrated with reference to the accompanying drawings. 
     
     
         72 . An apparatus substantially as described herein and/or illustrated with reference to the accompanying drawings. 
     
     
         73 . A radio substantially as described herein and/or illustrated with reference to the accompanying drawings.