Shielding flaw detection and measurement in orthogonal frequency division multiplexed (ofdm) cable telecommunications environment

Abstract

An egress signal from a broadband communication system (BCS) including orthogonal frequency division multiplex (OFDM) signals is distinguished from noise and broadcast transmissions and authenticated as such by detecting required or optional pilot signals by their frequency, pattern and/or other characteristics such as recurrence rate or cepstral interval. Detection is made more robust by detection of combinations of required or optional pilot tones. Different BCS plants in the same geographic area or overbuilt can be discriminated based on the frequencies and frequency intervals of required, fixed frequency PHY link channel (PLC) related pilots, pattern and frequency of the presence or absence of optional pilots at fixed frequency intervals and/or cepstral or recurrence intervals of required scattered pilots if different symbol rates are employed or frequency of scattered pilots if the OFDM bands do not overlap in the adjacent or overbuilt BCSs.

1 . A method of detecting an orthogonal frequency division multiplex signal among ambient electromagnetic signals comprising steps of
 receiving an ambient electromagnetic signal,   deriving a spectrum of said ambient electromagnetic signal, and   analyzing said spectrum to determine one or more characteristics of patterns of locations or intervals of spectral peaks.   
     
     
         2 . The method as recited in  claim 1 , wherein said characteristics of patterns of locations of spectral peaks comprise PHY Link Channel (PLC) related pilots. 
     
     
         3 . The method as recited in  claim 2 , wherein said PLC related pilots are at predetermined frequency intervals from a PLC frequency band. 
     
     
         4 . The method as recited in  claim 2 , wherein said patterns of locations of spectral peaks are:
 ±550 kHz for a narrow PLC or ±750 kHz for a wide PLC from the center frequency of the PLC or 350 kHz from the PLC edges,   ± 1000  kHz for a narrow PLC or ±1200 kHz for a wide PLC from the center frequency of the PLC or 800 kHz from the PLC edges,   ±1,550 kHz for a narrow PLC or ±1,750 kHz for a wide PLC from the center frequency of the PLC or 1,350 kHz from the PLC edges, and   ±2,150 kHz for a narrow PLC or ±2,350 kHz for a wide PLC from the center frequency of the PLC or  1950  kHz from the PLC edges.   
     
     
         5 . The method as recited in  claim 1 , wherein said one or more characteristics of patterns of locations or intervals of spectral peaks include
 optional pilot frequencies at selected frequency intervals from each other.   
     
     
         6 . The method as recited in  claim 5 , wherein said optional pilot signals are placed at fixed frequency intervals of 1.6 MHZ and/or 6.4 MHZ or multiples thereof. 
     
     
         7 . The method as recited in  claim 1 , wherein said characteristics of patterns of locations of spectral peaks comprise peaks of varying frequency at frequency separations of a predetermined frequency wherein said varying frequency repeats at a predetermined time interval. 
     
     
         8 . The method as recited in  claim 7 , wherein said predetermined time interval is a multiple of a symbol rate of a broadband communication signal of interest. 
     
     
         9 . The method as recited in  claim 8 , wherein said multiple of said symbol rate is 64 or 128 symbols. 
     
     
         10 . The method as recited in  claim 7  wherein said predetermined time interval is 2.56 msec. 
     
     
         11 . The method as recited in  claim 7 , wherein said characteristics of patterns of locations of spectral peaks comprise a cepstral line within a cepstral window. 
     
     
         12 . The method as recited in  claim 1 , wherein said spectral peaks are binary phase shift keying modulated. 
     
     
         13 . The method as recited in  claim 12 , wherein said modulation of said spectral peaks is synchronized. 
     
     
         14 . The method as recited in  claim 11 , wherein said spectral peaks are binary phase shift keying modulated by a pseudo-random binary signal sequence. 
     
     
         15 . The method as recited in  claim 14 , wherein said pseudo-random binary signal sequence is derived from a linear shift register with feedback from predetermined stages. 
     
     
         16 . The method as recited in  claim 1 , wherein said spectral peaks are unmodulated carrier wave signals. 
     
     
         17 . The method as recited in  claim 1 , wherein said patterns of locations differ between geographically proximate broadband communications systems. 
     
     
         18 . A method of authenticating a detected signal as an egress signal from a broadband communication system (BCS) system wherein a signal carried by said BCS system includes a plurality of pilot or marker signals, said pilot or marker signals being of different types characterized by frequency, pattern of frequency or rate of recurrence, said method comprising a step of
 detecting at least two types of said marker or pilot signals having particular frequencies, patterns of frequencies or rates of recurrence.   
     
     
         19 . The method as recited in  claim 18  wherein said pilot signals comprise PLC-related pilots, optional pilots and scattered pilots. 
     
     
         20 . Apparatus for detecting an egress signal from a broadband communication system, said apparatus comprising
 a tunable receiver to receive ambient electromagnetic signals,   a converter for digitizing samples of said ambient electromagnetic signals received by said tunable receiver, and   a processor configured to perform computations of
 a computation of a magnitude squared fast Fourier transform of said digitized samples, 
 a logarithm of results of said magnitude squared fast Fourier transform, and 
 an inverse fast Fourier transform of results of said computation of said logarithm. 
   
     
     
         21 . Apparatus as recited in  claim 20 , wherein said processor or another processor included in said apparatus is configured to additionally perform computations of
 a magnitude squared fast Fourier transform of said digitized samples, and   cross-correlation of results of said magnitude squared fast Fourier transform with a template.