Abstract:
A system for and method of detecting interference in a communication system. In an embodiment, a receiver acquires a communication signal, the communication signal comprising a carrier signal and an in-band interference signal. A signal processor conditions the communication signal and extracts the in-band interference signal without interrupting the carrier signal to form an error signal. The error signal is representative of the in-band interference signal. The signal processor is further configured to process the error signal to obtain one or more spectral properties of the error signal in a manner suitable for display.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/361,493, filed Mar. 4, 2002, the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to detection of interference and noise in a transmitted signal. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Interference (including noise) showing up in-band to a transmitted carder is a common problem in wireless communication systems. For example, in satellite communication systems, interference can be caused by, but is not limited to, isolation degradation of cross-polarized signals, adjacent satellite traffic, locally received terrestrial signals, or an unauthorized transmission. In many cases, interference can be very difficult to detect, however, its impact on the receive quality of the transmitted digital carrier can be significant. 
         [0004]    The most common approach to determining the presence of interference is to temporarily remove the service (the transmitted carrier) and inspect the received power spectrum with a frequency analysis device such as a spectrum analyzer. Although this approach can be effective, it causes a service interruption that can last for many hours. In some cases, interference is not the problem, and the service was interrupted unnecessarily. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The invention includes a method of and an apparatus for detecting and measuring noise and interference, which is in-band to a received communications carrier. To alleviate drawbacks to conventional approaches, the applicant has developed a non-intrusive interference detection and noise measurement approach. With this approach, interference and noise can be detected and measured without taking the carrier out of service. Rather, the measurements are made while the communications circuit (the transmitted carrier) is active. 
         [0006]    In one aspect, in-band interference in a carrier signal in a communication system is detected. A signal is acquired including the carrier signal and an interfering signal. The interfering signal is extracted from the carrier without interrupting the carrier. 
         [0007]    In another aspect, a signal is received, filtered, and digitized. Decimation is then performed and the signal resampled. Blind equalization and demodulation are performed thereby forming an error vector that is representative of the interference signal. 
         [0008]    In yet another aspect, a receiver acquires a digital signal. A signal processor conditions the digital signal, and a blind equalizer demodulator forms an error vector that is representative of an interference signal included in the carrier signal. 
         [0009]    These and other aspects are described in more detail herein. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  illustrates a flow diagram showing the main processing associated with this invention; 
           [0011]      FIG. 2  shows a detailed flow diagram of the processing associated with this invention; and 
           [0012]      FIG. 3  illustrates a system in accordance with an embodiment of the present invention; and 
           [0013]      FIG. 4  shows a graphical display that might be displayed to a user of this invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0014]      FIG. 1  shows a flow diagram of a process  100  in accordance with an aspect of the present invention. A signal, which contains noise and interference, is received in a step  101 . The signal is down-converted to an intermediate frequency (IF) and then digitized by a sampling device, in an acquisition step  102 . The digital samples are stored in memory for further processing, also in the acquisition step  102 . Using the stored samples, the signal is processed to create a re-sampled baseband version of the received signals, in a digital formatting step  103 . Using this re-sampled baseband signal, an equalized error signal is created, in an interference processing step  104 . This error signal is further processed to create a power spectrum of the underlying noise and interference contained in the received carrier, also in the interference processing step  104 . This power spectrum of the noise and interference can be measured for interfering signals as well as displayed to the user, in an output step  105 . 
         [0015]      FIG. 2  is a more detailed flow diagram of a process  200  in accordance with an embodiment of the present invention. An input signal may be a radio frequency (RF) signal from an antenna. Alternatively the input signal may be any communication signal in any frequency band i.e. RF, IF, microwave or optical. 
         [0016]      FIG. 3  illustrates a system  400  in accordance with an embodiment of the present invention. The system  400  detects and measures in-band interference and noise in an input signal  407  in accordance with the method of  FIG. 2 . An input signal  407  is received from a satellite  401  by a satellite dish  402 . The input signal  407  may be transmitted by means other than the satellite  401 , including but not limited to a radio transmitter, a cable transmitter, a cell tower, a microwave transmitter, or an optical transmitter. The input signal  407  may be received by means other than the satellite dish  402 , including but not limited to an antenna, a microwave dish, or an optical receiver. The present invention is applicable to any system that communicates a digital signal from a transmitter to a receiver, regardless of the medium or the carrier frequency. 
         [0017]    Referring to  FIGS. 2 and 3 , a receiver  403  may convert the input signal  407  from an RF input signal  407  to an IF signal in a step  201 . The IF signal may be at any frequency that makes the following detection simpler, cheaper, or more accurate. The receiver  403  may further filter and adjust the level of a signal, which is representative of the input signal  407  in a step  202 . The receiver  403  may filter the input signal  407  with a band-pass filter to limit the input signal  407  or its representative to the carrier and its modulation. An amplifier with automatic gain control may adjust the level of the filtered input signal  407  or its representative, also in the step  202 . The input signal  407  may be amplified to take full advantage of an A/D converter in the receiver  403 . The A/D converter is expected to perform best when the full amplitude bandwidth is used. 
         [0018]    The A/D converter, in the step  203 , produces a digitized version of the filtered IF signal. The A/D converter may sample the IF signal at a frequency at least twice the frequency of the highest frequency of interest in accordance with Nyquist&#39;s theorem, though another sampling frequency may be used. The digitized version of the IF signal is then stored as a snapshot  408  in a step  204 . The above steps  201 - 204  may comprise the acquisition step  102  of  FIG. 1 . 
         [0019]    A signal processor  404  may analyze the snapshot  408  to calculate parameters representative of the input signal  407  including, a bandwidth of the input signal  407 , a center frequency of the input signal  407 , a symbol rate of the input signal  407 , amplitudes of the carrier lines, frequencies of the cater lines, and maximums of the carrier lines. 
         [0020]    In a next step  205  of the process  200 , a power spectrum of the snapshot  408  is calculated by the signal processor  404 . Multiple power spectrums may be calculated and averaged together, to create a spectral density periodogram. The power spectrum may be calculated using conventional Fast-Fourier Transform (FFT) methods, for converting the IF signal from time space to complex frequency space. Other methods beside FFT may be used to convert the IF signal to frequency space. The power spectrum or the spectral density periodogram may be displayed to user at this time. 
         [0021]    The input signal may be a modulated carrier. The center frequency and bandwidth (BW) of the carrier may be calculated by a signal processor  404  in a step  206  using the power spectrum or the spectral density periodogram of the IF signal from step  205 . If the center frequency and the bandwidth are already known, however, then the steps  205  and  206  may be skipped. 
         [0022]    Once the center frequency of the carrier is known, down-converting of snapshot  408  to the baseband of the carrier may be performed in step  207  by the signal processor  404 . The snapshot  408  may be further filtered such that the signal is limited in bandwidth to that of the baseband signal, also in step  207 . Further, the snapshot  408  may be decimated also in a step  207 . Decimation may be performed at a frequency at least twice the frequency of the highest frequency of interest in accordance with Nyquist&#39;s theorem. 
         [0023]    The carrier signal may have multiple carrier lines. In a step  208 , information about the carrier such as symbol rate, and estimates of the amplitude and frequency of the carrier lines may be calculated. This information may be calculated by performing magnitude, square, cube and quad power transforms on the signal and recovering the maximums. 
         [0024]    The estimates of the amplitude and frequency of the carrier lines may be used to determine the modulation of the digital carrier and the frequency in a step  209 . By inspecting maximums of the carrier lines, the modulation of the carrier may be determined. Using information about the carrier frequency, any down-conversion error in the decimated signal may be removed in a step  210 . For example if the baseband signal is offset, it may be recentered such that any offset in the carrier frequency is removed. 
         [0025]    In a step  211 , the carrier signal may be re-sampled by the signal processor  404 , such as at a sample rate of two samples per symbol, and a resampled signal may be an output. This sample rate may be determined from the symbol rate calculated in the step  208 . The above steps  205 - 211  may be performed in the digital formatting step  103  of  FIG. 1 . 
         [0026]    A blind equalizer demodulator  405  may calculate an error vector that is representative of the interference signal in the input signal  407  in a step  212 . This step produces an error vector that may be used to calculate the interference signal that is in the input signal  407 . A digital communication system modulates a carrier wave for transmitting symbols to a receiver. In such a digital system, each symbol has discrete levels of amplitude and/or phase at which the carrier is modulated. A goal of the demodulator  405  is to determine the levels at which the carrier is modulated. It does this by making a first initial guess of the modulation levels and then calculating an error vector that represents the difference between the initial guess and the measured signal. Then the guessed modulation levels are adjusted to minimize an error function based on the error vectors. The guessed modulation levels are continuously adjusted until the error function has been minimized, at which point the modulation has converged. There are many ways to adjust the levels, including decision directed least mean square (DD-LMS) and constant modulus algorithm (CMA), both of which are well known in the literature. Conventionally the blind equalizer demodulator  405  is used to calculate the symbols in the input signal  407 . Here, the output of interest is the error vector as opposed to the prior art where the output of interest is the symbols. 
         [0027]    In a step  213 , a first M samples are removed from the error vector to produce a new en-or vector. Depending on the quality of the initial first guess, the first M samples may have large error vectors that do not truly represent the noise and interference in the input signal  407 . Before the blind equalizer demodulator  405  converges in the step  212 , the first M samples of the error vector may contain errors. The DC bias of the new error vector is removed in a step  214 , by subtracting the mean of the new error vector from the new error vector to produce an in-band vector that is representative of noise and interference in-band to the carrier. Any processing artifacts may also be removed in the step  214 . The power spectrum of the in-band vector is calculated to convert the complex time representation of the in-band vector into a frequency domain representation, in a step  215 . In a step  216  the spectral properties of the in-band vector are measured such as center frequency, BW, power, C/N and detected interference energy. The above steps  212 - 216  may comprise the interference processing step  104  of  FIG. 1  preformed by signal processor  405 . 
         [0028]    A power spectrum of the error vector, such as a trace  302  in a  FIG. 4 , may be sent to a display  406  for presentation to a user in a step  105 . A power spectrum of the input signal  407 , such as trace  301  in  FIG. 4 , may also be sent to the display  406  also in step  105 . Furthermore, the spectral properties of the en-or vector may be sent to the display  406 . Other calculations regarding the digital signal may also be sent to the display  406 . 
         [0029]    The system described in  FIG. 3  may also implement all the steps in the flowchart  200  of  FIG. 2  and Table 1 as follows. The system may be implemented in hardware and/or software. The system may be implemented in a standard PC and/or may be implemented with custom electronics. 
         [0030]    Table 1 presents the method of  FIG. 2  in tabular format. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Functional Block 
                 Input 
                 Description 
                 Output 
               
               
                   
               
             
             
               
                 RF Down-conversion 
                 From Antenna 
                 Convert RF signal to 
                 IF representation of 
               
               
                 (201) 
                   
                 IF Frequency 
                 signal 
               
               
                 Filtering and Gain Control 
                 IF Signal 
                 Band-limit signal and 
                 Filtered and amplified 
               
               
                 (202) 
                   
                 adjust signal level for 
                 signal 
               
               
                   
                   
                 A/D Converter 
               
               
                 A/D Conversion (203) 
                 Filtered IF Signal 
                 Convert analog signal 
                 Samples of IF signal 
               
               
                   
                   
                 at IF to digital 
               
               
                   
                   
                 samples 
               
               
                 Snapshot Memory (204) 
                 Samples of IF Signal 
                 Store IF samples 
                 Samples of IF signal 
               
               
                 PSD Computation (205) 
                 Samples of IF Signal 
                 Convert time domain 
                 Power spectrum 
               
               
                   
                   
                 representation of 
               
               
                   
                   
                 signal to frequency 
               
               
                   
                   
                 domain 
               
               
                   
                   
                 representation. 
               
               
                 Spectral Detection and 
                 Power Spectrum 
                 Detect and measure 
                 Center frequency and 
               
               
                 Measurement (206) 
                   
                 carrier of interest 
                 BW of carrier to 
               
               
                   
                   
                   
                 analyze 
               
               
                 Down-convert and 
                 Samples of IF signal, 
                 Down-convert carrier 
                 Decimated signal 
               
               
                 decimate (207) 
                 center frequency and 
                 to baseband, filter and 
                 (complex signal 
               
               
                   
                 BW estimation 
                 decimate 
                 representation) and 
               
               
                   
                   
                   
                 decimated sample 
               
               
                   
                   
                   
                 rate 
               
               
                 Carrier and Baud 
                 Decimated Signal 
                 Perform magnitude, 
                 Symbol rate, 
               
               
                 Recovery (208) 
                   
                 square, cube and 
                 estimates of 
               
               
                   
                   
                 quad power 
                 amplitude and 
               
               
                   
                   
                 transforms on signal. 
                 frequency of carrier 
               
               
                   
                   
                 Recover maximums 
                 lines 
               
               
                 Modulation Identification 
                 Estimates of amplitude 
                 Determine Modulation 
                 Modulation of digital 
               
               
                 (209) 
                 and frequencies of 
                 of digital carrier by 
                 carrier and Carrier 
               
               
                   
                 carrier lines 
                 inspecting carrier line 
                 frequency 
               
               
                   
                   
                 maximums 
               
               
                 Carrier Correction (210) 
                 Decimated signal and 
                 Remove 
                 Decimated signal 
               
               
                   
                 Carrier frequency 
                 down-conversion error 
               
               
                   
                   
                 from decimated signal 
               
               
                 Re-sampler (211) 
                 Decimated signal, 
                 Re-sample carrier to 2 
                 Re-sampled signal 
               
               
                   
                 Decimated sample 
                 samples/symbol 
               
               
                   
                 rate and symbol rate 
               
               
                   
                 of carrier 
               
               
                 Equalizer/Demodulator 
                 Resampled Signal 
                 Equalize and 
                 Estimated symbols 
               
               
                 (212) 
                   
                 demodulate signal 
                 and Error vector 
               
               
                   
                   
                 using blind 
               
               
                   
                   
                 equalizer/demodulator 
               
               
                   
                   
                 approach 
               
               
                 Remove M Initial samples 
                 Error Vector 
                 Remove first M 
                 New Error Vector 
               
               
                 (213) 
                   
                 samples from Error 
               
               
                   
                   
                 vector. First M 
               
               
                   
                   
                 samples contain 
               
               
                   
                   
                 errors from matched 
               
               
                   
                   
                 filter error 
               
               
                 DC bias removal (214) 
                 New Error Vector 
                 Remove mean from 
                 In-band vector 
               
               
                   
                   
                 New Error Vector 
                 (represents noise and 
               
               
                   
                   
                   
                 interference in-band 
               
               
                   
                   
                   
                 to carrier) 
               
               
                 PSD Computation (215) 
                 In-band vector 
                 Convert complex time 
                 In-band spectrum 
               
               
                   
                   
                 domain in-band vector 
               
               
                   
                   
                 to a frequency domain 
               
               
                   
                   
                 representation 
               
               
                 Spectral detection and 
                 In-band Spectrum 
                 Detect and measure 
                 In-band spectral 
               
               
                 measurement (216) 
                   
                 any spectral energy 
                 measurements 
               
               
                   
                   
                   
                 (Center frequency, 
               
               
                   
                   
                   
                 BW, power, and C/N 
               
               
                   
                   
                   
                 of any detected 
               
               
                   
                   
                   
                 interference energy) 
               
               
                 Display (217) 
                 In-band Spectrum and 
                 Display spectrum and 
                 Done 
               
               
                   
                 In-band spectral 
                 measurement results 
               
               
                   
                 measurements 
                 to user 
               
               
                   
               
             
          
         
       
     
         [0031]      FIG. 4  shows a graphical display  300  of data that the invention may present to a user. The trace  301  represents the power spectrum of the received carrier, and the trace  302  represents the power spectrum of the noise and interference, which are in-band to the received carrier. In this example, an interfering signal can be seen in the trace  302 , which is not visible in the received carrier trace  301 . 
         [0032]    Thus, a technique has been described for detecting and measuring interference within a digital carrier. The process can be completely blind, meaning that the process described above will work even when the digital carrier&#39;s RF and modulation parameters are unknown. The process described detects the RF carrier, measures its RF and modulation parameters, equalizes and demodulates the digital carrier, extracts an error vector, converts this error vector into a complex baseband estimate of the noise and interference. From this estimate, a power spectrum of the in-band noise and interference is created. This power spectrum is analyzed for spectral energy. The in-band spectrum and measurement results are displayed for a user. 
         [0033]    While the foregoing has been with reference to particular embodiments of the invention, it will be appreciated by those skilled in the art that changes in these embodiments may be made without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims.