Abstract:
A method and system for measuring noise and or interference in a communications signal without taking the signal out of service. In the present invention the communications signal is converted from an RF signal into an IF signal. The IF signal is then digitized and stored. The stored signal is processed to determine the interference signal. The interference signal is calculated from an error vector produced by a blind equalizer demodulator. The interference signal is extracted and presented to a user.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application Serial 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 carrier 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]    [0010]FIG. 1 illustrates a flow diagram showing the main processing associated with this invention;  
         [0011]    [0011]FIG. 2 shows a detailed flow diagram of the processing associated with this invention; and  
         [0012]    [0012]FIG. 3 illustrates a system in accordance with an embodiment of the present invention; and  
         [0013]    [0013]FIG. 4 shows a graphical display that might be displayed to a user of this invention. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0014]    [0014]FIG. 1 shows a flow diagram of a process  100  in accordance with an aspect of the present inveniton. 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 signal, 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]    [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]    [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 carrier 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 error 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 error 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.  
                           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]    [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.