Patent Publication Number: US-9419661-B2

Title: Impulse noise mitigation under out-of-band interference conditions

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 14/444,489 filed on Jul. 14, 2014 and now patented as U.S. Pat. No. 9,166,637, which is a continuation of U.S. patent application Ser. No. 13/941,604 filed on Jul. 15, 2013 and now patented as U.S. Pat. No. 8,792,543, which is a continuation of U.S. patent application Ser. No. 12/924,185 filed on Sep. 22, 2010, now patented as U.S. Pat. No. 8,488,663, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention is related to a wireless communication receiver and in particular impulse noise mitigation. 
     2. Description of Related Art 
     Noise and in particular impulse noise, which is generated in short bursts, can be disruptive to data (broadcasts) that are processed through an analog receiver and translated into a digital format to produce a quality output as one might experience in a received radio transmission. The impulse noise can be caused by many modern day sources in which ignition systems and domestic appliances represent a couple of sources. Elimination or mitigation of impulse noise is essential to a clear reproduction of the received analog signal into a digital signal format. 
     US Patent Application Publication 2010/0054150 (Oksman et al.) is directed to a method and system in which impulse noise is monitored and noise protection parameters are adjusted. In US Patent Application Publication 2009/0323903 (Cioffi et al.) a method and apparatus is directed to monitoring and adjusting noise abatement in a DSL link. In US Patent Application Publication 2009/0168929 (Liu et al.) a method and apparatus is directed to an adaptive impulse noise detection and suppression. In US Patent Application Publication 2003/0099287 (Arambepola) a method and apparatus is directed to detecting impulse noise in COFDM modulated TV signals. U.S. Pat. No. 7,676,046 B1 (Nelson et al.) is directed to a method of removing noise and interference from a signal by calculating a time-frequency domain of the signal and modifying each instantaneous frequency. U.S. Pat. No. 7,630,448 B2 (Zhidkov) is directed to a method to reduce noise in a multiple carrier modulated signal by estimating impulse noise and removing the noise as a function of the estimated impulse noise. U.S. Pat. No. 7,573,966 B2 (Kim et al.) is directed to a signal conditioning filter and a signal integrity unit to address equalization and noise filtering to improve signal fidelity. In U.S. Pat. No. 7,558,337 B2 (Ma et al.) a method and apparatus is directed to signal processing to mitigate impulse noise. In U.S. Pat. No. 7,499,497 B2 (Huang et al.) a method and apparatus is directed to suppression of impulse noise in an OFDM system. U.S. Pat. No. 7,302,240 B2 (Koga et al.) is directed to a communication apparatus that has an ADC to convert an analog signal to a digital signal before applying an noise detector. 
     U.S. Pat. No. 7,139,338 B2 (Wilson et al.) is directed to a receiver with a filter and an impulse response from which a controller adapts the impulse response to the filter. U.S. Pat. No. 7,035,361 B2 (Kim et al.) is directed to a signal conditioning filter and a signal integrity unit to address coupled problems of equalization and noise filtering. In U.S. Pat. No. 7,016,739 B2 (Bange et al.) a system and method is directed to removing narrowband from an input signal in which notch frequencies of notch filters are adjusted in accordance with a detected noise spectrum. In U.S. Pat. No. 6,920,194 B2 (Stopler et al.) a method and system is directed to correcting impulse noise present on an input signal. U.S. Pat. No. 6,795,559 B1 (Taura et al.) is directed to an impulse noise reducer, which detects and smoothes impulse noise on an audio signal. U.S. Pat. No. 6,647,070 B1 (Shalvi et al.) is directed to a method and apparatus for combating impulse noise in digital communication channels. U.S. Pat. No. 6,385,261 B1 (Tsuji et al.) is directed to an impulse noise detector an noise reduction system in an audio signal. U.S. Pat. No. 5,410,264 (Lechleider) is directed to an impulse noise canceller, which recognizes, locates and cancels impulse noise on an incoming signal. U.S. Pat. No. 5,226,057 (Boren) is directed to adaptive digital notch filters for use with RF receivers to reduce interference. U.S. Pat. No. 4,703,447 (Lake, Jr.) is directed to a mixer controlled variable passband finite impulse response filter. U.S. Pat. No. 4,703,447 (Lake, Jr.) is directed to a mixer controlled variable passband finite impulse response filter. 
     A primary purpose of a receiving tuner is to select a particular channel of interest and convert that frequency band to a baseband for digital signal processing. Shown in  FIG. 1  of prior art an output  11  of a tuner  10  is processed through an analog to digital converter (ADC)  12  to translate the analog output  11  of the tuner into a digital time domain waveform. Mitigation of sudden spikes in the time domain waveform, which are caused by impulse noise, prevent an accurate demodulation  16  of the digital signal produced by the ADC  12 . The output  13  of the ADC  12  is applied to an impulse noise mitigation circuit  14  and the output  15  of the impulse noise mitigation circuit is connected to a demodulator  16 . 
     Shown in  FIG. 2  of prior art is an expansion of the impulse noise mitigation  14  for time domain noise mitigation for impulse noise interference detection. An output of a magnitude function  20  is compared to a threshold using a standard comparator  22 . the output  23  of the comparator  22  is used as an impulse noise flag by the suppressor circuit  24 . The output of the suppressor circuit  15  is connected to the demodulator  16 . The detection threshold of the comparator  22  can either be a fixed predetermined value or the detection threshold can be dynamically calculated based on the output  21  of the magnitude function. The suppressor circuit  24  either clips the samples of the digital signal that are found to be impulse noise in the comparator or nulls out the corrupted samples of the digital signal caused by impulse noise. 
     A shortcoming of the time domain method of impulse noise mitigation, shown in  FIG. 2 , is the inability to detect the presence of impulse noise under normal or relatively high carrier to interference ratio. The impulse noise can often be buried under the average envelop of the desired signal. 
       FIG. 3  is an impulse noise mitigation scheme of prior art in the frequency domain. The output of the ADC  13 , shown in  FIG. 1 , is connected to a high pass filter  30  and the output  31  of the high pass filter is connected to a magnitude function  32 . The output of the magnitude function  33  is applied to a comparator circuit  34  having a detection threshold control. The output  35  of the comparator is connected to a suppressor circuit  36 , which connects  15  back to the demodulator shown in  FIG. 1   
     In the scheme shown in  FIG. 3  the detection threshold can be fixed to a predetermined value or dynamically adjusted based on the output  33  of the magnitude function  32 . The suppressor circuit  36  either clips the signal samples that are determined to be impulse noise or nulls out the corrupted samples. 
     The main drawback of the frequency domain method shown in  FIG. 3  is the inability to detect the presence of impulse noise under out of band interferers, especially adjacent interferers, where the signal at the output of the magnitude function  33  will contain the energy of both the impulse noise and the interferers thus making the detection of impulse noise unreliable. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide two complex filters to improve the frequency domain impulse mitigation. 
     It is still an objective of the present invention wherein a first filter admits only frequency components higher than the desired signal bandwidth in the positive frequency domain and the second filter admits frequency components lower than the desired signal bandwidth in the negative frequency domain. 
     It is further an objective of the present invention to measure the mean magnitude over a time interval T of each of the two high pass filters in order to select which of the two filters to use to detect and mitigate impulse noise. 
     In the present invention two complex high pass filters are used to mitigate impulse noise and other noise interferer signals. The response of each individual filter is measured over a time period T to determine which filter provides the best response. This measurement is the mean magnitude of the noise signal that is being removed from the signal being connected to the output of an impulse noise mitigation circuit, whereupon the filter producing the lowest mean magnitude value is chosen for impulse noise mitigation. This selection also dramatically reduces energy of other noise interferer signals. It should be noted that one of the two filters admits frequency components higher than the desired bandwidth on the positive frequency axis and the other of the two filters only admits frequency components lower than the desired signal bandwidth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention will be described with reference to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of a typical communication receiver of prior art; 
         FIG. 2  is a block diagram of time domain impulse noise mitigation of prior art; 
         FIG. 3  is a block diagram of frequency domain impulse noise mitigation of prior art; 
         FIG. 4  is a block diagram of the present invention of frequency domain impulse noise mitigation; 
         FIG. 5  is a frequency domain diagram showing the effects of the impulse noise mitigation relative to the desired signal, and 
         FIG. 6  is a method for impulse noise mitigation of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 4  is shown a block diagram of the frequency domain impulse noise mitigation  40  of the present invention. The output of an analog tuner  41  is connected to an ADC  42  to convert the receiver analog signal into a digital signal. The output  43  of the ADC  42  is coupled to the impulse noise mitigation circuit  40  for the purpose of impulse noise detection. The output of the ADC  43  is also connected to the suppressor circuit  57 . When impulse noise is detected from signal samples applied to the filters  44  and  48 , these samples will be flagged with an impulse noise indicator, and then the mitigation is applied to the corresponding ADC output connected directly to the suppressor circuit  57 . 
     Within the impulse noise mitigation circuitry  40  are two complex high pass filters  44  and  48 , filter 1 and filter 2 respectively. Each of filter output is connected to a magnitude function  45  and  49 , respectively, in which outputs of the magnitude functions are connected to accumulator circuits  46  and  50 , respectively. After accumulation over T samples, filter selection  52  selects one of Filter1  44  and Filter2  48  by comparing accumulator outputs  47  and  51 . The unselected filter can be disabled hereafter to reduce power consumption. The selected filter output is connected to a magnitude, or gain, function  55  that is connected to a comparator  56 . A detection threshold is either a fixed to a predetermined value, or dynamically adjusted based on the output of the magnitude function  55 . The suppressor circuit  57  connects the noise mitigated signal to the demodulator  58  and either clips the signal samples that are determined to be impulse noise, or nulls out the corrupted samples. A state machine  59  controls the operation of the impulse noise mitigation circuitry  40 , including which filter to activate, evaluation of the mean magnitude over T samples for each filter and the filter chosen to mitigate the impulse noise and any interferers. 
     It should be noted that it is within the scope of the present invention that a single programmable filter can be used, wherein both filters are integrated together and are separately selectable. The programmable filter is first configured similar to filter1  44  and the mean magnitude u1 is measured over T samples. Then the programmable filter is configured similar to filter2  48  and the mean magnitude u2 is measured over T samples. The two mean magnitudes u1 and u2 are compared, and the programmable filter is configured according to the method shown in  FIG. 6 . 
     It should also be noted that by using only one filter and disabling the other filter, power consumption can be improved. The purpose of using two filters is to detect impulse noise under out-of-band interference conditions, which is a drawback of the prior art shown in  FIG. 3 . Thus the present invention improves impulse noise detection by selecting a filter without out-of-band interference. 
       FIG. 5  demonstrates the effects of the two complex high pass filters of the impulse noise mitigation method of the present invention in the frequency domain. Filter1 only allows frequency components higher than the cut-off frequency of filter1 to pass through the impulse noise mitigation circuit  40  and filter2 only allows frequency components lower than the cut-off frequency of filter2 to pass through the impulse noise mitigation circuit. By collecting the mean signal data over a time duration (or samples) T for each filter the state machine  59  selects which filter to use to detect impulse noise under significant out-of-band interference condition. 
       FIG. 6  demonstrates the method for impulse noise mitigation of the present invention. Under control of the state machine  59  the response of the first filter to impulse noise and other interferer frequencies is measured  60 . The mean magnitude (u1) of the effects on the incoming signal is computed and stored for the first filter  61  over a time duration (or sample) of “T”. Then the response of the second filter to impulse noise and other interferers is measured  62 , and the mean magnitude (u2) of the effects on the incoming signal is computed and store for the second filter  63  over a time (or sample) duration of “T”. If the computed mean value “u2” is greater than the computed mean value “u1”  64 , then the first filter is selected and used for impulse noise detection and mitigation  65 . Otherwise if “u1” is greater than or equal to “u2”  66 , the second filter is selected and used for impulse noise detection and mitigation  65 . The unselected filter can be disabled hereafter to reduce power consumption. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.