Patent Publication Number: US-2012045027-A1

Title: Noise reduction in wireless communication applications

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a system and method of noise reduction in wireless communication networks. 
     BACKGROUND OF THE INVENTION 
     Wireless communication networks are commonly used to transmit digital data. In many cases the wireless transmissions are susceptible to interferences in the transmission range, since other transmitters share the medium. Such interferences are especially common in dense urban environments, where many transmitters may be using common bandwidths. Additionally, interference may exist in military applications, where jamming devices may be used to deliberately interfere with transmissions. Further additionally, electromagnetic devices may unintentionally, provide noise that interferes with transmissions of a specific frequency. 
     When using repeaters, for example to enhance the range or intensity of desired transmissions, also the undesired transmissions are enhanced. As a result the opposite effect may be achieved, for example interference signals may become enhanced so that wireless communication cannot take place. 
     In some cases the interference may be more powerful than the transmission signals. A repeater that converts an input signal from analog to digital with a limited conversion range (e.g. 8-10 bits) to process the signal would waste most of the significant bits representing the interference signal, whereas the weaker communication signal would be represented only by a few of the bits and be represented with less accuracy, thus the dynamic range of the desired signal is reduced. 
     Accordingly, it would be desirable to design a device such as a repeater or signal preprocessor that can minimize or overcome the above two problems, namely mitigate external interference signals and improve the dynamic range of a digital representation of the transmitted signals of interest relative to the interference. 
     SUMMARY OF THE INVENTION 
     An aspect of an embodiment of the invention, relates to a noise reduction system for use in a wireless communication network. The noise reduction system uses two or more antennas with distinct receiving characteristics, such that each antenna provides a distinct input signal that is a combination of the impending signals at the location of the noise reduction system. One of the input antennas serves as a base signal. The noise reduction system estimates the correlation between the signal provided by each antenna and the base signal to identify a dominant component of the signals and then mitigate the dominant component. 
     In an exemplary embodiment of the invention, a filter is coupled to each input antenna except the one providing the base signal. Optionally, the filter performs the estimation and mitigation. In an exemplary embodiment of the invention, a summator combines the filtered signal and the base signal and provides the combined signal as feedback to the filter. Optionally, all the filtered signals are combined together and an average of the combined signals is output from the noise reduction system. In some embodiments of the invention, the filters are adaptive filters. Optionally, the filters may be trained to identify specific signals and almost completely remove such signals, for example jamming signals or periodic interference signals. 
     In some embodiments of the invention, the input signals are converted from analog to digital before being filtered. Optionally, the base signal may either be converted to a digital representation or provided as an analog signal to enhance accuracy. In some embodiments of the invention, the digital representation of the base signal may include more bits of accuracy than the digital representation of the filtered signals. 
     In some embodiments of the invention, the output of the noise reduction system is transmitted wirelessly to a receiving device. Alternatively or additionally, the output of the noise reduction system is transmitted via a cable connection to a receiving device. 
     There is thus provided according to an exemplary embodiment of the invention, a noise reduction system in a wireless communication network, including:
         two or more input antennas adapted to receive wireless signals; wherein each input antenna has a different receiving characteristic, so that the signals provided by the input antennas constitute a distinct combination of the received signals;   a filter coupled to each of the input antennas except one, wherein the filter is adapted to estimate the correlation between the signal provided by its associated input antenna and the signal provided by the input antenna not coupled to a filter and mitigate a dominant component of the received signals responsive to the estimation;   a summator that is adapted to combine the filtered signal and the signal provided by the input antenna not coupled to a filter and provide the result as feedback to the filter; and   wherein the filtered signals from all the filters are combined to form an output signal.       

     In an exemplary embodiment of the invention, all the signals provided by the input antennas are converted from analog to digital. Alternatively, only the signals provided by the input antennas that are coupled to a filter are converted from analog to digital. Optionally, the filters are adaptive filters. In an exemplary embodiment of the invention, the summator is an analog summator. Alternatively, the summator is a digital summator. Optionally, the noise reduction system further includes a signal generator adapted to provide a training signal for output from the noise reduction system. In an exemplary embodiment of the invention, the noise reduction system according further includes a controllable switch to prevent output of the input signals from the noise reduction system. In an exemplary embodiment of the invention, the output signal is transmitted wirelessly to a receiving device. Alternatively, the output signal is transmitted through a cable to a receiving device. Optionally, the system is adapted to be trained to identify characteristics of a signal and then mitigate the identified signal from further received signals.
         There is further provided according to an exemplary embodiment of the invention, a method of reducing transmission noise in a wireless communication network, including:
           providing wireless signals using a device with two or more input antennas; wherein each input antenna has a different receiving characteristic, so that the signals provided by the input antennas constitute a distinct combination of the received signals;   coupling a filter to each of the input antennas except one;   estimating the correlation between the signal provided by its associated input antenna and the signal provided by the input antenna not coupled to a filter;   mitigating a dominant component of the received signals responsive to the estimation to form a filtered signal; combining the filtered signal and the signal provided by the input antenna not coupled to a filter and provide the result as feedback to the filter; and   forming an output signal by combining the filtered signals from all the filters.   
               

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and better appreciated from the following detailed description taken in conjunction with the drawings. Identical structures, elements or parts, which appear in more than one figure, are generally labeled with the same or similar number in all the figures in which they appear, wherein: 
         FIG. 1  is a schematic illustration of a noise reduction system, according to an exemplary embodiment of the invention; 
         FIG. 2  is a schematic illustration of a dominant signal, a typical signal and a digital bit template for sampling a combined signal, according to an exemplary embodiment of the invention; 
         FIG. 3  is a schematic block diagram of the control of a noise reduction system, according to an exemplary embodiment of the invention; 
         FIG. 4  is a schematic block diagram of the control of an alternative noise reduction system, according to an exemplary embodiment of the invention; and 
         FIG. 5  is a schematic block diagram of the control of a noise reduction system with more than two input antennas, according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of a noise reduction system  100 , according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, noise reduction system  100  uses two or more input antennas  110  to accept wireless signals. Optionally, noise reduction system  100  includes a processing unit  105  to process the input signals and an output port  140  that is connected to an output antenna  170  and/or an output cable  180  to output the processed signal. Optionally, the processed signal is transmitted to the output port  140  and from there either transmitted wirelessly via output antenna  170  to remote receivers (e.g. mobile telephones  130 ) or transmitted via output cable  180  to another device  160  (e.g. a cellular base station or other communication devices). Optionally, noise reduction system  100  may be incorporated into the other device  160  to serve as the signal input of the device. 
     In an exemplary embodiment of the invention, each of input antennas  110  has a different receiving characteristic, so that the signals received by input antennas  110  will each be a distinct combination of the approaching signals, for example the input antennas  110  may be separated geometrically, have a distinct polarization factor or be influenced by other differentiating parameter. 
       FIG. 2  is a schematic illustration of a dominant signal  214 , a typical signal  212  and a digital bit template  200  for sampling a combined signal made up from the two, according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, noise reduction system  100  serves as a repeater for wireless transmissions, for example signals transmitted by mobile telephones  130 . Optionally, the desired signals are represented by typical signal  212  and other strong transmissions in the vicinity that are picked up by noise reduction system  100  are represented by dominant signal  214 . In some embodiments of the invention, the dominant signals  214  may originate from an interference device  120 , which may be an intentional interference device such as a signal jammer. Alternatively, dominant signal  214  may originate from a non-intentional interference device  120  such as an electronic device that produces undesirable signals of a specific bandwidth or range of bandwidths that coincide with the bandwidths of typical signal  212 . Further alternatively, dominant signal  214  may originate from random interference. 
     In some embodiments of the invention, dominant signal  214  is stronger than typical signal  212 , and input antennas  110  receive a linear combination of both signals. Optionally, straightforward conversion from analog to digital of the input signals would provide a digital representation of the signal wherein the undesired part of the signal (dominant signal  214 ) is represented by most of the bits of template  200  (e.g. bits  1 - 8  of template  200  designated by  204 ) and the desired part of the signal (typical signal  212 ) is only represented by a fraction of the bits (e.g. bits  1 - 4  of template  200  designated by  202 ). 
     In an exemplary embodiment of the invention, processing unit  105  is provided to mitigate dominant signal  214  so that its power level is closer to the power level of typical signal  212 . Optionally, if dominant signal  214  is predictable, for example because it follows a pattern that can be studied when typical signal  212  is not applied, or prior knowledge is provided to processing unit  105 , then dominant signal  214  can essentially be completely removed. 
       FIG. 3  is a schematic block diagram of noise reduction system  100 , according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, the first input antenna  110 , receives an input signal that is a combination of dominant signal  214  and typical signal  212 . Optionally, the second input antenna  110  receives an input signal that is a different combination of dominant signal  214  and typical signal  212 . 
     In an exemplary embodiment of the invention, the received signals are transferred from input antennas  110  to a first receiving unit  315  and a second receiving unit  315 N. Optionally, the receiving units  315  and  315 N include analog receiving circuitry, for example filters and low noise amplifiers. In an exemplary embodiment of the invention, the receiving units  315 ,  315 N also include analog to digital converters (ADC)  320 ,  320 N that may include basic digital filtering capabilities. In an exemplary embodiment of the invention, the digitized signal from the first receiving unit  315  is provided to a filter  325  which provides a filtered digitized signal. The filtered digitized signal and the digitized signal at the output of  320 N are combined together and produce a combined signal (e.g. one subtracted from the other) to mitigate the dominant signal and to balance the power of the dominant signal  214  and typical signal  212  relative to each other. Optionally, filter  325  includes a digital processor to perform the calculations related to adaptation of filter  325  and applying filter  325  to the signal in real-time. Optionally, filter  325  is an adaptive filter that can be trained to enhance its ability to assure that the combined signal will have a reduced component of the dominant signal. In an exemplary embodiment of the invention, filter  325  produces a filtered signal that is provided to a summation element  355  to combine by adding or subtracting the filtered signal with the digitized signal from the second receiving unit  315 N. Optionally, the adaptive filter  325  is trained to minimize the correlation between the input signal and the combined signal therefore the combined signal will contain a higher component of the desired signal and lower component of the dominant signal. 
     In an exemplary embodiment of the invention, processing unit  105  includes a signal generator  330 , a control  335  and a switch  350 , to control the use of processing unit  105 . In some embodiments of the invention, switch  350  may be opened to prevent the summated signal from being output by processing unit  105  and in some cases switch  350  may be closed to allow the summated signal to be output by processing unit  105 . Optionally, control  335  will instruct switch  350  if to open or close the switch. In an exemplary embodiment of the invention, when switch  350  is closed the summated signal is output to output port  140  so that it may transmit, filter and/or convert the output signal from digital to analog. 
     In some embodiments of the invention, the training of filter  325  is achieved by feeding the signal from the output of summation element  355  back into filter  325  to estimate the correlation between the combined signal and the signal received by first receiving unit  315 . Optionally, by applying known training algorithms such as least mean square (LMS), recursive least square (RLS) or other methods, filter  325  can be perfected to achieve the desired mitigation of the dominant signal  214  and adjust the power of the dominant signal  214  relative to the typical signal  212 . In an exemplary embodiment of the invention, the filtered signal is returned to summator  355  to output it to output port  140 . 
     In some embodiments of the invention, switch  350  is opened during the training process of noise reduction system  100 , so that no signal is output from noise reduction system  100  while it is being trained. Optionally, control  335  may instruct signal generator  330  to provide a pre-selected signal during training or at other times so that noise reduction system  100  will train to eliminate its own output signals, and prevent feedback interference. 
     When noise reduction system  100  handles cellular applications, for example serving as a repeater for mobile telephones, the position of the transmitters (e.g. mobile telephones) varies rapidly. Optionally, filter  325  may be updated frequently by applying a frequent training process. Optionally, training may be performed periodically or as a function of a signal to noise ratio (SNR) that is measured from the received transmissions, for example if the signal to noise ratio goes below a pre-selected value filter  325  will be re-trained. 
     In some embodiments of the invention, filter  325  does not have any prior knowledge to distinguish between the desired signals and undesired signals, so that filter  325  aims to mitigate the dominant signal and equate between the power of the dominant signal  214  and typical signal  212  based on an estimate or on a feedback process. 
     Alternatively or additionally, filter  325  may have prior knowledge regarding some of the parameters of the dominant signal  214  or the typical signal  212 , for example the filter may know characteristics of a mobile telephone transmission and be able to use this knowledge to enhance mitigation of the dominant signal  214 . Likewise dominant signal  214  may have a distinct “fingerprint” that can be used to filter out dominant signal  214 . Optionally, filter  325  may be initially trained to identify the “fingerprint” of dominant signal  214 , for example before the typical signals  212  are transmitted or in a different location where only the dominant signal is available. Optionally, filter  325  may be locked with this information for a specific period of time or it may be used as a starting point and afterwards trained dynamically. In some embodiments of the invention, the typical signals  212  may be turned off for a specific time period so that filter  325  may train with the dominant signal alone. Optionally, if the dominant signal  214  is identified or has a periodic pattern it may be almost completely removed 
     In some embodiments of the invention, the number of quantization bits used for the analog to digital conversion of the signal provided by first receiving unit  315  is not the same as the number of bits used for the second receiving unit  315 N. Optionally, more bits are used for the second receiving unit  315 N that is not filtered and fewer bits are used for the elements of the first receiving unit  315  and filter  325 , thus improving the accuracy of the resulting signal without increasing the cost of the system significantly. 
     Taking this option one step further is achieved by replacing the second receiving unit  315 N with an analog representation of the signal instead of enlarging the digitization resolution.  FIG. 4  is a schematic block diagram of an alternative noise reduction system  400 , according to an exemplary embodiment of the invention. Optionally, noise reduction system  400  is similar to noise reduction system  100  except that the second receiving unit  315 N does not include an analog to digital converter. Additionally, since filter  325  is a digital filter, the output of filter  325  needs to be converted back to analog format with a digital to analog converter  465 , so that it can be summated using an analog summator  455  with the signal from the second receiving unit  315 N. Optionally, if the summated signal is provided as feed back to filter  325  it needs to be converted again to digital form by an analog to digital converter  460 . In some embodiments of the invention, switch  350  supports analog signals instead of digital signals. Optionally, output port  140  is also designed to handle analog signals instead of digital signals. 
       FIG. 5  is a schematic block diagram of the elements of a noise reduction system  500  with more than two input antennas  110  according to an exemplary embodiment of the invention. Optionally, noise reduction system  500  is similar to noise reduction system  300  except that it is designed to accommodate multiple input antennas  110 . Optionally, each antenna  110  feeds a signal to a receiving unit  515 . Optionally, each receiving unit  515  processes the signal converts it to a digital signal using an analog to digital converter  520  and provides the digitized signal to a filter  525 . In an exemplary embodiment of the invention, a summator  555  accepts the signal from the last receiving unit  515 N and combines it to the signal from each filter (e.g. by adding or subtracting the two signals). Optionally, the combined signal is provided to the corresponding filter  525  as feedback. Additionally, summator  555  combines the processed signals from all the filters and averages them out to form an output signal for providing to output port  140  via switch  350 . Optionally, the use of additional input antennas  110  improve the accuracy of separating between the dominant signal  214  and the typical signal  212 , so that the dominant signal may be mitigated. Optionally, the use of multiple antennas enables mitigation of multiple noise sources (e.g. up to N−1, where N is the number of antennas). 
     It should be appreciated that the above described methods and apparatus may be varied in many ways, including omitting or adding steps, changing the order of steps and the type of devices used. It should be appreciated that different features may be combined in different ways. In particular, not all the features shown above in a particular embodiment are necessary in every embodiment of the invention. Further combinations of the above features are also considered to be within the scope of some embodiments of the invention. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims, which follow.