Abstract:
An apparatus for detecting microwave signals is suggested. The apparatus comprises an antenna for sending and receiving electromagnetic waves a circuit for detecting microwave signals and the signalization stage for generating an indication signal. In addition to that a method for detecting the presence of microwave signals is proposed. The method comprises the following steps 
     receiving and detecting the microwave signal; 
     correlating the received microwave signal with the previously stored microwave signal; 
     generating an indication signal if the correlation between the received and the stored microwave signals exceeds a predetermined threshold value.

Description:
TECHNICAL FIELD 
       [0001]    The invention is related to an apparatus and a method for detecting a microwave emitting device such as a microwave oven. In particular, the present invention is related to apparatus according to claim  1  and a method according to claim  5 . 
       BACKGROUND 
       [0002]    The deployment of sensor based systems offers many opportunities for providing new services and applications in the home. In particular, in the area of home networking, a Wi-Fi home gateway platform may include an interface with an advanced search and recommendation engine allowing home users to access their preferred or personalized content. In addition to that, background algorithms may utilize additional information which is collected in the home of the user to improve recommendations for media consumption or other purposes. Such kind of information includes e.g. time, date, and ambient temperature. In addition to that, it has been found useful to have information about the operating status of a microwave oven in a home. In conjunction with calendar and time information, the information about the operating status of a microwave oven enables an adapted algorithm to provide more insight into the living habits inside a home. 
         [0003]    Information about the operational status of home appliances in general is interesting information also with regard to improving the management of power consumption in homes. In this context, there is an increasing demand for data about energy consuming devices. Energy disaggregation is a common keyword for this kind of research activities. 
         [0004]    Combining the data collection in the home of a user with a residential gateway makes sense because the residential gateway provides an interface between a home network and a public network such as the Internet. The residential gateway comprises the full interaction between services and devices supported by the residential gateway which provides a number of additional enablers for supporting the home user. Multiple home devices are able to handle multiple media streams. The flows of media streams are directed to the most appropriate devices while other devices are informed about the incoming stream. Recording of media streams and associated information is supported if needed. Thus, in the gateway there is already plenty of information available to generate recommendations to users with regard to media consumption. Consequently, it also makes sense for the gateway to capture context information such as information about the operating status of a microwave oven in the home. 
         [0005]    Modern gateways already support algorithms generating user recommendations based on a database about user preferences. Typically the database is built up over a long term. More advanced technologies also utilize the context information related to the user preferences and habits. 
         [0006]    The context information includes e.g. the location of the user, activity, ambient temperature, lighting and others. Such kind of information can also include for example the operating status of a microwave oven indicating that at least one of the inhabitants of a home is about to have a warm meal. 
         [0007]    A today existing possibility to detect the operation of a microwave oven is to connect the microwave oven to the mains with a radio controlled plug and socket, e.g. commercially available from Cleode (France). The radio controlled socket sends out its identifier in conjunction with the information that power is drawn from it. The combination of information allows concluding the microwave oven is operative once a monitoring software links a specific socket identifier with the microwave oven. 
         [0008]    Taking this as a starting point the present invention aims at an alternative approach for detecting the operation of a microwave oven. 
       SUMMARY 
       [0009]    According to a first aspect the present invention suggests an apparatus for detecting microwave signals. The apparatus comprises an antenna for sending and receiving electromagnetic waves, a circuit for detecting microwave signals, wherein the circuit ( 408 ) for detecting microwave signals comprises a memory ( 408 ) for storing the signature of a specific microwave signal and the signalization stage for generating an indication signal if the correlation between the received and the stored microwave signals exceeds a predetermined threshold value. 
         [0010]    According to an embodiment of the invention the apparatus can comprise an FM demodulator. 
         [0011]    In an advantageous embodiment the circuit for detecting microwave signals generates a feedback signal to the FM demodulator for correcting the center frequency of the FM demodulator. 
         [0012]    According to a second aspect the present invention suggests a method for detecting the presence of microwave signals. The inventive method comprises the following steps: 
         [0013]    receiving and detecting a microwave signal; 
         [0014]    storing the signature of a specific microwave signal;
       correlating the received microwave signal with the previously stored signature of the microwave signal;   generating an indication signal if the correlation between the received and the stored microwave signals exceeds a predetermined threshold value.       
 
         [0017]    The development of the inventive method further comprises the step of adapting the center frequency of an FM demodulator used for the detection of the microwave signal. 
         [0018]    In this case the method can further comprise the step of incrementing the center frequency of the FM demodulator to improve the detection of the microwave signal. 
         [0019]    Advantageously the method can comprise the step of storing the signature of the received and detected microwave signal during an initialization phase. 
         [0020]    In yet another embodiment the inventive method can further comprise the step of
       utilizing a first correlation threshold value during the initialization phase; and   utilizing a second correlation threshold value which is higher than the first correlation threshold value after the initialization phase.       
 
         [0023]    Further advantages of the present invention will become apparent when reading the detailed description in connection with the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]    In the drawing, an embodiment of the present invention is shown. The same or similar components are labeled with same or similar reference numbers. 
           [0025]      FIG. 1  schematically illustrates a room with a residential gateway, a set-top box, and a microwave oven; 
           [0026]      FIG. 2  displays the envelope of a microwave oven signal; 
           [0027]      FIG. 3  shows a spectrogram of the signal shown in  FIG. 2 ; 
           [0028]      FIG. 4  shows a schematic block diagram of the residential gateway of  FIG. 1 ; 
           [0029]      FIG. 5  shows a schematic block diagram of a modulation detector included in the residential gateway of  FIG. 4 ; and 
           [0030]      FIG. 6  displays a flow diagram illustrating an algorithm used by the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]      FIG. 1  schematically illustrates a room  100 . In the room  100  there is a gateway device  103  providing an access point to external networks such as PSTN, cable TV, and Internet. The access to external networks is symbolized in  FIG. 1  by the double-headed arrow  101 . Further details of the gateway  103  will be described further below in connection with  FIG. 4 . The gateway device  103  is provided with several transmission antennas  104 . In the room there is also a set-top box  105  which is provided with several antennas  106 . In  FIG. 1  only two of the antennas  104  and  106 , respectively, are shown. In other embodiments of the present invention the gateway device  103  and the set-top box  105  are provided with only one antenna each. The antennas  104  and  106  enable a wireless bi-directional communication between the gateway device  103  and the set-top box  105 . This wireless communication is based on transmitted and received electromagnetic waves establishing wireless data communication between the devices. In  FIG. 1  the electromagnetic waves travelling between the gateway  103  and the set-top box  105  are symbolized by arrows  107 . 
         [0032]    In the room  100  there is also a residential microwave oven  108 . Usual residential microwave ovens are not radiation tight. The microwave oven  108  radiates leakage microwaves into the room  100  which are illustrated with arrow  109  in  FIG. 1 . 
         [0033]    The residential microwave oven  108  comprises a magnetron to generate microwaves. The magnetron turns on and off at the mains frequency, i.e. at 50 Hz in Europe and 60 Hz e.g. in the US. In operation, the microwave oven generates microwaves in the ISM 2.4 GHz band. 
         [0034]    In the ON mode the microwave oven signal is similar to a frequency modulated signal with a fixed carrier signal at 2.4 GHz and an instantaneous frequency that changes with time. 
         [0035]      FIG. 2  shows the envelope of the microwave oven signal of an exemplary microwave oven which is operated at 60 Hz mains frequency. The envelope cycles between an ON and OFF state with a periodicity of 16.7 ms corresponding to 5 divisions on the abscissa in  FIG. 2  (3.33 ms per division on the abscissa in  FIG. 2 ). The frequency sweep of the radiated power is less than 20 MHz around a central frequency of 2.46 GHz.  FIG. 2  shows a frequency sweep of the microwave oven signal during a few milliseconds between two transient phases. During the sweep, the radiated signal can be considered as an FM modulated signal with almost sinusoidal variation of the power. 
         [0036]      FIG. 3  shows a spectrogram of the microwave oven signal. Transient signals right before and after the frequency sweep are clearly visible. 
         [0037]    The radiation leakage of microwave ovens allowed by regulations is comparable to the power transmitted by WiFi devices and is thus detectable by a properly designed detector. 
         [0038]    The present invention makes use of radiation leakage in order to detect if the microwave oven  108  is in its ON or OFF state in room  100 . 
         [0039]      FIG. 4  shows the gateway  103  in greater detail. The connection of the gateway  103  to external networks is symbolized with arrow  101  interfacing with a MIMO device  401 . One output  402  of the MIMO device  401  is connected with a power amplifier  403 . By means of a selection, switch  404 , the power amplifier  403  is connected to the antenna  104  when the gateway  103  is in a sending mode. When the gateway  103  is in a receiving mode to receive electromagnetic waves (RF signal), then the selection switch  404  changes its state and connects the antenna  104  with a low noise amplifier  405 . The output of the low noise amplifier  405  is provided to an RF coupler  406 . The RF coupler  406  provides an output signal on the one hand to an FM demodulator  407  and on the other hand to an input  408  of the MIMO device  401 . The MIMO device forwards the received input signal to perform conventional signal processing in the gateway  103 . The FM demodulator  407  filters the electromagnetic wave which is received by the antenna  104 . The output of the FM demodulator  407  is provided to a microwave signal detection and storage circuit  408 . 
         [0040]    Since there is a certain variation in the radiation characteristics of different microwave ovens, the detection and storage circuit  408  records the characteristic “signature” of the microwave oven  108  during an initialization phase to improve the detection reliability. Also the exact central frequency of the microwave oven changes as a function of the operating temperature of the magnetron. In order to still limit the frequency sweep of the FM demodulator  407  to 10 to 20 MHz, a central frequency tracking system within the detection and storage circuit  408  provides a feedback signal  409  about the detected central frequency to the frequency demodulator  407 . The circuit  408  communicates an output signal to a signalization stage  410  which generates an indication signal if the circuit  408  has detected a microwave signal. Then the signalization stage  410  produces an indication signal  411  for further usage in the gateway  103 . The group of components comprising the RF coupler  406 , the FM demodulator  407 , the detection and storage circuit  408 , and the signalization stage  410  form together a detection and signalization unit  412  shown with a dashed line in  FIG. 4 . 
         [0041]      FIG. 5  shows the detection and signalization unit  412  of  FIG. 4  in greater detail. The RF signal received by the antenna  104  is coupled to the detection and signalization unit  412  by the RF coupler  406  and is provided to the FM demodulator  407 . 
         [0042]    The FM demodulator  407  comprises a passband filter  501  and a demodulator stage  502 . The detection and storage circuit  408  comprises a detection stage  503  allowing detection of presence or absence of microwave. The circuit  408  also comprises the storage for storing the signature of the specific microwave signal which is characteristic for the specific microwave oven  108 . The circuit  408  provides a feedback signal  409  of the detected center frequency of the microwave signal back to the demodulator stage  502  permitting a small frequency scan of 10 to 20 MHz of the demodulator stage  502  in spite of the fact that the center frequency of the microwave signal drifts e.g. as a function of the changing operating temperature of the magnetron of the microwave oven  108 . 
         [0043]    The output of the detection and storage circuit  408  is provided to the signalization stage  410  which generates an indication signal  411  indicating the presence or absence of a microwave signal. 
         [0044]      FIG. 6  illustrates a flow diagram of an algorithm for identifying the signature of the microwave signal and for tracking its center frequency fc. After the algorithm is started (step  601 ) the next step  602  is to memorize the actual signature of the microwave signal leaking out of the microwave oven  108  during an initialization phase. Typically, the initialization phase takes place when the detection and signalization unit  412  detects the microwave signal for the first time. For the first detection the center frequency fc of the FM demodulator  407  is set to the lower threshold value fcmin of a known interval of typical center frequencies fc of microwave signals emitted by microwave ovens. The interval is described as [fcmin, fcmax] where fcmin is the lower limit and fcmax is the upper limit of the typical center frequency fc. Hence, the center frequency fc of the FM demodulator  407  is equal to fcmin after the start of the algorithm. Step  603  detects the microwave signal signature. In step  604  it is decided if the microwave signal detection is good or not based on the comparison with a predetermined correlation coefficient or correlation threshold value. If the signal detection is not good then the center frequency fc is incremented by 1 MHz. The increment value of 1 MHz is only an example and it can be chosen differently in other embodiments of the present invention. In step  606  it is decided if the center frequency of the demodulator  407  has already reached its maximum value fcmax. If this is not yet the case, then the loop including the steps  603 ,  604 ,  605 , and  606  continues until it is decided in step  604  that the microwave signal detection is good. 
         [0045]    Alternatively, if fc has reached its maximum value fcmax (fc=fcmax) then the process of detecting the microwave signal restarts at step  602 , i.e. fc is reset to fcmin. 
         [0046]    If the microwave signal is detected for the first time during the initialization phase the correlation threshold value which is used to decide if the detection is good or not is set to a low value to facilitate the detection during the initialization phase. This value is called here first correlation threshold value. For this purpose the parameter M is set to zero in step  602  (M=0). If the microwave signal has been detected for the first time after the initialization phase, the parameter M is still at M=0 when the algorithm arrives at step  607  and consequently the algorithm continues with step  608  where the signature of the microwave signal is stored and where the parameter M is changed and set to M=1. For all consecutive detections the parameter M remains at M=1 and the received microwave signal is compared with the microwave signal signature stored in the memory and the correlation threshold value is put to a higher value which is called here second correlation threshold value. 
         [0047]    When a microwave signal is detected again by the FM demodulator  407 , then it is correlated to the stored microwave signal signature. If the measurement correlation coefficient is above the fixed threshold value then the microwave oven  108  is ON and an indication signal is transmitted in step  609  for further processing by an application running in the gateway  103 . 
       REFERENCE SIGNS LIST 
       [0048]      100  room 
         [0049]      101  double-headed arrow 
         [0050]      103  gateway device 
         [0051]      104  antenna 
         [0052]      105  set-top box 
         [0053]      106  antenna 
         [0054]      107  arrow (electromagnetic waves) 
         [0055]      108  microwave oven 
         [0056]      109  arrow (leakage microwaves) 
         [0057]      401  MIMO device 
         [0058]      402  output of MIMO device 
         [0059]      403  power amplifier 
         [0060]      404  selections switch 
         [0061]      405  low noise amplifier 
         [0062]      406  RF coupler 
         [0063]      407  FM demodulator 
         [0064]      408  detection and storage circuit 
         [0065]      409  feedback signal 
         [0066]      410  signalization stage 
         [0067]      411  indication signal 
         [0068]      412  detection and signalization unit 
         [0069]      501  passband filter 
         [0070]      502  demodulator stage 
         [0071]      503  detection stage 
         [0072]      504  storage for microwave signature 
         [0073]      601 - 609  method steps