Abstract:
An alerting device allowing easy estimation of possible interference and useful suggestions to eliminate deteriorated communication quality due to interference is disclosed. A signal including a band used by a target radio communication system is selected from a received signal based on a prescribed system specification of the radio communication system. A detected signal level of the selected signal is detected and compared with the prescribed interference level. When the detected signal level is greater than the prescribed interference level, it is determined that interference occurs. Further, an interference source is estimated by analyzing the selected signal with respect to time and frequency components.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to interference monitoring techniques and in particular to a technique which alerts a user to deteriorated communication quality caused by occurrence of interference in a radio communication system. 
   2. Description of the Related Art 
   In recent years, it has become a common practice to obtain access to the Internet from ordinary households. In order to eliminate the inconvenience and disorder of cabling from a telephone line to information devices within a home, wireless connections for Internet access have received widespread attention. 
   As such a wireless communication system, the followings are known: Wireless LAN (Local Area Network) conforming to IEEE802.11b standards (e.g. IEEE std 802.11b-1999, Supplement to IEEE Standard for Information technology—Telecommunications and information exchanger between systems—Local and metropolitan area networks Specific requirements—Part 11: wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer (PHY) Extension in the 2.4 GHz Band); and Wireless system called “Bluetooth™” which is defined in Specification of the Bluetooth™ System Version 1.0B. These wireless systems are used in the 2.4 GHz band. 
   In radio communications, quality of communication is frequently affected by the presence of other electric equipment generating radio waves at frequencies in the same band. In order to avoid the effect of interference from such other electric equipment, several techniques have been proposed. In Japanese Patent Application Unexamined Publication No. 07-143090, a received electric field strength is compared with a reference level to produce a level signal and, when the period of high level is longer than the time width of a time slot, an alarm is generated. In Japanese Patent Application Unexamined Publication No. 2000-022712, a channel use state of its own radio communication system is measured for each channel and a channel use state (for example, interference power) of other radio communication system is also measured. When interference is detected, a channel to be used for its own radio communication system is automatically changed. 
   The above prior arts are designed to decrease the effect of interference when the radio communication system is in operation. In an ordinary household or the like, however, it is preferable to previously check whether interference occurs, and the result is used to design a radio communication system within the household to provide no interference. 
   In the case where a radio communication system has no interference avoiding technique such as the above prior art implemented therein, it is difficult to handle deteriorated performance problems cause by occurrence of interference. As a result, an expected communication performance cannot be obtained. 
   In Japanese Patent Application Unexamined Publication No. 05-136747, a directional coupler is connected between a radio transmitted an antenna to detect an unexpected reflected wave, which results in preventing damage to the transmitter and the antenna. In this publication, it is suggested that such technique can be also applied to interference detection. However this conventional technique can be applied to a radio transmitter. Accordingly, possible interference cannot be estimated until such a radio transmitter is possessed. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an alerting device and interference monitoring method allowing easy estimation of possible interference in a radio communication system. 
   Another object of the present invention is to provide an alerting device and method that can provide suggestions to eliminate deteriorated communication quality due to interference. 
   According to the present invention, an alerting device for alerting a user to occurrence of interference on a band used in a wireless communication system, includes: a determiner for determining whether an interference source exists, based on system identifying information prescribed in the wireless communication system; and an indicator for indicating that interference occurs on the band when it is determined that the interference source exists. 
   The determiner may include an interference detector for detecting occurrence of interference based on interference information included in the system identifying information. 
   Preferably, the determiner may further include an estimator for estimating an interference source from a received signal when the occurrence of interference is detected. 
   According to an aspect of the present invention, an alerting device for alerting a user to occurrence of interference in a radio communication system, includes: a signal extractor for extracting an interference component from a received signal based on system identifying information prescribed in the radio communication system; an interference detector for detecting whether the interference component deteriorates quality of communication, to produce an interference detection signal; and an indicator for notifying a user of occurrence of interference when the interference detection signal indicates deteriorated quality of communication. 
   According to another aspect of the present invention, an alerting device for alerting a user to occurrence of interference in a radio communication system, includes: a signal extractor for extracting an interference component from a received signal based on system identifying information prescribed in the radio communication system; an interference detector for detecting whether the interference component deteriorates quality of communication, to produce an interference detection signal; an estimator for estimating an interference source from the interference component when the interference detection signal indicates deteriorated quality of communication; and an indicator for notifying a user of occurrence of interference and the interference source when the interference detection signal indicates deteriorated quality of communication. 
   According to the present invention, an alerting method for alerting a user to occurrence of interference on a band used in a wireless communication system, includes the steps of: determining whether an interference source exists, based on system identifying information prescribed in the wireless communication system; and indicating that interference occurs on the band when it is determined that the interference source exists. 
   The determining step may include the step of: detecting occurrence of interference based on interference information included in the system identifying information. 
   The alerting method preferably further includes the step of: estimating an interference source from a received signal when the occurrence of interference is detected. 
   As described above, according to the present invention, an interference occurring in the radio communication system is detected based on specification data such as the center frequency and bandwidth of a band used by the radio communication system. When interference occurs, a user is notified by an alert such as sound, light, or vibration. 
   In addition it is preferable to notify the user of not only the occurrence of interference but also its interference source. 
   The 2.4 GHz band used as ISM(Industry Science and Medical) band is also used by not only wireless system such as wireless LAN conforming to IEEE802.11b and Bluetooth™ but also microwave ovens used in ordinary households. Accordingly, radio wave interference will increase more and more. Although a specialist in radio communications may easily estimate an interference source causing the interference, it is difficult for ordinary users. 
   It is possible to estimate influence on a target radio communication system by measuring power on its frequency band based on the center frequency and bandwidth used by the target radio communication system. For ordinary users, it is important to know whether interference occurs and further useful to know what causes the interference. 
   Envelope level patterns of radiated signals vary by electronic equipment. A signal envelope of a signal radiated by ordinary home-use microwave oven is a pulse-like signal having a width of approximately 20 msec, as indicated in Ad Kamerman, Nedim Erkocevic, “Microwave Oven Interference on Wireless LANs Operating in the 2.4 GHz ISM Band” (IEEE, Personal, Indoor and Mobile Communications, 1997. Waves of the year 2000, PIMRC&#39;97., The 8th IEEE International Symposium on, Volume:3, 1997). 
   As for Bluetooth™, a signal envelope of a radiated signal varies depending on a length of data to be transmitted. In the case of 1500-byte data, its signal envelope is a pulse-like signal having a width of approximately 1 msec. In the case of IEEE802.11b, the length of a transmission signal is defined to be one of 625 μsec, 625×3 (μsec) and 625×5 (μsec). Accordingly, its envelope of a radiated signal is a pulse-like signal having a width of one of these. 
   Further, a spectrum of IEEE802.11b has a bandwidth of 20 MHz per channel, a spectrum of Bluetooth™ has a bandwidth of 1 MHz per channel, and a spectrum of home-use microwave oven has a bandwidth of the order of 15 MHz. 
   Accordingly, it is possible to estimate an interference source by analyzing a frequency band used by a target communication system with respect to pulse length on time axis and spectrum on frequency axis. For example, when a microwave oven is estimated to be an interference source, it is easy for a user in an ordinary household to take an appropriate step, for example, to stop usage of the microwave oven. In this manner, the alerting device according to the present invention allows easy detection of interference in a radio communication system and can suggest what to do so as to avoid deterioration in communication quality cause by interference, 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is block diagram showing an alert generating device according to a first embodiment of the present invention; 
       FIG. 2  if a flow chart showing an  interference monitoring method employed in the alert generating device according to the first embodiment of the present invention; 
       FIG. 3  is a block diagram showing the circuit of a band selector in the first embodiment of the present invention; 
       FIG. 4  is a block diagram showing a circuit including an interference detector and an interference decision circuit in the first embodiment of the present invention; 
       FIG. 5  is a block diagram showing an alert generating device according to a second embodiment of the present invention; 
       FIG. 6  is A flow chart showing an interference monitoring method employed in the alert generating device according to the second embodiment of the present invention; 
       FIG. 7  is a block diagram showing the circuit of an interference analyzing section in the second embodiment as shown in  FIG. 5 ; 
       FIG. 8  is a block diagram showing the circuit of a time-domain analyzer as shown in  FIG. 7 ; 
       FIG. 9  is a block diagram showing the circuit of a frequency-domain analyzer as shown in  FIG. 7 ; 
       FIG. 10  is a block diagram showing the circuit of the interference analyzer as shown in  FIG. 7 ; 
       FIG. 11  is a diagram showing an example of contents stored in an interference pattern memory as shown in  FIG. 10 ; 
       FIG. 12  is a block diagram showing the circuit of the interference decision circuit as shown in  FIG. 5 ; 
       FIG. 13  is a diagram showing time-varying envelope levels of radiating electromagnetic waves in wireless LAN conforming to IEEE802.11b, Bluetooth™, and microwave oven; and 
       FIG. 14  is a spectrum diagram of radio-frequency signals generated by wireless LAN conforming to IEEE802.11b, Bluetooth™, and electromagnetic waves radiating from microwave oven; and 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   Referring to  FIG. 1 , an alerting device  1  according to a first embodiment of the present invention includes a band selector  2 , an interference detector  3 , an interference decision section  4 , and an interference indicator  5 . In this embodiment, the alerting device  1  is further provided with a baseband transceiver  6 , a radio-frequency transmitter  7 , a recording medium  8 , a receiving antenna  10 , an input terminal  11 , a transmitting antenna  12 , and an input/output terminal  13 . 
   The above functions of the alerting device  1  may be implemented with hardware or software. A program-controlled processor (not shown) may be provided in the alerting device  1  to implement the above functions and perform the interference monitoring and alert generating operations by running corresponding programs that are previously stored in the recording medium  8 . Hereafter, the interference monitoring and alert generating operations will be described with reference to  FIG. 2 . 
   1.1) Operation 
   Referring to  FIG. 2 , it is determined whether identifier information for identifying a target radio communication system is inputted at the input terminal  11  (step S 1 ). The identifier information includes three items of information as follows: 1) Center frequency of a frequency band used by the target radio communication system, 2) Band width of the frequency band, and 3) Interference level affecting the target radio communication system. Alternatively, the alerting device  1  inputs only an identifier for identifying a target radio communication system and convert it to the above three items of information. 
   When the identifier information is inputted (YES in step S 1 ), the information of center frequency Fc and bandwidth W are output to the band selector  2  and the interference level LI is output to the interference decision section  4 . 
   The band selector  2  inputs received radio signals from the antenna  10  and selects a radio signal on a band including a target band used by the target radio communication system depending on the information of center frequency Fc and bandwidth W that are inputted through the input terminal  11  (step S 2 ). The received signal selected by the band selector  2  is output to the interference detector  3 . 
   The interference detector  3  detects an interference level from the received signal to output it as an interference level signal to the interference decision section  4  (step S 3 ). 
   The interference decision section  4  inputs the detected interference level signal from the interference detector  3  and the interference level LI from the input terminal  11 . The interference decision section  4  compares them to determine which is greater (steps S 4  and S 5 ). 
   When the detected interference level signal is greater than the interference level LI (YES in step S 5 ), it is determined that interference occurs and the interference decision section  4  outputs an interference occurrence signal to the interference indicator  5  (step S 6 ). 
   When receiving the interference occurrence signal from the interference decision section  4 , the interference indicator  5  notifies a user of occurrence oh interference by appropriate means, for example, sound, light, vibration or the like. 
   The alerting device  1  according to the present embodiment is integrated with a radio transceiver for the target radio communication system. For this, the alerting device  1  is provided with the baseband transceiver  6 , the radio-frequency transmitter  7 , the transmitting antenna  12 , and the input/output terminal  13 . 
   Since the signals selected by the band selector  2  include signals on the band used by the target radio communication system, it is possible to perform a receiving operation of the transceiver by the baseband transceiver  6  demodulating the received signal to produce a baseband received signal, which is output to the input/output terminal  13 . A transmission operation of the transceiver can be performed by the baseband transceiver  6  modulating a transmission signal inputted through the input/output terminal  13  to produce a high-frequency transmission signal, which is transmitted by the RF transmitter  7  through the transmitting antenna  12 . 
   1.2) Band Selector 
   Referring to  FIG. 3 , the band selector  2  is composed of an oscillator  21 , a down converter  22 , and a band-pass filter  23 . The band selector  2  inputs received radio signals from an input terminal  20  connected to the receiving antenna  10 . An oscillation frequency of the oscillator  21  and a passing frequency band of the band-pass filter  23  are controlled depending on respective ones of the center frequency Fc and bandwidth W determined by the identifier information inputted through the input terminal  11 . 
   Accordingly, the received radio signals are converted into intermediate-frequency signals by the down converter  22  and then the intermediate-frequency signals are restricted by the band-pass filter  23  into signals on a controlled band and bandwidth that are used in the target radio communication system. At the same time, noise and other interference components can be filtered out by the band pass filter  23 . The received signal in this way is output to the interference detector  3  and the baseband transceiver  6  through an output terminal  24 . 
   1.3) Interference Detection and Decision 
   Referring to  FIG. 4 , the interference detector  3  is composed of an envelope detector  31 , and an analog-to-digital converter  32 . The interference decision section  4  is composed of a threshold memory  41  and a comparator  42 . 
   The envelope detector  31  inputs the selected signal from the band selector  2  through an input terminal  30  and detects an envelope thereof. For example, a log amplifier may be used as the envelope detector  31 . The detected envelope Env is converted into digital by the AD converter  32  and is then output to the comparator  42  of the interference decision section  4 . 
   The interference decision section  4  inputs the interference level LI from the input terminal  11  and stores it in the threshold memory  41 . The comparator  42  compares the detected envelope Env inputted from the AD converter  32  of the interference detector  3  with the interference level LI stored in the threshold memory  41 . When Env&gt;LI, it is determined that interference occurs and the comparator  42  outputs an interference occurrence signal to the interference indicator through an output terminal  43 . 
   Second Embodiment 
   According to a second embodiment of the present invention, an alerting device  100  can notify users of not only occurrence of interference but also source of interference. The details of the second embodiment will be described hereinafter. 
   Referring to  FIG. 5 , the alerting device  100  includes a receiving antenna  110 , an input terminal  111 , a band selector  200 , an interference analyzing section  300 , an interference detector  400 , an interference decision section  500 , and an interference indicator  600 . 
   In  FIG. 5 , the receiving antenna  110 , the input terminal  111 , the band selector  200 , and the interference detector  400  have the same functions as those of the first embodiment as shown in  FIG. 1  and therefore the details thereof will be omitted. 
   As in the case of the first embodiment, it is possible to integrate the above functions with a radio transceiver for the target radio communication system. 
   The above functions of the alerting device  100  may be implemented with hardware or software. A program-controlled processor (not shown) may be provided in the alerting device  100  to implement the above functions and perform the interference monitoring and alert generating operations by running corresponding programs that are previously stored in the recording medium  700 . Hereafter, the interference monitoring and alert generating operations will be described with reference to  FIG. 6 . 
   2.1) Operation 
   In  FIG. 6 , steps S 11 –S 13  are the same as the steps S 1 –S 3  of  FIG. 2 . Briefly, it is determined whether identifier information for identifying a target radio communication system is inputted at the input terminal  111  (step S 11 ). The identifier information includes three items of information as follows: 1) Center frequency of a frequency band used by the target radio communication system, 2) Band width of the frequency band, and 3) Interference level affecting the target radio communication system. When the identifier information is inputted (YES in step S 11 ), the information of center frequency FC and bandwidth W are output to the band selector  200  and the interference level LI is output to the interference analyzing section  300  and the interference decision section  500 . 
   The band selector  200  inputs radio signals from the antenna  110  and selects signals including signals on a target band used by the target radio communication system depending on the information of center frequency Fc and bandwidth W that are inputted through the input terminal  111  (step S 12 ). The received signal is output to the interference analyzing section  300  and the interference detector  400 . 
   The interference detector  400  detects an interference level from the received signal to output it as an interference level signal to the interference analyzing section  300  and the interference decision section  500  (step  313 ). 
   The interference analyzing section  300  analyzes the received signal inputted from the band selector  200  using interference level L 1  inputted from the input terminal  111  and the interference level signal inputted from the interference detector  400  to estimate a source of interference (step S 14 ) The estimation result is output to the interference decision section  500 . 
   The interference decision section  500  inputs the detected interference level signal from the interference detector  400 , the estimation result from the interference analyzing section  300 , and the interference level LI from the input terminal  111 . The interference decision section  500  compares the detected interference level signal with the interference level LI to determine whether interference occurs (steps S 15  and S 16 ) When the detected interference level signal is greater than the interference level LI (YES in step S 16 ), it is determined that interference occurs, and the interference decision section  500  outputs an interference occurrence signal to the interference indicator  600 . In addition, the interference decision section  500  outputs interference source information to the interference indicator  600  based on the estimation result inputted from the interference analyzing section  300  (step  317 ). 
   When receiving the interference occurrence signal and the interference source information from the interference decision section  500 , the interference indicator  600  notifies a user of not only occurrence of interference but also its interference source by appropriate means, for example, sound, light, vibration or the like. 
   2.2) Interference Analysis 
   Referring to  FIG. 7 , the interference analyzing section  300  includes a time-domain analyzer  320  a frequency-domain analyzer  330 , and an interference analyzer  340 . 
   The received signal is inputted from the band selector  200  through an input terminal  311  and are transferred to both the time-domain analyzer  320  and the frequency-domain analyzer  330 . The interference level signal is inputted from the interference detector  400  through an input terminal  312  and is transferred to the frequency-domain analyzer  330 . The interference level LI is inputted from the input terminal  111  through an input terminal  313  and is transferred to both the time-domain analyzer 320  and the frequency-domain analyzer  330 . 
   Based on these signals, the time-domain analyzer  320  and the frequency-domain analyzer  330  analyze the received signal to produce a duration of interference and a frequency spectrum thereof. These analysis results are output to the interference analyzer  340 . 
   The interference analyzer  340  estimates an interference source based on the time-domain analysis result and the frequency-domain analysis result and outputs the estimation result to the interference decision section  500  through an output terminal  314 . 
   2.2.1) Time-domain Analyzer 
   Referring to  FIG. 8 , the time-domain analyzer  320  includes a comparator  321 , a threshold memory  322 , an adder  323 , a maximum-value holding (max-hold) circuit  324 , a memory  325 , and a timer  326 . 
   Interference level signals are inputted from the interference detector  400  through an input terminal  327  and sequentially enter the comparator  321 . The interference level LI is inputted from the input terminal  111  through an input terminal  328  and is stored as a threshold value in the threshold memory  322 . 
   The comparator  321  sequentially compares the interference level signals with the interference level LI stored in the threshold memory  322 . When an interference level signal is greater than the interference level LI, the comparator  321  sets its output to 1. When the interference level signal is not greater than the interference level LI, the comparator  321  sets its output to 0. 
   The adder  323  adds the comparison output of the comparator  321  to a currently held value. stored in the memory  325  and the resultant value is stored in the memory  325  again. In other words, a currently held value of the memory  325  is incremented when the comparator  321  outputs 1. When the comparator  321  output 0, the memory  325  is reset to 0. Accordingly, the output of the adder  323  indicates the number of times the comparator  321  has successively outputted 1. 
   The output of the adder  323  is held in the max-hold circuit  324 , which holds the maximum number of times the comparator  321  has successively outputted 1. The timer  326  is set for a predetermined measurement time period. When time-out occurs, the value held in the max-hold circuit  324  is output as a time domain analysis result to the interference analyzer  340  through an output terminal  329 . Accordingly, the time-domain analysis result indicates a maximum time length during which the detected interference level signal exceeds the threshold value (interference level LI). In the words, the time-domain analysis result indicates a duration of interference. 
   2.2.2) Frequency-domain Analyzer 
   Referring to  FIG. 9 , the frequency-domain analyzer  330  has an input terminal  3301  for inputting the interference level LI from the input terminal  111 , an input terminal  3300  for inputting a detected interference signal from the interference detector  400 , and an input terminal  3302  for inputting a received signal from the band selector  200 . 
   The interference level LI is inputted through the input terminal  3301  and is stored as a threshold value in the threshold memory  3312 . The detected interference signal inputted through the input terminal  3300  enters a gain controller  3305 , which controls the gain of a variable-gain amplifier  3303 . The variable-gain amplifier  3303  amplifies the received signal with a controlled gain. In other words, the received signal is adjusted to an appropriate level based on the interference level signal detected by the interference detector  400 . 
   The received signal amplified by the variable-gain amplifier  3303  is quadrature-demodulated into a demodulated signal composed of I and Q-components by a quadrature demodulator  3304 . The quadrature demodulator  3304  outputs the demodulated signal composed of I- and Q-components (denoted by a slashed signal line in  FIG. 9 ) to a low-pass filter  3306 . The low-pass filter  3306  filters out components of double the frequency of the demodulated signal. The demodulated signal passing through the low-pass filter  3306  is converted to a digital signal by an analog-to-digital converter  3307 . The digital signal is outputted to a serial-to-parallel (S/P) converter  3308 , where the digital signal is converted every n serial signals to n parallel signals. The n parallel signals are output to a Fast-Fourier Transform circuit (FFT)  3309 . 
   The FFT  3309  performs FFT computation for n points to produce n frequency components included in the n serial signals which are output to a corresponding one of n power detectors  3310 - 1  through  3310 -n. The detected power for each signal is output to a corresponding one of comparators  3311 - 1  through  3311 -n, where the detected power is compared with the threshold value (the interference level LI) stored in the memory  3312 . 
   A comparator  3311 -i (i- 1 ,  2 , . . . , n) outputs 1 when a corresponding detected power is greater than the interference level LI and outputs 0 otherwise. Then output values each being 1 or 0 of the comparators  3311 - 1  through  3311 -n are converted to n serial values by a parallel-to-serial converter  3313 , for example, such that the n output values are sorted in ascending order of frequency. 
   The parallel-to-serial converter  3313  outputs a reset signal to a counter  3314  before outputting the n serial values. The counter  3314  is incremented when inputting “1” and is reset when inputting “0”. The count value of the counter  3314  is outputted to a max-hold circuit  3315 . Such an arrangement causes the max-hold circuit  3315  to hold a value indicating the number of consecutive bands having components exceeding the interference level LI. In other words, this value is equal to a maximum frequency bandwidth causing interference. 
   A timer  3316  is set for a predetermined measurement time period. When a time-out occurs, the value held in the max-hold circuit  3315  is output as a frequency-domain analysis result to the interference analyzer  340  through an output terminal  3317  (see  FIG. 7 ). 
   2.2.3) Interference Analyzer 
   Referring to  FIG. 10 , the interference analyzer  340  may be composed of an interference pattern memory  343 , The interference pattern memory  343  inputs time-domain and frequency-domain analysis results from the time-domain and frequency-domain analyzers  320  and  330  through input terminals  341  and  342 , respectively. 
   More specifically, the interference pattern memory  343  retrievably stores data of interference sources generating interference patterns, each of which is identified by a pair of time-domain and frequency-domain analysis results inputted from the time-domain and frequency-domain analyzers  320  and  330 . Accordingly, the time-domain and frequency-domain analysis results cab be used as an access address of the interference pattern memory  343  to estimate an interference source radiating that interference. An example of the data stored in the interference pattern memory  343  is shown in  FIG. 11 . 
   As shown in  FIG. 11 , the interference pattern memory  343  stores a table containing data of interference sources such as wireless LAN conforming to IEEE802.11b, Bluetooth™, and typical home-use microwave oven. The- rows of the table are labeled with frequency bandwidths in ascending order obtained by the frequency-domain analysis result and the columns are labeled with durations in ascending order obtained by the time-domain analysis result. Therefore, by inputting a pair of time-domain and frequency-domain analysis results, an interference source radiating the interference in question can be estimated. 
   For example, when the time-domain analysis result indicates that a duration of interference is 1000 μsec and the frequency-domain analysis result indicates that a frequency bandwidth is 8 MHz, the interference source is estimated to be “microwave oven”. 
   2.3) Interference Decision 
   Referring to  FIG. 12 , the interference decision section  500  includes a threshold memory  504 , a comparator  505 , and a multiplexer  506 . The interference level LI is inputted through an input terminal  501  and is stored as a threshold level in the threshold memory  504 . The detected interference level signal inputted from the interference detector  400  through an input terminal  502  enters the comparator  505 . 
   The comparator  505  compares the detected interference level signal with the threshold level (interference level LI) stored in the memory  504 . When the detected interference level signal is greater than the threshold level (interference level LI), it is determined that interference occurs and the comparator  505  output 1 to the multiplexer  506 . When the detected interference level signal is not greater than the threshold level (interference level LI), it is determined that no interference occurs and the comparator  505  output 0 to the multiplexer  506 . 
   The multiplexer  506  inputs the comparison result form the comparator  505  and the interference source estimation result from the interference analyzing section  300  through an input terminal  503 . The multiplexer  506  multiplexes the comparison result and the interference source estimation result to output it to the interference indicator  600 . 
   Examples of Interference Source 
   Referring to  FIG. 13 , A indicates an envelope of a signal radiated by microwave oven, B by Bluetooth™-compliant device, and C by IEEE802.11b-compliant device. 
   Envelope level patterns of radiated signals vary by electronic equipment. A signal envelope of a signal radiated by ordinary home-use microwave oven is a pulse-like signal having a width of approximately 20 msec, as indicated in Ad Kamerman, Nedim Erkocevic, “Microwave Oven Interference on Wireless LANs Operating in the 2.4 GHz ISM Band” (IEEE, Personal, Indoor and Mobile Communications, 1997. Waves of the year 2000, PIMRC&#39;97., The 8th IEEE International Symposium on, Volume:3, 1997). 
   As for Bluetooth™, a signal envelope of a radiated signal varies depending on a length of data to be transmitted. In the case of 1500-byte data, its signal envelope is a pulse-like signal having a width of approximately 1 msec. 
   In the case of IEEE802.11b, the length of a transmission signal is defined to be one of 625 μsec, 625×3 (μsec) and 625 ×5 (μsec). Accordingly, its envelope of a radiated signal is a pulse-like signal having a width of one of these. 
   Referring to  FIG. 14 , D- 1 , D- 2  and D- 3  indicate an example of spectrums on three ones of fourteen channels defined in IEEE 802.11b, F- 1  and F- 2  are an example of spectrums in Bluetooth™, and E is an example of spectrum of a signal radiated by a microwave oven. 
   A spectrum of IEE802.11b has a bandwidth of 20 MHz per channel, a spectrum of Bluetooth™ has a bandwidth of 1 MHz per channel, and a spectrum of home-use microwave oven has a bandwidth of the order of 15 MHz. 
   Accordingly, it is possible to estimate an interference source by analyzing a frequency band used by a target communication system with respect to pulse length on time axis and spectrum on frequency axis. For example, when a microwave  oven is estimated to be an interference source, it is easy for a user in an ordinary household to take an appropriate step, for example, to stop usage of the microwave oven. In this manner, the alerting device according to the present invention allows easy detection of interference in a radio communication system and can suggest what to do so as to avoid deterioration in communication quality cause by interference.