Method and apparatus for visualizing radio wave in certain area

A method for visualizing radio waves which can only one sensor station to accurately specify the positions of a plurality of radio wave sources operating in the same frequency band and which can monitor a state of utilization of the radio waves. In a sensor station performing a radio wave hologram observation, one or more scanning antennas for scanning along a predetermined path and a plurality of fixed antennas having different directions and positions are provided. A signal received at the scanning antenna interferes with another signal received at the fixed antenna for each fixed antenna while the scanning antenna scans along the predetermined path, thereby obtaining a complex scanning data matrix for which complex weighting and addition processing is performed so that distribution of the radio wave sources is reconstructed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method and an apparatus for performing
 radio monitoring, and more particularly, to a method and an apparatus for
 performing radio monitoring in a certain area where radio waves are used
 at a relatively high density to prevent the generation of radio
 interference.
 2. Description of the Prior Art
 With development of a mobile communication system, a wireless local area
 network (LAN) or the like, radio waves are used at a high density in urban
 areas and limited frequency resources are shared. Consequently, radio
 interference is generated to the extent that it can not be negligible.
 When a radio wave source which causes the radio interference is specified,
 a sensor station (monitoring station) is conventionally placed in each of
 a plurality of points, the Yagi-Uda antenna or a goniometer is used in
 each sensor station to observe a direction in which a radio wave comes
 from the radio wave source, and the radio wave incoming directions at
 respective sensor stations are plotted on a map to specify the position of
 the radio wave source based on the position of an intersection point of
 the directions.
 Since the above-mentioned conventional method specifies the position of the
 radio wave source based on the intersection point of the incoming
 directions of the radio wave observed in the sensor stations, at least two
 sensor stations are required. Thus, this method has a disadvantage that it
 can specify only a radio wave source of a relatively strong radiation
 receivable simultaneously at two or more sensor stations. Additionally,
 when two or more radio wave sources are present at positions near to each
 other at the same frequency band, radio waves transmitted therefrom can
 not separated for observation, thereby obtaining a result of observing the
 incoming radio wave direction in the sensor station in the form of a
 synthesized radio wave. Thus, it may be impossible to estimate each
 position of the radio wave source in this case.
 The radio interference in urban areas may frequently occur even between
 radio wave sources which are close to each other and emit relatively weak
 radio waves. Also, the sensor stations can not be placed at a high density
 in consideration of costs or the like. Consequently, according to the
 conventional method, it is impossible to completely specify the generation
 source of the radio interference and it is extremely difficult to find the
 cause of the radio interference.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide a method for visualizing
 radio waves in a certain area which can use only one sensor station to
 specify the positions of a plurality of radio wave sources operating in
 the same frequency band and which can monitor a state of utilization of
 radio waves.
 Another object of the present invention is to provide an apparatus for
 visualizing radio waves in a certain area which can use only one sensor
 station to specify the positions of a plurality of radio wave sources
 operating in the same frequency band and which can monitor a state of
 utilization of radio waves.
 The first object of the present invention is achieved by a method for
 visualizing radio waves in a specific area for observing radio waves from
 radio wave sources to reconstruct a radio wave hologram to visualize
 distribution of the radio wave sources, the method comprising the steps
 of: providing one or more scanning antennas and a plurality of fixed
 antennas having different directions and/or positions in a sensor station;
 causing a signal received at the scanning antenna to interfere with a
 signal received at the fixed antenna for each of the fixed antennas to
 thereby obtain a complex scanning data matrix while the scanning antenna
 scans along a predetermined path; performing complex weighting addition
 processing for the complex scanning data matrix to reconstruct
 distribution of the radio wave sources.
 The second object of the present invention is achieved by an apparatus for
 visualizing radio waves in a specific area for observing radio waves from
 radio wave sources to reconstruct a radio wave hologram to visualize
 distribution of the radio wave sources, the apparatus comprising: one or
 more scanning antennas for scanning along a predetermined path; a
 plurality of fixed antennas having different directions and/or positions;
 an interference unit for causing a signal received at each of the scanning
 antennas to interfere with a signal received at each of the fixed antennas
 to output an interference signal; a detector for complex detecting the
 interference signal to obtain a detection output; and operation means for
 temporally storing as a complex scanning data matrix the detection output
 for each combination of one of the scanning antennas and one of the fixed
 antennas and performing complex weighting addition processing for the
 complex scanning data matrix to reconstruct distribution of the radio wave
 sources.
 According to the present invention, by employing the above-mentioned
 configuration, one sensor station can be used to separate radio wave
 sources extremely close to one another for observation, and plane
 distribution (two-dimensional distribution) and characteristic
 distribution of radio wave sources can be accurately obtained even in an
 area where radio waves are used at a high density.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 A general outlines of radio wave visualization in a certain area based on
 the present invention will be described with reference to FIG. 1. Shown in
 FIG. 1 is a typical urban area, in which references f1 to f3 represent
 transmission antennas of radio stations operating at frequencies f1 to f3,
 respectively. Assume that a radio station operating at frequency f1
 provided at a rooftop of building 14 and a radio station operating at
 frequency f2 provided at car 15 running on a street are interference radio
 stations IS which cause radio interference.
 For radio monitoring in a surveillance area, sensor station 10 is placed at
 a position which offers a broad view of the surveillance area, for example
 at the rooftop of a high-rise building. Sensor station 10 performs a radio
 wave hologram observation to thereby obtain information on the positions
 of respective radio wave sources in the surveillance area. Sensor station
 10 is connected to center station 11 through line 12. Center station 11
 also collects information from other areas to produce and output radio
 wave utilization monitoring diagram 13 which displays a state of
 utilization of radio waves in real time.
 Next, a configuration of an apparatus for visualizing radio waves used in
 the radio wave hologram observation in sensor station 10 will be described
 with reference to FIG. 2.
 Since a radio wave is a kind of wave motions, a radio wave hologram can be
 observed similarly as in the case of a light hologram. The radio wave
 hologram is reconstructed to thereby obtain a radio wave reconstructed
 image, from which the distribution and intensity of wave sources can be
 investigated. The radio wave reconstructed image from the radio wave
 hologram observation may be obtained by, for example, providing a fixed
 antenna and a scanning antenna which successively moves in a scanning
 observation plane, correlating a received signal at the fixed antenna with
 another received signal at the scanning antenna at a predetermined
 observation frequency to derive a complex correlation value at each point
 in the scanning observation plane. The two-dimensional arrangement of the
 complex correlation values is a two dimensional complex interferogram.
 Then, the radio wave reconstructed image is obtained by reconstructing the
 two-dimensional complex interferogram.
 Since sensor station 10 is placed at the rooftop of a high-rise building or
 the like as described above, it is desirable that the radio wave hologram
 observation can be performed in all directions (360.degree.) in a
 horizontal plane. Thus, a circular scanning type hologram observation
 apparatus disclosed in Japanese Patent Laid-open Publication No.
 65406/1999 (JP, 11065406, A) by the present inventor can be used as an
 apparatus for performing the radio wave hologram observation. However, a
 conventional radio wave hologram observation apparatus using a pair of a
 fixed antenna and a scanning antenna has a limited resolution of a
 reconstructed image due to a limitation on the observation plane or the
 like. When a plurality of radio wave sources are placed extremely close to
 one another, a complicated calculation is required to separate them.
 Thus, in this embodiment, in a circular scanning type hologram observation
 apparatus, a plurality of fixed antennas having different directions and
 positions are provided and these fixed antennas are alternatively switched
 for use. Also, for the scanning antenna, a plurality of antennas are
 provided as required to be alternatively switched for use. In later
 operation, a weighting coefficient is manipulated in weighting process,
 thereby making it possible to separate a plurality of radio wave sources
 for observation.
 In an apparatus shown in FIG. 2, a plurality of fixed antennas 21 having
 different directions and positions are provided such that a received
 signal of one of these fixed antennas 21 is taken by switch 22. Variable M
 is used to indicate to which of fixed antennas 21 switch 22 is connected.
 A plurality of scanning antennas 23 are attached to an end portion of beam
 member 25 which rotates in a horizontal plane by means of motor 26.
 Scanning antennas 23 are arranged in a vertical direction such that
 respective scanning antennas 23 have scanning planes (i.e., revolution
 plane) with different heights. The angle between the direction center axis
 and the horizontal plane may vary among scanning antennas 23. Switch 24 is
 attached to beam member 25 such that one of scanning antennas 23 can be
 selected by switch 24. Variable N is used to indicate to which of scanning
 antennas 23 switch 24 is connected. A received signal of one of scanning
 antennas 23, selected by switch 24, is taken through rotary joint 27.
 Incidentally, it is assumed that the radius of revolution of scanning
 antenna 23 is r, the rotation angle of scanning antenna 23 is .o slashed.,
 and the observation frequency is f. Additionally, elevation angle .theta.
 and position angle .o slashed.' for a radio wave source viewed from sensor
 station 10 are defined as shown in FIG. 3. The arrow mark in FIG. 3
 indicates the direction for the radio wave source. For each of fixed
 antennas 21 and scanning antennas 23, a probe antenna shown in Japanese
 Patent Laid-open Publication 153725/1997 (JP, 09153725, A) by the present
 inventor can be preferably used, for example.
 A received signal of fixed antenna 21 selected by switch 22 is applied to
 band-pass filter 28 to be limited to a predetermined observation frequency
 band, and then outputted as signal S.sub.r (f). Similarly, a received
 signal of scanning antenna 23 selected by switch 24 is applied to
 band-pass filter 29 to be limited to the predetermined observation
 frequency band, and then outputted as signal S.sub.m (f). The pass band,
 that is, the observation frequency band, in band-pass filters 28 and 29
 can be set from the outside as "setting of observation frequency f".
 Interference unit 30 is provided for causing signals S.sub.r (f), S.sub.m
 (f) from respective band-pass filters 28 and 29 to interfere with each
 other to obtain interference signal .intg.S.sub.r.sup.* (f)S.sub.m (f)dt,
 and detector 31 is provided for complex detection of the output signal
 from interference unit 30, where t represents a time variable and *
 represents a complex conjugate. For interference unit 30 and detector 31,
 a device, in which a multiplier and a vector detector are combined,
 disclosed as a correlation function measurement device in Japanese Patent
 Laid-open Publication 133721/1997 (JP, 09133721, A) by the present
 inventor can be preferably used.
 In this embodiment, the radio wave hologram observation is performed with
 the plurality of fixed antennas 21 being switched by switch 22 and with
 the plurality of scanning antennas 23 being switched by switch 24. Thus,
 the output signal from detector 31 is a complex scanning data matrix
 E.sub.N,M (.o slashed.,f) whose rows and columns depend on which antennas
 are selected by switches 22 and 24, respectively.
 Buffer and operation unit 32 for performing a numerical operation for the
 complex scanning data matrix is provided on the output side of detector
 31. Specifically, buffer and operation unit 32 performs the operation for
 separating radio wave sources extremely close to each other. The detailed
 procedure thereof is described in Japanese Patent Application No.
 317418/1997 ("Multidimensional hologram data processing apparatus and
 method for extracting a plurality of peak points for multidimensional
 hologram data and an area occupied thereby using the same") by the present
 inventor. In the following, the procedure will be described in brief. In
 the following description, a radio wave source to be visualized is also
 referred to as a visualized station.
 Buffer and operation unit 32 receives parameters .alpha..sub.N,.beta..sub.M
 for selecting a field of view and a visualized station in addition to the
 output from detector 31. Buffer and operation unit 32 temporally
 accumulates the output from detector 31 and performs the operation
 represented by:
 ##EQU1##
 where .lambda.represents a wavelength of a radio wave (.lambda.=c/f where c
 is speed of light), and as parameters .alpha..sub.N,.beta..sub.M, and
 weighting function W(.o slashed.), the following are used:
 ##EQU2##
 where K represents a parameter for a field of view, R and L represent
 parameters for selecting a visualized station. It goes without saying that
 parameters and a weighting function other than those as shown above may be
 used for selecting the field of view and the visualized station.
 Consequently, buffer and operation unit 32 temporally stores the output
 from detector 31 as the complex scanning data matrix and performs
 processing of complex weighting and addition for this complex scanning
 data matrix E.sub.N,M (.o slashed.,f), thereby reconstructing the plane
 and characteristic distribution of radio wave source. In the complex
 weighting and adding processing, a weighting coefficient for each fixed
 antenna is used for separating different radio wave sources in the same
 frequency band. A weighting coefficient for each reception point, i.e.
 each scanning position of the scanning antenna is used for separating the
 direction and the elevation angle of the radio wave source. Since the
 circular scanning is employed in this embodiment, the scanning position is
 represented by rotation angle .o slashed.. Additionally, the weighting
 coefficient for each scanning antenna is used for adjusting the field of
 view in the direction and/or elevation angle of the radio wave source to
 be visualized. The use of some of the respective weighting coefficients
 can be omitted.
 Buffer and operation unit 32 calculates complex intensity distribution
 I(.theta.,.o slashed.',f) in accordance with equation (1) and the complex
 intensity distribution is applied to display unit 33. Display unit 33
 calculates absolute value [I(.theta.,.o slashed.',f)] to display the
 reconstructed result of the plane and characteristic distribution of the
 radio wave sources.
 As described above, in accordance with this embodiment, the plane and
 characteristic distribution of the radio wave sources can be reconstructed
 and displayed to obtain radio wave utilization monitoring diagram 13.
 Although a preferred embodiment of the present invention has been
 described, the present invention is not limited to the above-mentioned
 aspect.
 The radio wave hologram observation is not limited to the circular scanning
 type, and linear scanning or scanning on an arbitrary curved surface may
 be used, for example. Also, instead of using a plurality of fixed
 antennas, one fixed antenna may be used to change the direction and
 position thereof, thereby performing a plurality of observations.
 Additionally, for the scanning antenna, one scanning antenna may be used
 to perform a plurality of observations with varying height of the antenna
 scanning plane rather than arranging a plurality of scanning antennas in a
 height direction.
 Instead of using a switch for switching a plurality of fixed antennas, a
 plurality of band-pass filters, interference devices, and detectors may be
 provided to simultaneously observe a plurality of holograms. Similarly,
 instead of using a switch for switching a plurality of scanning antennas,
 a plurality of band-pass filters, interference devices, and detectors may
 be provided to simultaneously observe a plurality of holograms.
 It is to be understood that variations and modifications of the method and
 apparatus for visualizing radio waves disclosed herein will be evident to
 those skilled in the art. It is intended that all such modifications and
 variations be included within the scope of the appended claims.