Patent Publication Number: US-2016245913-A1

Title: Apparatus and method for measuring precipitation in the atmosphere using k-band frequency-modulated continuous wave (fmcw) weather radar system

Description:
BACKGROUND 
     The present invention relates to an apparatus and method for measuring the precipitation in the atmosphere using a K-band frequency-modulated continuous wave (FMCW) weather radar system. 
     Much research has been carried out since the International Telecommunication Union (ITU) approved the frequency of 24-24.25 GHz for long range radar (LRR) systems. Numerous researches have been done regarding frequency modulated continuous wave (FMCW) radar operation in K-band for distance measurement in multiple target situations. Moreover, continuous-wave operation makes FMCW radars less complex, thus cheaper and more reliable than pulse radars. 
     These properties have caused the widespread use of FMCW radar technology for example in automotive applications. Similarly, in rain radar and industry K-band FMCW has many significant applications. The electromagnetic wave is transmitted from the weather radar in the atmosphere and influence by rain drops, hail, graupel, and snowflakes. In K-band, the attenuation due to rain effect may be noticeable, but it is weak enough to be correctable with sufficient accuracy. 
     Wireless communication systems require a channel sounder for the characterization of radio channel. In the past few decades, many researches have taken place on channel characterization by using different sounding techniques. The major purpose of channel sounding is to attribute a radio channel by decomposing the radio propagation path into its individual components. 
     There are different techniques for the channel sounding. For example, a radio channel can be characterized by using vector network analyzer (VNA) as a measurement device. This technique is not cost effective but very sensitive because its accuracy strongly depends on the physical layout between the two ports. The scattering parameter (S 21 ) is only reliable for very close measurement when operating on higher frequency, as the movement of the cable such as bending can change the impedance of the cable. In addition, due to time varying channels the measurement of channel frequency response can be changed, which leads to inaccurate impulse response measurement. 
     Another technique for channel sounding is pseudo random binary sequence radar which uses the spread spectrum technique. A merit of this technique is that it has a strong immunity to noise signals and adjustable sensitivity by the control of chip length. In the time domain, it employs a rectangular pulse which can be a sinc function in frequency domain. The sinc-like spectrum spreads in the frequency range that is not suitable for observing the specific frequency band. 
     An alternative but significant way is frequency-modulated continuous wave (FMCW) radar can be utilized for channel sounding. This radar continuously transmits electromagnetic waves with varying frequency. Its system stability, repeatability and reliable measurement conditions are the reason for utilizing this method. 
     SUMMARY 
     The present invention is directed to measuring the precipitation in the atmosphere propagation using frequency modulated continuous wave (FMCW) weather radar system as a channel sounder. 
     In one general aspect of the present invention provides an apparatus for measuring precipitation in the atmosphere propagation in which FMCW radar is used as a channel sounder. The FMCW weather radar transmits a continuous radio wave frequency signal which is linearly swept. The transmitted signal hits the target and reflected back. Therefore, the received signal is the combination of these two signals. The frequency difference between the transmitted and received signals of FMCW radar indicates a delay ‘τ’ due to the propagation distance of radio frequency (RF) signal. This frequency difference is called an intermediate frequency (IF) signal and denoted as S IF . The IF signal can be utilized as a time domain signal as well as frequency domain signal, a unique property of linear frequency modulation (LFM). 
     In the LFM technique, the IF signal of the FMCW radar is interpreted in terms of time domain. It has been investigated that the change of the sweep time corresponds to the change of the modulated frequency in the LFM. Therefore, the time-domain signal in the slow modulation is represented in the frequency domain signal without Fourier transform algorithm. Hence, the scattering parameter S 21  in the radio channel is proportional to the conjugate of the IF signal S IF  of the FMCW radar. 
     Consequently, the relationship between S 21  and S IF  can be expressed as follows, 
     
       
         
           
             
               
                 
                   S 
                   21 
                 
                  
                 
                   ( 
                   
                     F 
                     i 
                   
                   ) 
                 
               
               = 
               
                 k 
                 · 
                 
                   
                     [ 
                     
                       
                         S 
                         IF 
                       
                        
                       
                         ( 
                         
                           F 
                           i 
                         
                         ) 
                       
                     
                     ] 
                   
                   * 
                 
               
             
             , 
             
               ( 
               
                 t 
                 = 
                 
                   
                     i 
                     
                       N 
                       - 
                       1 
                     
                   
                    
                   
                     T 
                     m 
                   
                 
               
               ) 
             
           
         
       
     
     where, k is an arbitrary constant, F t  the t-th frequency of IF signal, * the conjugate operation, i the number of intervals, N the number of sweep frequencies in the sweep period, and T m  the modulation time of the FMCW radar. 
     Similarly, the channel response can be extracted from the measured channel frequency response by performing an inversed discrete Fourier transform (IDFT) of S 21 . According to above equation, the normalized impulse response (h norm ) of the radio channel using FMCW radar is defined by the discrete Fourier transform (DFT) of S IF , because the IDFT of the S 21  corresponds to the DFT of the S IF . It can be written as, 
     
       
         
           
             
               
                 
                   
                     
                       h 
                       norm 
                     
                      
                     
                       ( 
                       
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                         , 
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                       ) 
                     
                   
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                       IDFT 
                       [ 
                       
                         
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                           21 
                         
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                               F 
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                         ( 
                         
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                               IDFT 
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                                     S 
                                     21 
                                   
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                                         F 
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                   = 
                     
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                                   F 
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                               DFT 
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                                       S 
                                       IF 
                                     
                                      
                                     
                                       ( 
                                       
                                         
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                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
     where, τ is the delay time and m the number of independent stirrer positions in the reverberation chamber. 
     Therefore, the normalized PDP can be defined by the impulse response as follows, 
     
       
         
           
             
               PDP 
                
               
                 ( 
                 τ 
                 ) 
               
             
             = 
             
               
                 〈 
                 
                   
                      
                     
                       
                         h 
                         norm 
                       
                        
                       
                         ( 
                         
                           τ 
                           , 
                           m 
                         
                         ) 
                       
                     
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                   2 
                 
                 〉 
               
               
                 max 
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                          
                         
                           
                             h 
                             norm 
                           
                            
                           
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                               τ 
                               , 
                               m 
                             
                             ) 
                           
                         
                          
                       
                       2 
                     
                     〉 
                   
                   ] 
                 
               
             
           
         
       
     
     where, &lt; &gt; is the expectation operator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  is a block diagram of frequency modulated continuous wave (FMCW) radar system according to one embodiment. 
         FIG. 2A  is the RF transmitter inside blocks according to one embodiment. 
         FIG. 2B  is the RF receiver inside blocks according to one embodiment. 
         FIG. 2C  is the first baseband unit inside blocks according to one embodiment. 
         FIG. 2D  is the second baseband unit inside blocks according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various forms. The following embodiments are described in order to enable those of ordinary skill in the art to embody and practice the present invention. 
     Although the terms first, second, etc. may be used to describe various elements, these elements are not limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of exemplary embodiments. The term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments. The singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, components and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. 
     Unless otherwise defined, all terms including technical and scientific terms used herein are to be interpreted as is customary in the art to which this invention belongs. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein. 
     With reference to the appended drawings, exemplary embodiments of the present invention will be described in detail below. Like numbers refer to like elements or elements corresponding to each other throughout the description of the figures, and the description of the same elements will not be reiterated. 
       FIG. 1  is a system block diagram of frequency modulated continuous wave (FMCW) radar utilized as a channel sounder for measuring the precipitation in the atmosphere using K-band frequency-modulated continuous wave (FMCW) weather radar system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , an apparatus for measuring the precipitation in the atmosphere using K-band frequency-modulated continuous wave (FMCW) to an exemplary embodiment includes a radio frequency (RF) transmitter  10 , a radio frequency (RF) receiver  20 , an attenuator  30 , a power divider  40 , a first baseband unit  50 , a second baseband unit  70 , a frequency synthesizer  60 , an analog-to-digital (A/D) converter  80 , a first band pass filter  100 , a second band pass filter  120 , a mixer  110 , an amplifier  130 , and a personal computer (PC)  90 . 
     The RF transmitter  10  modulates the signal generated by first baseband unit  50  and transmits a radio wave. As a result, RF receiver  20  receives a radio wave and demodulates that signal. This demodulated signal goes to mixer  110  and is mixed with a local oscillator to create the beat frequency. The isolation between the RF transmitter and receiver antennas is −100 dB for example. 
     The attenuator  30  is used to adjust the power of the transmission signal. The power divider  40  divides the signal from first baseband unit  50  into transmission path and local oscillating path. 
     First baseband unit  50  generates the frequency-modulated continuous-wave signal. This generated signal goes to the RF transmitter  10  to be transmitted and is also used as local oscillator to create the beat frequency. 
     Frequency synthesizer  60  device that generates frequencies from a fixed oscillator. It generates a reference signal for coherent operation of the overall system. This generated signal is inputted to first baseband unit  50 , second baseband unit  70 , RF transmitter  10 , and RF receiver  20 . 
     Second baseband unit  70  down-converts the beat signal from the mixer and generates baseband quadrature (I/Q) signal. The obtained analog I/Q signal is then converted to a digital signal by analog-to-digital converter  80 . 
     First band pass filter  100  suppresses harmonics generated by mixer  110  and only passes the beat frequency. While the mixer  110  mixes demodulated received signal with local oscillating signal from first baseband unit  50  to make beat frequency. 
     Second band pass filter  120  suppresses harmonics generated by amplifier  130  and only passes a power divided signal from first baseband unit  50 . The amplifier  130  is used to amplify the signal from first baseband unit  50  to supply enough power of local oscillating signal into mixer  110 . PC  90  receives the digital signal from A/D Converter  80  and processes that signal to get information. 
       FIG. 2A  shows the RF transmitter blocks included in the RF transmitter  10  according to one embodiment. The input signal is up-converted with transmitter phase locked loop (PLL)  107 . This signal is passed through first band pass filter  106  and doubled by the frequency doubler  105 . The signal then goes to mixer  104  and harmonics of up-converted are rejected by the first band pass filter  103 . Before transmission, the signal is amplified with amplifiers  102  (e.g., three amplifiers) and then transmitted with the transmission antenna  101 . 
       FIG. 2B  shows the RF receiver blocks included in the RF receiver  20  according to one embodiment. The received signal is down-converted by the receiver phase lock loop (PLL)  201 . Then the signal is filtered using second band pass filter  202  and amplified with an amplifier  203 . The received signal from reception antenna  206  is firstly amplified with an amplifier  207  and passes through second band pass filter  208 . Both band pass filter outputs are mixed using mixer  209 . 
       FIG. 2C  shows the first baseband unit blocks included in the first baseband unit  50  according to one embodiment. A direct digital synthesizer  501  signal is pass through a mixer  502  and filtered through a band pass filter  503  in one embodiment. 
       FIG. 2D  shows the second baseband unit blocks included in the second baseband unit  70  according to one embodiment. The main purpose of this main block is to down-convert the IF band signal with baseband phase lock loop  701  to a baseband signal. After this the baseband signal gets amplified with an amplifier  702  and passes through a band pass filter  703 . The signal is then inputted into a mixer  704  and then filtered  705  and amplified  706 . 
     While a few exemplary embodiments have been shown and described with reference to the accompanying drawings, it will be apparent to those skilled in the art that various modifications and variations can be made from the foregoing descriptions. For example, adequate effects may be achieved even if the foregoing processes and methods are carried out in different order than described above, and/or the aforementioned elements, such as systems, structures, devices, or circuits, are combined or coupled in different forms and modes than as described above or be substituted or switched with other components or equivalents. 
     Therefore, other implements, other embodiments, and equivalents to claims are within the scope of the following claims.