Abstract:
The present invention is related to a method for estimating a movement speed of a mobile unit in a mobile radio communication system. First, a signal from the mobile unit is received. Then, a first signal is generated by using a first filter having a first cut-off frequency to filter the signal. A first speed is obtained based on the first signal. A second signal is further generated by using a second filter having a second cut-off frequency to filter the signal. A second signal is obtained based on the second signal. Then the movement speed of the mobile unit is determined according to the first speed and the second speed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This Application claims priority to Taiwan Patent Application No.  091124966  entitled “Method and System for Estimating Movement Speed of Mobile Unit”, filed Oct. 25, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a method and system for estimating movement speed of a mobile unit, and more particularly, to a method and system for estimating movement speed of a mobile unit by using a plurality of filter units having different cut-off frequencies to filter signal.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a mobile radio communications system, information is exchanged between a mobile unit and a base station by radio signals. Since each base station can communicate with a mobile unit only within its radio coverage (cell), a plurality of base stations are needed to cover the whole service area. To avoid interruption of service when the mobile unit moves from one cell to another cell (roaming), a method is required to connect the mobile unit with the second cell at an appropriate time. The process of transferring between stations is called “handover.” 
           [0004]    Methods of implementing handover of a call to or from a mobile unit between cells in a mobile radio communication system affect the overall efficiency and quality of the service. Therefore, it is important to provide a method of achieving a fast and reliable handover between cells.  
           [0005]    Generally, the speed of mobile unit is an important factor to determine whether it is worthwhile implementing handover of a mobile unit between cells. The vehicular speed can be measured directly, but the method is not suitable for cellular radio applications. As an alternative, the vehicular speed may be estimated by monitoring the Doppler shift of a carrier frequency. FIG. 1 shows the conventional method of estimating movement speed of a mobile unit. First, a signal from the mobile unit is obtained in step  101 . An envelope of the signal is calculated and the envelope is squared in step  103 . According to the result of step  103 , a correlation coefficient is calculated in step  105 . Then, a corresponding Doppler frequency is obtained in step  107  by referring to a corresponding relationship between correlation coefficient and Doppler frequency. Lastly, the movement moving speed of the mobile unit is estimated according to the Doppler frequency in step  109 .  
           [0006]    However, this conventional method was limited. It can not accurately measure the movement speed of the mobile unit when the mobile unit moves at a low velocity. FIG. 2 shows the performance of the prior art. Three distinct curves A, B, and C respectively represent three mobile units having distinct powers of carrier signal. Obviously, when the three mobile units move below a specific speed, e.g. 18 km/hr, the curves A, B, and C bend down sharply. As such, one correlation coefficient mean value may map two distinct speeds, which prevents the system from determining the correct movement speed of the mobile unit.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method and system for estimating movement speed of a mobile unit in a mobile radio communication system. A wider range of measure and improved performance at low-velocity can be achieved by using the method of the present invention. In addition, by using the system of the present invention, the mobile radio communication system can measure the movement speed of the mobile unit with low velocity with even greater accuracy.  
           [0008]    First, a signal corresponding to a mobile unit transmitted signal is received. The present invention uses a first filter unit to filter the signal and generates a first signal. The first filter unit has a first cut-off frequency. Then, the present invention obtains a first speed by estimating the movement moving speed of the mobile unit based on the first signal. The present invention further uses a second filter unit to filter the signal and generates a second signal. The second filter unit has a second cut-off frequency. The present invention obtains a second speed by estimating the movement speed of the mobile unit based on the second signal. The present invention selects the first speed or the second speed to be the movement speed of the mobile unit.  
           [0009]    The present invention also provides a system for estimating a movement speed of a mobile unit. The system includes a receiving unit, a first filter unit, a first estimating unit, a second filter unit, a second estimating unit, and a selecting unit. The receiving unit receives a signal corresponding to a mobile unit signal. The first filter unit filters the signal to generate a first signal. The first filter unit has a first cut-off frequency. The first estimating unit estimates the movement speed of the mobile unit and obtains a first speed. The second filter unit filters the signal to generate a second signal. The second filter unit has a second cut-off frequency. The second estimating unit estimates the movement speed of the mobile unit and obtains a second speed. The selecting unit selects the first speed or the second speed to be the movement speed of the mobile unit. Furthermore, the selecting unit selects the second speed to be the movement speed while the first speed is slower than a predetermined speed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 shows a conventional method of estimating movement speed of a mobile unit;  
         [0011]    [0011]FIG. 2 shows the performance of the prior art;  
         [0012]    [0012]FIG. 3 is a flowchart of the present invention;  
         [0013]    [0013]FIG. 4 shows the performances of different filter units of the present invention;  
         [0014]    [0014]FIG. 5 ( a ) shows the first exemplary embodiment of the present invention;  
         [0015]    [0015]FIG. 5 ( b ) shows the estimation unit of the present invention; and  
         [0016]    [0016]FIG. 6 shows the second exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 3 is a flowchart of the present invention. First, the present invention receives a signal corresponding to a mobile unit transmitted signal in step  301 . The present invention, in step  303 , respectively uses a first filter unit and a second filter unit to filter the signal and generates a first signal and a second signal. The first filter unit has a first cut-off frequency and the second filter unit has a second cut-off frequency which is lower than the first cut-off frequency. The present invention, in step  305 , obtains a first speed by estimating the movement speed of the mobile unit based on the first signal and obtains a second speed by estimating the movement speed based on the second signal. In step  307 , the present invention selects the first speed or the second speed to be the movement speed.  
         [0018]    The method of estimating speed of a mobile unit is first to square the envelopes of the first signal and the second signal. Then, the method respectively calculates correlation coefficients of the signals and obtains Doppler frequencies of the correlation coefficients by referring to a corresponding relationship between correlation coefficient and Doppler frequency. The method further estimates the first speed and the second speed of the mobile unit according to the Doppler frequencies.  
         [0019]    [0019]FIG. 4 shows the performances of using different filter units of the present invention. This embodiment uses four filter units to filter the signal corresponding to the mobile unit signal. The signals are received from the mobile unit by the rake receiver of the base station. The cut-off frequencies of the four filter units respectively are 375 Hz, 250 Hz, 125 Hz, and 62.5 Hz.  
         [0020]    Curves A′, B′, C′ and D′ respectively represent the performances of using the filter units with the cut-off frequencies of 375 Hz, 250 Hz, 125 Hz, and 62.5 Hz. When the mobile unit moves below a specific speed, one mean of correlation coefficient value may map two distinct speeds in curve A′, which prevents the system from determining the correct movement speed of the mobile unit. Therefore, the present invention defines a T zone which is a common zone of curve A′ and B′. The minimum of the T zone is defined by the lowest point “a” of the curve A′ after bent. The maximum of the T zone is defined by the correlation coefficient mean “b” corresponding to the cut-off frequency of curve B′. When the movement speed falls into the T zone, the present invention selects the speed estimated according to curve B′ to be the movement speed of the mobile unit. The present invention further obtains a first predetermined speed based on the T zone. In other words, the present invention selects curve B′ when the mobile unit moves below the first predetermined speed. For example, the present invention selects the movement speed estimated by using the filter unit with 250 Hz cut-off frequency instead of the filter unit with 325 Hz cut-off frequency, when the mobile unit moves below a first predetermined speed of 120 km/hour.  
         [0021]    Similarly, when the mobile unit moves below a specific speed, one mean of correlation coefficient mean value may map two distinct speeds in curve B′, which prevents the system from determining the correct movement speed of the mobile unit. Therefore, the present invention defines a common zone for curve B′ and curve C′ and further obtains the movement speed. The present invention selects the movement speed estimated by using the filter unit with 125 Hz cut-off frequency instead of the filter unit with 250 Hz cut-off frequency, when the mobile unit moves below a second predetermined speed of 60 km/hour.  
         [0022]    When the mobile unit moves below a specific speed, one mean of correlation coefficient value may map two distinct speeds in curve C′, which prevents the system from determining the correct movement speed of the mobile unit. Therefore, the present invention defines a common zone for curve C′ and curve D′ and further obtains the movement speed. The present invention selects the movement speed estimated by using the filter unit with 62.5 Hz cut-off frequency instead of the filter unit with 125 Hz cut-off frequency, when the mobile unit moves below a third predetermined speed of 30 km/hour.  
         [0023]    [0023]FIG. 5 ( a ) shows the first exemplary embodiment of the present invention. The embodiment includes a receiving unit  501 , a first filter unit  5031 , a second filter unit  5033 , a third filter unit  5035 , a fourth filter unit  5037 , a first estimating unit  5051 , a second estimating unit  5053 , a third estimating unit  5055 , a fourth filter unit  5057 , and a selecting unit  507 . The cut-off frequencies of the first filter unit  5031 , second filter unit  5033 , third filter unit  5035 , and fourth filter unit  5037  respectively are 375 Hz, 250 Hz, 125 Hz, and 62.5 Hz.  
         [0024]    The receiving unit  501  receives the signal corresponding to the mobile unit transmitting signal. The first filter unit  5031 , second filter unit  5033 , third filter unit  5035 , and fourth filter unit  5037  respectively filter the signal to generate a first signal, a second signal, a third signal, and a fourth signal. The first estimating unit  5051 , second estimating unit  5053 , third estimating unit  5055 , and fourth filter unit  5057  respectively estimate the movement speed of the mobile unit based on the first signal, second signal, third signal, and fourth signal and respectively obtain a first speed, a second speed, a third speed, and a fourth speed.  
         [0025]    The first estimating unit  5051 , second estimating unit  5053 , third estimating unit  5055 , and fourth estimating unit  5057  respectively have a preference unit  5061 , a calculating unit  5063 , a look up unit  5065 , and an evaluating unit  5067 . As shown in FIG. 5( b ), each preference unit calculates the square of absolute value of the filtered signal. The calculating unit calculates a correlation coefficient ρ of the signal. The look up unit obtains a Doppler frequency of the correlation coefficient ρ referring to a corresponding relationship between correlation coefficient and Doppler frequency. The estimating unit estimates the movement speed of the mobile unit according to the Doppler frequency.  
         [0026]    The selecting unit  507  selects the movement speed among the first speed, second speed, third speed, and fourth speed by the method mentioned above.  
         [0027]    Selecting unit  507  selects the second speed estimated by using the second filter unit instead of the first speed estimated by using the first filter unit, when the mobile unit moves below a first predetermined speed. Selecting unit  507  selects the third speed estimated by using the third filter unit instead of the second speed estimated by using the second filter unit, when the mobile unit moves below a second predetermined speed. Selecting unit  507  selects the fourth speed estimated by using the fourth filter unit instead of the third speed estimated by using the third filter unit, when the mobile unit moves below a third predetermined speed.  
         [0028]    [0028]FIG. 6 shows the second exemplary embodiment of the present invention. The embodiment further saves hardware resource. The embodiment includes a receiving unit  601 , a first filter unit  6031 , a second filter unit  6033 , a third filter unit  6035 , a fourth filter unit  6037 , an estimating unit  6051 , and a selecting unit  507 .  
         [0029]    Similarly, the receiving unit  601  receives the signal corresponding to the mobile unit signal. The first filter unit  6031 , second filter unit  6033 , third filter unit  6035 , and fourth filter unit  6037  respectively filter the signal to generate a first signal, a second signal, a third signal, and a fourth signal. However, the embodiment only uses one estimating unit to obtain a first speed, a second speed, a third speed, and a fourth speed based on the first signal, second signal, third signal, and fourth signal. In order to save the hardware resource, the estimating unit  607  uses a switch to switch the transmission paths of the first signal, second signal, third signal, and fourth signal to estimate the movement speed of the mobile unit.  
         [0030]    The selecting unit  607  selects the movement speed among the first speed, second speed, third speed, and fourth speed by the method mentioned above.  
         [0031]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the discovered embodiments. The invention is intended to cover various modifications and equivalent arrangement included within the spirit and scope of the appended claims.