Abstract:
An apparatus and a method for sharing a temperature compensated crystal oscillator (TCXO) of a mobile terminal using a global positioning system (GPS). A communication system processor performs a process for a mobile communication system, computes a frequency variation according to a frequency change and outputs a value corresponding to the computed frequency variation to a system TCXO and a GPS processor. The GPS processor receives and processes a GPS signal, receives the value of the computed frequency variation from the communication system processor, and performs an operation for frequency compensation according to the value.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus and Method for Sharing a TCXO of a Mobile Terminal Using a Global Positioning System in a Mobile Communication System” filed in the Korean Intellectual Property Office on Feb. 15, 2006 and assigned Ser. No. 2006-14602, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to a global positioning system (GPS), and more particularly to an apparatus and method for sharing a temperature compensated crystal oscillator (TCXO) of a mobile terminal with a GPS function in a mobile communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     Global positioning system (GPS) satellites broadcast ephemeris and system time information while circling the earth along a designated orbit. A GPS receiver on the ground can set its position from the broadcast information. The GPS receiver sets an exact time and its position by computing relative reception times of GPS signals simultaneously received from at least four GPS satellites. On the other hand, the GPS receiver may use an assisted GPS (AGPS) mode and autonomous mode or stand-alone mode. In particular, in the autonomous mode or stand-alone mode, the GPS receiver independently and directly acquires a satellite signal from each GPS satellite without receiving satellite signal acquisition information from an assisted GPS server installed in a mobile communication base station, performs time-consuming tracking and decoding processes for the acquired signal, acquires ephemeris information of each satellite, and computes a position of the GPS receiver.  
         [0006]     Herein, the AGPS server connected to a code division multiple access (CDMA) network provides a mobile terminal with a positioning service, and includes a reference GPS receiver and an operation device. The reference GPS receiver successively tracks and/or monitors GPS satellite signals, provides information necessary for the positioning service of the mobile terminal, and provides an operation result relating to a position with respect to a measurement value obtained from the mobile terminal, and so on. A communication protocol between the AGPS server and the mobile terminal follows the IS-801 standard.  
         [0007]     As described above, when the GPS receiver operates in the autonomous mode or stand-alone mode, phase lock for a received GPS signal should be continuously maintained to demodulate navigation data including navigation information. To maintain the phase lock for the GPS signal, a temperature compensated crystal oscillator (TCXO) of the GPS receiver should not be varied due to influence of a different system for a predefined time.  
         [0008]     If a GPS operation unit and a communication modem sharing the TCXO of the mobile terminal operate in switching mode, a TCXO control operation of the communication modem, i.e., an automatic frequency control (AFC) operation, should be stopped when the GPS receiver operates in the autonomous mode. However, the conventional technology has a problem in that the AFC function cannot be continuously stopped for the normal operation of the communication modem.  
         [0009]     In a civil GPS satellite signal, a navigation data bit of a 20 ms period is conventionally modulated in a coarse acquisition (C/A) code of a 1 ms period.  FIG. 1  is a block diagram illustrating a structure of a receiver in a mobile terminal using a GPS according to the prior art. Referring to  FIG. 1 , the mobile communication terminal includes an antenna  100  for transmitting a data signal to and receiving a data signal from a GPS satellite, a modem TCXO  101 , a code division multiple access (CDMA) radio frequency (RF) processor  102  for down-converting the frequency of the CDMA RF signal into a baseband frequency, a GPS RF processor  103  for down-converting the frequency of the GPS RF signal into baseband frequency, a GPS TCXO  104 , an automatic frequency control (AFC) signal  105 , a CDMA baseband processor  106 , a GPS baseband processor  107 , and a memory  115 . The GPS baseband processor  107  includes a carrier numerically controlled oscillator (NCO)  108 , a correlator  109 , a code generator  110 , and a code NCO  111 .  
         [0010]     In  FIG. 1 , a modem TCXO  101  can control a frequency value by receiving an automatic frequency control (AFC) signal AFC  105 . As illustrated in  FIG. 1 , the GPS receiver of the mobile terminal measures relative arrival times of GPS satellite signals received from at least four GPS satellites, i.e., C/A code delay values, and sets an exact present time and position of the mobile terminal. At this time, the GPS receiver should have ephemeris information such as current position and speed information of each satellite in order to directly compute the position.  
         [0011]     As described above, when the GPS receiver operates in the autonomous mode or stand-alone mode, it directly obtains the ephemeris information by demodulating navigation data carried on a satellite signal. A process for directly obtaining the ephemeris information by demodulating the navigation data carried on the satellite signal will be described.  
         [0012]     A navigation data demodulator  114  obtains ephemeris information of each satellite by successively demodulating and decoding a navigation data bit corresponding to navigation information of a 20 ms period for about 30 seconds from a satellite signal detected through a signal detector  113  of  FIG. 1 . To correctly demodulate the GPS navigation data, a carrier-phase tracking loop of the GPS receiver controls a carrier numerically controlled oscillator (NCO)  108  and maintains phase lock for a received GPS signal.  
         [0013]     Using a shared TCXO and an absolute time of a CDMA system, the GPS receiver embedded in the mobile terminal can improve the reception sensitivity of the GPS signal. Also, the number of components and the cost and size of the mobile terminal can be reduced. However, when an AFC operation for controlling the TCXO is performed on the basis of a signal of a base station received in the CDMA mobile terminal in the case where the TCXO is shared, a reference frequency varies at a high rate as indicated in an AFC operation interval of  FIG. 2  which is a graph illustrating a reference error of a temperature compensated crystal oscillator (TCXO). This affects a carrier phase generated from the carrier NCO  108  within a GPS baseband processor  107 . As the carrier phase is affected, the phase lock is not maintained in the carrier-phase tracking loop and therefore navigation data cannot be correctly demodulated.  
         [0014]     When the GPS operation is performed in the autonomous mode or stand-alone mode, the navigation data should be successively demodulated for 30 seconds or more. In this situation, for example, when the control of the TCXO is stopped for 30 seconds or more in the case where the TCXO is shared, the reference frequency of the TCXO is slowly drifted as indicated in an AFC stop interval of  FIG. 2 . As a result, there is a problem in that the CDMA communication modem cannot operate normally.  
       SUMMARY OF THE INVENTION  
       [0015]     It is, therefore, an object of the present invention to provide an apparatus and method that can share a TCXO with a communication system when a GPS operation unit embedded in a mobile terminal operates in autonomous mode or stand-alone mode in a mobile communication system.  
         [0016]     It is another object of the present invention to provide an apparatus and method that can improve GPS performance and reduce manufacturing cost by sharing a TCXO with a communication system when a GPS operation unit embedded in a mobile terminal operates in an autonomous or stand-alone mode in a mobile communication system.  
         [0017]     In accordance with an aspect of the present invention, there is provided an apparatus for sharing a TCXO of a mobile terminal using a GPS, the apparatus including a communication system processor for performing a process for a mobile communication system, computing a frequency variation according to a frequency change and outputting the computed frequency variation to a system TCXO and a GPS processor; and the GPS processor for receiving and processing a GPS signal, receiving the computed frequency variation from the communication system processor, and performing an operation for frequency compensation according to the received value.  
         [0018]     In accordance with another aspect of the present invention, there is provided a method for sharing a TCXO of a mobile terminal for a GPS, the method including detecting a frequency variation of a communication system and comparing the frequency variation with a predetermined reference value; adjusting a frequency value of a system TCXO according to a comparison result; and synchronizing the TCXO to a base station signal according to an automatic frequency control operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above and other features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0020]      FIG. 1  is a block diagram illustrating a structure of a mobile terminal with a global positioning system (GPS) receiver according to the prior art;  
         [0021]      FIG. 2  is a graph illustrating a reference frequency error of a TCXO in the mobile terminal according to the prior art;  FIG. 3  is a block diagram illustrating a structure of a mobile terminal with a GPS receiver in accordance with the present invention;  
         [0022]      FIG. 4  is a block diagram illustrating main components of  FIG. 3 ;  
         [0023]      FIG. 5  is a graph illustrating a reference frequency error of a TCXO in the mobile terminal in accordance with the present invention; and  
         [0024]      FIG. 6  is a flowchart illustrating an operation for sharing the TCXO in the mobile terminal in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. For a better understanding of the present invention, particular components will be described. Those skilled in the art will appreciate that various modifications may be made without departing from the spirit and scope of the invention.  
         [0026]      FIG. 3  is a block diagram illustrating a structure of a mobile communication terminal including a GPS receiver in accordance with of the present invention. Referring to  FIG. 3 , the mobile communication terminal includes an antenna  200  for transmitting a data signal to, and receiving a data signal from, a GPS satellite, a code division multiple access (CDMA) radio frequency (RF) processor  201 , a GPS RF processor  202 , a system TCXO  203  for generating and/or providing a reference frequency to various components in the mobile communication terminal, and an automatic frequency control (AFC)  204  implemented with a frequency difference detector (FDD), a loop filer and a digital-to-analog converter (DAC) (not illustrated). In accordance with the present invention, the mobile communication terminal can include a CDMA baseband processor  205  for processing a CDMA signal. As illustrated in  FIG. 4 , the CDMA baseband processor  205  includes a digital-to-analog converter (DAC)  500  and a pulse density modulation (PDM) signal counter  501 . A PDM signal generated from a PDM signal generator within the CDMA baseband processor  205  is set to a predetermined bit output of the PDM signal counter  501 . An incremented value of the counter  501  is pre-computed by the CDMA baseband processor  205 . Thus, a frequency of the system TCXO  203  to be changed can be predicted in advance. Alternatively, the frequency of the system TCXO  203  may be read from a particular register.  
         [0027]     A GPS baseband processor  206  includes a frequency compensator  207  in accordance with the present invention. The GPS baseband processor  206  includes a carrier numerically controlled oscillator (NCO)  208 , a correlator  209 , a code generator  210 , and a code NCO  211 . In particular, the carrier NCO  208  includes a buffer register  502 , an adder  503 , and a phase decoder  504  as illustrated in  FIG. 4 . In particular, the buffer register  502  can store a value to be added by the adder  503  as a predetermined bit value. Thus, a frequency of a generated clock can be changed. Further, the phase decoder  504  performs an operation for changing a resulting value from the adder  503  to a corresponding sine or cosine value.  
         [0028]     An operation for sharing the system TCXO in accordance with the present invention will be described in detail with reference to  FIGS. 3, 4 , and  6 . In accordance with the present invention, an arbitrary value is set and can be dynamically changed according to state information. It is assumed that a PDM counter value for maintaining a GPS carrier tracking loop in a lock state is 2000, an output frequency of the system TCXO is 20 MHz, and a frequency variation Δƒ TCXO  corresponding to a 1-bit variation of the PDM counter with a predetermined bit is 0.02 MHz. Further, it is assumed that a buffer register value of the GPS carrier NCO  208  is 400 and an output frequency of the system TCXO is 5 MHz when the GPS carrier tracking loop is maintained in the lock state.  
         [0029]     With reference to  FIG. 6 , when the GPS carrier tracking loop is maintained in the lock state in step  600 , a frequency change of a CDMA signal input to the mobile communication terminal due to motion of the user is detected in step  605 . If a frequency change is detected in step  605 , the step  610  is performed. In step  610 , an operation for increasing a value of the PDM counter is performed for synchronization acquisition according to the frequency change of the CDMA signal input to the mobile communication terminal. In step  615 , an operation for computing a frequency variation according to the increased value of the PDM counter is performed. For example, assuming that the value of the PDM counter has been increased from 2000 to 2100, a frequency variation mapped to a difference of 100 becomes 2 MHz. In step  620 , an operation for changing an output frequency of the system TCXO is performed according to the computed frequency variation. Thus, a stabilization operation can be performed according to the output frequency change of the system TCXO and the frequency change of the CDMA signal.  
         [0030]     In the above-described example, the output frequency of the system TCXO is changed from 20 MHz to 22 MHz for the stabilization operation according to the frequency change of the CDMA signal. Thus, the PDM signal counter  501  of the CDMA baseband processor  205  outputs the changed frequency value to the frequency compensator  207  of the GPS baseband processor  206 . At this time, the frequency of the system TCXO is changed while carrier phase lock is maintained in the GPS baseband processor  206 . That is, as the frequency of the system TCXO is increased by 2 MHz and is set to 22 MHz, a GPS intermediate frequency (IF) value is changed. However, because the value of the buffer register  502  of the GPS carrier NCO  208  is set to 400 (5 MHz), the GPS carrier tracking loop is not maintained in the lock state. Thus, when the frequency of the system TCXO is increased by 2 MHz, an operation for computing a value of the buffer register  502  of the GPS carrier NCO  208  is performed by Equation (1) in step  630 . 
 
Δƒ=(GPS IF variation according to frequency variation of system  TCXO )×Δƒ TCXO    Equation (1) 
 
 For example, when a frequency variation Δƒ of the GPS carrier NCO computed by Equation (1) is 0.006 MHz, a value of the buffer register  502  of the GPS carrier NCO  208  is increased by 0.006 MHz. Thus, in step  635 , the lock state is maintained according to the increased value of the buffer register  502  of the GPS carrier NCO  208 , regardless of the AFC operation of the CDMA baseband processor. If frequency variation decreases, phase lock state is not maintained and therefore navigation data cannot be correctly demodulated. 
 
         [0031]      FIG. 5  is a graph illustrating a reference frequency variation according to a shared system TCXO in accordance with the present invention. As illustrated in  FIG. 5A , the mobile communication terminal maintains a system TCXO frequency and a CDMA response frequency in a predefined range by AFC through frequency adjustment according to a PDM signal variation time.  FIG. 5B  illustrates a variation in a GPS carrier NCO frequency. If a frequency variation value is not compensated when the system TCXO frequency is changed by the AFC coupled to the CDMA baseband processor, the GPS carrier NCO frequency, is out of a frequency error range as indicated by reference numeral  50 . When the GPS baseband processor and the CDMA baseband processor simultaneously operate and share the TCXO frequency, the frequency compensator  207  performs a frequency compensation operation according to a changed value of the PDM signal counter  501 . In this case, a value of the buffer register  502  of the GPS carrier NCO  208  is changed, such that the GPS carrier-phase tracking loop can be continuously maintained in the lock state.  
         [0032]     When a GPS operation is limited or an AFC operation of a CDMA baseband is alternately performed in the GPS operation in a state in which a GPS processor and a CDMA processor use their TCXOs or share one TCXO, the reception performance of a GPS signal may be conventionally degraded. However, the present invention can reduce the degradation of the reception performance of a GPS signal and can reduce a size of a mobile terminal.  
         [0033]     Although exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope of the present invention. Therefore, the present invention is not limited to the above-described embodiments, but is defined by the following claims, along with their full scope of equivalents.