Abstract:
A frequency management scheme for a hybrid cellular/GPS or other device generates a local clock signal for the communications portion of the device, using a crystal oscillator or other part. The oscillator output may be corrected by way of an automatic frequency control (AFC) circuit or software, to drive the frequency of that clock signal to a higher accuracy. Besides being delivered to the cellular or other communications portion of the hybrid device, the compensated clock signal may also be delivered to a comparator to measure the offset between the cellular oscillator and the GPS oscillator. The error in the cellular oscillator may be measured from the AFC operation in the cellular portion of the device. An undershoot or overshoot in the delta between the two oscillators may thus be deduced to be due to bias in the GPS oscillator, whose value may then be determined. That value may then be used to adjust Doppler search, bandwidth or other GPS receiver characteristics to achieve better time to first fix or other performance characteristics.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the field of communications, and more particularly to techniques for generating and managing precision frequency sources in cellular telephones or other communications devices having a location capability, such as Global Positioning System (GPS) or other location service.  
         BACKGROUND OF THE INVENTION  
         [0002]    Two important performance metrics for any GPS or other location-reporting services include the time needed to acquire synchronization with a signal source, and the ability to detect weak signals in noise. For GPS receivers these metrics correspond to time to first fix (TTFF) and receiver sensitivity, respectively. In a practical GPS receiver these metrics are dependent on the availability of an accurate frequency reference to drive the GPS receiver. Accuracies on the order of 0.5 ppm or better are required to attain acceptable GPS performance, for example TTFF ranges of a few tens of seconds. Conventional implementations require expensive precision components such as a temperature compensated crystal oscillator (TXCO) or oven controlled crystal oscillator (OCXO) in order to achieve this level of accuracy.  
           [0003]    As a result of the FCC-mandated E911 location service, GPS receivers are being integrated into cellular phones. Cellular networks use highly accurate clocks to maintain network synchronization. Cellular handsets typically contain their own stable reference clock which is locked to the cellular network by automatic frequency control (AFC) or other circuits. However the resulting frequency reference for cellular communications is generally different than that needed for GPS downconversion or other GPS operations. Other problems exist.  
         SUMMARY OF THE INVENTION  
         [0004]    The invention overcoming these and other problems in the art relates in one regard to a system and method for frequency management in a communications device having a positioning feature, such as a cellular phone equipped with GPS location capability, which can dynamically detect the error in a GPS receiver&#39;s reference oscillator without directly correcting that oscillator, but instead adjusting Doppler search or other control logic on the GPS side. This type of frequency aiding may be applied continuously or periodically to maintain very accurate frequency information, allowing a narrower bandwidth correlation to be used thereby improving the signal to noise ratio (SNR), and hence, sensitivity of the GPS receiver in a hybrid communications/location device. The frequency assist information may both narrow the carrier offset search space required during satellite acquisition thereby reducing the TTFF of a GPS receiver, as well as shorten the duration which the RF receiver must be powered on thereby reducing the power consumption of a GPS receiver, extending its battery life. Since existing cellular circuitry is made use of to enhance GPS operation, less expensive components may be used, enabling a cost reduction in an integral cellular handset/GPS receiver.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 illustrates an architecture for frequency management in a hybrid communications/positioning device, according to an embodiment of the invention.  
         [0006]    [0006]FIG. 2 illustrates an implementation of a frequency comparator, for use according to an embodiment of the invention.  
         [0007]    [0007]FIG. 3 illustrates frequency correction processing, according to an embodiment of the invention.  
         [0008]    [0008]FIG. 4 illustrates an architecture for frequency management in a hybrid communications/positioning device, according to an embodiment of the invention.  
         [0009]    [0009]FIG. 5 illustrates an architecture for frequency management in a hybrid communications/positioning device, according to an embodiment of the invention.  
         [0010]    [0010]FIG. 6 illustrates a timing diagram of messaging and other activity, according to embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0011]    [0011]FIG. 1 illustrates a hybrid communications/positioning device  10 , in which the communications portion of the hybrid device contains a cellular oscillator  30 , illustrated as a compensated 16.8 MHz oscillator, which is the reference for cellular circuitry including a radio frequency receiver  20 , frequency generation, timing or other components to transmit and receive cellular or other communications via communications antenna  50 . A positioning receiver such as a GPS receiver  90  or other location-calculating device may be driven by GPS oscillator  70 , illustrated as a free-running 24.5535 MHz oscillator, to drive the acquisition and tracking of satellite or other signals received via GPS antenna  100 . In embodiments, the communications/positioning device  10  may use or interface to other positioning systems such as the Russian GLONASS or other terrestrial or satellite-based services.  
         [0012]    As illustrated, automatic frequency control (AFC) including a frequency warping digital to analog converter (DAC warp)  110  may be implemented around the cellular oscillator  30  to maintain synchronization with the cellular network with which the hybrid communications/positioning device  10  is registered. This AFC arrangement may in embodiments employ comparatively coarse adjustment steps of 0.1 to 0.2 ppm to warp the frequency (16.8 MHz base) of the cellular oscillator  20  when a drift by that amount is sensed, by voltage or other feedback generated by the DAC warp  110 . Other coarse adjustment steps may be used. This AFC action maintains a frequency lock between the native frequency reference broadcast by the cellular network and the cellular oscillator  30  located in the handset or other hybrid communications/positioning device  10  to within the step value of the DAC warp  110  AFC mechanism, which is typically sufficient to ensure reliable cellular or other communications.  
         [0013]    The corrective effect of the DAC warp  110  on the cellular oscillator  30  may therefore however be limited to the quantization step of the DAC warp circuitry, which as noted in embodiments may be 0.1 or 0.2 ppm or more or less. However, further improved frequency accuracy may be attained by a second, software-based fine AFC process executing on processor  40 , which in embodiments may be or include a digital signal processor such as the DSP  56000  family manufactured by Motorola Corp. or others. The processor  40  may sense and output estimates of the residual error of the cellular oscillator  30  which are finer than the quantization step of the DAC warp  110  to a frequency correction unit  80 . Frequency correction unit  80  may in turn communicate the fine AFC data to the GPS receiver  90 . In embodiments, the fine AFC data so generated may reflect a 0.05 ppm or greater or lesser accuracy.  
         [0014]    Once ascertained, both the DAC warp  110  correction (also referred to as coarse AFC) and/or the fine AFC data may be communicated as frequency aiding intelligence information from the cellular portion of the hybrid communications/GPS location device  10  to the GPS receiver  90 . This aspect of operation includes at least a first process to measure the absolute offset between the cellular and GPS clock sources to determine a differential error between the two clock rates from their ideal frequency separation, and a second process to remove the error due to the cellular oscillator  30 . This processing leaves only the residual error in the GPS oscillator  70 , which may then be accounted for by adapting Doppler search or other control logic in GPS receiver  90 .  
         [0015]    In the first aspect of error analysis, clock signals from the cellular oscillator  30  and the GPS oscillator  70  may be communicated to a frequency comparator  60  that compares the frequency of the two clocks to produce an output which is a measure of the frequency difference between them. There are different ways known in the art to implement such a comparator. One implementation of frequency comparator  60  is illustrated in FIG. 2. This implementation uses a counter  120  illustratively clocked by the 24.5535 MHz or other clock signal of the GPS oscillator  70  or integer divide thereof, and gated by an integer divide of the 16.8 MHz or other clock signal of the compensated cellular oscillator  30 . At the end of the gate interval the counter value registered by the counter  120  is proportional to the error between the two clocks. That measure is given by:  
             Count   =       24.5535   16.8          N   M          (       1   +     ɛ   24         1   +     ɛ   16         )               Equation                 1                               
 
         [0016]    where ε 24  is the error of the 24.5535 MHz clock signal, ε 16  is the error of the 16.8 MHz clock in ppm/10 6 , and M and N are integers dividing the GPS oscillator  70  and cellular oscillator  30 , respectively. The gate time, averaging method, and number of measurements to be averaged can be adjusted per accuracy, latency or other implementation requirements.  
         [0017]    In the second processing aspect of the error analysis, the output of the frequency comparator  60  may be applied along with the fine AFC information to the frequency correction unit  80 . Frequency correction unit  80  may correct the error from the frequency comparator  60  by the measured error of the cellular oscillator  30  (at 16.8 MHz) as determined by the fine AFC. The fine AFC represents the error between the 16.8 MHz reference and the timing of the cellular or other communications network. The result of removing that remaining error due to the cellular oscillator  30  is the absolute error of the GPS oscillator  70 .  
         [0018]    [0018]FIG. 3 is a block diagram illustrating processing steps of the frequency correction unit  80 . As shown in that figure, the fine AFC data may be received from the communications transceiver  130 , for instance via a universal asynchronous receiver/transmitter (UART) or other channel, and processed along with the output of the frequency comparator  60 . After subtraction of the known contribution to total error by the cellular oscillator  30  according to Equation 1, the residual error ε 24 may  be generated which represents the error caused by the GPS oscillator  70  to within the measurement limits of the fine AFC calculation, or other limiting factor in the comparisons performed.  
         [0019]    The rate at which the frequency comparator  60  and frequency correction unit  80  may provide frequency aiding information to the GPS receiver  90  may be selected according to the drift rates of the cellular oscillator  30 , GPS oscillator  70  or other factors. In implementations, during a cellular interconnect call AFC information may be updated at multiples of 45 ms. For example if a frequency aiding message is sent to the GPS receiver  90  every 45 ms and the maximum error reported is 0.2 ppm, then the effective tracking rate would be 4.4 ppm/second. Other rates are possible.  
         [0020]    Thus far the techniques for analyzing error contributions and deriving frequency aiding information have in one regard been described. The following discussion describes techniques for transferring this information from the communications transceiver  130  portion of the hybrid communications/positioning device  10  to the GPS receiver  90  itself.  
         [0021]    [0021]FIG. 4 depicts an architectures for transferring frequency aiding information to the GPS receiver  90  according to an embodiment of the invention. In the arrangement shown in FIG. 4, derivation of the error in the GPS oscillator  70  (represented by ε 24 ) is performed wholly in the communications transceiver  130 .  
         [0022]    In the embodiment illustrated in FIG. 4, the frequency comparator  60  and frequency correction by the frequency corrector  80  operate in the communications transceiver  130 . In this case, a message generator  150 , which may be or include as a digital signal or other processor, may periodically send a completed message containing a computed ε 24  to the GPS receiver  90 . The communications transceiver  130  also contains a transmit/receive modem  140  to carry out cellular or other communications functions, while the cellular oscillator  30  is likewise corrected by DAC warp  110  while a fine AFC is also sensed and communicated to the frequency corrector  80 .  
         [0023]    Frequency corrector  80  receives data representing the comparison of the cellular oscillator  30  and the GPS oscillator  70 , and combined with the fine AFC values communicates the subsequent ε 24  value to message generator  150 . Message generator  150  may in turn communicate that data to message handler  160  within GPS receiver  90 , which transmits the data to a GPS receive modem  170 . Transmit/receive modem  140  and GPS receive modem  170 , as well as frequency corrector  80 , message generator  160  and message handler  160  and other parts may each for instance be, include or interface to hardware, software or firmware implementations, for instance using digital signal or other processors such as the DSP  56000  family manufactured by Motorola Corp., executing communications or other software modules or routines.  
         [0024]    The GPS receive modem  170  may adjust the Doppler search space, correlation bandwidth, and/or other control logic for GPS signal acquisition based on knowledge of the error in the GPS oscillator  70  which itself remains free-running. These adjustments enable improvements in TTFF, sensitivity, or other performance characteristics. In embodiments the GPS receive modem  170  may utilize the frequency error information to narrow the Doppler search space in order to improve TTFF. In embodiments the GPS receive modem  170  may utilize frequency error information to narrow correlation bandwidth in proportion to the Doppler search space, in order to improve receiver sensitivity. In embodiments the GPS receive modem  170  may utilize frequency error information to narrow both the Doppler search space and correlation bandwidth independently to achieve improvement in both TTFF and receiver sensitivity. Additionally the frequency error information may be used by the GPS receiver  90  to improve other performance characteristics. For instance, in embodiments the GPS receiver  90  may only be supplied with power, or with varying degrees of power, when GPS acquisition or tracking is activated to conserve batteries. In this case, achieving a faster TTFF may permit the amount of time that GPS receiver  90  is active to be reduced, thus extending battery life and service availability. In another regard, the use of an uncompensated or free-running GPS oscillator  70  may reduce the cost of manufacture of the hybrid communications/positioning device  10 .  
         [0025]    In an embodiment illustrated in FIG. 5, derivation of the ultimate error value for the GPS oscillator  70  may be split between the communications transceiver  130  and the GPS receiver  90 . In this embodiment, the GPS receiver  90  may use the fine AFC and the output of frequency comparator  80 , illustratively located within GPS receiver  90 , to complete processing such as that depicted in FIG. 3 to generate the residual error in the GPS oscillator  70 , ε 24 . In this case, the message generator  150  formats and transmits not a completed value for the error in the GPS oscillator, ε 24 , but instead a message indicating the amount of residual error in the cellular oscillator  30  after coarse AFC correction by the DAC warp  110 . That residual error is reflected in the fine AFC correction value, ε 16 .  
         [0026]    The GPS receiver  90  may then receive the AFC correction value, which may for instance be expressed in steps of 0.05 ppm or other values, via the message handler  160 . Message handler  160  may then communicate that data to the frequency corrector  80 , which also receives the results of comparison by frequency comparator  60  between the cellular oscillator  30  and the GPS oscillator  70 . Frequency corrector  80  therefore accepts an indication of the offset error between the cellular oscillator  30  and the GPS oscillator  70 , as well as the fine AFC (ε 16 ) which permits frequency corrector  80  to remove that portion of the error contributed by the cellular oscillator  30 , to within the precision of the fine AFC algorithm. The residual error may be assumed to be due to the bias in the GPS oscillator  70 , and is communicated as such (ε 24 ) to the GPS receive modem  170  to adjust the Doppler search or other control logic to increase positioning and other receiver performance.  
         [0027]    In the embodiments illustrated in both FIG. 4 and FIG. 5, a message is thus periodically sent from the communications transceiver  130  to the GPS receiver  90  to effectuate error detection and downstream compensation to account for bias in the GPS oscillator  70 . FIG. 6 illustrates a set of timing traces (A-E) for preparation and delivery of frequency aiding messages, in each case. The top trace, trace A in that figure represents the activity on the cellular communications channel, for instance in the 800/900 Mhz or 1.9 GHz or other bands. Trace B depicts the instantaneous frequency of the cellular oscillator  30 , generally showing a gradual upward drift in the 16.8 MHz clock of the cellular oscillator  30  until about midtrace.  
         [0028]    At the midtrace point in trace B, there is a sharp downward change in reference frequency as the effect of the DAC warp  110  is applied to the cellular oscillator  30 . Trace C illustrates messaging between the communications transceiver  130  and GPS receiver  90  according to the embodiment illustrated in FIG. 5, while trace D illustrates messaging between the communications transceiver  130  and GPS receiver  90  according to the embodiment illustrated in FIG. 4, respectively. The bottom trace, trace E annotates relevant timing events. The illustrated scenario shows a 3:1 interconnect frame with an overall period of 45 millisec.  
         [0029]    During the serving receive slot (shown as Rx) the communications transceiver  130  may measure the frequency offset of the cellular oscillator  30 . In embodiments, a snapshot of the frequency offset of the cellular oscillator  30  may be taken multiple times during the receive slot Rx, and for instance averaged or otherwise processed to arrive at a synthetic value. The fine AFC data may be updated just after the receive slot Rx, as shown in trace C during Frame 1. In the illustrated event, a DAC warp correction is appropriate because the accumulated drift in the cellular oscillator  30  exceeds step resolution of the DAC warp  110 . As shown in trace B, at midtrace after the coarse AFC of the DAC warp  110  is applied just before the receive slot Rx in Frame 2, the instantaneous frequency corrects to a lower value. If the cellular oscillator  30  is operating at 16.8 MHz, the value of coarse AFC correction may be, for instance, in the range of 200 Hz. Other values are possible.  
         [0030]    The message sent to the GPS receiver  90  contains the fine AFC measurement in the embodiment illustrated in FIG. 4, or the actual error in the GPS oscillator  70  in the embodiment of FIG. 5. In either case the frequency measurement in Frame 1 is based on the error in the cellular oscillator  30  before the ensuing DAC warp update. After the next receive slot Rx in Frame 2, the fine AFC may again be updated but before a new DAC warp update is evaluated. In this case the change in the value of the fine AFC adjustment may be comparatively large, because the measurement is based on the frequency of the cellular oscillator  30  after the effect of the DAC warp update. The error message (ε 16 ) k+1  shown in trace C which is sent to the GPS receiver  90  according to the embodiment of FIG. 5 may therefore reflect a comparatively sharp change or step increment in Frame  2 . The error message (ε 24 ) k+1  shown in trace D according to the embodiment of FIG. 4 in contrast may demonstrate a comparatively lesser change in value in Frame  2 , since the value of (ε 24 ) k+1  has already removed the effect of changes in the fine AFC leaving only the bias in the GPS oscillator  70  itself, which is not affected by the AFC operations on cellular oscillator  30 .  
         [0031]    In terms of performance advantage, in implementations according to the invention, the requirement for high tolerance parts such as high-precision TCXOs or other components for use in GPS oscillator  70  is significantly relaxed. Use of the invention consequently makes it possible to use oscillator parts with comparatively high frequency deviation, for example ±2.5 ppm or more, while maintaining equivalent performance to that of a highly accurate TCXO or other reference. The performance of ±3 ppm oscillator parts is typically as follows:  
                                                                                       TABLE 1                           Single Frequency Transfer (3 ppm, 100 us, 30 km)                TTFF                    (DOP &lt; 50)   Hor. Error (DOP &lt; 2)            Parameters   50%   95%   50%   95%                    C/No (dBHz) &gt; = 37   2.0   2.5   2.3   5.5       36 &gt; = C/No (dBHz) &gt; = 33   3.0   4.8   5.0   12.5       32 &gt; = C/No (dBHz) &gt; = 23   73.3   120.0   5.1   16.0                                  
 
         [0032]    With the benefit of the error tracking, frequency aiding and other aspects of the invention, the resulting GPS receiver performance may be at least equivalent to that of a hardware ±0.5 ppm TCXO or other part, as shown below:  
                                                                                       TABLE 2                           Single Frequency Transfer (.5 ppm, 100 us, 30 km)                TTFF                    (DOP &lt; 50)   Hor. Error (DOP &lt; 2)            Parameters   50%   95%   50%   95%                    C/No (dBHz) &gt; = 37   2.1   2.4   2.8   6.5       36 &gt; = C/No (dBHz) &gt; = 33   3.5   4.6   5.0   12.5       32 &gt; = C/No (dBHz) &gt; = 23   23.0   29.9   4.8   15.2                                  
 
         [0033]    The inventors have empirically confirmed that the invention may compensate or correct for errors of at least ±8 ppm in the GPS oscillator  70 , while still maintaining TTFF and other receiver performance equivalent to that of ±0.5 ppm hardware TCXOs or other parts. Compensation for significantly higher values of offset in the GPS oscillator  70  is possible.  
         [0034]    The foregoing description of the invention is illustrative, and modifications in configuration and implementation will occur to persons skilled in the art. For instance, while the invention has generally been described in terms of a hybrid cellular/GPS device, in embodiments other devices, such as two-way pagers, wireless network-enabled computers or other clients or devices may be configured with GPS capability according to the invention.  
         [0035]    Similarly, while the invention has generally been described in terms of oscillator parts which drive communications and positioning circuitry within a combined device, in embodiments one or more of oscillators, synthesizers, phase locked loops and other circuitry or software may be combined to deliver clock reference signals to those and other circuits of the platform. The scope of the invention is accordingly intended to be limited only by the following claims.