US 20060055595 A1 Abstract A method (
200) of frequency drift prediction for use by a positioning receiver (106) can include the steps of determining (202) a moving average of a frequency error, determining (206) a moving average of a frequency drift rate in a communication device, determining (212) a frequency drift rate uncertainty, and providing (214) the moving average of frequency error and frequency drift rate, and the frequency drift rate uncertainty to the positioning receiver. A point-to-point slope from the running average of the instantaneous frequency error and a running average of the point-to-point slope for a predetermined time period can be determined (208 & 210). The frequency drift uncertainty or window is determined using information determined from the moving average of the frequency drift rate. The positioning receiver can be a global positioning receiver. Claims(18) 1. A method of frequency drift prediction for use by a positioning receiver, comprising the steps of:
determining a moving average of a frequency error in a communication device; determining a moving average of a frequency drift rate of the communication device; determining a frequency drift rate uncertainty in a communication device; and providing the moving average of the frequency error, the moving average of the frequency drift rate, and the frequency drift rate uncertainty to the positioning receiver. 2. The method of 3. The method of 4. The method of 5. The method of 6. The method of 7. The method of 8. The method of 9. The method of 10. A method for generating a frequency reference in a hybrid communications device, comprising the steps of:
generating a clock signal at a base frequency; performing communications processing in a communications receiver based on an input of the clock signal at the base frequency; generating frequency tracking data containing a moving average of a frequency error, frequency drift rate and a drift rate uncertainty of the communications receiver; transmitting a control message to the positioning receiver to adapt positioning processing based on the frequency tracking data; and performing positioning processing in a positioning receiver based on an input of the clock signal at the base frequency. 11. A method according to 12. A system for generating a frequency reference in a hybrid communications device, comprising:
a clock source generating a clock signal at a base frequency; a communications receiver, the communications receiver performing communications processing based on an input of the clock signal at the base frequency, the communications receiver generating frequency tracking data containing a moving average of frequency error, a frequency drift rate and a drift rate uncertainty; a positioning receiver, the positioning receiver performing positioning processing based on an input of the clock signal at the base frequency; and a processor, the processor transmitting a control message to the positioning receiver to adapt positioning processing based on the frequency tracking data, the processor communicating with the communications receiver and the positioning receiver. 13. A system according to 14. A system according to 15. A system according to 16. A system according to 17. A system according to 18. The system according to Description This invention relates generally to the field of communications, and more particularly to a method and system for providing an improved frequency drift prediction scheme. Frequency drift as a result of thermal activity in a cell phone essentially affects all Global Positioning System (GPS) enabled cellular phones and their ability to quickly provide a location fix. Accurately predicting frequency drift rate in parts per million per second (ppm/sec) or parts per billion per second (ppb/sec) is difficult in a typical application due to dynamic signal conditions in a real life environment and the different rates of change of temperature experienced by the phone. The thermal factors typically experienced by a phone can be heavily dependent on ambient temperature, phone temperature, phone transmitter power, relative placement of the crystal (XTAL) or temperature corrected crystal oscillator (TCXO) to the heat generating components in the layout, charger activity, phone mode of operation (emergency call, idle, packet data, etc). Typically, an assumption is made by the software in the phone that accounts for the worst possible thermal drift rate. This assumption leads to longer GPS time to first fix (TTFF) times as the frequency search algorithms must be wide enough to account for these worst case conditions. Several companies discuss the use of Automatic Frequency Control (AFC) from the cell phone system to provide either a one time assist to the GPS engine or a continuous correction. No existing phone tries to predict or estimate frequency drift rate and drift rate uncertainty of the reference oscillator in the phone. As mentioned above, this drift rate will be different depending on the environmental and phone state. Embodiments in accordance with the present invention can provide a method by which the uncertainty in both frequency error and frequency drift rate can be narrowed considerably by a simple method of running averages. In this fashion, the phone's AFC (whose variation spreads under bad signal quality conditions in the downlink and improves under better conditions) is used in real-time to determine the frequency error, frequency drift rate and frequency drift rate uncertainty. The algorithm can be used on any cell phone application regardless of the temperature characteristics of the reference oscillator. In a first embodiment of the present invention, a method of frequency drift prediction for use by a positioning receiver can include the steps of determining estimates for a frequency error, a frequency drift rate, and a frequency drift uncertainty in a communication device based on moving averages and then providing the frequency error, the frequency drift rate and the frequency drift rate uncertainty to the positioning receiver. The step of determining the frequency drift rate can include the step of measuring a running average of a frequency error. Then a point-to-point slope can be calculated from this running average of the frequency error for a predetermined time period. The method can further include the step of determining a moving average of the point-to-point slope in a communication device. The method can further include the steps of determining a frequency drift rate uncertainty window from the difference between maximum and minimum of the drift rate. The positioning receiver can be a global positioning receiver and the step of providing frequency error, drift rate, and drift rate uncertainty can occur when the global positioning receiver is in a weak satellite signal condition. Optionally, the steps of determining the drift rate and the drift rate uncertainty is done using the automatic frequency control of the communication device in real-time. Using the method described above, a time-to-first-fix can be accelerated for the positioning receiver at weak satellite signal levels where dwell times are typically elongated and the wide frequency search windows degrade TTFF considerably. In a second embodiment of the present invention, another method for generating a frequency reference in a hybrid communications device can include the steps of generating a clock signal at a base frequency, performing communications processing in a communications receiver based on an input of the clock signal at the base frequency; and generating frequency tracking data containing frequency error, frequency drift rate and a frequency drift rate uncertainty of the communications receiver. The method can further include the steps of performing positioning processing in a positioning receiver based on an input of the clock signal at the base frequency and transmitting a control message to the positioning receiver to adapt positioning processing based on the frequency tracking data. The frequency tracking data can include an automatic frequency control message. In a third embodiment of the present invention, a system for generating a frequency reference in a hybrid communications device, can include a clock source generating a clock signal at a base frequency and a communications receiver that performs communications processing based on an input of the clock signal at the base frequency and generates frequency tracking data containing an offset frequency (or frequency error), a frequency drift rate, and a frequency drift rate uncertainty. The system can further include a positioning receiver that performs positioning processing based on an input of the clock signal at the base frequency and a processor that communicates with the communications receiver and the positioning receiver and transmits a control message to the positioning receiver to adapt positioning processing based on the frequency tracking data. The communications receiver can be a cellular telephone, a personal digital assistant, a messaging device, a two-way pager, a radio receiving device, a modem, a network-enabled wireless device, a radio receiving device, a transceiver, a wireless modem, a wired modem or an optical-receiver. The positioning receiver can be a GPS receiver. The frequency tracking data can be an automatic frequency control (AFC) message which can contain frequency deviation data generated by comparison to a base station signal. Other embodiments, when configured in accordance with the inventive arrangements disclosed herein, can include a system for performing the methods disclosed herein and a machine readable storage for causing a machine to perform the various processes and methods disclosed herein. While the specification concludes with claims defining the features of embodiments of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the figures, in which like reference numerals are carried forward. Embodiments in accordance with the present invention can provide a method by which the uncertainty in both frequency error and frequency drift rate can be narrowed considerably by a simple method of running averages. In this fashion, the phone's AFC (whose variation spreads under bad signal quality conditions in the downlink and improves under better conditions) can be used in real time to determine drift rate and drift rate uncertainty. An architecture or system The combined communications/positioning device as illustrated may contain a base oscillator The base oscillator The output of the loop filter The clock reference of the high-frequency oscillator The communications transceiver Once the frequency offset is determined according to embodiments of the invention, a DSP or processor Once the frequency error, drift rate, and drift rate uncertainty estimates are determined according to embodiments of the invention, the DSP or processor After the communications transceiver The GPS receiver circuitry The output of base oscillator In accordance with embodiments of the present invention, a frequency error, drift rate and drift rate uncertainty prediction algorithm was developed based on the measured history of a running average of cumulative frequency offset when a mobile locks to a base station. Such algorithm can assist in reducing the Time-To-First-Fix (TTFF) at a signal level down to a predetermined GPS receiver sensitivity level in dBHz. In this regard, note that the drift rate can be calculated as follows:
Referring to Referring to In one practical example using Motorola's iDEN radio technology, note that iDEN's slot-to-slot DSP frequency measurement error while locked to a base station signal has a current uncertainty specification of ±0.5 ppm relative to a GPS satellite clock due to various factors like Doppler effect, base station reference frequency accuracy, Received Signal Strength (RSS), and Carrier to Noise+Interference power ratio (C/(I+N)). This uncertainty specification also includes the assumption that the frequency drift due to temperature effects on the XTAL/TCXO circuit is such that it will not drift faster than about 5 ppb/sec in the time it takes to obtain a fix at low satellite signal levels. This imposes serious constraints on the layout placement and temperature drift specifications on reference oscillators that make the part expensive and impractical in cell phone designs, especially as form factors get smaller. If the frequency measurement uncertainty specification can be reduced and if the drift rate and drift rate uncertainty can be estimated in real time, then the GPS receiver First, baseline measurements of frequency offset were performed using a high stability signal generator to provide an accurate reference frequency Next, the signal generator was replaced by a low cost TCXO to provide a more realistic reference frequency and measurements were re-taken with the AFC still in open loop mode. The radio under test was soaked for ½ an hour in a temperature-controlled environment to stabilize the TCXO frequency. All offsets were calibrated to provide a zero frequency error at the stabilized temperature. New measurements were taken while the radio was set to receive with AFC in open loop mode. This measurement process was repeated at room and extreme temperatures. These measurements can provide a distribution of the frequency error in parts-per-billion (ppb) when a stable reference As a next step in the frequency uncertainty characterization, an interference condition was set up by supplying-the radio with an external interferer such as Co-Channel, Adjacent Channel, and Blocker interference types. Further, a set of channel fading profiles were used to test frequency error performance in a multipath environment. Industry standard profiles consisting of Static, BU5, BU50, and BU100 channels were tested with desired and interfering signals independently faded. Various levels of cellular signal carrier to interferer power ratios were tested to simulate real world conditions. All offsets were calibrated to provide a zero frequency error at the stabilized temperature. The results of this test revealed that the 99% spread in frequency error remained below ±0.24 ppm even under extreme Co-Channel interference conditions of C/I=15 dB. It is worth noting that in a real system, such an extreme level of degradation is uncommon, as handovers to better quality serving cells would normally occur long before conditions are permitted to degrade to this extent. Since Co-channel Interference was found to have the worst-case effect, further characterization focused on the use of this type cellular signal interferer. From the above result, it was concluded that the present uncertainty of ±0.5 ppm can be reduced to about ±0.25 ppm as long as a good predictor for drift rate (direction) and drift rate uncertainty (spread) can be estimated. As a final step of frequency uncertainty characterization, the closed-loop AFC tracking characteristics were measured. In this measurement a high stability signal generator was once again used to provide an accurate reference frequency. To measure tracking performance the generator was frequency modulated with a ramp waveform. All measurements in this analysis were performed while the AFC loop was closed (frequency measured and tracked/corrected). At the start of the characterization runs, all offsets were calibrated to provide a zero frequency error at the stabilized temperature. The reference modulating ramp was set up to simulate the effects of heating, due to full power cellular transmissions, on a reference circuit such as a XTAL or TCXO. An extreme case of ±30 ppb/sec ramp was used in the measurements. The iDEN DSP ( Observations from the characterization described above indicated that taking more points for the moving average of the frequency error and drift rate improves the accuracy of the drift rate and drift rate uncertainty estimates. Also, the frequency drift rate and the frequency drift rate uncertainty can be predicted based on a history of the moving average of the cumulative frequency error calculation. Further note that the approximation becomes better when the number of points needed to calculate the moving average of the frequency error and slope is increased. This is done at the expense of an increased wait time in the GPS session. In a typical iDEN call session, a wait time of 70×45 ms=3.15 seconds (after a call is initiated) is needed to start providing drift rate and drift rate uncertainty parameters to the GPS receiver Once again, a step-by-step description is provided as to how a hybrid communication device (such as a combined cellular phone and GPS receiver) in accordance with an embodiment of the invention can use an algorithm to provide a GPS receiver with frequency error, drift rate, and drift rate uncertainty parameters. First, a running average of the instantaneous frequency error measured by a DSP or other processor is kept by the processor (or in a lower level layer of a software system in a radio such as an iDEN phone). This running average is to be kept for predetermined number (X) of slots (slots are measured every 45 ms in an iDEN call). Second, the running average can be used for a point-to-point slope calculation. Third, a running average of the point-to-point slope is kept for X slots. This third step provides the GPS receiver with the parameter for drift rate. In a fourth step, a maximum and minimum delta from the last X points in the third step is calculated and divided by two. The divide by two provides the GPS receiver with the drift rate uncertainty as a ± parameter. The processing of the data described above can be done for example in an iDEN modem or in software within the hybrid communication device. Parameters for frequency error, drift rate, and drift rate uncertainty can be passed to the GPS software (within the positioning receiver) via messages between processors. The GPS receiver can use the information to narrow the frequency search, predict frequency trend, and improve time to first fix. In light of the foregoing description, it should be recognized that embodiments in accordance with the present invention can be realized in hardware, software, or a combination of hardware and software. A network or system according to the present invention can be realized in a centralized fashion in one computer system or processor, or in a distributed fashion where different elements are spread across several interconnected computer systems or processors (such as a microprocessor and a DSP). Any kind of computer system, or other apparatus adapted for carrying out the functions described herein, is suited. A typical combination of hardware and software could be a general purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the functions described herein. In light of the foregoing description, it should also be recognized that embodiments in accordance with the present invention can be realized in numerous configurations contemplated to be within the scope and spirit of the claims. Additionally, the description above is intended by way of example only and is not intended to limit the present invention in any way, except as set forth in the following claims. Referenced by
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