HK1074287B

HK1074287B - Base station time calibration using position measurement data sent by mobile stations during regular position location sessions

Info

Publication number: HK1074287B
Application number: HK05106707.6A
Authority: HK
Inventors: P.加尔; S．S．索利曼
Original assignee: 高通股份有限公司
Priority date: 2001-10-29
Filing date: 2002-10-29
Publication date: 2009-02-06

Description

Base station time alignment using position measurement data transmitted by mobile station during regular positioning sessions

Background

Technical Field

The present invention relates generally to mobile communications, and more particularly to determining the location of a mobile station in a mobile communications network. More particularly, the present invention relates to the calibration of base station time in order to maintain accuracy in determining the position of a mobile station.

Description of the related Art

Mobile communication networks offer increasingly sophisticated capabilities for locating the position of mobile terminals of the network. Administrative requirements for the authority may require the network operator to report the location of the mobile terminal when the mobile terminal makes a call to an emergency service, such as a 911 call in the united states. In Code Division Multiple Access (CDMA) digital cellular networks, position location capability may be provided by Advanced Forward Link Trilateration (AFLT), a technique for calculating the position of a Mobile Station (MS) from the time of arrival of radio signals measured by the mobile station from base stations. A more advanced technique is hybrid positioning, where the mobile station employs a Global Positioning System (GPS) receiver and calculates the position from both AFLT and GPS measurements. A further application of the hybrid technique is to use the time obtained from the GPS-synchronized cellular network in obtaining GPS measurements and calculating the position of the mobile station.

The accuracy of the position determined by AFLT or hybrid techniques depends in part on the accuracy of the time reference within each base station transmitter. For example, the IS-95a (cdma) standard published by the telecommunications industry association (Arlington, VA) allows for uncertainty in the transmission time from a base station to a mobile station of up to 10 microseconds. IS-95A, section 7.1.5.2, entitled "Base Station Transmission Time": "all base stations should transmit the pilot PN sequence within 3 mus of CDMA system time and will transmit the pilot PN sequence within 10 mus of CDMA system time. All CDMA channels transmitted by the base station will be within ± 1 μ s of each other. "since the radio signal propagates at the speed of light, i.e. at about 3 × 10⁸Meters propagate per second, so a 10 microsecond offset in transit time translates into a range error of 3 kilometers.

To maintain time synchronization among base stations, the base stations may be synchronized to each other or to a common time reference. For example, each base station may include a Global Positioning System (GPS) receiver using the GPS as a common time reference. The GPS system comprises a constellation of 24 satellites (plus back-up) in orbit in 11000 nautical miles above the earth. Each satellite has an atomic clock and emits a carrier signal modulated by a pseudo random code and a navigation message modulated at 50 bits per second. The navigation messages sent by each satellite include GPS system time, clock correction parameters, ionospheric delay model parameters, the ephemeris and integrity (health) of the satellite, and almanac and integrity data for other satellites. GPS signals from four or more satellites may be used to calculate GPS system time and the geographic location of the GPS receiver.

Although the GPS system can provide a stable time reference for the CDMA system, the reference point for the CDMA system time is the CDMA antenna at each base station. Due to differences in propagation delay and phase offset from the GPS antenna to the GPS receiver, from the GPS receiver to the CDMA transmitter, and from the CDMA transmitter to the CDMA antenna, each base station may have a corresponding time offset between GPS system time and GDMA signal transmission. Therefore, to reduce range errors in AFLT position decisions, and to reduce timing and range errors in hybrid position decisions, each base station must be individually calibrated with special test equipment after the base station installation is complete. The result of this calibration process is a time offset for each base station pilot. The time offset is stored in a database accessible during the calculation of the position of the mobile station. Any subsequent hardware changes require recalibration of the base station and updating of the database. All this represents a costly process.

There are other methods for synchronizing base stations to each other based on combining a pilot signal strength message (PSMM) sent by a mobile station in soft handoff with Round Trip Delay (RTD) measurements made by base stations in the active set. With this method, the base stations can be synchronized with each other; however, it is difficult to maintain overall synchronization with GPS time over a network of base stations.

Currently, to improve the accuracy of mobile terminal position determination, GPS receivers are incorporated into mobile terminals. The GPS receivers may be autonomous and perform all GPS acquisition functions and position calculations, or they may be non-autonomous (also known as wireless assisted) and rely on the cellular network for providing GPS acquisition data and possibly performing position calculations. By receiving GPS assistance data from the network, a mobile terminal with GPS capability can obtain time and location data from GPS satellites in about 10 seconds or less during a typical telephone call. Many, if not most, CDMA wireless telephones with GPS capability are expected to be wireless assisted GPS receivers with hybrid capability that provide GPS and AFLT position information after requesting a serving base station to process a call from the wireless telephone. The location session may be MS-assisted or MS-based, depending on where the location calculation takes place. In the MS-assisted case, the mobile station sends back raw or pre-processed measurement data to the base station. The network entity then calculates the location. In the MS-based case, the position calculation is performed within the mobile station.

The TIA/EIA Standard IS-801-. Pages 4-43 of the standard indicate: each base station transmits a GPS reference time correction for the base station antenna that transmitted the CDMA pilot Pseudorandom (PN) sequence.

Another positioning technique is where measurements are made by a network entity rather than by the mobile station. An example of these network-based methods is RTD measurement implemented by the serving base station. Measurements made by the mobile station may be combined with network-based measurements to improve the effectiveness and accuracy of the calculated position.

Disclosure of Invention

To calibrate base stations in a wireless telecommunications network to Global Positioning System (GPS) time, base station timing offsets are calculated from position measurement data obtained from one or more hybrid mobile stations during a regular position location session. The location measurement data includes GPS measurement data and measurement data based on propagation delay of signals transmitted between the base station and the hybrid mobile station. For example, the network is a Code Division Multiple Access (CDMA) wireless telecommunications network and the location measurement data includes GPS measurement data and Advanced Forward Link Trilateration (AFLT) measurement data.

In a preferred implementation, base station calibration is performed continuously to compensate for any interference in the base station. The collection of position measurement data from the hybrid mobile station occurs only when the hybrid mobile station makes or answers a wireless telephone call and the base station provides GPS acquisition data to the hybrid mobile station.

Detailed description of the drawings

Other objects and advantages of the present invention will become more apparent upon reading the following detailed description and upon reference to the drawings in which:

figure 1 shows a cellular telephone network according to the invention using a GPS system for positioning mobile telephone units and calibrating base stations;

figure 2 is a block diagram of a base station in the cellular telephone network of figure 1;

figure 3 is a block diagram of fixed components in the cellular telephone network of figure 1, including a position determining entity;

figures 4 to 7 together constitute a flow chart of a process performed by a position determination entity which uses base stations with hybrid (GPS and AFLT) position determination capabilities to calibrate the time references of the base stations; and

fig. 8 is a flow chart of a process performed by a position determination entity to manage calibration of a base station after the base station is installed or modified.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the form of the invention to the particular forms shown, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.

Detailed Description

Fig. 1 shows a CDMA cellular telephone network according to the present invention using a GPS system for positioning mobile telephone units and calibrating base stations. Fig. 1 also shows five CDMA base stations 11, 12, 13, 14, 15, which are deployed at fixed positions in a hexagonal array on the surface of the earth 16. Above the earth and at 11000 nautical miles, there are at least five GPS satellites 17, 18, 19, 20, 21 in communication with base stations 11 to 15. Within the communication range of the base station there are a number of mobile CDMA telephone units 22, 23, which are also referred to as Mobile Stations (MS) in the TIA standard documents mentioned above. These Mobile Stations (MSs) include AFLT only mobile stations, such as AFLT mobile station 22, and hybrid mobile stations, such as hybrid mobile station 23.

The CDMA network is capable of locating the position of the AFLT mobile station 22 and the hybrid mobile station 23 using the well-known AFLT technique used by mobile stations to measure the time of arrival of so-called pilot radio signals from base stations. The time of arrival is indicated by a pilot phase measurement relative to the mobile station time reference. To eliminate the effect of any time offset in the mobile station time reference, the difference between the pilot phase measurements from the corresponding pair of neighboring base stations is calculated. In most cases, the differences locate the mobile station on a particular hyperbola. The intersection of the hyperbolas provides the position of the mobile station.

The CDMA network is also able to locate the position of the hybrid station 23 using well-known GPS techniques. Each CDMA base station 11 to 15 has a GPS receiver for receiving the carrier wave and the pseudo-random code sequence of at least one of the GPS satellites 17 to 21 to provide a CDMA system time reference that is referenced to a GPS system time reference. The service reference may send GPS acquisition data to the hybrid mobile station when the hybrid mobile station is engaged in a positioning session with the CDMA network. The hybrid mobile station 23 may use the GPS acquisition data to obtain measurements of pseudoranges between the respective GPS17 through 21 and the mobile station in about 10 seconds or less. The hybrid mobile station 23 may send the pseudorange measurements to the serving base station. As further described below with reference to fig. 3, a Position Determination Entity (PDE) may calculate the geographic position of the hybrid mobile station 23 from four or more pseudorange measurements. Alternatively, in the case of an MS-based solution, the geographical location of the mobile station may be calculated by the mobile station itself.

Figure 2 shows functional blocks within each base station in the cellular telephone network of figure 1. The base station 11 includes a GPS receiver 31 that provides a base station time reference 32 that is referenced to GPS system time. The GPS receiver 31 obtains signals from a GPS antenna 39. The base station also includes a CDMA transceiver 33 for communicating with mobile stations in a CDMA network. The CDMA transceiver 33 obtains CDMA system time from the base station time reference 32. The CDMA transceiver 33 transmits and receives wireless signals through the CDMA antenna 40.

Figure 3 is a block diagram of the fixed components of the cellular telephone network of figure 1. A Mobile Switching Center (MSC)34 connects voice signals and telecommunications data between the base station 11 and a plurality of telephone lines 35, such as copper wires or optical fibers. A Mobile Positioning Center (MPC)36 is coupled to the mobile switching center 34. The MPC 36 manages the positioning application and connects the position data to an external data network via an interworking function (IWF)37 and a data network link 38. A Position Determination Entity (PDE)41 collects and formats positioning data. The PDE 41 provides wireless assistance to the mobile station and performs position calculations. PDE 41 is linked to MPC 36 and MSC 34. The PDE 41 manages the calibration database 42. The PDE 41 and the calibration data base 42 are implemented, for example, using a conventional digital computer or workstation. In practice, the processor of the computer executes a program, such as described below in the flowcharts of fig. 4 to 8, to function as the PDE 41. The calibration data base 42 is stored on a hard disk or in the memory of a digital computer or workstation.

As described above, there is a problem with calibrating the base station time reference (32 in fig. 2) when installing or modifying the base station. Due to variations in propagation delay or phase drift from the GPS antenna (39 in fig. 2) to the GPS receiver (31 in fig. 2), from the GPS receiver to the CDMA transceiver (33 in fig. 2), and from the CDMA transceiver to the CDMA antenna (40 in fig. 2), each base station will have a corresponding time offset between GPS system time and CDMA signal transmission. Therefore, to reduce range errors in AFLT position determination and range and timing errors in hybrid position determination, each base station should be calibrated after the base station installation is complete, for example, by saving the time offsets of the base stations in a calibration database (42 in fig. 3) for use by the position determination entity (PDE 41 in fig. 3). Furthermore, it is desirable to recalibrate the base station and update the database for any subsequent hardware changes.

As disclosed herein, this problem is addressed by calibrating base stations 11, 12, 13, 14, 15 using position measurement data obtained from one or more hybrid mobile stations 23 during a regular position location session. Thus, calibration data need not be obtained from outside the calibration instrument. Instead, the PDE (41 in fig. 3) may internally calculate the calibration data and continuously save the calibration data in the calibration data base (42 in fig. 3). Furthermore, to alleviate any privacy concerns, regular location sessions occur only when the operator of the hybrid mobile station makes or answers a wireless telephone call. In this case, the CDMA system does not determine the operator's location without the operator's knowledge and consent.

Figures 4 through 7 together constitute a flow chart of a process performed when a hybrid mobile station in a CDMA system implements a position location session. The operation for calibrating the serving base station is shown in the flow chart. In a first step 51, the calibration operation ends if the hybrid mobile station is not in the process of implementing a positioning session. Otherwise, execution continues to step 52.

In step 52, the PDE (41 in fig. 3) determines whether assistance data needs to be transmitted to the mobile station. If assistance is needed, the serving base station transmits assistance data to the hybrid mobile station in step 53, and execution continues to step 54. Otherwise, execution proceeds directly to step 54. In step 54, the hybrid mobile station acquires GPS code phase (i.e., pseudorange) measurements from at least five GPS satellites that should provide the best signal for determining the location of the hybrid mobile station. In the hybrid mobile station 23, code phase measurements should be taken with respect to the pilot phase received from the serving base station. These measurements may be taken directly or indirectly. In step 55, if the hybrid mobile station has not obtained pseudorange measurements of sufficient quality for five or more GPS satellites, the process ends. (note that a regular positioning session without base station calibration functionality will still continue.) the instruction for the pseudorange may be determined from the received signal-to-noise ratio, possibly from the shape of the observed correlation peak (a broad peak may indicate multipath error) or other factors. Otherwise, execution continues to step 56.

In step 56, the entity implementing the position location calculation receives pseudorange measurements from the hybrid mobile station of the various measured GPS satellites and calculates the position of the mobile station using known navigation solution techniques. In case of MS assisted method, the entity may be the PDE (41 in fig. 3), while in case of MS based method the entity is the mobile station itself. Thus, the navigation solution provides a mobile station position estimate, an average pseudorange bias (i.e., a mobile station clock bias), and a position solution cost (i.e., the RMS of the residual pseudorange error). In the MS-based case, both the position estimate and the mobile station clock bias are returned from the MS to the PDE. Since at least five measurements are used in the navigation solution, the solution cost is a valid indicator of the integrity of the GPS measurements. Thus, in step 57 of FIG. 5, if the solution cost is greater than a predetermined maximum value (CMAX), the calibration process ends. Otherwise, execution continues to step 58. When the solution cost is not available, (e.g., in an MS-based implementation that does not return it to the PDE), step 57 may be omitted. In this and all other cases, the solution cost threshold of step 57 may be replaced or augmented with a threshold based on a measurement standard deviation estimate. The standard deviation estimate may be based on measured signal characteristics, such as signal-to-noise ratio, or on statistical characteristics derived from measurements in the case of collecting multiple fixed points.

In step 58 of fig. 5, the PDE calculates the BS to MS range from the known fixed location of the serving base station and the GPS position calculated in step 56. In order to maximize the probability of signals in the line of sight between the serving base station and the MS, it is useful to apply a range threshold, thus minimizing the likelihood that serving pilot multipath will affect the mobile station system clock. Thus, in step 59, if the range is greater than the predetermined maximum Range (RMAX), the calibration process ends. Otherwise, execution continues to step 60. In step 60, the availability of RTD measurements is tested. If no RTD measurements (typically provided by the serving base station and corrected by the mobile station receive-transmit timing offset reported by the mobile station) are available, execution continues to step 62. Otherwise, execution continues to step 61. In step 61, the BS to MS range calculated in step 58 is compared to the value c RTD/2, where c is the speed of light. If the difference is greater than a predetermined maximum value (EMAX), or less than a predetermined minimum value (EMIN), the calibration process ends. (note that the observed differences can be used as a multipath correction term in the calibration calculations when the RTD measurements are known to be reliable.) otherwise, execution continues to step 62.

In step 62, the serving pilot signal strength measurements may be employed by the hybrid mobile station. In step 63, if the pilot signal strength is not greater than or equal to the predetermined minimum signal Strength (SMIN), the calibration process ends. Otherwise, execution continues to step 64.

In step 64, an estimate of the propagation time from the serving base station to the mobile station is calculated. The propagation time estimate may be based on the serving base station to mobile station distance calculated in step 58, or the RTD value used in step 61, or a combination thereof.

In step 65, a serving base station time offset estimate is calculated. The time offset is estimated as the difference between the mobile station clock offset calculated in step 56 and the serving base station to mobile station propagation delay calculated in step 64. (alternatively, the serving base station time offset may be estimated directly from the pseudorange measurements, knowing the GPS ephemeris, i.e., the position of the satellites in space, the theoretical GPS code phase observable by the hybrid mobile station may be calculated, and the difference between the returned pseudorange measurements and the theoretical code phase offsets the pseudorange.

In step 66, the serving base station time offset estimate obtained in step 65 is refined by applying various correction terms. The time offset estimation error caused by residual CDMA multipath effects can be estimated and corrected based on the known local signal propagation environment. For example, if it is known that a certain number of multipath excess delays are expected at the location determined in step 56, the expected delays may be applied as correction terms. If calibration data is available for a particular type of mobile station, base station time offset estimation errors due to internal asymmetries between CDMA and GPS processing in the hybrid mobile station can be compensated for. This may necessitate the PDE being passed information about the particular mobile station, such as the mobile station's electronic serial number. Alternatively, correction for internal asymmetries between CDMA and GPS processing in a hybrid mobile station may be omitted, in which case the mobile station time offset would be part of the base station calibration. This does not affect the positioning accuracy in any way as long as the internal asymmetry between CDMA and GPS processing in the hybrid mobile station is not compensated elsewhere.

Based on the collected statistics, the average lower bound of the corrected base station time offset estimate calculated in step 66 will represent the base station time alignment. It can be assumed that a higher base station time offset estimate has been affected by multipath propagation. The threshold eliminates the effects of multipath correlation errors. For example, in step 67 of FIG. 7, a threshold is calculated based on the collected statistics. In this example, the base station time offset estimation threshold is calculated as the average base station time offset plus two standard deviations. Since the time offset estimates due to multipath have a mean value that is not equal to zero, it may be beneficial to employ an asymmetric threshold level around the mean of the base station time offset estimates. If the actual base station time offset is expected to change abruptly, the threshold level may need to be relaxed, or more sophisticated statistical methods may be better used to determine the threshold. This is needed in order to enable the calibration process to continue after a jump in base station time offset; otherwise the calibration procedure may be disabled by considering that multipath affects all subsequent time offset estimates. Such statistical method may be, for example, to compute an age-weighted probability density function for all collected estimates, bias the local towards lower values, and select the highest peak. In step 68, if the base station time offset estimate is greater than a threshold, the calibration process ends because the base station time offset estimate is assumed to contain significant multipath error. Otherwise, execution continues from step 68 to step 69. The thresholding operation of step 68 may be omitted if the multipath free nature of the received serving pilot signal can be determined by other means.

In step 69, the base station time offset estimate is entered into the calibration data base. In step 70, the base station time offset estimate statistics, such as mean and standard deviation, are recalculated based on the base station time offset estimate added to the calibration database in the previous step 69. When there is not enough data to calculate the statistics, e.g. at the start of the first calibration procedure, a predetermined mean and (sufficiently large) standard deviation may be assumed in the initialization. In step 71, the timing offset of the management base station held in the management base station is updated with the new value of the mean base station time offset estimate calculated in the previous step 70.

Fig. 8 is a flow chart of a process performed by a position determination entity for managing calibration of a base station after installation or modification of the base station. When the base station is known to be uncalibrated, i.e., initially deployed or when timing instability is observed, the PDE can temporarily reject the corresponding pilot from the AFLT solution. Thus, in step 81, the base station attributes in the calibration database are set to "uncalibrated" and the PDE tests the attributes to temporarily reject the corresponding pilot from the AFLT solution. The PDE also tests this attribute to increase the GPS code phase window size for the mobile stations served by these pilots, step 82. After collecting calibration data in step 83 and ensuring that the statistics have stabilized in step 84, the PDE re-enables the AFLT and hybrid solution in step 85. For example, in step 84, the average base station time offset estimate for the base station is compared to the average base station time offset estimate entered into the calibration database for the last ten consecutive base station time offset estimates entered into the calibration database, and the average base station time offset estimate for the first ten consecutive base station time offset estimates entered into the calibration database, and if the average values differ by no more than a certain percentage, such as five percent, then the statistics of the base station time offset estimates are considered stable.

In view of the foregoing, a method of calibrating base stations in a wireless telecommunications network to GPS system time using position measurement data obtained from one or more hybrid mobile stations during a regular position location session has been described. Thus, calibration data need not be obtained from outside the calibration instrument, calibration occurring continuously to compensate for any interference in the base station. Privacy concerns are alleviated by using a regular position location session that occurs only when the operator of the hybrid mobile station makes or answers a wireless telephone call.

Claims

1. A method of calibrating base stations in a wireless telecommunications network to Global Positioning System (GPS) time, the method comprising:

calculating base station timing offsets from position measurement data obtained from one or more hybrid mobile stations during a regular position location session, wherein the position measurement data includes GPS pseudorange measurement data and position measurement data based on propagation delays of signals transmitted between the hybrid mobile stations and the base station;

storing the base station timing offset in a calibration database in response to the calculating step;

determining a base station timing offset statistic in response to said storing step; and

updating a timing offset of the base station in response to the determining step.

2. The method of claim 1, wherein calibration data for calibrating the base station to GPS time is not obtained externally from any calibration instrument for input into the wireless telecommunications network.

3. The method of claim 1, comprising performing base station calibration substantially continuously to compensate for any interference in a base station.

4. The method of claim 1, wherein collecting location measurement data from the hybrid mobile station occurs only when the hybrid mobile station makes or answers a wireless telephone call.

5. The method of claim 1, wherein the base station provides GPS acquisition data to a hybrid mobile station.

6. A method as claimed in claim 1, comprising rejecting position measurement data based on the propagation delay of signals transmitted between the hybrid mobile station and the base station when there is a substantial likelihood of significant propagation delay due to multipath propagation.

7. A method according to claim 1, comprising rejecting position measurement data based on propagation delay of signals transmitted between at least one base station and at least one hybrid mobile station when the distance between the at least one base station and the at least one hybrid mobile station exceeds a certain distance.

8. A method according to claim 1, comprising rejecting position measurement data based on propagation delay of signals transmitted between at least one mobile station and at least one base station when the strength of the signals transmitted between the at least one mobile station and the at least one base station is less than a certain signal strength.

9. A method according to claim 1, comprising rejecting a GPS position determination for at least one hybrid mobile station when a redundant GPS position fix cannot be calculated from pseudorange measurements from at least five GPS satellites.

10. A method according to claim 1, comprising rejecting a GPS position determination for at least one hybrid mobile station when there is more than a certain variance between redundant position fix points from at least five GPS satellites.

11. The method of claim 1 including collecting statistics of base station timing offsets and calculating an average lower bound for the calculated base station timing offsets based on the collected statistics to reject base station timing measurements related to multipath error.

12. The method of claim 11 including rejecting a base station timing offset measurement that is more than a certain number of standard deviations greater than the mean base station timing offset.

13. A method as claimed in claim 1, comprising installing or modifying at least one base station and declining to use signals from the at least one base station for position determination of mobile stations in the telecommunications network until the at least one base station has been calibrated by using position measurement data obtained from one or more hybrid mobile stations during a regular position location session.

14. The method of claim 13, comprising determining that the at least one base station has been calibrated when calibration data is observed to have stable statistics.

15. A method for calibrating a base station in a Code Division Multiple Access (CDMA) wireless telecommunications network to Global Positioning System (GPS) time, the method comprising:

calculating base station timing offsets from position measurement data obtained from one or more hybrid mobile stations during a regular position location session, wherein the position measurement data includes GPS pseudorange measurement data and Advanced Forward Link Trilateration (AFLT) position measurement data;

16. The method of claim 15, wherein base station calibration is performed continuously to compensate for any interference or drift in a base station.

17. The method of claim 15, wherein collecting position measurement data from the hybrid mobile station occurs only when the hybrid mobile station makes or answers a wireless telephone call.

18. The method of claim 15, wherein the base station provides GPS acquisition data to a hybrid mobile station.

19. The method of claim 15 including installing or modifying at least one base station and denying AFLT position determinations for mobile stations in the telecommunications network using pilot signals from the at least one base station until the at least one base station has been calibrated using position measurement data obtained from one or more hybrid mobile stations during a regular position location session.

20. A method for calibrating a base station in a Code Division Multiple Access (CDMA) wireless telecommunications network to Global Positioning System (GPS) time, the method comprising:

updating a timing offset of the base station in response to the determining step,

wherein base station calibration is performed substantially continuously to compensate for any interference in the base station,

collecting position measurement data from the hybrid mobile station only occurs when the hybrid mobile station makes or answers a wireless telephone call, and wherein the base station provides GPS acquisition data to the hybrid mobile station.