CN1294708C - Methods and apparatuses for using mobile GPS station to synchronize basestations - Google Patents

Abstract

The present invention relates to a synchronous method and a device for base stations in a cellular network. One demonstration method is characterized in that the space between at least two base stations of a first base station and a second base station carries out time synchronization. In the demonstration method, first date time and a first position of first MS are determined by a first satellite positioning system (SPS) positioned at the same position as a first cellular mobile receiver station (MS), and the first date time and the first position are transmitted to the first base station from first MS; the latter determines the date time of the first base station from the first date time, the first position, and the known position of the first base station. Likewise, in the demonstration method, second date time and a second position of second MS are determined by a second SPS receiver positioned at the same position as second MS, and the second date time and the second position are transmitted to the second base station from second MS; the latter determines the date time of the second base station from the second date time, the second position, and the known position of the first base station. The present invention also describes other methods and devices to synchronize the base stations in the cellular network.

Description

Method and device for synchronizing base station with mobile GPS station

Background of the invention

The present invention relates to the field of cellular communication systems, and more particularly to those systems in which the location of cellular mobile communication stations (MS) is determined.

For location location within a cellular network (eg, a cellular telephone network), several methods of triangulation based on timing information sent between various base stations and mobile devices are used. In one method called Time Difference of Arrival (TDOA), the times at which signals are received from a mobile station are measured at several base stations and these times are transmitted to a location determination entity, called a location server, which calculates the location of the mobile station. For this method to work, the time of day at each base station needs to be coordinate to provide an accurate position. Also, the location of the base station needs to be known exactly. FIG. 1 shows an example of a TDOA system in which the location server 24 measures the reception time (TR1, TR2 and TR3) of the same signal from the cellular mobile telephone 22 at the cellular base stations 12, 14 and 16. A location server 24 is coupled to receive data from the base stations via the mobile switching center 18 . The mobile switching center 18 provides signals (e.g., voice communications) to or derives signals from the landline Public Switched Telephone System (PSTS) so that signals can be passed to and from mobile phones to other phones (e.g., PSTS or other mobile phones) landline phone on the phone).

In some cases, the location server may also communicate with the mobile switching center over a cellular link. The location server may also monitor transmissions from several base stations in an attempt to determine the relative timing of these transmissions.

Another method, called EOTD, measures at the mobile station the time of arrival of signals emanating from each of several base stations. Figure 1 applies to this case, assuming the arrows for TR1, TR2 and TR3 are reversed. This calculation can be done at the mobile station itself or at the location server if the timing information obtained by the mobile station is transferred to this server via the link. Also, it is necessary to calibrate the time of day of the base stations and to accurately estimate their positions. In either method, the location of the base station is determined by standard survey methods and may be stored at the base station or in a server within the memory of some type of computer.

A third position location method uses a Global Positioning System (GPS) or other Satellite Positioning System (SPS) receiver within the mobile station. This approach could be entirely autonomous, or use the cellular network to provide assistance data or sharing within location calculations. Examples of this approach are described in US Pat. Nos. 5,841,396, 5,945,944, and 5,812,087. We refer to these methods as "SPS".

Combinations of either EOTD or TDOA or SPS systems are referred to as "hybrid" systems.

From the above description, it can be seen that for EOTD or TDOA, the time coordinates between the various cellular base stations are necessary to accurately calculate the position of the mobile station. The required time-of-day accuracy at the base station depends on the details of the positioning method used. In one approach, the signal is sent from the base station to the mobile station and back with a round trip time delay (RTD). In a similar alternative, the signal sent from the mobile station to the base station and back has a round trip time delay. These two round-trip delays are each divided by two to determine an estimate of the one-way time delay. Knowing the base station location plus the one-way delay constrains the mobile station location to within a circle on the earth. Then, another measurement by the second base station results in the intersection of the two circles, which in turn constrains the location to two points on the earth. This third measure resolves ambiguity. In terms of round-trip timing, it is important that the measurements of several base stations are scaled to a few seconds, at least so that if the mobile station is moving fast, the measurements will still correspond to those values occurring at the same location.

In other cases, it is not possible to make round-trip measurements to each of the two or three base stations, but only to the main base station with which the mobile station communicates. This is the case with the IS-95 North American CDMA cellular standard. Or, perhaps round-trip timing measurements are not possible at all due to device or signaling protocol limitations. In this case, it is more important to maintain accurate timing at the base station if a triangulation operation is to be performed since only the time difference between the mobile station-base station paths is used.

Another reason for accurate timing at the base station is to provide the mobile station with time for aiding GPS-based position calculations; such information may result in time reduction to a first fixed point, and/or improved sensitivity. The required accuracy in these cases can vary from a few microseconds to about 10 milliseconds, depending on the desired performance improvement. In a hybrid system, base station timing is used to improve TOA or TDOA operation as well as GPS operation.

Prior art approaches to network timing employ a particularly fixed location timing system called a location measurement unit (LMU) or timing measurement unit (TMU). These units typically include a GPS receiver which is capable of determining the exact time of day. The position of the unit can be measured, for example possibly with GPS from a measuring device.

Generally speaking, an LMU or TMU observes timing signals, such as existing framing marks within cellular communication signals sent from a base station, and attempts to time-tamp these timing signals with the local time found through a GPS device or other time-determining device . Messages may then be sent to base stations (or other infrastructure components), allowing these entities to track elapsed time. Special messages may then be sent over the cellular network to mobile stations served by the network, possibly on command or periodically, indicating the time of day relative to the framing structure of the signal. This is especially easy for systems like GSM where the total framing structure lasts for a period of more than 3 hours. Note that the location measurement unit may serve other purposes, such as acting as a location server - ie the LMU may actually perform time-of-arrival measurements from the mobile station in order to determine the location of the mobile station.

One problem with the LMU or TMU approaches is that they create new special fixtures at each base station or elsewhere within communication range of several base stations. This results in very high installation and maintenance costs.

Summary of the invention

The present invention provides various methods and apparatus for synchronizing cellular base stations within a cellular network. One exemplary method performs this synchronization between at least two base stations of a cellular communication system: a first base station and a second base station. In this exemplary method, a first time of day and a first location of a first MS are determined from a first satellite positioning system (SPS) receiver co-located with a first cellular mobile receiver station (MS), and the first The time of day and first location are communicated by the first MS to the first base station, which determines the time of day of the first base station from the first time of day and first location and from the known location of the first base station. Also in this exemplary method, a second time of day and second location of the second MS are determined from a second SPS receiver co-located with the second MS, and the second time of day and second location are communicated by the second MS to the second base station, which determines the time of day of the second base station from the second time of day and the second location and from the known location of the second base station. Since these mobile stations may be used for normal communication operations and are not necessarily fixed to buildings or structures, their use in timing networks avoids the high cost levels of maintaining fixed timing equipment. Other methods and apparatus are also described for synchronizing base stations within a cellular network.

Brief description of the drawings

The present invention is illustrated by way of example and is not limited to the description in the drawings, in which like references refer to like elements.

Figure 1 shows an example of a prior art cellular network which determines the location of a cellular mobile unit.

Figure 2 shows an example of a cellular mobile communication station which includes a GPS receiver and a cellular communication transceiver, in which the present invention can be used.

Figure 3 shows an example of a cellular base station that may be used in various embodiments of the present invention.

Figure 4 is a flow chart illustrating an embodiment of the method according to the invention.

5A and 5B are flow charts illustrating another embodiment of the method according to the present invention.

Figure 6A shows two signals processed according to an exemplary method of the present invention.

Figure 6B shows a signal representation at the base station showing how the base station updates its clock to synchronize with other base stations.

Figure 7 illustrates an example of a location server that may be used in some embodiments of the present invention.

Figure 8 shows the framing structure of a GSM cellular signal.

A detailed description

Various methods and apparatus are described herein for determining time at a cellular base station and for synchronizing cellular base stations within a cellular network. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. For example, the various configurations of base stations and mobile communication stations are provided for purposes of illustration and not limitation of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form for ease of explanation.

In one method described herein, a mobile communication station containing (or coupled to) a GPS receiver is used for determining time of day and location. FIG. 2 shows an example of such a mobile communication station. This GPS processing can be done in automatic mode if the received signal is large, or with the help of a device (server) within the infrastructure if the received signal-to-noise ratio is low. Note that a server device (eg, a location server as shown in FIG. 7 and detailed below) may also assist in time-of-day and location determination where improved performance is desired (see US Patent Nos. 5,945,944, 5,841,396, and 5,812,087).

In networks such as GSM, time-of-day information from a GPS receiver can be used to time-tamp the framing structure of received communication (eg, GSM) signals. For example, the start of a particular GSM frame boundary can be used, which occurs every 4.6 milliseconds (see Figure 8). There are 2048 such frames per superframe, and each superframe lasts 3.48 hours. Therefore, if this timing information is communicated to the base station (BS) via normal cellular signaling (eg, the cellular base station shown in Figure 3), the only major error remaining in the transmission time is from the mobile station (MS) (eg, the cellular base station shown in Figure 3). , the cellular mobile communication station in Figure 2) to the propagation time of the BS. Of course, there may be some other residual errors left, such as multipath delays and propagation delays through the MS hardware, and methods to compensate for these residual errors are described below.

Various methods can be used to estimate the above MS to BS propagation delay. The first and highly accurate method may be used when the MS and/or server have determined the MS location via the GPS unit, and the BS location is known accurately (eg, predetermined knowledge via measurements). In some cases, the propagation time can be determined (typically at some network entity) by dividing the BS-MS range by the speed of light. The BS can then determine the timing of the frame marker it sends out by simply subtracting the calculated propagation time from the frame marker timing provided by the MS. This approach is further described below in connection with Figures 5A, 5B, 6A and 6B.

The existing "timing advance" within the MS and BS enables a less accurate method of estimating the MS to BS propagation delay. The original purpose of this information concerns intra-cell traffic coordinates. However, timing advance metrics can be manipulated in a straightforward manner to produce these MS-to-BS delay estimates. The accuracy provided by such time alignment parameters is primarily determined by the time analysis of the communication bit intervals involved. Therefore, it is possible to realize propagation delay estimation accurate to several microseconds or tens of microseconds. Although the second method is less accurate than the first delay estimation method, it is particularly advantageous in situations where network manipulation of the exact MS location is excluded.

As previously mentioned, in some applications the base stations do not need to be synchronized to a microsecond type of accuracy, but only to a millisecond or even to a second type of accuracy. Compensating for MS-to-BS delays may not be effective for these cases, since these small delays, on the order of ten microseconds, are insignificant with respect to the required timing accuracy. Therefore, the rough time of day obtained at the MS can simply be used to time stamp the signal from the BS. This is sent to the BS without exact BS-MS range data. This situation is advantageous because GPS receivers are capable of coarse time scaling at signal levels much lower than required for accurate time tagging (see U.S. Patent No. 5,812,087, incorporated herein by reference, and Copending U.S. Patent Application Serial No. 09/062232, filed April 16, 1998). Furthermore, once a coarse time calibration is performed, its accuracy can be maintained over long periods of time due to the high frequency stability of the data transmitted by the base station.

Figure 4 illustrates an exemplary method in accordance with an embodiment of the present invention. In operation 151, the cellular mobile system determines an indication of the time of day at which the cellular mobile communication station is located. In an embodiment wherein a GPS receiver such as GPS receiver 52 is used within a cellular mobile communication station (designated as 50 shown in FIG. Get GPS time. Alternatively, techniques for determining time as described in US Patent 5,812,087 may be used. In this method, a sample of the GPS signal received at the mobile station may be transmitted to a location server or to some other server, where the record is processed to determine the time of receipt, as described in US Patent 5,812,087. Also, the time of day in operation 151 can either be calculated using one of the various methods described in pending US Application Serial No. 09/062232, filed April 16,1998. The method shown in FIG. 4 continues to operation 153 where a propagation delay between the cellular mobile communication station and the cellular base station, such as the cellular base station shown in FIG. 3, is determined. It will be appreciated that this operation is optional in some embodiments described above, where the time determined in operation 151 has more error associated therewith than the propagation delay. As also described above, this propagation delay can be determined by determining the position of the mobile station (by processing the GPS signal) and determining the position of the cell site. The position between these two positions divided by the speed of light determines the propagation delay in operation 153 .

In operation 155, if this optional operation is used, the time at the cellular base station is determined from the time of day at the mobile station (originated from the cellular mobile communication system) and from the propagation delay determined in operation 153 .

Cellular base stations within the network can use this procedure to synchronize all base stations relative to a time standard, such as GPS time. In this way, improved triangulation or range may be obtained from the use of timing information sent between the base stations and the mobile station. Many other uses of timing information can be made. These include allowing a mobile station's communications to be more efficiently "handed off" from one base station to the next, and allowing explicit time to be passed across the network for various purposes.

5A, 5B, 6A and 6B will describe a further example of an embodiment according to the present invention. This method can be implemented with a cellular mobile communication system, such as the system 50 shown in FIG. 2 and the cellular base station 101 shown in FIG. 3 .

The cellular mobile communication station 50 shown in FIG. 2 includes a GPS receiver 52 having a GPS antenna 51 and a cellular communication transceiver 54 including an antenna 53 . Alternatively, GPS receiver 52 may be contained in another chassis (and not integrated within the chassis holding components of mobile station 50, such as cellular communication transceiver 54), but be coupled to and communicate with cellular communication transceiver 54. The mobile station 54 is adjacent; in this case, the mobile station 50 does not include a GPS receiver, nor does it need a GPS receiver, as long as the GPS receiver is coupled to and co-located with the mobile station. The GPS receiver 52 can be a conventional, hardware correlator based GPS receiver, or it can be a matched filter based GPS receiver, or it can be a GPS receiver with a buffer to store the digitized GPS signal processed with a fast convolution receiver, or it may be a GPS receiver as described in U.S. Patent 6,002,363, wherein the components of the GPS receiver are shared with those of the cellular communication transceiver (see Figure 7B of U.S. Patent 6,002,363, which is hereby incorporated by reference ). The cellular communication transceiver 54 may be a modern cellular telephone operating on any known standard, including the GSM cellular standard, or the PDC communication standard, or the PHS communication standard, or the AMPS analog communication standard, or the North American IS-136 communication standard, or Synchronous wideband spread spectrum CDMA standard. GPS receiver 52 is coupled to cellular communication transceiver 54 to provide GPS time and location to cellular communication transceiver 54 in one embodiment (which then transmits this information to the base station). Also, the cellular communication transceiver 54 can provide assistance data such as Doppler information or time information to the GPS receiver, as described in US Patent Nos. 5,841,396 or 5,945,944. The coupling between the GPS receiver 52 and the cellular communication transceiver 54 can also be used to send a record to or receive a record from a cellular base station to match the record with another record to determine the time at the GPS receiver, such as the U.S. described in patent 5,812,087. In those cases or embodiments, where a location server is used to provide assistance data to a cellular mobile communication station in order to determine location or time at the system 50, or where the location server shares information processing (e.g., the location server determines the location of the mobile system 50) time or final location calculation), it will be appreciated that a location server (shown in Figure 7 and described further below) is connected to the cellular base station via a communication link to facilitate data processing. The location of a mobile station is generally not fixed, and is generally not predetermined.

Figure 3 shows an example of a cellular base station, which may be used in various embodiments of the invention. The base station 101 includes a transceiver 102 having at least one antenna 102a for transmitting and receiving signals to cellular mobile communication stations existing in the area served by the cellular base station 101. For example, cellular mobile communication station 50 may be one of the mobile stations served by cellular base station 101, depending on the range of signals typically transmitted by mobile system 50. Cellular transceiver 102 may be a conventional transceiver for transmitting and receiving cellular signals, such as GSM cellular signals or CDMA cellular signals. Clock 103 may be a conventional system clock that maintains the time of day at the cellular base station. The accuracy of the clock can be improved according to the method of the present invention so that the clock is synchronized with other clocks in other cellular base stations as described herein. In many cases, the clock may be highly stable, but over a period of time, the clock may drift a large amount relative to any initial time setting. Cellular base station 101 also typically includes a network interface that transmits and receives data to cellular transceiver 102 for coupling the cellular transceiver to a mobile switching center, as is known in the art. The cellular base station 101 may also include a digital processing system 105, which may either be located remotely from the cellular base station, or may be co-located with the cellular base station itself. Digital processing system 105 is coupled to clock 103 to adjust or recalibrate the clock time, thereby synchronizing the clock with other cellular base stations in accordance with the method of the present invention. In many cases, the clock is highly stable but idling, which can affect network operations and actually change the constant time tick. Instead, the time relative to the point in time can be adjusted. This is what "recalibration" means. The digital processing system 105 is also coupled to the network interface 104 for sending and receiving data or communications to the mobile switching center and for receiving data from the cellular transceiver 102, such as time-stamped frame markers sent from the mobile system, for communicating the clock 103 to other cellular Other clocks within the base station are synchronized.

The method shown in FIGS. 5A and 5B begins at operation 201 when a cellular base station transmits a cellular signal to a cellular mobile communication station. Optionally, the signal may include a request for synchronization information from the mobile system to allow the cellular base station to synchronize itself to other cellular base stations. The cellular base station provides a time stamp or marker within the signal it sends to the mobile system. This flag may be an inherent part of the signal framing structure. This is also shown in FIG. 6A, where base station 1 transmits a signal whose framing structure includes markers M1, M2, M3, M4, M5, M6, M7, M8 and M9, as shown in signal 301 of FIG. 6A. The mobile system within operation 203 of FIG. 5A receives the cellular signal with the logo. Concurrent with the reception of the cellular signal, the mobile station also receives a GPS signal from GPS satellites including GPS time, as is known in the art. The mobile station may then time-stamp the signature within the cellular signal received from the base station with GPS time representing, in GPS time, the time at which the signature was received at the mobile system. This is further shown in FIG. 6A by signal 303 , representing the signal received by mobile station 1 from base station 1 , delayed by propagation delay 307 . As shown in Figure 6A, a time stamp 305 has been applied to marker Ml, which represents the GPS time associated with the time of receipt of the marker at the mobile system. The mobile station in operation 205 determines its position simultaneously with time stamping the marker within the cellular signal. a GPS receiver within a mobile station may either determine its position autonomously (e.g., a GPS receiver based on a conventional hardware correlator may determine its position itself by reading ephemeris from GPS satellites), or it may determine its position with the assistance of a server , such as the location server shown in Figure 7, which is coupled with the cellular network. In operation 207, the mobile station sends its location (or pseudo-range, to allow the location server to determine its location) to the cell site along with the GPS time associated with the marker, which is time-stamped with the mobile station.

In operation 209, the cellular base station calculates its time of day by using the location of the mobile station and its known predetermined location to determine the propagation delay between the mobile station and the base station. This propagation delay is subtracted from the GPS time associated with the marker to determine the GPS time at which the marker was issued. This is shown in Figure 6B, where base station 1 receives time stamp TR1 from the mobile system. This time scale TR1 represents the GPS time relative to the marker M1. The propagation delay 307 is subtracted from the GPS time TR1 to derive the time T1 associated with marker M1. That is, the time T1 is the time stamp 309 relative to the marker T1 at the base station. The current time at the base station can then be updated by correlating the GPS time within the time stamp 309 with the current frame M9 to produce the current time 311 shown in Figure 6B. That is, when the framing structure of the signal 301 is known, there is a known time relationship between the marker M9 and the marker M1 in the signal 301 . The time difference between these two markers when the framing structure is known is added to the time T1 to generate the current time 311 . Thus, the current time at the cellular base station is updated from the GPS time associated with the transmitted signature, which has been time-stamped by the mobile station. This is shown at operation 211 of Figure 5B. Then in operation 213, the last time the clock was synchronized at the cell site is optionally saved in order to determine when it is appropriate to update the clock to synchronize the clock with other clocks in other cell sites. In operation 215, the cellular base station or a remote entity supporting the cellular base station may determine when to resynchronize. For example, a specified time of a few seconds can automatically trigger another synchronization process. Alternatively, other techniques may be used to determine when to resynchronize the clock at the base station with other clocks at other cellular base stations.

FIG. 7 shows an example of a location server 350 that may be used in various embodiments of the invention. For example, as described in U.S. Patent No. 5,841,396, a server can provide assistance data such as Doppler or other satellite assistance data to a GPS receiver in the mobile station 50, or a location server instead of the mobile station 50 can make the final position. is calculated (from which other data may be determined from the mobile station after receiving the pseudo-range or other data) and then forwards this position determination to the base station so that the base station can calculate the propagation delay. A location server generally includes a data processing unit such as a computer system 351, a modem or other interface 352, a modem or other interface 353, a modem or other interface 354, a mass storage device 355 (e.g., for storing software and data), and optionally GPS receiver 356 . The location server 350 may be coupled to three different networks, shown as networks 360 , 362 and 364 . Network 360 may include a cellular switching center or centers and/or a land-based telephone system switch; alternatively, modem 353 may be directly coupled to a cell site, such as cellular base station 101 . It will be appreciated that a plurality of cellular base stations are typically arranged to cover a geographical area with radio coverage, these different base stations being coupled to at least one mobile switching center, as is known in the art (as shown in Figure 1). Thus, multiple instances of base station 101 may be geographically distributed, but coupled together through a mobile switching center. Network 362 may be a network of reference GPS receivers that provide differential GPS information and possibly GPS ephemeris data for use in computing the position of the mobile system. The network is coupled to data processing unit 351 through a modem or other communication interface 354 . Network 364 includes other computers or network components, such as data processing system 105 shown in FIG. 3 (via optional interconnections not shown in FIG. 3 ). Also, network 364 may include computer systems operated by emergency operators, such as public safety answering points that answer 911 telephone calls. Various examples of the use of location server 350 have been described in a number of U.S. patents and patent applications, including U.S. Patents 5,841,396; 5,874,914; 5,812,087; incorporated herein by reference.

The method described above determines the effective transmission time towards the BS antenna. Using a larger number of MSs may tend to reduce error through the averaging process. This assumes that systematic bias can be eliminated.

Sufficient MS activity to support timing (eg, morning hours) can be improved by placing multiple MSs at various locations and making calls periodically.

Typical timing errors due to GPS processing at a single MS can be on the order of 10-30 nanoseconds. Therefore, other error sources such as multipath may dominate.

The stability of the BS oscillator may affect the frequency at which timing measurements need to be made and propagated. It is possible to model the drift of the BS oscillator versus time, thereby reducing such updates.

Several methods for calibrating mobile station receiver errors will now be described. In some embodiments of the present invention, a mobile station (eg, cellular mobile communication station 50 of FIG. 2 ) determines its position P _mobile = [x _m , y _m , z _m ], and a time T _mobile associated with that position. By simply measuring the delay from the position decision time (eg, in GPS time) to the framing mark time, this time can be correlated to the framing mark of the received cellular communication signal. Alternatively, the position decision may be made at a time equal to the set of frame marker times. Therefore, it is assumed without loss of generality that T _mobile is equal to the framing mark time as seen by the mobile station.

It is assumed that the mobile station also knows the location of the base station P _base = [x _b , y _b , z _b ]. Then, if the multipath delay is negligible, the range from the base station to the mobile station at time T _mobile is

R _Tm =[(x _m -x _b ) ² , (y _m -y _b ) ² , (z _m -z _b ) ² ,] ^1/2 .

Now, if there is no delay in the receiving circuit, the propagation delay between the base station and the mobile station will be in the range of R _Tm /c, where c is the speed of light.

For clarity, we refer to the transmission time of the framing flag at the base station as the occurrence time of the flag in front of the transmitting antenna of the base station. Thus, if there were no multipath delay (ie receiver delay), the transmission time of the frame marker at the base station antenna surface would be T _base =T _mobile -R _Tm /c.

Now, a GPS receiver may have a delay associated with its RF and digital signal processing, called _bGPS . Similarly, there may be a delay, referred to as b _comm , associated with the RF and digital signal processing of the communications receiver. Thus, referring to FIG. 2 , b _GPS is caused by delays within the GPS receiver 52 and b _comm is caused by delays within the cellular communication transceiver 54 . Also, there may be an additional propagation delay from the base station to the communication receiver due to multipath, called b _mult . This is assumed to dominate any multipath delays associated with GPS measurements. thus,

Instead of providing an unbiased measurement of transit time at the base station, a measurement biased by b _mult +b _comm -b _GPS is provided. In general b _mult may dominate other error sources, especially if receiver calibration operations are performed (discussed below). Therefore, under normal circumstances, the estimated transmission time of the framing mark will be delayed.

b _comm -b _GPS can be measured by using only a base station simulator which transmits the cellular signal with its framing structure and is directly connected to the antenna part of the mobile station and measures the reception time of the frame marker at the mobile station. In this process, the frame marks are time stamped by the mobile station's GPS receiver (GPS time received by the mobile station's GPS receiver). It is assumed here that the base station emulator uses a GPS receiver to synchronize its transmissions with the GPS time provided by the GPS receiver. Since the propagation delay from the base station to the receiver is zero, this method will determine b _comm -b _GPS without error (except for a small amount of measurement noise). This calibration step can be fully automated and easily incorporated within the receiver testing step during manufacture. Some simple modifications to the process are possible, such as transmitting simulated signals to the mobile station from a simulator very close to the mobile station.

Excessive multipath delay b _mult continues to be the dominant error source when synchronizing base stations. For paths within line of sight, this delay has a bias with a mean of zero. For reflective paths or paths combined with direct or transmitted paths, the mean is greater than zero (measured values are delayed relative to the actual direct path). For a short period of time, the base station will typically estimate from several mobile units, and possibly many times from each mobile unit, associated with each frame marker. Call these date-time estimates D ₁ , D ₂ , . . . , D _K . The minimum value in these estimates is generally much lower than the mean deviation of any individual measurement or measurement average. If the number of measurements K is large, you might rank the measurements from low to high, and maybe take an average of the smallest 10% of measurements, or some similar statistic. This greatly reduces mean deviation, but utilizes some averaging.

If the base station has a highly stable clock, this clock can be used to maintain time between updates from remote mobile units. Clocks can be used in the smoothing process to remove poor measurements from mobile stations due to multipath. Furthermore, the measurements from the mobile station can be used to measure the long-term stability of the base station clock due to eg aging. For example, a GSM superframe is about 3.48 hours, while a large frame is 6.12 seconds. Thus, a superframe is approximately 12528 seconds. A typical GPS time measurement without differential error correction should be accurate to around 100 nanoseconds. This accuracy allows measuring the long-term frequency of the base station oscillator, which is equal to about 100 nanoseconds/12528 seconds = 8×10 ⁻¹² . Measurements over a period of 6.12 seconds even allow a long-term frequency accuracy of about 1.6 x 10 ^-8 . Such long-term stability measurements are best accomplished by making several time-of-day measurements with the same mobile receiver. Therefore, static or slowly moving mobile stations are best suited for this. Continuous measurement of the position of the mobile station will provide the required information about the dynamics of the mobile station.

If there is significant user motion, it is important that any Doppler-related effects do not affect the timing measurements described above. In particular, if a mobile station measures time at one location and predicts the time of day relative to a cellular signal frame boundary occurring at a different location, errors will be introduced due to the movement of the mobile station. This occurs especially when the mobile station is moving fast and/or the differences at these points in time are large. There are many ways to deal with this kind of problem. For example, if the mobile station can determine its velocity, this data can be provided to the base station, which then compensates for errors due to Doppler related range velocity between the mobile station and the base station.

Although the method and apparatus of the present invention have been described with reference to GPS satellites, it will be appreciated that the inventive principles can also be applied to positioning systems using pseudolites or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters that broadcast a PN code (similar to a GPS signal), which may be modulated on an L-band carrier signal, generally synchronized to GPS time. Each transmitter can be assigned a unique PN code to allow identification of remote receivers. Pseudolites are useful in situations where GPS signals from orbiting satellites may not be available, such as in tunnels, mines, buildings or other enclosed areas. As used herein, the term "satellite" includes pseudolites or equivalents of pseudolites, and the term "GPS signal" as used herein includes GPS-like signals from pseudolites or equivalents of pseudolites.

In the foregoing discussion, the invention has been described with reference to the United States Global Positioning Satellite (GPS) system. However, it is obvious that these methods can also be applied to similar satellite positioning systems, especially the Russian Glonass system. The main difference between the Glonass system and the GPS system is that transmissions from different satellites are distinguished from each other by using slightly different carrier frequencies rather than different pseudo-random codes. The term "GPS" as used herein includes such alternative satellite positioning systems, including the Russian Glonass system.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will however be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the specification and drawings should be regarded as illustrative rather than restrictive.

Claims (16)

1. method of between two base stations, carrying out time synchronized at least, described at least two base stations comprise first base station and second base station of cellular communication system, described method comprises:

Determine when day and the position of first cellular mobile station;

By first cellular communication link when day of described first cellular mobile station and position are sent to described first base station;

When determining the day of described first base station during from described first cellular mobile station described day and the known location of described position and described first base station;

Determine when day and the position of second cellular mobile station;

By second cellular communication link when day of described second cellular mobile station and position are sent to described second base station;

When determining the day of described second base station during from described second cellular mobile station described day and the known location of described position and described second base station;

When wherein said first cellular mobile station and described second cellular mobile station have respectively used and be positioned at the satellite positioning system receiver that exists together and determine described first and second cellular mobile stations described day and described position.

2. the method for claim 1, it is characterized in that, during described first cellular mobile station described day is measured period with respect to the frame synchronization that exists in the cellular communication signal, and described cellular communication signal is sent from described first base station and received by described first cellular mobile station.

3. the method for claim 1 is characterized in that, described first base station during day be with respect to the frame synchronization period that exists in the cellular communication signal and definite, described cellular communication signal is sent from described first base station.

4. the method for claim 1, it is characterized in that described cellular communication link is used gsm communication standard, PDC communication standard, PHS communication standard, AMPS analog communication standard, North America IS-136 communication standard or do not had one of synchronous wide-band spread spectrum CDMA standard.

5. the method for claim 1 is characterized in that, described first cellular mobile station is identical station with described second cellular mobile station.

6. the method for claim 1 is characterized in that, described first cellular mobile station is a cellular mobile station different, that separate with described second cellular mobile station.

7. system that between two base stations, carries out time synchronized at least, described at least two base stations comprise first base station and second base station of cellular communication system, described system comprises:

First global position system (SPS) receiver, it can determine when day and the position of first cellular mobile station, first cellular mobile station and a described SPS receiver are positioned at and exist together, and wherein said first cellular mobile station and described position can be the described first honeycomb mobile receiver described day the time is delivered to described first base station;

With first measurement mechanism of described first base station coupling, when determining the day of described first base station when described first measurement mechanism can be from described day of described first cellular mobile station and the known location of position and described first base station;

Second global position system (SPS) receiver, it can determine when day and the position of second cellular mobile station, second cellular mobile station and described the 2nd SPS receiver are positioned at and exist together, and wherein said second cellular mobile station and described position can be the described second honeycomb mobile receiver described day the time is delivered to described second base station;

With second measurement mechanism of described second base station coupling, when determining the day of described second base station when described second measurement mechanism can be from described day of described second cellular mobile station and the known location of position and described second base station.

8. system as claimed in claim 7 is characterized in that, a described SPS receiver is integrated within described first cellular mobile station.

9. system as claimed in claim 8 is characterized in that, a described SPS receiver and described first cellular mobile station are shared at least one common component.

10. system as claimed in claim 7, it is characterized in that, during described first cellular mobile station described day with respect to the frame synchronization period that exists in the cellular communication signal and measured, described cellular communication signal is sent to described first cellular mobile station from described base station.

11. system as claimed in claim 7 is characterized in that, described first base station during day be with respect to the frame synchronization period that exists in the cellular communication signal and definite, described cellular communication signal is sent from described first base station.

12. system as claimed in claim 7, it is characterized in that described cellular communication link is used gsm communication standard, PDC communication standard, PHS communication standard, AMPS analog communication standard, North America IS-136 communication standard or do not had one of synchronous wide-band spread spectrum CDMA standard.

13. system as claimed in claim 7 is characterized in that, described first cellular mobile station is identical station with described second cellular mobile station.

14. system as claimed in claim 7 is characterized in that, described first cellular mobile station is a cellular mobile station different, that separate with described second cellular mobile station.

15. the method for claim 1 is characterized in that also comprising further method inclined to one side when determining cellular mobile station in the cellular mobile station, it combines and is positioned at the SPS receiver that exists together, and wherein said further method comprises:

Described cellular mobile station is placed near the cellular basestation simulator;

Described cellular basestation simulator is synchronized to the correct time benchmark;

Be positioned at when day that the SPS receiver that exists together is determined described cellular mobile station with described;

Inclined to one side when determining described cellular mobile station during with described day;

Be stored in partially in the memory that is attached to described cellular mobile station during described cellular mobile station.

16. method as claimed in claim 15 is characterized in that, described cellular mobile station be meant in described first cellular mobile station and described second cellular mobile station at least one of them.

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