Method and measuring device for base station frequency applied for cellular communication networks equipped with mobile receivers

TECHNICAL FIELD OF THE INVENTION

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

BACKGROUND

To determine the location in cellular networks (e.g., cellular telephone network), there are several approaches to using triangulation based on timing information transmitted between each of several base stations and mobile devices such as cellular phone. In one approach, referred to as "Time Difference of Arrival" (TDOA, TDOA), the times of reception of the signal from the mobile device is measured at several basestations, and these times are transmitted to the object position determination referred to as a location server, which calculates the position (position) of the mobile device with using these times of reception. For this approach to work you need to know the exact position (the position) of base stations, and the current time on these base stations must be coordinated to ensure accurate measurement of the location. The coordination on time - this is an operation to maintain, at a particular time, the current time associated with multiple base stations within a predetermined error.

1 shows an example of a TDOA system where the times of reception (VP1, VP2 and VP3) of the same signal from the mobile cellular telephone 111 are measured at cellular basestations 101, 103, and 105 and processed by the server 115 of the positioning. Positioning server 115 is connected to receive data from the base stations through the mobile switching center 113 link. Center 113 mobile switching provides signals (eg, voice) on a fixed public switched telephone network (PSTN, PSTS) and from it, which allows to transmit signals from the mobile phone to other phones (eg, phone landline PSTN or other mobile phones ) and back. In some cases the location server may also monitor the transmission of several base stations to determine the relative timing of these transmissions.

An alternative method, referred to as "enhanced observed time difference" (RNRV, EOTD) or "extended forward link trilateration (RTPL, AFLT), comprises measuring a time for the mobile device of signals sent from each of several base stations. 1 is applicable to this case, if you reverse the direction of the arrows VP1, VP2 and VP3. This timing data may then be used to calculate the position of the mobile device. This calculation can be performed on the mobile device or location server if the timing information obtained by the mobile device is transmitted to the location server through the communication line. Again, the current time on the mobile stations must be coordinated, and their location must be accurately determined. In each approach, the location of the base stations can be determined by standard surveying methods and be stored at the base stations, the location server, or elsewhere in the network computer memory of a particular type.

A third method of determining location involves the use in a mobile device with a global satellite positioning system (GPS, GPS) or other satellite positioning system (BSC, SPS). This method may be completely autonomous or may be accomplished using a cellular network to provide assistance data or to share in the position calculation. Examples of this process are disclosed in U.S. Patent number 5841396; Number 5945944 and № 5812087. These various methods will, in general, call MTP. In specific implementations, inexpensive mobile cellular communications receiver and the SPS receiver are united in one body, and may in fact share common circuitry.

The combination RNRV or TDOA system with the MSP called "mixed" system.

From the above description that the systems RNRV, RWP or hybrid systems coordination in time between cellular base stations needed to accurately calculate the position of the mobile device. The requirement for the accuracy of the current time in the base stations dependent on the specific positioning method.

According to another embodiment of the methods described above are round trip delay (delay in the passage of signals in the forward and backward directions) (DSZ, RTD) for the signals sent from the base station to the mobile device and returning back. According to the similar, but alternative methods are two-way delay for signals sent from the mobile device to the base station and returning back. Each of these values ​​bisect the round trip delay to determine an estimate of one-way delay. Knowing the location of the base station and the one-way delay can limit the location of the mobile unit circle on the earth's surface. Two such measurements provide the intersection of two circles, which in turn constrains the location to two points on the earth's surface. A third measurement (even an angle of arrival or cell sector) resolves the uncertainty. When using the round trip delay approach, it is important to measure DSZ are coordinated, in the worst case, up to a few seconds, in the event of rapid movement of the mobile measuring apparatus correspond to the mobile device, which is about the same place.

In many cases it is impossible to produce round-trip delay measurements for each of two or three base stations, but only one base station, which is a main communicating with a mobile device. For example, it may be the case of using the North American cellular standard IS-95 CDMA. Or maybe even impossible to carry out accurate measurements bilateral Hold (to within a fraction of a microsecond) due to limitations of equipment or the signaling protocol. This is possible in case of a GSM cellular communication. In these cases, even more important to maintain accurate timing (or relative timing) of the base station, if needed to implement the triangulation operation, since only the difference in time between different paths from the mobile device to the base station.

Another reason to maintain accurate timing information at basestations is to provide time to the mobile devices to assist the position calculation based on the GSM; and such information may result in reduced time to first fixed and / or hypersensitivity. Such examples are contained in U.S. Patent number 6150980 and number 6052081. The required accuracy for these cases can vary from a few microseconds to around 10 milliseconds, depending on the desired performance improvement. In a mixed system, the timing of the base station serves the dual purpose of improving the work of the RWP (or RNRV) and the work of the GSP.

Approaches that meet the state of the art, to coordinate timing involve the use of special systems of fixed timing of location, referred to as "location measuring units» (LMU) or "modules measuring timing» (TMU). These units typically include fixed location GPS receivers which enable to determine the exact current time. Location modules can explore, e.g., using equipment intelligence based on GPS. In alternative embodiments, the LMU or TMU can not rely upon an absolute time provided by a GPS receiver or other source, but simply based on the timing of one base station with respect to another base station in a differential sense. However, such an alternative approach (without using a GPS receiver) relies on the possibility to observe one object, multiple base stations. Furthermore, this approach can account for the accumulated error on the network.

Typically, LMU or TMU watch timing signals, such as framing marks present in the cellular communication signal transmitted from the base stations, and try to carry out temporary tagging these timing signals local time found via GPS devices or other time determination device. You can then send messages to the base stations (or other infrastructure) that allow these objects to track elapsed time. Then, on command or periodically, can be sent via cellular network to mobile devices served by the network, specific messages indicate the current time associated with the frame structure of the signal. This is particularly simple in systems such as GSM, where full frame structure lasts for a period in excess of 3 hours. Note that the location measurement units may be used for other purposes, e.g., they may act as a location server, i.e. LMU may actually perform measurement time of receipt of a signal from mobile devices to determine the location of mobile devices.

One problem associated with the LMU or TMU approach, is that they require a new special fixed equipment at each base station or elsewhere in the coverage area of ​​multiple base stations. This can lead to a substantial increase in the cost of installation and maintenance.

SUMMARY OF THE INVENTION

Described herein are methods and apparatuses for frequency synchronizing base stations in a cellular system.

According to one aspect of the invention, a method for predicting the transmission timing of the base station in a cellular communication system is receiving a first time tag for the first timing mark in the first cellular signal transmitted from the base station; receiving a second time tag for the second timing mark in the second cellular signal transmitted from the base station; and computing a frequency related to the base station using the first and second time tags. Each of the time tags are determined using at least one satellite positioning system signal received at the mobile station, which also receives the corresponding time marker contained in the cellular signal from the base station. In one example according to this aspect, the time tags are determined from the current-time messages in satellite positioning signals. In another embodiment corresponding to this aspect, the time difference between at least two time tags are determined based on the local reference signal, the frequency of which is determined by the signal processing of satellite positioning.

According to another aspect of the invention a method of measuring the frequency associated with the base station is receiving a mobile station on the at least one satellite positioning system signal; determining a frequency reference signal from the local oscillator of the mobile station based on the at least one satellite positioning system signal; receive at the mobile station cellular signal from a base station, the cellular signal modulates a carrier; measured carrier frequency using the reference oscillator signal; and determining a frequency associated with the base station using the carrier frequency.

The present invention provides a device for performing these methods, including data processing systems implementing these methods and machine readable media which when run on a data processing system, cause the systems to perform these methods.

Other features of the present invention will become apparent from the accompanying drawings and the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like elements are designated by similar reference numerals.

1 - an example of a cellular network according to the prior art, which provides for locating a mobile cellular device.

2 - an example of a mobile cellular communication station which may be used according to the present invention and which includes a GPS receiver and a cellular communication transceiver.

Figure 3 - a block diagram of a combined mobile station which may be used according to the present invention, and in which the GPS receiver and the cellular transceiver share common circuitry.

4 - an example of a cellular base station, which can be used according to various embodiments of the present invention.

5 - example of a server which can be used according to the present invention.

6 - network topology for measuring frequencies of signals from the base stations in accordance with one embodiment of the present invention.

7 - frame structure signal cellular GSM.

8 - a flowchart for determining the frequency of the base station according to one embodiment of the present invention.

9 - detailing a method for determining the frequency of base station signals using a framing period measuring base station signals according to one embodiment of the present invention.

Figure 10 - is another method for determining the frequency of base station signals using a framing period measuring base station signals according to one embodiment of the present invention.

11 - detailing a method for determining the frequency of base station signals using measurements of carrier frequency signals of the base station according to one embodiment of the present invention.

Detailed description

The following description and drawings are to illustrate the invention and should not be construed as a limitation of the invention. Numerous specific details are described to provide a thorough understanding of the present invention. However, in certain instances, well-known or conventional details are not described in order to avoid obscuring the present disclosure.

In most digital cellular systems, numbered framing marks are transmitted as part of the cellular system transmissions. In such a network, like GSM, the current time information from the GPS receiver can be used for time tagging frame structure (e.g., framing markers) of the received communication signal (e.g., GSM). For example, you can use the top of the frame boundaries of an individual GSM, which occurs every 4, 6 milliseconds (see para. 7). The hyperframe that lasts 3.48 hours, has 2,715,648 of such personnel; therefore, each such frame boundary is unambiguous for all practical purposes. In copending U.S. Patent Application No. 09 / 565.212, filed May 4 2000, describes a method for time coordination, according to which the mobile station (MS) containing GPS receivers are used to measure both the current time and position with high precision . Time information tag honeycomb framing measured at the mobile station is transmitted via regular cellular signaling to a base station (BS) (e.g., cellular base station, as shown in Figure 4) or other network entity (e.g., a server or a location server) to determine the current time of the base station. The delay due to the propagation time from the mobile station (MS) (e.g., the mobile cellular communication station shown in Figure 2) to the base station (BS, BS) may be determined (typically at the base station or other network entity) by dividing the distance BS-MS for the speed of light, since the mobile station to determine its position using the GPS unit and the base station knows its location (for example, through exploration). The base station may determine the timing of its mark transmitted frame by simply subtracting the estimated propagation time from the time of the tag label framing provided by the mobile station.

With time coordination between the base stations is closely related to frequency coordination (or synchronization) between the base stations. It is desirable that coordination in time, being installed, maintained for a long period of time. Otherwise, this would have to perform coordination often leading to complex and costly operation. For example, the base stations can coordinate their time by exchanging signals with each other over existing communication channels (e.g., cellular channels). The constant need for such alarms would lead to a waste of valuable resources links that can be used to transmit voice and other data.

To avoid frequent time coordination is desirable to have at each base station a precise measurement of the frequency of the primary signal source, or alternatively, the frequency of the source base station with respect to the sources of other base stations. If the frequencies of the primary signal source base station is known with high accuracy, the current time at these basestations, once coordinated, it can be maintained over a long period of time using the time interval counter.

In at least one embodiment of the present invention provides for the implementation of frequency coordination between base stations. The method of the present invention involve the use of conventional mobile cellular communication receivers that are equipped with GPS positioning means, without deploying fixed costly network resources.

One embodiment of the present invention involves the use of a cellular transmission timing markers (e.g., framing markers) for frequency synchronization. Measuring the transmission frequency tags crop base stations is used to accurately assess differences between the best and the true timing between successive crop marks. This discrepancy may be extrapolated forward in time as a function of label number using a standard curve fitting algorithm. Thus, the time of the marking of the frame can be used as an accurate clock for a long period of time after receiving the initial timing mark of the frame and get a good assessment of the frequency of shot mark (or deviation from the nominal frequency).

Another embodiment of the present invention provides the use of the carrier frequency of cellular transmissions for frequency synchronization. In most cases, the label and the carrier frequency of the framing signal from the cellular base stations are synchronized with the same reference signal generator at the base station. Thus, by simple mathematical calculation of the carrier frequency signal can be obtained cellular label base station signal framing.

In at least one embodiment of the present invention, framing mark frequency transmitted by the transmitter of the base station is determined for frequency coordination. However, labels and framing code signal (assuming digital modulation), as well as the frequency of the carrier signal is normally synchronized to a common main oscillator (e.g., oscillator 413 in Figure 4) in the digital cellular system. In several important cellular systems, including the system GSM, the Japanese system PDC and system WCDMA (WCDMA - wideband system multiple available CDMA), frequency timing signals (e.g., tags framing) and the carrier frequency is removed from the same the main generator. Thus, accurate measurement of the frequency of transmission or framing marks (symbol rate) or the carrier frequency of such transmissions can be used to achieve the same purpose. Carrier frequency can be used to display the transmission frequency and vice versa. Advantages and disadvantages of each measure related to the implementation details and precision measurements.

In one embodiment, one or more mobile stations make one or more timing measurements of received signals of the base station and transmit these time tags and optional additional information to a server, which in turn performs the calculation of the frequency.

According to another embodiment, the one or more mobile stations measure the carrier frequency of the received signals of the base station and transmit information about the carrier frequency and optional additional information to a server.

According to another embodiment, each of the one or more mobile stations produces at least two timing measurements for the received signals of the base station calculates a measure of the frequency (or, equivalently, the time interval) on the basis of these measurements, and transmits the frequency measure to a server.

In various embodiments, the server may collect a sequence of data from the mobile stations to perform further processing, in order to better estimate the frequency or to perform the curve fitting operation of frequencies depending on time.

Obviously, the transmission frequency of the mobile base station can be calculated at the base station (BS) for a mobile station (MS) or on a server (e.g., location server or other network entities).

Thus, for time synchronization of base stations (equivalently, determining transmission timing marks these base stations), the different methods of the present invention are intended to determine the frequency of these transmissions from the base stations, which can be an important part of the above problems time synchronization.

Figure 2 shows an example of a mobile station containing a GPS receiver, which can be used according to the present invention. GPS receiver can determine the time when the current reception signal (e.g., timing marks cellular signal received by transceiver 21) and the position of the receiver and the frequency of the incoming signal from the outside with high precision. Measurements of the current time, position, and frequency may be carried out in standalone mode if the received signal level is high, or via hardware infrastructure (servers) if the signal / noise ratio for the received signal is small (see., E.g., U.S. Patent number 5,945,944; number 5841396 and number 5,812,087).

Mobile station 210 is a cellular communication system shown in Figure 2, comprises a GPS receiver 211 connected to a GPS antenna 203, transceiver 213 and cellular antenna 201 connected to the communication. Alternatively, the GPS receiver 211 can be placed in another frame; In this case, the station 210 contains a GPS receiver and does not require it, because the GPS receiver is connected to station 210 and stored in one place with it.

Receiver 211 GSP may be a conventional GPS receiver based on hardware correlator may be a GPS receiver based on a matched filter or it may be a GPS receiver which uses a buffer to store digitized GPS signals processed by the rapid transformation of the convolution, and can also be GPS receiver as described in U.S. Patent number 6002363, according to which the components of the GPS receiver are shared with cellular transceiver components (see., e.g., Figure 7B of U.S. Patent number 6002363, which is incorporated herein by reference).

Transceiver 213 mobile can be a modern cell phone operating on any of the common cellular standards, including cellular standards GSM, Japanese standard connection a PDC or Japanese communication standard PHS, standard analog connection AMPS, the North American communication standard IS-136 standard or unsynchronized broadband spread spectrum CDMA.

GPS receiver 211 is connected to a transceiver 213 for transmitting cellular time and GPS position, according to one embodiment, transceiver 213 cellular network (which then transmits this information to the base station). In another embodiment, GPS receiver 211 provides support to accurately measure the carrier frequency of cellular signal received by transceiver 213.

In one embodiment GPS time may be obtained at the mobile station 210 by reading GPS time, GPS signals from GPS satellites. Alternatively, a method for determining time as described in U.S. Patent number 5812087. According to this approach, the sample GPS signals received at the mobile station, can be transmitted to the location server or to some other servers where the signal sample is processed to determine the time of reception, according to the patent number US 5812087. In addition, the current time may alternatively be computed using one of various methods described in U.S. Patent number 6,215,442.

In addition, cellular transceiver 213 may provide the GPS receiver auxiliary data such as Doppler information or time information, as described in U.S. Patent number 5,841,396 and number 5945944. The connection between GPS receiver 211 and cellular transceiver 213 may also be used to transmit the data record GPS to cellular base station or away from it for the purpose of matching that record with another record in order to determine the time at the GPS receiver, as described in U.S. Patent number 5812087. In those instances or embodiments, when / where a location server is used to provide auxiliary data the mobile stations of cellular communication in order to determine position or time at the system 210 or a location server shares the information processing (for example, the location server determines time or the final calculation of the position of the mobile system 210), it is obvious that the location server, for example, shown in 5 and described below, is connected to a cellular base station through a communication link to assist in processing data.

The position of the mobile station 210 is normally not fixed and is typically not pre-determined.

3 is a block diagram showing a combined mobile station which may be used according to the present invention and which is shared by a general scheme for the GPS receiver and cellular transceiver. The combined mobile station 310 includes circuitry to perform the functions required for processing GPS signals as well as the functions required for processing communication signals received through line 360 ​​to cellular base station 352 or away from it.

The mobile station 310 are combined GPS receiver and a transceiver provider. The circuit 321 of detection and tracking is connected to the GPS antenna 301, and a transceiver 305 connected to the communication antenna 311 communications. The generator 323 provides reference signals for the circuit 321 and the receiver 332 of communication. GPS signals are received through GPS antenna 301 and provided to circuitry 321 which detects the GPS signals received from various satellites. Processor 333 processes the data generated by circuit 321, for transmission by transceiver 305. Communication transceiver 305 contains a switch 1 transmission / reception which routes communication signals (typically RF) to an antenna 311 and communication therefrom. In some systems, etc., instead of the switch / per use a comb filter or duplexer. Received communication signals are received at transceiver 305 and transmitted to communication processor 333 for processing. Communication signals to be transmitted from the processor 333 are input to modulator 334, frequency converter 335 and power amplifier 336. U.S. Patent number 5874914, incorporated herein by reference, describes in detail a combined mobile station that contains a GPS receiver and a cellular transceiver and utilizes a communication link.

The carrier frequency signal from the cellular base station can be measured using the GPS receiver in different ways. According to one method, cellular receiver 332 carries out frequency synchronization and phase synchronization of the received carrier from the base station. This is usually used voltage controlled oscillator (VCO, VCO) (e.g., oscillator 323) having a circuit configuration with a frequency synchronization or phase synchronization, which can be controlled by a signal from a receiver communication line 340. In this case, the long-term frequency component of the VCO is proportional to the long transmitted carrier frequency component of the base station (after removing a Doppler frequency shift due to speed of the mobile station). The VCO output can be used as a frequency reference for downconversion circuitry of the GPS receiver (e.g., used in the circuit 321 detect and track). The signal processing order determined by the GPS receiver, frequency errors for the various received GPS signals received from several GPS satellites. Each received signal also contains a common component of frequency error due to the error of the VCO with respect to the ideal value. This frequency errors introduced by the VCO (a so-called frequency "shift") can be determined and scaled to determine the frequency of the base station after removing a Doppler frequency offset due to the motion of the mobile station.

It is common knowledge that such "common mode" frequency shifts can be obtained by GPS processing. Accepted frequency error due to a combination of movement and the receiver in phase shift. The motion is described by a three-component velocity vector. Thus, given the in-phase shear generally be determined four unknowns associated with frequency. Signals received from four different GPS satellites, typically addresses these four equations and hence the common mode bias error to find due to the VCO error. By implementing multiple sets of frequency measurements over a period of time can further reduce the number of GPS satellite signals that must be taken. Similarly, limiting the speed of the receiver (for example, assuming that the movement along the z axis is small), it is possible to further reduce the number of required received satellite signals.

According to an alternative approach, described GPS receiver can have a reference signal that is independent of the VCO used in the cellular transceiver. In this case, the GPS receiver again determines the frequency of its reference signal (typically from a crystal oscillator). The output of the cellular transceiver VCO and the reference signal for the GPS receiver can be sent to the reference frequency circuit, which determines, in a manner known in the art, the frequency ratio of the two reference signals. Since the frequency of the reference signal for the GPS receiver has been determined, the frequency of the cellular transceiver VCO can be determined from the frequency ratio. Since the VCO is synchronized in phase or frequency of the incoming signal with the carrier of the base station, the carrier frequency can be determined by a simple procedure signaling. In order to eliminate the Doppler frequency offset due to the motion of the mobile station relative to the base station, in addition to the speed of the mobile station usually needs to know the location of the base station. A server that performs the final calculation of the frequency of the base station usually knows the location of the base station.

Figure 4 shows an example of a cellular base station, which can be used according to various embodiments of the present invention. Base station 410 comprises cellular transceiver 411 coupled to at least one antenna 401 for transmitting signals to mobile stations and cellular communication between them, that are in the service area of the cellular base station 410. For example, the mobile stations 210 and 310 may be cellular be mobile stations served by the cellular base station 410. Cellular transceiver 411 may be a conventional transceiver used to transmit and receive cellular signals, such as GSM cellular signal or a CDMA cellular signal (CDMA). Generator 413 may be a conventional system oscillator which controls the signal frequency of the base station. The frequency of this oscillator may be measured by methods according to the present invention for frequency synchronization. In many cases oscillator 413 may be highly stable, but after a certain period of time, a small error in the oscillator frequency will result in the maintenance phase sync base station from ideal by a large amount. Accurate measurement of the frequency of the generator can be used to predict the timing error of the base station and framing error timing marks transmitted by the base station. Cellular base station 410 typically also includes a network interface 415 which transfers data to the cellular transceiver 411 and from the connection to the cellular transceiver 411 to the center 421 mobile switching as known in the art. Cellular base station 410 may also comprise a box and together with the data processing system 423. Alternatively, data processing system 423 may be removed from the base station 410. In some embodiments, the data processing system 423 is connected to the generator 413 to adjust or recalibrate Vreme audio clock to thereby synchronize the clock to other clocks in other base stations according to the methods described in co-pending US patent application number 09/565212 for, filed May 4, 2000 In many cases, 413 hours of highly stable, but work in the free-running mode, and it will have an impact on the network based on the actual change in the period of hours. Instead it is possible to regulate the time associated with periods of hours. This process is called "recalibration". Therefore, in order frequency synchronization may be no connection between the data processing system 423 and oscillator 413. The data processing system 423 is connected to the network interface 415 for receiving data from the cellular transceiver 411, such as time tag information for the frame markers measured by the mobile systems for synchronization with other cellular base stations, or for computing the frequency of transmission of framing markers. In practice, the base station may comprise a physical structure mast, one or more antennas and a set of electronics.

5 shows a data processing system that may be used in accordance with various embodiments of the present invention. For example, as described in U.S. Patent number 5841396 server may provide assistance data such as Doppler or other satellite assistance data to the GPS receiver of the mobile station 210. Additionally or alternatively, the final calculation of the position can perform the location server instead of the mobile station 210 ( after receiving pseudoranges from the mobile station or other data from which pseudoranges can be determined), which can then send the location information to the base station to the base station can calculate frequency. Alternatively, the frequency may be calculated location server or on other servers, or at other base stations. A data processing system, such as a location server typically includes communication devices 512, such as modems or network interface, and, optionally, connected to a co-located GPS receiver 511. The location server may be connected to several different networks through communication devices 512 (e.g., modems or other network interfaces). Such networks include a mobile switching center 525 or more centers of mobile switching switches 523 landline telephone system, cellular base station, other GPS signal sources 527, or other processors of other location servers 521 definition.

Multiple cellular base stations are usually capable of covering a geographic area of ​​radio coverage area, and these different base stations connected to at least one mobile switching center, which is known in the art (see, e.g., Figure 1). Thus, multiple instances of base station 410 are geographically separated but interconnected mobile switching center. Network 520 may be connected to a network of reference GPS receivers which provide differential GPS information and may also provide GPS ephemeris data for use in calculating the position of mobile systems. The network is connected via a modem or other communication interface to the processor 503. The network 520 may be connected to other computers or network components, such as data processing system 423 shown in Figure 4 (through an optional interconnection not shown in Figure 4). In addition, the network 520 may be connected to the computer systems used by operators of emergency (emergency) services such as public safety dispatch centers, which are responsible 911. Various examples of how to use the location server described in numerous US patents, including US Patent number 5841396; Number 5874914; Number number 5812087 and 6215442, which are incorporated herein by reference.

Positioning server 501, which is a data processing system 502 comprises a bus connected to the microprocessor 503, ROM 307, volatile RAM 505 and nonvolatile memory 506. The microprocessor 503 is connected to cache memory 504 as shown in the example in Figure 5. Bus 502 interconnects these various components. Although Figure 5 shows that the non-volatile memory is a local device, directly connected to the remaining components of data processing system, it is obvious that the present invention can use a nonvolatile memory, remote from the system, such as a network storage device connected to the data processing system through a network interface such as a modem or Ethernet interface. Bus 502 may include one or more buses connected to each other through various bridges, controllers and / or adapters are known in the art. In many cases the location server may perform its operations automatically without human assistance. In some designs, which require human interaction, the controller 509 I / O can communicate through the displays, keyboards and other input / output devices.

Note that while Figure 5 illustrates various components of a data processing system, it is not intended to represent any particular architecture or a particular method of joining the components together, because such details are not within the scope of the present invention. It is also obvious that the present invention allows the use of network computers and other data processing systems which have fewer components or perhaps more components.

From this description that aspects of the present invention may be implemented, at least in part, in software. Thus, a computer system or other data processing system can execute the methods of its corresponding processor executable sequences of instructions contained in memory such as ROM 507, volatile RAM 505, nonvolatile memory 506, cache 504 or a remote storage device. In various embodiments of the present invention in conjunction with the program instructions can use electronic circuitry. Thus, the techniques are not limited to any specific combination of hardware and software, and any particular source instructions executable by a data processing system. Also within this description, various functions and operations are described as being implemented or due to software code to simplify description. However, those skilled in the art will appreciate that these terms mean that the functions performed by a processor executing code, such as processor 503.

In some embodiments, the methods of the present invention may be implemented on computer systems which are simultaneously used for other functions, such as cellular switching, messaging services, etc. In these cases, some or all of the equipment shown in Figure 5, is shared for several functions.

Figure 6 shows a general system topology which may be used according to the present invention. For illustrative purposes, the figure is greatly simplified; however, it illustrates a number of different situations that may be used in practice.

6 shows three mobile stations (615, 616, and 617), two cellular base stations (613 and 614), the orbital group of the three GPS satellites (610, 611, and 612) and one server 618 of the positioning.

Server 618 positioning communicates with other infrastructure (usually) over a wireless link 622, communication lines 619 and 620 cellular infrastructure (typically wired), and through the communication infrastructure 621 (typically wired). Transfer the GPS satellites 623-625 shows unfilled. Transmissions from the base station 613 have shading (e.g., 626), and transmission from the base station 614 have a solid fill (e.g., 627). Reception signals of the mobile stations (with SPS receiver) followed by the same coding scheme. Thus, it follows from Figure 6 that MS 615 receives signals from the GPS satellites and from both BS 613 and BS 614.

For simplicity, all the mobile stations receive signals from all GPS satellites, but in practice it is not necessary. In practice, there may be a plurality of positioning servers, many more base stations and mobile stations; and each individual mobile station may monitor the transmission of more than two base stations. In addition, the location server may be combined with the base stations, or removed from the base station (as shown in Figure 6).

In the example shown in Figure 6, the mobile station 616 normally performs bidirectional communication with only one of the base stations from which it receives signals. For example, the MS 616 can perform two-way communication with the base station 613, but still can receive transmissions from both base stations 613 and 614. Thus, MS 616 in this case may perform synchronizing operations at the base stations 613 and 614, although in this example it transmits only synchronization information to the base station 613. In the prior art that cellular telephones monitor transmissions of other base stations in addition to the main or "serving" base station site to prepare for future transmissions, or "handover" to another base station.

6 also shows a location server, which can transmit data to and from mobile stations via their communication infrastructure and the cellular infrastructure. The location server may be located at the base station, but it is usually removed from the base stations, and in fact may communicate with several base stations. The timing information provided by the mobile stations normally receive one or more location server, which process this information to determine the absolute and relative timing of the base station.

Figure 7 shows the frame structure of traffic channels signals cellular GSM. In GSM traffic signal, a superframe occurs every 6.12 seconds and a hyperframe occurs every 2048 superframes, i.e. 3.4816 per hour. Therefore, the superframe period is convenient to measure the time interval. Alternatively, you can use the total number of frames, multiframes, etc., since their inception are uniquely determined as a multiple of the bit duration.

According to one embodiment of the present invention, the duration of transmission is measured between two framing markers contained in the cellular communication signal transmitted by a cellular base station. One or more mobile stations produces a set of measurements to determine the duration, i.e. timing later crop marks in relation to the earlier mark frame. The measured duration is compared (typically on a server) to an expected timing. The results are used to determine the error in the frequency of the base station generator against the desired value.

The error in the measurement can be set as a percentage of the true value and expressed in parts per million (ppm.). For example, if the time between specific labels framing is set to 1 second, but the measured value is equal to 1 second plus 1 microsecond, the error can be expressed as 1 microsecond / 1 second = 1 million hours. This is a convenient way to specify the error, since it also applies to errors other synchronized periods (e.g., bit rate) as well as the error in carrier frequency of the base station taking into account (which is actually the case) that the transmitted carrier frequency is synchronized to the framing markers.

Suppose one or more mobile stations measure the duration of the signal of the base station corresponding to 98 transmitted superframes, approximately equal to 10 minutes. The exact measurement of time may correspond to the beginning of a numbered multiframe. The mobile station monitors the number of unique multiframe by the signaling information transmitted in the programs of the main range. Therefore, the ideal measurement period accurately known, and is expressed in terms of the duration of the transmitted bit (bit period is 48/13 microseconds). The ideal measurement period is equal to 98 times the ideal period of each superframe, ie, 599.76 seconds. However, the actual measurement time is subject to errors in the transmitter clock and various errors associated with the measurement.

When the time interval between two predetermined marks crop, lasting about 600 seconds, measured with an error of less than 1 microsecond measurement error transmission frequency tags crop is less than 0,001,670 h. This accuracy is consistent with the long-term and short-term frequency stability of thermally stabilized crystal oscillators, are commonly used in cellular base stations, even though the absolute accuracy of such oscillators is often much lower. In fact, in many cases the framing frequency tags can be measured with much higher accuracy. Although the maximum absolute error in the oscillator frequency of GSM base stations is given of 0.05 ppm., The stability of these generators is typically much greater than this value.

The measured duration may be even several hours to achieve high accuracy of measurement, given that term stability of the oscillator of the base station supports such accuracy and long-term care characteristics (e.g., due to aging) follow a smooth curve. For example, the measurement period duration of one hour with an accuracy of 1 microsecond provides frequency accuracy 0.000278 ppm., Again consistent with the short-term stability of the thermally stabilized crystal oscillator of high quality. In fact, it is well known that high-precision crystal oscillators is ten times higher.

Thus, measuring the duration of transmission between two framing markers using a mobile station can provide a very accurate measurement of the frequency of transmission of framing markers, which can be associated with the frequency generator of the base station.

8 is a flowchart of determining transmission frequency of the base station according to one embodiment of the present invention. In step 801, the moments of signals cellular transmitted by the base station measured at different time points. Moments receipt framing markers (e.g., boundaries of certain frames) are measured using one or more mobile stations (e.g., MS 210, MS 310 or MS 615-617) with GPS receivers. Then we can calculate the transmission frequency of the base station using the arrival times of these cellular signals. Frequency framing marks can be calculated by dividing the known number of framing markers present in the measurement period, the duration of this period. Since the carrier signal frequency of the base station and the transmission frequency of framing markers are synchronized with the clock frequency of the base station can determine the frequency of the master oscillator of the base station and the carrier frequency signal of the base station. In some embodiments, computationally more convenient to compute the period of transmission from the base station.

According to the above definition of cellular transmitter frequency is usually done on the server, or so-called "object position determination» (OOM, SCEP), rather than a cellular base station, although OOM may be in the same location as the cellular base station. This server or OOM is a set of equipment which is in cellular infrastructure or communication network infrastructure which may pass messages to mobile stations and from the network through the communication network and cellular wireless links. Thus, when the mobile stations make measurements relating to the transmission timing of base stations, such measurements are transmitted over communication lines to the serving cellular base station and then through a fixed line infrastructure OOM. OOM uses these measurements to calculate the time and frequency related to future crop marks. This information can then be provided to the mobile stations or to other network entities wishing to utilize such information to improve system performance. In fact, in one embodiment, such timing information acts as assistance data that allows the mobile stations to more effectively carry out reception operation and future GPS measurement. This embodiment also includes an approach of "promotion", according to which the earlier GPS measurements produced by some mobile stations, significantly improves the performance of subsequent GPS measurements. This increase in productivity involves a considerable increase in sensitivity, reduced time to first fix, and increased efficiency, as described in U.S. Patent number 5841396 and number 5,945,944.

Figure 9 shows in more detail a method for determining the frequency of base station signals using a framing period measuring base station signals according to one embodiment of the present invention. In steps 901-909, the first mobile station (MS) receives the cellular signal from a base station (BS); Crop mark is contained in a cellular signal; He finds the current time and its own location using its GPS receiver; It assigns a temporary tag labels framing using the current time, found at step 905; and sends its location (or information for the determination of its location) and the time tags (or information for determining the time tags) to a server, such as the location server.

It is obvious that step 905 may be preceded by steps 901 and 903 or performed simultaneously with steps 901 and 903. The transmission path for sending the location information and time tag typically includes a cellular communication line in conjunction with additional landlines (e.g., telephone lines, local networks and so on.).

Cellular signal received in step 901, may receive over a communication link different from that used to transmit data in step 909. This means that the base station is observed at step 901 may not be a "serving" base station to the mobile station. This may be a base station that the mobile station monitors for a short time to determine a list of "neighboring" base stations, which can be used in the future for a handover operation. It often happens that a mobile station may observe 10 or more base stations, which are known in the art.

The second mobile station (or even the same base station) performs steps 911-919 similar to steps 901-909. Typically, the steps 911-919 are performed at times different from those made when steps 901-909. Obviously, the steps 911-919 can be performed on the same mobile station, which performs steps 901-909, but at different times.

At step 921 the server (e.g., a location server) processes the time tags received from the mobile stations, the locations of mobile stations and location information of the base station for calculating the frequency of the base station, such as the frequency associated with the frequency tags crop, or any other frequencies base station are synchronized with this frequency. The frequency can be expressed in terms of a nominal (ideal) frequency and an error, with the latter being expressed, for example, in dimensionless units of ppm. Since time tags correspond to the times of receipt tag framing the measured mobile station (or stations), the locations of mobile stations and the base station needs to convert from the time tags into time measures at the same location, in order to calculate the exact length of transmission. To this was subtracted from the time tags the signal delay on the cellular distribution of the transmitting base station to the measuring mobile stations.

In step 923 the time of occurrence of future crop marks the base station can be predicted using the measured transmission frequency. Such predictions may be transmitted to various network entities such as base stations or mobile stations upon request in step 925.

Since the information provided by the server in steps 909 and 919 also allows you to determine the current time associated with crop marks, coordination of time can also be performed according to methods described in co-pending US patent application № 09/565212, filed May 4, 2000

In step 927 mobile stations or base stations can use the projected timing periods to facilitate measuring the MTP or RWP or RNRV operations.

Although Figure 9 illustrates a method for determining the frequency of the base station using two mobile stations and one base station, in practice, it is usually employed much more mobile stations. In addition, each mobile station may simultaneously or sequentially view the timing periods of several base stations. Therefore, set operations such as the timing of operations 901-909 (or 911-919) may be performed in parallel in accordance with multiple base stations. The processing shown in Figure 9, may be performed on a continuous basis. As mentioned above, the steps shown in Figure 9, may be performed a single mobile station observing one or more base stations.

Errors in the projections period can be reduced by modeling the long term frequency characteristics of the time (maintenance) in the base station. In many cases, long-term care is smooth and well expect for high quality generators base stations. Thus, care characteristics can be determined from multiple measurements of the base station for very long periods of time. To predict the future care based on the characteristics of care can use curve fitting procedure. In typical curve fitting algorithms can be used polynomials.

According to the method shown in Figure 9, are not necessarily to the same mobile station produces a sequence of timing measurements. In fact, each measurement timing corresponding to a given base station may be performed by different mobile stations. When a time period produced a large number of measurements, it is possible to implement the various averaging operations, such as averaging the least squares method (LSM, LMS). Processing a large number of measurements not only reduces the measurement error significantly, but also allows measurement discard containing unusually high errors due to spurious effects, such as multipath reception of the base station. For such vybrakovyvaniya "emission" can first generate an initial estimate of the frequency using all measurements, then discard those measurements that appear to be very different from the initial measurement, and finally re-calculate the estimate using the measurements that have not been discarded. For vybrakovyvaniya emissions can also use other approaches, such as using the order statistic.

Cellular signal arriving at a mobile station, may be the result of reflection of the primary signal or the presence of multiple direct and reflected signals, so-called "multipath". In most cases, multipath results in a positive excess delay, i.e. a long delay in the transmission of the signal than the direct signal transmission in the line of sight. Delay for transmission in line of sight can be determined by dividing the distance between the base station and the mobile station at the speed of light. Because multipath rarely leads to a negative excess delay, simple averaging may not be the best approach to reduce the error due to multipath propagation.

Excess delay caused by multipath propagation, can be compensated by using weighted averaging. One method is to separate, i.e. giving greater weight to measurements derived from high quality signals, for example signals of high level (high signal / noise ratio) and signals with narrow, sharply defined waveforms. To determine the quality of the received signal, you can use a type of autocorrelation analysis. Signals of high quality tend to appear more frequently out of gear in line of sight, so they exhibit less excess delay than low quality signals. In some cases, at a sufficiently high level of received signal processing algorithms can be used to assess the amount of signal level and relative delay of the received signals from a given base station. In this case, to minimize the effect of excess delay can select the shortest delay.

Figure 9 illustrates a method, according to which the duration of transmission is computed server, and 10 shows another method where the duration of transmission is computed at the mobile station. In steps 1001-1007, the mobile station receives a cellular signal from a base station (BS) and the BS location; Crop mark is contained in the signal of cellular communication; finds your location and the current time using its GPS receiver; and assigns a time tag label framing using the current time found in step 1005. Similarly, a time tag for a second framing mark is determined in steps 1011-1017. In step 1019, the mobile station computes the duration of the transmission time using the time tags. In this case, typically require information about the location of the mobile station and the base station, since the mobile station may be moved between measurements and hence the change in the distance between the base station and the mobile station must be compensated. If it is known that the mobile station is stationary, then this information is not required. Transmission frequency framing markers for the base station can be determined and can be used to predict the timing of future framing markers of the base station. The duration or the measured frequency may be transmitted to the server, and the prediction of the timing may be performed on the server. In steps 1022 and 1023 forecasting can be provided to mobile stations or base stations to assist in measuring the MTP or in operations RNRV or RVP. The first and second cellular signals in Figure 10 typically correspond to two fragments of the cellular signal received at different times during the same telephone "call." However, they may also correspond to signals from the base station received during separate calls.

Figure 11 shows a detailed method for determining the frequency of base station signals using measurements of carrier frequency signals of the base station according to one embodiment of the present invention. In step 1101, the mobile station receives a cellular signal transmitted from the base station. It synchronizes the carrier frequency of the received cellular signal in step 1103. This is usually used in either the phase-locked loop (PLL, PLL), a diagram of automatic frequency control (AFC, AFC), each of which comprises a voltage controlled oscillator (e.g., VCO 323). According to the procedure of synchronization of the VCO must maintain a proportional relationship with the phase or frequency of the received signal.

At step 1105 the mobile station uses a GPS receiver (or BSC) to determine its position, speed, current time and its frequency reference signal LO. To determine the frequency of the base station main interest was to measure the frequency of the reference oscillator; however, the location information, speed, and current time typically is a byproduct of processing GPS. The location and velocity are required to determine the effect of the MS motion on the frequency measurement. As described above, the local reference signal used by a GPS receiver may provide the cellular transceiver VCO or may provide a separate crystal oscillator.

In step 1107, the mobile station determines the received carrier frequency of the base station signal of the VCO and GPS reference frequency measurement. As described above, this is a direct byproduct of GPS processing if the VCO is used as its frequency reference. Alternatively, to determine the ratio of the frequencies of the VCO signal and the GPS can be used individually scheme reference frequency. The frequency ratio and the value of the reference frequency GPS determined by processing GPS signals, provide accurate estimate of the frequency of the VCO, and hence the carrier frequency of the received base station signal.

At step 1109, the frequency information together with auxiliary data (e.g., current time, information identifying a base station, etc.) Is sent to the server. In step 1111 the information of the carrier frequency, which may be expressed in units of ppm. Or other units may be used to calculate the frequency of the oscillator of the base station and / or other frequencies (e.g., framing mark frequency). The location and velocity of mobile station is used together with the location of the base station to determine the frequency error due to the relative motion of the mobile station and the base station. This error can be eliminated to get an accurate estimate of the frequency of the base station. The server may combine several such frequency measurements together to further improve the estimate of the frequency of the base station. Finally, in steps 1113-1117 the server predicts the timing of future periods framing marks base station based on this frequency information and sends it to other network elements (e.g., mobile stations, base station or a location server) upon request to facilitate the measurements (e.g., measurements MOP or operations or RNRV RVP).

Although Figure 11 shows a scenario involving only a mobile station and a base station, in practice, may participate more mobile stations. Each mobile station may simultaneously and sequentially view the transmissions from multiple base stations. Hence, multiple sequences of operations (e.g., steps 1101-1109) may take place in parallel in accordance with multiple base stations. It is also obvious that the processing shown in Figure 11, can be carried out on a continuous basis.

Those skilled in the art that other embodiments of the method shown in Figures 8-11. For example, the mobile station can perform calculations 1111-1117 if it receives the location of the base station. 10, instead of measuring the current time at steps 1005 and 1015, the mobile station may compute the elapsed time after calibrate their clocks manner indicated in Figure 11 steps 1101-1107.

When the generator base station is quite stable, the calibration frequency of the base station can afford to accurately predict future periods of timing marks transmitted by the base station. Typically, the base station oscillator stability sufficient to be able to accurately predict timing for very long periods of time after being produced by the time coordinate.

At the base station as a reference frequency source commonly used high Thermo-stabilized crystal oscillator. Some base stations synchronize their usual reference signals with the signals transmitted from GPS satellites, in which case the long term stability of the base station is synchronized with a stable source of cesium and type suitable to accurately predict timing. In the following discussion, we assume that such synchronization the GPS is not used. In this case, the two main sources of instability in the generator base station as follows: 1) short-term frequency instability, which is usually characterized by measures of short-term frequency stability, such as methods of noise spectral density and the Allan variance; and 2) the frequency of long-term care, which is usually associated with the effects of aging. Long term frequency drift is usually about 0.0001 ppm. Per day or better and hence should not represent a significant source of error over relatively short periods of time (e.g., 15 to 30 minutes).

Most generators base stations using Thermo-stabilized crystal oscillators. Small changes in temperature of the thermostat or the voltage supplied to the thermostat, can contribute to the frequency error. In addition, certain short term frequency stability characteristics, such as a random walk frequency effects, produce a frequency error that grows as a function of observation time [see. J. Rutman and FL Walls "Description of frequency stability in precise frequency sources» (J. Rutman and FL Walls, Characterization of Frequency Stability in Precision Frequency Sources), protocols IEEE, m. 79, №6, June 1991, pp. 952-959]. Thus, it is important to check the magnitude of these effects in terms of devices and systems.

Considered here short term frequency stability is measured during a time interval from a few seconds to several hours. Measured during these periods of high-quality Thermo-stabilized generators have a short-term stability (fractional frequency deviation, or so-called Allan variance) about 0,000,010 hours. With such stability, timing signals from the base station can predict the future for a period of 10 minutes, with an accuracy of 6 nanoseconds and future period of 1 hour, with an accuracy of 36 nanoseconds.

Long term stability of high quality, thermally stabilized oscillators may be of the order of 0.001 ppm. Per day or better, corresponding to about 0,00004 ppm. Per hour [see. Fundamentals of Quartz Oscillators, Hewlett Packard Application Note 200-2]. Thus, the projections for the period of about one hour or more of the characteristics of the impact of aging can prevail.

In terms of measurement, Pickford (Pickford) examined the frequency drift between two base stations, based on the use of two-way delay measurements [see. Andrew Pickford, "Requirements for synchronizing the BTS and the refresh rate for the MIM RNRV 'technical performance Technical Subcommittee T1P1, October 8, 1999]. He found that after the elimination of the linear phase (or temporary) care (ie, a fixed frequency offset error) net mean square error of the time was about 66 nanoseconds, even during periods of more than 1 hour. He also demonstrated that the use of measurement for time period and extrapolation of an hour ahead provides similar accuracy. In addition, the test of its curves indicates that the residual error after removing the medium care prevails in the form of random errors. This may indicate that the major remaining errors due to measurement errors, or additive noise, rather than actual oscillator jitter. Note that the mean square error of 66 nanoseconds, measured over a period of 1 hour, equivalent to a frequency stability of around 0.000018 ppm., Which is typical for a high quality quartz generator.

In a similar article T. Rantallaynena (T. Rantallainen) et al. Provided similar results to the above [see. T. Ruutu Rantallaynen and B, "Method for Measuring DSS RNRV" technical presentation T1P1.5 / 99-428R0, July 8, 1999], approximately from 1500 to 2200 seconds. For non-linear characteristics of the plot of the phase of the time the explanations are not given. Most likely this behavior is explained by the aging characteristics of the crystal oscillator, as indicated above. Since the characteristics of aging inherent smoothness and predictability, polynomial approximation algorithm should work well. For example, depending on the approximation of the measured frame period of time second-order polynomial will be good to compensate for linear time-frequency drift.

Additional factors that can contribute to small changes in frequency over time include fluctuations reference frequency associated with the voltage and temperature. These factors may be manifested in the form of very small frequency changes. Base stations tend to have regulated voltages and temperatures to ensure reliability.

In the event of significant movements of the user, it is important that any Doppler effects do not significantly affect the measurement of the above timing and frequency. In particular, if the mobile station measures time at one time, and predicts the current time related to the frame border cellular signal, occurring at another point, the movement of the mobile station can lead to errors, especially if the mobile station is moving quickly, and / or gap between great moments in time. Can solve this problem in different ways. For example, when the mobile station can determine its velocity, the data rate of the mobile station can be transmitted to the server in order to compensate errors due to the Doppler effect associated with the rate of change of distance between a mobile station and a base station. This approach is shown in Figure 11. As described above, GPS signals can be processed to estimate the speed of the host platform. This information can be used to compensate for any errors caused by the movement of the mobile station.

There may be some residual error, for example due to multipath propagation delay and transient delay caused by the mobile station apparatus. However, the mobile station and / or base station are often able to determine the degree of deterioration and to give greater weight to these measurements as compared with the measurements having a smaller error.

The effective time of transmission (ie, the arrival time) are determined on the base station antenna. Using a large number of mobile stations can help reduce errors due to the averaging procedure. It is assumed that the displacement system can be eliminated or minimized by selection of proper measurement or through other bias estimation procedures.

Matters relating to the mobile station sufficient activity to support the timing (e.g. early morning hours) could be addressed by placing mobile stations at various locations and making calls periodically. However, they should not be fixed assets.

Typical timing errors due to the GPS processing at a single mobile station may be on the order of 10-30 nanoseconds. Thus, there may prevail other sources of error, such as multipath.

The stability of the base station generator depends on how often you need to produce and distribute measurement timing. It is possible to use a plurality of measurements from mobile stations to precisely determine not only the instantaneous frequency of the oscillator of the base station, but also higher order moments such as the rate of change of the frequency. As discussed above, generally the case that a simple approximation of the frequency response curve of the base station of the time can be maintained with extremely high accuracy over long periods of time.

Although the methods and apparatus of the present invention have been described with reference to GPS satellites, it is obvious that the inventive concept is equally applicable to positioning systems that utilize pseudolites or a combination of satellites from pseudolites. Pseudolites - a ground-based transmitters that broadcast a PN code (similar to the GPS signal) modulating the carrier signal is an L-band, generally synchronized with GPS time. Each transmitter may be assigned a unique PN code to the remote receiver can identify it. Pseudolites useful in cases where GPS signals from an orbiting satellite might be unavailable, such as in tunnels, mines, buildings or other enclosed areas. As used herein, the term "satellite" encompasses pseudolites or equivalents of pseudolites, as used herein, the term "GPS signals" includes GPS-like signals from pseudolites or equivalents of pseudolites.

In the foregoing specification, the invention has been described with reference to the global positioning satellite system, the United States. Obviously, these methods are equally applicable to similar satellite positioning systems, including the Russian Glonass System and the planned European Galileo System. Glonass system differs from the GPS system primarily in that the transmission of different satellites may be distinguished from each other by the use of slightly different carrier frequencies, rather than by using different pseudorandom codes. In this case, apply substantially all of the above schemes and algorithms. As used herein, "GSP" includes such alternative satellite positioning systems, including the Russian Glonass system and planned European system Galileo.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. Obviously, it is possible to propose various modifications without departing from the spirit and scope of the invention as defined by the following claims. Accordingly, the specification and drawings should be regarded in an illustrative sense rather than a restrictive sense.