Abstract:
An apparatus and method for generating an accumulated phase measurement of a communications signal over a predetermined time interval. A frequency estimate of the signal is generated; the frequency estimate is then converted to a coarse phase measurement. A phase error is generated based on the frequency estimate; the phase error is then converted to a fine phase measurement. The coarse and fine phase measurements are summed to yield an accumulated phase measurement.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to measuring characteristics of communications signals, and more specifically to obtaining an accumulated phase measurement of a communications signal. 
     II. Related Art 
     In general, wireless communications systems can be terrestrial or satellite-based. An example terrestrial wireless communications system includes at least one terrestrial base station and at least one user terminal (for example, a mobile telephone). The base station provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system. An example satellite-based wireless communications system includes at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system. 
     In such wireless communications systems, the need often arises for a receiver to obtain a measurement of the frequency of a received communications signal. For some applications, a high degree of accuracy is required. One such application is the determination of the location of a user terminal based on the characteristics of communications signals received by the user terminal. 
     Such frequency measurements are usually taken using an automatic frequency control (AFC) loop in a receiver. However, these measurements are generally of low accuracy. Current AFC techniques measure only “current” phase error, and so achieve phase error accuracy on the order of hundreds of degrees. In wireless communications system employing low signal to noise ratios, such as spread spectrum systems, this accuracy is insufficient. While low accuracy frequency measurements are adequate for acquisition and tracking, higher accuracy is required for applications such as position determination. What is needed is a way to measure phase error with an accuracy on the order of tens of degrees. 
     One approach to reducing this error is to increase the loop iteration period of the AFC loop (for example, by increasing the period of its integrate-and-dump accumulators). However, this adversely affects acquisition and tracking. For example, if the loop iteration period is increased so as to achieve a standard deviation on the order of 1 Hz, the acquisition time (that is, the time the user terminal requires to acquire the signal) grows to an order of several minutes. Mobile phone users will not tolerate such a delay. 
     What is needed, therefore, is a system and method for making accurate measurements of the frequency of a received signal in a wireless communications system without adversely affecting other aspects of user terminal performance. 
     SUMMARY OF THE INVENTION 
     The present invention is a novel and improved apparatus and method for accurately measuring the accumulated phase of an input signal over a given time interval. 
     The apparatus includes an automatic frequency control loop for generating a frequency estimate of the frequency of the input signal; a phase error estimator for generating a phase error based on the frequency estimate; and a phase detector for generating an accumulated phase measurement based on the frequency estimate and the phase error. 
     One advantage of the present invention is that it permits high-accuracy frequency measurements of a signal in a communications system. 
     Another advantage of the present invention is that its robust open-loop architecture is well-suited for use with communications signals subject to fading. 
     A further advantage of the present invention is that it employs a cascaded architecture that permits the use of an off-the-shelf automatic frequency control loop in the present invention without significant modification. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
     FIG. 1 depicts an exemplary wireless communication system; 
     FIG. 2 is a block diagram of an example transceiver for use in a user terminal; 
     FIG. 3 is a high-level block diagram of a system for measuring the accumulated phase of an input signal according to a preferred embodiment of the present invention; 
     FIG. 4 is a flowchart depicting the operation of the system illustrated in FIG. 3; 
     FIG. 5 is a circuit block diagram depicting the system of FIG. 3 in more detail; 
     FIG. 6A is a flowchart depicting the operation of an AFC Loop according to one embodiment of the present invention; 
     FIG. 6B is a flowchart depicting the operation of a Phase Error Estimator according to one embodiment of the present invention; and 
     FIG. 6C is a flowchart depicting the operation of a Phase Detector according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. Introduction 
     Preferred embodiments of the present invention are discussed in detail below. While specific steps, configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. After reading this description, it will become apparent to one skilled in the relevant art that other steps, configurations and arrangements can be used without departing from the spirit and scope of the present invention. 
     The present invention is described in two parts. First, an exemplary wireless communications system is described. Second, a system is described for obtaining an accumulated phase measurement of a signal in a satellite communications system according to preferred embodiments of the present invention. The present invention can be used in other types of communications systems, as would be apparent to one skilled in the relevant art. 
     II. An Exemplary Wireless Communications System 
     Before describing the present invention in detail, it is useful to provide a simplified description of an exemplary wireless communications system in which the invention can be implemented. 
     An exemplary wireless communication system in which the present invention is useful is illustrated in FIG.  1 . It is contemplated that this communication system uses CDMA type communication signals, but this is not required by the present invention. In a portion of a communication system  100  illustrated in FIG. 1, one base station  112 , two satellites  116  and  118 , and two associated gateways or hubs  120  and  122  are shown for effecting communications with two remote user terminals  124  and  126 . Typically, the base stations and satellites/gateways are components of separate communication systems, referred to as being terrestrial- and satellite-based, although this is not necessary. The total number of base stations, gateways, and satellites in such systems depend on desired system capacity and other factors well understood in the art. 
     The terms “base station” and “gateway” are sometimes used interchangeably in the art, with gateways being perceived as specialized base stations that direct communications through satellites and have more ‘functions,’ with associated equipment, to establish and maintain such communication links through moving repeaters, while base stations use terrestrial antennas to direct communications within a surrounding geographical region. Central control centers will also typically have more functions to perform when interacting with gateways and satellites. User terminals are also sometimes referred to as subscriber units, mobile units, mobile stations, or simply “users,” “mobiles,” or “subscribers” in some communication systems, depending on preference. 
     User terminals  124  and  126  each include a wireless communication device such as, but not limited to, a cellular telephone, a data transceiver, or a paging or position determination receiver, and can be hand-held or vehicle-mounted as desired. Here, the user terminals are illustrated as hand-held telephones. However, it is also understood that the teachings of the invention are applicable to fixed units where remote wireless service is desired, including “indoor” as well as “open air” locations. 
     Generally, beams from satellites  116  and  118  cover different geographical areas in predefined patterns. Beams at different frequencies, also referred to as CDMA channels or “sub-beams,” can be directed to overlap the same region. It is also readily understood by those skilled in the art that beam coverage or service areas for multiple satellites, or antenna patterns for multiple base stations, might be designed to overlap completely or partially in a given region depending on the communication system design and the type of service being offered, and whether space diversity is being achieved. 
     A variety of multi-satellite communication systems have been proposed with an exemplary system employing on the order of 48 or more satellites, traveling in eight different orbital planes in LEO orbits for servicing a large number of user terminals. However, those skilled in the art will readily understand how the teachings of the present invention are applicable to a variety of satellite system and gateway configurations, including other orbital distances and constellations. At the same time, the invention is equally applicable to terrestrial-based systems of various base station configurations. 
     In FIG. 1, some possible signal paths are illustrated for communications being established between user terminals  124  and  126  and base station  112 , or through satellites  116  and  118 , with gateways  120  and  122 . The base station-user terminal communication links are illustrated by lines  130  and  132 . The satellite-user terminal communication links between satellites  116  and  118 , and user terminals  124  and  126  are illustrated by lines  140 ,  142 , and  144 . The gateway-satellite communication links, between gateways  120  and  122  and satellites  116  and  118 , are illustrated by lines  146 ,  148 ,  150 , and  152 . Gateways  120  and  122 , and base station  112 , may be used as part of one or two-way communication systems or simply to transfer messages or date to user terminals  124  and  126 . 
     An exemplary transceiver  200  for use in a user terminal  106  is illustrated in FIG.  2 . Transceiver  200  uses at least one antenna  210  for receiving communication signals, which are transferred to an analog receiver  214 , where they are downconverted, amplified, and digitized. A duplexer element  212  is typically used to allow the same antenna to serve both transmit and receive functions. However, some systems employ separate antennas for operating at different transmit and receive frequencies. 
     The digital communication signals output by analog receiver  214  are transferred to at least one digital data receiver  216 A and at least one searcher receiver  218 . Additional digital data receivers  216 B- 216 N can be used to obtain desired levels of signal diversity, depending on the acceptable level of unit complexity, as would be apparent to one skilled in the relevant art. 
     At least one user terminal control processor  220  is coupled to digital data receivers  216 A- 216 N and searcher receiver  218 . Control processor  220  provides, among other functions, basic signal processing, timing, power and handoff control or coordination, and selection of frequency used for signal carriers. Another basic control function often performed by control processor  220  is the selection or manipulation of pseudonoise (PN) code sequences or orthogonal functions to be used for processing communication signal waveforms. Signal processing by control processor  220  can include a determination of relative signal strength and computation of various related signal parameters. Such computations of signal parameters, such as timing and frequency may include the use of additional or separate dedicated circuitry to provide increased efficiency or speed in measurements or improved allocation of control processing resources. 
     The outputs of digital data receivers  216 A- 216 N are coupled to digital baseband circuitry  222  within the user terminal. User digital baseband circuitry  222  comprises processing and presentation elements used to transfer information to and from a user of a user terminal. That is, signal or data storage elements, such as transient or long term digital memory; decoders; input and output devices such as display screens, speakers, keypad terminals, and handsets; A/D and D/A elements, vocoders and other voice and analog signal processing elements; etc., all form parts of the user digital baseband circuitry  222  using elements well known in the art. If diversity signal processing is employed, user digital baseband circuitry  222  can comprise a diversity combiner and decoder. Some of these elements may also operate under the control of, or in communication with, control processor  220 . 
     When voice or other data is prepared as an output message or communications signal originated by the user terminal, user digital baseband circuitry  222  is used to receive, store, process, and otherwise prepare the desired data for transmission. User digital baseband circuitry  222  provides this data to a transmit modulator  226  operating under the control of control processor  220 . The output of transmit modulator  226 , along with the data rate to be transmitted, is transferred to a power controller  228  which provides output power control to a transmit power amplifier  230  for final transmission of the signal from antenna  210  to a gateway. 
     Digital receivers  216 A-N and searcher receiver  218  are configured with signal correlation elements to demodulate or search for specific signals. Searcher receiver  218  is used to search for pilot signals, or other relatively fixed pattern strong signals, while digital receivers  216  A-N are used to demodulate other signals associated with detected pilot signals. Therefore, the outputs of these units can be monitored to determine the energy in, or frequency of, the pilot signal or other signals. These receivers also employ frequency tracking elements that can be monitored to provide current frequency and timing information to control processor  220  for signals being demodulated. 
     The present invention is described in terms of this example environment. Description in these terms is provided for convenience only. It is not intended that the invention be limited to application in this example environment. In fact, after reading the following description, it will become apparent to a person skilled in the relevant art how to implement the invention with other satellite-based communications systems having different architectures and levels of complexity. 
     III. An Accumulated Phase Measurement System 
     In communications receivers such as that described above with respect to FIG. 2, it is often desirable to obtain a phase measurement for a received communications signal. For example, it is often necessary to determine the position of a user terminal in a satellite-based communications system. Several approaches have been developed. Some of these approaches rely on measurements of the pilot signal frequency that are made by the user terminal. Several such approaches are described in commonly-owned copending applications entitled “Position Determination Using One Low-Earth Orbit Satellite” having application Ser. No. 08/723,751, “Passive Position Determination Using Two Low-Earth Orbit Satellites” having application Ser. No. 08/723,722, and “Unambiguous Position Determination Using Two Low-Earth Orbit Satellites” having application Ser. No. 08/723,725, which are incorporated herein by reference. 
     A high-accuracy frequency measurement can be obtained from an accurate measurement of the accumulated phase change of a signal over a short time interval. The required measurement is given by:              2        π   ·       ∫     t   1       t   2              f        (   t   )       ·                   t                   (   1   )                                
     where f(t)=frequency of the signal at time t. In a preferred embodiment of the present invention, the input signal is a pilot signal; however, in alternative embodiments, the present invention is used to obtain accumulated phase measurements of other similar signals. 
     FIG. 3 is a high-level block diagram of a system for measuring the accumulated phase of an input signal according to a preferred embodiment of the present invention. In one embodiment of the present invention, the system is part of searcher receiver  218  and digital data receivers  216  in user terminal transceiver  200 . In the embodiment illustrated, the system comprises Automatic Frequency Control (AFC) Loop  300 , Phase Error Estimator  320 , and Phase Detector  340 . 
     FIG. 4 is a flowchart depicting the operation of the system illustrated in FIG.  3 . AFC  300  generates an estimate of the frequency of the input signal, as shown in a step  402 . Phase Error Estimator  320  generates an estimate of the phase error based on the frequency estimate, as shown in a step  404 . Phase Detector  340  generates an accumulated phase measurement based on the frequency estimate and phase error, as shown in a step  406 . 
     FIG. 5 is a circuit block diagram depicting the system of FIG. 3 in more detail. FIG. 6A is a flowchart depicting the operation of AFC Loop  300  according to one embodiment of the present invention. AFC Loop  300  comprises Rotator  502 , Integrate-And-Dump Accumulators  504 , Cross-Product Generator  506 , and Loop Filter  508 . Rotator  502  receives the input signal and reduces its frequency, as shown in a step  602 . In a preferred embodiment of the present invention, the input signal is a quadrature phase-shift-key (QPSK) modulated pilot signal transmitted by gateway  102 . 
     The I-channel output of Rotator  502  is fed to Integrate-And-Dump Accumulator  504 A which operates over a loop iteration period of Δt 1 . In general, the loop iteration period Δt 1  is chosen according to the accuracy desired. In general, Δt 1  is chosen to be much shorter than the period of the input signal. In a preferred embodiment of the present invention Δt 1  is chosen so that                2        π   ·     Δt   1     ·     f   errmax         &lt;     π   6             (   2   )                                
     where f errmax  is the maximum steady-state AFC loop frequency error (that is, the steady-state output of Cross-Product Generator  506  ). In a conventional AFC loop f errmax  is generally less than 500 Hz. In a preferred embodiment of the present invention f errmax  is less than 100 Hz. 
     The Q-channel output of Rotator  502  is processed by Integrate-And-Dump Accumulator  504 B in a similar fashion. Integrate-And-Dump Accumulators  504  accumulate the reduced-frequency signals to produce signal samples, as shown in a step  604 . Cross-Product Generator  506  performs a cross-product operation between the current I and Q samples and the previous I and Q samples produced by Integrate-And-Dump Accumulators  504 , as shown in a step  606 . The output of Cross-Product Generator  506  represents the change in phase between the two samples, scaled by the energy of the pilot signal, and is given by: 
     
       
           I   k−1   ·Q   k−   Q   k−1   ·I   k   (3) 
       
     
     where 
     I k =current in-phase component of the pilot signal at time k; 
     Q k =current quadrature-phase component of the pilot signal at time k; 
     I k−1 =previous in-phase component of the pilot signal at time k−1; and 
     Q k−1 =previous quadrature-phase component of the pilot signal at time k−1. 
     The output of Cross-Product Generator  506  is then filtered by Loop Filter  508 , as would be apparent to one skilled in the relevant art, to produce a frequency estimate for the input signal, as shown in a step  608 . 
     Other circuit configurations can be employed to generate a frequency estimate without departing from the spirit and scope of the present invention. However, as discussed above, the accuracy of a typical AFC loop is not sufficient for position determination. Further, an AFC loop may “miss” a cycle of the input signal entirely, leading to erroneous phase measurements. Therefore, the invention incorporates a phase error estimator to generate a phase error to supplement the frequency estimate available from AFC Loop  300 . 
     In the embodiment illustrated in FIG. 5, Phase Error Estimator  320  comprises Phase Filters  522 , Cross-Product Generator  524 , and Normalizer  526 . Phase Error Estimator  320  is a cascaded structure from AFC Loop  300 . The use of such a cascaded structure produces two advantages. First, it permits any similar off-the-shelf AFC loop to be used in the present invention, without significant modifications, to obtain accumulated phase measurements. Second, it provides an open-loop architecture that is well-suited to fading signals. In a closed-loop architecture, the fluctuations in input signal strength for a fading signal are fed back and thus enhanced. Thus, because an open-loop architecture has no feedback, it is more suitable for fading signals. 
     FIG. 6B is a flowchart depicting the operation of Phase Error Estimator  320  according to one embodiment of the present invention. The output of Integrate-And-Dump Accumulator  504 A is filtered by Phase Filter  522 A; likewise, the output of Integrate-And-Dump Accumulator  504 B is filtered by Phase Filter  522 B, as shown in a step  622 . This filtering operation improves the signal-to-noise ratio of the signal, which yields improved accuracy in the phase measurement. The design and operation of Phase Filters  522  will be apparent to one skilled in the relevant art. Cross-Product Generator  524  performs a cross-product operation between the current and previous outputs of Phase Filters  522 , as shown in a step  624 . The output of Cross-Product Generator  524  is normalized by Normalizer  526  to produce a phase error, as shown in a step  626 . The phase error is given by:                      I   _       k   -   1       ·       Q   _     k       -         Q   _       k   -   1       ·       I   _     k               (         I   _       k   -   1     2     +       Q   _       k   -   1     2       )     ·     (         I   _     k   2     +       Q   _     k   2       )                 (   4   )                                
     where: 
     {overscore (I)} k =current in-phase component of the phase-filtered pilot signal at time k; 
     {overscore (Q)} k =current quadrature-phase component of the phase-filtered pilot signal at time k; 
     {overscore (I)} k−1 =previous in-phase component of the phase-filtered pilot signal at time k−1; and 
     {overscore (Q)} k =previous quadrature-phase component of the phase-filtered pilot signal at time k−1. 
     Other circuit configurations can be employed to generate a phase error without departing from the spirit and scope of the present invention. 
     In the embodiment illustrated in FIG. 5, Phase Detector  340  comprises Integrate-And-Dump Accumulators  510   a,b,  Multiplier  512 , and Summer  514 . FIG. 6C is a flowchart depicting the operation of Phase Detector  340  according to one embodiment of the present invention. In the illustrated embodiment, AFC Loop  300  produces a frequency estimate which is fed to Integrate-And-Dump Accumulator  510 A to produce a coarse frequency estimate, as shown in a step  642 . The coarse frequency estimate is multiplied in Multiplier  512  by twice the product of Δ and the loop iteration period Δt 1  of AFC Loop  300  to produce a coarse phase measurement, as shown in a step  644 . The coarse phase measurement over a measurement interval Δt 2  is given by:              2        π   ·       ∑     k   =     t   1         t   2                           f   _     k     ·     Δ     t   1                     (   5   )                                
     where 
     {overscore (f)} k =AFC loop frequency estimate at time k 
     Δt 1 =AFC loop iteration period. 
     The duration of measurement interval Δt 2 =t 2 −t 1  is selected according to various factors, including positioning requirements, as would be apparent to one skilled in the relevant art. In a preferred embodiment of the present invention, the duration of measurement interval Δt 2  is approximately two seconds. 
     Phase Error Estimator  320  produces a phase error, which is fed to Integrate-And-Dump Accumulator  510 B. Integrate-And-Dump Accumulator  510 B, which operates over the measurement interval Δt 2 , produces a fine phase measurement, as shown in a step  646 . 
     The coarse and fine phase measurements are summed by Summer  514  to produce an accumulated phase measurement for the input signal, as shown in a step  648 . 
     IV. Conclusion 
     The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.