HK1074705B - Aiding beam identification in a satellite system - Google Patents

Description

Aiding beam identification in a satellite system

Background

RELATED APPLICATIONS

Priority is claimed In this application for U.S. provisional patent application entitled "Method and System for AidingBeam Identification In A LEO Satellite System", serial No. 60/342,925, filed on 25/10.2001, which is hereby incorporated by reference In its entirety.

I. Field of the invention

The present invention relates generally to satellite communications and satellite communications systems, and more particularly to estimation and compensation of propagation delays associated with identification of satellite beams in a satellite communications network.

II. background of the invention

Conventional satellite communication systems include one or more terrestrial base stations (hereinafter gateways), user terminals, such as mobile telephones, and one or more satellites for propagating communication signals between the user terminals and the gateways. The gateway receives and transmits signals from and to the satellite that may enter Low Earth Orbit (LEO), processes the communication link or call, and interconnects or transfers the call to the appropriate terrestrial network if necessary. That is, the gateway provides a ground-based link to allow system users to communicate with other system users, or to provide a communication link to a ground service provider, such as a Public Switched Telephone Network (PSTN), a data network, a wireless communication system, or other satellite gateway.

While mobile telephones or wireless user terminals provide enhanced mobility and flexibility to users, the proliferation of such telephones has increased the demand for related communication systems. For example, in the case of satellite-based communication systems, determining the location of a user of the system is extremely important relative to others in establishing a communication link with a telephone, determining what services are used by the provider, and providing position location services to the user of the telephone. However, the flexibility of mobile phone roaming complicates this process.

Most communication satellites transmit a "footprint" comprising a number of radio links or communication signal beams grouped to provide coverage for communication with system users in the geographic area covered by the footprint. A particular user terminal may be assigned, albeit temporarily, a particular satellite beam is used to transmit communication signals depending on the geographical location of the user terminal. Therefore, the satellite communication system gateway must know the user terminal location to provide the appropriate communication service to a particular user through the appropriate serving satellite beam. Thus, knowledge of the particular satellite bundles that serve a particular user or geographic area is essential to the ability of the gateway to provide communication services.

Another aspect of the process is to properly establish communication links to other communication service providers, such as the PSTN and data networks. These service providers are typically associated with a particular terrestrial geographic area and only handle communication links associated with their respective areas. For example, the network may have government licenses or various business agreements with consumers to serve a particular area. Knowledge of the location of the user terminal is also required before these geo-related services are provided. The identification of the satellite beam is a necessary step in determining the position of the user terminal.

There are many conventional methods for determining the position of a satellite communication system user. For example, some techniques must measure the distance between the user terminal and the associated satellite and determine the rate of change associated with the determined distance. When these distance measurements are combined with other data, the location of the user terminal may be accurately determined. Techniques for determining the Position of a user terminal Using the distances and rates of change of the distances of Satellite user terminals are disclosed in U.S. Pat. No. 6,078,284 entitled "Passive Position Determination Using Two Low Earth Orbit Satellites", 6,327,534 entitled "unknown Position Determination Using Two Low Earth Orbit Satellites", 6,107,959 entitled "Position Determination Using one Low Earth Orbit Satellites". In addition, U.S. Pat. No. 6,137,441 entitled "Accurate Range And Range Determination In A Satellite communications System" discloses a technique for compensating for Satellite motion to enhance the accuracy of user terminal location information.

However, although the movement between the satellite and the associated user terminal may be determined, errors often occur in these measurements due to, for example, effects such as antenna gain characteristics, or, for example, because a particular satellite may be at a lower altitude relative to the user terminal. These errors often result in the gateway misidentifying the service or communication satellite beams and thus misjudging the user terminal location. The end result of misidentification of satellite beams is a frequent denial of system service, or even a complete failure of radio link acquisition for the associated user.

Another source of error may occur as part of the implementation of a particular communication technology or user access method used by the communication system to accommodate multiple system users. There are a variety of techniques for providing multi-system users with access to a communication system. Two well-known multiple access techniques include Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA), the basis of which is well known in the art. However, spread spectrum modulation techniques, such as Code Division Multiple Access (CDMA), are significantly more desirable due to their ability to accommodate multiple users in increasingly bandwidth-limited environments.

The use of CDMA technology In Multiple Access Communication systems is disclosed In U.S. Pat. No. 4,901,307, entitled "Spread Spectrum Multiple Access Communication System Using Satellite orthogonal repeats," And U.S. Pat. No. 5,103,459, entitled "System And methods for generating Signal Power In A CDMA Cellular Telephone System," both assigned to the assignee of the present invention And incorporated herein by reference. The method for providing CDMA Mobile communications IS standardized by the Telecommunications industry Association in the United states under the designation "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wireless band Spread spectrum cellular System", referred to herein as IS-95, TLA/EIA/IS-95-A. Other communication systems or techniques are described in IMT-2000/UM, or International Mobile Telecommunications System 2000/GSM, and the covered standards are known as Wideband CDMA (WCDMA), cdma2000 (e.g., cdma 20001 x or 3x standard), or TD-SCDMA.

In a satellite communication system using CDMA, a large number of user terminals, each having a transceiver, communicate using spread spectrum communication signals through satellites and gateways. By using CDMA, the associated spectrum may be reused multiple times, thereby allowing an increase in system user capacity. Thus, CDMA is more spectrally efficient than other user access technologies. While CDMA is spectrally efficient, CDMA is somewhat vulnerable to the problems associated with mobile station or terminal mis-location errors discussed above. One area in which CDMA systems are particularly vulnerable is the area in which users are handed off.

Handoff occurs when a mobile station communication session or link, such as an ongoing call or conversation, is transferred from one satellite beam to another. There are generally two types of handoff, hard handoff and soft handoff. In hard handoff, when a mobile station moves from within the coverage area of one beam to the coverage area of another destination or target beam to be serviced, the terminal interrupts its communication link with the serving beam and establishes a new communication link with the target beam. However, in soft handoff, the mobile station establishes a link with the target bundle before interrupting its communication with the current bundle. This technique is known in the art as make-before-break. In addition, in soft handover, the determination of the correct identification of the target beam is related to the location of the serving beam. Thus, in soft handoff, the mobile station communicates with both the serving beam and the target beam.

Soft Handoff techniques are disclosed in U.S. Pat. No. 5,267,621 entitled "Mobile Station-assisted Soft Handoff in a CDMA Cellular Communications System," assigned to the assignee of the present invention and incorporated herein by reference. In the system of the' 621 patent, the soft handoff process is predicated on using measurements of the strength of pilot signals transmitted for each beam to facilitate access to the satellite by a particular mobile station. By way of background, access by a mobile station to a CDMA-based communication system or communication signal is provided in the forward link, i.e., in the direction from the satellite to the mobile station. The forward link includes three types of supplemental channels: at least one pilot channel, a synchronization channel, and one or more paging channels. These supplemental channels are used by the system to establish and manage communication sessions with the mobile station.

The pilot channel comprises the transmission of a pilot signal as a beacon for potential system users or subscribers and is used by user terminals or mobile stations to acquire initial system synchronization and provide robust time, frequency and phase tracking of the signals transmitted by the base stations. In spread spectrum communication systems, such as those based on IS-95, the base station IS characterized or distinguished by a phase offset in a pseudo-random noise (PN) code used to spread the communication signal, also referred to as the PN offset of the pilot signal. Typically, each terrestrial base station uses the same spreading code, but differs in code phase offset. Alternatively, it is more typical for satellite systems to use a list of PN codes based on unique PN polynomials in the communication system, with different PN codes possibly being used for different gateways and satellites for each orbital plane. Those skilled in the art will readily appreciate that more or fewer PN codes can be assigned as needed to identify a particular signal of a source or repeater in a communication system.

In a satellite-based system, to determine the correct target satellite beam from a number of candidate beams, i.e., which beam covers the location of the user terminal, the user terminal searches for the appropriate pilot signal by determining the pilot signal strength and the PN code or code phase offset. This is accomplished by performing a correlation operation on each potential code and code phase offset, wherein all received pilot signals are correlated with a particular set of PN code offset values. Methods And apparatus for Performing the related operations are described In detail In U.S. Pat. No. 5,805,648 entitled "Method And apparatus for validating Search In A CDMA Communication System," assigned to the assignee of the present invention And incorporated herein by reference.

To initially establish a communication link with a communication system, a user terminal must first acquire a pilot signal associated with the system. The user terminal receives the PN code and phase offset information of the pilot signal when demodulating the pilot signal and system timing by demodulating the synchronization channel. However, before the user terminal is handed off to a new satellite beam, the received pilot signal must be correlated with a set of PN codes and code offset values to determine the PN offset for the most likely target satellite beam.

The amount of propagation delay between the communication system satellite and the user terminal is often significant but uncertain and can result in an unknown shift in the PN offset value to be detected. I.e. the subscriber terminal detects a larger phase shift due to the delay, instead of the original signal source. These shifts may cause the user terminal to misidentify a new handoff or target satellite beam. In the cited reference, unknown propagation delays may occur especially when the new target satellite is low in the horizontal plane relative to the currently serving satellite.

Accordingly, there is a need for a technique to compensate for the effects of propagation delay by allowing a user terminal to independently verify the PN offset measurements associated with a target or new target satellite in a manner that estimates the propagation delay.

Brief description of the invention

The present invention provides a system and method for independently validating PN offset measurements. The range to the satellite can be estimated in the user terminal by using the Doppler (Doppler) and Doppler rate of change of the target satellite. By filtering these measurements and using integer mathematics, the user terminal can convert the doppler measurements into range estimates and then into PN offset estimates. This PN offset estimate can then be compared to the PN offset value measured by the user terminal. If the difference between the PN offset estimate and the measured PN offset is greater than a predetermined value, an error is indicated or the measurement is discarded.

Consistent with the principles of the present invention, as embodied and broadly described herein, the present invention comprises a method for determining a timing offset error in a communication system, such as a satellite system. The method includes determining, at the terminal, a first timing offset associated with a first transmitter signal from a first transmitter (such as a first beam from a first satellite). The method also includes determining, at the terminal, a second timing offset associated with a second transmitter signal from a second transmitter (such as a second beam from a second satellite) based on the first timing offset. A first range from the user terminal to the first transmitter or satellite is estimated and a second range from the user terminal to the second transmitter or satellite is estimated. Finally, an error between the signal time difference of arrival and the estimated difference between the first and second ranges is determined.

In a further aspect, estimating the first and second ranges includes determining doppler frequencies associated with the first and second satellite beams and a rate of change of the determined doppler frequencies, respectively.

A method is also provided for performing beam identification in a user terminal configured to communicate using a low earth orbit satellite system, the user terminal including a processor configured to receive a first satellite beam including a first timing offset from a serving satellite and facilitate a handoff from the first satellite beam to a second satellite beam radiated from a target satellite. The method comprises the following steps: acquiring a second satellite beam at the user terminal; determining a second timing offset for the second satellite beam based on the first timing offset; identifying a second satellite beam based on the second timing offset, estimating a first range from the user terminal to the serving satellite; estimating a second range from the user terminal to the target satellite; determining a timing difference based on a difference between the estimated first range and the estimated second range; calculating a difference between the second timing offset and the timing difference; and confirming identification of the second satellite beam when a difference between the second timing offset and the timing difference is greater than a predetermined value.

An example user terminal constructed in accordance with the invention includes: a receiving module configured to receive and demodulate first and second transmitter signals associated with the first and second transmitters, respectively, such as first and second satellite beam signals associated with the first and second satellite beams, respectively. The first and second satellite beams are received from respective first and second satellites. The user terminal also includes a processor coupled to the receiving module. The processor is configured to (i) determine a first timing offset associated with the first transmitter or satellite beam signal, the first timing offset being indicative of a first beam ID of the first satellite beam, (ii) determine a second timing offset associated with the second transmitter or satellite beam signal, the second timing offset being determined from the first timing offset and being indicative of a second beam ID of the second satellite beam, and (iii) measure doppler characteristics associated with the first and second satellite beam signals, respectively, at the user terminal to verify the first and second beam IDs, respectively.

The respective doppler characteristics are used to determine first and second ranges associated with the user terminal and each of the first and second satellites, respectively. Finally, the user terminal is adapted to convert the first and second ranges into a relative timing difference and to determine the error present in the second beam ID based on a comparison of the timing difference and the estimated range difference in combination with the beam ID.

Features and advantages of the present invention include the ability to independently verify a PN offset or beam ID measurement of a user terminal. This capability enhances the accuracy of identifying the satellite beams, thereby increasing the certainty with which the user terminal gains access to the communication system. The unknown propagation delay due to the low-level satellite and the resulting PN offset error can be determined and compensated for. Accordingly, the accuracy of the user terminal in determining the satellite beam ID of the target satellite beam can be improved. Accordingly, a user may use a satellite communication system with increased accuracy in which desired system services are not interrupted or completely denied.

Brief description of the drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the objects, advantages, and principles of the invention. In the drawings:

FIG. 1 is a system diagram illustrating a conventional low earth orbit satellite communication system;

FIG. 2 is an illustration depicting a soft handoff of a user terminal from one satellite to another in the system of FIG. 1;

FIG. 3 is a diagram depicting the format of one satellite beam pattern or footprint in the system of FIG. 1;

FIG. 4 depicts users within the coverage area of two satellite footprints in the system of FIG. 1;

FIG. 5 is a timing diagram depicting beam identification errors due to propagation delays;

FIG. 6 is a diagram depicting exemplary satellite terminal position determination;

fig. 7 is a diagram illustrating a user terminal positioning error due to the propagation delay shown in fig. 5.

FIG. 8 is a block diagram of a user terminal constructed and arranged in accordance with the invention;

FIG. 9 is a flow chart describing a method for determining timing offsets associated with first and second satellites from respective satellites;

FIG. 10 illustrates the relationship of several vectors associated with a satellite and a user terminal;

FIG. 11 is a flow chart describing a method for determining a range from a user terminal to associated first and second satellites; and

FIG. 12 is a flow chart describing a method for determining a timing difference associated with a voyage determined by the method of FIG. 10.

Detailed description of embodiments of the invention

The following detailed description of the invention refers to the accompanying drawings that illustrate exemplary embodiments consistent with this invention. Other embodiments may be utilized, and modifications may be made to the embodiments within the spirit and scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense. The scope of the invention is, of course, defined by the appended claims.

Those skilled in the art will appreciate that the present invention, as described below, may be implemented in many different hardware, software, firmware, and/or physical embodiments as illustrated. Any actual software code with the specialized control hardware to implement the embodiments is not limiting of the invention. Thus, having regard to the level of detail presented herein, the operation and behavior of the present invention will be described with the understanding that modifications and variations of the embodiments are possible.

Fig. 1 illustrates an exemplary satellite communication system in which the present invention may be used. Although the communication system is contemplated to use CDMA communication protocols and signals, this is not required. In fig. 1, an exemplary LEO satellite communication system includes first and second satellites, 102 and 104, respectively. Also included are gateways 106 and 108 and a portable user terminal 110, which user terminal 110 includes a communication unit 111 that sends and receives signals through an antenna 109. Finally, a mobile user terminal 113 is provided. Gateways 106 and 108 handle calls associated with portable user terminal 110 and mobile terminal 113 and provide communication links to telephone network 114 and data network 115. Satellites 102 and 104 transmit radio frequency signals, i.e., satellite beams, to provide communication links between gateways 106 and 108 and user terminals 110 and 113. In particular, satellite 102 transmits satellite beam B112₂、B114₂、B116₂And B118₂Satellite 104 transmits satellite beam B119₄And B120₄。

In this example, each of the user terminals 110 and 113 has or includes an apparatus or wireless communication device such as, but not limited to: a cellular telephone, a wireless handset, a data transceiver or a pager or a position determination receiver. Each user terminal may also be hand-held, portable or fixed () installed in a vehicle (including, for example, cars, trucks, boats, trains, and planes), as desired. For example, fig. 1 illustrates that the user terminal 110 is a handheld device and the user terminal 113 is a device installed in a portable vehicle. A wireless communication device, also sometimes referred to as a mobile wireless terminal, user terminal, mobile wireless communication device, subscriber unit, mobile station, mobile radio, or simply a "user," "mobile," "terminal," or "subscriber" in other communication systems, is determined by preference.

Typically, a satellite provides multiple beams within a "footprint" that is indicated as covering separate, typically non-overlapping, geographic areas. Here, satellite beams B112, B114, B116, and B118-B120 provide satellite coverage for different geographic areas in a predetermined pattern. Typically, multiple beams of different frequencies, referred to as CDMA channels, "sub-beams" or Frequency Division Multiplexed (FDM) signals, frequency bins or channels, may overlap the same geographical area. One embodiment of the exemplary system 100 includes a plurality of satellites traveling in different orbital planes at a height of about 1,400 kilometers to serve a large number of user terminals. However, the present invention is not limited to this configuration and may be adapted to a variety of different satellite system and gateway configurations, including other track heights, distances, and constellations, among others. In the example system of fig. 1, the gateways 106 and 108 also control the allocation to particular satellites of the user terminals. When a user terminal transitions from one geographic coverage area to another, a handoff occurs from one satellite beam to another or from one satellite to another to provide continuous user terminal coverage.

Figure 2 illustrates the process of a user handing off from one satellite beam to another satellite beam. In fig. 2, the portable user terminal 110 travels along a timeline 200. At t₁At that time, terminal 110 is within the coverage of serving satellite beam B114. As the terminal 110 continues to advance in the x-direction, at t₂At the moment, the user terminal is located at the satellite 102Satellite beam B114₂And target satellite beam B120 of satellite 104₄Within the common coverage area of (c). Here, before a serving gateway, such as gateway 106, can deliver messages and calls to terminal 110, terminal 110 must communicate to gateway 106 the precise identification of satellite bundles B114 and B120. In one example embodiment, an individual satellite may include up to 16 or more satellite beams on each frequency within a given footprint. Thus, each satellite may provide a communication link to the user over any of 16 different satellite bundles, as shown in FIG. 3.

In fig. 3, an example coverage area or footprint of a satellite is shown. That is, the footprint of each of satellites 102 and 104 includes an individual satellite beam B105-B120, each having its own position or pattern within the footprint. Within the footprint are satellite beams B112, B114, B116, and B118-120 shown in FIG. 1. Also, as described in more detail below, different satellites may share the same or similar beam construction and beam identification numbers. Individual beams within the same satellite may be distinguished from each other by their PN code offsets, as discussed above. Beams from adjacent satellites may be distinguished from each other according to different PN polynomials, as discussed above. Different PN offsets may also be used in some systems, such as systems with differing PN sequence parameters, such as length, to distinguish beams from adjacent satellites having the same beam ID number.

FIG. 4 provides a satellite beam B114₂And B120₄For a more detailed description of the user terminals 110 in the coverage area. In fig. 4, terminal 110 must be able to clearly identify target beam B120₄. Although bundle B114₂And B120₄Similar pilot signals are used, however the associated PN offsets and/or PN polynomials for these beams are different, plus some unknown propagation delay. Referring back to fig. 3, each satellite beam B105-B120 has a unique PN offset corresponding to a common timing sequence shared by the multiple beams. In particular, in the example communication system of fig. 1, the timing (code offset) difference between individual satellite beams is about 15 milliseconds (ms). Thus, propagation delays near or exceeding 15ms may prevent user terminal 110 from shifting PNAccurate measurement of. As a result, satellite beam B120 may appear₄Is not marked. The PN phase offset associated with one satellite beam is closely related to its beam ID. In addition, other factors, such as elevation and antenna gain differences, can also contribute to the degree of error in accurately identifying the associated satellite beams.

Fig. 5 illustrates the effect that propagation delays between satellites may have on satellite beam identification. In FIG. 5, the relative timing associated with each satellite 102 and 104 is illustrated along respective timelines 502 and 504 for comparison. As discussed above, beams from the same satellite have similar pilot signals, although each signal has a different PN offset in this example embodiment. However, satellite beams from adjacent satellites are distinguished from each other by differences in arrival times measured at the user terminal and caused by propagation delays between the satellites.

Timeline 502 depicts satellite beam B112 of satellite 102₂-B115₂The timing relationship between them. Time line 504 illustrates satellite beam B120 of satellite 104₄And satellite beam B114 of satellite 102₂Potential timing relationships between. As shown, a small timing error 506 is tolerable in this timing relationship. The small timing error 506 is approximately equal to the maximum expected propagation delay between satellites. This delay is about plus or minus 7.5 ms. However, propagation delays greater than this value can translate into PN code offset measurement errors and ultimately lead to misidentification of the associated satellite beam.

For illustrative purposes, satellite beam B114 from satellite 102 is shown₂At t along the timeline 502₂The moment arrives at the user terminal 110. However, satellite beam B120 from satellite 104₄Shown along time line 504, may be at satellite beam B114 from satellite 102₂Before and after arrival at the user terminal 110. I.e., beam B120 from satellite 104₄May be at t of the timeline 502₂Previously occurred, the value of the phase difference being equal to t₂X, possibly also at some point later, may differ by t at most due to propagation delay₂+ x. Thus, fig. 5 illustrates propagation delays of plus or minus about 8.5 ms. In an example communication systemThe spacing between adjacent satellite beams is only about 15ms in 100. Therefore, delays near or greater than about 15ms will introduce errors in the PN offset measurements. The propagation delays of plus or minus 8.5ms illustrated in fig. 5 represent 17ms delays that may introduce PN offset measurement errors. Thus, the user terminal 110 will misidentify the satellite beam B114 of the target satellite 104₂. The significance of this misidentification is more clearly illustrated in fig. 6 and 7.

Fig. 6 depicts a timing relationship between the user terminal 110, the serving satellite 102, and the target satellite 104. In particular, FIG. 6 shows satellite beam B114₂Is shifted in phase by PN₁₁₄And satellite beam B120₄Is shifted in phase by PN₁₂₀The relationship between them. As noted above, the PN offsets for different satellite beams from the same satellite are correlated based on the beam ID. The PN offsets of satellite beams from different satellites are also correlated. In FIG. 6, for example, PN₁₂₀Is an offset PN₁₁₄Plus a function of the expected maximum propagation delay PD between the user terminal 110 and the target satellite 104. However, if the actual propagation delay differs from the expected maximum propagation delay PD by a value greater than the value 506, PN₁₂₀It may not be accurately determined. The user terminal searches for the PN offset of the target satellite beam by searching a window of available PN offset values (all possible offsets). Such search techniques are well known in the art. However, these techniques are not fail-safe and may eventually produce a location or identification error.

Fig. 7 illustrates the effect of erroneous PN code offset measurements. In fig. 7, an error 702 may be generated by an error in the measurement of the propagation delay PD by the user terminal 110. The error 702 may eventually cause the gateway 106 to believe that the satellite 104 is at an incorrect location 704 rather than its actual location 706. The gateway 106 may also believe that the user terminal is located at an incorrect location 708 rather than its actual location 710 and, as a result, may allocate an incorrect service or terminate access for the user terminal 110. Thus, by using conventional techniques, the user terminal 110 cannot recognize the PN₁₁₄And sum of PD and PN₁₂₀Error 702 is also included. This would be especially true if the error 702 exceeded the expected timing between satellite beams for the same satelliteIt is important.

The present invention provides the user terminal 110 with the ability to independently validate PN code phase offset measurements for a target satellite beam. This independent authentication may establish a more reliable communication link between the user terminal and the gateway of the example communication system 110.

As shown in fig. 8, the communication unit 111 includes a transceiver 802 for transmitting and receiving signals through the antenna 109 and a modulation/demodulation unit 804 for performing modulation and demodulation on the transmitted and received signals, respectively. The processor 806 is electrically connected to the communication unit 111 to process signals. The processor 806 may include standard components or generalized functions known as such or general purpose hardware including various types of programmable electronic devices, or a computer operating under commands, firmware, or software instructions to perform the desired functions. Examples include a software-controlled controller, a microprocessor, one or more Digital Signal Processors (DSPs), application specific circuit modules, and an Application Specific Integrated Circuit (ASIC). When the user terminal initially acquires the serving satellite and then hands off to the target satellite, the method or process illustrated in fig. 9 is executed in the processor 806 to independently verify the PN offset measurements for the target satellite.

In fig. 9, a user terminal 110 establishes a communication link using the example communication system 100. Thus, terminal 110 initially acquires satellite beam B114 from satellite 102₂As shown in step 900. The pilot signal, which is a beacon, informs the user terminal 110 of the presence of satellite beam B120. Since the user terminal 110 may be in the coverage of other satellite beams, the user terminal 110 measures the strength of all received pilot signals to ensure a connection with the strongest satellite beam of the closest satellite. When the received pilot signal has a signal strength greater than a predetermined value, the pilot signal is transferred from the receiver 802 to the demodulator 804, which performs demodulation.

Once the user terminal 110 successfully acquires the pilot signal from the satellite 102 and is thus allowed to demodulate the synchronization channel, the user terminal 110 determines the PN code offset of the acquired pilot signal. The PN offset represents the beam ID, as shown in step 902. In the present exemplary embodiment, user terminal 110 now knows that it hasAcquiring satellite beams B114 of satellites 102₂. The CDMA synchronization channel does not need to be demodulated again as long as the user terminal 110 remains active. Thus, demodulation of the pilot channel and the synchronization channel need only occur once in any particular communication session for a given user terminal. In this session, the user terminal 110 is notified of the satellite beam ID.

Knowledge of the satellite beam ID is critical because it is critical for the associated gateway to provide the user terminal with access to the paging channel. The paging channel is the medium on which messages and paging information associated with other communications are forwarded to the user terminal. Thus, if the user terminal 110 is to reserve its communication link, access to the paging channel is required.

Eventually, the service or communication link for the user terminal 110 will be from the currently serving satellite bundle B114₂The handoff procedure to the target or new target satellite beam is transmitted as shown in figure 2 and described in step 904 of figure 9. When a user terminal initially receives a new pilot signal associated with a potential target satellite beam, it initially has no way of knowing the associated beam ID. To determine the bundle ID, the user terminal 110 must first determine the PN offset of the new bundle, as depicted at step 906. To determine the PN offset for the new pilot signal, the user terminal 110 searches a window of all available PN phase offset values (all possible offsets), which may be stored in a memory (not shown) of the processor 806. To perform this search, user terminal 110 may use serving satellite bundle B114₂As a reference point. As more clearly illustrated in FIG. 6, the target satellite beam PN₁₂₀The PN offset value of (a) is the serving satellite beam PN₁₁₄And the sum of the maximum expected propagation delay PD and the PN offset value of (d). The user terminal 110 performs this process according to the standard timing relationship between satellite beams of the same satellite, as discussed above.

In the example communication system 100, the timing or offset of pilot signals of adjacent satellite beams within the same satellite are approximately 15ms apart. Thus, when the user terminal 110 searches for and finds a pilot signal of a target satellite beam from a different satellite, it uses a predetermined timingIncrements and establishes a tentative bundle ID based on the actual timing interval. This process is described in step 908. In the communication system 100, the reference PN timing offset determined in step 902 is the PN shown in fig. 6₁₁₄。

As described above, this process accommodates small PN offset errors, such as error 506 of fig. 5. This degree of error may be less than or equal to the maximum desired propagation delay and less than a beam interval of 15 ms. However, a propagation delay that exceeds the error value 506 may cause the user terminal 110 to incorrectly measure the PN offset of the newly acquired pilot signal. However, in the exemplary communication system 100, an independent PN offset validation technique is used as a verification or validity check of the PN offset determination process of step 906.

The independent validation technique uses the doppler and doppler rate of change of the currently serving satellite 102 and the destination or target satellite 104 that is expected to provide future service in order to estimate their respective range from the user terminal 110. That is, the user terminal 110 is configured to periodically measure the doppler and doppler rate of change of the satellite pilot signals. The use of doppler characteristics provides a means of measuring the signal propagation delay caused by the range difference between the satellite and the user terminal 110.

By filtering the doppler measurements, the user terminal 110 can convert the doppler measurements into range estimates. The difference between the range estimates is then converted to an estimated PN code offset value. This estimated PN offset value is then compared to the value determined in step 906. If the estimated PN offset value matches the value determined in step 906 within the desired propagation delay (less than or equal to the threshold), the beam ID of the target satellite beam determined in step 908 may be confirmed or verified. A more detailed discussion of doppler and doppler change rate determination is provided below with reference to figure 10. One technique for measuring Range And Range velocity In satellites is explained In U.S. patent No. 6,137,441 entitled "Accurate Range And Range Determination In satellite communications system," assigned to the assignee of the present invention And incorporated herein by reference.

Fig. 10 depicts the vector relationship that exists between a satellite 102 or 104 and a user terminal 110. In particular, the user terminal 110 measures the doppler shift and doppler rate of change of the satellite beam. Before discussing this process, table 1 shows several relevant known quantities.

TABLE 1 constants

Constant quantity	Value of	Description of the invention
			F	2.49*109Hz	Forward link frequency
C	2.998*108m/s	Speed of light
			V	7.152*103m/s	Satellite velocity
R	6.378*106m	Radius of the earth
			R	1.414*106m	Altitude of satellite
R	7.792*106m	Radius of orbit of satellite, R+R

The idea of the diagram of fig. 10 is that a satellite 102 or 104 orbits the earth 1002 in a near-circular orbit. Although the earth has an irregular shape and is therefore not perfectly circular, this irregularity can be ignored for measuring the doppler characteristic. In fig. 10, vector v represents the velocity of the satellite, vector v' represents the acceleration of the satellite, and vector r represents the direction vector pointing from the user terminal 110 to the satellite 102 or 104. An angle theta is formed between the velocity vector v and the satellite range vector r.

The purpose is to calculate the length R of the direction vector, R, also defined as the range from satellite 102/104 to user terminal 110. The user terminal 110 must also determine how fast the flight R changes, i.e., the flight speed R'. The range speed R' is given by the following expression:

using the definition of R' above, and using the derivative across equation (1), a range acceleration value is generated, represented by:

where v 'is the acceleration of the satellite as a function of earth gravity and r' is equal to the satellite velocity.

Next, each of the three terms of equation (2) can be written as:

see fig. 10

The three terms are substituted into the formula (2) to obtain,

r' can be rewritten and substituted into (3):

thereby, the course acceleration R' becomes

(5)

Equation (5) can rearrange R

To solve R in formula (6), sin θ and R ″ are required. sin θ is solved by using (1-cos2 θ), which is related to a known quantity, i.e., the doppler shift. R' is derived from another known quantity Doppler rate of change. Thus, the formula can be rewritten, expressing Doppler as f:

solving for cos θ and using sin2 θ ═ 1-cos2 θ, sin2 θ becomes:

next, equation (4) can be rearranged as:

by using the derivative of R 'with respect to t, R' becomes:

f' in equation (10) is the Doppler rate of change df/dt, a quantity that can now be estimated using known techniques. Using equations (8) and (10), R in equation (6) can now be successfully calculated. In the communication system 100, a range R may be calculated for both the serving satellite 102 and the target satellite 104.

Next, the calculated range R must be converted to an estimated PN code offset value to check for the PN offset originally determined in step 906. To convert range R to a PN offset value, the range difference of the first and second satellites 102 and 104 is divided by the speed of light:

an example of this process is described with reference to fig. 2, 11 and 12, which describes the next process of the user terminal 110 to check the PN offset initially determined in step 906. First, the processing circuit or element (e.g., ASIC) in user terminal 110 must be configured to perform the operations expressed in equations (1) - (11) above. Configured in this manner, user terminal 110 is able to determine target satellite beam B120₄As depicted in step 1100 of figure 11 and shown in figure 2. Next, the user terminal 110 measures and correlates with satellite beam B114₂Associated doppler shift and doppler rate of change as described in step 1102. When the satellite beam B114 is known₂And B120₄After correlating the Doppler and Doppler rate of change, the user terminal may determine the range R of satellite 104 from user terminal 110₁₀₄As depicted in step 1104. Likewise, user terminal 110 determines the range R of satellite 102 from user terminal 110₁₀₂As depicted at step 1106.

In fig. 12, the target satellite beam B120 is expressed as formula (10)₄May be determined from the range R determined above₁₀₄And R₁₀₂And (6) obtaining. Thus, as shown in step 1200User terminal 110 determines R₁₀₄And R₁₀₂And converts the difference to a satellite beam B120₄The PN difference of the pilot signal. Thus, user terminal 110 now has the estimated PN difference from step 1200 and the determined PN difference from step 906. The user terminal 110 may now compare the estimated PN difference with the determined PN difference, as depicted in step 1202.

If the difference between the determined PN difference and the estimated PN difference is less than a predetermined value, the user terminal 110 may confirm the earlier satellite beam B120 from step 908, as depicted in step 1204₄Is detected. In the present exemplary embodiment, the predetermined value is approximately 10 ms. A value of about 10ms is sufficient to accommodate the maximum expected propagation delay between satellites 102 and 104, i.e., about +/-7.5ms, as described above.

On the other hand, if the estimated timing (offset) PN difference from step 1200 is greater than the predetermined value, as depicted in step 1206, the earlier satellite beam B120₄The identification is considered to be an error and discarded. Although exemplary embodiments of the present invention use correlation calculations or search windows of approximately 10ms size to determine whether the determined PN code offset and the estimated PN code offset match or are within a preselected period of correlation with each other, those skilled in the art will appreciate that other values or window sizes may be selected as desired.

An alternative technique is provided for determining range. Here, the course may be determined using a quadratic method, as follows:

wherein G is equal to a gravity constant; and M_EEqual to the mass of the earth.

This expression can be rearranged, resulting in:

aR²+bR+d＝0，

wherein

a＝GM_E；

b＝2R³ _SR'; and

the course information can be derived using the quadratic expression of equation (12) and simulations show that one root is always positive and the other is always negative.

By using the techniques described above, the user terminal 110 can increase the accuracy of identifying a target satellite beam by compensating for propagation delays between satellites using beam ID validation techniques. When the target satellite beam is accurately identified, the user terminal 110 may more reliably access the system resources of the example communication system 100. The present invention thus provides a confirmation and proof of identity for conventional satellite beams.

The foregoing description of the preferred embodiments provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. It is therefore intended that the scope of the invention be defined by the claims and their equivalents.

Claims (26)

1. A method for determining a timing offset error in a low earth orbit satellite system, the low earth orbit satellite system (100) including at least one user terminal (110, 113) configured to (i) receive a first satellite beam from an originating satellite (102), the first satellite beam including a first timing offset, and (ii) facilitate a handoff from the first satellite beam to a second satellite beam, the second satellite beam radiating from a target satellite (104), the method comprising:

(a) determining (906) a second timing offset for the second satellite beam as a function of the first timing offset;

(b) estimating (1104) a first range from the user terminal (110, 113) to an originating satellite (102);

(c) estimating (1106) a second range from the user terminal (110, 113) to a target satellite (104); and

(d) an error in the second timing offset is determined (1206) from the estimated difference between the first and second ranges.

2. The method of claim 1, wherein the first and second timing offsets represent PN code phase offsets used in pilot signals.

3. The method of claim 2, wherein the PN phase offset is associated with a code division multiple access based communication system.

4. The method of claim 1, wherein at least one of (b) and (c) comprises: doppler characteristics associated with the user terminal (110, 113) and the first and second satellites (102, 104) are determined.

5. The method of claim 4, wherein the Doppler characteristics include Doppler frequency and a rate of change of Doppler frequency.

6. The method of claim 5, wherein the first and second voyages are approximated from the expression:

where R 'is range acceleration, R' is range rate, R is range from the user terminal (110, 113) to the satellite, and R is range from the user terminal (110, 113) to the satellite_EIs the radius of the earth, R_sIs the satellite orbit radius, v is the velocity vector, G is the gravity constant, M_EIs the earth mass.

7. The method of claim 1, wherein the step of estimating the first and second ranges comprises determining (i) doppler frequencies associated with the first and second satellite beams and (ii) rates of change of the determined doppler frequencies, respectively.

8. The method of claim 7, wherein the first and second timing offsets represent PN code phase offsets used in pilot signals.

9. The method of claim 8, wherein the PN phase offset is associated with a code division multiple access based communication system.

10. The method of claim 7, wherein the first and second ranges are derived from the expression:

where R 'is the range acceleration, R' is the range rate, R is the range from the user terminal to the satellite, R_EIs the radius of the earth, R_sIs the satellite orbit radius, v is the velocity vector, G is the gravity constant, M_EIs the earth mass.

11. The method of claim 1, wherein (d) comprises:

(i) measuring Doppler characteristics associated with the first and second satellite beams;

(ii) determining a range associated with the first and second satellites (102, 104); and

(iii) a timing difference is determined based on the determined flight.

12. The method of claim 11 wherein all of said timing offsets and timing differences are differences in pilot signal PN code phase.

13. The method of claim 12, wherein said pilot signal PN phase offset and said timing difference are associated with a code division multiple access based communication system.

14. The method of claim 1, wherein the method further comprises:

acquiring a second satellite beam at the user terminal (110, 113);

identifying the second satellite beam according to the second timing offset;

determining a timing difference based on a difference between the estimated first range and the estimated second range;

calculating a difference between the second timing offset and the timing difference; and

confirming identification of the second satellite beam when the difference between the second timing offset and the timing difference is greater than a predetermined value.

15. The method of claim 14, wherein the predetermined value is user selectable.

16. The method of claim 1, wherein the first and second timing offsets are beacon signal phase offsets.

17. The method of claim 1, wherein the first and second timing offsets must match within a predetermined time window.

18. The method of claim 17, wherein the predetermined time window is less than a beam spacing between the first and second satellite beams.

19. A user terminal (110, 113) comprising:

a receiving module (802, 804) configured to receive and demodulate first and second satellite beam signals associated with first and second satellite beams, respectively, received from first and second satellites (102, 104), respectively; and

a processor (806) coupled to the receive module and configured to (i) determine a first timing offset associated with the first satellite beam signal, the first timing offset representing a first beam ID of the first satellite beam, (ii) determine a second timing offset associated with the second satellite beam signal, the second timing offset determined from the first timing offset and representing a second beam ID of the second satellite beam, and (iii) measure respective doppler characteristics associated with the first and second satellite beam signals at a user terminal to verify the first and second beam IDs, respectively;

wherein the respective Doppler characteristics are used to determine first and second ranges associated with the user terminal (110, 113) and each of the first and second satellites (102, 104), respectively; and

wherein the user terminal (110, 113) is adapted to convert the first and second ranges into a third timing offset and to determine the presence of an error in the second beam ID based on a comparison of the second and third timing offsets.

20. The user terminal (110, 113) according to claim 19, wherein the first and second satellite beam signals comprise codes according to a code division multiple access mode.

21. The user terminal (110, 113) according to claim 19, wherein the doppler characteristic comprises at least a doppler frequency and a rate of change associated therewith.

22. The user terminal (110, 113) according to claim 19, wherein all of said timing offsets comprise PN code phase offsets for pilot signals.

23. The user terminal (110, 113) according to claim 19, wherein the first and second timing offsets have to match within a predetermined time window.

24. The user terminal (110, 113) according to claim 23, wherein said predetermined time window is smaller than a position interval between said first and second satellite beams.

25. The user terminal (110, 113) of claim 23, wherein the predetermined time window is greater than a known propagation delay between the first and second satellite beams.

26. A user terminal (110, 113) comprising:

a receiving module (802, 804) configured to receive and demodulate first and second transmitter signals associated with first and second transmitters, respectively; and

a processor (806) coupled to the receive module and configured to (i) determine a first timing offset associated with a first transmitter signal, the first timing offset representing a first transmitter ID, (ii) determine a second timing offset associated with the second transmitter ID, and (iii) measure respective doppler characteristics associated with the first and second transmitter signals at the user terminal to verify the first and second transmitter IDs, respectively;

wherein the respective Doppler characteristics are used to determine first and second ranges associated with the user terminal and each of the first and second transmitters, respectively; and

wherein the user terminal (110, 113) is configured to convert the first and second flights to a timing difference and to determine the presence of an error in the second transmitter ID based on a comparison between the second timing offset and the timing difference.