Title of Invention

METHOD FOR ESTIMATING VELOCITY OF A TERMINAL IN A WIRELESS COMMUNICATION SYSTEM

Abstract

Techniques to estimate the velocity of a terminal in a wireless communication system. Movement by the terminal results in a Doppler shift in the frequency of each transmitted signal received at the terminal. In one method, the positions of the terminal, a base station, and each of two or more satellites are initially determined. A residual rate of a change of pseudo-range may also be determined for each satellite, e.g., based on (1) an estimated baseband frequency error that includes the Doppler frequency shift due to the terminal's movement and (2) an estimated Doppler frequency shift due to movement by the satellite. A set of equations is then formed based on the determined positions of the terminal, the base station, and the satellites and the determined residual rates of change of pseudo-ranges for the satellites. The velocity of the terminal may thereafter be estimated based on the set of equations.

Full Text

BACKGROUND Field [1001] The present invention relates generally to communication systems, and moors particularly to techniques for estimating the velocity of a terminal in a wireless communication system. Background [1003] Signals fro— satellites and/or base stations may be used to estimate the location of a terminal. By receiving and processing the signals transmitted from these transmitters, the amount of time required for the signals to travel from the transmitters to the terminal may be estimated and used to compute the distances (or ranges) between the transmitters and the terminal. The signals themselves may further include information indicative of the locations of the transmitters. By accurately determining the distances to three or more transmitters at known locations, the position of the terminal may be determined using dilatation. [1##4] In certain instances and for certain applications, the velocity of the terminal [nay also need to be ascertained, in one simple technique for estimating velocity, a aeries opposition fixes are itetermined for the terminal and used to estimate its velocity, - this technique has several shortcomings. One showcasing relates to the use of position fixes to estimate velocity. If the position fixes are determined at short time intervals, then small errors in the position fixes may result in large errors in the velocity estimate. However, if the position fixes are determined at longer time intervals, then the position fixes may be more indicative of the average velocity of the terminal instead of the instantaneous velocity. [1005] A second shortcoming is related to the shared resources at the terminal. In many terminal designs, some or all of the elements used for voice and/or data communication are also used for position determination. These terminal designs typically do not allow the shared elements to be used simultaneously for both communication and position determination. Consequently, communication is typically inhibited while position is being determined, and vice versa. Obtaining several consecutive position fixes would then require the terminal to stay in a GPS mode for an extended period of time or to repeatedly interrupt communication. [1006] There is therefore a need in the art for techniques to efficiently and accurately estimate the velocity of a terminal in a wireless communication system. SIMMARY [1007] Aspects of the invention provide techniques to estimate the velocity of a terminal in a wireless communication system. Movement by the terminal results in a Doppler shift in the frequency of each transmitted signal received at the terminal. This Doppler frequency shift is related to the terminal"s velocity, which may be accurately estimated by processing the received signal to provide a set of frequency errors in the transmitted signals (as received at the terminal) for a number of satellites. \"various scenarios are described in further detail below, and the terminal"s velocity may be estimated (1) based on signals from both base station and satellites or based only on signals from satellites and (2) for a 3-dimensional (e.g., earth-centered, earth-fixed) or a 2-dimensional (e.g., east, north) frame. [1008] A specific embodiment of the invention provides a method for estimating the velocity of a terminal in a wireless communication system. In accordance with the method, the positions of the terminal, a base station, and each of two or more satellites are initially determined. A residual rate of change of pseudo-range may also be diseased each satellite. A set of equations is then formed based on the determined aositjois of the terminal, the base station, and the satellites and the determined residual rates of change of pseudo-ranges for the satellites. The velocity of the terminal may thereafter be estimated based on the set of equations. [1009] To determine the residual rates of change of pseudo-ranges for the satellites, the received signal (which includes the signals transmitted from the satellites) is initially downconverted to provide a baseband signal. The frequency error of the baseband signal is then determined for each satellite. The Doppler shift in the frequency of the signal from each satellite is also estimated. The residual rate of change of pseudo-range for each satellite is then determined based on the estimated baseband frequency error and Doppler frequency shift for the satellite. [1010] For certain scenarios, the terminal"s velocity may be estimated without using the base station. In this case, the residual rates of change of pseudo-ranges are determined for three or more satellites, and the frequency error in the oscillator used to downcsmvert the received signal becomes an additional unknown that can be solved for using son additional satellite measurement. The velocity estimation techniques are described in further detail below. [1011] The invention further provides other methods, computer program products. receiver units, terminals, and apparatus and elements that implement various aspects, embodiments, and features of the invention, as described in further detail below. BRIEF DESCRIPTION OF THE DRAWINGS [1012] The features, nature, and advantages of the present invention will become more apparel from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: [1013] FIG. 1 is a simplified diagram of a system wherein various aspects and embodiments of the invention may be implemented; [1014] FIG. 2 is a block diagram of an embodiment of some of the processing performed by a terminal to estimate its position and velocity; and [1015] FIG. 3 is a flow diagram of an embodiment of a process for estimating the velocity of the terminal based on signals from satellites and a base station. DETAILED DESCRIPTION [1016] FIG. 1 is a simplified diagram of a system wherein various aspects and embodiments of the invention may be implemented. A terminal 110 whose position and velocity are to be ascertained receives signals transmitted from a number of transmitters, which may be base stations 120 of a wireless communication system and/or satellites 130 of the Global Positioning System (GPS). In general, any type of transmitter having locations that are known or can be ascertained may be used to estimate position and velocity. [1017] Terminal 110 may be any device capable of determining the arrival times of transmitted signals with respect to a reference time. In one embodiment, terminal 110 is a cellular telephone capable of receiving signals from a number of transmitters. In other embodiments, terminal 110 may be an electronics unit (e.g., a computer tenninal, a persor.’ digital assistance (PDA), and so on) having a wireless modem, a stand-alone GPS BBceiver, a receiver capable of receiving signals from satellites and/or base stations, or any otiier rj"pe of receiver. [1018] The positicHi and velocity of terminal 110 may be estimated based on signals received at the terminal (e.g., such as those transmitted from the GPS satellites and/or base stations) plus the locations of the transmitters from which the signals originated. The position and velocity of the terminal may be estimated by the terminal, a Position Determining Equipment (PDE) 140 in the wireless communication system, a base station, or some other entity. The entity performing the position and velocity estimation is provided with the necessary information (e.g., the pertinent measurements and either the locations of the transmitters or the means to determine these locations). [1019] The locations of the GPS satellites may be ascertained by processing the signals transmitted by the satellites. Each satellite transmits "Almanac" information, which includes information regarding coarse locations of all satellites in the constellation. Each satellite further transmits "Ephemeris" information, which includes a higher accuracy version of its own orbit, as tracked and reported by tracking stations on earth. The locations of the base stations may also be made known (e.g., via messages) to the entity performing the position and velocity estimation for the terminal. roc exsnple, the terminal may include a database of the locations of the base stations mid/csr ■atp-llire."; or these locations niay be provided by a PDE or base station. Aitemaiively, the base station or PDE may perform the position and velocity estimation for the terminal and may have the information for the satellite and/or base station locations. The location information for the satellites and/or base stations may also be transmitted via messages. [1020] The GPS satellites and base stations may be used as reference points to estimate the location of a terminal. By accurately measuring the distances to three transmitters at known locations, the position of the terminal can be determined using trilateration. The terminal can estimate the distance to each transmitter by measuring the time required for a signal to travel from the transmitter to the terminal. If the time the signal is transmitted from the transmitter is known (e.g., stamped into the signal), then the travel time of the signal can be determined by observing the time the signal is received at the terminal (based on its internal clock). Typically however, the amount of time between transmission and reception cannot be exactly determined because of offs betwren the clocks at Ae transmitter and terminal. Thus, a "pseudo-range" is typically obtained based on the difference between a reference time and the time thar the signal is recsived. [1621] HG. 2 is a block diagram of an embodiment of some of the processing performed by a terminal 110a to estimate its position and velocity. The signals transmitted by the base stations and/or satellites are initially received by an antenna 212 and provided to an amplifier/filter block 214, which conditions (e.g., filters and amplifies) the received signal to provide a conditioned radio frequency (RFj signal. A mixer 216 then downconverts the RP signal to baseband with a local oscillator (LO) signal prcNided by an oscillator 218. The baseband signal may further be amplified and filtered by an amplifier/filter block 220 and then digitized by an analog-io-digital converter (ADC) 222 to provide (complex) data samples. [1022] In a typical receiver design, there may be one or more stages of amplifier, filter, mixer, and so on. For example, the received signal may first be downconverted to an intermediate frequency (IF) with a first LO signal and thereafter (quadrature) downconverted to baseband with a second LO signal. For simplicity, these various signal conditioning stages are lumped together into the blocks shown in FIG. 2. For example, mixer 216 may represent one or multiple downconversion stages (e.g., from RF dowTj to IF, and from IF down to baseband). Tl§231 In the embodiment shown m FIG. 2, the data samples are provided to a "XMja. 224 that translates the center frequency of the data samples with a carrier signal position. If the terminal is located within 10 km of the base station, then the satellite"s Doppler frequency shift, as estimated at the base station, would have less than 10 Hz of error at the terminal, which is acceptable for many applications. The satellite"s Doppler frequency shift may thereafter be more accurately estimated at the terminal"s position (instead of the base station"s position) once the terminal"s position has been estimated based on any number of position determination techniques known in the art. The Doppler frequency shift may be estimated for each satellite to be used to estimate the terminal"s velocity. [1031] The baseband frequency error, f’’, may be estimated and used to estimate the terminal"s velocity in a manner described in further detail below. In an embodiment, the baseband frequency error, /t,j,,, may be estimated based on a frequency ccmrrol loop used to acquire and track the frequency of the signal from a transmitter. Tne received frequency of the signal (as tracked by the frequency control loop) may be subtracted from the signal"s nominal carrier frequency to provide the baseband frsqasncy error, f’., for the transmitter. One frequency control loop may be used 10 acquire and track the signal from each transmitter. [1032] In another embodiment, the baseband frequency error, /’i,,, is estimated by performing signal processing on the data samples. To satisfy the requirements mandated by the Federal Communications Commission (FCC) for an enhanced emergency 911 (E-911) service, the terminal needs to work in difficult environments (e.g., dense urban areas and indoor) and at low signal-to-noise-plus-interference ranos (SNUS). In order to achieve this, coherent integration of the received signal for longer periods of time is needed to detect the signal in the presence of noise. And to integrate the signal longer without suffering significant losses, it is necessary to better estimate the frequency of a transmitted signal, as received at the terminal, so that the frequency error is as small as possible. The frequency error may be reduced by estimating and removing the satellite"s Doppler frequency shift and the carrier frequency offset. [1033] In an embodiment, the satellite carrier frequency offset, f’’j, is estimated oased on the Ephemeris information transmitted by the satellite. The estimated satellite earner frequency offset, /Q’, , may be combined with an initial estimate of the satellite DoopJer frequency shift, fy’-, and provickd to the terminal. coarse estimate of the terminal"s carrier frequency error so that /Q,’’ is as small as possible, and further removes the initial estimate of the satellite Doppler frequency shift from the data samples. The code phase selection determines the timing of each transmitted signal, as received at the terminal. The timing may thereafter be used to estimate the pseudo-range to the transmitter. And the fine frequency selection estimates the rotator frequency error, f’’.. Other processing orders and/or other schemes may also be used to derive the necessary timing and frequency error information, and this is within the scope of the invention. [1037] For a code phase search, the received signal is processed to determine the timing for each transmitted signal used to estimate the terminal"s position and velocity. The baseband frequency error, f’’, is a non-pure tone since the transmitted signal includes data that may be spread with a spreading code. This spreading code is typically a pseudo-random noise (PN) sequence, such as the Gold PN code used for GPS satelEtes. A transmitted signal"s timing may be determined at the terminal by correlating liie rotated data samples with the same spreading sequence used at the iTHnsmitter. [1038] In particular, the rotated data samples are correlated with a locally generated PN sequence at various offsets (or phases), with each PN phase corresponding to a hypothesis for the transmitted signal"s timing. For each hypothesis, short segments of data samples (i.e.. short with respect to the baseband frequency error) may be correlated with the PN sequence for the hypothesis, and the correlation results for multiple segments may be (non-coherently) accumulated to provide a correlation result for the hypothesis. By removing the satellite Doppler frequency shift, the frequency error in the rotated data samples is reduced and a longer integration time (i.e., longer segments) may be used for the coherent integration to improve signal detection. The higher energy from the correlations also facilitates demodulation to extract data from the baseband signal. iW39] For the fine frequency search phase, the hypotheses are tested for correlation osing coherent integration and non-coherent integration to determine the amount of easrgy in ffl: various frequency offsets. The coherent integration may be performed tjased on a fast Fourier transform (FFT), or some other suitable rotate and accumulate ;2£i=iasion technique. In particular, the rotated data samples may be correlated with a «caa> aeaeafcd sequence that is encoded with a PN code associated with the particular [1041] As shown above, the baseband frequency error may be estimated without use of a frequency control or tracking loop. In this case, the baseband frequency error may be estimated by correlating over a number of PN phase hypotheses and a number of frequency bms. The size of the frequency bin is linked to the coherent integration period. Longer coherent integration periods require smaller frequency bin sizes in order to reduce the integration losses, and therefore result in higher accuracy. [1042] If the terminal"s oscillator is slaved or locked to the carrier frequency of the base station and if the frequency base of the communication system is either (1) accurate (derived from the GPS system or from a very accurate clock) (2) stable but off by some known amount (measured against an accurate source)), then the unknown part of the terminal"s oscillator frequency offset,/oter, corresponds to the Doppler frequency shift due to the motion of the terminal relative to the base station. [1043] The velocity of the terminal, Vj, may be estimated based on the estimated baseband frsqiKncy error, /’ ,, as follows. Initially, a pseudo-range measurement to each transmitier (i£„ each satellite and/or each base station) to be used to estimate the aiUiiui"s veJocnt) and position may be determined based on the arrival time of tiie signal transmitted by the transmitter (e.g., as identified by the PN offset from the code phase search). The position of the terminal may then be estimated using the pseudo-range measurements to the transmitters and their locations. The position determination may be performed using various techniques known in the art. [1044] A residual rate of change of pseudo-range may then be derived based on the estimated baseband frequency error shown in equation (6). To estimate this baseband frequency error, the Doppler frequency shift due to the motion of each transmitter (e.g., each satellite) may be initially estimated (e.g., at the base station position) and provided to the terminal (or the PDE or some other entity). The baseband frequency error may then be estimated in the manner described above for each transmitter (e.g., each satellite) to be used for position and velocity estimation. For the embodiment described above, the baseband frequency error may be estimated by determining the frequency error of the rotated data samples and adding back the initial estimate of the transmitter"s Dappkr frequency shift. For each transmitter, the baseband frequency error estimate, f’ . may be expressed as shown in equadon (6), which is: frequency offset, fo’’,, and (4) the wavelength of the oscillator frequency, A. Equations (8) and (10) also show thai the residual rates of change of pseudo-ranges for the satellites are due to the terminal Doppler frequency shift, f’’,., and the terminal carrier frequency error,/oier- [1047] Equation (10) may be used to estimate the terminal"s velocity, Vj., for various scenarios. For example, equation (10) may be used when signals from both satellites and base stations are used for position and velocity estimation, or when only signals from satellites are used for position and velocity estimation. Also, equation (10) may be applied to any coordinate frames and dimensionality. Some of these scenarios are described in further detail below. 3D Velodrr Estimation for a Terminal with Satellites and a Single Base Station [1048] If the carrier frequency of a signal transmitted from the serving base station is equal to a ncsninal carrier frequency (or that any offset in the carrier frequency is known and can be adjusted or accounted for), then any oscillator frequency error./oer. at the lermiriai would be due to movement of the terminal relative to the serving base station. In this case, the terminal carrier frequency error would not need to be handled as an independent unknown, but would be a function of the terminal velocity. Hence, one of the unknowns in the velocity estimation may be eliminated and the number of required measurements is reduced by one. However, because of the proximity to the base station, it would be necessary to accurately know the terminal position before performing the velocity computation. [1049] To estimate a 3-dimensional (3D) velocity of the terminal based on signals transmitted from the satellites and a single base station, the following notations are used: x = (x y z) is the terminal"s coordinates in an ECEF estimated for each satellite (e.g., by the tensional) and the satellite Doppler frequency shift, /vs.,,, estimated for each satellite at the terminal position, as described above in equation (10). Other techniques to estimate the residual rate of change of pseudo-range may also be used and are within the scope of the invention. [1083] A set of equations is then formed based on the determined positions of the terminal, the base station, and the two or more satellites and the determined residual rates of change of pseudo-ranges for the satellites. This set of equations may be as shown in equation (12) for a 3-dimensional (e.g., ECEF) frame or equation (17) for a 2-dimensional (e.g., east north) frame. The velocity of the terminal is then estimated based on the set of equations, as shown in equation (14) for the 3-D frame or equation (19) for the 2-D frame. [1084] The flow diagram shown in FIG. 3 may be modified for the embodiments described above’ wherein the terminal"s velocity is estimated based only on signals transmitted from us satellites (and not from the base stations). For these embodiments, the base station"s position is not needed and step 314 may be eliminated. Moreover, the residual rates of chaise of Seiko-ranges are determined for three or more satellites, in step 318, and the set of equations further include an known for the terminal carrier frequency offset,/osier, as shown in equations (30) and (32). [1085] The computations to estimate the terminal"s velocity may be performed at the terminal, the base station, or some other entity capable of forming and solving the proper set of equations, as described above. The entity performing the velocity estimation is provided with the required information, which may include (1) the position of the terminal, (possibly) the base station, and the satellites, (2) the estimated baseband frequency errors for the satellites, and (3) the estimated Doppler frequency shifts for the satellites, or some equivalent information. [1086] The velocity estimation techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the elements used to estimates the terminal"s velocity may be implemented within one or more application speeds ring’s- electronic units designed to perform the functions described herein, or a combination thereof. [1087] For a software implementation, the velocity estimation techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described harem. The software codes may be stored in a memory unit (e.g., memory 242 FIG. 2) and executed by a processor (e.g., controller 240). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art. [1088] Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification. [1089] The description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications tic these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be Airmailed to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. WE CLAIM: 1. A method for estimating a velocity of a terminal in a wireless communication system, comprising the steps of: determining a position of the terminal; Determining a position of a base station in communication with the terminal; determining a position of each of two or more satellites; Determining a residual rate of change of pseudo-range for each satellite; forming a set of equations based on the determined positions of the terminal, the base station, and the two or more satellites, wherein the set of equations is further formed based on the determined residual rates of change of pseudo-ranges for the two or more satellites; and estimating the velocity of the terminal based on the set of equations; wherein the residual rate of change of pseudo-range for each satellite is determined as: 2. The method as claimed in claim 1, comprising the steps of: downconverting a received signal having included therein a signal transmitted from each of the two or more satellites to provide a baseband signal; and estimating a frequency error of the baseband signal for each satellite, and wherein the residual rate of change of pseudo-range for each satellite is determined based in part on the estimated baseband signal frequency error for the satellite. 3. The method as claimed in claim 2, comprising the steps of: estimating a Doppler shift in the frequency of the signal transmitted from each satellite, and wherein the residual rate of change of pseudo-range for each satellite is further determined based in part on the estimated Doppler frequency shift for the satellite. 4. The method as claimed in claim 3, comprising the steps of: estimating an offset in a carrier frequency of each satellite, and wherein the residual rate of change of pseudo-range for each satellite is further determined based in part on the estimated satellite carrier frequency offset. 5. The method as claimed in claim 3, wherein the Doppler frequency shift for each satellite is estimated at the terminal position. 6. A method for estimating a velocity of a terminal in a wireless communication system, comprising the steps of: determining a position of the terminal; determining a position of a base station in communication with the terminal; determining a position of each of two or more satellites; determining a residual rate of change of pseudo-range for each satellite. forming a set of equations based on the determined positions of the terminal, the base station, and the two or more satellites, wherein the set of equations is further formed based on the determined residual rates of change of pseudo-ranges for the two or more satellites; and estimating the velocity of the terminal based on the set of equations; wherein the set of equations comprises: M = Au + N , where M is a vector of the determined residual rates of change of pseudo-ranges for the two or more satellites, A is a matrix of elements derived based on the determined positions of the terminal, the base station, and the two or more satellites, u is a vector for the terminal velocity, and N is a noise vector. 7. The method as claimed in claim 6, wherein the velocity of the terminal is estimated as : 1,. is a unit vector directed from an /-th satellite position to the terminal position, where i is an index for the two or more satellites, and Ig is a unit vector directed from the terminal position to the base station position. 9. The method as claimed in claim 1, wherein the velocity of the terminal is estimated for a 3-dimensional frame. 10. The method as claimed in claim 9, wherein the 3-dimensional frame is based on an ECEF (earth centered, earth fixed) frame. 11. The method as claimed in claim 1, wherein the velocity of the terminal is estimated for a 2-dimensional frame. 12. The method as claimed in claim 11, wherein the 2-dimensional frame is based on an ENU (east, north, up) frame. 13. A method for determining a velocity of a terminal in a wireless communication system, comprising: determining an offset of a local oscillator at the terminal, wherein the determining the local oscillator offset includes, performing a coherent integration based on a fast Fourier transform (FFT) to provide FFT results, and non-coherently integrating the FFT results; sorting the non-coherently integrated FFT results into a plurality of frequency bins; estimating a first Doppler shift in the frequency of a first signal received at the terminal from a first transmitter; estimating a second Doppler shift in the frequency of a second signal received at the terminal from a second transmitter; estimating a third Doppler shift in the frequency of a third signal received at the terminal from a third transmitter; and determining the velocity of the terminal based on the local oscillator offset and estimates of the first, second, and third Doppler frequency shifts. 14. The method as claimed in claim 13, wherein the estimates of the first, second, and third Doppler frequency shifts are provided by a base station in the wireless communication system. 15. The method as claimed in claim 13, comprising the steps of removing the estimates of the first, second, and third Doppler frequency shifts from the first, second, and third signals, respectively. 16. The method as claimed in claim 15, wherein non-coherently integrating comprises squaring and accumulating the FFT results. 17. A method for determining a velocity of a terminal in a wireless communication system, comprising the steps of: determining an offset of a local oscillator at the terminal, wherein the determining the local oscillator offset comprises, performing a coherent integration based on a fast Fourier transform (FFT) to provide FFT results, and non-coherently integrating the FFT results, wherein non-coherently integrating the FFT results includes squaring and accumulating the FFT results; estimating a first Doppler shift in the frequency of a first signal received at the terminal from a first transmitter; estimating a second Doppler shift in the frequency of a second signal received at the terminal from a second transmitter; estimating a third Doppler shift in the frequency of a third signal received at the terminal from a third transmitter; determining the velocity of the terminal based on the local oscillator offset and estimates of the first, second, and third Doppler frequency shifts; and despreading the non-coherently integrated FFT results. 18. The method as claimed in claim 17, wherein the despreading comprises correlating the FFT results with a pseudo-random noise (PN) sequence at a plurality of PN phases. 19. The method as claimed in claim 18, comprising identifying for each transmitter a PN phase associated with a maximum correlated value. 20. The method as claimed in claim 19, comprising identifying for each transmitter a frequency offset corresponding to an FFT bin with the maximum correlated value. 21. The method as claimed in claim 30 , comprising: deriving a pseudo-range measurement for each transmitter based on the identified PN phase; and deriving a residual rate of change in pseudo-range based on the identified frequency offset. 22. The method as claimed in claim 21, comprising determining a position of the terminal based on pseudo-range measurements for the first, second, and third transmitters. 23. The method as claimed in claim 22, wherein the residual rate of change of pseudo-range for each transmitter is determined by multiplying the identified frequency offset for the transmitter by a wavelength for the signal from the transmitter. 24. The method as claimed claim 23, comprising estimating the velocity of the terminal based on an estimated position of the terminal and estimated positions of the transmitters.

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