A METHOD FOR RAPID SIGNAL ACQUISITION IN A SATELLITE COMMUNICATIONS SYSTEM

Abstract: The present invention enables a radio to rapidly acquire a pilot signal from a satellite after the radio has been out of contact with the satellite for a length of time. The radio first acquires a satellite pilot signal and determines its position in relation to the earth. If the radio is now shut off or loses track of the pilot signal for some reason, the radio keeps track of the length of time that it has been out of contact with the satellite. This length of time enables the radio to estimate the extent of possible changes and, therefore, which satellites might be in view and their relative positions to the radio. This allows the radio to make assumptions about variables such as the Doppler shift of the signal that reduces the uncertainty inherent in the acquisition process and the size of the window that must be searched.

Description: A METHOD FOR RAPID SIGNAL ACQUISITION IN A SATELLITE COMMUNICATIONS SYSTEM BACKGROUND OF THE INVENTION I. FIELD OF THE INVENTION The present invention relates to satellite communications. More particularly, the present invention relates to acquiring satellite signals. II. DESCRIPTION OF THE RELATED ART When a radiotelephone is turned on in a cellular or satellite radiotelephone environment, it typically must search for and acquire the forward link signal transmitted by a base station. This forward link signal, referred to as the pilot signal in a CDMA system, is continuously transmitted by the base station. The pilot signal is used by the radiotelephone to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the signals from the base station. In a CDMA radiotelephone system, the radiotelephone cannot transmit until the pilot signal is acquired. This signal, therefore, must be acquired rapidly after the radiotelephone is turned on since a radiotelephone user typically does not want to wait to make a telephone call after turning on the radiotelephone. In a ground based CDMA cellular system, pilot signal acquisition typically takes only a few seconds. This is largely due to the known proximity of the cell sites to the radiotelephone and the fact that the cell is stationary. The pilot signal, therefore, has a relatively short delay to reach the radiotelephone and the Doppler shift of the frequency of the pilot signal is slight. Acquiring a pilot signal in a satellite based communication system, however, may take significantly longer since there is a greater frequency uncertainty that must be searched. The frequency uncertainty may be due to the range of possible Doppler shift caused by the rapidly moving low earth orbit (LEO) satellites. Other sources of uncertainty, such as the larger possible round trip time delay of the signal through the satellite can magnify the acquisition problem. There is a resulting need for a method for rapidly acquiring a signal transmitted from a satellite communication system. SUMMARY OF THE INVENTION The process of the present invention enables a mobile radio to rapidly acquire a signal from a satellite or a number of satellites that are part of a constellation of satellites. The radio has a real-time clock and memory containing the ephemeris of each of the satellites. The process begins by the radio first determining its spatial position in relation to the earth. The time that this position was determined is also noted. Both the position and the time are stored for future use. If the radio is turned off and back on or the signal is lost for some other reason, a second time from the real-time clock is determined. The radio then searches for the signal in response to a difference between the first and the second times. Knowing the position of the user allows assumptions to be made that simplify acquisition. Knowing the time since the last position fix allows varying degrees of confidence in these assumptions. The smaller the difference, the smaller the frequency or time uncertainty window to be searched, thus reducing acquisition time. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a satellite based communication system of the present invention. FIG. 2 shows a block diagram of a typical radio of the present invention. FIG. 3 shows the frequency band being searched by the process of the present invention. FIG. 4 shows a flowchart of the process of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A typical satellite communication system is illustrated in FIG. 1. In the preferred embodiment, this communication system is comprised of 48 satellites (101) that act as repeaters for the signals transmitted to and from mobile radios (102) located on the earth or in an airplane at some altitude above the earth. The satellite bounces the signal back to a ground receiving station (103) connected with the public switched telephone network (PSTN). The PSTN switches the call to another cellular network or to a land-line telephone. In the preferred embodiment, the radios are CDMA-type radio¬ telephones. U.S. Patent No. 4,901,307 to Gilhousen et al. and assigned to Qualcomm, Inc. discloses the operation of such a CDMA-type radio¬ telephone in a terrestrial CDMA system. Alternate embodiments use time division multiple access (TDMA) or frequency division multiple access (FDMA) type radiotelephones. The satellite communications system of the present invention operates by the radio first locating a pilot carrier signal transmitted by the satellites. In the preferred embodiment, this pilot signal is an unmodulated PN sequence. However, alternate embodiments have a pilot signal comprised of a preamble sequence, a preamble sequence mixed with data, a sufficiently strong information-bearing signal, or other signal types. In the preferred embodiment, the pilot signal is transmitted by each satellite using a known frequency and pseudo noise (PN) code, possibly with different spread spectrum code generators or phase offsets, thus allowing the radio to distinguish the different satellites. Since the satellites use a known PN code and code phase, the radiotelephone's internal real-time clock synchronizes to the system timing by a search through all code generators and phases. The strongest signal found by the radio corresponds to the best satellite signal. The satellite's rapid movement across the sky causes the frequency of the pilot signals from the various satellites to be changed due to Doppler shift. If the radio searches the frequency spectrum at the pilot signal's transmitted frequency, the signal will only be found if the satelUte is directly overhead. This is also true of the PN code phase. The radio, therefore, must search a time uncertainty for the PN code phase and the frequency uncertainty to acquire the satellite. The process of the present invention reduces this uncertainty by using knowledge of satellite and radio position. A typical radio (200) of the present invention is illustrated in FIG. 2. The radio (200) is comprised of memory (201) that stores the ephemeris of each satellite. In the preferred embodiment, this memory (201) is nonvolatile random access memory (RAM). This allows the radio (200) to update it's ephemeris data when a satellite is out of service or changes orbit. Alternate embodiments use programmable read only memory in which each satellite's ephemeris is permanently stored. Still another alternate embodiment uses battery backed-up RAM. The radio (200) also consists of a real-time clock (205) that is discussed later, a processor (210) to control the radio, a vocoder (215) to encode and decode the user's voice, a modulation/demodulation circuit (220), and the radio frequency electronics (225) that convert the modulated signals to higher frequencies for transmission and received signals to lower frequencies. The process of the present invention, illustrated in FIG. 4, begins with the radio synchronizing its internal real-time clock to the satellite system time (401), kept by the satellites' or ground stations' internal clocks. In the preferred embodiment, the satellites simply reflect the clock signals from the ground station. Alternate embodiments use a clock in each satellite that is synchronized to the other clocks in the satellite system. Since the radio now knows the time and the ephemeris of each satellite, it knows the location of the satellites in relation to Earth. The radio, however, has to determine its position in relation to Earth in order to know its position in relation to the satellites. The radiotelephone's position is determined (405) by triangulation with different satellites or a single satellite in different positions over time. Since the radio knows the satellites' positions, it measures the time required for a signal to reach and return from each satellite to determine its position in relation to Earth. In an alternate embodiment, it is possible to use an alternate positioning system such as the global positioning system (GPS) or a user entered position. The radio's spatial position is stored in memory. If the radio is turned off, the time is stored with the present spatial position (410). After the radio is turned back on, this turn-on time is determined from the real¬ time clock (415). The difference between the time stored at power-down and the time at power-up allows an estimate of how far the user may have moved from the previous known position. When the estimate of the radiotelephone's possible new position is combined with known satellite positions, a list of possible and likely satellites in view can be determined along with the possible and likely relative position of each. The radio then determines the Doppler shift and PN sequence that will be experienced from a signal being transmitted through each satellite. The Doppler shift gives the new frequency to find and the length of time the radio was off determines the width of the search window (420) about this frequency. If the radio was only turned off a short time, it could not have traveled very far. In this case, the search window is going to be relatively small. In the preferred embodiment, the search window starts at 5 kHz for times on the order of an hour. If the radio has been off for a long time, such as greater than 24 hours, there is still a good chance that the radio did not move from its previous power on location. The user may have just turned the radio off and left it sitting. The search window, in this case, is again relatively small. In the preferred embodiment, this window is 5 kHz. As the time that the radio is off increases, the search window increases. This is true up to the preferred embodiment's 24 hour point as discussed above. In the preferred embodiment, the search window increases by 5 kHz for every hour that the radio was turned off. If the process of the present invention does not find the frequency of the satellite's pilot signal with the initial search window, the window is widened and the search done again. In the preferred embodiment, the window is widened 10 kHz. This widening and searching is repeated until the frequency of the signal is found or a decision is made that the user position is in error or something else is wrong with the ability to acquire the signal. An example of a search of the frequency spectrum is illustrated in FIG. 3, showing the frequency of the pilot signal as f. The expected Doppler shift is denoted as Δ. The frequency band to the right of f is f + Δ while the frequency band to the left of f is f - Δ. The -Δ side of the spectrum is due to the satellite moving away from the radio while the +Δ side is due to the satellite moving towards the radio. If the radio has determined that the pilot frequency will be shifted by +Δχ due to the satellite's relative position, a window (301) around +Δι is searched to find the pilot signal frequency. This window increases as the time that the radio is turned off increases. This is due to the increasing inaccuracy of the frequency determination the longer the radio is turned off. Therefore, if the radio has been turned off only a few minutes, the window to be searched will only be 5 kHz wide in the preferred embodiment. If the radio is off for a couple of hours, this window increases. Alternate embodiments search different size frequency windows. The process of the present invention also reduces uncertainties produced by mechanisms other than Doppler shift of the received signal. In alternate embodiments, different satellites use different frequencies, PN codes, and/or other differentiating characteristics. The radio's knowledge of the location of the satellites relative to the radio enables it to determine which frequencies and PN codes are available and should be searched to acquire the signal. The signal acquisition process of the present invention also enables a radio to search a PN code phase that has been affected by a Doppler shift. In the same manner as the above described preferred embodiment, the process searches a window in the PN code phase, by estimating the Doppler affects on the PN code phase, to locate a particular PN code. Alternate embodiments of the process of the present invention searches different size windows depending on the time that the radio is turned off. Alternate embodiments also continue to increase the size of the search window to other maximum times, such as 48 hours, before assuming the radio has not left its previous position and reverting back to the initially small search window. The process of the present invention greatly reduces the time required for a radio to find a signal in a satellite based communication system. By estimating the position of the radio after the radio has been shut down for a period of time, the Doppler shift can be estimated and, therefore, the window of the frequency band and other uncertainties that must be searched is substantially reduced. I claim:

Claims: CLAIMS 1. A method for a radio to rapidly acquire a signal, having a frequency, from at least one satellite of a constellation of satellites, the radio having a clock and an ephemeris of the at least one satellite, the constellation of satellites communicating with a plurality of ground base stations, the method comproing the steps of: determining a spatial position for the radio, the position determined at a first time as indicated by the clock; saving the spatial position and the first time; determining a second time from the clock; estimating a cond spatial position in response to a difference between the first time and the second time; and searching a predetermined frequency band for the frequency in response to the second spatial position. 2. The method of claim 1 and further including each satellite of the constellation of satellites having a clock, each satellite clock being synchronized with the other satellite clocks. 3. The method of claim 2 and further including the step of synchronizing the radio clock to the satellite clocks. 4. The method of claim 1 and further including at least one ground station of the plurality of ground stations having a clock that is transmitted to the constellation of satellites. 5. A method for a radiotelephone to rapidly acquire a signal, having a frequency, from at least one satellite of a constellation of satellites, the radio having a clock and an ephemeris of a sub- constellation of satellites of the constellation of satellites, the constellation of satellites communicating with at least one ground base station, the at least one ground base station having a clock that is transmitted to the constellation of satellites, the method comprising the steps of: synchronizing the radiotelephone clock to the ground base station clock; determining a spatial position for the radiotelephone, the spatial position determined at a first time as indicated by the radiotelephone clock; saving the spatial position and the first time; determining a second time from the radiotelephone clock; estimating a second spatial position in response to a difference between the first time and the second time; and searching a predetermined frequency band for the frequency in response to the second spatial position. 6. The method of claim 5 wherein the bandwidth of the predetermined frequency band is proportional to the difference between the first and the second times. 7. A method for a code division multiple access radiotelephone to rapidly acquire a pilot signal, having a frequency, from a first satellite of a constellation of satellites, the radiotelephone having memory, a clock, and an ephemeris of each satellite of the constellation of satellites stored in the memory, each satellite communicating with a plurality of ground base stations, each ground base station having a clock, a first ground base station transmitting a clock signal from the clock to the first satellite, the method comprising the steps of: synchronizing the radiotelephone clock to the clock signal; determining a spatial position for the radiotelephone, the position determined at a first time as indicated by the radiotelephone clock; storing the spatial position and the first time in memory; determining a second time from the radiotelephone clock; estimating a second spatial position in response to a difference between the first time and the second time, the second spatial position being at * an increasing distance from the first spatial position as the difference between the first and second times increases up to a predetermined maximum difference; and searching a predetermined frequency band for the frequency in response to the second spatial position. 8. A method for a radio to rapidly acquire a signal, having a predetermined frequency, from a first satellite of a constellation of satellites, the radio having a clock and an ephemeris of the first satellite, the first satellite communicating with a ground base station, the method comprising the steps of: determining a first spatial position for the radio, the first spatial position determined at a first time as indicated by the clock; in response to the ephemeris, determining a spatial position for the first satellite; and searching a predetermined frequency band for the predetermined frequency in response to the first spatial position of the radio and the spatial position of the satellite. 9. The method of claim 8 wherein the predetermined frequency band is determined by the steps of: saving the first spatial position for the radio and the first time; determining a second time from the clock; estimating a second spatial position in response to a difference between the first time and the second time; and determining the predetermined frequency band in response to the second spatial position and the spatial position for the first satellite. 10. A method for a radio to rapidly acquire a signal, having a predetermined pseudo noise code, from a first satellite of a constellation of satellites, the radio having a clock and an ephemeris of the first satellite, the first satellite communicating with a ground base station, the method comprising the steps of: determining a spatial position for the radio, the spatial position determined at a first time as indicated by the clock; in response to the ephemeris, determining a spatial position for the first satellite; and searching a predetermined pseudo noise code phase for the predetermined pseudo noise code in response to the spatial position of the radio and the spatial position of the satellite. 11. The method of claim 10 wherein the predetermined pseudo noise code phase is determined by the steps of: saving the spatial position for the radio and the first time; determining a second time from the clock; estimating a second spatial position in response to a difference between the first time and the second time; and determining the predetermined code phase in response to the second spatial position and the spatial position for the first satellite.

Inventors: CARTER STEPHEN S, CARTER, STEPHEN, S.
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 1995
Main classification: H04B7
IPC codes: G01S1, H04B7