MXPA96001558A - Frequency error correction in a satellite-mobile communications system - Google Patents

Abstract

Mobile stations correct their frequencies using a signal received from the moving relay station including correction of any Doppler shift caused by the satellite's movement. The mobile station includes a receiver for receiving a paging channel signal broadcast by the moving relay station and demodulators and decoders for decoding the information in the paging channel signal. Using he decoded information, the mobile station can determine an estimate of its position within the communication system. The mobile station can also determine a frequency error and a Doppler shift using the decoded information and the position estimate. A frequency controlling signal is determined using the determined frequency error and the Doppler shift. A controlled reference oscillator then uses the frequency correcting signal as a control input signal. Finally, the mobile station contains a transmitter for transmitting a signal using the voltage controlled oscillator as a reference.

Description

CORRECTION OF FREQUENCY ERROR IN A SYSTEM D? MOBILE COMMUNICATION BY SATELLITE FIELD OF THE INVENTION The present invention relates to a bi-directional communication system with mobile telephones through a relay station in motion, such as an orbiting satellite. In a mobile satellite communication system, the movement of the satellite causes the radiofrequency of satellite transmissions, as perceived by mobile stations, to be augmented by the Doppler effect. If the mobile stations add their internal frequency norms to the lost satellite signal, there will be an error and thus the frequencies transmitted from the mobile stations back to the satellite will also be incorrrerated. In addition, the signals of the newly satellite mobile stations will again be shifted by the Dopple shift so that the frequency as perceived on the satellite is doubly wrong. More specifically, the present invention relates to a method and apparatus by which the mobile stations can correct their frequencies using a signal received from the mobile relay station including correction of any Doppler deps 1 caused by the satellite movement.

BACKGROUND OF THE EXPOSURE Since the relay or movement station is satellite, it is assumed that the communication station is known, the main ground station can, in principle, precompensate the frequency used to spread the signal in a way, it is perceived directly by a mobile station. However, since the Doppler shift perceived in a mobile station depends on the position of the mobile station, different precompensation is needed for each signal that is transmitted to each mobile station. In this way, the precompensation technique is not a soft solution for cases where all mobile phones receive the same signal, for example, the call channel of a mobile satellite-based telephone system. The prior art contains examples of broadcasting a frequency or time norm from a moving satellite La. Global Satellite Placement (GPS) is an example of that subject. The Global Satellite Collocation system, as its name implies, is a satellite navigation system whose main purpose is to allow the base receivers on the ground to determine their position. Knowing their position and details of the systematic movement of the satellite, the Doppler shift can be determined at each mobile station and the satellite signals corrected accordingly, thus providing a frequency reference. In order to determine the three spatial coordinates and the unknown time / frequency parameter, necessary to determine the Doppler shift, the receiver is capable of receiving signals from four satellites simultaneously. The GPS system is not a communication system and the navigation receivers do not have associated transmitters that transmit the signals to the satellites. In addition, GPS receivers usually do not try to determine a reference-based position or frequency by listening to the signals of a single satire 1 ite. The prior art contains examples of reference oscillators based on remembering a previous correction in a temperature search box memory. New frequency errors subsequently determined at a temperature found above are averaged with the value prior to that temperature in order to update the contents of the table. This system has been used in portable cell phones manufactured by Ericsson-GE in Lynchburg, Va, since 1991 and has been manufactured in Lund, Sucecia since 1987. In this way, it is possible to use this method of the previous technique to provide a Good calculation of the required frequency corrections by measuring the temperature and using the current average of previous corrections made at the same temperature, even before a new frequency error determination has been completed. However, the prior art method does not include Doppler correction before updating the TCXO memory.

SUMMARY OF THE EXHIBITION An object of the present invention is to allow the mobile stations to determine their approximate position by listening to a call channel of a satellite with the purpose of after finishing the approximate Doppler shift of the received signal with the help of data describing the spread of satellite movement from the satellite in the calling channel. The Doppler shift. then it is eliminated before the vile stations correct their reference oscillators. In addition, the computed Dopoler computation is also used to terminate a frequency deviation in relation to the reference oscillator that is going to. apply when generating a transmitted signal from the mobile station so that it is received on the satellite at a correct and desired frequency. In accordance with one embodiment of the present invention, the mobile station contains a plurality of elements which are combined for communication through a relay station. First, the mobile station includes a receiver element for receiving a broadcast call signal, for the relay station in motion and demodulation and decoding elements for decoding the information in the call signal. Using the decoded information, the mobile station can determine a calculation of its position within the communication system. The mobile station also has elements to determine a frequency error and a Dopple shift using the decoded information and the position calculation. A frequency correction element determines a frequency control signal using the determined frequency error and the Doppler shift. A reference oscillator with trimming then uses the frequency correction signal as one. control input signal. Finally, the mobile station contains a transmitting element for transmitting a signal using the voltage controlled oscillator as a reference. In accordance with another embodiment of the present invention, the mobile station contains a plurality of elements which combine to communicate through a relay station in motion. First, the mobile station includes a receiver pa receiving a call channel signal broadcasted by the relay station and decoding decompression elements to de-code the information in the call channel signal, using the decoded information, the mobile station can Determine a position calculation of your position within the communication system. The mobile station also has elements for terminating a frequency error and a Doppler shift using the decoded information and the position calculation. The mobile station also includes a temperature sensor element for making an approximate temperature measurement and a memory element for storing the averaged frequency signal values for each of a number of temperatures. One-element correction correction determines a frequency control signal based on the frequency error determined by the Doppler shift or based on the stored values and the approximate temperature measurement. An averaging element averages the stored average values with the frequency correction signal values and stores the promed values in the memory at positions determined by the temperature measurements. A controlled crystal oscillator element has a control input for the frequency correction signal. Finally, the mobile station contains a transmitting element for transmitting a signal using the controlled oscillator as a reference.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the invention will be readily apparent to one of ordinary skill in the art from the following written description, used in conjunction with the drawings, in which: Figure 1 illustrates a system communication system 1 ite general; Figure 2 illustrates a mobile co-ordination station with one embodiment of the present invention; Figure 3 illustrates a mobile station in accordance with another embodiment of the present invention; and Figure 4 illustrates a flow chart of a modality of the present invention.

DETAILED DESCRIPTION OF THE EXPOSURE Figure 1 illustrates a block diagram of a satellite communication system. An orbiting satellite 110 is in communication with at least one ground station or core station 100 as well as a plurality of mobile stations 120. The satellite contains a multi-beam antenna. The mobile stations each receive service by an appropriate antenna spot beam from the multi-beam antenna. The core-station 100 communicates with the satellite using, for example, the C-band or Ka-band frequencies, while the satellite communicates with the mobile stations using, for example, L-band frequencies in the uplink direction. S-band in the downlink direction. In most cases, most of the calls will be between the mobile stations and the ordinary telephones connected to the Public Switched Telephone Network (PSTN) 130. The 100 d core station accepts calls from the PSTN 130 and relays them to the stations 120 mobile through the satellite 110 and conversely, accepts calls from the mobile stations 120 and retransmits them from the satellite 110 and connects the calls to the PSTN 430. A small percentage of the calls can be calls from mobile station to mobile station, in where the core station 100 directly connects the mobile stations to each other without necessarily involving the PSTN. In some systems, two or more core stations located in different parts of the world communicate with the same satellite. In this case, mobile-to-mobile calls may involve connections from core station to core station that can be achieved through international trunk lines that can be part of the PSTN system. Alternatively, the satellite-core station links can be distributed to the na capacity for communication from core station to core station through the satellite for said occurrences, thus avoiding terrestrial line tariffs. In each antenna beam, one of the retransmitted signals performs the function of call channel (foliation channel) broadcast channel. This channel carries system information required by the mobile stations in that beam (the information diffused, eg, ID, satellite movement information, etc.) and also information directed to individual mobile stations (calls or foliages originated in Since each call end is intended for reception only by mobile stations in its beam, the Doppler shift can be corrected approximately by the element perceived by all the mobile stations in the beam, for example, the Doppler shift can arranged to be zero in the center of the beam, or example of ground station and satellite architectures suitable for implementing the invention may be found in US Patent Application No. 08 / 179,953, entitled "A System of Cellular / Satellite Communications with Improved Frequency Reuse ", which is incorporated herein by reference In low orbiting satellites, the displacement Doppler varies from place to place on the earth and the Doppler shift at the edges of the beam will differ from the Doppl shift at the center of the beam. Consequently, the Doppler shift at the edges of the cell will be non-zero even though the Doppler shift has been previously compensated for? s zero in the ce-Hro of the cell. Maximum cell edge Doppler errors occur in this case when the satellite is closest up and the Doppler shift rate with position is the highest. In this way, in order to more accurately correct the Doppler shift, a mobile station needs some knowledge of its position within the cell or correct beam. This position information, in contrast to GPS, must be obteinable by listening to a single satellite. A mobile station in accordance with one embodiment of the present invention is illustrated in Figure 2. The mobile station 2 contains a receiver 202 and a transmitter 216 which are used to communicate with a core station through a transmitting station. moving. As stated above, the relay or satellite station broadcasts a call signal in each antenna beam. When the receiver 202 receives a satellite channel signal from the satellite, the received signal is demodulated in a known manner in a demodulator 20 The demodulated signal is then decoded in a decoder 206 to extract the information contained in the received signal .

Using the decoded information, the mobile station can determine an approximate calculation of its position within the antenna beam or cell. One of these methods the mobile station can use to determine its location in the satellite navigation solution called TRANSIT. The TRANSI satellite receivers determine the Doppler shift several times using the satellite's upper saje. With various measurements on the Doppler shift curve with time, more information diffused by the satellite in its orbital trajectory, a mobile station can compute its position within a few hundred meters of precision. This procedure usually takes a period of time that would be considered excessive in the current context, in which the mobile station must be in a position to make or answer calls within a few seconds after connecting. However, the TRANSIT method provides much more precise than what is needed for satellite communications. Typically, the spot beam diameters in the ground in a satellite system can be empty hundreds of kilometers in diameter and the differential Doppler shift after precompensation through that cell can be +/- 2KHz. In this way, the knowledge of the position of the mobile within a cell up to a precision of a few kilometers will allow the de-termination of Doppler displacement not compensated to approximately +/- 20 Hz. As a result, it is possible that a modification of the TRANSIT solution may be appropriate for the present invention and the present invention is not limited to any particular modification. However, a preferred method is described in U.S. Patent Application No. 08 / 179,958, entitled "Position Register for Cellular Satellite Communication Systems" and U.S. Patent Application No. 08 / 179,953, "A Cellular Communications System / Satelite with Enhanced Frequency Reuse", both of which are commonly assigned and are expressly incorporated herein by reference. A system described therein comprises radiation from the satellite of a large number of largely translational beams with gradually stepped overlap. Each beam overlap is used for a particular communications channel. In the present invention, the term "channel" can mean either frequency (in a FDMA system), a time slot (in a TDMA system) or a dispersion coding (in the CDMA system) or any combination of the same in hybrid modulation systems. The beam of channel 1 has, for example, 90% overlap with a beam of channel 2, 80% overlap with cann 3 beam, 70% overlap with the beam of channel 4 and so on to an overlap of zero with the beam of channel 11, which therefore can reuse the same frequency, time slot or coding of channel 1. The previous example was simplified to a one-dimensional pattern of beams for the purpose of illustration and in practice is used a two-dimensional reuse pattern of gradually stepped overlap. A mobile station will receive more strongly the channel whose beam associated with the mobile is more closely placed centrally. In this way, by means of relative signal strength measurements in the overlapping channels, the mobile station can obtain an approximate approximate position calculation for the purposes of this invention. In particular, when the satellite signal is a TDMA signal and the partially overlapping beams are linked in time slots, the mobile receiver can only hear the signal frequency of the TDMA carrier and sample signal strength in sequential time slots during the frame. In addition, the repetition of the frame allows measurements to be averaged across several frames for improved prision. Once the position of the mobile station has been determined, a frequency error detector 210 uses the determined position as well as the information broadcast by satellite that describes the systematic movement of the satellite to terminate a frequency error and a Doppler shift of received signal. According to one embodiment of the present invention, the satellite can precompensate its transmission frequency so that the error due to Doppler is canceled in the vicinity of the cell or beam. However, the error will be of zero zero of center in one direction in a plane containing the light d vision and the orbital velocity vector. The satellite may differ in the magnitude of the rate of frequency change along an East-West line and a North-South line from the center of the line. Since the mobile determines its position with respect to the cell center, it can calculate the magnitude of the unbalanced Doppler shift in the location of the mobile and subtract the magnitude of the Doppler shift from a positive frequency measurement made by the mobile receiver. For example, if in the beam the satellite broadcasts information that the frequency changed off center by f7 HZ / mg along the East-West line -3 HZ / kg along the North-South line and the mobile determines that it is 100 km west and 200 km south of the center, then the frequency error is a (7x100) - (2x200) = 100HZ. Alternatively, if the satellite broadcasts that the frequency error changes to 10 HZ / km was cell center along the North 40 west line, and the mobile determines that it is 100 Km North 30 East of the cell center, then Frequency error will be calculated as (10HZ) x (cos (70)). The above methods are illustrative only. In general, the satellite could spread not only Doppler calculation parameters for each beam, but also the formula for using the parameters. All these methods are considered to be within the scope of the present invention. The frequency correction element 212 d then uses the error signal to calculate a frequency correction signal to correct the satellite signal frequency perceived by the non-compensated Doppler shift. The frequency correction signal is then used as a voltage controlled oscillator 314 to correct the reference signal supplied to the transmitter 216 for transmission back to the satellite when the mobile station calls or answers a call. It is optional if the mobile station applies a Doppler shift precompensation additional to its transmission as it is received correctly on the satellite. Whether this is necessary depends on the channel spacing between different mobile signals. If the frequency spacing is small and the Doppler shift is a significant fraction, the precompensation in the return link (uplink) would be apr piate. In this case, any residual Doppler shift experienced by the satellite may be due to errors in mobile station position terminations caused by errors in the information broadcast by the satellite. By averaging the residual errors through many independent mobile station signals, the satellite can correct the dissemination of information to eliminate systematic errors and, thus, the system becomes autocalculating. For this purpose, the mobile station will report information related to position during the calls, appropriately using for this purpose the Slow Associated Control Channel (SACCH) in the uplink direction. The SACCH information is multiplexed with traffic information (eg, speech) in mobile-to-satellite transmissions. On the other hand, if the link transmissions are high band signals compared to the asynchronous link Doppler shift, then it is not essential to think of the uplink Doppler shifts. In this case, the satellite retransmits the signals to the ground network station that determines the uplink Doppler shift for each mobile station during the demodulation of its retransmitted signals. The value of the uplink Doppler shift compared to the cell center value is provided provides an indication of the mobile offset from the cell center that complements any additional footprints such as signal strength measurements. This information can be used in determining an optimal channel allocation for use to communicate with each mobile station. Another embodiment of the present invention is illustrated in Figure 3, wherein the mobile station illustrated in the Figure also has the ability to measure the temperature when a call signal is received. The mobile station 300 contains a receiver 302, a demodulator 304, a decoder 306, a position detector 308 and a frequency error detector 310, all of which operate in a manner similar to those elements of the descriptions in Figure 2. In this embodiment of the present invention, the frequency correction element 312 can use various methods to determine a frequency correction signal. In the first method, the frequency correction element 312 can use an error signal produced by the frequency error detector 310 to determine the frequency correction signal as described above with respect to Figure 2. Alternatively, the The frequency correction element 312 may use frequency control values stored in the memory element 322 to determine the frequency correction signal. When the mobile station 300 receives a call signal, the temperature sensor 314 measures the temperature, close to the reference oscillator of the mobile station. The frequency correction element 312 then selects a frequency control signal value from the memory 312 based on the temperature measured by the temperature sensor 314. In addition, the mobile station 300 may contain a protation element 320 for averaging the frequency control signal values stored with the frequency correction signal produced. The averaged value can then be stored in memory 322 in the correct position determined by the temperature dida. The frequency correction signal output by the frequency correction element 312 is used to adjust the frequency of the reference signal produced by the controlled reference oscillator 316, which is supplied to the transmitter 3.

Another embodiment of the present invention provides a method by which a mobile station can correct its frequency using a signal received from a satellite. As illustrated in Figure 4, after the mobile station receives a signal in the call channel in step 400, the received signal is demodulated and decoded in steps 402 and 404, respectively. The mobile station then uses the decoded information to determine an approximate calculation of its position within a beam, in step 406. After the mobile station has determined its position, a frequency error and a Doppler shift in the step 408. Using the determined frequency err and Doppler shift, the mobile station calculates a frequency correction signal in step 410. Finally, the mobile station can use the frequency correction signal to adjust the reference signal produced by the controlled reference oscillator, such as or voltage controlled oscillator, in step 412, which is then applied to the transmitter of the mobile station. In addition, the Doppler shift and the reference frequency can be used to produce a precompensated transmission signal in step 414. Finally, the method can be linked to any update during the wait, in step 416, to set continuously updated during the traffic passing the stage 418. The invention described above avoids the need to include a high-precision expensive reference frequency source in mobile phones, allowing the reference frequency to be determined by listening to the channel of li mada. The Doppler shift in the call channel signal is corrected by the use of an approximate position calculation in order to obtain the frequency reference that is then used to determine an accurate transmission frequency from the mobile station 1. It will be observed by those of ordinary experience in the field that the. This invention can be modalized in other more specific fora without abandoning the essential spirit or character of it. The modalities currently described, therefore, are considered in all respects as ilsutrative and not restrictive. The scope of the invention is indicated by the appended clauses in lieu of the foregoing description and, all changes that fall within the meaning and scope of equivalent thereof are intended to be encompassed therein.

Claims (3)

CLAIMS:

1. - A mobile station for use in a satellite communication system with at least one mobile retransmission station, comprising: a receiving element for receiving a called channel signal broadcasted by the relay station; an element for demodulating the call channel signal; an element for decoding the demodulated signal; an element for determining a position calculation of the mobile station from the decoded signal; an element for determining a frequency error and Doppler shift using the decoded signal and the position buffer; a frequency correction element for determining a frequency correction signal based on the determined Doppler frequency and shift error; a controlled oscillator element having a control input for the frequency correction signal; and a transmitting element to generate a transmission signal-using the controlled oscillator as a reference.

2. The mobile station according to claim 1, wherein the relay station is a satellite in orbit.

3. - The mobile station according to claim 1, wherein the position calculation is within several ki meters of a real position of the mobile station. a.- The mobile station according to claim 1, wherein the call sign contains information about the movement of the relay station. 5. The mobile station according to claim 1, wherein the transmitting element generates a transmission signal using the controlled oscillator and the determined Doppler shift so that the retransmission station receives the transmission signal with displacement. Doppler that has been compensated. 6. A mobile station for use in a satellite communication system with at least one mobile relay station, comprising: a receiving element for receiving a call channel signal broadcast by the relay station; an element for demodulating the signal of the flame channel; an element for decoding the demodulated signal; an element for determining a position calculation of the mobile station 1; an element for determining a frequency error and a Doppler shift using the decoded signal and the position calcu- lation; a temperature sensing element for making a temperature measurement; a memory element for storing the average frequency control end values for each of a plurality of temperatures. a frequency correction element for determining a frequency correction signal based on the determined Doppler frequency and displacement error or based on the stored values and the temperature measurement; an averaging element for averaging the stored average values with the frequency correction signal values and storing the average values in the memory in po tions determined by the temperature measurements; a controlled oscillating element having a control input for a frequency correction signal; and a transmitting element for generating a transmission signal using the controlled oscillator as a reference. 7. A mobile station according to claim 6, wherein the retransmitting station is in motion and an orbiting satellite. 8. The mobile station according to claim 6, wherein the position calculation is within several kilometers of the actual position of the mobile station. 9. The mobile station according to claim 6, wherein the call sign contains information about the movement of the relay station. 10. A method for correcting a frequency of a mobile station for use in a teletext communication system with at least one mobile retransmission station, comprising the steps of: receiving a call channel signal broadcasted by the user; relay station; demodulating the call channel signal; decoding the demodulated signal; determining a position calculation of the mobile station from the decoded signal; determine a frequency error and a Doppler shift using the decoded signal and the positional calculation; determining a frequency correction signal based on the frequency error and Doppler displacement determined to adjust a controlled oscillator element using the frequency correction signal. 11. The method according to claim wherein the retransmission station in motion is an orbited satellite. 12. The method according to claim wherein the position calculation is within several kilometers of the actual position of the mobile station. 13. The method of conformance with the re-identification 10 wherein the call sign contains information about the movement of the relay station.