HK1057781B - Method and apparatus for reducing code phase search space - Google Patents

Description

Method and apparatus for reducing code phase search space

Background

(1) Field of the invention

The invention relates to a reduction of the code phase search space of a receiver in a distributed system.

(2) Description of the related Art

Satellite positioning systems include a set of orbiting satellites (also known as space vehicles or "SVs") that broadcast signals from which receivers can determine their position. Two such systems are the NAVSTAR GPS system (as described in "global positioning system, standard positioning services signal specifications, second edition, published 2.6.1995, united states coast defense navigation center, alexander, VA") and the global orbiting navigation system (GLONASS) maintained by the russian republic. To determine its three-dimensional position in such a positioning system, the receiver must first acquire the signals of the four SVs. The initial acquisition of each SV signal is typically computationally intensive and may take up to several minutes.

In order to acquire a GPS signal, the receiver must automatically track the frequency of the carrier signal and the phase of the code modulated on the carrier. The frequency of the received carrier may vary because of the movement of the SV with respect to the receiver and the resulting doppler shift. Inaccuracies in the local oscillator of the receiver can result in additional frequency errors. Thus, automatically tracking the carrier requires the receiver to search for a signal over a range of frequencies.

Each SV transmits a signal spread by direct sequence spread spectrum modulation. In particular, each SV transmits a signal spread by a digital pseudorandom (or "pseudo-noise") code known as Coarse Acquisition (CA). The periodic code has a chip rate of 1.023MHz and repeats every 1023 symbols (i.e., once every msec). The signal received by the receiver may be a composite signal transmitted by several SVs.

The code phase of the received SV signal is determined by the location at a predetermined location within the CA code of the signal. Because the CA code is periodic, the possible locations of the predetermined positions (i.e., the possible code phases) may appear as points along the circumference, as shown in fig. 1. The determination of the code phase of the received signal requires searching for a correlation at each position on the circumference (e.g., between the receiver output and the code sequence based on the particular CA code) until the code is located within the received signal (e.g., as indicated by the occurrence of a correlation peak).

Because the nominal carrier frequency of the GPS signal is 1.575GHz, maintaining signal synchronization under a cover such as indoors, in a car, and/or in a tree is difficult. When a portable GPS receiver loses signal synchronization, the receiver will suffer from inconvenient suspension of position location capabilities and consumption of computing resources when attempting to reacquire the signal. Since the frequency offset changes rather slowly, only a limited effort is required to re-establish the frequency synchronization after a short break. However, the code phase of the received signal changes faster and it is necessary to search for the missing signal over the entire 1023 symbol code phase circle. For applications where accurate positioning information must be available on demand, such delays may not be acceptable. Of course, it is also advantageous to avoid long delays during initial acquisition.

It is desirable to expand certain wireless systems for mobile communications by increasing the ability to locate the position of a particular mobile unit. One reason is a rule promulgated by the Federal communications Commission (release 6/10 1999, third report and rule adopted 15/9 1999, record number 94-102). This rule requires that all cellular carriers in the united states before 10 months 2001 be able to locate locations within 50 meters for 67% of calls made to 911 calls (and within 150 meters for 95% of calls). Other applications of location capability in wireless communication systems include value-added user features like navigation and fleet management support.

One option for adding location fixes to such a communication system is to add GPS reception capability to the mobile unit. However, such approaches suffer from difficulties in maintaining reliable reception of GPS signals in many areas where mobile units are typically suitable, such as indoors and in vehicles. On the other hand, base stations in such a system are generally well-arranged in terms of satellite visibility, and the base stations may assist the mobile station by collecting information on the SV signals (including code phase) and forwarding it to the mobile station.

In a code division multiple access CDMA system for wireless communications, the operation of a mobile station and a base station are synchronized on a common time reference (see fig. 1). Because of this property, the base station can transmit code phase information relative to the time reference that is meaningful to the mobile station. Due to the difference in the location of the base station and the mobile station, and due to inaccuracies in the mobile unit's local oscillator, the code phase information sent by the base station may not accurately coincide with the code phase of the GPS signal received by the mobile unit. However, this process can greatly reduce the size of the code phase search reference (e.g., from 1023 symbols to only 30 symbols).

However, in analog systems widely used for wireless communications in the united states, such as the Advanced Mobile Phone Service (AMPS) system, there is no such time reference between the mobile station and the base station. In practice, the operation of the two stations cannot even be synchronized within one millisecond (i.e., the time to traverse the entire code phase circle). Thus, there is no system reference point (see fig. 2) for which the base station would send useful code phase information. Thus, in an AMPS system that supports GPS location capability via a mobile station, any acquisition and reacquisition of satellite synchronization requires a search through the code phase circle. It is desirable to reduce the code phase search space in such distributed systems.

SUMMARY

Systems, methods, and apparatus for reducing code phase search space in a code division multiple access receiver, such as a GPS receiver. The reduction is obtained by applying information about the time relationship between the two received signals. This time relationship provides the ability to know the code phase of the second signal if the code phase of the first signal is known. Knowing the code phase of the second signal reduces the search space because the searcher can directly steer to the desired code phase.

Brief description of the drawings

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

FIG. 1 illustrates a CDMA system with a system reference time;

FIG. 2 illustrates an AMPS system without a system reference time;

FIG. 3 illustrates how the code phase of a signal is determined from (1) the code phase of another signal and (2) the time difference between the code phases;

FIG. 4 illustrates one method of representing the time difference between the encoded phases of more than two signals;

FIG. 5 illustrates another method of representing the time difference between the encoded phases of more than two signals;

FIG. 6 illustrates a system in accordance with one embodiment of the present invention and a plurality of SVs 100;

FIG. 7 shows a block diagram of an apparatus 120 according to an embodiment of the invention;

FIG. 8 shows a flow diagram of a method according to an embodiment of the invention;

FIG. 9 shows a flow diagram of a method according to another embodiment of the invention;

FIG. 10 shows a flow diagram of a method according to a further embodiment of the invention;

FIG. 11 shows a block diagram of an exemplary implementation of an apparatus 120 according to one embodiment of the invention;

FIG. 12 shows a block diagram of an apparatus 110 according to an embodiment of the invention;

FIG. 13 shows a block diagram of an exemplary implementation of an apparatus 110 according to one embodiment of the invention; and

FIG. 14 illustrates a block diagram of another exemplary implementation of an apparatus 110 according to one embodiment of the invention.

Detailed description of the drawings

The following detailed description refers to the accompanying drawings that illustrate embodiments of the present invention. Other embodiments are possible and modifications may be made to the embodiments without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense. Rather, the scope of the invention is defined by the appended claims.

In a system, method and apparatus according to one embodiment of the present invention, the code phase of the second received signal is located by using the following entry of time information: (1) the code phase of the first received signal and (2) the time relationship between the code phases of the two received signals (e.g., the time difference shown in fig. 3). This method may be extended to allow locating further received signals by providing time difference increments (i.e. with respect to another received signal, as shown in figure 4) and/or accumulating time differences (i.e. with respect to the first received signal, as shown in figure 5).

Fig. 6 shows a block diagram of a system according to an embodiment of the invention, which includes a field receiver 110 and a reference receiver 120. The reference receiver 120 receives signals from at least the first and second SVs 100 and determines the code phase of these received signals (e.g., by correlating with a local copy of a known CA code). Information pertaining to the time relationship between the code phases of the received signal is then transmitted to field receiver 110. After determining the code phase from a first SV 100, field receiver 110 uses the time correlation information to reduce the size of the interval it needs to search in determining the code phase of the signal from a second SV 100.

Fig. 7 shows a block diagram of a reference receiver 120 according to an embodiment of the invention. Within reference receiver 120, a Radio Frequency (RF) receiver 210 receives modulated carrier signals from at least two SVs and outputs corresponding demodulated signals to a correlator 220. Correlator 220 determines the code phases of the received signal and outputs information about the differences in these code phases to transmitter 230 (e.g., as shown in tasks P110 and P120 of fig. 8).

As shown in task 140 of fig. 8, transmitter 230 transmits the information output by correlator 220. In one example, correlator 220 determines the difference between the code phases and transmitter 230 transmits the difference (e.g., as shown in tasks P130 and P145 of fig. 9). In another example of fig. 10, transmitter 230 transmits information regarding the code phases of the received signal (task P142), and the receiver of this information (e.g., field receiver 110) operates on tasks to determine the time difference between the code phases.

Fig. 11 shows an exemplary embodiment of the reference receiver 120. In this example, RF receiver 210 receives signals from SV 100 via a GPS antenna. The encoded phase information discussed above is then transmitted by transmitter 230 (e.g., to one or more field receivers 110) via communication antenna 250.

The reference receiver 120 may be co-located and/or integrated with a base station of a wireless communication system. In this case, the position of the reference receiver 120 can be generally obtained with high accuracy.

As shown in FIG. 12, a field receiver 110 according to one embodiment of the present invention includes a receiver 310 that receives signals from at least two SVs. The rf receiver 310 also receives a reference signal from which the time relationship (e.g., difference) between the code phases of the SV signals is derived when received by the reference receiver.

For signals received from the first SV, the correlator 320 determines its code phase. Correlator 320 combines the code phase information with the time relationship between the first SV signal and the second SV signal to reduce the search space for the code phase of the second SV signal.

In the exemplary embodiment of field receiver 110 shown in FIG. 13, the RF receiver receives signals from SVs via GPS antenna 340 and reference signals (e.g., from reference receiver 120) via communication antenna 350. RF receiver 310 may be an integrated unit or RF receiver 310 may comprise two separate units (GPS receiver 310-1 and communication receiver 310-2) as shown in fig. 14.

In the client server architecture of the present invention shown in fig. 2, the central server has its own GPS receiver that accurately knows the position of the satellites in the air, the frequencies of those satellites, and the time differences between the satellites and the server from other information. The server may send information to the client that is identifying the satellites in view so that the client need not search for each satellite but only the satellites that are likely to receive the signal. For example, the server may forward information about the encoded sequences corresponding to the SVs in the field of view (e.g., the sequences themselves, or one or more indices corresponding to a predetermined encoded sequence list). The server may also transmit the doppler frequency information of the satellite to the client. In addition, the server may send the timing of the satellites (e.g., one or more time differences between the encoded phases) to the client. It is desirable for the server to send these three types of information in the order given above.

Thus, the present invention greatly reduces the search space or time required by the GPS receiver of the client. In an off-the-air GPS client-server configuration according to one embodiment of the invention, the transmission by the server of relative timing with respect to satellites reduces the search space even in the absence of an available timing reference at the client or an available common time reference between the client and the server.

The previous description of the described embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments are possible, and the generic principles set forth herein may be applied to other embodiments as well. For example, the invention may be implemented in part or in whole as a hardwired circuit, a circuit configuration constituting an application specific integrated circuit, or a firmware program loaded into non-volatile memory or a software program loaded from or onto a data storage medium such as machine-readable code (which may be instructions executable by an array of logic cells such as a microprocessor or other digital signal processing unit). Thus, the present invention is not intended to be limited to the embodiments shown above but is to be accorded the widest scope consistent with any principles and novel features disclosed herein.

Claims (21)

1. A method for reducing a code phase search space of a positioning signal received by a mobile station, comprising:

determining, at a receiver remote from the mobile station, a code phase for each of a plurality of received signals, wherein the received signals are positioning signals; and

determining a time difference between the encoded phases of the first and second positioning signals in the plurality of received positioning signals and transmitting the time difference to the mobile station;

receiving the first positioning signal at the mobile station and determining a first code phase;

the second positioning signal is received at a mobile station and a second code phase of the second positioning signal is determined in response to the first code phase and the transmitted time difference.

2. The method of claim 1, wherein each of the plurality of received signals has a corresponding periodic code, and

wherein each of the code phases is associated with a predetermined position in the corresponding periodic code.

3. The method of claim 1, wherein each of the plurality of received signals is based at least in part on a corresponding direct sequence spread spectrum modulated signal.

4. The method of claim 1, wherein each of the plurality of received signals is based at least in part on a corresponding direct sequence pseudonoise modulated signal.

5. The method of claim 1, further comprising receiving a composite signal,

wherein each of the plurality of received signals is based at least in part on at least a portion of the composite signal.

6. The method of claim 5, wherein the determining of the code phase of each of the plurality of received signals comprises calculating, for each of the plurality of received signals, a correlation between the corresponding code sequence and a signal based at least in part on the composite signal,

wherein each of the plurality of received signals has a corresponding periodic code, and

wherein each code phase is associated with a respective predetermined position within a respective periodic code, and

wherein the coding sequence is at least partially associated with a corresponding periodic code.

7. A method for reducing a code phase search space of a positioning signal received by a mobile station, comprising:

receiving, at the mobile station, a time difference between the first positioning signal and the second positioning signal, the time difference being transmitted by the reference receiver;

determining, at a mobile station, a code phase of the first positioning signal;

determining, at a mobile station, a code phase of the second positioning signal; wherein the determination of the code phase of the second positioning signal is responsive to the received time difference and the code phase of the first positioning signal.

8. The method of claim 7, wherein the first received signal has a corresponding periodic coding and the second received signal also has a corresponding periodic coding, and

wherein the code phases of the first received signal and the second received signal each relate to a respective predetermined position within the respective periodic code.

9. The method of claim 7, wherein each of the first received signal and the second received signal is based at least in part on a corresponding direct sequence spread spectrum modulated signal.

10. The method of claim 7, wherein each of the first received signal and the second received signal is based at least in part on a corresponding direct sequence pseudo-noise modulated signal.

11. The method of claim 7, further comprising receiving a composite signal, wherein each of the first received signal and the second received signal is based at least in part on at least a portion of the composite signal.

12. The method of claim 11, wherein the determining of the code phase of the first received signal comprises calculating a correlation between the code sequence and a signal based at least in part on the composite signal,

wherein the first received signal has a corresponding periodic coding and the second received signal also has a corresponding periodic coding, an

Wherein the code phases of the first received signal and the second received signal are each associated with a respective predetermined position within a respective periodic code, and

wherein the code sequence is at least partially associated with a periodic code corresponding to the first received signal.

13. An apparatus in a reference receiver for reducing a code phase search space of a field receiver, comprising:

a reference position location receiver configured to receive a plurality of positioning signals;

a correlator configured to determine a code phase for each of a plurality of received positioning signals; and

a transmitter configured to transmit a time difference between encoded phases of at least one pair of signals among a plurality of received signals to the field receiver.

14. The apparatus of claim 13, wherein each of the plurality of received signals has a corresponding periodic code, and

wherein each of the code phases is associated with a predetermined position within the corresponding periodic code.

15. The apparatus of claim 13, wherein each of the plurality of received signals is based at least in part on a corresponding direct sequence spread spectrum modulated signal.

16. The apparatus of claim 13, wherein each of the plurality of received signals is based at least in part on a corresponding direct sequence pseudonoise modulated signal.

17. The apparatus of claim 13, wherein the correlator is further configured to determine a code phase for each of the plurality of received signals at least in part by computing a correlation between a corresponding code sequence and the plurality of received signals for each of the plurality of received signals,

wherein each of the plurality of received signals has a corresponding periodic code;

wherein each of the plurality of code phases is associated with a respective predetermined position within a respective periodic code, an

Wherein the corresponding coding sequence is at least partially associated with the corresponding periodic coding.

18. A mobile station having a reduced code phase search space, comprising:

a position location receiver configured to receive the first and second positioning signals and to receive a signal comprising a time difference between code phases of the first received signal and the second received signal, an

A correlator configured to determine a code phase of at least one of the first and second received signals with respect to a predetermined code and to correlate the other of the first and second received signals with the predetermined code in response to a time difference between the first and second received signals.

19. A system, comprising:

a reference receiver configured to receive periodic positioning signals from a plurality of spacecraft and to transmit information; and

a field receiver configured to receive the periodic positioning signals from a plurality of spacecraft and to receive the information from the reference receiver,

the reference receiver determines a reference code phase for each of at least a first and a second of the periodic positioning signals, an

Said information being at least about the time difference between the reference code phases of said first and second signals, an

The field receiver determines the field code phase of the first locating signal, an

The field receiver determines a field code phase of a second one of the positioning signals in response to the time difference and the field code phase of the first one of the positioning signals.

20. The system of claim 19, wherein the periodic positioning signals comprise one of GPS and GLONASS signals.

21. The system of claim 19, wherein said field receiver and said reference receiver are asynchronous in time.