U.S. patent application number 10/334199 was filed with the patent office on 2003-07-17 for method of despreading gps spread spectrum signals.
Invention is credited to Dooley, Saul R., Yule, Andrew T..
Application Number | 20030132879 10/334199 |
Document ID | / |
Family ID | 9892655 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030132879 |
Kind Code |
A1 |
Dooley, Saul R. ; et
al. |
July 17, 2003 |
Method of despreading GPS spread spectrum signals
Abstract
A method of despreading a GPS spread spectrum signal received at
a mobile unit MS1 with assistance from a base station BS1 is
disclosed comprising the steps of (a) receiving and despreading GPS
signals at the base station and deriving GPS signal information
therefrom; (b) estimating the location of the mobile unit; (c)
modifying the GPS signal information so as reflect the GPS signal
characteristics as would be observed at the estimated location of
the mobile unit; (d) transmitting the GPS signal information from
the base station to the mobile unit; (e) generating replica signals
containing PRN codes corresponding to those of the GPS signal using
the modified GPS signal information; and (f) performing a
correlation of the GPS signal and the replica signals in order to
acquire the GPS signal.
Inventors: |
Dooley, Saul R.; (Reigate,
GB) ; Yule, Andrew T.; (Felbridge, GB) |
Correspondence
Address: |
PHILIPS ELECTRONICS NORTH AMERICAN CORP
580 WHITE PLAINS RD
TARRYTOWN
NY
10591
US
|
Family ID: |
9892655 |
Appl. No.: |
10/334199 |
Filed: |
December 31, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10334199 |
Dec 31, 2002 |
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09862291 |
May 22, 2001 |
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6525689 |
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Current U.S.
Class: |
342/357.64 |
Current CPC
Class: |
G01S 19/06 20130101;
H04J 13/0022 20130101; G01S 5/0027 20130101 |
Class at
Publication: |
342/357.09 |
International
Class: |
G01S 005/14 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2000 |
GB |
0013148.2 |
Claims
1. A method of despreading a GPS spread spectrum signal received at
a mobile unit with assistance from a base station comprising the
steps of: (a) receiving and despreading GPS signals at the base
station and deriving GPS signal information therefrom; (b)
estimating the location of the mobile unit; (c) modifying the GPS
signal information so as reflect the GPS signal characteristics as
would be observed at the estimated location of the mobile unit; (d)
transmitting the GPS signal information from the base station to
the mobile unit; (e) generating replica signals containing PRN
codes corresponding to those of the GPS signal using the modified
GPS signal information; and (f) performing a correlation of the GPS
signal and the replica signals in order to acquire the GPS
signal.
2. A method according to claim 1 wherein step (c) is done before
step (d).
3. A method according to claim 2 wherein the initial estimate of
the location of the mobile unit is determined in the mobile unit
and transmitted to the base station.
4. A method according to any of the preceding claims wherein the
GPS signal information comprises Doppler information.
5. A method according to any of the preceding claims wherein the
GPS signal information comprises code phase information.
6. A method of providing base station assistance to a mobile unit
for despreading GPS spread spectrum signals received at the mobile
unit substantially as hereinbefore described with reference to the
accompanying drawings.
7. The combination of a base station and a mobile unit for
despreading a GPS spread spectrum signal received at a mobile unit
with assistance from a base station by a method according to any
preceding claims.
8. The combination of a base station and a mobile unit for
despreading a GPS spread spectrum signal received at a mobile unit
with assistance from a base station; wherein the base station
comprises a GPS receiver for receiving and despreading GPS signals
at the base station and deriving GPS signal information therefrom,
a processor arranged to modify the GPS signal information so as
reflect the GPS signal characteristics as would be observed at an
estimated location of the mobile unit; and a transmitter for
transmitting the modified GPS signal information from the base
station to the mobile unit; and wherein the mobile unit comprising
a GPS receiver for receiving the GPS signal, a communications
receiver for receiving GPS signal information from a base station;
and a processor arranged to generate replica signals containing PRN
codes corresponding to those of the GPS signal using the modified
GPS signal information and perform a correlation of the GPS signal
and the replica signals in order to acquire the GPS signal.
9. The combination of a base station and a mobile unit for
despreading a GPS spread spectrum signal received at a mobile unit
with assistance from a base station; wherein the base station
comprises a GPS receiver for receiving and despreading GPS signals
at the base station and deriving GPS signal information therefrom,
and a transmitter for transmitting the modified GPS signal
information from the base station to the mobile unit; and wherein
the mobile unit comprising a GPS receiver for receiving the GPS
signal; a communications receiver for receiving GPS signal
information from a base station; and a processor arranged to modify
the GPS signal information so as reflect the GPS signal
characteristics as would be observed at an estimated location of
the mobile unit, generate replica signals containing PRN codes
corresponding to those of the GPS signal using the modified GPS
signal information and perform a correlation of the GPS signal and
the replica signals in order to acquire the GPS signal
10. The combination of a base station and a mobile unit for
despreading a GPS spread spectrum signal received at a mobile unit
with assistance from a base station substantially as hereinbefore
described with reference to the accompanying drawings.
11. A base station for providing assistance to a mobile unit for
despreading GPS spread spectrum signals received at the mobile unit
comprising a GPS receiver for receiving and despreading GPS signals
at the base station and deriving GPS signal information therefrom,
a processor arranged to modify the GPS signal information so as
reflect the GPS signal characteristics as would be observed at an
estimated location of the mobile unit; and a transmitter for
transmitting the modified GPS signal information from the base
station to the mobile unit.
12. A base station for providing assistance to a mobile unit for
despreading GPS spread spectrum signals received at the mobile unit
substantially as hereinbefore described with reference to the
accompanying drawings.
13. A mobile unit for despreading a received GPS spread spectrum
signal with assistance from a base station comprising a GPS
receiver for receiving the GPS signal; a communications receiver
for receiving GPS signal information from a base station; and a
processor arranged to modify the GPS signal information so as
reflect the GPS signal characteristics as would be observed at an
estimated location of the mobile unit, generate replica signals
containing PRN codes corresponding to those of the GPS signal using
the modified GPS signal information and perform a correlation of
the GPS signal and the replica signals in order to acquire the GPS
signal
14. A mobile unit for despreading a received GPS spread spectrum
signal with assistance from a base station substantially as
hereinbefore described with reference to the accompanying drawings.
Description
[0001] This invention relates to a method of despreading a GPS
spread spectrum signal received at a mobile unit with assistance
from a base station.
[0002] It is well known to provide a GPS receiver in which replica
GPS satellite pseudorandom noise (PRN) code signals are continuous
generated and correlated with received GPS signals in order to
acquire them. Typically, as the replica codes are likely to have a
different code phase to those of the received GPS signals and also
a different frequency due to Doppler shift between the receiver and
orbiting satellites, a two dimensional code frequency/phase sweep
is employed whereby such a sweep will eventually result in the
incoming PRN code having the same frequency and code phase as that
of the locally generated replica. If detected, the code is acquired
and tracked, and the pseudorange information may be retrieved from
which the position of the receiver may be calculated using
conventional navigation algorithms.
[0003] It is further known to provide a mobile cellular telephone
incorporating such a GPS receiver for the purpose of enabling
operators of cellular telephone networks to determine the location
from which a call is made and, in particular, for an emergency call
to the emergency services. Of course for an emergency call, it is
desirable for the call location to be available as soon as
possible, however, from a "cold start" where the GPS receiver does
not have access to up to date ephemeris data or even worse from a
"factory cold start" where the GPS receiver does not have an up to
date almanac, the time to first fix (TTFF) can be anywhere between
30 seconds and 5 minutes.
[0004] In order to reduce the TTFF, a GPS receiver may be provided
with base station assistance in order to acquire GPS signals more
quickly. Such assistance may include the provision by the base
station to the receiver of a precision carrier frequency reference
signal for calibrating the local oscillator used in the GPS
receiver and, as obtained by the base station, the data message for
up to date satellite almanac and ephemeris data from which Doppler
shift for satellites in view can be determined together with the
current PRN code phase. With such assistance, it is possible to
sweep only a narrowed range of frequencies and code phases in which
the GPS PRN code is known to occupy, thereby reducing the number of
code instances that need to be checked and thus reducing the time
for code acquisition. Base station assistance is further described
in U.S. Pat. No. 5,841,396 and 5,874,914 which are incorporated
herein by reference.
[0005] It is an object of the present invention to provide an
improved method of despreading a GPS spread spectrum signal
received at a mobile unit with assistance from a base station with
which the TTFF may be reduced.
[0006] According to the present invention, there is provided such a
method comprising the steps of:
[0007] (a) receiving and despreading GPS signals at the base
station and deriving GPS signal information therefrom, ideally
including Doppler and code phase information;
[0008] (b) estimating the location of the mobile unit;
[0009] (c) modifying the GPS signal information so as reflect the
GPS signal characteristics as would be observed at the estimated
location of the mobile unit;
[0010] (d) transmitting the GPS signal information from the base
station to the mobile unit;
[0011] (e) generating replica signals containing PRN codes
corresponding to those of the GPS signal using the modified GPS
signal information; and
[0012] (f) performing a correlation of the GPS signal and the
replica signals in order to acquire the GPS signal.
[0013] The method of the present invention enables a narrower range
of frequencies and/or code phases in which the GPS PRN code is
known to occupy to be identified. As such, fewer code instances
need to be checked and the time for GPS signal acquisition and thus
the TTFF may be reduced.
[0014] Step (c) may be done at the base station in which case the
modified GPS signal information is transmitted from the base
station to the mobile unit, i.e. step (c) is done before step (d).
Where this is so, whilst it may be convenient for the base station
to transmit the modified GPS signal information to the mobile unit
using direct RF communication, transmitting the modified GPS signal
information from the base station to the mobile unit is also
intended to include indirect transmission such as via land lines or
using repeater stations. Furthermore, the initial estimate of the
location of the mobile unit may be determined in the mobile unit
and transmitted to the base station.
[0015] Also, provided in accordance with the present invention the
combination of a base station and a mobile unit for despreading a
GPS spread spectrum signal received at a mobile unit with
assistance from a base station; wherein the base station comprises
a GPS receiver for receiving and despreading GPS signals at the
base station and deriving GPS signal information therefrom, a
processor arranged to modify the GPS signal information so as
reflect the GPS signal characteristics as would be observed at an
estimated location of the mobile unit; and a transmitter for
transmitting the modified GPS signal information from the base
station to the mobile unit; and wherein the mobile unit comprising
a GPS receiver for receiving the GPS signal, a communications
receiver for receiving GPS signal information from a base station;
and a processor arranged to generate replica signals containing PRN
codes corresponding to those of the GPS signal using the modified
GPS signal information and perform a correlation of the GPS signal
and the replica signals in order to acquire the GPS signal. In
addition, a further combination is provided wherein the base
station comprises a GPS receiver for receiving and despreading GPS
signals at the base station and deriving GPS signal information
therefrom, and a transmitter for transmitting the modified GPS
signal information from the base station to the mobile unit; and
wherein the mobile unit comprising a GPS receiver for receiving the
GPS signal; a communications receiver for receiving GPS signal
information from a base station; and a processor arranged to modify
the GPS signal information so as reflect the GPS signal
characteristics as would be observed at an estimated location of
the mobile unit, generate replica signals containing PRN codes
corresponding to those of the GPS signal using the modified GPS
signal information and perform a correlation of the GPS signal and
the replica signals in order to acquire the GPS signal.
[0016] Further provided in accordance with the present invention is
a base station for providing such assistance comprising a GPS
receiver for receiving and despreading GPS signals at the base
station and deriving GPS signal information therefrom, a processor
for modifying the GPS signal information so as reflect the GPS
signal characteristics as would be observed at an estimated
location of a mobile unit; and a transmitter for transmitting the
modified GPS signal information from the base station to the mobile
unit.
[0017] Yet further provided in accordance with the present
invention is a mobile unit comprising a GPS receiver for receiving
the GPS signal; a communications receiver for receiving GPS signal
information from a base station; and a processor arranged to modify
the GPS signal information so as reflect the GPS signal
characteristics as would be observed at an estimated location of
the mobile unit, generate replica signals containing PRN codes
corresponding to those of the GPS signal using the modified GPS
signal information and perform a correlation of the GPS signal and
the replica signals in order to acquire the GPS signal
[0018] The above and other features and advantages of the present
invention will be apparent from the following description, by way
of example, of an embodiment of a mobile cellular telephone
comprising a GPS receiver for use in a cellular telephone network
with reference to the accompanying drawings in which:
[0019] FIG. 1 shows, schematically, the geographic layout of a
cellular telephone network;
[0020] FIG. 2 shows, schematically, the mobile cellular telephone
MS1 of FIG. 1 in greater detail; and
[0021] FIG. 3 shows, schematically, the base station BS1 of FIG. 1
in greater detail.
[0022] The geographical layout of a conventional cellular telephone
network 1 is shown schematically in FIG. 1. The network comprises a
plurality of base stations BS of which seven, BS1 to BS7, are
shown, situated at respective, mutually spaced geographic
locations. Each of these base stations comprises the entirety of a
radio transmitter and receiver operated by a trunking system
controller at any one site or service area. The respective service
areas SA1 to SA7 of these base stations overlap, as shown by the
cross hatching, to collectively cover the whole region shown. The
system may furthermore comprise a system controller SC provided
with a two-way communication link, CL1 to CL7 respectively, to each
base station BS1 to BS7. Each of these communication links may be,
for example, a dedicated land-line. The system controller SC may,
furthermore, be connected to a the public switched telephone
network (PSTN) to enable communication to take place between a
mobile cellular telephone MS1 and a subscriber to that network. A
plurality of mobile cellular telephones MS are provided of which
three, MS1, MS2 and MS3 are shown, each being able to roam freely
throughout the whole region, and indeed outside it.
[0023] Referring to FIG. 2, mobile cellular telephone MS1 is shown
in greater detail comprising a communications transmitter (Comm Tx)
and receiver (Comm Rx) 21 connected to a communications antenna 20
and controlled by a communications microprocessor (Comm .mu.c) 22
for communication with the base station BS1 with which it is
registered. The design and manufacturing of such telephones for
two-way communication within a cellular telephone network are well
known, those parts which do not form part of the present invention
will not be elaborated upon here further.
[0024] In addition to the conventional components of a mobile
telephone, telephone MS1 further comprises a GPS receiver (GPS Rx)
24 connected to a GPS antenna 23 and controlled by a GPS
microprocessor (GPS .mu.c) 25 receiving GPS spread spectrum signals
transmitted from orbiting GPS satellites. When operative, the GPS
receiver 24 may receive NAVSTAR SPS GPS signal through an antenna
23 and pre-process them, typically by passive bandpass filtering in
order to minimise out-of-band RF interference, preamplification,
down conversion to an intermediate frequency (IF) and analog to
digital conversion. The resultant, digitised IF signal remains
modulated, still containing all the information from the available
satellites, and is fed into a memory of the GPS microprocessor 25.
The GPS signals may then be are acquired and tracked for the
purpose of deriving pseudorange information from which the position
of the mobile telephone can be determined using conventional
navigation algorithms. Such methods for GPS signal acquisition and
tracking are well known, for example, see chapter 4 (GPS satellite
signal characteristics) & chapter 5 (GPS satellite signal
acquisition and tracking) of GPS Principles and Applications
(Editor, Kaplan) ISBN 0-89006-793-7 Artech House. The GPS
microprocessor 25 may be implemented in the form a general purpose
microprocessor, optionally common with the communications
microprocessor 22, or a microprocessor embedded in a GPS
application specific integrated circuit (ASIC).
[0025] Cellular telephone network base station BS1 is shown
schematically in FIG. 3. In additional to the conventional
components of a base station, it further comprises a GPS antenna
34, receiver 35 and microprocessor 36 which are in substantially
continual operation whereby the base station is in constant
possession of up to date GPS satellite information. This
information includes which of the orbiting satellites are presently
in view (such satellites are likely to be common to both telephone
and associated base station for even macrocells, obscuration
aside); the GPS data message containing an up to date almanac and
ephemeris data, and the Doppler shift and current code phase of the
GPS satellites signals as observed by the base station. In
accordance with the present invention, this information is modified
so as to reflect the position of the mobile unit, i.e. to described
the Doppler shift and code phase of the GPS signals as would be
observed at the mobile unit. In the event of the user of the mobile
cellular telephone MS1 making an emergency call and under the
control of the system controller SC via a two-way communication
link CL1, the base station BS1 may provide this modified
information to the telephone whereby it is then only required to
sweep a narrowed range of frequencies and code phases in which the
GPS PRN code is known to occupy, ensuring rapid code acquisition
and TTFF. The position of the mobile telephone is then transmitted
to the base station from the telephone, and then on to the
emergency services operator, termed the Public Safety Answer Point
(PSAP) in the US.
[0026] In the above example, the base station which provides the
assistance to the mobile telephone to despread the incoming signal
is also the communications base station with which the mobile
telephone communicates the emergency call and its position, i.e.
that which it is registered with. Of course this need not be the
case, the base station for providing assistance to despread the
incoming signal may be a different base station to that with which
the mobile telephone is registered, for example, where the
assistance is provided by a dedicated unit serving more than
network cell. In such a case, the base station providing the
assistance will transmit the modified GPS signal information to the
mobile telephone via the base station with which the mobile
telephone is registered.
[0027] An example of a method of modifying the Doppler shift and
current code phase of the GPS satellites signals as observed by the
base station to reflect the position of the mobile unit is provided
below.
[0028] If basestation BS1 is at position b=(X.sub.b, Y.sub.b,
Z.sub.b) and receives a GPS signal transmitted from a GPS satellite
at position s=(X.sub.s, Y.sub.s, Z.sub.s) travelling with velocity
v=(X.sub.v, Y.sub.v, Z.sub.v), then the theoretical Doppler shift
f.sub.b on those signals as observed at base station BS1
transmitted from the satellite at the base station is: 1 f b = F c
d _ sb v _ [ Equation 1 ]
[0029] where F is the GPS signal carrier frequency (1575.42 MHz in
the case of the NAVSTAR GPS L1 signal), c is the speed of light and
d.sub.sb is the unit direction vector of the satellite from the
basestation given by: 2 d _ sb = s _ - b _ | s _ - b _ | [ Equation
2 ]
[0030] In practice various minor effects, including selective
availability when operative, will cause the measured frequency
f.sub.b' to be slightly different. This frequency offset
.differential.f can be determined by the base station by: 3 f = f b
' - F c d sb v _ [ Equation 3 ]
[0031] If mobile cellular telephone MS1 is at position m=(X.sub.m,
Y.sub.m, Z.sub.m), the base station may then supply a more accurate
approximation to the Doppler shift at the mobile f.sub.m' given by:
4 f m ' = f + F c d _ sm v _ [ Equation 4 ]
[0032] where d.sub.sm is the unit direction vector of the satellite
from the mobile given by: 5 d _ sm = s _ - m _ | s _ - m _ | [
Equation 5 ]
[0033] Similarly for the code phase estimate, the theoretical code
phase cp.sub.b as observed at the base station can be expressed as
follows: 6 cp b = ( C | s _ - b _ | c ) mod L [ Equation 6 ]
[0034] where C is the chipping rate of the GPS PRN code (1.023 MHz
in the case of C/A code), c is the speed of light and L is the code
length (1023 chips). Again, there will be a small deviation
.differential.cp due to uncompensated range errors caused by
atmospherics, selective availability etc., which can be determined
at the basestation:
.differential.cp=cp.sub.b'-cp.sub.b [Equation 7]
[0035] where cp.sub.b' is the measured code phase. Therefore, the
expected code phase at the mobile can be estimated to be: 7 cp m '
= cp + ( C | s _ - m _ | c ) mod L [ Equation 8 ]
[0036] An initial estimate of the position of a mobile unit may be
provided in a variety of conventionally ways, for example, by
retrieving a last known GPS derived position of the mobile unit
from a memory store at the base station or at the mobile phone or
elsewhere and supplied to the base station upon request.
Alternatively, time of arrival, time difference of arrival and/or
angle of arrival measurements may be made between the mobile unit
and at least one base station, by either the mobile unit or the or
each base station, from which an position estimate may be provided.
A further alternative using a series of base stations is to
estimate a position by providing an average or weighted average of
the position of the base stations that a mobile unit can "see",
i.e. those base stations which the mobile unit can detect signal
transmissions from. A yet further alternative is to use an
independent system such as dead-reckoning or map matching,
especially where the mobile unit is vehicle based. As such methods
of providing a position estimate are known, they will not be
described here further.
[0037] As well as Doppler and code phase data for the satellites in
view, the GPS signal information might also include other GPS SV
information such as azimuth, elevation and signal strength
data.
[0038] Also, as an alternative to the early-late correlation
methods, fast convolution methods and in particular, involving Fast
Fourier Transforms (FFTs), may be used in order to acquired the PRN
codes. Such convolution methods are described in a paper entitled
"FFT processing of direct sequence spreading codes using modern DSP
microprocessors" by Robert G Davenport, IEEE 1991 National
Aerospace and Electronics Conference NAECON 1991, volume 1, pages
98 to 105, and also in U.S. Pat. No. 5,663,734. The method of the
present invention is equally is applicable such convolution
methods.
[0039] At present GPS is most notably associated with the
Navigation System with Time and Ranging (NAVSTAR) GPS, an all
weather, spaced based navigation system developed and operated by
the US Department of Defense, however, the general principles
underlying GPS are universal and not merely limited to NAVSTAR.
Accordingly, GPS refers to any positioning system comprising a
plurality of CDMA radio transmitters at different locations and a
receiver which determines its location based on the time of arrival
of the transmissions of the radio transmitters.
[0040] From a reading of the present disclosure, other
modifications will be apparent to the skilled person skilled and
may involve other features which are already known in the design,
manufacture and use of GPS receivers and component parts thereof
and which may be used instead of or in addition to features already
described herein. Although claims have been formulated in this
application to particular combinations of features, it should be
understood that the scope of the disclosure of the present
application also includes any novel feature or any novel
combination of features disclosed herein either explicitly or
implicitly, whether or not it relates to the same invention as
presently claimed in any claim and whether or not it mitigates any
or all of the same problems as does the present invention. The
applicants hereby give notice that new claims may be formulated to
such features and/or combinations of such features during the
prosecution of the present application or of any further
application derived therefrom.
* * * * *