U.S. patent number RE38,808 [Application Number 10/230,104] was granted by the patent office on 2005-10-04 for cellular positioning system (cps).
This patent grant is currently assigned to ITT Manufacturing Enterprises, Inc.. Invention is credited to Ronald Bruno, Lloyd Engelbrecht, Leonard Schuchman, Aaron Weinberg.
United States Patent |
RE38,808 |
Schuchman , et al. |
October 4, 2005 |
Cellular positioning system (CPS)
Abstract
Positioning system for locating a mobile body comprising a
plurality of earth based spread spectrum (SS) broadcasting stations
arranged geographically in a cellular pattern. Each SS broadcasting
station include a modulator providing a channel signal structure
which is substantially orthogonal with respect to adjacent stations
in the cellular pattern, each channel signal including navigation
beacon data including a unique beacon identification, station
latitude and longitude, time_slot and phase characterizations and
selected parameters of adjacent stations. In one embodiment, each
modulator provides a chirped SS signal in which the navigation
beacon is a frequency tone that is repeatedly swept over a selected
frequency band for each station. In a further embodiment, each
modulator provides a GPS like direct sequence SS signal in which
the navigation beacon is a PN coded broadcast. A receiver on the
mobile body receives the SS signals from at least three of the SS
broadcasting stations and determines the location thereof. A fourth
SS broadcasting station provides altitude CPS satellite signals can
be used for timing control.
Inventors: |
Schuchman; Leonard (Potomac,
MD), Bruno; Ronald (Arlington, VA), Weinberg; Aaron
(Potomac, MD), Engelbrecht; Lloyd (The Sea Ranch, CA) |
Assignee: |
ITT Manufacturing Enterprises,
Inc. (Wilmington, DE)
|
Family
ID: |
35005252 |
Appl.
No.: |
10/230,104 |
Filed: |
August 29, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
363773 |
Dec 23, 1994 |
5604765 |
|
|
Reissue of: |
393312 |
Feb 23, 1995 |
06111538 |
Aug 29, 2000 |
|
|
Current U.S.
Class: |
342/357.31;
340/7.2; 342/361; 342/386; 342/464 |
Current CPC
Class: |
H04Q
9/00 (20130101); H04B 1/69 (20130101); H04B
15/00 (20130101); G01S 19/21 (20130101); G01S
19/46 (20130101); G01S 5/14 (20130101); G01S
19/11 (20130101); G01C 21/206 (20130101); H04L
27/103 (20130101); G01S 5/145 (20130101); G01S
5/0236 (20130101); G01S 1/68 (20130101); H04B
2001/6912 (20130101); H04L 5/0048 (20130101); H04L
5/0053 (20130101); H04L 5/1469 (20130101) |
Current International
Class: |
G01S
5/02 (20060101); H04B 7/185 (20060101); G01S
1/00 (20060101); G01S 1/08 (20060101); G01S
001/08 (); G01S 005/02 (); H04B 007/185 () |
Field of
Search: |
;342/386,464,361,357.09,357.1,357.12 ;455/456,422.1 ;340/7.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Issing; Gregory C.
Attorney, Agent or Firm: Edell, Shapiro & Finnan,
LLC
Parent Case Text
REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No.
08/363,773 filed Dec. 23, 1994 .[.now pending.]. .Iadd.now U.S.
Pat. No. 5,604,765 .Iaddend.entitled "POSITION ENHANCED
COMMUNICATION SYSTEM INCLUDING SYSTEM FOR EMBEDDING CDMA NAVIGATION
BEACONS UNDER THE COMMUNICATIONS SIGNALS OF A WIRELESS
COMMUNICATION SYSTEM", incorporated herein by reference.
Claims
What is claimed is:
1. Positioning system for locating a mobile body comprising a
plurality of earth based spread spectrum (SS) broadcasting stations
arranged geographically in a hexagonal cellular pattern, each said
SS broadcasting station including a modulator providing a channel
signal structure which is substantially orthogonal with respect to
adjacent SS broadcasting stations in said hexagonal cellular
pattern, each channel signal including navigation beacon data
including: (a) a unique beacon identification, (b) station latitude
and longitude, (c) time slot frequency and phase characterizations
and (d) selected parameters of adjacent stations.
2. The positioning system defined in claim 1 wherein each said
modulator provides a GPS like direct sequence SS signal in which
the navigation beacon data is a PN coded broadcast.
3. The positioning system defined in claim 1 including receiver
means for receiving the SS signals from at least three of said SS
broadcasting stations and determining the location thereof.
4. The positioning system defined in claim 1 in which GPS satellite
signals are received in said hexagonal cellular pattern and
including means at each said broadcasting station for receiving
said GPS satellite signals and deriving therefrom a reference
frequency signal and a time epoch signal, said modulator means
being adapted to receive said reference frequency and time epoch
signals and be timed thereby.
5. Positioning system for locating a mobile body comprising a
plurality of earth based spread spectrum (SS) broadcasting stations
arranged geographically in a hexagonal cellular pattern, each said
SS broadcasting station including a modulator providing a channel
signal structure which is substantially orthogonal with respect to
adjacent SS broadcasting stations in said hexagonal cellular
pattern, each channel signal including navigation beacon data
including: (a) a unique beacon identification, (b) station latitude
and longitude, (c) time slot frequency and phase characterizations
and (d) selected parameters of adjacent stations, each said
modulator providing a GPS like direct sequence SS signal in which
the navigation beacon is a PN coded broadcast, and receiver means
for receiving the direct sequence SS signals from at least three of
said SS broadcasting stations and determining the location
thereof.
6. The positioning system defined in claim 5 in which each said SS
broadcasting station includes means to receive GPS satellite
signals and means for deriving therefrom a reference frequency
signal and a time epoch signal, said modulator means being adapted
to receive said reference frequency and time epoch signals and be
timed thereby..Iadd.
7. A positioning apparatus suitable for use in an earth based
spread spectrum (SS) broadcasting station among a plurality of
earth based spread spectrum (SS) broadcasting stations arranged
geographically in a cellular pattern in a communication system for
locating a mobile body, each said SS broadcasting station
outputting a channel signal that is substantially orthogonal with
respect to channel signals output by adjacent SS broadcasting
stations in said cellular pattern, the apparatus comprising a
modulator suitable to output such a substantially orthogonal
channel signal including navigation beacon data including: (a) a
unique beacon identification, (b) station latitude and longitude,
(c) time slot frequency and phase characterizations, and (d)
selected parameters of adjacent stations..Iaddend..Iadd.
8. The positioning apparatus of claim 7, further comprising a time
transfer receiver configured to receive GPS satellite signals and
deriving therefrom a reference frequency signal and a time epoch
signal, said modulator being connected to the receiver and adapted
to receive and be timed by said reference frequency and time epoch
signals..Iaddend..Iadd.
9. The positioning apparatus of claim 7, wherein the modulator is
configured to output a GPS-like direct sequence SS (DSSS) signal
having the navigation beacon data that is PN
coded..Iaddend..Iadd.
10. The positioning apparatus of claim 9, wherein the DSSS signal
output from the modulator has at least one of a PN code and a PN
phase that differs from PN codes and PN phases of the channel
signals output by the adjacent SS broadcast
stations..Iaddend..Iadd.
11. The positioning apparatus of claim 7, wherein the modulator is
configured to output a chirped spread spectrum (CSS) signal as the
substantially orthogonal channel signal having chirp
characteristics that are different from chirp characteristics of
the channel signals output by the adjacent SS broadcasting
stations..Iaddend..Iadd.
12. The positioning apparatus of claim 11, wherein the different
chirp characteristics include at least one of a chirp rate and a
chirp phase..Iaddend..Iadd.
13. A positioning apparatus suitable for use in a mobile body in a
communication system having a plurality of earth based spread
spectrum (SS) broadcasting stations arranged geographically in a
pattern suitable for facilitating cellular communications, each SS
broadcast station transmitting a navigation beacon that is
substantially orthogonal with respect to navigation beacons
transmitted from adjacent SS broadcasting stations in the cellular
pattern, wherein the navigation beacons include: (a) a unique
beacon identification, (b) station latitude and longitude, (c) time
slot frequency and phase characterizations, and (d) selected
parameters of adjacent stations, the positioning apparatus
comprising a receiver configured to receive said navigation beacons
from at least three of said SS broadcasting stations in the
communication system and determine a location of the mobile body
therefrom..Iaddend..Iadd.
14. The positioning apparatus of claim 13, wherein the navigation
beacons transmitted by said at least three of said SS broadcasting
stations are direct sequence SS (DSSS) signals having different PN
characteristics, and the receiver is configured to receive such
DSSS signals..Iaddend..Iadd.
15. The positioning apparatus of claim 14, wherein the different PN
characteristics include at least one of a PN code and a PN
phase..Iaddend..Iadd.
16. The positioning apparatus of claim 13, wherein the navigation
beacons transmitted by said at least three of said SS broadcasting
stations are chirped spread spectrum (CSS) signals having different
chirp characteristics, and the receiver is configured to receive
such CSS signals..Iaddend..Iadd.
17. The positioning apparatus of claim 16, wherein the different
chirp characteristics include at least one of a chirp rate and a
chirp phase..Iaddend..Iadd.
18. A method of determining a location of a mobile unit operating
within a wireless communication system having a plurality of earth
based broadcasting stations each transmitting a navigation beacon
substantially orthogonal to navigation beacons transmitted by
adjacent broadcasting stations, the method comprising: detecting on
a control channel of a communication signal broadcast by one of the
plurality of broadcasting stations a navigation message containing
information concerning the navigation beacons transmitted by the
plurality of broadcasting stations; acquiring the navigation
beacons transmitted by at least three of the broadcasting stations
using the information contained in the detected navigation message
and determining from the acquired navigation beacons range
measurements to said at least three broadcasting stations; and
determining a position of the mobile unit based on the range
measurements, wherein the navigation message includes identifiers
of at least the adjacent broadcast stations and characteristics of
the navigation beacons transmitted by the adjacent broadcast
stations..Iaddend..Iadd.
19. The method of claim 18, wherein characteristics of the
navigation beacons transmitted by the adjacent broadcast stations
include for each navigation beacon at least one of: a time slot
identifier of a time slot within a communication signal in which
the navigation beacon is broadcast; phase information concerning a
phase of the navigation beacon; and a frequency of the navigation
beacon..Iaddend..Iadd.
20. The method of claim 19, wherein the navigation beacons
transmitted by the adjacent broadcast stations are direct sequence
spread spectrum (DSSS) signals, and the characteristics of the
navigation beacons include a PN code and PN code
phase..Iaddend..Iadd.
21. The method of claim 19, wherein the navigation beacons
transmitted by the adjacent broadcast stations are chirped spread
spectrum (CSS) signals, and the characteristics of the navigation
beacons include a chirp rate and chirp phase..Iaddend..Iadd.
22. The method of claim 18, wherein at least one of said navigation
beacons is overlaid with a communication signal, and the method
further comprising processing said communication
signal..Iaddend..Iadd.
23. The method of claim 22, further comprising transmitting to a
call destination mobile unit position information based on the
determined position of the mobile unit..Iaddend..Iadd.
24. The method of claim 23, further comprising transmitting with
the mobile unit position information, information identifying the
mobile unit..Iaddend.
Description
BRIEF DESCRIPTION OF THE INVENTION
Wireless communications are rapidly augmenting conventional
telephone communications. For many types of wireless calls such as
911 or calls for roadside automotive repair/towing, knowing and
conveying the location of the call origin is vital. However, since
most users of wireless communications are mobile, their location is
typically not known and can encompass a large uncertainty region.
As shown in application Ser. No. 08/363,773, assigned to the
assignee hereof, there are several alternative systems for position
location by mobile users, but none of these current systems are
adequate for the wide variety of environments for wireless
communications. By far the largest segment of mobile communications
in the US is the current cellular voice system, and this system
presents an opportunity to establish a cellular array of spread
spectrum navigation beacons that can be used for position
determination by users of cellular telephony and other public
wireless services. This concept was described in the
above-referenced patent application. The present invention
discloses about how a set of spread spectrum navigation beacons can
be uniquely designed and arranged in a cellular pattern, and how
the required navigation receiver signal processing can be
efficiently integrated into a cellular phone via novel application
of state-of-the-art technology. As shown later herein, the cellular
array of navigation beacons can be a stand-alone navigation system,
or it can be co-located and integrated with an existing or future
cellular communications system.
DESCRIPTION OF THE INVENTION
The object of the invention is to provide a system of spread
spectrum navigation beacons arranged geographically in a cellular
pattern that supports position determination at a mobile or
portable cellular telephone or other wireless communications
terminal.
The invention features the following:
1) In one embodiment, the use of a set of direct sequence spread
spectrum signals (with properties described by time slot of
operation, specific PN code, PN code phase, and carrier frequency)
to comprise a cellular array of navigation beacons that is used for
position location by mobile or portable terminals. The system of
beacons may be a stand-alone system, or an overlay of a cellular
communications system in which the beacons occupy the same spectrum
as the communications system.
2) In another embodiment, the use of a set of chirped spread
spectrum signals to comprise a cellular array of navigation beacons
that is used for position location by mobile or portable terminals.
Again, the system of beacons may be a stand-alone system, or an
overlay of a cellular communication system in which the beacons
occupy the same spectrum as the communications system.
3) Navigation beacons that use a common frequency and a common PN
code, but are distinguished by a different phase offset of the PN
code epoch relative to the 1 msec time epoch. In the terrestrial
environment, a unique phase offset in the code relative to the 1
msec epoch can provide a unique signature for a navigation beacon
in a local geographical region.
4) Chirped navigation beacons that use a common frequency, but are
distinguished by a different phase offsets of the chirp epoch
relative to the 1 msec time epoch, and different sweep rates a
common frequency band. In the terrestrial environmental, a unique
phase offset in the chirp relative to the 1 msec epoch can provide
a unique signature for a navigation beacon in a local geographical
region composed of a number of otherwise identical beacons.
5) In a cellular communications system with a cellular positioning
system (CPS) overlay, the provision of the cellular system control
channels to convey the navigation "almanac" to mobile and portable
users. The "almanac" is comprised of the data needed to convert a
set of pseudorange measurements into a position, and includes a
list of the cellular broadcast locations and a characterization of
the navigation beacons that are broadcast from each location.
6) The use of NVRAM for the storage of the bulk of the "almanac"
data which is unchanging except insofar as the cellular system
and/or its navigation beacons are modified as part of system
evolution.
7) Direct Sequence Spread Spectrum (DSSS) or Chirped Spread
Spectrum (CSS) navigation beacons that are uniquely characterized
in a local region by their assigned signal characteristics so that
data modulation of the beacons is not required for beacon
identification.
8) Frequency notching in a DSSS or CSS navigation receiver to
filter out the interference caused by the occupied narrow band
communications channels of a cellular communications system. In the
chirped spread spectrum CSS receiver, the use of signal attenuation
when the chirped spread spectrum CSS signal sweeps through the
occupied communications channels as a novel implementation of the
frequency notch. The use of the cellular system broadcast control
channels to convey knowledge of the occupied slots so that they may
be notched from the receiver.
9) The fact that a common antenna and RF front end is applicable
for both communications and navigation is a unique and novel
feature of this invention.
10) The implementation of the cellular positioning system CPS
navigation receiver/processor using time-domain and
frequency-domain approaches.
DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the
invention will become more apparent when considered with the
following specification and accompanying drawings wherein:
FIG. 1 illustrates a mobile terminal taking pseudo-range
measurements to three base stations,
FIG. 2 illustrates a hexagonal array of base stations covered by
three orthogonal navigation beacons (NB) represented by different
shades of gray and seven PN codes (or seven phases of a single PN
code) which is represented by the numbers (1,2,3 . . . 7) in the
array,
FIG. 3 illustrates a cellular positioning system of seven
navigation beacons (NBs) that cover a hexagonal region of cells
having seven chirp phases of a single chirped signal,
FIG. 4A is a general block diagram of a stand alone base station
used in this invention,
FIG. 4B is a block diagram of a direct sequence spread spectrum
(DSSS) embodiment base station used in this invention,
FIG. 4C is a block diagram of a chirped spread spectrum (CSS)
embodiment base station used in this invention,
FIG. 4D is a block diagram of a cellular base station to which the
navigation beacon signals of FIGS. 4A, 4B and 4C has been
added,
FIG. 5 is a general block diagram of a cellular position system
receiver used in this invention,
FIG. 6 is a block diagram of a cellular position system processor
with PN navigation signals using a time domain approach,
FIG. 7 is a block diagram of a cellular position system processor
with PN navigation signals using a frequency domain approach,
FIG. 8 is a block diagram of a cellular position system processor
with chirp signals, and
FIG. 9 is a block diagram illustrating the time-gated approach for
frequency notching in a chirp receiver.
DETAILED DESCRIPTION OF THE INVENTION
The system is illustrated in FIG. 1 which shows a mobile user MU
surrounded by base stations 10-1, 10-2, 10-3 . . . 10-N of a
cellular positioning/communications system. At any point in time,
the mobile unit MU tracks the navigation signals from at least 3
base stations and measures the pseudorange to each of them by
processing their navigation beacons. Measurements from three base
stations are required for a 2D solution and if a 3D solution is
desired, then measurements from 4 base stations are required. This
positioning system is referred to as the Cellular Positioning
System (CPS). In the cellular positioning system (CPS), each
cellular base station (BS) broadcasts a spread spectrum navigation
beacon (NB) whose frequency and timing are synchronized and slaved
to GPS. The system of beacons may be a stand-alone system (FIG.
4A), or an overlay of a cellular communication system (FIG. 4D) in
which the beacons occupy the same spectrum as the communications
system. In such an overlay system, the power of the spread spectrum
signals described below can be set sufficiently low so as not to
interfere with the communications channels, yet sufficiently high
to support a good signal for a navigation receiver. Two types of
spread spectrum navigation signals are disclosed herein:
1) A GPS-like direct sequence spread spectrum (DSSS) signal in
which the navigation beacon is a PN coded broadcast (see FIG. 4B).
One example code has a length of 1023 and a code period of 1 msec
so that the chipping rate of the navigation beacon is 1.023 Mcps.
The resulting signal is spread over about 2 MHz of spectrum.
Depending upon the detailed cellular positioning system (CPS)
design, the navigation beacon signal may have a different code
length and chipping rate, and may support little or no data.
2) A chirped spread spectrum (CSS) signal in which the navigation
beacon is a frequency tone that is repeatedly swept over a chosen
frequency band (see FIG. 4C). A 10 MHz band with a period of 1 msec
or greater are example parameters for a chirped signal. Depending
upon the chosen CPS design, chirp navigation beacon signals may
also incorporate different frequencies, sweep size and sweep rate,
and may support little or no data.
According to this invention, an efficient set of navigation beacons
for a cellular positioning system CPS must conform to a channel
structure that supports orthogonal or near orthogonal beacons that
do not significantly interfere with each other. The degree of
orthogonality is important since within a cellular array of
navigation beacons, when a user is near a cellular BS, the
navigation beacon signal power from that station may be 30 dB or
more stronger than the power of navigation beacon signals from far
(i.e., adjacent) BSs. With complete orthogonality, the strong
navigation beacon will never interfere with a weak navigation
beacon. Complete orthogonality is needed to solve the near-far
interference problem in the cellular positioning system CPS. With
less than complete orthogonality, however, the near-far
interference is a problem whenever the near signal power is some
threshold number of dB stronger that far navigation beacon
signal.
In a cellular positioning system CPS, the same channels can
typically be reused for beacons over a wide geographic region
because range attenuation effectively provides separation between
distant beacons that are using the same channel. Current and
proposed cellular communications systems use frequency division,
time division, code division and data/tone markers to create
distinct channels. The DSSS navigation beacons in a cellular
positioning system CPS similarly can use frequency, time, code and
data to distinguish them from each other. In general, each
individual DSSS navigation beacon is characterized by the
parameters in Table 1.
TABLE 1 Parameters of DSSS Navigation Beacons time.sub.--slot for
example, each NB may be on for 1/3 second every second. of The
time_slot would thus be 1, 2 or 3, indicating which operation: part
of a 1 second frame the NB is broadcasting in. A scheme of 3 times
slots supports a 3-fold orthogonality that is complete and
therefore sufficiently robust to solve the near-far problem.
Schemes employing as few as 1, and as many as 7 time slots are
practical and feasible in a CPS. chosen one of the library of PN
codes with good cross correlation PN code: properties. Different PN
codes provide a partial orthogonality that is roughly proportional
to the spreading gain (which is defined as the receiver signal
averaging time divided by the chip time duration). Thus for an NB
with a 1.023 Mcps chipping rate supporting 50 bps of data, the
spreading gain is about 43 dB which means the signal power of a
strong signal interferer is suppressed by 43 dB phase of the phase
of the PN code at the msec time epochs. NP signals the PN may use
the same PN codes but broadcast the codes with suf- code: ficient
phase offset to provide good orthogonality. The code phase of the
NB may be in-phase (i.e., with the first chip aligned to the 1 msec
time epoch) or out-of-phase (i.e., delayed by some specified number
of chips so that the nth chip is aligned to the 1 msec epoch) A
1023 chip code will provide roughly 60 dB separation (1023 .times.
1023) between two NBs with a different code phase (when averaged
over one code cycle and the frequencies of the NBs at the receiver
are coherent over the code cycle period). If the frequencies are
not coherent, then the orthogonality that is achieved is roughly
the same as for two different PN codes. A PN code with a period of
1 msec provides a substantial amount of range ambiguity resolution
in the terrestrial environment. For example, if a PN code phase is
offset 1/2 the code period (0.5 msec) with respect to a reference
code, the code phase can resolve up to 75 km of range ambiguity;
thus if the two PN codes are broadcast from towers that are closer
than 75 km, each signal can be resolved distinguished from the
other. The use of more than two code phases states for signals will
provide proportionately less capability to resolve the range
ambiguity. frequency Each NB occupies about 2 MHz of spectrum since
the band- of the NB: width between the first nulls of an unfiltered
1.023 Mcps signal is 2.046 MHz. In a system where multiple signals
are used for the NBs the nulls of a central frequency are good
candidates for addition NB frequencies. The 1st null results in a
completely orthogonal NB as long as the frequencies are coherent
over the receiver signal integration time. Even if the frequencies
are not coherent, NBs at the 1st, 2nd, 3rd and other nulls can
achieve 30-60 dB or more of separation by appropriate filtering.
unique ID Each NB may support 8-16 bits of data which will be a of
the NB: unique identifier of that NB in the local cellular
region.
The chirped spread spectrum CSS navigation becomes in a cellular
positioning system CPS similarly can use frequency, time, sweep
phase, sweep rate, and data to distinguish them from each other.
The characterization parameters in a system of chirped navigation
beacons are described in Table 2 below. There is much commonalty
with the DSSS navigation beacons but there are some significant
differences that are noted.
TABLE 2 Parameters of Chirped Navigation Beacons time_slot same as
for DSSS Navigation Beacons; chirps occupying of different time
slots are completely orthogonal. operation: chirp Different sweep
rates of chirped signals is one form of sweep distinct modulation
that can provide good orthogonality; rate: for example, two chirped
signals that occupy the same frequency and time slot, but with
chirp period of 1 msec and 2 msec respectively. phase the phase of
the chirp at the msec time epcohs. NB signals of the may broadcast
the same chirp but with sufficient phase chirp: offset to provide
good orthogonality. The chirp phase of the NB may be in-phase
(i.e., with the frequency minimum aligned to the 1 msec time epoch)
or out-of-phase (i.e., delayed by some specified amount of time).
Chirp signals that differ by a constant phase offset are
orthogonal. frequency roughly the same as for the DSSS; chirps at
different of the NB: frequencies are completely orthogonal. unique
ID Each NB may support 8-16 bits of data which will be a of the NB:
unique identifier of that NB in the local cellular region.
While the above parameters in Tables 1 and 2 apply to the most
general cellular positioning system CPS, in any specific
application, the use of this parameter set can be limited to a
subset that fills out the defined cells with navigation beacons
with suitable signal characteristics that uniquely define them (in
a local region) and solve the near-far interference problem. In
order to solve the near-far problem in a hexagonal cellular array,
it is necessary to have at least 3 navigation beacon signals that
are virtually completely orthogonal (i.e., separable by at least 60
dB) under all circumstances. Of the set of DSSS parameters listed
above, only time-slot and frequency satisfy this requirement. While
they do not support complete orthogonality, the parameters of PN
code and code phase, serve to uniquely identify the navigation
beacon in a local region without having to read a unique ID that
may be modulated onto each navigation beacon. Table 3A illustrates
a sample of DSSS navigation beacon signal sets that satisfy the
orthogonality requirements to both solve the near-far interference
problem and uniquely identify the navigation beacon in its local
environment. Note that the value define 21 distinct navigation
beacon signals that are suitable for allocation across a hexagonal
array of cells. As illustrated in the cellular positioning system
CPS in FIG. 2, the hexagonal array is covered by allocating the 3
orthogonal navigation beacon signals over the array (represented by
the different shades of gray) and 7 PN codes (or 7 phases of a
single PN code) which is represented by the numbers in the cellular
array. The 3 different shades can be viewed as either 3 orthogonal
time slots or 3 orthogonal frequencies. In this manner a group of
21 cells is formed and is replicated repeatedly to fill any
cellular array. Note that in these systems, the separation distance
between navigation beacons with the same signal parameters is quite
large (roughly 9 cell radii).
TABLE 3A Alternative Systems of DSSS Navigation Beacons Solve
Near-Far Interference Provide Unique Signature System time slots
frequencies PN codes PN phases A 3 1 1 7 B 1 3 1 7 C 3 1 7 1 D 1 3
7 1
As indicated in Table 3A, many other combinations are possible for
the specification of a set of navigation beacons. Clearly, the
variety of options for parameters of time_slot, frequency, PN code,
and PN phase ensure that there are many sets of suitable navigation
beacons that can be generated in a tailored fashion for the
particular cellular layout and constraints on frequency
availability and other signal parameters. In an analogous fashion,
the parameters of Table 2 that characterize the chirped navigation
beacons can be exercised to generate a suitable set of unique
beacon signals for any reasonable cellular layout.
TABLE 3B Alternative Systems of CSS Navigation Beacons Provide
Unique Solve Near-Far Interference Signature System time slots
frequencies chirp phase chirp rates A 3 1 7 1 B 1 3 7 1 C 1 1 21 1
D 1 1 7 1 E 1 1 7 2
Table 3B illustrates a sample of CSS navigation beacon signal sets
that satisfy the orthogonality requirements for solving the
near-far interference problem and for uniquely identifying the
beacons in their local environment. For three systems (A-C), 21
distinct navigation beacons are defined, for one system (D), only
seven are defined, and for one (E) fourteen are defined. Note that
for chirp signals, chirps of different phase are orthogonal. Thus
different phases alone solve the near-far interference problem, and
so in two systems (C and D), the navigation beacons are defined
only by a distinct phase of the chirp. FIG. 3 illustrates the
allocation of 7 distinct chirp phases (as in System D of Table 2)
over a hexagonal array. The seven phases are assigned to seven
hexagons of a local cluster, and this pattern is replicated to fill
an entire coverage area. In general, it can be shown that in a
system of beacons distinguished only by chirp phase the following
constraint must be satisfied in choosing the number of phase states
and the period of the chirp for a given cellular coverage area:
##EQU1##
where
D=the diameter of cells
n=the number of distinct phase slates
.tau.=the period of the chirp, and
c=the speed of light
3.0 Operational Scenarios of CPS Utilization
3.1 A Stand-Alone CPS
A cellular positioning system CPS can be a stand-alone system, or
part of a cellular communications system or a combination of
stand-alone stations and cellular sites with a positioning signal.
In a stand-alone system (FIGS. 4A-4C), the position is determined
at a navigation receiver (NR) by acquiring, tracking and
demodulating the navigation beacons. Accordingly, in a stand-alone
system, all the navigation data required for position determination
is broadcast by the navigation beacons. An example of the data that
would be conveyed by each navigation beacon is illustrated in Table
4. Note that this information conveys the position of the BS as
well as the signal parameters of the navigation beacon that is
broadcast by that BS.
TABLE 4 BS Navigation Beacon Data Unique BS/Beacon ID Station
Latitude (1 meter quantization) Station Longitude (1 meter
quantization) Station Altitude (1 meter quantization) Time_Slot and
Code_Phase and/or other characterization of NB) Selected Parameters
of NBs of adjacent BSs
In order to determine position, a navigation receiver NR needs to
acquire and measure the pseudorange on at least 3 navigation
beacons for a 2D solution (time plus lat/lon), and at least 4
navigation beacons for a 3D solution (time plus lat/lon/alt). In
addition, the navigation receiver NR would also have to demodulate
the data on each of the navigation beacons. When the navigation
receiver NR has measured the required number of pseudoranges and
has read the data on each navigation beacon, the position of the
navigation receiver NR can be solved for. Excess measurements can
be used to generate added measurement precision and robustness via
standard techniques for minimization of measurement variance and
elimination of out of bounds measurements. The typical scenario for
position determination for a stand-alone cellular positioning
system CPS is as follows:
Step 1: At navigation receiver NR turn-on, the navigation receiver
NR searches the signal parameter space, acquires the first
navigation beacon, and measures the pseudorange. The navigation
receiver NR then reads the data on the navigation beacon.
Step 2: With or without the aid of the data from the first acquired
navigation beacon, the navigation receiver NR continues a search
for subsequent navigation beacons from other BSs. The navigation
receiver NR continues until it makes a pseudorange measurement and
acquires the data from at least two additional navigation beacons
broadcast from adjacent BSs.
Step 3: The navigation receiver NR computes a 2D or 3D position,
depending upon the number of navigation beacons acquired and
processed.
Step 4: The navigation receiver NR continues to acquire and track
navigation beacons as the navigation receiver NR may move though
the geographic region that is covered by the cellular positioning
system CPS, and it periodically recalculates the navigation
receiver NR position according to some defined algorithm.
3.2 A CPS Embedded in a Cellular Telephone System
In a cellular positioning system CPS that is integrated with a
cellular communications system, the navigation beacons may share
the same spectrum as the communications systems (as described in
the above-referenced patent application). In addition, much or all
of the required navigation data may be conveyed by the broadcast
control channels of the communications system. Thus, in a cellular
positioning system CPS integrated with a communications system the
data supported by each navigation beacon is minimal: at most an
8-16 bit identifier that is unique with in the local cellular
region. The rest of the navigation data needed is transmitted by
the cellular control channels that are broadcast from each cellular
BS. The scenario for position determination in this combined
communications-navigation system is described below via the
operation of a cellular phone and a phone navigation receiver NR.
For illustration purposes, the set of navigation beacon signals
illustrated as System A in Table 3A is assumed.
Step 1. Comm Initialization via Listening to the Cellular Control
Channel: At the start of this scenario, the phone is just turned
on. The phone searches for, finds and listens to a cellular
communications control channel and initializes itself according to
the applicable cellular communications standard or protocol.
Step 2. Nav Initialization via Listening to the Cellular Control
Channel: In listening to the control channel, the phone also
initializes with respect to cellular navigation. When this task is
complete, the phone has determined that it is near a specified base
station, and has acquired the lat/lon/alt/code_phase/time_slot for
all the base stations in the local environment. This data is
conveyed via navigation overhead messages that are broadcast on
each control channel; these overhead messages contain a data set as
illustrated in Table 4, but also include the data for all of the
adjacent cellular BSs as well as an issue number that will change
with cell system evolution, and the spectral occupancy of
communications channel broadcast by the BS. This information will
be stored in non-volatile random access memory (NVRAM) (FIG. 5) so
that the data is preserved from call to call. The NVRAM is of
sufficient size so the IDs and locations of a reasonable sized
region of cells can be stored. Every time the phone is in an
initialization state, the NVRAM is updated so that the NVRAM always
stores the most current broadcast of the cellular navigation
parameters. Thus, in making a call for which position is desired,
the parameters of the local navigation beacons will be known
(assuming that the received "issue number" in the navigation data
matches the "issue number" of the data stored in NVRAM for that
cellular base station). If the received unique ID and issue number
do not both match a value stored in NVRAM, then the navigation data
would need to be collected before the positioning call could
proceed. As the phone explores new territory, the list of cell base
station locations would expand to fill the NVRAM. For a phone that
covers a great deal of territory, new locations would be written
over old locations whenever the capacity of the NVRAM is reached.
This navigation data that is stored in the NVRAM is referred to as
the navigation almanac.
Step 3. Search and Acquisition of a Navigation Beacon (NB): This
step can be done in parallel with Step 1 and Step 2. Every cellular
base station transmits a navigation beacon with a set of signal
parameters designated in the communication broadcast control
channel. The navigation receiver NR of the phone searches various
time slots and code phase states for a navigation beacon that is
strong enough to acquire the PN code and possibly to read a "unique
ID" that is coded onto the navigation beacon. When this task is
completed, the phone navigation receiver NR has made a pseudorange
measurement on the navigation beacon and has reached
synchronization with the navigation system, meaning that it now has
an absolute reference for both time_slot and code_phase. In the
local environment, each navigation beacon is uniquely specified by
the time_slot/code_phase pair so that additional navigation beacons
can be rapidly acquired, and the identity of each navigation beacon
is known a priori (thus the data on these beacons does not need to
be read in order to determine the identity of the navigation
beacon).
Step 4. Search and Acquisition of Other Navigation Signals:
Having achieved synchronization with code_phase and time_slot of a
navigation signal in Step 3, the phone navigation receiver NR looks
up (in the NVRAM) the code_phase and time_slot of other navigation
beacons in the local environment, the phone navigation receiver NR
then proceeds to acquire these signals and make pseudorange
measurements. Note that since code_phase and time_slot uniquely
specify the navigation beacon in its local environment, the data on
these navigation beacons is not read. Note also that acquisition of
navigation beacons in this Step may be aided by using the data on
the spectral occupancy of communications channels via insertion of
frequency notches in the navigation receiver NR to reduce the
interference that is created by a strong communications signal
(from a nearby cellular base station) on a weak navigation signal
(broadcast by a distant base station). Such notching would
typically be required for the AMPS and TDMA cellular systems in
which the communications channels are contained in 30 KHz frequency
slots that are dispersed throughout the cellular allocation. In a
Q-CDMA system, such frequency notching would not be required. If
pseudorange measurements on at least 2 other navigation beacons are
made, the phone navigation receiver NR can proceed to Step 5.
Step 5. Calculation of Position: With a total of 3 or more
pseudorange measurements, the navigation receiver NR can generate a
2D solution, solving for navigation receiver NR time and location
(lat/lon). With a total of 4 or more pseudorange measurements, the
navigation receiver NR can generate a 3D solution, solving for time
and location (lat/lon/alt). Excess measurements can be used to
generate added measurement precision and robustness via standard
techniques for minimization of measurement variance and elimination
of out of bounds measurements. When Step 5 is completed, the phone
displays a "position fixing" indicator analogous to the roaming
indicator. The "position fixing" indicator will tell the user that
the phone knows its position at that particular moment in time.
Step 6. Recalculation of Position: In the idle state, the phone
will continue to listen to the communications control channel.
During this time, the phone navigation receiver NR may or may not
(e.g., where power is scarce) continue to operate. In general, the
phone navigation receiver NR will recalculate its position
according to a programmed algorithm. Recalculation could be done
continuously, or in response to an expired time or event as
described below:
continuous With continuous recalculation the NR stays operand
recalculation: and continuously recalculates its position. This
algorithm would be suitable for applications that have sufficient
power to support the continuous operation of the phone NR. periodic
In this mode, recalculation is initiated periodically after
recalculation: the expiration of a set amount of time; thus, the
phone NR would normally be in a power conserving sleep mode, but
would wake up periodically (e.g., every 5 minutes) in order to
recalculate its position. The time interval between wake-ups would
be programmed by the phone user. recalc on In this mode, the NR
would normally be in a power control conserving sleep mode, but
would wake up whenever the channel phone receiver changes the
control channel that it turnover: listens to; this turnover in
control channels occurs whenever the signal strength of the initial
control channel fades down to a specified level and the phone
searches for and locks onto a stronger control channel. 4.0
Operational Scenario of CPS-Cellular Communications Coordination
for Call Processing
At the start of this scenario, it is assumed that the phone and
phone navigation receiver NR has completed the Steps 1 to 3 that
are described above. At the completion of Step 3, the phone
displays a position fixing indicator that tells the phone user that
the phone is ready and prepared to make a "position enhanced" (PE)
phone call. The call processing that then takes place is a
follows:
Step 1. Initiating the Call: The phone user initiates a PE phone
call in the same manner as normal calls (nominally by dialing the
number and pressing the Send key). Depending upon the way
positioning service is used, the phone user may convey a desire for
a PE call via the pressing of some specified key combination. For
911 calls, a PE call would be the default.
Step 2. Phone Response: In response to the user call initiation,
the phone seizes the access control channel and sends a digital
message in accordance with cellular system specifications. This
message contains the phone electronic serial number (ESN), the user
mobile phone number (MIN), and other such data. In a PE phone call,
the digital message would contain an additional cellular control
word that conveys the lat/lon of the phone location in accordance
with a standard compressed format. For example, to convey the
lat/lon of the phone (with a 10 meter quantization) relative to the
lat/lon of the base station would require about 24 bits.
Step 3. MTSO Response. With the completion of Step 2, the MTSO has
the location of the phone prior to call setup, and can therefore
use this information in call processing. Thus, in a 911 call, the
MTSO could use location knowledge to find the appropriate emergency
service center for the phone location, and then route the call and
the location data to that emergency service center. For PE calls
other than 911, the MTSO would also send the location of the
calling phone to the call destination. This can be accomplished
in-band via modem (in accordance with an established standard) or
out-of-band via SS7.
Step 4. Call Servicing: Call servicing of a "one-shot" positioning
call proceeds the same as a normal cellular call. In response to
the request by the phone, the MTSO assigns an available voice
channel to the phone via a message on the control channel; the
phone then switches to that channel while the MTSO proceeds to
patch the call through to the dialed number. However, with
"continuous" position fixing during a call, the phone navigation
receiver NR must continue to measure pseudoranges on all the
navigation beacons to update the location estimate of the phone. In
order to do this as the phone moves through cell during the call
progress, it is clear that the phone will need to continue to
receive a control channel in order to maintain the navigation data
it needs to acquire and track the navigation beacons. The process
for a "continuous" position fixing call is described in the
succeeding steps.
Step 5. Call Servicing for "Continuous" Position Fixing: In this
mode, the phone navigation receiver NR must continue to receive and
monitor a control channel, since the ability to position fix
depends upon the navigation overhead data that is broadcast on the
control channels. Thus, as phone moves through cells during a call,
the phone must continue to monitor control channels and switch to a
stronger one as required in order to maintain a current file of the
navigation data. As it does this, the phone navigation receiver NR
continues to calculate its position. Each position update may then
be periodically sent to the MTSO via a specially-defined message on
the voice control channel. Alternatively (or in addition), the
message can be sent in-band via a simultaneous "data-in-voice"
modem. The continuous monitoring of a control channel after
switching to a voice channel is a departure from normal cellular
telephony operations, but it is not inconsistent with such
operations, and may even provide telephony benefits. For example,
if the cellular telephone continues to monitor a control channel,
the MTSO has a means to offer such services as "call waiting" by
alerting a phone via the control channel that a call is being
placed to their busy number.
Receiver Processing
This section presents a description of selected embodiments of a
navigation receiver signal processor. While novel features will be
explicitly identified, it should be emphasized that other
embodiments would also be included as part of this invention. An
overview of receiver processing, within the user's unit, is shown
in FIG. 5. Note that the user's unit may be a cellular car phone,
portable phone, or other receiving device that may, for example,
operate in a general PCS environment. As seen in FIG. 5, the
composite incoming RF signal is received by a single antenna (AN)
and is amplified and conditioned by a single RF front end (RFE)
(e.g., Low Noise Amplifier). It should be emphasized that the
composite incoming signal, which includes the combination of
communication-channel and control-channel signals, typically spans
tens of MHz (e.g., >20 MHz for cellular). The fact that a common
antenna AN and RF front end RFE is applicable for both
communications and navigation is a unique and novel feature of this
invention.
As further seen in FIG. 5, following the RF front end RFE, the
signal is split SPL into two parts. One path goes to the
communications/control-channel CPCR processing portion of the
device, and the other path goes to the navigation portion NP of the
device. In the present discussion, only the navigation portion NP
of the device is described in detail, but as seen, the navigation
processing requires "notching" data that is provided by the Control
Channel (CC). This "notching" aspect was addressed and is expanded
upon further below.
As further shown in FIG. 5, the signal output of the RF front end
RFE is downconverted via mixer MX, and a single fixed
downconversion frequency, to a convenient intermediate frequency
(IF). While a single mixing stage is shown, this invention
encompasses the m ore general case of one or multiple
downconversion stages. The IF signal is filtered by Bandpass Filter
BPF, whose bandwidth is approximately 2 Mhz, to accommodate the
1.023 Mcps spread spectrum signal or to an alternative bandwidth to
accommodate the chirped signal.
The BPF is followed by the following unique system elements:
1. The Cellular Positioning System CPS Processor, CPSP, which is
amenable to a miniaturized/low-power implementation, executes all
the required signal processing functions on the chirped or PN
spread-spectrum signals of interest, to enable highly accurate
navigation. The outputs of the CPSP block are the relative PN code
or chirp timing epochs of each of the cellular base station BS
spread-spectrum signals being tracked; as discussed earlier, up to
seven such signals may be tracked for a typical hexagonal cellular
configuration. Also, as discussed in earlier sections, a minimum of
three "high quality" signals must be tracked to enable a highly
accurate 2D navigation solution. As such, considerable diversity
and robustness is built into this navigation approach.
2. The timing and other data output from the CPSP is fed into the
Navigation Processor block NP, which converts the relative timing
information from CPSP into actual user position (e.g.,
latitude/longitude).
While several embodiments of the CPSP block may exist, two unique
embodiments for DSSS navigation beacons are illustrated in FIGS. 6
and 7, respectively. In addition, one embodiment of the CPSP block
for CSS navigation beacons is illustrated in FIG. 8. It should be
emphasized, however, that other embodiments that may exist would
also be included in this invention. Key features of each of the
processing approaches are described in the following.
CPs Processor 1 (for DSSS Navigation Beacons)--Time Domain Approach
using CCD Correlators
The heart of the CPSP in FIG. 6 is the serial combination of the
two Charge Coupled Device (CCD) blocks shown. The processing shown
here is a unique application and extension of the IF sampled CCD
demodulation described in the above-referenced patent and another
patent application that is currently pending. The key features of
this processing are as follows:
1. The IF is sampled at a carefully selected rate, so that
successive samples effectively represent in-phase and quadrature
baseband components. This sampling rate is (4/k).times.the IF,
where k is an odd integer. In other words, baseband components are
generated here without the need for mixer components. Also, these
operations are performed at high speed and low power consumption
without A/D conversion, since the CCD is an analog device. As seen
in FIG. 6, a single A/D converter is required after all CCD
processing is complete. Further details are described in the above
cited patent and patent application, which also discusses the
significant benefits in size, power consumption, and programmable
flexibility.
2. The first CCD is implemented as a programmable transversal
filter, CTF, whose tap weights may be programmed with multi-bit
(e.g., 8) quantization, to enable shaped filtering. In the present
application, the specific CTF of interest "notches" out the
high-power, narrowband communication signals that may degrade
spread-spectrum signal demodulation.
a. When the user's unit is first turned on, it tunes to the
cellular Control Channel (CC) designated to the BS for the cell
that the user is in. For AMPS and TDMA, this represents a 30 KHz
channel, while for QCDMA, the control channel is another
spread-spectrum signal. For AMPS and TDMA, immediately following
turn-on, the CTF shown is bypassed--i.e., the switch shown is
closed--thereby precluding notching. The reason for this is that
notching is not required to acquire and accurately track the spread
spectrum signal transmitted by the BS that is closest to the user;
this has been explicitly shown in our previous patent application
(Position Enhanced Communication System Including System for
Embedding CDMA Navigation Beacons Under the Communications Signals
of a Wireless Communication System)). The CCD following the
transversal filter is used to acquire and track the desired
spread-spectrum signal this CCD is discussed further below.
b. The CTF is always bypassed for QCDMA, since notching is never
needed when operating in the QCDMA spread-spectrum environment.
c. The AMPS or TDMA CC provides the CPSP with the frequencies of
the communication channels employed by the BS in the user's cell,
once this data is received by the CPSP, it programs the taps of the
CTF to "notch" out these "strong" narrowband signals, thereby
virtually eliminating the "near-far" problem. This notching remains
in place as navigation processing is in progress. Furthermore, when
the user moves to another cell, and the frequencies of the "strong"
communication channels change, the CTF tap weights are
appropriately adjusted based on data provided by the new CC
designated to the user's new cell BS.
3. The CTF is followed by another CCD, CCF, that is sampled at the
same rate as the CTF, CCD, CCF has fixed tap weights matched to the
PN code of interest, and provides the extremely rapid PN code
acquisition that is essential for applications such as 911. The
uniqueness of this implementation is noted. Specifically, for the
Table 3A System A of interest, a single CCD PN matched filter
correlator is all that is needed to process all CPS spread spectrum
signals across the entire cellular system. This is because the same
PN code is transmitted by all cell BSs, with discrimination among
sites being executed via the combination of the time diversity and
code phase diversity described in Sections 3 and 4.
4. The CCD, CCF output is A/D converted, to enable efficient
post-processing as shown. The A/D operation is advantageous here
since the CCD output SNR is much higher than at the input.
Furthermore, CCD technology has advanced to the point where an
ultra-low power AID may be directly incorporated onto the CCD chip
itself, wherein the A/D is implemented using "charge-domain"
processing techniques.
5. The A/D output is averaged and algorithmically processed to
determine the PN code correlation peaks--hence, their relative
timings--thereby yielding the desired timing data for transfer to
the Navigation Processor. As also noted in FIG. 6, the algorithmic
processing of the Digital Processor, DP, further provides AFC
frequency control corrections, as necessary, to compensate for
offsets in the user's local oscillator. These frequency corrections
are executed by suitable adjustment of the CCD IF sampling
rate--another unique feature of this implementation!
CPS Processor 2 (for DSSS Navigation Beacons)--Frequency Domain
Approach
A frequency domain equivalent to the above is shown in FIG. 7. In
this scenario, the IF input to the CPSP is first A/D converted, and
all subsequent processing is performed digitally:
1. The procedure at service turn-on is virtually identical to the
time domain approach, in that the CC of the user's BS is first
processed to identify the spectral locations of the "strong"
communication channels associated with the user's BS.
2. For this FFT process fixed blocks of data are collected and
stored at a time. In the present illustrative case, wherein the IF
is sampled at four times the PN chip rate, each block of
4.times.1023 samples encompasses the full PN code cycle. The
collected samples are then used to generate the associated FFT
(zero padding may also be used, as necessary or desired for
additional resolution).
3. For initial acquisition and tracking of the user BS's cellular
positioning system CPS signal, no notching is required, as
discussed in the time domain approach. Once locations of the
"strong" signals are identified via the CC, notching is readily
implemented by zeroing out the appropriate portions of the FFT.
4. PN code correlation/despreading is implemented in the frequency
domain by multiplying the FFT by the complex conjugate of the FFT
of the cellular positioning system CPS PN code; this complex
conjugate is stored as the array II*(w).
5. The result of the multiplication is processed by an inverse FFT
(IFFT) to yield the time-domain correlation function. Once this
IFFT is averaged, to enhance SNR, the desired correlation
peak--and, hence, the desired PN code epoch--is readily extracted.
The IFFT may further be processed to yield frequency correction
information, to correct for local oscillator offset, and the result
fed back to the Complex Multiply block shown, CM.
6. As in the time-domain approach, for System A of Table 1, three
PN code epochs are obtained from each IFFT, wherein the epochs are
suitably and unambiguously spaced.
7. Because the FFT and IFFT operations are computationally
intensive--especially when the length is on the order of 4000 or
greater--the FFT/IFFT operations need not be performed on
contiguous data. Thus, for example, a 1 ms data block (1 PN code
cycle) is collected and processed over several ms, followed by
additional data collection and processing. Because each FFT/IFFT
encompasses three PN code epochs, this mode of operations still
offers efficient acquisition and tracking at acceptable levels of
computation power.
CPS Processor (for CSS Navigation Beacon)
A comparable processing chain for CSS beacons is illustrated in
FIG. 8. The CSS beacons are assumed to have a period of 5 msec
during which they sweep over 10 MHz of frequency. In this
implementation, the key features of the signal processing are as
follows:
1. The signals enter at RF and are mixed with the output of a
synthesizer SYN. The synthesizer SYN produces a frequency staircase
at 10 KHz steps (each lasting 5 .mu.sec). Thus, in 5 msec (1000
stairs), the synthesizer moves over 10 MHz of frequency. The
nominal intermediate frequency (IF) resulting from the mixing
process is 70 MHz. In general, there will be a number of chirped
navigation signals arriving at the navigation receiver, along with
a number of narrowband signals of the communications system. At the
outset, the navigation receiver conducts a search to first acquire
a strong navigation signal. The process by which this is done is a
"largest of" detection algorithm based upon a search over all
phases of chirp. This process of acquisition and tracking of the
first signal is described in items 2 and 3 below. The acquisition
and tracking of subsequent navigation beacons is described in item
4 below.
2. During acquisition, the output of the mixing process due to a
received chirped navigation beacon is a series of short chirps each
with a duration of 5 usec. If the synthesizer is roughly in phase
with the navigation signal, the chirps will be within 10 KHz of the
70 MHz IF. If the synthesizer is out of phase with the incoming
chirped signal, the chirps after the mixing process can be up to
.+-.MHz offset from the 70 MHz IF. The CCD sampling rate is 20 MHz
and the CCD has fixed binary tap weights so that alternate output
samples of the CCD are a moving sum of the in-phase (I) and
quadrature (Q) components of the input signal. The moving sum over
5 .mu.sec is essentially a low-pass filter with a first null out at
200 KHz. The output of the CCD is then sampled by the A/D stage of
the receiver at 5 .mu.sec intervals (after each frequency stair).
Thus the sampling rate of the A/D for I/Q pairs is 200 KHz, and
further processing by the navigation receiver is all digital.
a. Following the A/D step, the signal is further accumulated
coherently for 0.1 msec which is 20 samples of the CCD; with this
step, the filter bandwidth of the receiver has been collapsed down
to 10 KHz to the first null. The maximum allowable time for
coherent accumulation is controlled by the drift in the local
oscillator (that is driving the synthesizer) relative to the
incoming received navigation signal. If the local oscillator (LO)
is accurate to .+-.1 part in a million, the relative frequency
drift for a 900 MHz navigation signal will be about .+-.900 Hz.
Over 0.1 msec, a 900 Hz offset will result in about a 30.degree.
drift in the phase of the I and Q samples. For drifts larger than
this, cancellation among the accumulated samples will occur.
b. This coherent accumulation is followed by envelope accumulation
for 5 msec (50 coherent accumulation intervals); this value is then
dumped and stored in memory. This value stores the detection
statistic for a search of an incoming chirp signal with a bandwidth
that is roughly within 5 KHz of the chirp that is generated by the
synthesizer.
c. At this point, the timing of the frequency staircase is advanced
by 2.5 .mu.sec and the accumulation resumes again for 5 msec; the
resulting output is again dumped to memory. The effect of the 2.5
.mu.sec advance in the phase of the synthesizer is a shift in the
frequency search bandwidth of 5 KHz.
d. This process continues for 2000 iterations so that all phases of
the code are explored with a quantization of 2.5 .mu.sec in time
and 5 KHz in frequency (e.g., 2000.times.5 KHz=10 MHz); the largest
of the 200 iterations is chosen, and with this detection, the phase
of the strongest chirped navigation signal has been acquired. The
estimated acquisition time for this first signal is thus
2000.times.5 msec or about 10 seconds.
3. During tracking, the control of the synthesizer is turned over
to a tracking loop. As in acquisition, the CCD output is a series
of I/Q pairs at a 200 KHz rate. The DSP then implements an
accumulator over a time interval of from 1 msec to 10 msec which
corresponds to a tracking bandwidth of 1000 Hz and 100 Hz,
respectively. The tracking accurate (.DELTA.x) is dependent upon
the resolution of the frequency tracking bandwidth and is governed
by the following formula: ##EQU2##
where,
.DELTA..function.=frequency resolution
.tau.=the period of the chirp=5 msec
C=the speed of light, and
F=chirp swing=10 MHz
Thus, a 100 Hz resolution corresponds to a 15 meter accuracy for
the pseudorange tracking measurement. However, multipath delays of
up 1 .mu.sec (300 meters) can be expected; thus, at times this will
create a broadening of the frequency on the order of 1000 Hz. Thus,
it is necessary to have an adaptive tracking loop that spans the
frequency band created by the multipath.
4. Upon acquisition and tracking of a first navigation signal, the
parallel receiving channel in FIG. 8 begins operation. The
acquisition and tracking for subsequent navigation signals is
similar to that of the first navigation signal, but there are some
significant differences. Firstly, with the acquisition of a first
navigation signal, the search space for the chirp phase is reduced
by about a factor of 10 so that subsequent acquisitions are
accomplished in about 1 second. In addition, there is a reasonable
chance that the subsequent navigation signals will be interfered
with by strong narrowband communications signals from a nearby cell
site. In this case, this interference must be mitigated by a
time-gated switch that eliminates this interference by nulling out
the received signal when the synthesizer passes through the
occupied narrowband communications channels. In this manner, the
communications channels can be attenuated by 40 dB or more, and
allow the reception of much weaker navigation signals from more
distant cell site. To achieve this nulling, a sliding window of FIR
filter is implemented that has a sharp roll-off so that interfering
signals that fall outside the band are greatly attenuated; when the
interfering signals cross the FIR bandwidth, the time gate nulls
out the incoming signal. Thus the FIR filter, in combination with
nulling at the appropriate times, effectively and dramatically
reduces the interference resulting from strong communications
signals during both acquisition and tracking. This concept is
illustrated in the functional receiver design shown in FIG. 9. The
wideband chirped signal and the narrowband interferer from wideband
filter WF are correlated in correlator COR with signals from chirp
generator CG (with frequency offset) and under control of
accumulator/detector ACD. An electronic switch or time gate ES is
opened when the interferer crosses the passband frequency. The
correlation peak of the incoming chirp and the mooring peak of the
interfering signals are shown in the inset in the lower right side
of FIG. 9.
SUMMARIZING THE INVENTION HAS THE FOLLOWING NOVEL FEATURES
The use of a set of direct sequence spread spectrum signals (with
properties described by time slot of operation, specific PN code,
PN code phase, and carrier frequency) to comprise a cellular array
of navigation beacons that is used for position location by mobile
or portable terminals. The system of beacons may be a stand-alone
system, or an overlay of a cellular communications system in which
the beacons occupy the same spectrum as the communications
system.
The use of a set of chirped spread spectrum signals to comprise a
cellular array of navigation beacons that is used for position
location by mobile or portable terminals. The system of beacons may
be a stand-alone system, or an overlay of a cellular communication
system in which the beacons occupy the same spectrum as the
communications system.
The use of navigation beacons that use a common frequency and a
common PN code, but are distinguished by a different phase offset
of the PN code epoch relative to the 1 msec time epoch. In the
terrestrial environment, a unique phase offset in the code relative
to the 1 msec time epoch can provide a unique signature for a
navigation beacon in a local geographical region.
The use of chirped navigation beacons that use a common frequency,
but are distinguished by different phase offsets of the chirp
relative to the 1 msec time epoch, and different sweep rates within
a common frequency band. In the terrestrial environment, a unique
phase offset in the chirp relative to the 1 msec time epoch can
provide a unique signature for a navigation beacon in a local
geographical region composed of a number of otherwise identical
beacons.
In a cellular communications system with a cellular positioning
system CPS overlay, the use of the cellular system control channels
to convey the navigation "almanac" to mobile and portable users.
The "almanac" is comprised of the data needed to convert a set of
pseudorange measurements into a position, and includes a list of
the cellular broadcast locations and a characterization of the
navigation beacons that are broadcast from each location.
The use of NVRAM for the storage of the bulk of the "almanac" data
which is unchanging except insofar as the cellular system and/or
its navigation beacons are modified as part of system
evolution.
The use of DSSS or chirped spread spectrum CSS navigation beacons
that are uniquely characterized in a local region by their assigned
signal characteristics so that data modulation of the beacons is
not required for beacon identification.
The use of frequency notching in a DSSS or chirped spread spectrum
CSS navigation receiver to filter out the interference caused by
the occupied narrow band communications channels of a cellular
communications system. In the chirped spread spectrum CSS receiver,
the use of signal attenuation when the chirped spread spectrum CSS
signal sweeps through the occupied communications channels as a
novel implementation of the frequency notch. The use of a cellular
system broadcast control channels to convey knowledge of the
occupied slots that they maybe notched from the receiver.
The fact that a common antenna and RF front end is applicable for
both communications and navigation is a unique and novel feature of
this invention.
The implementation of the cellular positioning system CPS
navigation receiver/processor using time-domain and
frequency-domain approaches.
While the invention and preferred embodiments have been shown and
described, it will be appreciated that various other embodiments,
modifications and adaptations of the invention will be readily
apparent to those skilled in the art.
* * * * *