Note: Descriptions are shown in the official language in which they were submitted.
CA 02605880 2007-10-30
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CHIRP SLOPE MULTIPLE ACCESS
The present invention relates to chirp radio communication systems and, more
specifically, the invention relates to increasing the number of transmitters,
or users, that can be
simultaneously accommodated in the communication system and the data capacity
of a chirp
radio communication systems without requiring additional radio frequency
bandwidth.
Many radio communication systenis require predetermined bands of radio
frequency
for accomplishing conununications. The frequency bandwidth required for
accomplishing
communications depends on a plurality of factors. A prlinary factor is the
required data
capacity of the communication system. In a chirp radio communication system, a
choke-point
io for data capacity is typically the ability of the system to correlate and
detect chirp signals.
In gerieral, increasing frequency bandwidth increases data capacity and allows
more
users to be simultaneously accommodated in the communication system. However
the amount
of radio frequency available for public use is limited. Regulations may limit
the acquisition and
licensing of frequency bands. In addition, the cost of acquiring frequencies
appears to have
is substantially increased over the years.
An object of the present invention is to provide a method for increasing data
capacity
of the communication system without requiring additional bandwidth.
Another object is to provide a novel system for increasing the number of
transmitters
dr users that can be simultaneously accommodated in the communication system
operating
20 within a geographic area without increasing frequency bandwidth, and also
to provide a chirp
radio communication having plural transmitters transmitting chirp radio
signals of different
slopes as a method of increasing the data capacity.
The present invention also includes a chirp radio communication system for
transmitting
and receiving a first chirp signal of a first predetermined slope, the first
predetermined slope
25 continuously increasing in frequency over a predetermined frequency band
during a
predetermined amount of tiune, characterized in that the transmitting and
receiving a second
chirp signal having a different slope from the first predetermined slope over
the predetermined
frequency band in the same geographic area, so that the data capacity of the
system is
significantly increased without increasing the first predetermined frequency
band.
30 The invention will now be described, by way of example, with reference to
the
accompanying drawings, in which:
Figure 1 is a simplified pictorial representation of a communication system of
the present
invention;
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CA 02605880 2007-10-30
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Figure 2 is a timing diagram illustrating an encoding technique used in a
system of the
present invention.
Figure 3 is a graph illustrating a chirp signal which is in a system of the
present
invention.
Figure 4 is functional block diagram illustrating a technique for
discriminating among
transmitter types in a system of the present invention.
Figure 1, illustrates a chirp radio communication system that include plural
transmitters
10, which may be stationary or mobile, which are in communication with one or
more receiving
stations or base stations 12. The receiving stations 12 communicate with a
central station 14.
io This communication is by way of conventional telephone circuits 13. The
central station 14
include a control console 16, a storage unit 18, and means for communicating
20 with other
central stations 14 or external systems. A common clock signal 22 provided by
a geosta tionary
satellite system to each of the base stations 12. '
In operation, the transmitters 10 periodically or aperiodically transmit a
beacon or signal
is to the base station(s) 12 within the range of its transmitted signal. The
receiving station(s) 12
receive the beacon signal from the transmitters 10 and may associate with such
signals a time
of arrival. Information regarding the signal which was received and the time
of its arrival may
be communicated by the receiving station(s) 12 through conventional means to a
central station
14. The receiving station(s) 12 coordinated in time through the receipt of a
clocking signal 22
20 from a common source, such as the satellite system 24.
The signals transmitted by the transmitters 10 may include an ideritification
of the
specific transmitter 10 which sent the signal, an indication that one or more
events have
occurred at the transmitter 10, a data portion relating to an activity or
condition at the
transmitter 10 (such as, without timitation, a temperature, a flow rate, a
pressure reading, etc.),
25 an indication that emergency assistance is required at the transmitter 10,
and practically any
other condition, indication, information, or circumstance which may be
digitally encoded.
When the signals transmitted by the transmitters 10 are received at the
receiving stations
12, information regarding the signals and their times of arrival may be
cornmunicated to the
central station 14 for further analysis. Depending upon the type(s) of signals
being
so communicated, the central station 14 may store related information on the
storage unit 18, may
transmit information regarding the signals to other central stations 14 or to
other systems (not
shown).
. For example, if the signals represent events which have occurred at a
transmitter 10,
information regarding the occurrence (and time) of the event may be sent to
another system for
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operations or control purposes. Such a system could include a detector which
detects an
improper entry into a building and triggers an event signal at a transmitter
10. When the entry
event is received by the central statfon 14, the central station 14 may notify
=a local police
department of the event and the location of the transmitter 10 for appropriate
police response.
For another example, the signals could include data from a medical sensor
attached to
a user of the transmitter 10. When passed to the central station 14, the
signals could be used to
determine the present health of the user or to record (using the storage unit
18) the physical
characteristics of the user over time. If the user's health were determined to
need assistance, the
signals from the receiving stations 12 could be analyzed to determine the
geolocation of the user
io so that medical personnel could be directly dispatched. Finally, the
signals could merely
identify the transmitter 10 and its location. Such a system could be used, for
example, to
monitor the instantaneous or history of the location of each truck in a fleet
of delivery trucks.
The signals from the transmitters 10 may have been encoded with digital
information
(signifying the identification of the transmitter, events, data, etc.). A
method of encoding data
on the chirp waveform for this embodiment is depicted in Figure 2. It is to be
understood that
the particular frequencies, times, number of starting points to each chirp,
the number of chirps
in the series, etc. in the following embodiment are illustrative only and are
not intended to limit
the scope of the invention. The transmitter sends a series of six chirps
(which are simply a linear
sweep from one frequency to another) nominally from fl to f2, a bandwidth of
approximately
4 MHz. Each chirp occurs in 8.192 milliseconds. The total time occupied by the
waveform is
therefore 49.152 milliseconds and the sweep rate is 0.5 MHz per millisecond
o:r 500 Hz per
microsecond.
Data is encoded on the waveform by starting the second and subsequent chirps
in the
series (the "subsequent chirps") atfrequencies that may be differentfrom the
non-dnal frequency
2s by a specific amount depending on the data to be encoded. The encoding
technique assigns one
of 32 possible starting points to each subsequent chirp, thus conveying 5 bits
of digital data (25
= 32) in each subsequent chirp. The increment of the offset for the starting
points is chosen as
1 kHz, which represents 2 microseconds at the sweep rate of the chirp.
Therefore, each of the
subsequent chirps in the series are started at a frequency of f, = fl +(m *
1kHz) where m is a
value from 0 to 31 representing the desired 5-bit synibol. The value of m can
be the same or
different for each of the subsequent chirps.
The chirp waveform can be received by a correlator which acts as a matched
filter by
comparing the value of each sample of the received waveform to the desired
waveform and
outputs a numerical value equal to the number of samples that matched the
desired waveform.
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It is a property of such receivers that the correlation of a chirp with a time
shifted value of itself
is poor. Thus the output of the correlator resembles a series of spikes which
occur at the point
that the incoming waveform best aligns with a replica of the transmitted
chirp. This spike
occurs at different times depending on the starting frequency of the
transmitted chirp. In this
9 example, the spikes occur at a time
t- ((n-1) * 8.192 milliseconds - m*2 microseconds) where n is the number of
the chirp and m is
the transmitted symbol for the chirp n. By measuring the time delay from the
spike indicating
correlation of the first or reference chirp to the spikes for each of the
subsequent chirps, the
value of the symbol transmitted in each of the subsequent chirps and the
corresponding binary
io data can be recovered.
With reference to Figure 3, a chirp signal as used herein is a signal having a
continuously
varying frequency over a finite period of time. As depicted in Figure 3, a
chirp signal used in
the present invention may be a signal having 4 MHz bandwidth which is swept
over an 8
millisecond period. While the sweep of the chirp is shown in Figure 3 as being
upward over
15 time, the present invention is not so limited and the chirp may sweep
downward in frequency.
It is also desirable that the sweep be linear, that is, the plot of the
frequency of the signal over
time is a straight line. The use of a linear sweep permits the receiver of the
signal to use a time
invariant matched filter to decode the transmitted signal, even with a signal
that has a relatively
large frequency offset. Thus, the transmitter may be built using a relatively
inexpensive timing
20 source, such as an inexpensive oscillator with poor frequency stability and
the system will
perform satisfactorily.
The exact timing of the length of the chirps, the number of chirps in a group,
the number
of bits encoded by the frequency used in initiation of a chirp, or the amount
of delay induced
in the detection of a chirp associated with a particular bit sequence are not
limited to the times
25 and numbers used in the exemplary embodiment and can be set to any
practical values
depending upon the sensitivity and accuracy of the transmitting and receiving
equipment.
With reference to Figure 4, a chirp radio communication systern may include
plural
transmitters or tags of a first type 42, plural transmitters or tags of a
second type 44, plural
receiving stations or base stations of a first type 46, plural receiving
stations or base stations of
30 a second type 48, and a central station 50. The receiving stations of the
first type and the second
type may be co-located in a combined receiving station 52. As discussed above,
the receiving
stations 46, 48, and 52 may be in communication with the central station 50.
Plural central
stations may be located in a geographic area to communicate with different
types of receiving
stations. A difference in type may indicate a difference in chirp signals.
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In operation, transmitters of the first type maytransmit chirp signals having
a first slope
and transmitters of the second type may transmit chirp signals having a second
slope. The first
slope may be different from the second slope in magnitude and/or polarity
(e.g., the sweep of
a first chirp signal may be upward over time as in Figure 3, and the sweep of
a second chirp
signal may be downward in frequency). Preferably, the first slope is of the
same magnitude and
of opposing polarity as the second slope (il.g., opposing slopes). Preferably,
the chirps signals
sweep over the entire frequency band of the communication system.
The receiving stations of the first type 46 may include correlators for
detecting the
magnitude and polarity of chirp signal having the first slope. The receiving
stations of the
io second type 48 may include correlators for detecting the magnitude and
polarity of chirp signals
having the second slope. Preferably, the correlators in the receiving stations
may discriminate
between two types of chirp signals by detecting the chirp signal polarity.
Combined receiving stations 52 may include two separate correlators for
independently
detecting the first and second slopes. A receiving station of either type 46
or 48 may be
designed to be expandable into a combined receiving station so that
correlators for differing
chirp signal may operate from the same antenna. The correlators in a combined
receiving
station may feed a single communications processor which effectively doubles
system capacity
with a modest infrastructure increase. All of the receiving stations may
communicate with a
single central office 50. Altematively, two central stations may operate
separately to process
2o chirp signals transmitted by the two types of transmitters.
Together, the transmitters of the first type 42, the receiving stations of the
first type 46,
and a central station may be considered to be a first chirp radio
communication network and
the transmitters of the second type 44, the receiving stations of the second
type 48, and a second
or co-located central stations may be considered to be a second chirp radio
communication
network. Transmitters in one network will not be recognized by the other
network. The first
and second chirp radio communication networks may include combined receiving
stations 52.
One of the two networks may be "open" and the other "closed." "Open" may
indicate that a
chirp radio communication service is provided by subscription and "closed" may
indicate that
a chirp radio communication service that is provided for a specific customer
or set of customers.
"Open" and "closed" may also indicate a distinction between national and local
services.
A national system may allow roaming and must provide enough "address space"
for a
nationwide customer base which limits the data space. A local service may
require a smaller
"address space" and allow higher data transfer.
The use of two chirp signals provides a simple means for sharing the load in a
network
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by only increasing processor requirement and re-using antenna equipment,
communications
processor, and possibly a location processor at each receiving station. If one
chirp signal
increases over time and the other decreases over time, then the two of types
of transmitters may
coexist in the same network with a modest increase in equipment at each site
or multiple
networks may be established based on types of transmitters.
A chirp radio communication system for increasing the number of transmitters,
orusers,
that can be simultaneously accommodated in the communication system and the
data capacity
of the system without increasing frequency bandwidth. A system may include
plural
transmitters, receivers, and a central station for establishing chirp radio
communications. Each
receiver may discriminate between chirp radio transmitter types based on the
differing chirp
signal characteristics.
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