Note: Descriptions are shown in the official language in which they were submitted.
I 6
BACKGROUND OF THE INVENTION
The present invention relates generally to radio
communications systems, and more particularly to an imp
proved method and apparatus for dynamically selecting one
of a plurality of radio frequency signal transmitters for
transmitting message signals from a primary station to
remote stations of a data communications system.
In radio communications systems covering large
geographical areas, the location of remote stations such
as portable or mobile radios must be known with reason-
able accuracy in order to provide good quality commune-
cations. In such communications systems, the geographical
area may be divided up into a number of zones or cells,
each of which is covered by at least one radio
transmitter and radio receiver associated with a primary
station. For establishing communications between the
primary station and a remote station, knowledge of the
remote station's location is necessary in order that the
radio transmitter covering the zone in which the remote
station is located may be selected at the primary
station.
The problem of selecting the radio transmitter which
covers the zone in which a remote station is located has
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been solved with a limited degree of success in several
different ways. According to one technique, the radio
receiver receiving the strongest RF signal from a
selected remote station is used to define the location of
that remote station. The primary station simply selects
the radio transmitter covering the geographical area of
the receiver receiving the strongest signal from the
selected remote station.
According to another technique, each remote station
0 it assigned to a specific geographical area. In other
words, a remote station is permanently associated with
the zone or zones covered by a specific radio
transmitter. This technique works reasonably well as
long as the remote station remains within the
geographical area covered by the assigned radio
transmitter. However, this technique is inadequate for
communications systems where each remote station is free
to move about throughout a very large geographical area,
making it impossible to limit a remote station to the
coverage area of a single radio transmitter.
According to yet another technique that is utilized
in cellular radiotelephone systems, the remote station
determines the zone in which it is located by selecting
the radio transmitter having the largest signal strength.
This technique requires that each radio transmitter have
a different frequency, and that communications from the
primary station to a selected remote station be sent in
all zones in order for the remote station to make its
choice known on demand. This technique is adequate for
radio telephone systems where the average message length
is much longer than the minimum message length, but is
inadequate or data communications systems where the
average message length is not much larger than the mini-
mum message length. Therefore, in order to provide good
quality communications in data communications systems, it
~,~Z6626
is necessary to have a reasonably accurate determination
of the location of each remote station in the system.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide
an improved method and apparatus for dynamically
selecting one of a plurality of radio frequency signal
transmitters for transmitting message signals from a
primary station to a selected remote station of a data
communications system.
It is another object of the present invention to
provide an improved method and apparatus for dynamically
determining the location of remote stations of a data
communications system providing communications between a
primary station and a plurality of remote stations lo-
acted throughout a large geographical area.
It is yet a further object of the present invention
to provide an improved method and apparatus for
simultaneously transmitting message signals to two or
more remote stations located in different zones of a data
communications system.
Briefly described, the present invention encompasses
a method for use in a primary station of a communications
system for communicating message signals via a
communications medium, such as a radio channel, between
the primary station and a plurality of remote stations,
such as portable and mobile radios located anywhere in a
large geographical area that is divided into a plurality of
cells or zones. The primary station further includes a
communications controller, a plurality of transmitters
for transmitting signals modulated on a first carrier
signal of the radio channel and a plurality of receivers
for receiving signals modulated on a second carrier
signal of the radio channel. Each zone of the commune-
cations system is covered by one of the transmitters and
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covered by at least one receiver. The remote stations
each further include a transmitter for transmitting
signals modulated on the second carrier signal and a
receiver for receiving signals modulated on the first
carrier signal. The method of the present invention
enables the communications controller to select one of
the transmitters for transmitting signals from the
primary station to a selected remote station. The novel
method practiced by the controller at the primary station
comprises the steps of measuring the signal strength of
the carrier signal received by each primary station
receiver during each transmission from the selected
remote station; computing an adjusted signal strength
for each zone by adjusting the measured signal strength
for each primary station receiver by corresponding
predetermined factors associated with the zone and
combining the adjusted signal strengths selecting the
; primary station transmitter covering the zone which had
the largest adjusted signal strength for the last
transmission from the selected remote station. The
communications controller may include apparatus such as a
computer or microcomputer that is suitably programmed to
execute each step of the transmitter selecting method.
According to another feature of the present
invention, the location of each remote station may be
dynamically determined by the communications controller.
The unique locating method practiced by the controller
comprises the steps of measuring the signal strength of
the carrier signal received by each primary station
ED receiver during each transmission from each remote
station, computing an adjusted signal strength for each
zone by adjusting the measured signal strength for each
primary station receiver by corresponding predetermined
factors associated with the zone and combining the
adjusted signal strengths for each remote station, and
` selecting for each remote station the zone having the
: .
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largest signal strength for the last transmission from
that remote station. The inventive locating method can
further include the step of storing the zone having the
largest adjusted signal strength and the zone having the
second largest adjusted signal strength. Then, when
transmitting a message signal to a selected remote
station, both zones can be tried.
According to another feature of the present
invention, the communications controller can
simultaneously transmit message signals to two or more
remote stations located in different zones. The unique
method practiced by the controller comprises the steps of
assigning only one of the primary station transmitters
for covering a zone, determining the zone in which
each remote station is located, modulating a first
message signal on the first carrier signal of the primary
station transmitter assigned to a first zone in which a
first remote station is located, and modulating a second
message signal on the first carrier signal of the primary
station transmitter assigned to a second zone in which a
second remote station is located provided that the
primary station transmitter assigned to said second zone
does not cover a portion of said first zone. Since
message signals can be simultaneously transmitted to many
different remote stations, message throughput of the data
communications system is greatly enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of a data communications
system that may advantageously utilize the present
invention.
Figure 2 is a diagram of a geographical area that is
divided up into a number of zones.
Figure 3 is a block diagram of the circuitry in the
receivers in Figure 1.
~2~6Ç;2~;
-- 6 --
Figure 4 is a block diagram of the circuitry in the
channel communications modules in figure 1.
Figure 5 is a block diagram of the circuitry in the
general communications controller in Figure 1.
Figure 6 is a flow chart used by the general
communications controller for processing signal strength
data received from the channel communications modules in
Figure 1.
Figure 7 is a flow chart used by the general
lo communications controller for selecting a transmitter on
which data signals are transmitted to a selected portable
radio in Figure 1.
Figure 8 is a flow chart used by the channel
communications module for measuring the signal strength
of signals transmitted by the portable radios in Figure
1.
Figure 9 is a block diagram of the circuitry in the
portable radios in Figure l.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Figure 1, there is illustrated a data
communications system that communicates message signals
between a primary station, such as a general
communications controller (GCC) 104, by way of a
communications medium, such as a radio frequency (RF)
: communications channel, to a plurality of remote
stations, such as mobile or portable radios 130, 132 and
134. Although described in the context of a data only
communications system, both data signals and analog
signals such as voice signals can be communicated over
the RF communications channel to the portable radios 130,
132 and 134. The data communications system covers a
large geographical area which is divided into a plurality
of cells or zones. Located throughout the geographical
area are a number of channel communications modules (CAM)
106, 108, 110 and 112, which are each coupled to and
~2~t~6~5
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control a number of RF signal transmitters 114, 120, and
124 and RF signal receivers 116, 118, 122, 126 and 128.
The RF communications channel is preferably comprised
of first and second carrier signals which may be modulated
with the message signals. Transmitters 114, 120 and 124
may be operative on the first carrier signal, while receivers
116, 118, 122, 126 and 128 may be operative on the second
carrier signal of the radio communications channel. Each
zone of the radio communications system is covered by an
10 assigned one of the transmitters 114, 120 and 124 and by at
least one of the receivers 116, 118, 122, 126 and 128.
Transmitters 114, 120 and 124 and receivers 116, 118, 122,
126 and 128 may be any suitable commercially available
transmitters and receivers such as those described in Motorola
15 Instruction Manual POW. Cams 106, 108, 110 and
112 may be co-located with their corresponding transmitters
and receivers or may be remotely located and coupled to
their corresponding transmitters and receivers by means of
a suitable remote control system, such as, for example,
20 the tone remote control system described in US. Patent
Number 3,577,080.
Portable radios 130, 132 and 134 may be either
commercially available mobile radios of the type shown
and described in Motorola instruction manual no. POW
25 or commercially available hand-held portable radios of the
type shown and described in US. patent numbers 3,906,166,
3,962,553 and Number 4,486,624 entitled "Microprocessor
Controlled Radiotelephone Transceiver", and invented by
Larry C. Purl et at. Portable radios 130, 132 and 134 each
30 include a transmitter operable on the second carrier signal
and a receiver operable on the first carrier signal. The
transmitter and receiver in portable radios 130, 132 and
134 may be any suitable commercially available
2;266~i
-- 8 --
conventional transmitter and receiver, such as, for example,
the transmitter and receiver described in Motorola
instruction manual no's. POW and 68P81014C65. These
and the other Motorola Instruction Manuals referenced herein
are available from the Service Publications Department of
Motorola, Inc., 1301 East Algonquin Road, Schaumburg,
Illinois or from Motorola C & E Parts, 1313 East Algonquin
Road, Schaumburg, Illinois.
GCC 104 of the data communications system in Figure 1
may be coupled to a host computer 102 which may control
a number of GCC's 104 that are located in different
geographical areas, such as, for example, different cities.
Thus, host computer 102 may gather data from, and dispatch
data to, portable radios located in several different
cities. GCC 104 may be coupled to host computer 102 and
Cams 106, 108, 110, and 112 by means of commercially
available modems and associated dedicated telephone lines.
GCC 104 in Figure 1 transmits message signals to and
receives message signals from portable radios 130, 132
and 134. The message signals may include coded data
packets which each may contain a binary preamble, a
predetermined synchronization word and an information
word containing a command, status or data. The format of
the data packets may be any of a number of existing data
formats, such as, for example, those described in US.
patent numbers 3,906,445, 4,156,867 and 4,354,252, and in
Canadian Patent Application Serial Number 433,332 entitled
"Data Signaling System", filed July 27, 1983 and invented
by Timothy M. Burke et at.
Message signals are routed by GCC 104 to a solitude
CAM 106, 108, 110 and 112 for transmission by its
corresponding transmitter. Since the message signals are
not transmitted on all transmitters simultaneously, as in
simulcast systems of the type described in US. Patent
Number 4,188,522, it is necessary that GCC 104 have a
6~6
reasonably accurate determination of the location of each
portable radio 130, 132 and 134 so that GCC 104 may
select the transmitter 114, 120 or 124 which covers the
zone in which a particular portable radio is located.
The improved method and apparatus of the present
invention enable GCC 104 to dynamically select the
transmitter 114, 120 or 124 for transmitting a message
signal to a selected portable radio 130, 132 or 134.
According to another important feature of the
lo present invention, two or more of the transmitters 114,
120 or 124 can be operated simultaneously for
communicating with different portable radios located in
different zones provided that transmissions from the two
transmitters do not interfere with reception in the
particular zones where the two portable radios are
located. As a result, data throughput of the data
communications system illustrated in Figure 1 can be
significantly increased by reuse of the RF commune-
cations channel. In other words, by taking advantage of
reuse, a single RF communications channel can serve
thousands of portable radios in a geographical area
covering several states and their major cities.
Referring to figure 2, there is illustrated a
geographical area of a data communications system that is
divided into seven zones, Z1-Z7, and that includes three
Cams 210,220 and 230 and corresponding transmitters and
receivers. Transmitter To of CAM 210 has a coverage area
within circle 212, transmitter To of CAM 220 within
circle 222, and transmitter To of CAM 230 within circle
232. Each time a portable radio transmits, signal
strength readings are taken by receivers R1, R2 and R3.
These readings can be expressed by the following signal
strength SKI matrix:
[SKI] = SUE SUE SUE].
- 35 According to the present invention, the signal
strength readings taken by receivers R1, R2 and R3 can be
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used to compute an adjusted signal strength for each zone
Z1-Z7 by adjusting the measured signal strength for each
receiver R1, R2 and R3 by corresponding predetermined
factors associated with the particular zone and then come
brining the adjusted signal strengths. The predetermined
factors used to compute the adjusted signal strength
depend on a number of factors such as the terrain, the
height and gain of the antennas, and the sensitivity of
the receivers. In other words, the predetermined factors
lo associated with each zone are empirically determined and
depend upon the characteristics of the equipment and
terrain in each data communications system. The pro-
determined factors can be arranged in a zone selection
ZSEL matrix, such as, or example, the exemplary ZSEL
matrix hereinbelow:
15.5 0 0 10.7 10.4 0 7-7
[ZSEL] = 0 15.3 0 0 9.8 10.2 7.5
I 0 15.7 lo 0 if 7.4~
The adjusted signal strength ZADJ matrix for each of the
zones Z1-Z7 is then computed according to the following
matrix formula:
[ZADJ] = [SKI] X [ZSEL]; or
[ZADJ] = [ZlADJ Z2ADJ Z3ADJ Z4ADJ Z5ADJ Z6ADJ Z7ADJ]
Then, using the ZADJ matrix, GCC 104 can select the zone
which has the largest adjusted signal strength for a
particular transmission from a portable radio. The
selected zone can be stored together with other data in a
location of the memory of GCC 104 associated with that
portable radio.
Whenever transmitting a message signal to that
particular portable radio, GCC 104 will first transmit
the message signal on the carrier signal of the
transmitter that covers the zone which had the largest
adjusted signal strength for the last transmission from
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that portable radio. Both that zone and the transmitter
covering it are stored in the memory of GCC 104. If the
portable radio does not acknowledge the transmission of
the message signal from GCC 104, GCC 104 may attempt one
or more retransmission of the message signal by means of
that selected transmitter. If the retransmission
likewise are not acknowledged by the portable radio, GCC
104 may then transmit the message signal via the
transmitter covering the zone which had the second
largest adjusted signal strength for the last
lo transmission from that portable radio. Again, if the
portable radio does not acknowledge the transmission from
GCC 104, GCC 104 may resend the message signal one or
more times by means of that selected transmitter. If GCC
104 does not reach the selected portable radio by means
of these two transmitters, GCC 104 may either select
another transmitter covering that portable radios "home"
zone, or initiate a polling sequence in which the
selected portable radio is polled in every zone in the
data communications system starting with the portable
radio's "home" zone.
Assuming that the SKI matrix is 10, 10, 10 for a
transmission from a selected portable radio, the ZADJ
matrix will be 155, 153, 157, 207, 202, 212, 226 using
the predetermined factors in the above ZSEL matrix. For
this particular transmission from that portable radio,
the zone having the largest adjusted signal strength is
zone Z7 and the zone having the second largest adjusted
signal strength is zone Z6. Referring to Figure 2, the
portable station is most probably located in zone Z7
which is approximately midway between Cams 210, 220 and
230. The second most likely location of the portable
station is zone Z6 which is between Cams 220 and 230.
The transmitters To, To and To in Figure 2 can be
assigned to cover the seven zones as follows Zone Z1 is
I covered by To, zone Z2 is covered by To, zone Z3 is
'~,Z~6~?26
- 12 -
covered by To, zone Z4 is covered by To, zone Z5 is
covered by To, zone Z6 is covered by To, and zone Z7 is
covered by To. For transmitting a message signal to the
portable radio, transmitter To is used first since zone
Z7 has the largest adjusted signal strength. If the
portable radio does not acknowledge the first transmit-
soon or subsequent retransmission from transmitter To,
the message signal is next transmitted by transmitter To
for covering zone Z6, which had the second largest
lo adjusted signal strength for the last transmission from
the portable radio.
Assuming that on a subsequent transmission from the
portable radio the SKI matrix is 10, 10, 0, the ZADJ
matrix is 155, 153, 0, 107, 202, 102, 152. In this case,
zone Z5 has the largest adjusted signal strength, and
zone Z1 has the second largest adjusted signal strength.
Therefore, a message signal would first be transmitted by
; transmitter To for covering zone Z5, and thereafter
transmitted by transmitter To for covering zone Z1.
Again, assuming that a subsequent transmission from
the portable station results in an SKI matrix that is 0,
10, 10, than the ZADJ matrix is 0, 153, 157, 100, 98,
212, 149. In this case, zone Z6 has the largest adjusted
signal strength, and zone Z3 has the second largest
adjusted signal strength. Since transmitter To covers
both zone Z6 and zone Z3, a message signal transmitted by
transmitter To will reach the portable radio if it is in
either zone Z6 or zone Z3. For a subsequent
transmission, zone Z2 has the third largest adjusted
signal strength and is covered by transmitter To.
Next, the transmitter reuse feature of the present
invention may be illustrated by the seven zone arrange-
- mint in Figure 2. First of all, there is no transmitter
; interference for communications to portable radios
located in zones 21, Z2 or Z3. That is, transmitter To,
To and To can be operated simultaneously for
I
- 13 -
communicating with portable radios in zones Z1, Z2 and
Z3, respectively. However, for zone Z4, transmitter To
must be off; for zone Z5 transmitter To must be off; for
zone Z6 transmitter To must be off; and for zone Z7
transmitters To and To must be off. Using the foregoing
interference criteria, transmitter reuse is possible for
all zones except for zone Z7. For example, if the
portable radio is located in zone Z4, transmitter To is
used to communicate with that portable radio, and
lo transmitter To can be simultaneously operated for
communicating with portable radios in zone Z2.
Similarly, while transmitter To is used for communicating
with a portable radio in zone Z6, transmitter To must be
off and transmitter To can be on. In this case, trays-
miller To could be on and communicating with a portable
radio located in zone Z1. Both a transmitter selection
SWAHILI matrix and a zone interference ZIP matrix can be
used to show the above criteria. The SWAHILI matrix is as
follows:
To To To
Z1 1 0 0
Z2 0 1 0
Z3 0 0
[ SWAHILI] c Z4 1 0 0
Z5 0 1 0
Z6 0 0
Z7 1 0 0
A one in the SWAHILI matrix indicates that the trays-
miller in that column is used for communicating with a
portable radio located in the zone in that row.
The ZIP matrix is as follows:
~.22~
- 14 -
To To To
Z1 0 0 0
Z2 0 0 0
Z3 0 0 0
SOPHIE] = Z4 0 0
Z5 1 0 0
Z6 0 1 0
Z7 0
A one in the ZIP matrix means that the transmitter in
lo that column cannot be transmitting if it is desired to
communicate with a portable radio located in the zone in
that row.
Both of these matrices can be provided by tables
that are stored in the memory of GCC 104 in Figure 1.
GCC 104 uses both of this matrices during the process of
selecting a transmitter for communicating a message
signal to a selected portable radio. For example,
assuming a portable radio is in zone Z5, transmitter To
is used and transmitter To must be off.
Referring to Figure 3, there is illustrated a
detailed circuit diagram of the receivers 116, 118, 122,
126 and 128 associated with Cams 106, 108, 110 and 112
in Figure 1. Each receiver includes two antennas spaced
at a predetermined distance from one another and a
maximal ratio predetection diversity combiner 312, 314,
316, 318, 320, 322, 324, 326 and 328 for combining the
signals received by each of the antennas. The space
diversity provided by the two antennas is utilized to
prevent degradation in communications which results when
an antenna is located in an RF signal null. Rapid and
deep RF signal nulls, called Raleigh fading, are
experienced in communications systems operating at RF
signal frequencies in the new 800 to 900 mHz frequency
range. The maximal ratio predetection diversity combiner
coiffeuses the RF signals from each antenna and linearly
adds the cophased signals to provide a composite signal
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- 15 -
having components that are proportional to the square of
the RF signals from each antenna. Therefore, strong
signals are emphasized much more than weak signals. In
other words, communications are not adversely affected if
a very weak signal is received by one antenna and a
reasonably good signal is received by the other antenna.
In the diversity receiver in Figure 3, the frequency
of local oscillator 306 determines the radio channel
to which the diversity receiver is tuned. The RF signal
lo received by each antenna is combined by mixers 302 and
304 with the signal from local oscillator 208 to provide
corresponding IF signals. The IF signal from mixers 302
and 304 is then applied to IF band pass filters 308 and
310, respectively, which may be a monolithic band pass
filter of conventional design similar to that described
in US. patent no. 3,716,808. The filtered IF signals
from filters 308 and 310 are split and fed forward via
two paths to mixers 312, 324 and 314, 326, respectively.
; First portions of the IF signals are applied to mixers
324 and 326, and second portions of the IF signals are
applied to mixers 312 and 314 together with the composite
- IF signal which is fed back from amplifier 330. By
feeding back the composite IF signal, the IF strip of the
diversity receiver forms a closed feedback loop that is
regenerative on noise. Thus, the randomly varying phase
of the IF signals from filters 308 and 310 relative to
the composite IF signal is added into the closed loop via
mixers 312 and 314 and then subtracted out at mixers 324
and 326, respectively. By this process, the random phase
variations are removed from the If signals in relation
; to the composite IF signal. The result is that each of
the IF signals is cophased to the composite IF signal.
The product signals from mixers 312 and 314 at the
difference frequency are applied to filters 316 and 318,
respectively, which each provide a variable phase shift.
Filters 316 and 318 may be two-pole crystal filters. The
'2t:;~;Z~
- 16 -
signals from filters 316 and 318 are linearly amplified
by amplifiers 320 and 322, respectively, and applied to
the second input of mixers 324 and 326, respectively.
Mixers 324 and 326 multiply the signals from amplifiers
320 and 322, respectively, with the IF signals from
filters 308 and 310, respectively, to provide product
signals that are cophased with the composite IF signal.
The product signals from mixers AYE and 326 are both
cophased and proportional to the square of the level of
the IF signals from filters 308 and 310, respectively.
The product signals from the mixers 324 and 326 are
linearly added by summer 328 to form one composite IF
signal. The composite IF signal may be coupled via
amplifier 330 to a conventional FM detector 332 which has
an output signal providing demodulated message signals.
The output signal of FM detector 332 is coupled to its
corresponding CAM 106, 108, 110 or 112 in Figure 1.
Further details of the circuitry in the diversity
receiver in Figure 3 are illustrated and described in the
20 instant assignee's US. Patent No. 4,369,520 entitled
"Instantaneously Acquiring Sector Antenna System", and
invented by Frank J. Corny, Jr. and James J. Mikulski,
and in Canadian Patent No. 1,141,431 entitled "Large
Dynamic Range Multiplier for a Maximal Ratio Diversity
Combiner", and invented by Frank J. Corny, Jr.
Figure 3 also illustrates the circuitry 340, 348 and
350 comprising the signal strength detector that is
located in the receivers. Summer 340 is coupled to the
signals from filters 308 and 310 and provides a composite
signal which is coupled to amplifier 348. The output of
amplifier 348 is coupled to envelope detector 350 which
provides an SKI signal that is proportional to the maxima
of the composite signal from amplifier 348. A separate
amplifier 348 and envelope detector 350 can be provided
35 for each of the signals from filters 308 and 310 if it is
sty
desired to measure each separately. The SKI signal from
envelope detector 350 is coupled to its corresponding
CAM 106, 108, 110 or 112 in Figure 1, where it is
digitized. Many other types of commercially available
signal strength detecting circuitry can be utilized in
place of summer 340, amplifier 348, and envelope detector
350.
Referring to Figure 4, there is illustrated a block
diagram of the circuitry in Cams 106, 108, 110 and 112 in
Figure 1. Each CAM includes a microcomputer 402 having a
memory with stored program therein for communicating with
GCC 104 and portable radios 130, 132 and 134 in Figure 1.
Microcomputer 402 can be any suitable commercially available
microcomputer such as, for example, the Motorola type
MCKEE, MCKEE or MCKEE microprocessor, or those micro-
processors described in US. patent numbers 4,030,079 and
4,266,270, and the patents and patent applications referred
to therein.
Microcomputer 402 is coupled to RS232 interface 404
which may be coupled by a modem to a dedicated telephone
line from GCC 104 in Figure 1. Message signals received
by microcomputer 402 from the GCC may be coupled
to filter 406 and thereafter applied to its corresponding
transmitter. The message signals may be coded according
to fre~uency-shift keying, phase-shift keying or any
other suitable existing encoding scheme. Suitable
message signal coding schemes are described in the alone-
mentioned US patent nos. 3,906,445, 4,156,867 and
4,354,252 and Canadian patent application serial no. 433,332.
Message signals received from portable radios by the
Cams receiver are coupled to filter 408 and thereafter
to limiter 410 which converts the analog signals into a
non-return-to-zero binary signal. The output of limiter
410 is applied to an input port of microcomputer 402.
Microcomputer 402 also takes signal strength readings
while it is receiving message signals. The SKI
signal from its corresponding receiver is coupled to Aye
converter 412, which may continuously convert the analog
SKI signal to a digitized SKI signal. The digitized SKI
signal from A/D converter 412 is applied to an input port
of microcomputer 402. Several A/D conversions are
performed while a message signal is being received. The
digitized SKI signals for the several conversions are
averaged by microcomputer 402. The average SKI signal is
appended to the received message signal which is sent by
lo microcomputer 402 via RS232 interface 404 to GCC 104 in
Figure 1.
Referring to Figure 5, there is illustrated a block
diagram of the circuitry in the general communications
controller 104 in Figure 1. The GCC includes a micro-
computer 500 having a memory with a stored program for
communicating with Cams 106, 108, 110 and 112 in Figure
1. Microcomputer 500 is coupled to RS232 interfaces 504,
505 and 506 which may be coupled by modems to dedicated
telephone lines from each CAM. Microcomputer 500 is also
coupled to RS232 interface 502 which may be coupled to a
dedicated telephone line from host computer 102 in Figure
1. Information in message signals received from
portable radios by way of Cams 106, 108, 110 and 112 is
forwarded by microcomputer 500 to host computer 102.
Conversely, information to be sent to portable radios
from host computer 102 is transmitted to microcomputer
500 and incorporated into message signals transmitted to
designated portable radios. Microcomputer 500 receives
signal strength information from each of the Cams
whenever a portable radio transmits a message signal and
processes the signal strength information to determine
the zone in which that portable radio is presently
located.
Microcomputer 500 stores for each portable radio the
zone having the largest adjusted signal strength for the
last transmission, the zone having the second largest
foe
- 19 -
adjusted signal strength for the last transmission, the
"home" zone assigned to that portable radio, and the last
zone used for communications with that portable radio.
For subsequent transmissions of message signals to a
portable radio, the GCC accesses the zone location
information for that portable radio and selects a
transmitter for transmitting a message signal in the zone
in which the portable radio is most likely located.
Microcomputer 500 also keeps track of which transmitters
are in use and which transmitters interfere with
communications in a particular zone. Thus, when
transmitting a message signal in the zone where a
selected portable radio is located, microcomputer 500
inhibits the use of other transmitters which would
interfere with communications in that zone. If
transmission of a message signal to a portable radio
would interfere with a transmission already under way,
microcomputer 500 queues that message signal for trays-
mission when the interfering transmitter has completed
its transmission. Microcomputer 500 can be any suitable
commercially available microcomputer, such as, or example,
a Motorola type MCKEE, MCKEE or MCKEE microprocessor,
or those microprocessors described in US. patent
numbers 4,030,079 and 4,266,270 and the patent application
referred to therein.
Referring next to Figure 8, there is illustrated a
flow chart including the process steps used by Cams 106,
108, 110 and 112 in Figure 1 for measuring the signal
strength of RF signals transmitted by portable radios.
The flow chart in Figure 8 provides a detailed descrip-
lion of the process steps required for execution by
microcomputer 402 in Figure 4. The coding of the process
steps of the flow chart in Figure 8 into the instructions
of a suitable commercially available microcomputer is a
mere mechanical step for a rottener skilled in the art.
Zoo
- 20 -
Entering the flow chart in Figure 8 at start block
800, a check is made to see if the SKI flag is set at
decision block 802. If the SKI flag is not set, NO
branch is taken to decision block 820 where it is
determined whether or not a SYNC (synchronization) word
has been detected. The SYNC word is part of each data
packet in a message signal and is followed by
alphanumeric information. The particular bit pattern of
the SYNC word is detected by microcomputer 402 in Figure
4. Signal strength measurements need not be taken until
a SYNC word is detected. Once a SYNC word has been
detected, several signal strength measurements can be
taken at different times during receipt of the message
signal and then averaged to obtain a more realistic
estimate of the signal strength for the portable radio
transmitting that message signal.
If a SYNC word has not been received, NO branch is
taken from decision block 820 to block 822 to exit from
the flow chart in Figure 8. Otherwise, YES branch is
taken from decision block 820 to block 824 where the SKI
running average is cleared. Next, at block 826, the SKI
flag is set, and then at block 828 the SKI timer is sex
to twelve milliseconds. Assuming that a data packet has
a length of approximately twenty-four milliseconds, the
SKI timer is set at twelve milliseconds so that two
signal strength measurements will be taken for each data
packet. Next, the flow chart is exited at block 830.
Returning back to block 802 in Figure 8, the SKI
flag is set whenever a message signal is being received
from a portable radio. Assuming the SKI flag was
previously set, YES branch is taken from decision bloc
802 to decision block 804 where it is determined if the
SKI timer is equal to zero. Assuming that microcomputer
402 in Figure is interrupted once every millisecond, the
SKI timer may be decrement Ed and the flow chart in Figure
8 may be executed every millisecond in response to each
~.Z~fi~
- 21 -
interrupt. As a result, the SKI timer will be zero
twelve milliseconds after a SYNC word has been received.
If the SKI timer is not equal to zero, NO branch is taken
to exit from the flow chart at block 80~. Otherwise, YES
branch is taken to block 80~ where the digitized SKI
signal is read from A/D converter 412 in Figure 4. Next,
at block 810, the newly read digitized SKI signal is
averaged with the SKI running average
Proceeding to decision block 812 in Figure 8,
lo check is made to determine if the end of the portable
radio message signal has been reached. If the end of the
message signal has not been reached, NO branch is taken
to block 82~ where the SKI timer is set to twelve
milliseconds for taking another signal strength measure-
mint. otherwise, YES branch is taken from decision block
812 to block 814, where the SKI running average is
appended to the message signal which is sent to GCC 104
in Figure 1. Next, at block 816, the SKI flag is cleared
in preparation for receipt of subsequent message signals,
and the flow chart is exited at block 818.
The process steps of the flow chart in Figure 8 are
designed to take two signal strength measurements for
each data packet in a message signal received from a
portable radio. For example, if there are four data
packets in a message signal, eight signal strength
measurements are taken and averaged. All Cams 106, 108,
110 and 112 in Figure 1 receiving the same message signal
from a portable radio are likewise taking two signal
strength measurements per data packet and appending the
average signal strength to the message signal that is
routed to the GCC. Therefore, within a short period of
time, the GCC will be receiving several different average
signal strength measurements from the Cams that receive
the same message signal from a portable radio.
Referring to Figure 6, there is illustrated a flow
chart used by GCC 104 for processing the average signal
Jo z2~jfi2~
- 22 -
strength measurements received from each of the Cams
106, 108, 110 and 112 in Figure 1. The flow chart in
Figure 6 is entered at start block 600 whenever a message
signal together with an average signal strength
measurement is received from a CAM. Next, at block 602 a
message timer is set to one-hundred milliseconds to
provide a time interval during which the same message
signal is received by other Cams and sent together with
an average signal strength measurement to the GCC. All
Cams should receive, if at all, the same message signal
lo at approximately the same time. The one-hundred
millisecond message time interval is utilized to alloy
fry CC~ processing and transmission delays. Assuring
that microcomputer 500 in Figure 5 is interrupted once
every millisecond, the message timer may be decrement Ed
in response to each interrupt.
Next, at block 604 in Figure 6, the average signal
strength measurement received with a message signal is
entered into the SKI matrix in the position for the
receiver that took the measurement. Proceeding to
decision block 606, a check is made to see if another
average signal strength measurement has been received
from another CAM. If so, YES branch is taken back to
block 604. Otherwise, NO branch is taken to block 608
where the message timer is decrement Ed once every
millisecond. Next, at decision block 610 a check is made
to see if the message timer is equal to zero. If not, NO
branch is taken back to decision block 606 to check to
see if another average signal strength measurement has
been received. Otherwise, YES branch is taken to block
612 for processing the average signal strength
measurements that have been received during the previous
one-hundred millisecond time interval.
Proceeding to block 612 in Figure 6, an adjusted
signal strength is computed for each zone using the newly
received average signal strength measurements that have
been entered into the SKI matrix and the predetermined
factors previously entered into the ZSEL matrix. The
ZADJ matrix is computed by multiplying the SKI matrix
and the ZSEL matrix according to the formula:
[ZADJ] = [SKI] x USE
The resulting ZADJ matrix has one adjusted signal
strength for each zone in the data communications system.
Since some of the zones may be in different cities, some
of the adjusted signal strengths may be zero. For the
zone configuration in Figure 2, it is possible that
transmissions from a portable radio will be received by
all three receivers Al, R2 and R3, producing an adjusted
signal strength for all seven zones Zl-Z7.
According to another feature of the present invent
lion, the SKI matrix can be stored and later used in
combination with the SKI matrix for the next transmission
from the same portable radio. or example, the signal
strength measurements in the stored SKI matrix can be
; decreased on the basis of the time interval between
the previous and newly received transmission from the
portable radio. Next, the decreased signal strength
measurements and the new signal strength measurements
- may be averaged for each CAM receiver, and the average
signal strength measurements may be used to calculate
: 25 the ZPDJ matrix in block 612. the average signal
strength measurements may then be stored in the SKI ma-
-- trip for use with the signal strength measurements taken
: for a subsequent transmission from the same portable radio
'
:
iffy
- aye -
Next, at block 614 in Figure 6, the zone having the
largest adjusted signal strength in the ZADJ matrix come
putted in block 612 is selected and stored in zone lo-
cation I for the portable radio whose transmitted
message signal was received by each of the Cams The
number of Cams receiving a message signal and making
a signal strength measurement for a portable radio will
vary depending both on the location of the portable
radio and the terrain and location of receivers in the
geographical area of the data communications system.
In other words, depending on the location of a portable
radio, as Jew as one and potentially all of the CC'~I
receivers may receive the same message signal from a
portable radio.
Next, at block 616, the zone having the second
largest adjusted signal strength in the ZADJ matrix
is selected and stored in zone location I for the
particular portable radio. Zone locations Al and
I are the most likely zones in which that portable
radio is located. Every time the portable radio trays-
mitt a message signal, new signal strength measurements
are taken and the zones stored in zone locations I
and I are updated. Therefore, according to the present
I
- 24 -
invention, the location of each portable radio is updated
every time that portable radio transmits a message signal
using the average signal strength measurement taken by
all of the CAM receivers that receive its message signal.
Since the signal strength measurements from all CAM
receivers receiving the same message signal are used, a
reasonably accurate determination ox the portable radio's
location can be made. To insure that location
information does not become stale, GCC in Figure 1 can
initiate a shortwhere-are-you message signal for those
portable radios that have been inactive for a relatively
long period of time.
Whenever it is desired to transmit a message signal
from GCC 104 in Figure 1 to a selected portable radio,
the flow chart in figure 7 is utilized by the GCC for
selecting the CAM transmitter covering the zone in which
the selected portable radio is most likely to be located.
Entering the flow chart in Figure 7 at start block 700, N
is set equal to 1 at block 702 and M is set equal to one
at block 704. N is an integer number used to determine
which zone location Z~1), Z(2~, I or I is selected,
and M is an integer number used to determine the number
of retransmission made to a particular zone.
Next, at block 706 in Figure 7, the GCC selects the
transmitter covering zone location I for the selected
portable radio. Initially, the GCC selects zone location
I. As previously explained, zone location I is the
zone having the largest adjusted signal strength or the
last transmission prom the selected portable radio, zone
location I is the zone having the second largest
adjusted signal strength for the last transmission from
the selected portable radio, zone location I is the
"home" zone for the selected portable radio, and zone
location I is the zone location used for the last
transmission to the selected portable radio.
.,
;26
- 25 -
Proceeding next to decision block 708 in Figure 7, a
check is made to see if an interfering transmitter is in
use. The interfering transmitters are determined by rev-
erroneous to the ZIP matrix, which identifies transmitters
that interfere with communications in zone location I.
If an interfering transmitter is in use, YES branch is
taken to block 710 where the message signal is queued for
later transmission to the selected portable radio, and
the flow chart is exited at block 712. If an interfering
lo transmitter is not in use, MO branch is taken to block
714 where a message signal is transmitted to the selected
portable radio using a transmitter selected from the SWAHILI
matrix for covering zone location I. At the same
time, interfering transmitters selected from the ZIP
matrix for zone location I may be inhibited from
transmitting while the message signal is being sent to
the selected portable radio.
Next, at block 716 in Figure 7, the GCC waits for
one hundred milliseconds to determine if an
acknowledgement message has been received from the
selected portable radio. If the selected portable radio
is actually in zone location I and receives the
transmitted message signal, it will transmit an
acknowledgement signal indicating that the message signal
has been properly received. Proceeding to decision block
718, a check is made to see if an acknowledgement signal
has been received. If so, YES branch is taken to block
720 and the flow chart is exited. In other words,
the message signal has been successfully communicated to
the selected portable radio. If an acknowledgement
signal has not been received, NO branch is taken to block
722 where M is incremented by 1. The variable M is used
to provide for one or more retransmission of the
message signal to the same zone location. In the
preferred embodiment, one retransmission is allowed.
Therefore, at decision block 724 a check is made to see
1.2;~t;~Z~
- 26 -
if M is greater than or equal to three. If M is less
than three, NO branch is taken back to block 706 for
retransmitting the message signal to zone location I.
If M is greater than or equal to three, YES branch is
taken to block 726 for preparing to transmit the message
signal in the next zone location.
At block 726 in figure 7, N is incremented by one
for selecting the next zone location. Proceeding to
decision block 7~8, a check is made to see if N is
greater than or equal to five. If N is less than five,
NO branch is taken to block 704 where M is set equal to
one and the process steps are repeated for the next zone
location I. The process steps are repeated beginning
at block 704 for each of the zone locations I, I,
and I so that a message signal is transmitted, and
retransmitted once, in all four stored zone locations in
an attempt to communicate a message signal to a selected
portable radio. If N is greater than or equal to five,
YES branch is taken from decision block 728 to block 730
where the GCC alerts host computer 102 in Figure 1 that
the portable radio is either inactive or lost. At this
point in time, the host computer may decide to poll the
portable radio in every zone of the data communications
system. Such a poll would be conducted on a low priority
basis using a minimum length message signal. Next, the
flow chart in Figure 7 is exited at block 732.
The flow charts in figures 6 and 7 provide a
detailed description of the process steps used by GCC
microcomputer 500 in Figure 5 for communicating message
signals to portable radios. The coding of the process
steps of the flow charts in Figure 6 and 7 into the
instructions of a suitable commercially available
microcomputer is a mere mechanical step for a rottener
skilled in the art. By way of analogy to an electrical
circuit diagram, the flow charts in Figures 6, 7 and 8
are equivalent to a detailed schematic for an electrical
- , ~.2~jfiZ~
- 27 -
circuit where provision of the exact part valves for the
electrical components in the electrical schematic
corresponds to provision of microcomputer instructions
for blocks in the flow charts.
Referring to Figure 9, there is illustrated a block
diagram of the circuitry in portable radius 130, 132 and
134 in Figure 1. Each portable radio includes a radio
transceiver 340, a microcomputer 320, an alphanumeric
display 310, and a keyboard 312. Alphanumeric display
310 may be any commercially available display, such as an
LCD display or gas discharge display, that provides for
the display ox one or more lines of alphanumeric
information. Display 310 is controlled by I/O device 321
of microcomputer 320. Keyboard 312 may be any
commercially available keyboard having both numeric and
alphanumeric keys. Keyboard 312 is coupled to I/O device
321 of microcomputer 320, which senses activation of its
various keys.
Radio transceiver 340 in Figure 9 may be any suit-
able commercially available transceiver, such as that de-
scribed in the aforementioned Motorola instruction manual
no. POW and in Motorola instruction manual no.
68P81014C65. Radio transceiver 340 includes two an-
tennis spaced at a predetermined distance from one
another for providing receiver diversity. Receiver 341
is coupled directly to one antenna and coupled by
duplexes 342 to the other antenna. Duplexes 342 may be
any suitable commercially available duplexes such as that
described in US. patent number 3,728,731. Receiver 341
may include suitable commercially available circuits for
selecting between the two antennas, such as, for example,
the antenna selection circuitry in the aforementioned
Motorola instruction manual no. 68P8103gE25. Receiver
~3~1 demodulates message signals transmitted from the CAM
- 35 transmitters. The demodulated message signals are
filtered by filter 316 and limited by limiter 31~ and
it
- 28 -
thereafter applied to I/O device 321 of microcomputer 320.
Message signals from I/O device 321 of microcomputer 320 are
applied to filter 318 and thereafter to transmitter 343 for
transmission to CAM receivers. Transmitter 343 is turned on
in response to the TX key signal from I/O device 321 of
microcomputer 320. The output of transmitter 343 is coupled
to one of the radio transceiver antennas by way of duplexes
342.
Microcomputer 320 in Figure 9 includes I/O devices 321,
microprocessor (MU) 322, random-access memory (RAM) 326,
read-only memory (ROM) 323, and I.D. ROM 324. MU 322
may be any suitable commercially available microprocessor,
such as, for example, the Motorola type MCKEE, MCKEE or
MCKEE microprocessors, or those microprocessors described
in US. patent numbers 4,030,079 and 4,266,270 and the
patent application referred to therein. Similarly, I/O
device 321, RUM 326, ROM 323 and I.D. ROM 324 may be any
commercially available devices that are suitable for operation
with the type of microprocessor selected for MU 322. I.D.
ROM 324 is a removable device that includes a specific
identification code or address that is assigned to a
portable radio. ROM 323 stores the control program that
is executed by MU 322 for communicating message signals
and acknowledgement signals to GCC 104 in Figure 1. RAM 326
includes both a scratch pad area used by MU 322 during
execution of the control program stored in ROM 323 and
a number of register locations allocated for storing the
identification code read in by MU 322 from I.D. ROM 324,
information displayed by display 310, information
entered from keyboard 312, and other status information.
The contents of specific registers in RAM 326 may be
loaded from message signals received from GCC 104 in
Figure 1 or may key included in message signals sent by
MU 322 to the GCC. The formatting of register
information into message signals may be accomplished as
- 29 -
described in the aforementioned Canadian patent application,
serial number 433,332, which application also includes a
listing of suitable control program.
The portable radio illustrated in Figure 9 may be
either a mobile radio that is installed in a vehicle or a
portable radio that is small enough to be hand-carried
from place to place (See the aforementioned Motorola
instruction manual Number 68P81014C65). Although the
portable radio in Figure 9 is primarily adapted to
transmit and receive message signals including
alphanumeric information, the portable radio may also
provide voice communications by means of a speaker
connected to the output of receiver 341 and a microphone
connected to the input of transmitter 343. A portable
radio adapted to communicate both alphanumeric information
and voice signals is described in the instant assignee's
US. Patent No. 4,430,742, entitled, "Data Muting Method
and Apparatus for Radio Communications System", and
invented by Thomas A. Free burg et at.
In summary, unique methods and apparatus for trays-
miller selection and transmitter reuse in data commune-
cations systems have been described. By selecting the
proper transmitter for transmitting message signals to
portable radios, unnecessary transmissions are
eliminated, freeing up the radio channel for communique-
lions with other portable radios. Moreover, transmitters
which do not interfere with communications already
underway to a particular zone can be simultaneously
transmitting message signals to portable radios in other
zones, thus greatly enhancing message signal throughput.
