Note: Descriptions are shown in the official language in which they were submitted.
~2~6039
METHOD AND APPARATUS FUR
COrirIUNICATING VARIABLE LENGTH MESSAGES
BETWEEN A PRIMARY STATION AND REMOTE STATIONS
OF A DATA COMMUNICATIONS SYSTEM
Background of the Invention
The present invention relates generally to radio
communications systems, and more particularly to an im-
proved method and apparatus for coding and communicating
variable length messages between a primary station and
5 remote stations of a data communications system.
In prior art data communications systems, as
exemplified by the system described in U.S. patent no.
4,156,867, messages communicated from a central station
to mobile stations included a start code followed by one
10 or more data blocks each including a parity word and a
ntunber of digital words with a data and parity portions.
The bits of the data block were interleaved so as to be
more tolerant of burst errors. However, the interleaving
scheme of these prior art data communications systems
15 typically rendered blocks susceptible to the loss of
either a group of data bits or a group of parity bits due
to burst errors. Furthermore, simultaneous transmissions
from two or more mobile stations could interfere with one
another such that neither was adequately received by the
2U central station.
,.~ I,
.f C.r
~22~039
-lA-
Moreover, in these prior art data communications
systems, no information is provided in the first or
second block as to the ntunber of following blocks, and no
provision was made for efficiently and reliably
25 accomodating long strings of data. The bits of the data
block were interleaved so as to be more tolerant of burst
errors. However, the interleaving scheme of these prior
art data communications systems typically rendered blocks
susceptible to the loss of either a group of data bits or
30 a group of parity bits due to burst errors. Furthermore,
simultaneous transmissions from two or more mobile
X226039
-2-
stations could interfere with one another such that
neither was adequately received by the central station.
Obiects and Summarv of the Invention
It is an object of the present invention to provide
an improved method and apparatus for coding messages
communicated between a primary station and remote
stations of a data communications system.
It is another object of the present invention to
provide an improved method and apparatus for
communicating variable length messages between a primary
station and remote stations of a data communications
system.
Briefly described, the present invention encompasses
a signalling system for use in stations of a
communications system for cornmunicating variable length
messages via a communications medium, such as a radio
channel, between a primary station and a plurality of
remote stations, such as portable and mobile radios. The
signalling system includes a variable length message
which is transmitted between an originating station and
received by at least one destination station. The
variable length message includes a bit synchronization
field, a message synchronization field and a plurality of
channel data blocks. According to one arrangement of
channel data blocks, the first and second channel data
blocks including an information field including control
data and a check word, and a parity field coded from the
information field according to a predetermined code, the
'nits of the fields being interleaved such that
consecutive bits of a field are separated by a
predetermined number of bits; and said third and
following channel data blocks including an information
field including data, and a parity field coded from the
information field, the bits of the fields being
1226039
-2A-
35 interleaved such that consecutive bits of a field are
separated by a predetermined number of bits, and the
information field of the last channel data block further
including a check word coded from the data in the
information field of the third and following channel data
40 blocks.
According to another arrangement of channel data
blocks, the first channel data block includes an
information field including at least one command and a
station address, and a parity field coded from the
45 information field according to a predetermined code. The
second channel data block includes an information field
including a block count indicating the number of
following channel data blocks, a message sequence number
incremented for each message and a check word, and a
5U parity field coded from the information field according
to a predetermined code. The third and following channel
data blocks include an information field including data,
and a parity field coded from the information field. The
bits of the fields in each channel data block are
55 interleaved such that consecutive bits of a field are
separated 'by a predetermined number of bits.
l2~so39
-3-
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 data messages in a typical
S data stream.
Figure 3 is a diagram of the preferred bit pattern
of the message synchronization field in Figure 2.
Figure 4 is a diagram of the structure of the
physical messages in Figure 2.
Figure 5 is a diagram of the structure of the
channel data blocks in Figures 2 and 4.
Figure 6 is a diagram of the bit interleaving matrix
used in coding a channel data block.
Figure 7 is a diagram of the structure of a Type-1
basic information unit in Figure 5.
Figure 8 is a diagram of the structure of a Type-2
basic information unit in Figure 5.
Figure 9 is a diagram of a geographical area that is
divided up into a number of zones.
Figure 10 is a block diagram of the circuitry in the
receivers in Figure 1.
Figure 11 is a block diagram of the circuitry in the
channel communications modules in Figure 1.
Figure 12 is a block diagram of the circuitry in the
general communications controller in Figure 1.
Figure 13 is a block diagram of the circuitry in the
portable radios in Figure 1.
Figures 14A and 14B taken together are a flow chart
used by the channel communications modules and portable
and mobile radios in Figure 1 for building, coding and
transmitting messages to another station.
1.226039
-4-
Figures 15A and 15B taken together are a flow chart
used by the channel communications modules and portable
and mobile stations in Figure 1 for receiving and
decoding messages from another station.
Figure 16 is a diagram of the structure of variable
length messages.
Figure 17 is a diagram of the structure of the con-
tents of the variable length message control register.
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 portable radios 130, 132 and 134 and
mobile radios 136 and 138. 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 and mobile radios 130, 132, 134, 136 and 138.
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 (CCM) 106,
108, 110 and 112, which are each coupled to and 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 assigned one of
122603
-5-
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
Instruction Manual 68P81013E65. CCM's 106, 108, 110 and
112 may be co-located with their~corresponding
transmitters and receivers or may be remotely located
located and coupled to their corresponding transmitters
and receivers by means of a suitable remote control
system, such as, for example, the tone remote control
system described in U.S. 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.
68P81039E25 or commercially available hand-held portable
radios of the type shown and described in U.S. patent
numbers 3,906,166 and 3,962,553. Mobile radios may
be commercially available radios of the type shown and
described in Motorola instruction manual no. 68P81039E25.
Portable and mobile radios 130, 132, 134, 136 and 138
each include a transmitter operable on the second carrier
signal and a receiver operable on the first carrier
signal. The transmitter and receiver in portable and
mobile radios 130, 132, 134, 136 and 138 may be any
suitable commercially available conventional transmitter
and receiver, such as, for example, the transmitter and
receiver described in Motorola instruction manual no's.
68P81039E25 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.
1226039
-6-
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 and mobile radios located
in several different cities. GCC 104 may be coupled to
host computer 102 and CCM's 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 and mobile radios
130, 132, 134, 136 and 138 by way of CCM's 106, 108, 110
and 112. The message signals may include a binary
preamble, a predetermined synchronization word, an
information word containing a command, status or data,
and one or more data words.
As shown in Figures 1 and 2, data messages between
GCC 104 and portable and mobile radios 130, 132, 134,
136, and 138 are structured as a stream of binary
information (i.e., bits). The preferred embodiment of
the signalling system utilizes digital frequency
modulation at 4800 bits per second.
This data transmission stream 200 in Figure 2 can
contain one or more physical messages 203, 204. A
physical message 203 or 204 within the data transmission
stream 200 is preceeded by a preamble 201 or 202. The
preamble 201 or 202 is required to establish bit and
message synchronization between the transmitting and
receiving ends of the link.
The preamble 201 or 202 assumes different forms
depending on its position in the data transmission stream
200. The initial preamble 201, which occurs once at the
beginning of a data transmission stream, contains one of
two fields: bit sync field 204 and message sync field
205.
1226039
_7_
The bit sync field 204 is a predefined pattern of at
least twenty bits of alternating 1's and 0's, ending in a
zero. Bit sync is used only at the beginning of a data
transmission stream.
The message sync field 205 consists of a pattern of
40 bits that mark the beginning of a physical message.
Each physical message in a data transmission stream is
preceded by a message sync field 205. The message sync
field 205 precedes all physical messages in the data
transmission stream. The preferred bit pattern of the
message sync field 205 is shown in figure 3.
A physical message 401 (refer to figure 4) is
composed of one or more channel data blocks (CDB's) 403,
405, 407. The channel data block is the fundamental
envelope for all information that is sent through the
"ether", and is a fixed number of bits (112 bits) that is
the final result of the encoding process described
below.
Ignoring the exact order of occurrence of the bits,
a channel data block can be logically segmented into
three component fields as shown in Figure 5. These
fields are: a 48-bit Basic Information Unit (BIU) field
503, a 56-bit Parity field 505, and an 8-bit channel
state field 507.
The basic information unit (BIU) field 503 carries
the actual control and user information. There are two
formats the basic information unit field 503 can assume
which are developed in detail later.
The parity field 505 contains 56 bits that are the
result of the operation of a rate 1/2, K=7 convolutional
encoding algorithm on the Basic Information Unit. A
systematic convolutional code is used (G1(D)=1) with
parity generator:
G2(D)=1 + D2 + D5 + D6
1226039
_8_
The parity bits are inverted prior to transmission.
The implementation of this code is discussed in "An
Introduction to Error Correcting Codes", by Shu Lin
(Prentice Hall, 1970).
In this code, the last 2 parity bits and last 8 data
bits will always be zero. Eight of these bits are used
to convey the channel state information by changing their
sense prior to transmission. These bits logically belong
to the channel state field 507. The channel state field
507 contains eight bits, seven of which are used to
indicate the state of the inbound channel (inbound with
respect to CCM's) to terminals that have potential
inbound traffic, so that these terminals will inhibit
transmission until the inbound channel is free, thus
preventing message collisions. There are two states:
busy and idle or free. A binary one indicates a busy
state, and a binary zero indicates a free state. In
other words, if the channel is busy, all bits of the
channel state field 507 are set to a binary one state,
and vice versa. In the case of portable and mobile
radios, the channel state field 507 may be used to
indicate predetermined status of the radio, such as, for
example an idle or in-use condition or an emergency or
normal condition.
The next step in the encoding process is the
interleaving. Refering to Figure 6, as the channel data
block is encoded, the bits are stored in columns of a
matrix. Each column holds 7 bits. The entire 112 bit
channel data block fills up exactly 7 rows and 16
columns. The matrix is filled such that every other bit
is from parity field 505, i.e, bits 0, 2, 4, 6, 8, 10,
etc, are from parity field 505. When the matrix is
filled, the bits are removed for transmission row by row.
The sequence of bit numbers transmitted is 111, 104, 97,
. . . , , , . . . 7, 0. Matrix bit #111 is
sent first. Bit numbers in parentheses designate channel
~22~6039
-9-
state bit locations in the matrix. The locations are bit
numbers 1, 3, 5, 7, 9, 11 and 13.
At the receiver, the bits are stored in rows of a
matrix. Each row holds 16 bits and the entire 112 bits
fills up exactly 7 rows. When the matrix is filled, the
bits are removed (for decoding) column by column. This
exactly complements the interleaving process of the
encoder.
The channel data block decoder takes in the 112 bits
and, if possible, extracts an exact copy of the Basic
Information Unit and channel state bits that were
transmitted. As the bits are removed from the
de-interleaving matrix, channel errors are corrected
according to the decoding process for such codes, as will
be known to one versed in the art, and as explained in
the reference mentioned above.
The Type-1 basic information unit is shown in Figure
7. This form is used primarily to communicate control
information (i.e., terminal address, message control
parameters, etc.). The first channel data block of a
physical message is always of Type-1.
To properly perform control functions, error
detection is required to prevent any uncorrected bit
errors from inadvertently causing an undesired control
function. This error protection is provided by dividing
the 48-bit type 1 basic information unit into two fields:
a 32-bit command field 702 and a 16-bit cyclic redundancy
check (CRC) field 703. The cyclic redundancy check field
703 is computed from the command field 702 prior to
transmission and later checked upon reception.
The error detection code defined here is commonly
used in the data communications industry, and is known as
CRC-CCITT. The command field 702 to be checked is
divided by the generating polynomial:
G(x) - X16 + X12 + X5 + 1
-- 1.226039
-10-
Integer quotient digits are ignored, and the CRC
field 703 is filled with the complement of the resulting
remainder value. The remainder is complemented (1's
complement) to provide protection against bit sync loss.
Upon message reception, the same process is
followed. The result is compared with the received CRC
and if they exactly match, the message is acceptable.
The type-2 basic information unit in Figure 8 is
used primarily to communicate user data (e.g., text) and
error checks on user data.
The format and coding of the type-1 basic
information unit in Figure 5 is described in detail in Canadian
patent application serial number 433,332, entitled
"Data Signalling System", filed July 27, 1983 and
invented by Timothy M. Burke, et. al.
As explained in aforementioned patent application
serial no. 433,332, the type-1 basic information unit
provides for instruction blocks or register blocks.
Tnstruction blocks contain identifier (3 bits) op-code (5
bits), argument (8 bits) fields. The instruction blocks
manipulate the contents of registers in the portable and
mobile radios (see registers in RAM 626 of microcomputer
520 in Figure 13). The various instruction blocks and
their coding are summarized hereinbelow in Table I.
1,226039
-11-
TABLE I
R A I
/ / / OP-CODE ARG
I N O 7 DESCRIPTION MNEMONIC
________________ ___ _____-______________ _________
0 0 x 0 0010 Read Sub-register RSR
0 x x 0 0011 Discrete Operations DISOP
0 x x 0 0101 Vehicle Status WSR 1,1
0 x x 0 0110 User Status WSR 1,2
x x 0 0111 Message WSR 1,3
0
0 0 1 1 0100 Read Register short RRS
0 0 x 1 0101 0 Read Register Long RRL
0 x x 1 0101 1 Write Register Long WRL
R/I Bit; 0=Instruction, 1 - Register.
A/N Bit; 0 = NO ACK, 1 - ACK Required.
I/0 Bit; 1 - Outbound from CCM, 0 = Inbound to CCM.
ARG 7; Most Significant Bit of Argument Field.
A variable length physical message can be
constructed by transmitting a variable number of channel
data blocks continuously. Variable length data (e.g.,
text, transparent binary information, etc.) is divided
(blocked) into an integral number of segments such that
each segment fills a chanel data block. The channel data
blocks are transmitted, along with appropriate control
information, and are de-blocked to recover the original
form of the data upon reception.
Referring to Figure 16, variable length physical
message 203 is preceeded by preamble 201, and consists of
channel data block 207, which is built from a type-1
basic information unit, and contains addressing and
command information; channel data block 209, which is
also built from a type-1 basic information unit, and
contains further control information to be described
below; a number of channel data blocks 211, each built
from a type-2 basic information unit and containing up to
_. 1226039
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48 bits of the user data to be transmitted; and
terminated by channel data block 213, which is built from
a type-2 basic information unit, and contains up to 32
bits of the user data in addition to a 16 bit cyclic
redundancy check character.
Since the Type-2 basic information unit format is
used for the user data and its definition contains no
facility to detect errors that may escape error
correction, the message level CRC is defined. This CRC
uses the same algorithm that is defined for use in the
type-1 basic information unit, and is computed over all
of the user data.
Referring to Figure 17, a variable length message
control register (VLMCR) is loaded with the contents of
the second channel data block in the variable length
message. The VLMCR then instructs the terminal with
format and control information that directs the
processing of the message.
The block count field 241 specifies the size of the
variable length message. This field provides the
terminal with information to perform buffer management
and enables the base end controllers to assess the size
of inbound variable length messages.
The message format 243 field specifies the format
and padding of the user data. The format field is
divided into two sub-fields; the information coding
sub-field (ICSF) and the informatin padding sub-field
(IPSF). The ICSF specifies one of four possible ways in
which the informatin portion of the message may be coded,
e.g. ASCII, binary data, etc. The IPSF specifies the
amount of padd ing that is required to make the message an
integer number of blocks.
The physical message sequence number field (PMSNF)
247 is divided into two sub-fields; a 1-bit
re-synchronization sequence flag (RSF), and a 3-bit
physical message sequence number (PMSN). The physical
1226039
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message sequence number provides the terminal and base
ends of the R/F network with the capability to reject
duplicate copies of the same message. The duplicate
copies can result when ACKs are lost and the message is
re-transmitted. The capability to re-sync the PMSN is
also defined when RSF = 1.
The PMSN is incremented by the sender when an
acknowledgement is received to the transmission. A
transmission procedure is initiated with the first
transmission of a physical message. Subsequent
re-transmission of the same physical message, in an
attempt to set an acknowledgement, are part of the same
transmission procedure and use the same PMSN. That is,
repeat transmissions of non-acknowledged physical
messages will not cause the number to be incremented.
The sequence number, both inbound and outbound, can
assume incrementing values modulo-7, including zero (0).
Thus, a continuous sequence of physical messages to the
same destination will have an incrementing sequence
number (...0, 1, 2, ... 5, 6, 7, 0, ....).
The receiver will only accept messages that have a
monotonicly increasing sequence number (modulo-7) that is
one greater than the last one it received. The receiver
will send a negative acknowledgement, indicating to the
sender that the message is out of physical sequence.
The PMSN is re-sync'ed with a physical message that
has the RSF set. The re-sync sequence flas (RSF = 0)
indicates the PMSN is to be checked as described above.
When (RSF = 1) the PMSN check will not be made and the
physical message will be accepted by the receiver (and
ACK'ed if specified). Additionally, the receiver's
current PMSN will be updated to that of the PMSN of the
physical message it rode in on.
The logical message sequence number field 249 in
Figure 17 provides the capability to manage the
communicator of large logical messages by segmenting the
1226039
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logical message into several physical messages. It also
supports the capability to interrupt the transmission of
a long logical message, composed of many physical message
segments, with the transmission of a single segment
logical message. This interruption must occur between
physical segment transmission.
The LMSNF is further divided into two sub-fields;
logical message sequence number (LMSN), and last segement
flag (LSF). The LSF indicates that the associated
message segment is the last segment in the logical
message (LSF=1) or it is not the last segment (LSF=0).
The LMSN sequentially numbers and identifies physical
message segments within a logical messge. In a
multi-segment message, the LMSN is initalized to zero (0)
upon the transmission of the first physical message
segment. The LMSN is incremented with each succeeding
message segment and wraps around to the value of one (1)
(e. g., 0, 1, 2.....7, 1, 2....).
Note that zero (0) has been left out of all but the
start of the sequence. This permits a single
transmission logical message (OIC) to interrupt and
resume multi-transmission (multiple physical messages
from one logical message) logical messge. If the LMSN is
found to be out of sequence upon reception, the logical
message may be aborted and restarted from the beginning.
If the logical message consists of several physical
message segments, the first physical message in the chain
will be identified by: LSF=0 and LMSN=0. The physical
messages that are in the middle (i.e., not the first nor
the last physical messages) are identified by: LSF=0 and
(1 <= LMSN <= 7). Note that the LMSN is allowed to wrap
from 7 back to 1. This message may be interrupted by a
physical message marked by (OIC) and resumed with the
next sequential physical message of the logical message
chain. If the logical message consists of several
1226039
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physical message segments, the last physical message will
be identified by: LSF=1 and (1 <= LMSN <= 7).
The user header offset field 253 in Figure 17
defines the size of the user header information that may
precede the displayable portion of the character oriented
message. Essentially, the variable length portion of
the message is divided into two parts; user header and
user data. The user header contains variable length user
supplied information (i.e. transaction I.D., program
function key, data presentation parameters etc.) that is
not displayed, but is passed to user applications
programs as a set of parameters. The user data portion
is the part of the message that is displayable.
Since the user header is variable length, and in
fact optional, it can change in structure and content
depending on the message. Also, changes to the user
header will not impact the system software of the
terminal or base since its contents are transparent to
that software.
The message priority field 251 provides the terminal
with the capability to process messages at four (4)
different priority levels.
Referring back to Figure 1, message signals are
routed by GCC 104 to a selected CCM 106, 108, 110 and 112
for transmission by its corresponding transmitter. Since
the message signals are not transmitted on all transmit-
ters simultaneously, as in simulcast systems of the type
described in U.S. Patent Number 4,188,522, it is neces-
sary that GCC 104 have a reasonably accurate determina-
tion 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 porta-
ble radio is located. Utilizing unique processes, the
GCC 104 dynamically selects the transmitter 114, 120 or
124 for transmitting a message signal to a selected
portable or module radio 130, 132, 134, 136 or 138.
1.22609
-16-
According to another unique feature of the novel
data communications system in Figure 1, 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 re-use of
the RF communications channel. In other words, by taking
advantage of re-use, a single RF communications channel
can serve thousands of portable and mobile radios in a
geographical area covering several states and their major
cities.
Referring to Figure 9, there is illustrated a
geographical area of a data communications system that is
divided into seven zones, Z1-Z7, and that includes three
CCMs 210, 220 and 230 and corresponding transmitters and
receivers. Transmitter T1 of CCM 210 has a coverage area
within circle 212, transmitter T2 of CCM 220 within
circle 222, and transmitter T3 of CCM 230 within circle
232. Each time a portable or mobile radio transmits,
signal strength readings are taken by receivers R1, R2
and R3. These readings can be expressed by the following
signal strength SSI matrix:
[SSI] - [SSI1 SSI2 SSI3].
The signal strength readings taken by receivers R1,
R2 and R3 can be used to compute an adj usted 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 combining the adj usted 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,
1.226039
_17_
and the sensitivity of the receivers. In other words,
the predetermined factors associated with each zone are
emperically determined and depend upon the
characteristics of the equipment and terrain in each data
communications system. The predetermined 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
[ZSELJ - 0 15.3 0 0 9.8 10.2 7.5
0 0 15.7 10 0 11 7.4
The adjusted signal strength ZADJ matrix for each of the
zones Z1-Z7 is than computed according to the following
matrix formula:
[ZADJ] - [SSI] X [ZSEL] ; or
[ZADJ] - [ZIADJ Z2ADJ Z3ADJ Z4ADJ ZSADJ 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 or mobile 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 or mobile 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
that portable or mobile radio. Both that zone and the
transmitter covering it are stored in the memory of GCC
104. If the portable or mobile radio does not
acknowledge the transmission of the message signal from
GCC 104, GCC 104 may attempt one or more retransmissions
of the message signal by means of that selected
transmittter. If the retransmissions likewise are not
acknowledged by the portable or mobile radio, GCC 104 may
then transmit the message signal via the transmitter
1226039
_ 18-
covering the zone which had the second largest adjusted
signal strength for the last transmission from that
portable or mobile 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 or mobile radio by means of
these two transmitters, GCC 104 may either select another
transmitter covering that portable or mobile radios
"home" zone, or initiate a polling sequence in which the
selected portable or mobile radio is polled in every zone
in the data communications system starting with the
portable or mobile radio's "home" zone.
The transmitter re-use features of the novel data
communications system in Figure 1 are described in .
further detail in the instant assignee's Canadian
patent application, serial number 439,421, entitled
"Method and Apparatus for Dynamically Selecting
Transmitters For Communications Between A Primary Station
and Remote Stations of a Data Communications System",
filed October 20, 1983, and invented by Thomas A.
Freeburg.
Referring to Figure 10, there is illustrated a
detailed circuit diagram of the receivers 116, 118, 122,
126 and 128 associated with CCM's 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 Rayleigh fading, are
experienced in communications systems operating at RF
signal frequencies in the new 800 to 900 mHz frequency
122603
-19-
range. The maximal ratio predetection diversity combiner
cophases the RF signals from each antenna and linearly
adds the cophased signals to provide a composite signal
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 10, the
frequency of local oscillator 306 determines the radio
channel to which the diversity receiver is tuned. The RF
signal received by each antenna is combined by mixers 302
and 304 with the signal from local oscillator 208 to pro-
vide corresponding IF signals. The IF signal from mixers
302 and 304 is then applied to IF bandpass filters 308
and 310, respectively, which may be a monolithic bandpass
filter of conventional design similar to that described
in U.S. 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 substracted 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.
-20-
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
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 324 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 CCM 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
instant assignee's U.S. Patent
No. 4,369.,520, Issued January 18, 1983, entitled
"Instantaneously Acquiring Sector Antenna System", and
invented by Frank J. Cerny, Jr. and James J. Mikulski,
and in Canadian Patent No. 1,141,437, Issued February 15,
1983, entitled "Large Dynamic Range Multiplier for a Maximal
Ratio Diversity Combiner", and invented by Frank J. Cerny, Jr.
Figure 10 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
.~ ~,~so39
-21-
provides an SSI 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
for each of the signals from filters 308 and 310 if it is
desired to measure each separately . The SSI signal from
envelope detector 350 is coupled to its corresponding CCM
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 11, there is illustrated a block
diagram of the circuitry in CCM's 106, 108, 110 and 112
in Figure 1. Each CCM includes a microcomputer 402
having a memory with stored program therein for
communicating with GCC 104 and portable and mobile radios
130, 132, 134, 136 and 138 in Figure 1. Microcomputer
402 can be any suitable commercially available
microcomputer such as, for example, the Motorola type
MC6800, MC6801, MC6805 or MC68000 microprocessor, or
those microprocessors described in U.S. patent numbers
4,030,079 and 4,266,270, and the patents and patent
applications referred to therein.
Microcomputer 402 is coupled to conventional 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 assembled into variable length messages 203,
204 in Figure 2 and coupled to filter 406 and thereafter
applied to its corresponding transmitter. The preferred
embodiment of the transmitter utilizes digital frequency
modulation at 4800 bits per second for coding message
signals for transmission. A suitable modulation circuit
is described in U.S. patent no. 3,414,842. Other
suitable message signal coding schemes are described in
the aforementioned US patent nos. 3,906,445, 4,156,867
and 4,354,252 and patent application serial no. 433,332.
126039
-22-
Message signals received from portable or mobile radios
by a CCM's receiver are coupled to filter 408 and there-
after 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 which decodes the information and data therein.
Microcomputer 402 also takes signal strength
readings while it is receiving message signals. The SSI
signal from its corresponding receiver is coupled to A/D
converter 412, which may continuously convert the analog
SSI signal to a digitized SSI signal having eight bits.
The digitized SSI 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 SSI signals for the
several conversions are averaged by microcomputer 402.
The average SSI signal is appended to the received
message signal which is sent by microcomputer 404 via
RS232 interface 404 to GCC 104 in Figure 1. The
information and data in a received message signal may be
coded in any suitable conventional format for
transmission to GCC 104.
Referring to Figure 12, 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 CCM's 106, 108, 110 and 112 in Figure
1. Microcomputer 500 is coupled to conventional RS232
interfaces 504, 505 and 506 which may be coupled by
modems to dedicated telephone lines from each CCM.
Microcomputer 500 is also coupled to a conventional 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 CCM's 106, 108, 110 and 112 is forwarded
by microcomputer 500 to host computer 102. Conversely,
~.2zso3s
-23-
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 CCM's
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
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 interefere with a transmission already under way,
microcomputer 500 queues that message signal for trans-
mission when the interfering transmitter has completed
its transmission. Microcomputer 500 can be any suitable
commercially available microcomputer, such as, for
example, a Motorola type MC6800, MC6801, MC6805 or
MC68000 microprocessor, or those microprocessors
described in U.S, patent numbers 4,030,079 and 4,266,270
~.22~039
-24-
and the patents and patent applications referred to
therein.
Referring to Figure 13, there is illustrated a block
diagram of the circuitry in portable and mobile radios
130, 132, 134, 136 and 138 in Figure 1. Each portable or
mobile radio includes a radio transceiver 640, micro-
computers 620 and 660, an alphanumeric display 610, and a
keyboard 612. Alphanumeric display 610 may be any
commercially available display, such as an LCD display or
gas discharge display, that provides for the display of
one or more lines of alphanumeric information. Display
610 is controlled by I/O device 621 of microcomputer 620.
Keyboard 612 may be any commercially available keyboard
having both numeric and alphanumeric keys. Keyboard 612
is coupled to I/0 device 612 of microcomputer 620, which
senses activation of its various keys.
Radio transceiver 640 in Figure 13 may be any suit-
able commercially available transceiver, such as that de-
scribed in the aforementioned Motorola instruction manual
no. 68P81039E25 and in Motorola instruction manual no.
68P81014C65. Radio transceiver 640 includes two antennas
spaced at a predetermined distance from one another for
providing receiver diversity. The two antennas may also
be located Within the radio housing as shown and
described in the instant assignee's copending latent
applications, 458,095, Filed July 4, 1984, entitled
"Homotropic Antenna System", invented by Thomas F.
Kneisel et al, and 457,944,
entitled "Signal Quality Steered
Diversity" invented by Eugene J. Bruckert et al, and
filed on July 6, 1983. Receiver 641 is coupled directly
to one antenna and coupled by duplexer 642 to the other
antenna. Duplexer 642 may be any suitable commercially
available duplexer, such as that described in U.S. patent
number 3,728,731. Receiver 641 may include suitable
commercially available circuits for selecting between the
3.26039
-2s-
two antennas, such as, for example, the antenna selection
circuitry in the aforementioned patent applications,
458,095 and 457,944 and in
Motorola instruction manual no. 68P81039E25. Receiver
641 demodulates message signals transmitted from the CCM
transmitters. The demodulated message signals are
filtered by filter 616 and limited by limiter 614 and
thereafter applied to I/O device 661 of microcomputer
660. Message signals from I/O device 661 of micro-
computer 660 are applied to filter 618 and thereafter to
transmitter 643 for transmission to CCM receivers.
Transmitter 643 is turned on in response to the TX key
signal from I/O device 661 of microcomputer 660. The
output of transmitter 643 is coupled to one of the radio
transceiver antennas by way of duplexer 642.
Microcomputers 620 and 660 in Figure 13 each include
I/O devices 621 and 661, microprocessor (MPU) 622 and
662, random-access memory (RAM) 626 and 666, and
read-only memory (ROM) 623 and 663, respectively.
Microcomputer 620 also includes a removable I.D. ROM 624,
which contains the sixteen bit address assigned to the
portable or mobile radio. MPU's 622 and 662 may be any
suitable commercially available microprocessor, such as,
for example, the Motorola Type MC6800, MC6801, MC6805 or
MC68000 microprocessors, or those microprocessors
described in U.S. patent numbers 4,030,079 and 4,266,270
and the patent applications referred to therein.
Similarly, I/O device 621 and 661, RAM 626 and 666, ROM
623 and 669 and I.D. ROM 624~may be any commercially
available devices that are suitable for operation with
the type of microprocessor selected for MPU's 622 and
662. ROM's 623 and 663 store the control programs that
are executed by MPU's 622 and 662, respectively for
communicating message signals and acknowledgement signals
between the portable or mobile radio and GCC 104 in
Figure 1. RAM 626 includes both a scratch pad area used
x.226039
-26-
by MPU 622 during execution of the control program stored
in ItOM 623 and a number of register locations allocated
for storing the address or identification code of the
portable or mobile radio read in by MPU 622 from I.D. ROM
_624, information displayed by display 610, information
entered from keyboard 612, and other status and control
information. The contents of specific registers in RAM
626 may be loaded or read-out by way of message signals
received from GCC 104 in Figure 1.
Microcomputer 660 in Figure 13 receives messages
203, 204 in Figure 2 by way of receiver 641 from the GCC.
Microcomputer 660 demodulates, de-interleaves and decodes
the information and data in each message and loads the
data into shared memory 650, which includes semaphore 651
and RAM 652.
Microcomputer 620 then accesses the new information and
data from shared memory 650 and processes it.
Information and data to be transmitted is formatted into
information and data words (see Figures 2, 5 and 7) and
loaded into shared memory by microcomputer 620.
Microcomputer 660 reads out the information and data
words from shared memory 650 and assembles the
information and data words into messages 203, 204 as
illustrated in Figure 2 and automatically transmits the
messages to the GCC.
The portable radio illustrated in Figure 13 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 or mobile radio in Figure 13 is primarily
1226039
- 27 -
adapted to transmit and receive message signals including
alphanumeric information, the radio may also provide
voice communications by means of a speaker (not shown)
connected to the output of receiver 641 and a microphone
(not shown) connected to the input of transmitter 643. A
portable radio adapted to communicate both alphanumeric
information and voice signals is described in the instant
assignee's co-pending application serial
number 439,421, filed October 20, 1983, entitled, "Data
Muting Method and Apparatus for Radio Communications
System", and invented by Thomas A. Freeburg et. al.
Referring next to Figures 14A and 14B taken
together, there is illustrated a flow chart including the
process steps used by CCM's 106, 108, 110 and 112 and
portable and mobile radios 130, 132, 134, 136 and 138 in
Figure 1 for building, coding and transmitting messages
to another station. The flow chart in Figures 14A and
14B provides a detailed description of the process steps
for enabling microcomputer 402 in Figure 11 and
microcomputer 660 in Figure 13 to transmit variable
length messages. The coding of the process steps of the
flow chart in Figures 14A and 14B into the instructions
of a suitable commercially available microcomputer is a
mere mechanical step for a routineer skilled in the art.
Referring to Figure 14A, the flow chart for
transmitting variable length messages is entered at block
710 where the bit synchronization pattern is transmitted
first. Next, a block 712, the message synchronization
pattern is transmitted. Proceeding to block 714, the'
commands for the type-1 BIU in the first and second
channel data blocks is built. Then, at block 716, the
CRC is computed for each BIU and appended thereto. The
parity for each BIU is computed in block 718 using a
preselected convolution code. Then, at block 720, the
BIU, parity and channel state are interleaved, as
illustrated and described with respect to Figure 6
3.226039
- 28 -
hereinabove, and the first and second channel data blocks
are transmitted from the originating station. At any
point during the transmission of a channel data block,
the binary state of the bits in the channel state field
are dynamically changed to reflect predetermined status,
such as, for example, whether or not the inbo and channel
is busy or free, or whether or not an emergency condition
exists at the originating station. Since the binary
state of the bits in the channel state field are
dynamically changed, the predetermined status conveyed by
the channel state field can be detected by the
destination station without first decoding the
information in a channel data block.
Referring next to Figure 14B, program control
proceeds from block 720 in Figure 14A to block 724 where
a check is made to see if 48 bits of data are available.
If so, YFS branch is taken from decision block 724 to
block 730 where a type-2 BIU is built with the next 48
bits of data. Then, at block 732, the CRC is updated to
reflect the present BIU. The parity is then computed for
the present BIU at block 734 with the preselected
convolution code. Then, at block 736, the present BIU,
parity and the channel state are interleaved and the
channel data block is transmitted by the originating
station. Program control then proceeds back to decision
block 724 to check for any remaining data bits to be
transmitted.
If 48 bits of data are not available, NO branch is
taken from decision block 724 to decision block 726 where
a check is made to determine if there are 32 or less data
bits remaining. If not, NO branch is taken from decision
block 726 to block 728 where null data is appended to the
more than 32 data bits to create a 48 bit field. Program
control proceeds from block 728 to block 732 for
transmitting the newly formed 48 bits of data.
1226039
- 29 -
If 32 or less data bits are available, YES branch is
taken from decision block 726 to block 740 where the CRC
is updated to reflect the present BIU. Then, at block
742, the CRC and any necessary null data is appended to
the 32 or less data bits to form a 48 bit BIU. The
parity is then computed for the present BIU at block 744
with the preselected convolution~code. Finally, at 746,
the BIU, parity and channel state are interleaved and the
last channel data block is transmitted by the originating
station.
Referring to Figures 15A and 15B taken together,
there is illustrated a flow chart used by CCM's 106, 108,
110 and 112 and portable and mobile radios 130, 132, 134,
136 and 138 in Figure 1 for receiving and decoding
messages from another station. The flow chart in Figures
15A and 15B provides a detailed description of the
process steps for enabling microcomputer 402 in Figure 11
and microcomputer 550 in Figure 13 to receive variable
length messages. The coding of the process steps of the
flow chart in Figures 15A and 15B into the instruction of
a suitable commerically available microcomputer is a mere
mechanical step for a routineer skilled in the art.
Referring to Figure 15A, the flow chart for
receiving and decoding messages is entered at decision
block 820, where a check is made to determine if message
sync has been received. If not, NO branch is taken and
program control waits for reception of message sync.
Otherwise, YES branch is taken from decision block 820 to
block 822, where the next 112 data bits are buffered and
the channel state bits are checked. If three or more
consecutive bits of the channel state field indicate that
the channel is busy, the transmission of messages by the
receiving station are inhibited until three or more
consecutive bits of the channel state field indicate that
the channel is free. Thus, according to an important
feature of applicants' signalling system, collisions
x.226~39
- 30 -
between simultaneously transmitting stations can be
prevented by utilizing the information in the channel
state field.
Next, at block 824, the buffered channel data block
is deinterleaved and decoded using the preselected
convolution code. Then, at decision block 826, a
calculated CRC for the received BIU is compared with the
received CRC. If they are not the same, NO branch is
taken to return back to derision block 820. Otherwise,
YES branch is taken to decision block 828 where the
received address is checked. If the received address is
not valid, NO branch is taken back to decision block 820.
The validity check is made by portable and mobile
stations by comparing the received address with their
preassigned address stored in ID ROM 624 in Figure 13.
In the case of GCC, the received address is valid if it
is assigned to an active mobile or portable radio in the
data communication system. If the received address is
valid, YES branch is taken from decision block 828 to
block 830 where the next 112 data bits are buffered and
the channel state field is checked. Then, at block 832,
the received CDB is deinterleaved and decoded using the
preselected convolution code.
Program control proceeds from block 832 in Figure
15A to decision block 834 in Figure 15B, where a
calculated CRC for the received BIU is checked with the
received CRC. If they are not the same, NO branch is
taken to return to decision block 820 in Figure 15A.
Otherwise, YES branch is taken to block 836 where the
control information from the second CDB is stored in an
appropriate register. In the case of portable and mobile
radios, a register in RAM 626 in Figure 13 is loaded with
the control information. Then, at block 838, the next
112 data bits of the next CDB are buffered and the
channel state is again checked. The received CDB is
deinterleaved and decoded using the preselected
~.~25039
-31-
convolution code at block 840. Then, at block 842, the
data in the received CDB is stored in a buffer in the
receiving station. Next, at decision block 844, a check
is made to determine if the received CDB is the last CDB
of a message. If not, NO branch is taken back to block
838. Otherwise, YES branch is taken to decision block
846 where a calculated CRC for the received BIU is
compared to the received CRC. The last CDB of a message
contains the CRC for all CDB's carrying data. If they
lU are not the same, NO branch is taken back to decision
block 820 in Figure 15A. Otherwise, YES branch is taken
to block 848, where an acknowledgement message (ACK) is
transmitted by the receiving station to
indicate to the originating station that the message has
been properly received. The transmission of the ACK
message may have to be delayed depending on the state of
the bits in the received channel state field.
The flow charts in Figures 14 and 15 provide a
detailed description of the process steps used by CCM
microcomputers 402 in Figure 11 and portable and mobile
microcomputers 660 in Figure 13 for communicating message
signals between stations. The coding of the process
steps of the flow charts in Figure 14 and 15 into the
instructions of a suitable commercially available
microcomputer is a mere mechanical step for a routineer
skilled in the art. By way of analogy to an electrical
circuit diagram, the flow charts in Figures 14 and 15 are
equivalent to a detailed schematic for an electrical
circuit where provision of the exact part values for the
electrical components in the electrical schematic
corresponds to provision of microcomputer instructions
for blocks in the flow charts.
In summary, a unique method and apparatus for coding
and communicating variable length messages between stations
of a data communications systems has been described. The
novel signalling system results in messages that are more
1226039
- 32 -
tolerant of burst errors than heretofore possible.
Furthermore, the channel state, i.e. busy or free status,
is dynamically communicated to all stations for
minimizing message collisions.
