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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to the mobile radio art. Specifically, a
communication network is described for a mobile radio service utilizing
meteor bursts as a communication medium.
Mobile radio networks are commonly found in public use. Generally, such
systems cover small geographic areas utilizing VHF frequency
communications. In order to extend the range of these geographically
limited systems, complex interconnections between base stations are
provided by microwave link, satellite link or other dedicated services.
It has been known in the past to utilize the reflective capabilities of
meteor bursts for communicating from point to point. Meteor burst
communication systems are based on the use of meteor trails which are
generated when particles enter the earth's atmosphere and ionize a path
over which the meteors travel. These trails are typically 50-75 miles
above the earth's surface. Radio frequencies in the frequency range of
40-50 megahertz are reliably reflected from these ionized trails. As a
result of the height of these trails, over the horizon communications at
distances up to 1200 miles become practical.
High speed meteor burst digital communications have been realized between
fixed points by various government agencies. One such system for
collecting meteorological data is described in U.S. Pat. Nos. 4,277,845
and 4,630,314. The described digital communication system is used for
sending meteorological data from remote locations to a central data
collection point.
A major system limitation is imposed by the fact that, although billions of
meteor trails are created every day, their duration in time is usually
from a few milliseconds to a few seconds. This means that the viability of
the reflective ionized trail lasts for a brief moment in which the data
packet must be sent from a distant transmitter.
The present invention provides a nationwide communications network for a
mobile radio service utilizing the propagating capabilities of meteor
bursts. A specific protocol has been designed for the communications
system, as well as a specialized transmit-receive base station facility to
avoid the limitation imposed by the brief duration of the ionized meteor
trail.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a nationwide mobile radio data
service.
It is a more specific object of this invention to provide a mobile digital
radio service utilizing the reflective capabilities of meteor trails.
These and other objects are provided by at least one master station and a
plurality of mobile radio stations which communicate over brief meteor
bursts occurring in the upper atmosphere. Utilizing two master stations,
an entire area the size of the continental United States may be covered,
permitting mobile communication from anywhere in the United States to one
of the two master stations.
Each of the master stations includes a unique transmit station which can
transmit either a polling data message or an information message in any of
a plurality of individual azimuth sectors permitting transmission in any
direction from a master station. Messages for transmission to the various
mobile radio units are composed at the master station. The messages are
transmitted in a frequency range for reflection from a meteor trail. The
transmit station includes a circular array of individual transmit sector
antennas which provide 360.degree. of azimuth coverage.
A receiving antenna array is provided at each master station which provides
for 360.degree. of azimuth coverage for detecting incoming signals. The
receive array comprises a plurality of antennas generally spaced among the
transmit antennas in the circular array providing a plurality of receive
sectors which can be monitored for incoming messages. The combined receive
sectors provide receive coverage coextensive with the transmit coverage of
the combined transmit sectors.
A unique protocol is provided for transmitting and receiving messages
between a mobile and master station. When a master station has a message
to send, it will initiate a polling message in each of the transmit
sectors, identifying by address one or more stations to which
communication is desired.
Upon receipt of a recognizable polling address belonging to a mobile
station, the mobile station will initiate an acknowledgement on a fixed
frequency different from the master station transmit frequency. The master
station, upon receipt of the acknowledgement, will immediately transmit
the outbound message.
The master station receive and transmit antenna arrays are operated to note
the sector from which an incoming message is being received, and to
transmit on a sector which is known to contain the mobile station for
which a message is destined.
Messages originated from a mobile unit are composed at the mobile unit. The
mobile unit will listen to the frame of data being sent from the master
station. When the mobile unit detects a master station transmission, the
mobile unit responds with its message transmission. Collisions between
simultaneous transmissions of two mobile stations are avoided with the
selective antenna receive pattern at the master station. If two
simultaneous mobile transmissions are received in the same receive sector,
the master station will not acknowledge any message containing errors as a
result of the collision between simultaneous transmissions.
If the master station hears the transmitted signal it will decode the
message, and if validated as having been correctly received after
appropriate error correction, an acknowledgement will be sent by the
master station, acknowledging receipt of the message.
DESCRIPTION OF THE FIGURES
FIG. 1 is an overall block diagram of the master station in the network.
FIG. 2 is an illustration of the transmit and receive lobes generated at
the master station.
FIG. 3 is an illustration of the various message protocols exchanged
between the master and mobile stations.
FIG. 4 illustrates the beam scanning technique for scanning each sector
received beam in azimuth and elevation.
FIGS. 5A and 5B are flow charts illustrating the operation of the system
under control of the host computer for each of the transmit and receive
sectors.
FIG. 6 is a block diagram of a transceiver for communicating with a base
station.
FIG. 7 is a flow chart of the microprocessor controlled radio unit for
receiving and transmitting messages.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown the general layout of a master station
for receiving and transmitting digital messages to a plurality of mobile
stations 11. As the effective range for a master station is estimated to
be 1200 miles, two of such stations can effectively handle all traffic in
the continental United States. A central station 12 serving as a message
center is connected by a telephone trunk or other local communication
medium, for receiving digital messages to be transmitted through the
master station, and for receiving and relaying those which are received by
the master station to a subscriber. The central station message center 12
may include a modem for transmitting digital data packets between the
trunk and personal computer at the message center 12, or other device
which can transmit and receive digital messages from a host computer 14,
and transmit the same over a standard link 13, such as an RS 232. The
central station message center 12 may, of course, be a manually operated
facility wherein a dispatcher receives messages to be composed and
transmitted, as well as distributes digital message data received by the
master station from a mobile station.
The master station includes an array of eight transmit towers 5 and sixteen
receive antenna towers 6. The eight towers divide the effective transmit
coverage area in eight sectors, each subtending an arc of approximately
45.degree.. The eight antenna towers 5 each comprise two five-element
cross polarized yagis stacked vertically at a height of 5 wavelengths,
providing separate polarization senses in the horizontal, vertical or
circular polarization senses. Depending on propagation conditions, the
system will use the most advantageous polarization sense.
Sixteen receive towers 6 are located in approximately the same
circumferential arc as each of the transmit antenna towers 5. The sixteen
receive towers 6 each generate a steerable sector beam capable of covering
a beam width of approximately 221/2.degree. indexable in three azimuth
positions, and in three elevation positions. The steerable beams of the
sixteen receive antennas 6 are controlled from the host computer 14 such
as to provide a sector scan of nine different positions for each tower.
The receive towers may provide both horizontal and vertical polarization
signals.
With the aforesaid antenna structure, it is possible to transmit on one
frequency in any one or all of eight different azimuth sectors and receive
on a separate duplex frequency, different from the transmit signal in any
one of the sixteen sectors or on all of the sixteen sectors as required.
The azimuth plane of the receive and transmit sectors is shown in FIG. 2.
The transmit sectors are seen to comprise a transmit lobe having a beam
width of 45.degree.. Two receive sectors are shown contained in each
transmit lobe. The receive sectors are defined as lobes which can be
steered in azimuth either side of their axis, as well as positioned
vertically in three different positions. The adjacent transmit sectors
have transmit lobes which intersect at the 3db points. Adjacent transmit
beams may overlap in a sector which comprises approximately 37.degree.
azimuth coverage. The 37.degree. of overlap in the transmit sector
coverage can result in adjacent sector interference for mobile stations
which lie in a transmit path contained within the 37.degree. transmit
sector overlap. One solution for avoiding interference from adjacent
sector overlap is to time multiplex, on an alternate basis, transmission
from adjacent transmit tower sectors.
Thus, it is seen that a measure of directivity for increasing the transmit
and receive gain for the master station is provided by the individual
sector arrays for both the transmit and receive function.
Each of the sixteen receive antenna towers is connected to a beam steering
network 17 for positioning the receive sector beams in three different
azimuth positions and three different elevation positions. The nature of
the receive towers, which permit the generation and steering of the
receive beams will be described more particularly with FIG. 4.
Continuing with the description of FIG. 1, each of the beam steering
networks 17 is connected for control by the host computer 14. Each of the
receive beams are simultaneously stepped during an initial programming
mode through each of the nine positions.
A receiver 18 is shown for each of the receive antenna towers. The receiver
18 will provide decoded digital data which is received in any of the
sixteen receive sectors.
The controller 19 is shown in each of the sixteen tower receive stations
which will send a control signal to the host computer, indicating a
message is being received in a given receive sector. The host computer 14
will initiate an INHIBIT signal to the beam steering network 17 in
response to the control signal. Following the successful decoding of a
signal by the receiver 18, the individual controller units 19 will enable
host computer 14 to continue steering the receive beams along a
preprogrammed position format.
Shown associated with each of the transmit towers is a transmitter 15 and
respective controller unit 20 for the transmit towers. The transmitters
1-8, operating at a frequency offset with respect to the frequency of
receivers 18, and the frequency of the transmitter of the mobile units 11,
will transmit message and polling data in one or more of each of the
sectors defined by a transmit tower antenna array. Each transmitter 15 may
have a peak power of 10 kW. The transmitter 15 produces duobinary FSK
modulated data from data received from a respective controller 20.
Each controller 20 will decode a polarization command to switch
polarization of each transmit tower. Host computer 14 will determine
conditions which require a polarization change by tallying the number of
receive acknowledgements for each initiated polling command. Polarization
switch 21 will provide one of two signal outputs representing vertical or
horizontal polarization feed lines connected to each transmit tower 5.
Each of the eight transmitters 15 receives data for transmitting from the
host computer 14. In operation, the receive portion of the master station
will identify a sector containing a given mobile unit 11 when the mobile
unit 11 sends either an acknowledgement or message to the master station.
Host computer 14 can then address any subsequent messaging to a given
mobile unit through the appropriately identified transmit sector, avoiding
utilizing channel bandwidth for other transmit sectors not needed for
communicating with a given mobile unit 11.
Associated with each transmitter 15 is a controller 20 which receives the
data from host computer 14 over an RS232 link. The controller 20 is an
interface which may be a standard interface such as the MDI Model 4031.
The host computer 14 downloads the components of the messages shown in
FIG. 3. The controller formats the message components along with error
correction bits to provide a duobinary FSK modulation signal for each
respective frequency modulated transmitter 15 of a sector. Messages are
encoded with standard FEC error correction techniques. Burst errors
resulting from man-made interference or fading are efficiently corrected
with the Reed Solomon code. The outbound data rate transmitted by each
transmitter 15 may be 4800 bps, with a carrier frequency deviation of 4
kHz. The transmitter may have a carrier frequency of 47 mHz which is known
to produce reliable meteor burst reflections.
Referring to FIG. 3, there is shown the various message types which are
transmitted through the master station and received through the master
station for communicating with a plurality of mobile radio stations 11.
The first message type shown in FIG. 3 is a polling command which is
initiated any time the master station receives messages through the
message center 12 for transmission to any one of a plurality of mobile
radio stations 11. During those periods of time in which no messages are
to be sent to the mobile stations, the polling command is continually
transmitted with dummy addresses providing a signal for the mobile station
to verify that the communication channel is open and available. The
polling command could, upon initialization, be sent to all transmit
sectors, as a given mobile radio station's location may be unknown. A
polling command consists of a series of sync bits followed by a one bit
message number distinguishing successive polls. The type of address which
follows indicates whether it is an individual mobile unit 11, or a group
of mobile units. A 1 bit in this location indicates group polling, a 0 is
individual polling. The address sent in a group poll is the group
identification.
The remaining digital bytes are the addresses of a particular mobile unit
11 which is to receive a message when an individual poll is being
conducted. A message number bit is included which toggles between 1 and 0
to distinguish between successive polls.
The control station center 12 receives and compiles messages to be sent to
an individual mobile radio station. These outbound messages are shown in
FIG. 3 as comprising the requisite synchronization data, the address of an
individual mobile unit to receive the following message, a message type,
followed by the text and an end of message (EOM) character. The outbound
message will be transmitted through a transmit sector after an
acknowledgement is received by successfully polling the individual mobile
radio station.
When the mobile radio station 11 hears its address on the receive
frequency, it will initiate an inbound acknowledgement shown in FIG. 3.
The inbound acknowledgement includes synchronization pulses as well as an
address identifying the origin of the particular inbound acknowledgement.
Thus, the polling command could be polling any number of mobile radio
stations 11 in each frame of a cycle and individual acknowledgements will
be received and identified by the address contained in the inbound
acknowledgement.
The master station, having received an inbound acknowledgement, will then
transmit the outbound message shown in FIG. 3. As the particular receive
sector receiving an acknowledgment has been identified by the receiver 18
and controller 19 for the receive sector, the outbound message may be sent
over a single corresponding transmit sector to avoid redundant broadcasts
in transmit sectors which do not include a particular mobile radio station
11.
Messages may also be originated from a mobile radio station. The mobile
radio station 11 will compose an inbound message and include therein, as
shown in FIG. 3, the requisite synchronization bits, its address, the type
of message, the message text, and an end of message character. The inbound
message is sent any time that the mobile radio station 11 can hear the
frame being transmitted from the master station. In the event two or more
simultaneous mobile station transmissions suffer a collision at the master
station, and generate message errors in excess of a tolerable level, the
master station will not acknowledge the mobile radio station
transmissions, requiring the mobile radio stations to retransmit the
messages. Each mobile radio station using standard collision avoidance
protocol techniques, waits a random time and attempts a subsequent
retransmission of the message when the master station carrier is heard. To
distinguish a collision from other circumstances which do not permit the
master stations to decode and acknowledge the message, at least two
attempts of transmitting the mobile radio station message is made before
entering a random retransmit collision avoidance mode.
Each master station receiving an inbound message will stop beam scanning
within the sector receiving the inbound message and process the inbound
message through a respective receiver 18 and controller 19. The stationary
receive beam will process the incoming inbound message.
Following receipt and decoding of the inbound message, the host computer 14
will forward the decoded message with its appended address as shown in
FIG. 3 to the central message center 12. The message center may, in a more
simplified system, print out the message with its address for forwarding
by a human operator over a trunk line or other means to a destination
indicated in the inbound message.
Upon successful decoding of the inbound message, an outbound
acknowledgement is sent to the transmitter 11 as part of the transmit
frame for the transmit station, identifying all of the mobile radio
stations for which a message has been correctly received.
Having generally described the operation of the master and mobile radio
stations, a more detailed description of the transmit and receive array
will be made.
Referring in particular to FIG. 4, there is shown one of the receiving
towers of FIG. 1 and its associated beam steering circuitry. Each of the
receive towers 9 includes a stacked array of eight pairs of 12-element
yagi antennas 10. Each antenna of a pair 10 is disposed along a horizontal
axis. Each antenna of each pair of antennas 10 are shown connected to an
individual amplifier 21 which in turn is connected to a respective
variable phase delay network 22. By changing the phase between
horizontally adjacent yagi antennas, it is possible to steer the sector
beam formed by the array of antenna pairs three positions in azimuth for
the sector beam formed from the tower. The receive beamwidth of the array
formed by each of the receive towers is approximately 14.degree. in
azimuth and 7.degree. elevation, steerable in azimuth for a total
effective beamwidth of 21.degree.. In elevation, the beam is steerable to
cover an elevation angle of +31/2.degree. to 241/2.degree..
Steering in azimuth is accomplished by selecting a phase delay between
adjacent horizontal yagi arrays 10 which is proportional to the angle
.phi. identifying the beam position with respect to the horizontal axis
10a as follows
.phi..sub.A =2cos.phi..multidot.360.degree.
where .phi..sub.A is the phase between adjacent antennas of each pair.
To steer the receive lobes in elevation, the phase delays 22 between
vertically adjacent antennas 10 are selected as follows
##EQU1##
where each one of the eight pairs of antennas takes on an odd value of n,
.theta. being the off axis angle with respect to the horizon, and
.phi..sub.n is the phase between vertically adjacent antennas.
Depicted in FIG. 4 is a scheme for a single polarization wherein each
antenna of the pair 10 will be vertically polarized. It is possible, of
course, to add a second polarization sense, wherein the summing networks
24, phase delays 22 and amplifiers 21 are repeated for the other sense of
polarization. A digital voting circuit known to the communication industry
is interposed between the receiver and associated controller of each
polarization sense. The voting circuit will forward non-duplicative valid
messages from controllers to the host computer. However, for purposes of
description the single vertical polarization yagi elements are shown.
A beam controller microprocessor 26 is shown for stepping the phase delays
22 in a sequence which will move the radiation beam formed from the tower
1 in azimuth three positions, as well as in elevation three positions. The
scanning of the radiation beam can be inhibited by the host computer 14
when a signal has been detected in receiver 18, such as to permit the beam
to remain positioned while the signal is being decoded. The software
description for the host computer 14 to permit control over the beam
scanning will be described at a later time.
Each of the transmitter towers comprises an array similar to that of each
receive tower 9, but without the ability to steer the beam. Each
transmitter tower comprises a vertical stacking of two vertically and
horizontally polarized five-element Yagi-Uda arrays with dual polarization
selection capability. One or the other will be selected by feeding energy
to one or the other of two sets of elements on each antenna. The pattern
resulting from such a stacked array has an azimuth beam width of
approximately 48.degree. which is substantially the desired sector of
45.degree. for each transmit tower as shown in FIG. 2.
Each receiver 18 of the master station may be the receiver of a
commercially available mobile radio station used in voice communications.
Filtering stages of such a conventional receiver which are intended to
filter demodulated voice are removed to enhance the ability to recover
duobinary FSK data signals.
The controller 19 for each receiver may also be a commercially available
interface which receives demodulated duobinary FSK data. The controller 19
has built in error correction circuitry for error correcting the FEC
encoded digital data.
Having thus generally described the receive and transmit functions, the
program necessary to carry out the transmission and reception protocol for
the master station is shown more particularly in FIG. 5A and 5B. FIGS. 5A
and 5B show a flow chart of the programming operation executed by computer
14 for each of the eight transmit sector and sixteen receive sectors.
Each of the operations in the transmit and receive sectors can be
considered to begin at start 41. A decision block 42 will determine
whether or not there are any new outbound messages which are to be sent to
a given mobile station 11. If the host computer 14 has received from the
central station message center 12 a new message to be sent, the host
computer 14 will determine whether or not the location of that mobile 11
station has been previously determined. If so, the mobile station 11 which
is to receive a message will be polled in a single sector in step 47.
Thus, one of the transmitters 1 through 8 of FIG. 1 which is dedicated to
broadcasting in the identified sector, will receive a polling message such
as is shown in FIG. 3, including the address of the new message addressee.
If the location of the addressee is not known, the poll command will be
entered in all sectors in step 46, such that each of transmitters 1
through 8 broadcasts a polling command shown in FIG. 3.
Assuming there are outbound messages which must be sent as determined in
step 43, an algorithm is entered through step 48 to either increase or
decrease the number of addresses contained in a given polling command. In
the event that acknowledgements received after a poll command exceed a
certain rate, the rate of polling addresses can be decreased in step 51.
This will depend on whether a minimum number of addresses have been
successfully polled as determined in decision block 50. Also, in the event
that the number of acknowledgements received exceeds a certain rate, the
number of addresses in a given poll ar increased in step 49.
In the event no messages are in the message queue of central station
message center 12, each of the transmitters 1 through 8 will enter into an
idle probe mode in step 44. This will maintain a carrier on the transmit
sectors to permit the mobile unit 11 to sense the carrier. This carrier
sense is necessary in order to determine whether mobile stations should
attempt access to the master station receive channel.
In the event that a given mobile unit 11 poll has not been acknowledged
within a certain time-out period, decision block 55 will either change the
schedule for polling the mobile unit or drop the poll from the polling
frame. In the event that polling has been limited to less than all the
sectors, decision block 57 will expand the poll to include all sectors in
step 58, until an acknowledgement is received. If the mobile station is
already being polled in all sectors, step 59 will reduce the rate of
polling for the particular mobile station or alternatively drop the mobile
station from the polling schedule.
Each of the controllers of the receivers of the master station is locally
polled by the host computer in step 53 to determine whether there is a
message present in any of the receive sectors. A detected message will
result in the receive sector scanning being inhibited. When a signal is
detected to be present, it is decoded in step 60 of FIG. 5B. Decision
block 61 will determine whether the decoded signal is either an
acknowledge from a previous poll or is a message originated from a mobile
unit 11. If the message was received from mobile unit 11, the message is
decoded, the error corrected in step 60 and validated in decision block
62. In the event that the message has not been validated, indicating the
message was decoded incorrectly, the scan for the receive sector is
restarted in step 64.
In the event a message is correctly decoded, an acknowledgement is sent in
step 65 to the mobile unit. Following the acknowledgement, the scan for
the receive sector in which the signal was received is restarted in step
66. A dispatch queue in the central station message center 12 receives the
decoded message in step 67 which permits it to be either transmitted via
modem to a leased private line or manually conveyed by an operator to its
destination.
When an acknowledge is detected as being sent from one of the mobile radio
stations, step 63 will determine whether or not it has been decoded
correctly. If it has not been correctly decoded, the scan of the receive
sector is restarted in step 72. In the event it has, the decoded
acknowledge will include an address of the originating mobile unit which
has transmitted the acknowledge. Step 70 will determine whether or not the
acknowledge is from one of the stations being polled. If it is, that
station is dropped from the poll command in step 71. If not, the step 73
repeats the polling message command in search of an acknowledgement from a
mobile unit 11 to which the master station wishes to send a message.
When the acknowledge has been received from a mobile unit 11, the sector
which has received the acknowledgement is noted in step 76, and a message
is transmitted in a corresponding transmit sector which includes the
receive sector producing the acknowledgement.
After the message has been sent, and if the mobile unit has not responded
with an acknowledgement, as determined in step 77, indicating that it has
not received the message it expects to receive, the time that the message
was sent is logged in step 81, and a rescan of the receive sectors is
started at 82, and the program returns to central point B of FIG. 5A.
In the event an acknowledgement of the transmitted message has been
received from a respective mobile station 11, a tallying step 78 registers
a number of acknowledgements received for a given period, as well as the
time logged for the delivery time of the message in step 79. The tallying
step will provide the system operator with up to date propagation
conditions. The tally step may be used to initiate a transmit polarization
change when the rate of received acknowledgements per transmitted polling
messages falls below a predetermined threshold. An additional decision
step 87 will compare the rate of acknowledgement received with a threshold
value n. When the rate of acknowledgements is below n, the host computer
14 will effect a change in polarization for each of the transmit sectors
in step 88. The receive array sector scan is restarted in step 83, and the
new polarization is effected for transmitting when control returns to B of
FIG. 5A.
Thus, the overall functioning of the master station of FIG. 1 has been
explained in terms of the flow chart of FIG. 5.
The mobile unit 11 includes a standard mobile data terminal which composes
messages, and inserts them in a message queue for transmission.
The mobile units 11 have a common protocol which is described in detail in
FIG. 7. The mobile units are a standard duplex transmit receive unit which
receives digitally composed data as well as displays digitally received
data. The transmit frequency for the mobile unit 11 is, of course,
selected to be the same as the receive frequency for the master station.
The receive frequency of the mobile unit 11 is selected to be the same as
the transmit frequency for the master station.
Referring now to FIG. 6, there is shown a block diagram of one embodiment
of a mobile transceiver which may be used in the meteor burst
communication system of the present invention. The receiver includes a
double conversion superheterodyne receiver, typical of many mobile
communications transceivers. The antenna 201, shown as a whip antenna in
the general system description of FIG. 1, is connected to a
transmit-receive (t-r) switch 204. Also connected to the t-r switch 204 is
the receive RF amplifier 206, the power amplifier 229 for the transmit
portion of the transceiver, and a position location unit 221. The position
location unit 221 provides position data over a standard interface such as
an RS-232 to the encoder 224 for formatting as a message to be
transmitted. The position location unit 221 may be a Loran-C position
location system which receives on antenna 201 until a message is to be
transmitted. The t-r switch 204 may be an isolator or an
electromechanically controlled switch operated from the microprocessor in
order to isolate transmit power from entering either the position location
unit 221 or RF amplifier 206. The position location unit 221 can, under
control of microprocessor 220, output a recent position determination to
encoder 224. The encoder 224 will, under control of the microprocessor
220, format a position location message for transmission to the master
station. The transmission may occur from either a poll received from the
master station requesting the position of the mobile unit, or transmitted
on a periodic basis as determined by the system operator.
RF amplifier 206 is tuned to the receive frequency of 49 megahertz. A first
local oscillator 209 and mixer 207 provide the first conversion. The first
intermediate frequency signal is amplified in IF amplifier 210. A second
local oscillator 213 and associated mixer 212 generate a second
intermediate frequency signal, amplified by second IF amplifier 214.
Limiter discriminator 215 demodulates the frequency modulated carrier
bearing the messages from the master station. The discriminator 215 is
selected to demodulate FM signals having a frequency deviation of 4.0 KHz.
The demodulated duobinary FSK signals appear as a polarity change signal
alternating between polarities.
A threshold detector 217 checks the polarity of the pulses from the
discriminator and provides an indication of a binary 1 or 0 level. A
filter 216 is included, having a Butterworth frequency response with a
bandwidth slightly greater than the bit rate of the data being
demodulated.
A decoder 218 provides for the error correction and decoding of the
received signal in cooperation with microprocessor 220. Decoder 218 will
store detected messages and microprocessor 220 will effect the error
correction therein. An alphanumeric display 219 will be provided to
display each decoded message
The transmitter portion of the transceiver of FIG. 6 includes a standard
keyboard 223 and the encoder 224. The encoder 224, in cooperation with
microprocessor 220, will format messages to be sent to include parity and
error correction bits. A filter 225 prefilters the signals. The
premodulation filter will provide a signal to the phase modulator 226
providing for duobinary FSK baseband modulated data signals. The resulting
modulated baseband signal is frequency multiplied in doubler/tripler 228
to the required transmit frequency of 49 megahertz. Power amplifier 229
will provide the necessary gain to a level of approximately 100 watts for
coupling to a whip antenna 201.
The operation of the transceiver of FIG. 6 is, as was stated, under control
of the microprocessor 220. The programming for the microprocessor 220
which will result in the messages being correctly decoded and encoded for
transmission will be described with respect to FIG. 7. The operation of
the mobile unit in terms of the programming of microprocessor 220 will now
be explained with regard to FIG. 7.
An operator input 101 is provided via a keyboard which will permit the
composing of a message and storage of the message in a message queue.
The receiver continuously listens in step 103 to the master station
transmit channel. Signals on the receive channel are decoded in step 105.
If no message has been composed by the operator to send, decision block 107
will transfer control to decision block 108 which determines whether or
not a poll command is being received.
The mobile units may advantageously include a position location system such
as a Loran C or other well known navigation device. If such information is
available, it is stored for use by the system. In the event that the
master station, in a poll addressed to the mobile unit 11, desires the
position location for the mobile unit 11, decision block 110 will detect
this request. The location data stored from the Loran C navigation
equipment is retrieved in step 116 from the Loran C equipment 118 and sent
through the mobile unit transmitter in step 113.
In the event a poll of the mobile unit is detected and position location
information is not needed, an acknowledgement is prepared in step 112 and
sent via the mobile unit transmitter in step 113.
Having thus sent a message or an acknowledge to the master station, a wait
loop is entered in step 114. While waiting, the receiver listens for a
message or an acknowledgement from the master station in step 115. The
message is decoded and error corrected using standard FEC techniques in
step 115. In step 119, the validity of the corrected message is
determined. In the event no message or acknowledgement is received when
expected, a counter is increased by one in step 120. This counter will
keep track of the number of times the system has attempted to communicate
with the master station without receiving a properly validate | | |
