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Claims  |
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What we claim is:
1. A communication system for sending messages from central communication
stations to one or more of a plurality of mobile receivers through Earth
orbital relay stations using Time Division Multiplexed communication
signals, comprising:
at least one central communication station having transmission means for
transmitting a TDM communication signal which comprises a series of N
channels, with D data channels, for D<N, used exclusively for transmission
of message data and A address channels, where A=N-D, used for transmission
of address information in the form of receiver addresses for receivers
designated to receive messages from said station and corresponding ones of
said D channels on which said receiver message is transmitted.
2. The communication system of claim 1, wherein said central communication
station further comprises:
receiver assignment means for detecting which of said plurality of
receivers each message is directed to;
channel assignment means for assigning at least one channel to each of said
messages at a time from said D channels;
address control means for transferring a receiver address and an associated
channel assignment onto one of said A channels in response to input
messages intended for a receiver; and
message input means connected to said channel assignment means for
receiving digital messages and transferring said messages onto said D
channels in response to an assignment from said channel assignment means
at a rate of one message per channel.
3. The communication system of claim 2, wherein said central communication
station further comprises:
at least one narrow-beam antenna;
conversion means for converting information to be transmitted to a receiver
into a digital message at a predetermined transfer rate.
4. The communication system of claim 3, wherein said transfer rate is at
least 5,000 bits per second.
5. The communication system of claim 3 wherein said digital means signals
have predetermined transmission periods.
6. The communication system of claim 1 further comprising:
at least one mobile receiver station, having receiver means for detecting,
demodulating and decoding said TDM communication signal, said receiver
means being adjusted to receive at least one of said A address channels
when not receiving a message; and
channel selection means connected to said receiver means for adjusting said
receiver means to demodulate only a designated one of said D message
channels of said TDM communication signal according to a channel
assignment received for said mobile receiver on said at least one of said
A address channels.
7. The system of claim 6 wherein said a mobile receiver, comprises:
a narrowbeam antenna;
receiver means for receiving said communicating signal;
a demultiplexer, comprising:
address storage means for recording and storing a preassigned address for
said receiver; and
means for demuliplexing, said TDM communication signal over a time interval
equal to at least one time interval for channels in a signal frame
depending upon the encounter of said preassigned address for said
receiver.
8. The system of claim 7 wherein said mobile receiver further comprises a
message display device connected to said demultiplexer for displaying in
visual form a series of alphanumeric characters corresponding to a
respective message.
9. The communication system of claim 1, wherein said of said messages has a
predetermined maximum length M and a ratio of A to D is approximately
equal to a ratio of a sum of digital bits used to define a receiver
address and channel designation to a sum of bits used to define each of
said messages.
10. A method for sending messages from central communication stations to
mobile receivers through Earth orbital relay stations, comprising:
generating a TDM communication signal which is divided into a series of N
transmission channels;
providing digital messages to be transmitted on said channels;
assigning at least one receiver destination to each message;
assigning at least one of D data channels to each of said messages where
D<N;
transferring each of said messages on at least one of said D channels;
transferring said channel assignment onto one of A address channels where
A=N-D;
detecting, demodulating and decoding said TDM communication signal by at
least one mobile receiver;
adjusting said receiver to detect only of said D data channels of said TDM
communication signal in response to said address assignments transmitted
on said A address channels; and
demodulating said one data channel according to said assignment.
11. The method of claim 10 further comprising the steps of applying a
Golay[24,12] block coding process to said digital message prior to
transmission and applying a Golay[24,12] block decoding process to symbol
data received on said one demodulated data channel for each receiver.
12. The method of claim 11 wherein said Golay coding step generates digital
symbol data at a rate on the order of 10,000 bits a second.
13. The method of claim 10 further comprising the steps of performing a
checksum computation on input digital messages in said transmission means
and on corresponding demodulated symbol data in said receiver.
14. The method of claim 10 further comprising the step of hopping a
transmission frequency of said TDM communication signal on a predetermined
periodic basis over a preselected number of frequencies.
15. A communication system for sending messages from at least one central
communication station to one or more of a plurality of mobile receivers
through Earth orbital relay stations using Time Division Multiplexed (TDM)
communication signals, said system comprising:
at least one central communication station comprising:
transmission means for transmitting a TDM communication signal which
comprises a series of N channels, with D data channels where D<N, said D
data channels used for transmission of message data, and with A address
channels where A=N-D, said A address channels used for transmission of
address information in a form of receiver addresses for receivers
designated to receive messages from each station and corresponding ones of
said D channels on which said receiver message is transmitted;
receiver assignment means for detecting which of said plurality of
receivers each message is directed to;
channel assignment means for assigning at least one channel to each of said
messages at a time from said D channels;
address control means for transferring a receiver address and an associated
channel assignment onto one of said A channels in response to input
messages intended for a receiver;
message input means connected to said channel assignment means for
receiving digital messages and transferring said messages onto said D
channels in response to an assignment from said channel assignment means
at a rate of one message per channel; and
at least one mobile receiver station comprising:
receiver means for detecting, demodulating and decoding said TDM
communication signal at a rate of about 1/N times a transmission rate of
said communication signal, said receiver means being adjusted to receive a
selected one of said A address channels when not receiving a message; and
channel selection means connected to said receiver means for adjusting said
receiver means to demodulate only a designated one of said D message
channels of said TDM communication signal according to a channel
assignment received for said mobile receiver on said selected one of said
A address channels.
16. The communication system of claim 15, wherein said central
communication station further comprises:
at least one narrow-beam antenna;
conversion means for converting information to be transmitted to mobile
receiver station into a digital message at a predetermined transfer rate.
17. The communication system of claim 15, wherein said transfer rate is at
least 5000 bits per second.
18. The communication system of claim 15 wherein said digital message
signals have predetermined transmission periods.
19. The system of claim 15 wherein said mobile receiver comprises:
a narrowbeam antenna;
receiver means for receiving said communication signal;
a demultiplexer, comprising:
address storage means for recording and storing a preassigned address for
said receiver; and
means for demultiplexing, said TDM communication signal over a time
interval equal to at least one time interval for channels in a signal
frame depending upon the encounter of said preassigned address for said
receiver.
20. The system of claim 19 further comprising a message display device
connected to said demultiplexer for displaying in visual form a series of
alphanumeric characters for displaying a message.
21. The communication system of claim 15, wherein each of said messages has
a predetermined maximum length M and a ratio of A to D is approximately
equal to a ratio of a sum of digital bits used to define a receiver
address and channel designation to a sum of bits used to define each of
said messages. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communication systems and more
particularly to telecommunication systems employing central message
transmitting stations and Earth orbit relay satellites to send messages to
mobile receivers. The invention further relates to a method and apparatus
for transferring messages to one or more of a plurality of receivers using
narrowband, multiplexed, receiver addresses and channel assignments to
reduce signal demodulation requirements for each receiver.
2. Background of the Art
There is an ever increasing need for a variety of half-duplex communication
systems to provide message delivery or brief transfers of information from
a central control station to a multiplicity of remote system users. There
are a substantial number of commercial, governmental, and private
applications requiring the delivery of messages to a large number of
geographically dispersed terminals, or mobile receivers, often on an
irregular basis. Applications for message services include such diverse
uses as government services (military, law enforcement, legislative) where
secure communications are desired, forestry services, disaster relief or
coordination, and commercial transport or message delivery services.
Other examples include the interstate trucking industry where dispatchers
wish to communicate short messages to trucks located anywhere in
continental U.S. Presently such communication is restricted to periodic
telephonic communications between drivers and a central dispatcher or
contact person. Currently, truck drivers must call long distance from many
remote geographic locations to retrieve messages or to update delivery and
pick-up schedules. However, it is hard, if not impossible, for drivers to
consistently "call in" at fixed, scheduled, times since telephone services
are not always readily available in many areas. The conventional "call in"
approach also creates accounting problems and major personal inconvenience
by requiring vehicle drivers to use money or charge cards for long
distance calls at substantial cost even when no updated message or
information is transferred.
Aside from conventional telephone systems, other communication systems have
attempted to address the mobile market. Radio telephone, cellular
telephone, and portable radio transceivers (CB) are all capable of
providing some form of communication between a mobile receiver and a base
unit. However, these communication systems suffer from several drawbacks
and have proven inadequate as message communication systems for serving a
large number of widely dispersed users.
Current mobile communication services operate on a limited number of high
frequency, low data rate, channels and have many more potential users than
system capacity. Many systems, like cellular telephone, employ frequency
reuse across an array of cells to increase capacity, with each cell having
a short transmission range to increase frequency reuse and reduce
interference. However, these lower power transmissions are more prone to
frequency selective fading and signal blocking and require highly mobile
users to frequently change channels as new cells are traversed. These
systems are also prone to sudden communication loss when no channels are
available in a new cell.
Cellular systems economically serve large metropolitan areas but leave many
urban and most rural areas without any service or direct coverage.
Messages for these areas are switched to conventional telephone or
satellite carriers for long distance transfers.
Direct communication, non-cellular, using traditional mobile radio
transceivers also requires constant monitoring of a variety of frequencies
all of which are crowded with existing traffic. These services, like
cellular telephone, are subject to frequent system overload and signal
degradation from several interference sources which makes them incapable
of handling a large volume of traffic. These transceivers also have a low
range imposed by the need to prevent interference with other communication
systems.
Communication systems based on Earth orbital relay satellites are proposed
increasingly as the new approach to solve many communication problems,
especially coverage of geographically diverse, low user density or rural
areas. Several systems have been developed to operate through orbital
relay satellites and central communication stations. Examples of such
systems are found in U.S. Pat. Nos. 4,555,651 and 4,504,946.
However, the high gain, low interference requirements imposed on such
systems, especially in regards to adjacent Earth orbit relay satellites or
concurrent satellite users, requires the use of large receiver antennas on
the order of 4 feet or more in diameter, which eliminates mobile
applications. In addition, current satellite systems must contend with low
data transfer rates which limit the number of users and amount of data
transferred. Otherwise, tracking and demodulating a high data rate
satellite signal requires very high speed, complex and expensive,
receivers that consume a lot of power for signal scanning and processing
even when not receiving messages.
What is needed is a communication system that allows continuous delivery of
messages as well as related communication parameters or control
information to a large number of users over a large geographical area. The
system also needs to be cost effective, simple to operate, maintain, and
install, and minimize antenna requirements.
SUMMARY OF THE INVENTION
With the above problems of the art in mind it is a purpose of the present
invention to provide a waveform or modulation scheme that provides
performance comparable to a predetermined signal to interference ratio in
any interference scenario encountered.
Another purpose of the invention is to use a novel modulation, coding, and
multiplexing technique which allows an inexpensive mobile receiver with a
very small antenna suitable for use on a moving vehicle, to receive and
transmit data reliably using a satellite repeater.
Yet another purpose of the invention is to provide a modulation and coding
scheme that is implemented digitally, preferably as firmware in signal
processing chips of the VLSI type and in microcomputers.
One advantage of the present invention is a high performance, cost
effective, and flexible implementation.
Another advantage is that the present invention provides a modulation and
coding scheme that allows mass production of circuits and modules to be
used for the remaining analog functions for the mobile terminals.
An object of the invention is to allow flexibility in the data transfer
rate so that receivers of differing transfer rates or capabilities are
efficiently accommodated.
Yet another advantage of the system that results from this latter object is
that more energy per bit can be used to compensate for unequal downlink
power density across large geographical areas.
These and other purposes, objects, and advantages are realized in a message
communication system for sending messages from central communication
stations to one or more mobile receivers through Earth orbital relay
stations using Time Division Multiplexed communication signals, by
employing at least one central communication station having a transmitter
producing a TDM communication signal which comprises N serial channels,
with D data channels, for D<N, used exclusively for transmission of
message data and A address channels, where A=N-D, used for transmission of
address information in the form of receiver addresses where messages are
designated to be sent from the station and corresponding data channels on
which the message is transmitted. An exemplary embodiment employs 25
channels with 4 address channels and 21 data or message channels.
The central communication station employs a receiver assignment device
which determines the receiver each message is directed to and a channel
assignment device for assigning each message to at least one of the data
channels for transmission. An address controller transfers one receiver
address and an associated channel assignment onto one of the address
channels in response to input messages intended for each receiver.
Alternatively, a plurality of receivers are designated where a message is
intended for several receivers in the system. A message input control
device is connected to the channel assignment device and receives digital
messages and transfers them onto designated data channels in response to
an assignment from the channel assignment device at a rate of one message
per channel. The input message data is nominally transferred at a rate on
the order of at least 5,000 bits per second.
In a preferred embodiment the communication system applies Golay[24,12]
block coding to the digital messages prior to transmission and applies a
corresponding Golay[24,12] block decoding process to resulting symbol data
when received. The Golay coding generates 2 digital symbols for each data
symbol resulting in a channel transmission rate on the order of 10,000
symbols per second. In addition, a checksum computation is performed on
input digital messages with a series of bits being added to the end of the
message for checksum verification by the receiver.
In further aspects of the invention, the operating frequency of the TDM
communication signal is frequency hopped on a predetermined periodic basis
over a preselected number of frequencies to decrease the power density of
the communication signal.
Each message has a predetermined maximum bit length. The ratio of the
number of address channels to data channels is approximately equal to the
ratio of the number of digital bits used to define a receiver address and
channel designation to the number of bits used to define each of the
message. Digital message signals are generally padded with zeros where
necessary to have predetermined transmission lengths.
The central communication station uses a conversion means to convert
incoming message information into digital messages at a predetermined
transfer rate on the order of 5,000 bits a second. The digital messages
are then used to modulate a carrier to form the communication signal. The
transmitter is connected to at least one narrow-beam antenna which directs
communication signals to a preselected orbiting relay satellite.
The communication system of the present invention uses mobile receivers
having small, portable, directable, narrow-beam antennas connected to a
demultiplexer and other means for detecting, demodulating, and decoding
the TDM communication signal to receive message data. The receiver
demultiplexes and demodulates the communication signal at a rate only of
1/N times the transmission rate when receiving data. An address storage
means records and stores a preassigned address used exclusively for that
receiver. Alternatively, the address storage can store system or class
type addresses for polled or broadcast messages and may be reprogrammed
according to new address assignments transmitted from a central
communication facility.
When an address assignment is received along with an associated data
channel designation, the receiver is automatically adjusted by a channel
selector to only receive and demodulate the designated data channel
portion of the communication signal. Coded data received on the designated
channel is demodulated, decoded where necessary, and transferred to a
message display device for displaying the message in visual form, as a
series of alphanumeric characters on a display screen. Alternatively, a
small print out such as by a small thermal printer can provide hard copies
of messages for later reference.
If reception of the communication signal is lost prior to completion of the
message or the message reaches a natural termination point, the channel
selector automatically readjusts the receiver to demultiplex the
communication signal and demodulate address channels.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features of the present invention may be better understood from
the accompanying description when taken in conjunction with the
accompanying drawings in which:
FIG. 1 illustrates an overall diagram of a communication system operating
according to the principles of the present invention;
FIG. 2 illustrates an overview of the frame and channel multiplexing of
communication signals used on the system of FIG. 1;
FIG. 3 illustrates an overview of the channel assignment, data transfers,
demultiplexing, and decoding steps used in operating the system of FIG. 1;
and
FIG. 4 illustrates a schematic representation of exemplary circuit
functions employed in a receiver for the system of FIG. 1.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention provides a method and apparatus for communicating
messages or other information to one or more remote, mobile receivers. The
communication system provides this information transfer for a large number
of users without requiring each receiver to demodulate or decode a wide
bandwidth signal on a continuous basis. This is accomplished by
establishing a multi-channel Time Division Multiplexed (TDM) communication
signal and dedicating one or more of the channels to transmit address
information that designates message recipients and reception channels for
each message. The address information is received and demodulated by each
receiver at a rate much less than that required to demodulate the entire
TDM communication signal. The address information is demodulated by each
receiver until a message for that receiver, and its corresponding channel
assignment, are detected, at which time that receiver switches to the
designated data channel for message reception.
An overview of a communication system operating according to the principles
of the present invention is illustrated in FIG. 1. In FIG. 1, a
communication system 10 is illustrated having a mobile receiver (not
shown) mounted in a vehicle such as a truck 12. The truck 12 represents
any of a variety of vehicles whose occupants desire to obtain at least
occasional updated information, status reports, or messages from a central
communication source. As previously discussed, truck drivers or various
drayage personnel often find automatic access to periodic messages
extremely useful for more efficient operation.
A message is transmitted to the truck 12 from a central transmission
facility or terminal 14 referred to as a Hub facility. The central
terminal or Hub 14 can be placed at a location such as a trucking terminal
or central dispatch office allowing lower site costs and direct access for
message transmission equipment.
Alternatively, the Hub 14 is located in a remote location more ideally
suited for low interference ground-to-satellite transmission or reception.
In this case, one or more system user facilitates in the form of central
dispatch offices, message centers, or communication offices 16 are tied
through a telephonic, optical or other communication link to the Hub 14.
In addition, for large numbers of remote customer message c | | |
