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Claims  |
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We claim:
1. A local area network comprising:
a plurality of satellites disposed within a given geographic area, each of
said plurality of satellites being connected to selected ones of
computers, word processors, printers and other related equipment as well
as digitally encoded audio signals and at least one unidirectional
antenna, each of said plurality of satellites having a first transmitter
coupled to said unidirectional antenna transmitting first signals at a
first frequency to others of said plurality of satellites according to a
predetermined protocol and a first receiver coupled to said unidirectional
antenna receiving second signals at a second frequency from said others of
said plurality of satellites according to said predetermined protocol,
said first and second frequencies being different; and
a relay disposed in said geographic area having an omnidirectional antenna,
each of said unidirectional antennas of said plurality of satellites being
in focused communication with said omnidirectional antenna, a second
receiver coupled to said omnidirectional antenna to receive said first
signals according to said predetermined protocol and a second transmitter
coupled to said second receiver and said omnidirectional antenna to
convert said first signals to said second signals and to transmit said
second signals to said others of said plurality of satellites according to
said predetermined protocol;
each of said first and second signals including digital data combined with
a single predetermined pseudo-random spreading code having N bits common
to all of said plurality of satellites, where N equals to a predetermined
number greater than two.
2. A network according to claim 1, wherein
said first transmitter includes
a source of digital data having a given bit rate for transmission,
a code generator to produce said spreading code having a bit rate greater
than said given bit rate, and
a first means to combine said digital data and said spreading code such
that such said spreading code is inverted when a data bit is "1" and said
spreading code is not inverted when a data bit is "0".
3. A network according to claim 2, wherein
said first transmitter further includes
a differential encoder coupled to the output of said first means,
a source of carrier signal at said first frequency, and
a biphase modulator coupled to said differential encoder and said source of
carrier signal to provide said first signals.
4. A network according to claim 3, wherein
said first receiver includes
second means coupled to said unidirectional antenna responsive to said
second signal to recover therefrom a waveform containing said combined
data and spreading code,
third means coupled to said second means to sample the amplitude of said
waveform at a predetermined rate greater than said given bit rate and
provide two channels of alternating samples expressed in digital form,
fourth means coupled to said third means to match said digital samples of
each of said two channels against a signal produced from a stored copy of
said spreading code, and
fifth means coupled to said fourth means to respond to the strongest match
occurring in said two channels and to detect the beginning of said digital
data to recover said digital data.
5. A network according to claim 4, wherein
said fourth means includes
a pair of correlators each coupled to a different one of said two channels,
each of said pair of correlators matching its stream of samples against
said signal produced from a stored copy of said spreading code and
determining how well the preceding said predetermined number of bits of
said samples match said spreading code, each of said pair of correlators
providing a correlation peak indicating a strong match independent of each
other every N bit periods.
6. A network according to claim 5, wherein
said first and second signals further include a predetermined number of
preamble bits having the same binary value indicating the beginning of a
data packet, and
said fifth means includes
sixth means coupled to said pair of correlators to detect a given number of
said predetermined number of said preamble bits,
seventh means coupled to said pair of correlators to detect predetermined
binary conditions of a given number of bits disposed at the beginning of
each frame of said data packet, and
eighth means coupled to said pair of correlators to detect said correlation
peak at the output of said pair of correlators,
said sixth, seventh and eighth means cooperating to synchronize said first
receiver to said first transmitter transmitting said combined data and
spreading code received by said first receiver.
7. A network according to claim 1, wherein
said first receiver includes
first means coupled to said unidirectional antenna responsive to said
second signal to recover therefrom a waveform containing said combined
data and spreading code,
second means coupled to said first means to sample the amplitude of said
waveform at a predetermined rate greater than said given bit rate and
provide two channels of alternating samples expressed in digital form,
third means coupled to said second means to match said digital samples of
each of said two channels against a signal produced from a stored copy of
said spreading code, and
fourth means coupled to said third means to respond to the strongest match
occurring in said two channels and to detect the beginning of said digital
data to recover said digital data.
8. A network according to claim 7, wherein
said third means includes
a pair of correlators each coupled to a different one of said two channels,
each of said correlators matching its stream of samples against said
signal produced from a stored copy of said spreading code and determining
how well the preceding N bits of said samples match said spreading code,
each of said pair of correlators providing a correlation peak indicating a
strong match independent of each other every N bit periods.
9. A network according to claim 9, wherein
said first and second signals further include a predetermined number of
preamble bits having the same binary value indicating the beginning of a
data packet, and
said fourth means includes
fifth means coupled to said pair of correlators to detect a given number of
said predetermined number of said preamble bits,
sixth means coupled to said pair of correlators to detect predetermined
binary conditions of a given number of bits disposed at the beginning of
each frame of said data packet, and
seventh means coupled to said pair of correlators to detect the highest
correlation peak at the output of said pair of correlators,
said fifth, sixth and seventh means cooperating to synchronize said first
receiver to said first transmitter transmitting said combined data and
spreading code received by said first receiver.
10. A long distance communication system comprising:
a plurality of transceivers disposed in a given geographic area, each of
said plurality of transceivers having at least one unidirectional antenna,
a first transmitter coupled to said unidirectional antenna to transmit
first signals at a first frequency to others of said plurality of
transceivers according to a predetermined protocol and a first receiver
coupled to said unidirectional antenna to receive second signals at a
second frequency from said others of said plurality of transceivers
according to said predetermined protocol, said first and second
frequencies being different; and
a relay transceiver disposed in said geographic area having an
omnidirectional antenna, each of said unidirectional antennas of said
plurality of transceivers being in focused communication with said
omnidirectional antenna, a second receiver coupled to said omnidirectional
antenna to receive said first signal according to said predetermined
protocol and a second transmitter coupled to said second receiver and said
omnidirectional antenna to convert said first signals to said second
signals and to transmit said second signals to said others of said
plurality of transceivers according to said predetermined protocol;
each of said first and second signals including digital data combined with
a single predetermined pseudo-random spreading code having N bits common
to all of said plurality of transceivers, where N equals a predetermined
number greater than two.
11. A system according to claim 10, wherein
said first transmitter includes
a source of digital data having a given bit rate for transmission,
a code generator to produce said spreading code having a bit rate greater
than said given bit rate, and
a first means to combine said digital data and said spreading code such
that said spreading code is inverted when a data bit is "1" and said
spreading code is not inverted when a data bit is "0".
12. A system according to claim 11, wherein
said first transmitter further includes
a differential encoder coupled to the output of said first means,
a source of carrier signal at said first frequency, and
a biphase modulator coupled to said differential encoder and said source of
carrier signal to provide said first signals.
13. A system according to claim 12, wherein
said first receiver includes
second means coupled to said unidirectional antenna responsive to said
second signal to recover therefrom a waveform containing said combined
data and spreading code,
third means coupled to said second means to sample the amplitude of said
waveform at a predetermined rate greater than said given bit rate and
provide two channels of alternating samples expressed in digital form,
fourth means coupled to said third means to match said digital samples of
each of said two channels against a signal produced from a stored copy of
said spreading code, and
fifth means coupled to said fourth means to respond to the strongest match
occurring in said two channels and to detect the beginning of said digital
data to recover said digital data.
14. A system according to claim 13, wherein
said fourth means includes
a pair of correlators each coupled to a different one of said two channels,
each of said pair of correlators matching its stream of samples against
said signal produced from a stored copy of said spreading code and
determining how well the preceding N bits of said samples match said
spreading code, each of said pair of correlators providing a correlation
peak indicating a strong match independent of each other every N bit
periods.
15. A system according to claim 14, wherein
said first and second signals further include a predetermined number of
preamble bits having the same binary value indicating the beginning of a
data packet, and
said fifth means includes
sixth means coupled to said pair of correlators to detect a given number of
said predetermined number of said preamble bits,
seventh means coupled to said pair of correlators to detect predetermined
binary conditions of a given number of bits disposed at the beginning of
each frame of said data packet, and
eighth means coupled to said pair of correlators to detect said correlation
peak at the output of said pair of correlators,
said sixth, seventh and eighth means cooperating to synchronize said first
receiver to said first transmitter transmitting said combined data and
spreading code received by said first receiver.
16. A system according to claim 10, wherein
said first receiver includes
first means coupled to said unidirectional antenna responsive to said
second signal to recover therefrom a waveform containing said combined
data and spreading code,
second means coupled to said first means to sample the amplitude of said
waveform at a predetermined rate greater than said given bit rate and
provide two channels of alternating samples expressed in digital form,
third means coupled to said second means to match said digital samples of
each of said two channels against a signal produced from a stored copy of
said spreading code, and
fourth means coupled to said third means to respond to the strongest match
occurring in said two channels and to detect the beginning of said digital
data to recover said digital data.
17. A system according to claim 16, wherein
said third means includes
a pair of correlators each coupled to a different one of said two channels,
each of said correlators matching its stream of samples against said
signal produced from a stored copy of said spreading code and determining
how well the preceding N bits of said samples match said spreading code,
each of said pair of correlators providing a correlation peak indicating a
strong match independent of each other every N bit periods.
18. A system according to claim 17, wherein
said first and second signals further include a predetermined number of
preamble bits having the same binary value indicating the beginning of a
data packet, and
said fourth means includes
fifth means coupled to said pair of correlators to detect a given number of
said predetermined number of said preamble bits,
sixth means coupled to said pair of correlators to detect predetermined
binary conditions of a given number of bits disposed at the beginning of
each frame of said data packet, and
seventh means coupled to said pair of correlators to detect the highest
correlation peak at the output of said pair of correlators,
said fifth, sixth and seventh means cooperating to synchronize said first
receiver to said first transmitter transmitting said combined data and
spreading code received by said first receiver. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to communications systems and more
particularly to a digital radio communications system capable of being
employed in a small geographic area, such as a local area network, and in
a large geographic area, such as a long distance communications system.
A local area network is a data communications system which links computers,
word processors, printers and other related equipment as well as digitally
encoded audio signals within a small geographic area. The local area
networks allow computers to exchange information. A local area network
enables computers to share resources and communicate with computers in
other networks, including minicomputers and mainframe computers. In a
local area network, information processing devices, such as desktop
computers, have access to one another and to hard disk storage, databases,
communications software and printers. The use of local area networks has
grown at a rapid rate in the past decade as computer systems have become
prevalent in the workplace.
Local area networks consist of both hardware and software. In its common
form, a local area network has the following elements: (1) network
interface units, which functionally and physically connect workstations to
the network, may be a printed circuit card inserted into the computer, or
may be a separate box attached with a plug to the computer; (2) software,
which manages and coordinates the flow of information on the network; (3)
file server computer, which acts as the system control unit; and (4) in
the past twisted pair wire, coaxial cable or fiber optic cable have been
employed to link the components of the local area network.
In a conventional local area network, network interface cards embedded in
each computing device control the transmission of data. As originally
configured, data is transmitted over coaxial cable, twisted pair wire or
fiber optic cable.
In order for a local area network to function properly, all connected
devices must obey rules or "protocols" that govern access to the network
and control communications with other devices in the local area network.
The International Standards Organization (ISO) has developed a framework
for network protocols so that incompatible devices will be able to
communicate. The ISO model separates the communications and computing
services provided by local area networks into seven hierarchical layers.
Standards for the two lowest layers in the ISO model, the physical layer
and the data link layer, set parameters for media, transmission speed,
physical architecture and access method of the local area network. The
five higher layers in the ISO model, the software layers, address the
methods by which information is reliably transmitted between sending and
receiving systems, and the way such information is processed and presented
to the user. The data link layer forms a bridge between hardware and
software in the local area network. The software layers control the
exchange of information.
Local area networks generally use three basic architectures: star, ring and
bus. In a star type network, each device is connected with a separate link
through a central file server. An example is ATT's STARLAN. In a ring,
such as IBM's TOKEN RING, devices are connected to one another in a closed
loop. In a bus architecture, devices are attached to a single open-ended
cable. Examples are ETHERNET and ARCNET.
While originally local area networks had their computers connected to one
another through twisted pair wires, coaxial cables or fiber optic cables,
more recently the computers have been connected in a local area network by
the means of radio frequency (RF) waveforms. Such a radio communications
system for a local area network has been disclosed in U.S. Pat. No.
4,789,983. In this patent there is disclosed a local area network using RF
waves rather than wires to connect users to a central node or station. The
users timeshare different RF frequencies for transmission and reception.
In this local area network, the users operate one at a time, do not employ
focused antennas and the interference and multipath fading is overcome by
employing diversity techniques.
The communications system of the present invention is not limited to local
area networks, but rather can also be used in long distance communications
systems of the type employing relay stations for line-of-sight
communications over a large geographic area or employing an orbiting
satellite as the relay station for the long distance communication system.
A satellite communication system is disclosed in U.S. Pat. No. 3,611,435
which discloses a satellite communication system where each terminal of a
system transmits data to a satellite repeater or relay station on a
carrier signal which has a frequency different for each terminal and each
terminal of the system receives data on a carrier signal which is
different for each terminal and different for each of the transmitted
carrier signals.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a communications system of
the digital type which may be employed in either a local area network or a
long distance communication system.
Another object of the present invention is to provide a communication
system for a local area network which links the computers therein with RF
waves, instead of coaxial cables, twisted pair wires or fiber optic
cables.
A further object of the present invention is to provide a local area
network using a predetermined protocol to connect nodes with wireless RF
links instead of hard wire links.
Still another object of the present invention is to provide a communication
system capable of being employed in a local area network and a long
distance communication system employing spread spectrum techniques.
A feature of the present invention is the provision of a local area network
comprising a plurality of satellites disposed within a small geographic
area, each of the plurality satellites being connected to selected ones of
computers, word processors, printers, and other related equipment, as well
as digitally encoded audio signals, and at least one unidirectional
antenna, each of the plurality of satellites having a first transmitter
coupled to the unidirectional antenna transmitting first signals at a
first frequency to others of the plurality of satellites according to a
predetermined protocol and a first receiver coupled to the unidirectional
antenna receiving second signals at a second frequency from others of the
plurality of satellites according to the predetermined protocol, the first
and second frequencies being different; and a relay disposed in the small
geographic area having an omnidirectional antenna in communication with
the unidirectional antennas of the plurality of satellites, a second
receiver coupled to the omnidirectional antenna to receive the first
signals according to the predetermined protocol and a second transmitter
coupled to the second receiver and the unidirectional antenna to convert
the first signals to the second signals and to transmit the second signals
to the others of the plurality of satellites according the predetermined
protocol.
Another feature of the present invention is the provision of a long
distance communication system comprising a plurality of transceivers
disposed in a large geographic area, each of the plurality of transceivers
having at least one unidirectional antenna, a first transmitter coupled to
the unidirectional antenna to transmit first signals at a first frequency
to others of the plurality of transceivers according to a predetermined
protocol and a first receiver coupled to the unidirectional antenna to
receive second signals at a second frequency from the others of the
plurality of transceivers according to the predetermined protocol, the
first and second frequencies being different; and a relay transceiver
disposed in the large geographic area having an omnidirectional antenna in
communication with the unidirectional antennas of the plurality of
transceivers, a second receiver coupled to the omnidirectional antenna to
receive the first signals according to the predetermined protocol and a
second transmitter coupled to the second receiver and the omnidirectional
antenna to convert the first signals to the second signals and to transmit
the second signals to the others of the plurality of transceivers
according to the predetermined protocol.
BRIEF DESCRIPTION OF THE DRAWING
Above-mentioned and other features and objects of the present invention
will become apparent by reference to the following description taken in
conjunction with the accompanying drawing, in which:
FIG. 1 is a block diagram illustrating a local area network disposed in a
small geographic area employing a communication system in accordance with
the principles of the present invention;
FIG. 2 is a diagram defining the symbols employed in FIGS. 1,3,4 and 5;
FIG. 3 is a block diagram of a long distance communication system in a
large geographic area employing the communication system of the present
invention;
FIG. 4 is a block diagram of the satellite of FIG. 1 and the data
transceiver of FIG. 3;
FIG. 5 is a timing diagram illustrating the output of certain components of
FIG. 4;
FIG. 6 is a timing diagram illustrating the transmit and receive packet in
accordance with the principles of the present invention;
FIG. 7 is a block diagram of the relay of FIG. 1 and the relay transceiver
of FIG. 3;
FIGS. 8 and 9 show embodiments of the spreading code generator of FIG. 4;
FIG. 10 is a block diagram illustrating one embodiment of the differential
encoder of FIG. 4;
FIG. 11 is a block diagram of one embodiment of the correlators of the dual
correlator of FIG. 4;
FIG. 12 is a block diagram illustrating one embodiment of one of the
circuits of the synchronization and tracking circuit of FIG. 4; and
FIG. 13 is a flow chart illustrating the operation of the synchronization
circuit of FIG. 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, the communication system of the present invention is
illustrated as being included in a local area network covering a small
geographic are such as a building 1. Personal computers 2 are
interconnected for the transmission of information therebetween by means
of satellites 3, relay 4 and focused antennas including unidirectional
antennas 5 and omnidirectional antenna 6 rather than by the hardwired
connections employed in the prior art. While it is illustrated that
personal computers 2 are interconnected for transfer of information
therebetween by the radio waves of satellites 3 and relay 4, word
processors, printers and other related equipment could also be
interconnected for information transference by satellites 3 and relay 4.
In accordance with the principles of the present invention, satellites 3
all transmit at a first frequency and receive at a second frequency
different than the first frequency. In addition the information
transmitted from and received at satellites 3 utilizes unidirectional
antenna 5 in a focused communication with an omnidirectional antenna 6
coupled to the relay 4. As illustrated in FIG. 1 some of the personal
computers, such as personal computers 7 are tied together by hardwiring,
such as by coaxial cable, twisted pair wire or a fiber optic fiber
arrangement. These computers 7 are hardwired to file server 8 which then
is coupled to satellite 3, information thereof being transmitted on
unidirectional antenna 5 to omnidirectional antenna 6 of relay 4. The
operation of personal computers 7 is determined by the ETHERNET protocol
provided by the ETHERNET cards 9 while the transmission of the information
from personal computers 2 is controlled by any predetermined protocol,
such as the ARCNET protocol provided by the ARCNET card 10 associated with
each of the personal computers 2. While the ARCNET protocol has been
employed in the hardwired local area network any predetermined protocol
can be employed in the local area network in accordance with the
principles of the present invention employing RF waves to interconnect a
plurality of personal computers for transfer of information therebetween.
According to the ARCNET protocol, data communication links between as many
as 255 computers, word processors, printers and other related equipment
can be interconnected. An ARCNET system has electronic boards that plug
into each computer 2 in the network and the ARCNET board has an
intelligent controller which directs the flow of information to and from
its host computer. The ARCNET controllers communicate with short burst of
information called packets. There are two general types of packets, data
packets and message packets. Data packets vary in length up to 4,064 data
bits. The controllers use message packets to send signals to each other.
In an ARCNET protocol, only one controller may transmit at a time. Any of
the other controllers connected to the network may receive the
transmission. The controllers take turns sending data packets. If a
controller, in turn, has information to transmit it sends a packet. It
then passes the right to transmit on to the next controller. The
controllers use message packets to verify that the destination is ready to
accept a transmission and to insure that the next controller receives the
right to transmit.
When a computer 2 has information to send, its ARCNET controller 10 waits
for the right to transmit. As soon as it obtains the right to transmit, it
sends a message to the destination via relay 4 to verify that the
controller at the destination is ready to receive a packet. If the
receiving controller can accept the packet, it sends an "acknowledge"
message and the sending computer transmits a packet. If the receiving
controller can not accept the packet, it returns a "negative acknowledge"
message. The sending controller then passes the right to transmit to
another computer on the network. When the receiving controller accepts a
transmission, it tells the sending controller if it has received the
packet successfully. If transmission was successful, the sending
controller passes the right to transmit in the normal manner. If
transmission was unsuccessful, the receiving controller does nothing.
Eventually the sending controller will get another chance to rebroadcast
the packet.
In addition to employing a first transmission frequency for each of the
satellites and a second receive frequency for each of the satellites to
enable transmission of information between the computers 2, the
communication system in accordance with the principles of the present
invention employs spread spectrum techniques to substantially reduce
interference and multipath fading problems along with the use of focused
antennas, such as omnidirectional antenna 6 at the relay 4 and
unidirectional antenna 5 at each of the satellites 3.
Spread spectrum communications systems use special modulation techniques
that spread the energy of the signal being transmitted over a very wide
band. The information to be conveyed is modulated onto a radio frequency
carrier by some conventional technique, such as amplitude modulation,
frequency modulation or phase modulation, and the bandwidth of the signal
is then deliberately widened by means of a spreading code or function. The
spectrum spreading technique used in the transmitter is duplicated in the
receiver to enable detection and decoding of the signal. Spread spectrum
systems offer two important technological advantages over conventional
transmission systems. First, the spreading technique reduces the power
density of the signal at any frequency within the transmitted band,
thereby reducing the probability of causing interferences to other signals
occupying the same spectrum. Second, the signal processor in spread
spectrum systems tends to suppress undesired signals, thereby enabling
such signals to tolerate strong interfering signals. This results in
significantly higher signal-to-noise ratios than can be achieved by
conventional systems, such as amplitude modulation that use no bandwidth
spreading. The improvement in signal-to-noise ratio is termed "processing
gain" and is greater than that achieved by the diversity techniques of the
prior art.
Direct sequence spread spectrum systems, such as employed in the
communication system of the present invention, combine the information
signal, which is digital, with a much faster stream of binary code. The
combined information and code signal is then used to modulate an RF
carrier. Since the binary spreading code dominates the modulating
function, it is the direct cause of the wide spreading of the transmitted
signal. The spreading code is a fixed-length, pseudo-random sequence of
bits, with random characteristics, but not truly random. The system
continuously recycles the same binary spreading code.
The spread spectrum system's performance in terms of minimizing
interference to other signals and improving processing gain, is determined
in large part by attributes of the spreading code used to spread the RF
carrier. The degree to which interference to other signals is reduced
depends on the length of the spreading code, i.e., the number of bits, and
the relative rate at which the bits of the spreading code are generated
compared to the rate data bits are transmitted.
While the communications system of the present invention preferably employs
a longer code, the description and illustrations employed in the present
application will be concerned with a 7-bit spreading code for purposes of
describing the operation of the communications system of the present
invention.
As previously mentioned with respect to FIG. 1, the local area network in
accordance with the principles of the present invention employs satellites
3 and relay 4 for the transfer of information between personal computers.
One or more personal computers 2 may be connected to each satellite, for a
total of 255 personal computers connected to all satellites 3. Standard
ARCNET wired local area network interface cards 10 are plugged into each
personal computer 2. ARCNET is an established local area network protocol
that operates at 2.5 Mbps over cable. The cables connecting the personal
computers 2 to the satellite 3 are standard twisted pair cables with RJ11
telephone type modular connectors. More than one personal computer can be
connected to a single satellite 3 by daisy-chaining personal computers
together in a single string as illustrated by personal computers 11.
Network access and protocol is identical for all personal computers 2 and
11, whether linked by wire or by radio waves.
The Federal Communications Commission has allocated three different bands
for spread spectrum radio communication, 902-928 MHz, 2400-2483 MHz and
5725-5875 MHz. While the communication system of the present invention can
employ any of these bands, or any other band, the preferred band is the
2400 MHz (2.4 GHz (gigahertz)) and the 5725 MHz (5.7 GHz) bands.
The relay unit 4 is placed centrally with respect to all satellites 3 in
the local area network. The satellites 3 transmit to the relay 4 on the
2.4 GHz band. The relay 4 receives on the 2.4 GHz band and rebroadcasts to
the satellites 3 on the 5.7 GHz band. Satellites 3 use highly directional
antennas on both transmit and receive while the relay 4 uses
omnidirectional antennas on both receive and rebroadcast.
There are three advantages to the communications system of the present
invention as opposed to using individual omnidirectional antennas of the
prior art at each satellite 3 without a relay. First, the focused antennas
on the satellites 3 achieve greater signal penetration through office
walls than would omnidirectional antennas. Second, the directional
antennas on the satellites 3 focus most of the radiated power toward the
center of the network. This reduces interference between neighboring
networks.
Third, and most critical to the operation of the local area network, the
relay system solves the problem of multipath interference. High frequency
radio transmissions reflect off physical objects within the office
environment. This effect, called multipath, can create multiple pathways
through which a transmitted signal can reach a receiver. Signals received
from different paths can interfere with or even cancel each other. As a
result of this interference there are some placements of receiving and
transmitting antennas for which reception is poor. An antenna position
which produces excellent reception may be just a few inches away. For a
single pair of omnidirectional antennas it is not difficult to find
relative positions for which reception is good. However, as you add
additional antennas to a network, finding a position for each antenna
which provides good reception with all other antennas in the network
becomes more and more difficult. In a relay system as presented herein,
each satellite has one point of focus, omnidirectional antenna 6 of relay
4. Thus, each satellite antenna 5 added to the network is no more
difficult to position than the first. Moreover, a focused antenna on a
satellite 3 is less susceptible to multipath interference than an
omnidirectional antenna at the satellite because it is not sensitive to
radiation received from direction outside its narrow angle of focus.
Referring to FIG. 3, the communication system of the present invention can
be employed in a large geographical area including a data transceivers 12
with unidirectional antennas 13 being focused upon omnidirectional antenna
14 of the relay receiver 15. The data transceivers 12 are coupled to the
data source and data utilization equipment 16 with the protocol of this
large area system being again dictated by a predetermined protocol, such
as, for example, an ARCNET protocol provided by ARCNET card 17. Everything
that has been stated with respect to the local area network of FIG. 1
applies to the operation of the communication system in a large geographic
area as illustrated in FIG. 3. As in the local area network of FIG. 1 the
transceivers 12 transmit on a first frequency, such as 2.4 GHz, and
receive on a second frequency different than the first frequency, such 5.7
GHz. The relay receiver 15 receives the first frequency and converts or
rebroadcasts the data on the first frequency to the second frequency for
transmission to all of the transceivers 12. Also the spread spectrum
technique mentioned hereinabove with respect to FIG. 1 is employed in the
large geographic area communications system of FIG. 3.
Referring to FIG. 4, one embodiment of the satellite 3 of FIG. 1 and the
transceiver 12 of FIG. 3 is illustrated in block diagram form. The twisted
pair cables 18 connect the ARCNET card 10 of FIG. 1 and ARCNET card and 17
of FIG. 3 to the satellite 3 of FIG. 1 and the transceiver 12 of FIG. 3. A
simple device, called a balun, can easily adapt an ARCNET card with a
coaxial connector to twisted pair. The controllers within each ARCNET card
10 or 17 will follow a protocol which insures that only one ARCNET card in
the communications system is transmitting data at any given time. Data
comes over a twisted pair cable 18 as a pulse train in packets up to 4,064
data bits long.
The twisted pair cable 18 is connected to a cable driver and receiver 19
which translates the data of the pulse train on the twisted pair cable 18
to digital data for transmission. The driver and receiver 19 also
translates the received data into a pulse train which drives the twisted
pair cable 18. The transmit interface 20 buffers transmit data.
Spread code generator 21 generates a repetitive pseudo-random spreading
code of predetermined number of bits. As an example, in the describing of
the operation of the components of the satellite 3 or transceiver 12 we
will employ a 7-bit code. The transmitted data is coupled to EXCLUSIVE-OR
gate 22 and, as an example, is shown in Curve A of FIG. 5. This
transmitted data is combined in the EXCLUSIVE-OR gate 22 with the
spreading code from generator 21 which has the characteristics shown in
Curve B of FIG. 5. The output of the EXCLUSIVE-OR gate 22 is as is shown
in Curve C of FIG. 5 and is coupled to differential encoder 23. It should
be pointed out that gate 22 functions to superimpose the transmit data of
Curve A, FIG. 5 on the spreading code of Curve B, FIG. 5. The 7-bit
spreading code is repeated once for each data bit. The EXCLUSIVE-OR logic
function inverts the spreading code for a data bit of "1" and leaves it
uninverted for a data bit of "0" as illustrated in Curve C of FIG. 5. The
combined data and spreading code output of gate 22 is coupled to
differential encoder 23. The communication system of the present invention
modifies the phase of the carrier to transmit data. Between transmission
and reception the signal may undergo a phase inversion. This phase
inversion is unpredictable and depends on the physical environment of the
path that the signal travels. Instead of transmitting the spreading code
directly, encod | | |
