U.S. patent number RE35,736 [Application Number 08/344,534] was granted by the patent office on 1998-02-24 for distributed antenna system.
This patent grant is currently assigned to Allen Telecom Group, Inc.. Invention is credited to Alan J. Powell.
United States Patent |
RE35,736 |
Powell |
February 24, 1998 |
Distributed antenna system
Abstract
The present invention provides a distributed antenna system
comprising a primary antenna (14A, 16A) and a plurality of
secondary antennas (36, 40), a .[.fibre.]. .Iadd.fiber
.Iaddend.optic network (20, 22) connected between the primary
antenna (14A, 16A) and the secondary antennas (36, 40), first means
(.[.16.]. .Iadd.24.Iaddend., 44) associated with a first one of the
antennas (16A, 40) which transmits signals received by that antenna
(16A, 40) into the .[.fibre.]. .Iadd.fiber .Iaddend.optic network
(20, 22), and second means (32, 50) associated with a second one of
the antennas (14A, 36) which causes that antenna (14A, 36) to
transmit signals received by the second means (32, 50) from the
.[.fibre.]. .Iadd.fiber .Iaddend.optic network (20, 22).
Preferably, the use of a heterodyne circuit is avoided and the
light signals travelling within the .[.fibre.]. .Iadd.fiber
.Iaddend.optic networks (20, 22) are modulated at radio
frequency.
Inventors: |
Powell; Alan J. (Seal Farnham,
GB2) |
Assignee: |
Allen Telecom Group, Inc.
(Solon, OH)
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Family
ID: |
27263764 |
Appl.
No.: |
08/344,534 |
Filed: |
November 23, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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865325 |
Apr 8, 1992 |
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Reissue of: |
250928 |
Sep 29, 1988 |
04916460 |
Apr 10, 1990 |
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Foreign Application Priority Data
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Jan 29, 1988 [GB] |
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8801975 |
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Current U.S.
Class: |
343/853;
398/115 |
Current CPC
Class: |
H01Q
3/2676 (20130101); H04B 10/25753 (20130101); H04B
10/25759 (20130101) |
Current International
Class: |
H01Q
3/26 (20060101); H04B 10/12 (20060101); H01Q
021/00 (); H04B 009/00 () |
Field of
Search: |
;343/711,853
;359/145,146 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3135231 |
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Apr 1983 |
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DE |
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1439399 |
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Jun 1976 |
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GB |
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1546604 |
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May 1979 |
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GB |
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2177572 |
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Jan 1987 |
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GB |
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Other References
Umechi et al. "Optical Fiber Transmission For Separate Receiving
System of TV Translators," NTC 78 Conference Record of the IEEE
1978 National Telecomm. Conf., Birmingham, Ala, USA, 3-6 Dec. 1978,
pp. 5.6.1 to 5.6.5. .
IEE Proceedings, vol. 132, Pt. F., No. 5, Aug. 1985, entitled
"Towards a High-Capacity Digital Cellular Mobile Radio
System"..
|
Primary Examiner: Wimer; Michael C.
Attorney, Agent or Firm: Sidley & Austin
Parent Case Text
RELATED APPLICATION
This is a continuation of U.S. patent appl. Ser. No. 07/865,325
filed Apr. 8, 1992 abandoned.
Claims
What is claimed is:
1. A distributed antenna system comprising:
a primary antenna means for receiving modulated RF carrier signals
from a remotely positioned radio frequency antenna means;
a circuit means including a first means connected to the primary
antenna means for converting the modulated RF carrier signals to
corresponding modulated light signals, an optic fiber network means
having a first end and a plurality of second ends, the first end
being connected to the first means for receiving and transporting
the modulated light signals to the plurality of second ends, a
plurality of second means connected to the plurality of second ends
for receiving the modulated light signals and outputting
corresponding modulated RF carrier signals, and a corresponding
plurality of secondary antenna means connected to the plurality of
second means .Iadd.each .Iaddend.for receiving and transmitting
.Iadd.all .Iaddend.the corresponding modulated RF carrier signals,
wherein modulated RF carrier signals are received by the first
means of the circuit .Iadd.means .Iaddend.for conversion to light
modulated signals for transport through a radio frequency
interference environment location by the optic fiber network
.Iadd.means .Iaddend.and conversion back to modulated RF carrier
signals by the second means for transmission by .Iadd.all
.Iaddend.the plurality of secondary antenna means.
2. A distributed antenna system according to claim 1 wherein the
primary antenna means includes a receiver means for producing
modulated control signals and said circuit first means includes a
laser means connected to the receiver means for receiving the
modulating control signals and outputting modulated light signals
through the .[.fiber.]. optic .Iadd.fiber .Iaddend.network
.Iadd.means.Iaddend., and said plurality of the .[.circuits.].
.Iadd.circuit .Iaddend.second means including a plurality of
photodetector means for receiving the modulated light signals and
outputting modulated RF carrier signals to the plurality of
secondary .[.antennas.]. .Iadd.antenna means .Iaddend.for
transmission.
3. A distributed antenna system comprising:
a primary antenna means for transmitting modulated RF carrier
signals to a remotely positioned radio frequency antenna means;
and
a circuit means including: a first means connected to the primary
antenna means for converting modulated light carrier signals to
modulated RF carrier signals for transmission by the primary
antenna means to the remotely positioned radio frequency antenna
means, a fiber optic network means having a first end and a
plurality of second ends in communication with the first end,
.Iadd.wherein all optical signals input into each of the second
ends of the fiber optic network means are combined optically into a
composite optical signal communicated to the first means,
.Iaddend.said first end being connected to the first means for
inputting modulated light signals received from the plurality of
second ends into the first means for conversion .Iadd.of the
composite optical signal .Iaddend.to modulated RF carrier signals,
a plurality of second means for converting modulated RF carrier
signals into modulated light signals, said plurality of second
means being connected to the plurality of second ends for inputting
the modulated light signals into the fiber optic network
.Iadd.means.Iaddend., and a plurality of secondary antenna means
for receiving RF modulated carrier signals, said plurality of
secondary antenna means being connected to the plurality of second
means for inputting any received modulated RF carrier signals.
4. A distributed antenna system according to claim 3 wherein the
plurality of second means for converting modulated RF carrier
signals into modulated light signals includes a plurality of
receiver amplifier means.Iadd., .Iaddend.for receiving the
modulated RF carrier signals and producing control signals, and a
plurality of laser means connected to the plurality of receiver
amplifier means for receiving the control signals and outputting
modulated light signals for the fiber optic network means and the
circuit first means connected to the primary antenna means
including a photodetector means connected to the fiber optic
network means for receiving the .Iadd.composite .Iaddend.modulated
light signals and outputting corresponding modulated RF carrier
signals for transmission by the primary antenna means to a remotely
positioned antenna means.
5. A distributed antenna system comprising:
a first primary antenna means for receiving modulated RF carrier
signals from a remotely positioned transmitter;
a first plurality of secondary antenna means for transmitting
modulated RF carrier signals; and
a .Iadd.first .Iaddend.circuit means interconnecting the
.Iadd.first .Iaddend.primary antenna means to the plurality of
secondary antenna means, said .Iadd.first .Iaddend.circuit means
including: a first means connected to the first primary antenna
means for receiving the modulated RF carrier signals and outputting
corresponding modulated light signals, a fiber optic network having
a first end connected to the first means for receiving the
modulated light signals and a plurality of second ends .Iadd.each
.Iaddend.for outputting .Iadd.therefrom all .Iaddend.the modulated
light signals .Iadd.input into the first end.Iaddend., a plurality
of second means connected to the plurality of second ends for
receiving the modulated light signals and outputting corresponding
modulated RF carrier signals to the first plurality of secondary
antenna means.Iadd., whereby each secondary antenna means transmits
all the modulated RF carrier signals that correspond to the
modulated light signals input into the first end of the fiber optic
network.Iaddend.; and
a second primary antenna means for transmitting modulated RF
carrier signals to a remotely positioned .[.transmitter,.].
.Iadd.receiver;.Iaddend.
a second plurality of .Iadd.secondary .Iaddend.antenna means for
receiving modulated RF carrier signals.[.,.]..Iadd.;
.Iaddend.and
a .Iadd.second .Iaddend.circuit means interconnecting the second
primary antenna .Iadd.means .Iaddend.to the second plurality of
secondary antenna means, said .Iadd.second .Iaddend.circuit means
including a plurality of first means connected to the second
plurality of .Iadd.secondary .Iaddend.antenna means for receiving
and converting modulated RF carrier signals to corresponding
modulated light signals, a fiber optic network having a plurality
of first ends connected to the plurality of first means .Iadd.of
the second circuit means .Iaddend.for receiving the modulated light
signals and a second end for outputting the modulated light
signals, and a second means .Iadd.of the second circuit means
.Iaddend.connected to the second end for receiving and converting
the modulated light signals to corresponding modulated RF carrier
signals, said second primary antenna means being connected to the
second means .Iadd.of the second circuit means .Iaddend.for
receiving and transmitting the modulated RF carrier signals to
.[.a.]. .Iadd.the .Iaddend.remotely positioned .[.radio antenna
means,.]. .Iadd.receiver;.Iaddend.
wherein two-way modulated RF carrier signals are converted to
modulated light signals for transport by the fiber optic networks
through a location containing an RF interference environment and
re-converted to modulated RF carrier signals for re-transmission
whereby two-way radio communication is provided .[.on each side
of.]. .Iadd.in .Iaddend.the location containing the RF interference
environment.
6. A distributed antenna system comprising:
a primary antenna means for either receiving or transmitting
modulated RF carrier signals, respectively, to or from a remotely
positioned radio antenna means;
a plurality of secondary antenna means for either transmitting or
receiving modulated RF carrier signals; .[.and.].
a circuit means interconnecting the primary antenna means to the
plurality of secondary antenna means, said circuit means including
a first subcircuit having a first means connected to the primary
antenna means for converting received modulated RF carrier signals
into modulated light carrier signals, a fiber optic network having
a first end connected to the first means for transporting .Iadd.all
.Iaddend.the modulated light carrier signals .Iadd.input into the
first end .Iaddend.to .Iadd.each end of .Iaddend.a plurality of
second ends of the .[.filter.]. .Iadd.fiber .Iaddend.optic network,
and a plurality of second means connected to the plurality of
second ends for receiving and converting the modulated light
carrier signals to modulated RF carrier signals for transmission by
the plurality of secondary antenna means.Iadd., whereby all said
secondary antenna means transmit RF carrier signals corresponding
to the modulated light carrier signals input to the first end of
the fiber optic network.Iaddend.; and
a second sub-circuit having a plurality of first means connected to
the plurality of .[.second.]. .Iadd.secondary .Iaddend.antenna
means for receiving and converting modulated RF carrier signals to
modulated light carrier signals, a fiber optic network having a
plurality of ends connected to the plurality of first means for
transporting the modulated light carrier signals to an end opposite
the plurality of ends, and a second means connected to the end
opposite the plurality of ends of the fiber optic network for
receiving and converting the modulated light signals into modulated
RF carrier signals for transmission by the primary antenna
.Iadd.means, .Iaddend.wherein two-way modulated RF carrier signals
are converted to modulated light carrier signals for transport
through a radio frequency interference environment location for
conversion back to modulated RF carrier signals for transmission.
.Iadd.7. A distributed antenna transmission system for transmitting
RF signals in a radio frequency interference environment,
comprising:
a primary receiver for receiving RF signals from a remotely located
radio frequency transmitter;
a receiver converter connected to the primary receiver for
converting the RF signals to corresponding light signals;
a fiber optic network for use in said radio frequency interference
environment, and having a first end and a plurality of second ends,
the first end of the fiber optic network being connected to the
receiver converter for receiving and transferring all the light
signals input thereto to each end of the plurality of second ends
of the fiber optic network,
a plurality of transmitter converters, each transmitter converter
connected to a respective second end of the fiber optic network for
receiving the light signals and for converting the light signals to
corresponding RF signals;
a transmitter connected to each transmitter converter for
transmitting RF signals; and
an antenna associated with each said transmitter for transmitting
the RF signals, whereby RF signals are received by the receiver
converter for conversion to corresponding light signals for
transfer through a radio frequency interference environment by the
fiber optic network, and for conversion back to corresponding RF
signals by the transmitter converter
for transmission by the plurality of antennas..Iaddend..Iadd.8. A
distributed antenna transmission system according to claim 7
wherein the primary receiver includes means for producing control
signals, and said receiver converter includes a laser connected to
the primary receiver for receiving the control signals and
outputting corresponding light signals through the fiber optic
network, and each said transmitter converter includes a
photodetector for receiving the light control signals and
outputting RF signals to a respective said antenna for
transmission
thereof..Iaddend..Iadd.9. A distributed antenna receiving system
for receiving RF signals in a radio frequency interference
environment, comprising:
a primary transmitter for transmitting RF signals to a remotely
located radio frequency receiver;
a transmitter converter connected to the primary transmitter, and
having a single optical input for converting light signals to
corresponding RF signals for transmission by the primary
transmitter to the remotely located radio frequency receiver;
a fiber optic network for use in the radio frequency interference
environment, and having a first end and a plurality of second ends,
the first end of said fiber optic network being connected to the
single optical input of the transmitter converter for inputting a
composite light signal representative of all the light signals
received from the plurality of second ends to the transmitter
converter for conversion of the composite light signals to
corresponding RF signals;
a plurality of receiver converters for converting RF signals into
corresponding light signals, each receiver converter being
connected to a respective second end of the fiber optic network for
inputting the light signals into the fiber optic network;
a plurality of secondary receivers for receiving RF signals, each
said secondary receiver being connected to a respective said
receiver converter for inputting received RF signals thereto;
and
an antenna associated with each said secondary receiver for
receiving RF signals in the radio frequency interference
environment, whereby the received RF signals are converted by the
receiver converters to corresponding light signals and transferred
through the fiber optic network, for conversion back to
corresponding RF signals by the transmitter converter for
transmitting by the primary
transmitter..Iaddend..Iadd.10. A distributed antenna receiving
system according to claim 9 wherein the plurality of receiver
converters for converting RF signals into corresponding light
signals includes a plurality of receiver amplifiers for receiving
the RF signals and producing control signals, a plurality of lasers
connected to the plurality of receiver amplifiers for receiving the
control signals and outputting corresponding light signals into the
fiber optic network, and the transmitter converter connected to the
primary transmitter includes a photodetector connected to the fiber
optic network for receiving the composite light signals and
outputting corresponding RF signals for transmission by the primary
transmitter to the remotely located radio
frequency receiver..Iaddend..Iadd.11. A distributed transmitting
and receiving antenna system for transmitting and receiving RF
signals in a radio frequency interference environment,
comprising:
a primary receiver for receiving RF signals from a remotely located
transmitter;
a plurality of secondary transmitters for transmitting RF signals;
and
a receiver/transmitter circuit interconnecting the primary receiver
to the plurality of secondary transmitters, said
receiver/transmitter circuit including: a first converter connected
to the first primary receiver for receiving the RF signals and
outputting corresponding light signals, a fiber optic network for
use in the radio frequency interference environment, and having a
first end connected to the first converter for receiving the light
signals, and a plurality of second ends of the fiber optic network
each for outputting all the light signals input to the first end
thereof, a plurality of second converters connected respectively to
the plurality of second ends of the fiber optic network for
receiving the light signals and outputting corresponding RF signals
to the first plurality of secondary transmitters, whereby all the
secondary transmitters transmit RF signals corresponding to all the
light signals input to the first end of the fiber optic network;
and
a primary transmitter for transmitting RF signals to a remotely
located receiver, a plurality of secondary receivers for receiving
RF signals, and a transmitter/receiver circuit interconnecting the
primary transmitter to the plurality of secondary receivers, said
transmitter/receiver circuit including a plurality of third
converters connected to the plurality of secondary receivers for
receiving and converting RF signals to corresponding light signals,
a fiber optic network having a plurality of first ends connected to
the plurality of third converters for receiving the light signals,
and a second end for outputting the light signals, and a fourth
converter connected to the second end of the fiber optic network
for receiving and converting the light signals to corresponding RF
signals, said primary transmitter being connected to the fourth
converter for receiving and transmitting the RF signals to the
remotely located receiver, wherein two-way RF signals are converted
to corresponding light signals for transport by the fiber optic
networks through a location exhibiting an RF interference
environment and re-converted to RF signals for re-transmission,
whereby two-way radio communication is provided in the location
exhibiting the RF interference
environment..Iaddend..Iadd. A distributed transmitting and
receiving antenna system for transmitting and receiving RF signals
in a radio interference environment, comprising:
a primary receiver/transmitter system for either receiving or
transmitting RF signals, respectively, to or from a remotely
located radio frequency transmitter/receiver system;
a plurality of secondary transmit and receive antennas for
respectively transmitting and receiving RF signals; and
circuit means interconnecting the primary receiver/transmitter
system to the plurality of secondary antennas, said circuit means
including a first sub-circuit having a first means connected to the
primary receiver/transmitter system for converting received RF
signals into corresponding light signals, a fiber optic network
having a first end connected to the first means for transporting
all the light signals input to the first end to each end of a
plurality of second ends of the fiber optic network, and a
plurality of second means connected to the plurality of second ends
for receiving and converting the light signals to RF signals for
transmission by the plurality of secondary transmit antennas,
whereby each said secondary transmit antenna transmits RF signals
corresponding to all the light signals input to the first end of
the fiber optic network; and
a second sub-circuit having a plurality of first means connected to
the plurality of secondary receive antennas for receiving and
converting RF signals to corresponding light signals, a fiber optic
network having a plurality of ends connected to the plurality of
first means of the first sub-circuit for transferring the light
signals to an end opposite the plurality of ends, and a second
means connected to the end opposite the plurality of ends of the
fiber optic network for receiving and converting the light signals
into RF signals for transmission by the primary
receiver/transmitter system, wherein two-way RF signals are
converted to light signals for transfer through a radio frequency
interference environment and for conversion back to RF signals
for
transmission..Iaddend..Iadd.13. A method of receiving RF signals in
a zone of interference and transmitting the RF signals at a remote
location, comprising the steps of:
locating a plurality of RF receivers at spaced apart locations in
the zone of interference for receiving free space transmissions of
the RF signals;
converting the free space transmissions of RF signals to
corresponding light signals;
transferring the light signals through a single optical medium from
said zone of interference to a location remote from the zone of
interference to a primary transmitter;
optically combining the light signals converted from RF signals
received by each of the RF receivers to a composite light
signal;
reconverting the composite light signals to corresponding RF
signals; and
transmitting the RF signals by said primary transmitter so that the
RF signals received within the zone of interference are transferred
therefrom by the optical medium and
retransmitted..Iaddend..Iadd.14. The method of claim 13 further
including locating the spaced apart receivers in a
vehicle traffic tunnel..Iaddend..Iadd.15. A method of transmitting
RF signals within a zone of interference, as received from a remote
location, comprising the steps of:
locating a plurality of RF transmitters at spaced apart locations
within the zone of interference;
locating a primary RF receiver at said remote location for
receiving RF signals;
converting the RF signals received at the remote location into
corresponding light signals;
transferring all the light signals through a single optical medium
from the remote location into the zone of interference to each said
RF transmitter; and
converting all the light signals carried by the optical medium to
corresponding RF signals for transmission of the RF signals by each
said RF transmitter in free space in said zone of
interference..Iaddend..Iadd.16. The method of claim 15 further
including branching from the optical medium all the light signals
for conversion thereof to the RF electrical signals and for
transmission in free space of the RF signals..Iaddend..Iadd.17. The
method of claim 15 further including locating the RF transmitters
at spaced apart locations in a vehicle traffic tunnel..Iaddend.
Description
The present invention relates to a distributed antenna system.
BACKGROUND OF THE INVENTION
It is well known that the transmission and reception
electromagnetic radiation at frequencies such as radio frequencies
is severely impaired by any significant mass of solid material such
as the walls of a building or the ground above a tunnel. The
inability to transmit and/or receive radio signals within a tunnel
or from one part of a building to another can be a severe
disadvantage.
In order to mitigate the described disadvantage, it has previously
been proposed to establish a distributed antenna system, which
system is sometimes referred to as a "leaky feeder". This comprises
the provision of a co-axial cable with holes in the shielding of
the cable at strategic locations whereby a radio frequency signal
injected into the cable "leaks out" at the strategically placed
holes. This arrangement does, to some extent, mitigate the above
.[.dscribed.]. .Iadd.described .Iaddend.disadvantage. However,
attenuation of the radio frequency signal within the cable is
severe and typically a repeater may be required at 100 yard
intervals with a maximum practical length of cable being about 1
mile. Beyond this distance, it is extremely difficult to
distinguish the original signal from the background noise, despite
the use of the repeaters. It will be appreciated that a relatively
high power signal is used and consequently the co-axial cable must
have relatively high power specifications, which inevitably result
in a relatively high expense. The "leaky feeder" co-axial system is
not appropriate for use with the radio signals at the frequencies
used for cellular radio telephone systems.
SUMMARY OF THE INVENTION
With a view to providing an improved system, the present invention
provides a distributed antenna system comprising a primary antenna
and a plurality of secondary antennas, a fiber optic network
connected between the primary antenna and the secondary antennas,
first means associated with a first one of the antennas which
transmits signals received by that antenna into the fiber optic
network, and second means associated with a second one of the
antennas which causes that antenna to transmit signals received by
the second means from the fiber optic network.
In one embodiment, the first antenna is the primary antenna and
each of the secondary antennas is provided with a respective one of
said second means. In another embodiment, the second antenna is the
primary antenna and each of said secondary antennas is provided
with a respective one of said first means. More preferably, an
embodiment of the invention provides a distributed antenna system
in which both the aforementioned arrangements are provided. That
is, the system provides for distributed transmission and
distributed reception.
Most beneficially, the signal transmitted into the fiber optic
network comprises direct radio frequency modulation of the output
of a laser.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described by way
of example only and with reference to the accompanying drawings, in
which:
FIG. 1 is a schematic diagram illustrating an embodiment which
provides for both distributed transmission and distributed
reception, and
FIG. 2 is a more detailed block diagram illustrating the reception
and distributed transmission system of FIG. 1, and
FIG. 3 is a more detailed block diagram illustrating the
distributed reception and central re-transmission system shown in
FIG. 1.
DETAILED DESCRIPTION
A preferred embodiment of the invention is illustrated
schematically in FIG. 1. A conventional radio transmission and
reception system is indicated by units 10-16, each of which has an
associated antenna, 10A-16A. Units 10 and 12 are respectively a
transmitter and a receiver established at a first location. Units
14 and 16 are respectively a transmitter and a receiver established
at a second location. Two way radio communication between the first
and second locations takes place in a purely conventional manner,
as is to be found in any free space radio link. However, units 14
and 16 act as the input and output of a distributed antenna
system.
In the example illustrated in FIG. 1, it is desired to provide a
full free space radio transmission and reception system within a
tunnel. The tunnel is indicated by reference numeral 18. One
circuit, 20, provides distributed transmission of a radio signal
within the tunnel and another circuit 22, provides for the
distributed reception of radio signals from within the tunnel. Of
course, in practice, there may only be a requirement for either
distributed reception or distributed transmission, in which case
only one of the circuits 20 and 22 would be provided. However, the
illustrated two circuit arrangement may be desirable, for example
if the aim is to provide cellular radio telephone facilities within
the tunnel.
Each circuit 20 and 22 comprises a primary antenna and a plurality
of secondary antennas, with the primary antenna being connected to
the secondary antennas via a fiber optic network. In the case of
circuit 20 (FIG. 2), the receiver units 16 associated with primary
antenna 16A provides control signals to modulate the output of a
laser 24. Laser 24 transmits light signals into fiber optic network
26 which has network branches 28 which feed respective secondary
antenna systems 30. The secondary antenna system 30 comprises a
photo detector 32 which receives light signals from fiber optic
branch 28, and a transmitter amplifier 34. Amplifier 34 receives
electrical signals from the photo detector 32 and supplies signals
to secondary antenna 36, whereby the original radio frequency
signal is re-transmitted within the tunnel. A number of secondary
antenna units 30 may be spaced along the length of the tunnel,
effectively providing local "drop off" nodes for the radio
frequency signal.
Circuit 22 (FIG. 3) is of similar configuration except for the fact
that the signals travel in the opposite direction. That is, each
secondary antenna unit 38 comprises a secondary antenna 40 which
receives radio frequency signals from within the tunnel and
supplies these to a receiver amplifier unit 42 which uses the
received signals to control the output of a laser 44. Laser 44
transmits light signals into the fiber optic network 46 via a fiber
optic network branch 48. A photo detector 50, filter 56 and power
amplifier 58 are associated with primary antenna 14A and
transmitter 14. That is, photo detector 50 receives light signals
from fiber optic network 46 and supplies transmitter 14 with radio
frequency electrical signals which are used to cause primary
antenna 14A to re-transmit the radio signals. A plurality of
secondary antenna units 38 may be provided along the length of the
tunnel.
The .[.laser.]. .Iadd.lasers .Iaddend.employed in the illustrated
arrangement are of conventional construction. These lasers are,
however, of the so-called "linear" type and operate in an analog
rather than a digital mode. The components used for the various
transmitter and receiver units are also conventional. More detail
of these units is given with reference to FIGS. 2 and 3. It is to
be noted that in the arrangement described with reference to FIG.
1, the light signals travelling within the fiber optic networks are
modulated at radio frequencies. The only conversion is between
electrical and light signals. No heterodyne circuit is used.
FIGS. 2 and 3 illustrate in more detail the respective circuits 20
and 22 shown in FIG. 1. That is, FIG. 2 shows the .[.detail.].
.Iadd.details .Iaddend.of a circuit suitable for a `Base to Mobile`
distribution antenna system.Iadd., .Iaddend.whereas FIG. 3 shows
the .[.detail.]. .Iadd.details .Iaddend.of a circuit suitable for a
"Mobile to Base" distributed antenna system. As stated above, it is
expected that the two circuits will usually be used together,
although each could be used separately as the circumstances
.[.requires.]. .Iadd.require.Iaddend.. Essentially, FIGS. 2 and 3
.[.shown.]. .Iadd.show .Iaddend.that the radio frequency to fiber
optic converters and fiber optic to radio frequency converters
comprise conventional components. Specifically, the radio frequency
to fiber optic converters comprise a filter 52, a laser power
supply circuit 54, and the laser itself 24 and 44. The fiber optic
to radio frequency .[.to fiber optic.]. converters comprise a photo
detector .[.30.]. .Iadd.32 and 50.Iaddend., a filter 56 and an
amplifier .Iadd.34 and .Iaddend.58. The design of these components
is within the skill of the person skilled in the art and
consequently will not be described herein.
In comparison, with the co-axial cable used in the known "leaky
feeder" system, a suitable fiber optic network could be established
at a very significantly reduced cost, perhaps as high as an 80%
saving. Propagation of light signals within the fiber optic
network, as is commonly known, .[.are.]. .Iadd.is .Iaddend.subject
to remarkably little attenuation. It is considered possible for a
signal to be transmitted in the fiber optic network over a distance
of about 30 miles before it is necessary to introduce a repeater.
This is a very striking contrast with the above described use of
repeaters in the co-axial system and may well be of profound
significance for many modern vehicle tunnels. The fiber optic
network is, of course, physically very flexible and easily conforms
to the configuration required by the structure within which it is
located. Signals travelling within the fiber optic network are
unaffected by radio frequency interference and thus the network may
be located adjacent power cables, which is not possible with the
conventional co-axial system. Moreover, the bandwidth of the fiber
optic network is considerably better than that of a co-axial
system. The fiber optic bandwidth can cover essentially all radio
frequencies and in particular those used by the cellular radio
telephone system.
In particular, the receiver units and respective lasers comprise a
linear analog system. That is, the laser .[.38.]. .Iadd.44
.Iaddend.(FIG. 3) is modulated in its linear region of operation.
Specifically, the receiver units modulate the radio frequency on
the DC power supply of the laser. This results in radio frequency
baseband signals being transmitted in the form of light waves.
Typically, the radio frequencies used with the above described
embodiment might be in the range 100 MHz to 1 GHz.
For use in the described tunnel system, the power of the signal
transmitted at each secondary antenna 36 may be of the order of a
few milliwatts. In a conventional system using a "leaky feeder"
co-axial system or an injection aerial located at the entrance to
the tunnel, the power of the transmitted signal would typically be
of the order of tens of watts and the maximum penetration into the
tunnel will be significantly less than can be achieved with the
described fiber optic network system. It is to be noted that the
fiber optic network may take any suitable form whether tree-like or
linear.
One application of the present invention, namely use in tunnels,
has been described. However, it will be readily apparent to those
skilled in the art that the present invention has numerous
applications. As a further example, the system is particularly
useful within buildings, especially larger office accommodation and
hospitals or the like. Such application of the invention is
particularly beneficial in combination with cordless telephone
systems and cellular radio telephones. With use of the present
invention, it may be feasible to locate a radio telephone "cell"
within a single building.
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