Note: Descriptions are shown in the official language in which they were submitted.
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TWO WAY CABLE SYSTEM WITH NOISE-FREE RETURN PATH
REFERENCE TO RELATED APPLICATIONS
This application is a Continuation in Part of Application Serial No. 09/541,
I87
filed April 3, 2000 to the applicant of the present application.
FIELD OF THE INVENTION
This invention relates to cable systems and more particularly to such systems
with
a sufficiently noise free return path to support high bandwidth two-way
broadband,
multimedia content delivery to and from the home.
BACKGROUND OF THE INVENTION
It is well known that the return path in a cable system is noisy and is
frequently
referred to as a "noise funnel". There are three primary sources of such
noise: Thermal,
fiber optic link and ingress. Thermal noise is generated in each of the active
components
(amplifiers and receivers). The fiber optic link noise is generated in the
return laser, fiber
and headend receiver. Ingress noise arises through home wiring and connections
and
constitutes the major source of noise. A complete discussion of the return
path and the
noise characteristics is provided in "Return Systems for Hybrid Fiber/Coax
Cable TV
Networks" by Donald Raskin and Dean Stoneback, 1998 Prentice Hall, Inc.
Traditional cable systems have a major trunk along which signals are
transmitted
from a headend in a forward direction to set-top boxes located in homes or
business
facilities connected to the feeder lines. Connection of set-top boxes to a
feeder line is
provided by connecting each set-top box to the feeder line via a tap. In the
usual
organization of a cable system there are many set-top boxes connected to each
feeder line.
Moreover, each feeder and/or trunk line includes bi-directional amplifiers
which pass
signals in a high frequency band in the forward (downstream) direction, and in
a low
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frequency band in the return (upstream) direction, which is well understood in
the art.
Signals in the low frequency band originate at set-top boxes and are used to
communicate
in the upstream direction to the headend.
The problems with present return paths in cable systems arise from the fact
that the
path from the set-top to the tap in the feeder line (the inside wiring and the
drop) is
characterized by an unacceptable level of noise (ingress) which is picked up
in the home
wiring and in drop cable in the low frequency band where the set-top box
transmits.
Further, no other band (relatively free of such ingress noise) in a low-split
cable system is
available for transmission from the home to the headend. Present low-split
cable systems
are wedded to transmission from the cable headend in a high frequency band and
transmissions from set-top boxes in a low frequency band.
Yet the financial expectations of two way, broadband channels via a cable
system
are so compelling that significant resources are being dedicated towards
solving the
ingress noise problems in the return paths. The present remedial solutions are
expensive,
cause system shut down, cause system instability, require repeated service
calls to
subscribers facilities, and frequent home and drop rewiring or installation of
special traps.
Moreover, with corrosion and deterioration of lines and connectors, there is a
high
likelihood that continued attention by cable operators will be necessary.
In the last ten years the cable industry has been retrofitting its cable
infrastructure
to allow for two-way communications on the cable plants. This is referred to
in the
industry as activating the return path; the return path being in the 5-40 MEiz
frequency
band. The design of the return path started with rebuilds in the late 70's. In
the late 80's
the larger cable companies began to segment their service area into smaller
groups called
"nodes", and changed their trunk system in many cases from using just co-axial
cable and
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trunk amplifiers to a hybrid fiber/co-axial cable system (HFC). At the same
time active
and passive devices were replaced to increase the frequency spectrum in the
downstream
direction from 50-350 MHz plants to 50-750 MHz, in some cases up to 850 MHz.
The
increased downstream frequency band allows cable companies to offer more
channels of
video services. The increased bandwidth also can be used for digital services
in the
forward direction. Also, by now activating the return path, two-way services
such as
impulse pay-per view, interactive TV, cable modems, telephone service, and
additional
services can be offered.
In the activation of the return path, it has been found by most of the cable
companies, that the 5-40 MHz frequency band, especially the 5-15 MHz spectrum
is
extremely noisy. Because of the presence of the noise, most of the services
presently
available in the lower frequency band are digital services that can often work
with low
carrier to noise signal levels. But since the noise is not consistent,
services are seriously
impaired at times. Thus, a large number of cable companies are currently
looking for ways
to reduce the noise in the 5-40 MHz frequency band. Most of the approaches
have been to
reduce the number of homes connected to each node thereby reducing the total
accumulated noise collected in each segment of the node. There have also been
approaches involving the installation of 5-50 MFIz blocking filters to reduce
the noise
from inactive subscriber's homes in the 5-50 MHz frequency band from entering
the main
cable distribution network. The current best approach is to divide the cable
system into
many nodes which service as few as fifteen homes which is in effect providing
a system of
small clusters of homes, each connected directly to the node.
BRIEF DESCRIPTION OF THE INVENTION
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The present invention is based on the realization that a portion of the
downstream
frequency band (i.e. 50-750 MHz) can be used, in part, to carry the return
path signal from
a set-top box. That portion of the frequency band is presently used to provide
TV signals
and digital signals from the headend to the home. But that portion of the band
cannot
presently be used to carry return path signals.
In accordance with the principles of this invention, the noise picked up in
home
appliances, drops, connectors, etc and transported to the corresponding node
in the feeder
line is avoided by reconfiguring the set-top box to transmit in the high
frequency band
rather than in the low frequency band where most of the noise occurs. The
signals from
the set-top box proceed in the downstream direction to the feeder line end,
which in
addition is equipped with a receiver (optical transmitter) and a fiber back to
the node or
directly to the headend. The result is that set-top box transmissions travel
in the forward
direction to the feeder line end where they are received and transmitted
(optical
transmitter) via a fiber to the node or the headend. The noise (home to feeder
line tap) is
avoided since the lower frequency band in which the majority of the ingress
noise is
located is not utilized. In this context, each feeder line end has
terminators) and the
receiver (i.e. optical transmitter) plus fiber link may be placed just after
the last amplifier
(terminal) in the feeder line and before the feeder line separates to
different line
terminator(s). The portion of the feeder line between the last amplifier
(terminal) and the
position where the feeder line separates line terminators) is referred to
herein as the
feeder line end.
Specifically, applicant herein adds to the cable system relatively inexpensive
equipment which permits the set-top box to feeder line end portion of the
return path to
function as a forward path. This is accomplished, in one embodiment, by
providing at
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each feeder line end a optical transmitter and fiber link. The optical
transmitter receives
the signals in the high portion of the band and transmits the signal via a
fiber link back to
the node or headend. The nature of this system is that it virtually eliminates
ingress noise
from house wiring and the drop, which is shown schematically on page 57 of the
above-
noted publication. It provides a further advantage of allowing the system
operator to
choose the size and location of the return band within the 50-750 MHz
frequency
spectrum.
In another embodiment, each feeder line end includes a receiver and a
demodulator
to decode the received data. The decoded data is then used to modulate a new
signal. The
regenerated signal does not contain the noise that was contained in the
received signal. It
is in effect a noise free signal. A optical transmitter than send the signal
via the fiber to
the node or headend.
Thus, in accordance with the principles of this invention, a technique is
provided
for eliminating the ingress noise in the low frequency band from house wiring,
devices)
in the home, and the drop from entering the cable system by not using the
lower frequency
band where most of the ingress noise resides. Each of the forward amplifiers
already has a
high pass filter blocking the low frequency band being amplified in the
forward direction.
The higher frequency band is used to carry the return signal from the feeder
line end to the
node or the headend. Due to the substantial noise reduction, much higher
modulation
schemes such as QAM-16, QAM-32, QAM-64, QAM-256 etc may be employed. Current
modulation schemes also become much more reliable and have much lower bit
error rates.
The return signals are not restricted to modulated signals. All kinds of
signals (i.e. video
signals, radio signals) can originate from the subscriber locations. Overall
it makes the
return path in a cable system much more usable. With the resulting higher
reliability there
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is likely to be fewer customer calls for service and higher customer
satisfaction. The size
of the return path is totally flexible and the system operator can choose any
size and
location in the frequency band for the return signal. Furthermore, since the
return path for
each feeder line end can be brought back separately to the headend, the
effective size of
the return path for the overall system is substantially larger than the
existing system could
allow.
This invention, illustratively, utilizes a portion of the 50-750 MHz frequency
band
to carry the return signal from the subscriber locations, rather than the 5-40
MHz
frequency spectrum. But the return signal is transmitted first forward to the
feeder line
end where it is picked up and brought back to the cable headend. At the end of
each of the
feeder lines is a receiver that operates, illustratively in the 50-750 MHz
band to receive the
"return" signal. For example, the 300-350 lVYf3z band could be used to carry
the return
signal "forward" to the feeder line end. The signals in this band are received
by the
receiver (i.e. optical transmitter) at the end of the feeder line. The signals
are then send
via a fiber link to the node or to the cable headend The ingress noise in the
lower
frequency band is total avoided since the lower frequency band is not
utilized. The system
also does not require reverse amplifiers and thereby also avoiding the need to
align reverse
amplifier signal levels that is time consuming and painstaking work.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic representation of a prior art cable system including
cable
headend, trunk, nodes and illustrative set-top box locations;
Fig. 2 is a graphic representation of portions of the frequency band presently
used
for cable headend, set-top box, and bi-directional amplifier operation in
prior art cable
systems;
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Fig. 3 is a graphic representation of typical ingress noise levels for
transmissions in
the low frequency band of Fig. 2;
Fig. 4 is a schematic representation of a cable system in accordance with the
principles of this invention;
Fig's SA - SD are graphical representations of portions of the frequency band
used
for the headend, the set-top box, the bi-directional amplifier, and the
optical transmitter of
the arrangement of fig 4.
Fig's 6A- 6F and 7A- 7F are representations of signal processing schemes and
related graphical representations of portions of the frequency band used for
feeder line
ends in cable systems in accordance with the principles of this invention;
Fig 8 shows a set of related graphs of signal level versus frequency for a
cable
headend, a set-top box, an amplifier and an optical transmitter for a cable
system in
accordance with the principles of this invention;
Fig 9 shows a graph of ingress noise and portion of the frequency spectrum in
which set-top boxes transmit in accordance with the principles of this
invention;
Fig 10 is a schematic representation of an alternate embodiment of this
invention;
Fig 11 is a set of related graphs of signal level versus frequency for a cable
headend, a set-top box, an amplifier and optical transmitter respectively for
a cable system
in accordance with the principles of this invention;
Fig 12 is a graphical representation of a band pass filter for use in
embodiments of
this invention; and
Fig's 13, 14, and 15 are schematic representations of a prior art set-top box
and
alternative set-top boxes useful in embodiments of this invention.
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DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THIS
INVENTION
Fig. 1 shows a schematic block diagram of a prior art cable system to
establish a
point of reference and terminology for the description of illustrative
embodiments of this
invention: Specifically, Fig. 1 shows a cable system 10 with a cable headend
11 and a
major trunk 12. Trunk 12 typically comprises a coaxial cable and is connected
to node or
hubl3. Node 13 is connected to the cable headend via optical fiber (or a
coaxial cable) 14
and (for the former) includes a optical transmitter for providing return
signals from a
subscriber set-top box to the cable headend.
The major trunk includes a plurality of bi-directional amplifiers represented,
illustratively, at 17 and 18. The trunk also includes bi-directional bridger
amplifiers 20
and 21 to which feeder lines 22, 23 and 24 are connected as indicated. Also
shown is a
auxiliary feeder line 26 which also includes bi-directional amplifiers
(represented at 27)
and tap 28 to which a drop cable 29 to a set-top box is connected.
A trunk cable end terminator is present at the end of trunk 12 as indicated at
30.
The end of a feeder line 24 has line terminators at 31 and 32.
Fig. 2 shows a set of related graphs of signal level versus frequency for the
headend, the set-top box, and the bi-directional amplifiers respectively, for
a prior art
cable system. In the prior art system, the cable headend illustratively,
receives signals in
the 5-40 MHz band and transmits over the entire, illustratively, 50-750 MHz
band. The
set-top box operates in just the opposite manner. Specifically, the set-top
box transmits in
the 5-40 MHz band and receives signals in the 50-750 MHz band.
The bi-directional amplifiers pass signals forward, (away from the headend) in
the
50-750 MHz band and pass return (toward the headend) signals in the 5-40 MHz
band.
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Thus, signals from a set-top box in the 5-40 MIiz band occur exactly where
most of the
ingress noise occurs. Fig. 3 shows a curve 33 representing the accumulated
ingress noise
with maximum ingress in the 5-40 IVfriz band. It is clear that the usefulness
of the present
return path can be severely limited by ingress noise.
Fig. 4 is a block diagram of a cable system in accordance with the principles
of
this invention. The system 40 comprises a headend 41 connected to a node (or
hub) 42 by
fiber optic (or coaxial) cable 43. The node contains one or more return
optical
transmitters (for fiber optic systems). The system also includes a major trunk
45 with
amplifiers 47 and 48 (there usually are more amplifiers and they are located
usually 500-
1500 feet apart) with bridger amplifiers 50, and 52. In one embodiment a
feeder line 56 is
shown connected to bridger amplifier 50 and line end terminator 58. A high
pass filter 59
and an optical transmitter 60 is located between the last amplifier (terminal)
57 and feeder
line terminator 58 . The optical transmitter feeds into fiber 91 that goes to
node 42. It
could be routed directly to cable headend 41 and the principals of this
invention would
still apply. The frequency spectrum modulating the optical transmitter 60 is
shown in Fig.
5A.
In another embodiment, a feeder line 61 has feeder line ends at terminators 62
and
65. A band pass filter 63 and an optical transmitter 64 are located after the
last amplifier
53 and before terminators 62 and 65. The band pass falter only passes signals
in the 300-
350 MHz frequency band, the frequency band in which the set-top boxes
transmit. The
optical transmitter 64 feeds the signal into fiber 92 that goes to node 42.
The frequency
spectrum modulating optical transmitter 64 is shown in Fig. 5B.
Illustrative of a further embodiment, a feeder line 66 has a feeder line end
terminator 69 which includes a band pass filter 67, a demodulator 70,
modulator 71, and
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an optical transmitter 68. The band pass filter only passes signals in the 300-
350 MHz
frequency band; the frequency band in which the set-top boxes transmit. Only
one
demodulator is shown in Fig. 4 for illustrative purposes. There can be
multiple
demodulator and modulator pairs, all separated in the frequency spectrum over
the 300-
350 MHz frequency band. The combined output of the modulators can be over any
frequency spectrum. In the illustrative embodiment of fig. 4, the modulator 71
output is
over the 140-190 MHz frequency spectrum. The frequency spectrum modulating
optical
transmitter 68 is shown in Fig. SC. In a further embodiment a feeder line 110
has a feeder
end at 80 which includes a band pass filter 81, a frequency band block
converter 83 which
converts the block of frequency band 300 to 350 MHz to 50 to 100 MI3z, and an
optical
transmitter 82. Again, the band pass filter only passes signals in the 300-350
MHz
frequency band; the frequency band in which the set-top boxes transmit. The
frequency
spectrum modulating optical transmitter 82 is shown in Fig. SD. Use of a block
converter
allows different frequency bands to be used for communication back to the
node, where
I S the various signal for the feeder line ends are combined together into a
single signal but
separated in the frequency band, and sent back to the cable headend.
In Fig. 4, fiber 92 is shown connected to node 42. The fiber could be routed
directly to the headend with no additional processing of the signal at node
42. The
complete frequency spectrum 50-750 MHz could then be received at the headend.
This
would provide the system operator the capability of checking the quality of
the signals
sent on the network plus the receive the signals sent back by the set-top
boxes.
Fig. 6A shows one possible embodiment of signal processing at node 42 of Fig.
4.
In this particular case all the signals received from feeder line ends at node
42 are
combined into a signal and the combined signal is carried to the headend via a
single fiber
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cable. Fig. 6A shows a fiber input into optical receiver 1 S 1. The input to
optical receiver
1 S 1 could be the signal transmitted on fiber 92 of Fig. 4. Fig. 6B shows the
frequency
spectrum output of optical receiver 1 S 1. The optical receiver 1 S 1 output
signal is fed into
a combiner 1SS. Similar outputs of optical receiver 152, 153, and 154 are also
fed into
S combiner 1SS. Fig. 6C shows the frequency spectrum of optical receiver 152.
Fig. 6D
shows the frequency spectrum output optical receiver 153. Fig. 6E shows the
frequency
spectrum output of receiver 154. The frequency spectrum output of combiner 1SS
is
shown in Fig.6A. The frequency spectrum of optical receivers 1 S 1, 1 S2, 1
S3, and 1 S4 are
combined and overlap in the frequency output spectrum of combiner 1 SS. In
this
particular case only one of the set-top boxes transmits at a particular
frequency at the same
time. The output of each of the set-top boxes is in-effect time division
multiplexed. Fig.
6A shows only four fiber returns but the principal would be the same where the
number of
return fibers is substantial greater. The output of combiner 1SS is fed into
optical
transmitter 156. The output of optical transmitter 1 S6 is a fiber cable back
to the headend.
1 S The signal processing at the headend is similar to that of the prior art
system. Table 1
shows various other kinds of devices and mediums that can replace an optical
transmitter
and fiber at the feeder line end and still achieve the same objective of
getting the received
signals from the set-top box at the feeder line end back to the headend. A
person skilled in
the art would recognize, that if the device and medium are changed at the
feeder line end,
a receiver that is compatible with the medium and transmitter device would be
required at
the cable headend to receive the signal. Any required signal conversion could
also be
made at the feeder line.
Fig. 7A shows another embodiment of signal processing at node 42 of Fig. 4. In
this embodiment the signals received from each of the feeder lines is
frequency division
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multiplexed into a single signal. The frequency division multiplexed signal is
sent back
via fiber to the headend. Fig. 7A shows a fiber input into optical receiver
141. The input
to optical receiver 141 could be the signal transmitted on fiber 92 of Fig. 4.
Fig. 7B shows
the frequency spectrum output of optical receiver 141.
High Pass Filter 59 of Fig. 4 is operative to receive signals illustratively
in the 50
to 750 MHz band. The set-top boxes in the system of Fig. 4 are also operative
to transmit
in the 50 to 750 MHz band. Thus, transmissions from a set-top box (the return
transmissions) are received first by receivers at the feeder line ends before
they are
transmitted via the fiber link to the cable headend. The principals of this
invention would
still apply where the optical transmitter and fiber link are changed to a
cable amplifier and
co-axial cable.
Table 1.
Device at feeder Return Medium
line end
Optical Transmitter Fiber Optic Cable
Amplifier Co-axial cable
Wireless transmitterAir
Microwave transmitterAir
Satellite TransmitterAir and Space
Telephone Modem Telephone Line
It is to be understood that in accordance with the principles of this
invention,
signals from a set-top box are in a frequency band which travels to a receiver
at the feeder
line end rather than in a return path to the cable headend.
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But each feeder line end, also in accordance with the principles of the
invention,
includes means for receiving those signals and transmitting those signals back
to the node
or the cable headend. In the Fig. 4, the means for receiving signals in the 50-
750 MHz
band are optical transmitters 60, 64, 68 and 82. The optical transmitters feed
the signal
into the fiber and those signals are received at the node or cable headend.
Fig. 8 shows a set of related graphs of signal level versus frequency for a
cable
headend, a set-top box, an amplifier, and an optical transmitter respectively
for a cable
system in accordance with the principles of this invention. As shown in Fig.
8, the
headend can receive in the 50-750 MHz band, but does not transmit over the
entire 50-
750 MHz band. The 300-350 MHz portion is notched out. The set-top box
transmits in the
300-350 MHz portion and receives in the 50-300 MHz and in the 350-750 MHz
bands.
The amplifiers operate only in the forward direction and carry signal away
from the
headend. Not having reverse amplifiers removes the necessity of having to
align the
reverse amplifiers which is time consuming and painstaking work.
I S It is clear from Fig. 8 that signals transmitted by set-top boxes in the
system of Fig.
4 are passed in a "forward" direction to the corresponding feeder line end
where they are
received, and transmitted back to the node or cable headend.
Fig. 9 shows a graph of ingress noise 100, corresponding to that of Fig. 3,
along
with the portion of the frequency spectrum 300-350 MHz in which set-top boxes
transmit
in accordance with the principles of this invention. It is clear that ingress
noise is
insignificant over the portion of the spectrum now used by set-top boxes in
the system of
Fig. 4, Thereby providing return signals virtually free of ingress noise in
the return path to
the cable headend.
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A system, in accordance with one of the embodiments of the principles of this
invention, also includes band stop (notch) filters (i.e. 112) at the start of
auxiliary feeder
lines (i.e. 110) in the system (of fig. 4) to ensure that transmissions from a
set-top box in
the 50-750 MHz band are only received by one feeder end in the system. Such a
band
stop filter 112 is located at the start of auxiliary feeder line 110 to ensure
that the signal
for each set-top box is received only at one feeder end (i.e. the signal from
set-top box 55
is received by band pass filter 67 only, since band stop filter 112 blocks the
signals from
being received by band pass filter 81.
Fig. 10 shows a system similar to that of Fig. 4 where the bi-directional
amplifiers
of the prior art system of Fig. 1 are maintained to provide a system where the
prior art set-
top boxes can continue to function as before in this new combined system that
allows for
new set-top boxes to transmit in the 50-750 MHz band and prior art set-top
boxes to
transmit in the 5-40 MHz band.
Fig. 11 shows a set of related graphs of signal level versus frequency for a
cable
headend, a set-top box, an amplifier, and optical transmitter respectively for
a cable
system in accordance with the principles of this invention. As shown in Fig. 1
l, the
headend can receives in the 50-750 MHz band and also in the 5-40 MHz as in the
prior
art, but does not transmit over the entire 50-750 MHz band. The 300-350 MHz
portion is
notched out. The new set-top box transmits in the 300-350 MHz portion and
receives in
the 50-300 MHz and in the 350-750 MHz bands. The bi-directions amplifiers
operate as
before to carry 5-40 MHz signals towards the headend and 50-750 MHz signal in
the
forward direction away from the headend. This embodiment shows a combined
system
where the prior art set-top boxes and the novel set-top boxes of this
invention operate in a
combined system. This combined system offers a solution to cable operators to
continue
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to serve existing customers with their existing (prior art) set-top boxes and
new customer
using the set-top boxes in line with the principles of this invention. Thereby
retaining the
existing system and upgrading the system in line with the principles of this
invention.
Fig. 12 shows a graphical representation of a band stop filter which passes
signals
in the 5-750 MHz band except for signals in the 300-350 MHz (notch) portion of
the
band. The presence of such filters prevents signals from a set-top box (in the
300-350
MHz band) from being received by more than one feeder end.
Fig's. 13 and 14 show schematic representations of a prior art set-top box and
a
set-top box in accordance with the principles of this invention, respectively.
In the prior
art set-top box of Fig. 13, a high pass filter 104 excludes signals in the 5-
40 MHz band
and passes signals in the 50-750 MHz band. The set-top box also includes a low
pass
filter 101 which excludes signals in the 50-750 MHz band and passes signals in
the 5-40
MHz band.
The set-top~box of Fig. 14 is considerably different. Specifically, the set-
top box
of Fig. 14 includes a band stop filter 102 which passes SO-750 MHz but notches
out
signals in the 300-350 MHz band. The set-top box also includes a band pass
filter 103
which passes signals in the 300-350 MHz band. Thus, the set-top box ofFig. 14
receives
and transmits in the same (high) band (i.e. 50-750 MHz) whereas the set-top
boxes of the
prior art receive and transmit in high and low (considerably different) bands
respectively.
Fig 14, further shows an optional feature in the set-top box that would allow
the set-top
box to also transmit a return signal in the low frequency band (5-40 MHz) as
done by the
prior art set-top box. This optional feature would work with the system of
Fig. 10 that
allows the headend to receive in the 5-40 MHz band and in the 50-750 MHz band.
This
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new set-top box would now have two separate frequency bands in which the
headend can
receive signals from the set-top box.
Fig. 4 also shows an auxiliary feeder line 110 extending from feeder line 66.
It is
important that a transmission from a set-top box of the system of Fig. 4 be
received only
by the receiver at the end of one feeder line to which the transmitting set-
top box is
connected. In order to prevent signals from, for example, a set-top box
connected to
feeder line 66 being received by a receiver 82 (optical transmitter) connected
to an
auxiliary feeder line (110), the auxiliary feeder line includes a band stop
filter 112 to
exclude such transmissions as discussed herein before.
Alternatively, the cable headend may be configured to poll (i.e. enable) a set-
top
box and the corresponding feeder line end optical transmitter simultaneously
so that only
signals from that receiver are received at the headend. The cable headend will
of course,
require additional software in this case. This would allow the cable operator
to choose the
size and location of the return frequency band. Frequency agile band stop
filters and
frequency agile band pass filters can also be used in the system to utilize
any portion of
frequency band desired by the system operator. The frequency bands selected
herein are
only illustrative and other bands and/or notches may be suitable as is clear
to one skilled
in the art. For example, the operator could use the 700 MHz and up band for
the return
path. In this case the configuration of the set-top box would change to that
shown in Fig.
15. The optional feature shown in Fig. 15 allows this new set-top box to also
send signals
to the headend via the 5-40 MHz band. This set-top box can communicate with
the
headend in the 5-40 MHz frequency band and in the 700 MHz and up frequency
band.
There are various ways for the signals received at node 42 of Fig. 4 to be
brought
back to the cable headend 40 of Fig. 4. The embodiment of Fig. 7A shows
receipt of fiber
16
CA 02407321 2002-10-24
WO 01/82618 PCT/CA01/00556
92 of Fig. 4 being received by optical receiver 141. Fig 7B shows the
frequency spectrum
received by optical receiver 141. The 300-350 MHz frequency band ofFig. 7B is
directly
mapped to 300-350 MHz frequency band of Fig. 7A. Fig. 7C shows the frequency
band
received by optical receiver 142. Block frequency converter 146 of Fig. 7A
converts the
300 MHz (f~) to 350 MHz (fd) frequency band to 240 MHz (fd) to 290 MHz (f~)
frequency
band. The frequency spectrum of optical receiver 141, block frequency
converters 146,
147, and 148 are combined by combiner 149. The frequency spectrum output of
combiner
149 is shown in Fig. 7A. Fig. 7A shows that each of the feeder line outputs is
separate in
the frequency spectrum of combiner 149. This allows one set-top box in each of
the feeder
lines to transmit at the same frequency and at the same time as another set-
top box in
another feeder line. Thereby provide a substantially higher effective return
bandwidth for
the cable system.
It is anticipated that the novel set-top boxes shown herein may have wireless
capability added to them to allow them to communicated wireless to other
devices in the
home and business facilities such as personal computers, videophones,
telephone etc.
The optical transmitters, optical receivers, high pass filters, band pass
filters, band
stop filters, converter receivers, transmitters and other components herein
are all
commercially available or easily configured from available components. Any
such
components operative as required herein may be used in accordance with the
principles of
this invention.
It is to be understood that although the invention has been described
illustratively
in terms of a set-top box, any two-way communication device, such as a cable
modem,
can be used.
17
