Note: Descriptions are shown in the official language in which they were submitted.
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Multi-band Coax Extender for In-building Digital
Communication Systems
Cross-Reference to Related Applications
This application claims priority to a provisional application filed February
7, 2001
with U.S. Serial No. 60/267,046. This application provides a way to boost the
signal
carrying capacity of a system to provide High Speed Data Communication Over
Local
Coaxial Cable as described in co-pending application 09/482,836 based on
Provisional
Application No. 60/115,646 filed January 13, 1999. Another application
assigned to
common assignee coaXmedia, Inc that describes the environment of the present
invention
is Architecture and Method for Automated Distributed Gain Control for Internet
Communications for MDUs and Hotels (Application No. 09/818,378 based on
Provisional Application No. 60/193,855). The '855 application has the filing
date of
March 30, 2000.
For the convenience of the reader, applicant has added a number of topic
headings
to make the internal organization of this specification apparent and to
facilitate location
of certain discussions. These topic headings are merely convenient aids and
not
limitations on the text found within that particular topic.
In order to promote clarity in the description, common terminology for
components is used. The use of a specific term for a component suitable for
carrying out
2o some purpose within the disclosed invention should be construed as
including all
technical equivalents which operate to achieve the same purpose, whether or
not the
internal operation of the named component and the alternative component use
the same
principles. The use of such specificity to provide clarity should not be
misconstrued as
limiting the scope of the disclosure to the named component unless the
limitation is made
explicit in the description or the claims that follow.
Background of the Invention
Technical Field
The present invention adds to the field of data communications. More
particularly
the invention is one of the ongoing improvements in the area of data
communications
,addressing the use of tree and branch distribution systems for upstream and
downstream
data communication between a hub-server and a set of two or more client
modems.
Preferably, the client modems are adapted to allow a plug and play connection
or other
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easy connection between a laptop and the tree and branch network. The tree and
branch
network is preferably connected to the Internet. Thus, the present invention
can be used
in a hotel or Multiple Dwelling Units (MDU's) or analogous buildings to allow
plug and
play access to the Internet over existing coax television networks. Note, the
present
invention is not limited to installations in a hotel or Multiple Dwelling
Units (MDU's) or
analogous buildings, these are examples of locations that can use the benefits
of the
present invention.
The '836 application describes a system that allows the connection of devices
such as personal computers to special modems that connect to a legacy tree and
branch
coax network in a hotel, Multiple Dwelling Units (MDUs), or analogous
building. The
system described in the '836 application used two bands outside of the range
used for
cable TV. Thus, the system would have one frequency range for a downstream
data
channel and one frequency range for an upstream data channel. As this is a
tree and
branch network, all communications heading downstream must identify which
modem
~ 5 device (or devices) are being addressed since all modem devices will
receive the
communication. Conversely, the communication from the many individual modem
devices to the upstream end of the network must be controlled so that only one
modem
device is sending an upstream communication at any one time in order to avoid
distortions to the upstream data resulting from more than one client modem
transmitting
20 on the same frequency at the same time ("bus contention"). The method of
control used
in the referenced applications is based on a polling and response model.
The present invention improves prior work by assignee coaXmedia, Inc. by
providing a way to increase the capacity of the main feeder cables to carry
communications to and from client modems.
25 In the preferred embodiment, the client modems are all mass-produced to
operate
at the same pair of upstream and downstream frequency bands.
The situation addressed by both the referenced applications and the current
invention is shown generally in FIGURE 1.
Environment
3o The previously described solution can be summarized by FIGURE 1. In
FIGURE 1, the bandwidth between 50 MHz and 860 MHz (108) is allocated for
downstream transmission of television signals. The band of SMHz to 42 MHz
(104) is
used for the existing services that use upstream traffic such as pay-per-view.
Much of the
frequency band between 860 MHz and 900 MHz (112) is used for other
applications such
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as cellular telephones. Due to the relatively high field-strength radiation of
portable
cellular handsets, it is prudent to avoid using frequencies close to those
used for cellular
telephones.
The legacy coax distribution networks have sputters and couplers that operate
satisfactorily up to approximately 1 GHz (1000 MHz). Thus, the '836
application and the
'378 application suggested having a downstream frequency for data and an
upstream
frequency for data, both in the band between 900 MHz and 1000 MHz. In FIGURE
1,
the upstream frequency is shown at 915 MHz (116) and the downstream frequency
is
shown at 980 MHz (120). A single pair of upstream and downstream frequencies
was
~ o thought sufficient to serve the statistical two-way Internet access needs
of fifty to one-
hundred users or client modems.
The '378 application taught that additional downstream spectra can be
allocated in
bands between 1 GHz and about 1.6 GHz provided that existing components are
replaced
with components that work adequately in this frequency band. This solution
would
require a means for the client modem to recognize a request to switch from the
normal
downstream channel of 980 MHz to the high frequency channel. Thus, in addition
of the
cost to upgrade the components of the legacy coax network, there would be a
need to
provide more expensive client modems that can operate on multiple downstream
frequencies.
2o Problem being addressed
As illustrated in FIGURE 2, larger Multi-Dwelling Unit (MDU) in-building coax
cable TV distribution systems commonly have many more than fifty coax
receptacles.
These larger distribution systems normally have a mix of local services 604 in
addition to
the TV channels. In a hotel the local services might include a digital video
server, cheek
out information and information about the hotel restaurants.
The local services 604 and cable television channels 608 would be combined at
element 612 and amplified by central location amplifier 620 before the feeder
cable 624
(sometimes called a coax riser).
An even larger system might include one or more central location sputters 630
to
feed additional pairs of an amplifier 634 and another long feeder cable 638.
To avoid
clutter in the drawing, the local distribution networks connected to long
feeder cable 638
are not shown. These distribution systems require intermediate amplifiers 650
to boost
the signal levels that have been attenuated by coax cable, splitter and
directional tap
losses, in order that sufficient signal levels be provided to television sets
and/or other
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entertainment equipment. These intermediate amplifiers 650 are distributed
within an
MDU at some distance from the central feed point to the building which may
provide
services from CATV, TV broadcast antenna or via means such as fiber optics.
These
intermediate amplifiers 650 normally carry TV channel signals in one direction
only,
usually at frequencies in the range SO MHz to 750 MHz. In some cases these
amplifiers
are equipped with a reverse direction amplifier that can carry signals in the
frequency
range S MHz to 42 MHz. The reverse channel is sometimes used to carry command
signals for requesting pay-per-view (PPV) television services or, with
increasing
frequency, the upstream channel of a cable modem used for Internet access.
1o When the TV coax distribution system is utilized to carry data outside of
the
CATV frequency band, there is a need to provide bypass amplifiers for each
signal
direction, connected to the coax cables via frequency selective diplexers.
Thus, when
implementing a system to carry data on an existing cable television network,
there is a
need for circuitry such as shown in FIGURE 3 to boost the data signals.
FIGURE 3 operates without interference to the operation of existing CATV line
extender amplifier 650. The amplifier 650 is isolated by a pair of low-pass
filters 654 in
diplexers 660. A high frequency bypass around the existing amplifier 650 is
provided by
a pair of high-pass filters 658. The bypass is split into a downstream channel
and an
upstream channel by sputters 664. The downstream channel and the upstream
channel
are isolated from one another by shielding 668.
For a system using 980 MHz as the downstream frequency and 915 MHz as the
upstream frequency, the downstream channel is comprised of a 980 MHz bandpass
filter 672, a variable attenuator 676, an amplifier 680, and a 915 MHz band-
stop
filter 684. The upstream channel is comprised of a 915 MHz bandpass filter
688, a
variable attenuator 676, an amplifier 692, and a 980 MHz band-stop filter 696.
When too many users share the data distribution system, there may be
insufficient
capacity. Insufficient capacity can lead to service degradation in the form of
lost or
delayed data packets. The number of users that is "too many" is a function of
the type of
data needs for the individual users. How many users are "too many" users? It
depends
on whether the users are likely to be connected at the same time, the need to
receive or
transfer large amounts of data and the sensitivity of the applications to
delays in receiving
data packets. As the amount of data communicated to a single connected user
increases
with the evolution towards multimedia, video conferencing, and other data
intensive
applications, the number of users that can be supported by the data networks
will drop.
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Low latency applications such as video conferencing or voice over IP (Internet
Protocol)
exacerbate the problem.
While it may seem attractive to simply use additional frequencies for the
upstream
and or the downstream channel, this is not an attractive solution.
s There are several advantages to having a set of client modems that are tuned
to
receive a single downstream frequency and to transmit on a single upstream
frequency.
For example, manufacturing and set up costs are reduced if there is not the
need to
provide modems that can be tuned to operate on a range of receive or transmit
frequencies.
Even if a designer was willing to forego the advantages of using the same pair
of
transmit and receive frequencies for an entire set of client modems, there are
practical
limits to the number of frequency bands available above 900 MHz. One problem
is that
approximately 1 GHz is an effective frequency ceiling. This limitation comes
from the
reality that the sputters, directional taps, connectors and sometimes the coax
cable itself
in the distal portions of the coax distribution tree and branch network
frequently perform
poorly at frequencies much beyond 1 GHz.
Using several frequency channels in the spectrum above 900 MHz and below
I GHz has its own problems. One problem is that adding additional channels
will result
in increased total signal power. This additional signal power will then
increase the risk of
2o signal overload in the active elements of the network. The overload can
adversely impact
the delivery of TV services. An additional problem is that adding more
channels will
increase the complexity of filters required to separate the individual
channels.
Fortunately, the main (feeder) coax distribution cables (624, 638) connecting
TV
signals between the feed point to the building and the distributed "booster"
amplifiers 650
are usually able to carry frequencies well above 1 GHz, as these feeder cables
do not
usually include directional taps or splitters. Even if there are a few taps or
splitters before
the booster amplifiers it will be easy to replace or upgrade the components.
It will be
easy because even if there are taps or sputters before the booster amplifiers,
there will
only be a few and they are easily accessible. This is in sharp contrast to the
situation after
3o the booster amplifiers where there are many taps and most are difficult to
access.
BRIEF SUMMARY OF DISCLOSURE
The present invention solves the prior art limitations by utilizing a two-
stage
system. In the preferred embodiment, the feeder cable stage takes advantage of
the
capacity of the feeder cable to carry multiple bands of data in the frequency
spectrum
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above 1 GHz. The local stage converts these bands of data into corresponding
bands in
the frequency range 900 MHz to 1 GHz, at the TV "booster" amplifier locations,
and
amplifies these downstream communications for onward transmission to end users
connected in groups to individual local tree and branch networks in the TV
coax
distribution system. Likewise, at least some of the upstream communications
are shifted
to a frequency above 1 GHz for upstream transmission on the feeder cable. The
solution
of the present invention offers significantly higher data capacity in a system
in which all
data interface "modems" can be identical and without complex tuning functions.
Thus,
the modems for use at the end user termination points of the tree and branch
network can
be mass produced and preset for given upstream and downstream channels as the
many
upstream and downstream bands are converted into standard upstream and
downstream
frequency channels for the local stage of the distribution. The modems can be
used
interchangeably on several different local tree and branch networks.
Optionally, one set of upstream and downstream communications can travel on
the feeder cable at the frequencies used by the client modems so that no
frequency
shifting is required for this fraction of the communications. While using the
same
frequencies for all client modems may be desirable for administrative or
economic
reasons, the present invention is not limited to networks where all client
modems operate
solely on one pair of upstream and downstream frequencies. Alternative
frequencies
2o bands, other than above 1 GHz, are suggested in the discussion of
alternative
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the frequency bands used in the related applications to convey
data upstream (116) and downstream (120) over a legacy tree and branch
distribution
network for cable television.
Figure 2 illustrates the relationship between the feeder cables (624 and 638)
with
local coax distribution networks 762, 766, 768, and 770.
Figure 3 illustrates the components in a line extender used to provide
amplified
signals for the data sent over the legacy tree and branch distribution
networks.
Figure 4 illustrates one embodiment of the present invention using three
different
downstream frequencies over the feeder cable 624 but only one upstream
frequency over
the feeder cable 624.
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Figure 5 illustrates another embodiment of the present invention using three
different downstream frequencies over the feeder cable 624 and three different
upstream
frequencies over the feeder cable 624.
DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS
Overview of Figures 4 and 5
FIGURES 4 and 5 show two principal embodiments of the present invention.
Both embodiments are shown on a combination of a Figure A that shows the
equipment
upstream of the feeder cable 624 and a Figure B that shows the equipment
downstream of
the feeder cable 624.
Both embodiments have one central system to feed one or more feeder cables
(624 or 638) and ultimately a number of local networks. In the preferred
embodiment,
each local network would use standard client modems with pre-set frequencies
for
transmit and receive.
~ 5 FIGURE 4 differs from FIGURE 5 in that FIGURE 4 anticipates a situation
where one upstream frequency is adequate for the entire set of client modems
but the
downstream data requirements exceed the bandwidth of a single downstream
frequency.
In both FIGURE 4 and FIGURE 5, the system uses several frequencies to carry
downstream transmissions on the feeder cable before conversion to the standard
2o downstream frequency for transmission on the parallel local networks. The
embodiment
of FIGURE 4 will be suitable for many situations with much more information
sent
downstream to client modems than sent upstream from the client modems. Web
browsing is one example of an application with this downstream/upstream
imbalance.
Much more downstream capacity is needed to convey the data necessary to
construct a
25 web page than is necessary to communicate upstream the simple request to
display that
web page. An additional load on downstream capacity is Value Added (VA)
services,
such as local digital video services, that require broadband capacity. The
combination of
downstream data from the Internet service provider with the bandwidth
intensive Value
Added services will frequently lead to a need for more downstream capacity
than
30 upstream capacity. In many situations, there will be too much downstream
traffic for the
existing feeder cables to carry it all on one downstream frequency in the 900
MHz to 1
GHz spectrum.
The system illustrated in FIGURE 5 is like FIGURE 4 in that the illustrated
system has multiple downstream frequencies for the feeder cable. The
embodiment
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shown in FIGURE 5 differs from FIGURE 4 in that it has more than one upstream
frequency for the upstream travel through the feeder cable. FIGURE 5 is
adapted to
work in situations where both the upstream and downstream traffic exceed the
bandwidth
for a single frequency on the feeder cable. Email or voice over IP are
examples of
applications that have a more even distribution between upstream and
downstream data.
Details of Figure 4
The cable television signals from coax 608 connected to the CATV service drop
are amplified at amplifier 312 before reaching the low-frequency leg of
diplexer 316.
The high-frequency leg of diplexer 316 receives data from Internet access,
local
Value Added services (if any), and from the digital video server 712 (if any).
More
specifically, the connection to the Internet 704 can be split from the CATV
service drop
cable 608, or come through another communication route such as fiber, cable
modem, or
wireless.
In FIGURE 4A, the functions of a central hub are allocated across a set of
~ 5 components. The conversions from the Internet protocol to local network
protocol occur
in a central server 708. Typically, the conversions will be from Ethernet to
PPPoE (PPP
over Ethernet) in the downstream direction and the reverse for upstream
transmission.
Optionally, other local value added services can be administered in central
server 708.
Part of the local Value Added services can include a request for delivery of
content from
2o digital video server 712.
The downstream data including data from the digital video server 712 pass to a
router 716 that distributes the data to a set of two or more central modems
(720, 722, and
724). As this embodiment is set for a system with relatively small amounts of
upstream
traffic, only one central modem 720 is used to receive upstream traffic. In
the example
25 shown in FIGURE 4A, the downstream traffic is carried to one set of client
modems on
feeder cable frequency 980 MHz. Downstream traffic to another set of client
modems is
carried on feeder cable frequency 1.05 GHz to take advantage of the capacity
of the
feeder cable to carry frequencies above one gigahertz. Downstream traffic to
yet another
set of client modems is carried on feeder cable frequency 1.10 GHz.
30 In the preferred embodiment, there would be an additional modem for each
additional feeder cable frequency used for downstream traffic. As will be
evident in the
description of FIGURE 4B, the use of the downstream frequency used by the
client
modems as one of the feeder cable frequencies reduces the amount of components
used in
FIGURE 4B. Alternatively, the system could be set up to use downstream feeder
cable
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frequencies above one gigahertz for all central modems and then convert all
the
downstream traffic to the downstream frequency used by the client modems.
The upstream traffic from all the client modems is transmitted on the single
upstream feeder cable frequency of 915 MHz which is the same frequency used by
the
client modems. The coax cables from each of the three central modems are
connected to
combiner 734 which is connected to the high-frequency leg of diplexer 316.
FIGURE 4B illustrates a mufti-band coax extender for use with FIGURE 4A.
As an overview, the mufti-band coax extender receives each of the three
downstream
bands and, using local frequency synthesizers and mixer elements, converts two
of the
received bands into two separate streams having bands identical to the third
spectrum
carried downstream on the main feeder. Each of these streams are then
introduced, using
spectral diplexers, into separate coax cable branches which may feed perhaps
fifty or
more client modems (such as the coaXmedia SandDollarTM client modem). In the
upstream direction, using directional taps, same-spectrum signals from each of
the
separate coax cable branches are combined together, filtered to remove out-of
band noise,
and amplified prior to insertion, as an upstream signal, onto the feeder cable
624 and back
to the central modem 720 having an upstream receiver.
The system as described generally above is implemented in one embodiment with
the following details shown in FIGURE 4B. Starting at the distal end of feeder
2o cable 624 as shown in FIGURE 4B, the feeder cable 624 feeds diplexer 750.
In one
preferred embodiment, diplexer 750 is set with low pass from DC to 865 MHz and
with
high pass set to 905 MHz and above. The low-frequency leg of the diplexer 750
feeds the
input to the television amplifier 650, which in turn feeds diplexers 754, 756,
and 758.
Each of the diplexers (754, 756, and 758) feeds a local coax distribution
network 762,
766, or 770.
Depending on the anticipated loading, the distribution networks service
approximately fifty end users. The distribution network terminates with
equipment such
as set forth in block 400. The details for one of the many blocks are shown on
FIGURE
4B. The actual layout of components within block 400 is not important for
purposes of
3o this invention and the sample given should not be interpreted as a
limitation of the scope
of the invention. For purposes of illustration, the components within block
400 are as
follows:
Within cluster 400, a client modem 408 connects to the high-pass port on
diplexer 406. Diplexer 406 is connected to the coax receptacle 404. Sample
values for
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the downstream legs of the diplexer 406 are LP SMHz to 860MHz and HP 900 MHz
to 1
GHz. A conventional TV coax cable 412 connects a television 416 to the low-
pass port
on the diplexer 406. The client modem 408 is shown as a sand dollar in
deference to the
assignee's trademarked name for assignee's client modem.
5 The user may connect a downstream device 420 to the data cord of client
modem 408. The user's downstream device 420 could be a personal computer
("PC").
While the downstream device 420 is likely to be either a desktop or laptop
personal
computer, it could be some other device capable of interfacing with an
external source of
digital data. One such example is the range of devices known as PDAs
("Personal Digital
Assistants"). Thus, the present invention allows for communication between the
downstream device 420 and the Internet through substantial use of existing
infrastructure
used to deliver cable TV signals to user's television 416.
Each of the three diplexers (754, 756, and 758) receives downstream
transmissions at
980 MHz and upstream transmissions at 91 S MHz. While the aggregate downstream
traffic for all three local coax distribution networks (762, 766, and 770) is
too much to be
carried on one frequency on the feeder cable 624, there is no problem having
all the
downstream traffic on the same frequency once it is divided among the three
parallel
local networks.
The components in block 800 handle the conversion from three feeder cable
frequencies to three parallel local networks. The downstream path starts with
diplexer
750 upstream of amplifier 650. The high-frequency leg of the diplexer 750
feeds sputter
804. The downstream path continues from the sputter 804 to amplifier 808. The
portion
of the downstream traffic at 980 MHz passes through a band pass filter 812 set
at 980
MHz (passing plus or minus 20 MHz -as do band pass filters 836 and 852). Since
980
MHz is the standard frequency used by the client modems 408, no conversion is
necessary and the downstream traffic passes through the directional tap 816 to
the high-
frequency leg of diplexer 754 on route to local coax distribution network 762.
In parallel with the path for downstream traffic to local coax distribution
network
762, there is a path for downstream traffic to local coax distribution network
766.
3o Downstream traffic for network 766 at feeder cable frequency 1.05 GHz exits
the
amplifier 808 and passes through high-pass filter 820 set to pass frequencies
above 1.02
GHz. The high-pass filter 820 is used to prevent residual lower-band spectrum,
which
could potentially pass directly through either of the mixers (832 or 848),
from interfering
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with similar spectrum in the 980 MHz range created by the down-conversion from
higher
spectrum bands of the downstream traffic for local coax distribution networks
756 or 758.
Through use of oscillator 824, synthesizer 828, and mixer 832, the downstream
traffic is shifted to 980 MHz and passes through band pass filter 836 and
directional
tap 840 to reach the high-frequency leg of diplexer 756. (Typical synthesizer
output
values would be 70 MHz or 2.03 GHz.) Diplexer 756 is connected to local
distribution
network 766.
In a similar way, the downstream traffic for local coax distribution network
770
travels on coax feeder 624 at 1.10 GHz. The downstream traffic passes through
high-pass
filter 820. Through use of oscillator 824, synthesizer 844 and mixer 848, the
downstream
transmission is shifted to 980 MHz and passes through band pass filter 852 and
directional tap 856 to reach the high-frequency leg of diplexer 758. (Typical
synthesizer
output values would be 120 MHz or 2.08 GHz.) Diplexer 758 is connected to
local
distribution network 770.
t 5 As mentioned in connection with FIGURE 4A, the downstream traffic to local
coax distribution network 762 could have been carried on the feeder cable 624
on a
frequency other than the standard downstream frequency (980 MHz) used by the
client
modems 408. This choice would require an additional synthesizer and mixer
along with
adjustments to the filter scheme.
2o The upstream traffic from the three local coax distribution networks is
sent on
standard frequency 915 MHz. The upstream path is from diplexers 754, 756, and
758
through directional taps 816, 840, and 856 to combiner 860.
The combined upstream traffic passes through band pass filter 864 set for
915MHz (plus or minus 10 MHz). The upstream traffic is amplified at 868 and
passes
25 through splitter 804 to the high-frequency leg of diplexer 750 to feeder
cable 624.
FIGURE 4 illustrates a system with three modem pairs servicing three local
distribution networks. In practice, any number of modem pairs may be combined
in this
matter, taking into consideration the required downstream capacity. Two small
local
distribution networks can share one pair of a modem and a feeder cable
frequency. The
3o present invention can be used in situations with two or more local coax
distribution
networks.
Details of Figure 5
FIGURE SA illustrates a similar arrangement to that shown in FIGURE 4A with
the exception that each central modems (720, 726 and 728) includes an upstream
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receiver. Each receiver is tuned to a different coax feeder upstream
frequency. The
advantage of this arrangement is the multiplication of upstream capacity. The
specific
frequency bands shown are by way of example only as the principle may be
applied
independently of frequencies or spectrum used. As with the downstream
frequencies,
there is a slight advantage to using the standard transmit frequency for the
client
modems 408 as one of the coax feeder upstream frequencies. However, it is not
required
that one of the coax feeder upstream frequencies be the same as the standard
transmit
frequency for the client modems 408.
FIGURE 5B illustrates a similar arrangement to that shown in FIGURE 4B with
exception that the same-spectrum upstream bands from two of the separate local
coax
distribution networks are frequency-shifted before being combined for
transmission in an
upstream direction on the main coax feeder 624. In this example, the
downstream traffic
at splitter 804 is carried on frequencies 980 MHz, 1.11 GHz, and 1.24 GHz. The
upstream traffic at sputter 804 is carried on frequencies 915 MHz, 1.045 GHz,
and 1.175
i s GHz.
More specifically, in the preferred embodiment, the upstream communications
from local coax distribution network 762 passes through diplexer 754,
directional
tap 816, and band pass filter 872, to combiner 860 without modification of the
upstream
frequency of 915 MHz. (Typical values for band pass filters 872, 876, and 880
are 915
20 +/- 20 MHz)
The upstream traffic from local coax distribution network 766 is also at 915
MHz
but after passing through diplexer 756, directional tap 840, and band pass
filter 876, the
upstream traffic is shifted to 1045 MHz by mixer 884 using synthesizer 838
output at 130
MHz. The shifted upstream traffic passes through band pass filter 892 set for
1045MHz
25 +/- 20 MHz.
Similarly, the upstream traffic from local coax distribution network 770 also
starts
at 915 MHz. After passing through diplexer 758, directional tap 856, and band
pass
filter 880, the upstream traffic is shifted to 1075 MHz by mixer 888 using
synthesizer 844
output at 260 MHz. The shifted upstream traffic passes through band pass
filter 896 set
3o for 1075MHz +/- 20 MHz.
Alternative Embodiments
In the example shown in FIGURE 5, a single heterodyne frequency source,
provided by a synthesizer, is used to frequency shift both a downstream and an
upstream
signal. Thus, the amount of frequency shifting for both directions of
transmission will be
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identical. Alternatively, separate heterodyne frequencies may be employed,
thus enabling
a more flexible frequency plan.
The system as set forth on FIGURES SA and SB uses 915 MHz for upstream
communication and 980 MHz for downstream communication in the local coax
distribution networks (762, 766, and 770). As shown in FIGURE SB, one of the
pairs of
frequencies transmitted on the feeder cable 624 is 915 MHz and 980 MHz, which
is used
without frequency shifting by one of the local coax distribution networks 762.
This
eliminates the need for an additional set of components to frequency shift
these signals.
While this is advantageous, it is not required and all of the bands may be
frequency
A
shifted without deviating from the scope of the present invention.
The band-pass filters included in FIGURE SB may be conveniently and
economically created using printed circuit board stripline elements. Other
forms of filter,
such as ceramic or surface acoustic wave types may alternatively be employed.
The solution employed could have multiple local coax distribution networks
using
~ 5 the same upstream or downstream frequency over the feeder cable 624
providing that the
aggregate traffic on the feeder does not exceed its carrying capacity for a
given
frequency. Thus, several local coax distribution networks may use the same
feeder cable
frequencies as are used in the local coax distribution network. One or more of
the other
local coax distribution networks would shift one or both of the communication
2o frequencies to add to the carrying capacity of the feeder cable 624.
The method described may be used in digital transmission systems using any
form
of modulation, or different forms of modulation on any portion of the coax
distribution
system.
The title and the disclosed embodiments of the present invention are given in
the
25 context of data communication using legacy cable television coax tree and
branch
networks. The frequencies chosen for the upstream and downstream communication
reflect this environment. Note that one of skill in the art could select other
frequencies or
modulation schemes to implement this invention, especially in any tree and
branch
network that is not a coax network for use in distributing cable television
signals, or in a
3o tree and branch network that does not use coax.
When used in connection with data communication using legacy cable television
coax tree and branch networks, the preferred embodiment uses frequencies on
the feeder
cable above the useful frequency range of the local distribution networks
(typically
frequencies above 1.0 GHz). Those of skill in the art could use the teachings
of this
CA 02433611 2003-06-30
WO 02/063880 PCT/US02/03805
14
invention to use additional carrier frequencies on the feeder cable to
increase the
bandwidth of the feeder cable through use of frequencies below 1.0 GHz.
Generally,
there are surmountable obstacles in using these other frequencies. The band of
5 to 42
MHz could be used, especially for an extra feeder cable downstream frequency,
but this
band is subject to a variety of uses that will change over time.
The frequency band set aside for television channels extends up to 860 MHz.
Many systems do not use the frequency band from approximately 750 MHz to 860
MHz.
This bandwidth could be used for additional feeder cable frequencies. A
downside of
using this band of frequencies is that cable television providers in some
zones may
1 o already be using the 750 MHz to 860 MHz band, so this solution may not be
universally
applied. Another possible place to put additional feeder cable frequencies is
in unused
television channels within the band of frequencies used for television
channels.
Depending on the modulation and filter equipment used to convey the feeder
cable
frequencies, it may be necessary to fmd several contiguous unused television
channels in
order to carry one feeder cable frequency. A problem with using unused
channels is that
cable television providers rearrange the channels that are used to convey the
television
signals from time to time. A rearrangement by the cable television provider
might cause
a conflict with the plan to have extra feeder cable frequencies when unused
television
channels become active television channels, thus triggering a need to adjust
the
2o equipment to use a different frequency.
The frequency band of approximately 900 MHz to 1.0 GHz is yet another possible
band to carry additional feeder cable frequencies. As noted above, there would
be
possible problems from aggregate signal power and the need for a more rigorous
filter
scheme in order to add additional feeder cable frequencies to this band as the
preferred
embodiment already uses 915 MHz and 980 MHz. While these factors point towards
using the band above 1.0 GHz, the band between 900 MHz and 1.0 GHz could carry
three
or more feeder cable frequencies rather than two feeder cable frequencies.
Those skilled in the art will recognize that the methods and apparatus of the
present invention have many applications and that the present invention is not
limited to
3o the specific examples given to promote understanding of the present
invention.
Moreover, the scope of the present invention covers the range of variations,
modifications, and substitutes for the system components described herein, as
would be
known to those of skill in the art.
CA 02433611 2003-06-30
WO 02/063880 PCT/US02/03805
The legal limitations of the scope of the claimed invention are set forth in
the
claims that follow and extend to cover their legal equivalents. Those
unfamiliar with the
legal tests for equivalency should consult a person registered to practice
before the patent
authority which granted this patent such as the United States Patent and
Trademark
5 Office or its counterpart.
