Burst-mode combining of reverse path radio frequency signals

Details Burst-mode combining of reverse path radio frequency signals Burst-mode combining of reverse path radio frequency signals

2002-04-29

2004-03-30

Fan, Chieh M. (Department: 2634)

Pulse or digital communications

Transmitters

Antinoise or distortion

C375S285000, C375S346000, C398S182000

Reexamination Certificate

active

06714598

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to broadband communications systems, such as cable television systems, and more specifically to burst-mode combining of reverse path radio frequency (RF) signals that are generated in the broadband communications systems.
BACKGROUND OF THE INVENTION
FIG. 1
is a block diagram illustrating an example of a conventional broadband communications system
100
, such as a two-way hybrid fiber/coaxial (HFC) communications system, that carries optical and electrical signals. Such a system may be used in a variety of networks, including, for example, a cable television network; a voice delivery network; and a data delivery network to name but a few. The communications system
100
includes a headend facility
105
for generating forward, or downstream, radio frequency (RF) signals (e.g., video, voice, or data signals) that are transmitted in a forward frequency band. A typical forward frequency band ranges from 50 Mega Hertz (MHz) to 860 MHz. Numerous application devices
110
,
175
,
176
,
177
,
178
,
179
located within the headend facility
105
generate the forward RF signals. For example, a digital network control system (DNCS)
110
controls the routing of digital video broadcast signals and provides the signals to, for example, quadrature amplitude modulation (QAM) modulators
115
a-n
and/or digital audio/visual council (DAVIC) modulators
120
that modulate the signals with a desired forward carrier signal. A combiner
125
combines the modulated RF signals with other modulated signals being supplied from other modulators and provides the signals to a broadcast optical transmitter
130
. In a known conventional manner, the broadcast optical transmitter
130
first converts the signals to an optical signal and an erbium-doped fiber amplifier (EDFA)
135
then amplifies the optical signal. A splitter
140
then splits the optical signal for transmission downstream through a long haul fiber distribution network
145
.
A forward optical receiver (FORU) (not shown) that is included in each of a plurality of fiber nodes
150
a-h
receives the split optical signal and converts the signal back to RF signals in a known manner. The RF signals are then routed through an RF distribution network
155
for delivery to connected network terminal devices
160
a-h
. It will be appreciated that the network terminal devices
160
a-h
can be a variety of different communication devices that are tuned to receive the broadcast RF signals at specific forward frequencies. By way of example, device
161
may be a cable modem tuned to receive signals that include DOCSIS cable modem termination system (CMTS) signals; device
162
may also be a cable modem tuned to receive signals that include pre-DOCSIS CMTS signals; device
163
may be a status monitoring device that receives status monitoring signals; and device
164
may be a telephone that receives cable telephone signals, to name but a few.
In the reverse frequency band, which typically ranges from 5 MHz to 42 MHz, electrical signals are provided from the network terminal devices
160
a-h
to the headend facility
105
through the RF and fiber distribution networks
155
,
145
. Periodically, the network terminal devices
160
a-h
each sends reverse carrier signals in predetermined reverse frequency bands to the application devices. It will be appreciated, however, that these reverse carrier signals are not sent by the network terminal devices
160
a-h
at all times. This periodic transmission of carrier signals is colloquially known in the art as “burst mode” transmissions. Moreover, the normal functioning and protocol of each application device
110
,
175
-
179
controls the timing of the reverse carrier signals. For example, the DNCS
110
allows one set-top device to transmit signals at a specific frequency at a specific time and, when provided, receives the reverse carrier signal from the set-top device via DAVIC modulator
180
. This conventional reverse protocol insures that there is no ambiguity by the application devices
110
,
175
-
179
as it receives signals from the plurality of network terminal devices
160
a-h
.
FIG. 2
illustrates a typical reverse band and the frequencies allocated to various services that may be used by the network terminal devices
160
a-h
for the purpose of sending reverse carrier signals.
Unfortunately, however, in addition to the desired reverse carrier signals that are sent through the networks
155
,
145
, unwanted noise signals also enter the RF distribution network
155
by numerous means and conditions. A large portion of the unwanted noise signals enter the system through, for example, defective connectors, poorly shielded cable, and other cable components located at the subscriber location or throughout the RF distribution of the network
155
. Consequently, these unwanted noise signals degrade the ability of the respective application device
110
,
175
-
179
to effectively process the desired reverse carrier signals.
A reverse optical transmitter (ROTU) (not shown) is also included in each of the plurality of fiber nodes
150
a-h
. The ROTU converts the reverse RF signal(s), which includes both the carrier signals and the noise signals, to an optical signal and provides the optical signal via the fiber distribution network
145
to a corresponding reverse optical receiver (RORU)
165
a-h
. It will be appreciated that separate reverse fiber paths (not shown) are routed between each of the reverse optical transmitters (ROTUs) and the respective reverse optical receiver (RORU)
165
a-h
. Typically, this is required because reverse optical signals of the same wavelength cannot conventionally be combined and, therefore, require a direct fiber link between an optical transmitter to an optical receiver in the reverse path.
The RORUs
165
a-h
each convert the optical signals back to electrical signals in a conventional manner. The reverse signals provided by each of the RORUs
165
a-h
are then electrically combined through passive combiner
170
. Application devices
110
,
175
-
179
are tuned to a specific reverse frequency band (e.g.,
205
,
210
,
215
,
220
(FIG.
2
)) in order to receive just the desired portion of the combined reverse signals, which includes the desired carrier signal(s). By way of example, a DOCSIS CMTS
175
may be tuned to receive carrier signals within reverse frequency band
205
, a status monitoring device
177
may be tuned to receive carrier signals within reverse frequency band
210
, a cable telephone device
178
may be tuned to receive carrier signals within reverse frequency band
215
, and a pay-per-view device
179
may be tuned to receive carrier signals within frequency band
220
. Commonly eight to ten independent application devices offering specific services utilize the return frequency band. Each of these applications orchestrates the timing of their associated network terminal device (e.g.,
160
a-h
) such that only one network terminal device transmits within the application's return frequency band at a time. This orchestration of singular transmission within a reverse frequency band may also be used to orchestrate the behavior of elements that are or are not the linking application to its targeted network terminal device.
Unfortunately, as mentioned, noise signals, also referred to as ingress signals, can enter the system at any time and travel to the headend facility
105
, regardless of whether or not a desired reverse carrier signal is being transmitted. Once ingress signals are present in the system, the ingress signals are transmitted back through the HFC reverse path along with any desired carrier signal(s). Of particular concern is the fact that the undesired ingress signals from multiple premises tend to be combined through the system and, therefore, to build in relative amplitude. The aggregate of these undesired ingress signals could pose a considerable threat to the ability of the system to successfully transmit and process the desired carrier signals. More specifically, after c

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