Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
2002-08-06
2004-06-15
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
Reexamination Certificate
active
06751271
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to broadband communications systems, such as cable television networks, and more specifically to a carrier-detect device that detects the presence of a carrier signal that is transmitted in the reverse path of the broadband communications system.
BACKGROUND OF THE INVENTION
FIG. 1
is a block diagram illustrating an example of one branch of a conventional broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) network, that carries optical and electrical signals. Such a network may be used in a variety of systems, including, for example, cable television networks, voice delivery networks, and data delivery networks to name but a few. The communications system
100
includes headend equipment
105
for generating forward, or downstream, signals (e.g., voice, audio, video, or data signals) that are transmitted to subscriber equipment
145
. Initially, the forward signals are transmitted as optical signals along a first communication medium
110
, such as a fiber optic cable. In most networks, the first communication medium
110
is a long haul segment that carries light having a wavelength in the 1550 nanometer (nm) range. The first communication medium
110
carries the forward signal to hubs
115
, which include equipment that transmits the optical signals over a second communication medium
120
. In most networks, the second communication medium
120
is an optical fiber that is designed for shorter distances, and which carries light having a wavelength in the 1310 nm range.
From the hub
115
, the signals are transmitted to an optical node
125
that converts the optical signals to radio frequency (RF), or electrical, signals and provides the signals to one or more output ports that are connected to separate branches of the system. The electrical signals are then transmitted along a third communication medium
130
, such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers
135
a-c
positioned along the communication medium
130
. Taps
140
further split the forward signals in order to provide signals to subscriber equipment
145
, such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one branch of the network connecting the headend equipment
105
with the plurality of subscriber equipment
145
is shown for simplicity. However, those skilled in the art will appreciate that most networks include several different branches connecting the headend equipment
105
with several additional hubs
115
, optical nodes
125
, amplifiers
135
a-c
, and subscriber equipment
145
.
In a two-way network, the subscriber equipment
145
generates reverse RF signals, which may be generated for a variety of purposes, including e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals from the set-top terminal. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream through the reverse path to the headend equipment
105
. The reverse electrical signals from various subscribers are combined via the taps
140
and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment
145
. The combined reverse electrical signals are amplified by one or more of the distribution amplifiers
135
a-c
and typically converted to optical signals by the optical node
125
before being provided to the headend equipment
105
. It will be appreciated that in the electrical, or RF, portion of the network
100
, the forward and reverse electrical signals are carried along the same coaxial cable
130
. In contrast, the forward and reverse optical signals on the first and second communications media
110
,
120
are usually carried on separate optical fibers.
The reverse RF carrier signals are generally transmitted within a frequency range from 5 MHz to, for example, 42 MHz.
FIG. 2
illustrates a typical reverse band and the frequencies allocated to various services that may be used by the subscriber equipment
145
for the purpose of sending reverse carrier signals. It will be appreciated that the combined reverse carrier signals may include a plurality of reverse carrier signals from a plurality of subscriber equipment. The combined carrier signals may also include a variety of signals in a plurality of frequencies. In addition to the carrier signals, noise and interference is often present in the system. Typically, the noise signals can be viewed with test equipment as essentially a constant level, or noise floor, that most particularly affects the reverse path signals. Disadvantageously, the noise signals interfere with the processing of the valid carrier signals with the headend equipment.
When necessary, the presence of a valid carrier signal in the reverse path is typically detected among the noise signals by examining the instantaneous power level of the RF signal. For example, if the power level of the RF signal is above a predetermined threshold for a predetermined amount of time, e.g., at least 2 microseconds during any 8 microsecond window, the RF signal is treated as a valid RF carrier signal. Otherwise, the reverse signal is treated as just noise and interference. This carrier-detect scheme works adequately for narrowband noise that has a predictable noise floor; however, wideband noise, which has a high peak to average power ratio in the time domain, can exceed the preset threshold for 2 microseconds in an 8 microsecond window even though its average power level is low.
The present invention is, therefore, directed to an improved carrier-detect circuit that detects a valid reverse carrier signal. As a result, the HFC network's reverse path signaling capacity, quality, and reliability are greatly enhanced.
SUMMARY OF THE INVENTION
The present invention is directed toward an improved carrier-detect circuit that detects a valid reverse carrier signal. More specifically, a power averaging carrier-detect device determines the presence of at least one desired RF carrier signal. The device determines if a reverse carrier signal is present by averaging the power level of the reverse signals by analyzing each input sample bit by bit to determine the power level over a certain period of time.
REFERENCES:
patent: 3978407 (1976-08-01), Forney et al.
patent: 5563914 (1996-10-01), Sogabe
patent: 5796503 (1998-08-01), Junginger et al.
patent: 6137607 (2000-10-01), Feldman et al.
patent: 2003/0032441 (2003-02-01), Ofuji et al.
Scientific-Atlanta, Inc. Pending Application Ser. #09/840,767, Filing Date: Apr. 23, 2001, Title: “Burst-Mode Digital Transmitter,” Inventors: Joseph Mobley and Lamar E. West, Jr.
Scientific-Atlanta, Inc. Pending Application Ser. ·09/840,753, Filing Date: Apr. 23, 2001, Title: “Burst-Mode Analog Transmitter,” Inventors: Lamar E. West, Jr. and Donald C. Sorenson.
Scientific-Atlanta, Inc. Pending Application Ser. #10/026,283 Filing Date: Dec. 21, 2001, Title: “HFC Reverse Path Using an Intelligent Dynamic Switch,” Inventors: Donald C. Sorenson, David M. Job and Lamar E. West, Jr.
Ao Jiening
Kien Thai-Bao H.
West, Jr. Lamar E.
Odom Curtis
Scientific-Atlanta, Inc.
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