Coded data generation or conversion – Analog to or from digital conversion – Using optical device
Reexamination Certificate
2000-04-21
2001-11-27
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Using optical device,
C455S426100, C370S902000
Reexamination Certificate
active
06323793
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a cable television hybrid-fiber-coax (CATV HFC) return path system that uses digital conversion and transport. More particularly, the present invention describes a system that implements a digital transport method at the node, so as to replace analog laser technology with a high-speed baseband digital technology, thereby providing immunity from the troublesome analog laser impairments, enabling longer distances to be covered, and potentially avoiding the need for hub repeater hardware required in analog systems, among other benefits.
BACKGROUND OF THE INVENTION
Traditional HFC systems have implemented the return path, a 5-40 MHz bandpass in North America, using bi-directional RF amplifiers in the coaxial plant, and a linear return path laser in the fiber optic node, driving a return fiber for the optical trunking. At the hub or HE, the optical power is converted back to RF. The technology is the same analog AM-based optics approach used to transport the broadcast forward path video signals. There are numerous design and implementation issues that make this approach difficult and costly, including analog laser specifications, laser second order response characteristics, optical link length constraints, optical receiver specification, and testing of the components. All of these factors contribute to the overall cost issue of developing high performance analog optics. Node laser issues are exacerbated by the fact that this component must operate in an outdoor environment, specified over a very wide temperature range.
These problems have previously been addressed with continued development in improved analog lasers and return path receiver performance, higher power lasers and modification of HFC architectures. However, each of the above-noted issues are still presented.
The present invention is therefore directed to the problem of developing a return path system that lowers the cost of return path products and improves performance such that greater architectural flexibility can be achieved, resulting in, among other possibilities increased bandwidth and capacity, and faster connection speeds to subscribers.
SUMMARY OF THE INVENTION
The present invention implements a digital transport method at the point that the RF plant terminates and the optical trunk begins inside the node. To do this, analog laser technology is replaced by high-speed baseband digital technology. As a digital signal, immunity from the troublesome analog laser impairments is obtained and longer distances can be covered, potentially avoiding the need for hub repeater hardware required in analog systems, among other benefits.
There are many analytical and design issues associated with digitizing the return path at the node. It can be shown that an analog-to-digital (A/D) converter is a mathematical analogue to the AM modulated laser technology traditionally used. The A/D converter quantization noise can be treated as the effective “analog” optical link noise. This can be compared with the known performance capabilities of the lasers currently used. Additionally, the distortion performance, in particular the laser clipping aspect, is similar to the phenomenon of overdriving AID input thresholds. The second order and third order distortions also can be kept very low in A/D's and, furthermore, the second order distortions do not degrade to poor values as analog lasers can. This can be an important issue in broadband applications. Ultimately, the strength of the digitizing technique is furthered by the functions and processing that can be applied once the information is represented completely as bits.
According to one embodiment of the present invention, a method for providing a digital return path in a hybrid fiber-coax cable television system using baseband serial optical transport, in which the return path signal from the fiber optic node to the headend is represented by encoding it entirely as ones and zeroes, includes the steps of (1) converting a composite return path waveform to a sequence of digital words whose value represent analog signal samples; (2) arranging said digital words into a serial stream with appropriate synchronization information to identify the boundaries between words and to recover timing of the bits themselves; (3) converting the electrical digital signal to an optical digital signal, and transmitting the optical ones and zeroes across an optical fiber; and (4) inverting steps (1)-(3) at the receive side.
Another embodiment of the invention is directed to a system for providing a digital return path in a hybrid fiber-coax cable television system using baseband serial optical transport, in which the return path signal from the fiber optic node to the headend is represented by encoding it entirely as ones and zeroes.
An aspect of one embodiment of the invention includes means for converting the sequence of digital words received at the headend to analog signal samples. Yet a further aspect of one embodiment of the invention includes means for providing the parallel digital words to a direct interface to an application's digital receiver.
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Brophy Timothy J.
Howald Robert Landis
Metz Erik Christopher
General Instrument Corporation
Mayer Fortkort & Williams PC
Nguyen John
Williams Esq. Karin L.
Young Brian
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