Multiplex communications – Channel assignment techniques – Combining or distributing information via time channels...
Reexamination Certificate
1998-04-13
2001-09-04
Chin, Wellington (Department: 2664)
Multiplex communications
Channel assignment techniques
Combining or distributing information via time channels...
C370S395430, C370S436000, C714S755000, C714S774000
Reexamination Certificate
active
06285681
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the transmission of data over the physical layer in a multilayer data communication scheme, and is particularly applicable to the transmission of upstream messages in a hybrid fiber coaxial transmission system. The invention is useful, for example, in communicating messages from a subscriber terminal to a cable television headend for purposes of providing, e.g., Internet access over a cable television network.
In the past, cable television systems have utilized coaxial cable to carry signals from a headend to individual subscriber terminals. With the advent of new digital television services as well as the desire to carry data to and from subscriber terminals, the increased bandwidth provided by fiber optic cable has become attractive. Implementation of optical fiber networks all the way from the headend to individual subscriber terminals is not currently a practical alternative due to the high cost of building an optical fiber plant in which fiber is run all the way to individual homes. As a compromise, hybrid fiber coax (HFC) plants are being implemented.
In HFC systems, fiber is run from the headend to neighborhood hubs. Existing coaxial cable is then coupled to receive the signals from the optical fiber, for distribution to individual homes.
Various interactive services to be provided by digital transmission systems require a bidirectional link between the headend and individual subscriber terminals. One way to provide a return path from the subscriber terminals back to the headend is to rely on existing telephone lines. However, it would clearly be advantageous to provide bidirectional communication over the same plant, such as an HFC plant, in which the downstream television signals are provided to the subscriber terminals. In order to accomplish this, proposals have been made to develop robust upstream communication systems over HFC plants. For example, single carrier frequency and time division multiple access (F/TDMA) have been proposed to provide a low risk, high capacity approach which offers suitable characteristics for upstream modulation over an HFC system.
In order to implement a practical upstream channel, it would be advantageous to provide various choices to a system operator to accommodate different needs. For example, it would be advantageous to offer a trade-off between data throughput (i.e., bandwidth efficiency), error rate performance (i.e., robustness) and latency. In offering such flexibility, it would be particularly advantageous to offer system operators a frequency agile carrier, choices between different modulation techniques, such as quadrature phase shift keyed (QPSK) and quadrature amplitude modulation (QAM), multiple symbol rates, flexible forward error control (FEC) coding, and flexible frame and preamble structure for the data packets transported over the communication channel. Such choices would allow many opportunities for different system operators to find satisfactory sets of operational modes to serve the needs of subscribers economically and efficiently.
The present invention provides a flexible communication scheme enjoying the aforementioned and other advantages.
SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus is provided for communicating variable or fixed length data packets in variable length bursts over a physical layer in a multilayer data communication scheme. Each burst contains (i) information data and (ii) overhead including forward error control (FEC) data. A programmable block processor groups the information data into blocks according to a selected one of a plurality of available grouping modes. A programmable FEC encoder encodes the blocks with the FEC data according to a selected one of a plurality of possible coding levels. An interface is provided for selecting a burst mode that uses a particular grouping mode provided by the block processor (e.g., in response to a controller associated with the block processor) and a particular coding level established by the FEC encoder to achieve an associated transmission bandwidth efficiency and burst transmission robustness for bursts containing the blocks. The selection of a burst mode can, in addition, enable different latencies to be chosen.
The apparatus can further comprise a programmable modulator for modulating the encoded blocks for transmission according to one of a plurality of available modulation modes. The modulator is responsive to the burst mode selected via the interface for providing a particular one of the modulation modes. For example, the modulation modes can include choices between QPSK and QAM.
In an illustrated embodiment, the bursts are transmitted using time and frequency division multiple access (F/TDMA) with frequency agility over a communication channel.
Each of the grouping modes is associated with a particular symbol rate for the transmission of symbols carried by the bursts. In one embodiment, the block processor blocks protocol data units (PDUs) containing the information data into units that are independent of the PDU length and provides a preamble of a selected length in response to the burst mode selected via the interface. The FEC encoder encodes data from said PDU length-independent units into a particular number of codewords dictated by the selected burst mode. The block processor can allow a final unit derived from the PDUs whose data is contained in a burst to be shorter than previous units contained in the burst. In a departure from conventional practice, which typically seeks to maximize the symbol rate, one or more of the grouping modes may advantageously lower the symbol rate. For example, in certain situations a lower symbol rate may provide a more efficient transmission scheme, such as for constant bit rate users, where the lower rate will allow a closer match of the bit rate to the symbol rate to provide lower latency.
The FEC encoder can be programmable to provide codewords of different lengths. An interleaver can be provided for interleaving codeword symbols whenever the FEC encoder encodes the blocks into two or more codewords.
In one embodiment described, the block processor provides modes of varying burst length, each having one codeword per burst. These bursts are convolutionally coded using at least one of a convolutional FEC code and trellis coded modulation. The convolutional coding can be provided as an inner code concatenated with an error-correcting outer code. With concatenated coding, the outer code can still block the data into multiple codewords, allowing the same trade-offs as without the inner code. In an illustrated embodiment, the blocks comprise asynchronous transfer mode (ATM) cells, and the block processor allocates a particular number of the cells to each block based on the burst mode selected via the interface.
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Anderson Steven E.
Hou Victor T.
Kolze Thomas J.
Chin Wellington
Duong Frank
General Instrument Corporation
Lipsitz Barry R.
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