Coding scheme for cable modems
AssigneeTexas Instruments Incorporated
InventorsAlan Gatherer, Murtaza Ali
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Abstract
A modem (12) including a least-significant bit convolutional coding scheme is disclosed. In the transmit side of the modem (12), an encoder (28) is included, within which convolutional coders (35I, 35Q) are used to each encode one bit of each symbol applied to a phase and amplitude modulation constellation, preferably the least significant bits, such that the encoded bits select one of a plurality of sub-constellations in the modulated signal. Each of the coders (35, 35′) are arranged as finite state machines, of either thirty-two or sixty-four states. The minimum Hamming distance (dfree) provided by the codes is four, such that the resulting coding gain of the modem is improved over conventional encoding schemes.
Claims17
A communications device for transmitting data signals over a communications facility, comprising:
- a computer interface for receiving data words corresponding to data signals to be transmitted;
- an encoder, comprising:
- a first convolutional coder, coupled to the computer interface to receive a first selected subset of a group of digital bits;
- a second convolutional coder, coupled to the computer interface to receive a second selected subset of the group of digital bits;
- and
- a phase and amplitude modulation map function for arranging digital symbols into points of a phase and amplitude modulation constellation, each constellation point corresponding to a digital value, the phase and amplitude modulation map function coupled to the computer interface to receive a group of digital bits therefrom that are not coded by the first and second convolutional coders, and coupled to the first and second convolutional coders to receive coded digital bits therefrom;
- wherein the first and second convolutional coders each comprise a finite state machine having at least thirty-two states, and are for respectively generating first and second groups of coded bits by way of a convolutional coding scheme, the first and second groups of coded bits each having more bits than the first and second subsets, respectively, and wherein the convolutional coding scheme has a minimum Hamming distance of four;
- a modulator, coupled to the encoder, for generating a phase and amplitude modulated signal responsive to the points of the phase and amplitude modulation constellation;
- and
- a network interface, coupled to the modulator, for driving a communications facility with the modulated signal.
A method of encoding digital information for transmission over a communications facility, comprising the steps of:
- receiving the digital information in a bitstream;
- for each of a plurality of groups of bits in the received bitstream:
- applying a convolutional code of at least thirty-two states to a first subset of the group of bits, the convolutional code having a minimum Hamming distance of four, to generate a first set of coded bits;
- applying a convolutional code of at least thirty-two states to a second subset of the group of bits, the convolutional code having a minimum Hamming distance of four, to generate a second set of coded bits, the first and second sets of coded bits having the same number of bits;
- combining one bit from each of the first and second sets of coded bits to at least one of the group of bits not in the first and second subsets to form a plurality of constellation points in a phase and amplitude modulation constellation;
- and
- generating a phase and amplitude modulated signal responsive to a sequence of the plurality of constellation points.
Dependencies are inferred from the claim text.