Programmable signal decoder

Details Programmable signal decoder Programmable signal decoder

1997-12-12

2001-03-20

Pham, Chi H. (Department: 2631)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C375S341000, C714S794000

Reexamination Certificate

active

06205187

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to data communications systems and, more specifically, to a programmable signal decoder that performs decoding functions specified by decoder configuration data that has been downloaded into the decoder.
BACKGROUND OF THE INVENTION
Data communications systems transfer digital information over media such as copper wires or fiber optic cables by converting the digital information to signals that drive the media. In general, there is direct correspondence between the bandwidth of a signal and the rate at which that signal transmits information. However, in every transmission medium, there is a practical upper limit on the data rate of the signal which the medium can reliably pass.
Numerous data modulating schemes have been developed to increase the rate at which digital information can be passed through a medium with noise and limited bandwidth. One prevalent modulating scheme, known as quadrature amplitude shift keying (“QASK”), involves representing information by the amplitude and phase of the transmitted signal. The relative vector amplitude of the signal at predetermined instances in time represents a particular value, called a “symbol.” The graphical representation of the vector relationships is referred to as a “constellation.” Although systems that use QASK may achieve higher effective data throughput, ambient noise and other signal interference may affect the accuracy with which the equipment that receives the signal detects the relative amplitude of the signal.
To increase the likelihood that the signal generated by the receiver is the same as the transmitted signal, encoding schemes such as forward error correction (“FEC”) encoding have been developed that enable a receiver to correct errors that result from noise and other signal interference. One encoding scheme, known as Wagner encoding, encodes information similar to parity information into the transmitted signal.
Another encoding scheme, known as trellis coded modulation (“TCM”), combines the separate processes of forward error correction coding and modulation into one process. A symbol transmitted by a transmitter that uses trellis coded modulation depends on previous symbols. This dependency is achieved by limiting the choices for each transmitted symbol based on the current and previous symbols. Both the transmitter and receiver contain information specifying the predetermined, limited choices for a symbol based on the previous symbols.
At the receiver, the sequence information of the received symbols is used to select a next symbol most likely to have been transmitted. Thus, if a symbol gets corrupted during transmission, by knowing the sequence of previously received signals, the receiver may be able to identify the correct symbol. Trellis coded modulation techniques are well known in the data communication art. For example, one TCM technique is disclosed in U.S. Pat. No. 4,980,897 to Decker et al., entitled MULTI-CHANNEL TRELLIS ENCODER/DECODER, the contents of which are incorporated herein by reference.
Several variations of the trellis coded modulation technique are used in conventional data communications systems. U.S. Pat. No. 4,077,021 to Csajka and Ungerboeck and the article “Channel Coding with Multilevel/Phase Signals”, IEEE Transactions on Information Theory, Vol. IT-28, No. 1, January, 1982, both of which are incorporated herein by reference, disclose a coding system using a conventional two-dimensional signal constellation having 2
N
signal points, wherein the size of the constellation is increased to 2
N+1
signal points. The encoder in this system introduces redundancy into the transmitted signal by adding one bit of information to each N bit symbol according to the state of a finite state machine internal to the encoder. The resulting N+1 bits for each symbol are mapped into one of the 2
N+3
signal points of the constellation. The signal points are organized into disjointed subsets and arranged so that the minimum Euclidean distance between two signal points belonging to the same subset is greater than the minimum distance between any two signal points in the constellation. The memory of the finite state machine is arranged so that the sequence of subsets from which signals are drawn is predetermined to provide maximum distance between signal points.
The encoder operates by selecting the subset from which each signal is to be drawn. This coding permits only certain sequences of signals to be transmitted. In essence, each signal, as part of the sequence, carries historical information which is used by the decoder. The decoder uses a maximum likelihood sequence estimation technique to decode the actual sequence of transmitted signals. One sequence estimation technique is the Viterbi algorithm which is described in Forney, “The Viterbi Algorithm”, Proceedings of the IEEE, Vol. 61, No. 3, March, 1973, the contents of which are incorporated herein by reference.
Some systems use a coding technique known as multi-dimensional coding. In multi-dimensional coding, transmitted signals are grouped, each group consisting of at least two symbols. Each symbol is drawn from a two-dimensional signal constellation. An interdependence is introduced among the signal points drawn for a particular group. Through this interdependence, data correction may be accomplished using lower symbol processing rates than are used by other coding schemes.
Some systems use a layered coding technique known as multi-level coded modulation (“MLCM”). In the case of two level encoding, one group of bits is encoded by a first encoder. The output of this encoder is used as an input to a second encoder which encodes the remaining bits. If desired, the multi-level technique can be extended to more than two levels.
From the above, it can be seen that a multitude of encoding techniques are used in conventional data communication systems. In practice, a data communications user may use various services each of which employs signals that are encoded using different encoding schemes. For example, a user may subscribe to a new data service that transmits data using a different encoding technique than was used by the user's old data service. Alternatively, a user may, on a regular basis, receive more than one type of service over the same line, where each service uses a unique encoding scheme. For example, a consumer may subscribe to a video on demand service and a computer service, both of which are provided to the consumer's house via a common cable from a telephone central office.
Conventional decoders are implemented and configured for a single encoding scheme. They cannot be dynamically switched between different encoding schemes, different levels of encoding, or different coding dimensions. Thus, to provide the appropriate decoding for new services, a user would have to obtain additional decoder equipment. This can significantly increase the cost of the service to the user.
Consequently, a need exists for a decoder that can conveniently and economically decode signals that are encoded in a variety of coding formats.
SUMMARY OF THE INVENTION
A programmable signal decoder processes incoming signals using any one of several predefined decoding schemes. The decoder is programmed with a selected encoding scheme by downloading configuration data associated with the decoding scheme into programmable logic in the decoder. The decoding scheme is changed as needed by simply downloading new configuration data. Accordingly, various decoding schemes are supported by providing the appropriate configuration data.
The configuration data defines the input signals, output signals and logic operations performed by the programmable logic. To generate the configuration data for a given decoder, a circuit designer develops a circuit capable of performing the desired functions. This circuit information is converted into data suitable for downloading into the programmable logic. This data is stored in a data memory connected to the programmable decoder. System configuration circuitry

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