Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-09-28
2001-08-21
Decady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S752000, C714S762000
Reexamination Certificate
active
06279132
ABSTRACT:
This application is related to U.S. patent application Ser. No. 09/069,681, entitled Concatenated Coding System for Satellite Communications, filed Apr. 29, 1998.
BACKGROUND OF THE INVENTION
The present invention relates to error control coding in communications systems. In particular, the present invention relates to inner codes for use with concatenated codes.
In digital satellite communication systems, error correcting coding is often used to obtain more efficient link power usage at the expense of less efficient spectral usage. In other words, additional error coding allows transmitters to send with less power at the expense of somewhat increased bandwidth. Applying error correcting codes to information in the uplink signal at the ground station helps reduce the effect of numerous sources of corrupting interference. Thus, error correcting codes attempt to lower the Bit Error Rate (BER) (generally defined as the ratio of incorrectly received information bits to the total number of received information bits) of the coded information, as received, to acceptable levels.
Among the available techniques for such error correction coding is concatenated coding. Generally, concatenated coding refers to the sequence of coding in which a second coding operation is performed upon already encoded data. The two levels of codes are generally referred to as the inner code and the outer code. The outer code of the concatenated code is the first code applied at the ground station, while the inner code is the second code applied. At the receiver, such as a receiving satellite, the inner code (if decoded at all) is decoded first followed by the outer code.
The first code the ground station applies is typically a block code. A codeword in a block code consists of K information bits, and R parity bits. The codeword is therefore N=K+R bits in length.
Block codes are generally organized on the basis of bits. A variety of block codes known as Reed-Solomon codes may be used as the outer code to encode the uplink signals. Reed-Solomon block codes, however, are organized on the basis of groups of bits referred to as codeletters. To form codeletters, an incoming serial bit stream may be reorganized as sequences of m individual bits (a codeletter). The Reed-Solomon code has K information codeletters (rather than bits), R parity codeletters, and a total codeword length of N=K+R codeletters. For 8-bit codeletters, a Reed-Solomon codeword is typically 255 codeletters in length. Allowing the codeword to correct up to 16 or fewer codeletters requires 32 parity codeletters, thereby leaving 223 data codeletters (for an effective code rate of {fraction (223/255)} (approximately ⅞)).
An inner code that is often applied is a convolutional code. A convolutional code is an error correcting code which transforms an input sequence of bits into an output sequence of bits through a finite-state machine, where additional bits are added to the data stream to allow for error-correcting capability. Typically the amount of error-correction capability is proportional to the amount of additional bits added and the memory present in the finite-state machine (encoder). The constraint length, L, of a convolutional code is proportional to the finite-state machine's memory. The rate of the convolutional code (k
, with k<n) describes how many additional bits are added for every k information bits (input) (i.e., n-k bits added for each k information bits.) Viterbi decoding may be used to decode a convolutional code. Unfortunately, the decoding complexity of a convolutional code increases exponentially with the constraint length.
As an additional measure, uplink signals may be interleaved before transmission. The process of interleaving reorders coded sequences prior to transmission and is inverted after reception and decoding of the inner code but prior to decoding of the outer code. Interleaving is most beneficial when the output of the inner decode contains a burst of errors. This is because interleaving spreads burst errors out in the sequences deinterleaved after reception and thus increases the likelihood of accurate decoding. Unfortunately, interleaving coded sequences takes time, power, and computational resources, and adds to the overall complexity of the communication system.
Commonly used coding methods present two difficulties when processing information received on an uplink. First, an interleaver is impractical in most forms of uplink access because of the significant latency (i.e., additional delay) arising when interleaving is applied to a low speed application. Second, the process of Viterbi decoding, though readily realizable, consumes a significant amount of spacecraft power, which is a scarce resource. The second problem is due primarily to the fact that, for convolutional inner codes, decoding complexity increases exponentially with the constraint length.
A need has long existed for a simple and effective error control method that may be used in the uplink of a processing satellite communications system.
BRIEF SUMMARY OF THE INVENTION
One object of the present invention is to produce a coding gain on the uplink without interleaving and the latency associated therewith.
Another object of the invention is to apply coding to information transmitted in an uplink that produces a coding gain, but that requires relatively low spacecraft power to decode.
An additional object of the invention is to apply coding to information transmitted in an uplink that produces significant coding gain over simple repetition coding.
Yet another object of the present invention is to apply a relatively short inner block code in a concatenated coding scheme, such that a codeletter of the outer code corresponds to one or more codewords of the inner code.
A still further object of the present invention is to apply a relatively short inner block code in a concatenated coding scheme that produces biorthogonal signal waveforms.
The preferred embodiment of the present invention provides a method for coding information transmitted in an uplink to a satellite. The method includes the step of applying an outer code to an information block to form an outer coded block. The outer coded block is then inner coded by applying a short block code without an intervening interleaver to the code letters at the inner code, thereby producing a concatenated coded block. The concatenated coded block is then transmitted to a satellite.
The step of inner coding preferably applies an (8,4) block code to produce codewords which form a biorthogonal constellation. The step of inner coding may alternatively apply another short block code such as a Nordstrom-Robinson (16,8) code. The short block code applied may, for example, include an identity sub-matrix and a code sub-matrix to generate a systematic code. The step of inner coding may generate, for example, sixteen or two-hundred-fifty-six unique inner codewords for the (8,4) or (16,8) case respectively.
Another embodiment of the invention is a coded information signal embodied in a carrier wave. The carrier wave, for example, may be a 30 GHz uplink carrier. The coded information signal includes a plurality of transmitted waveforms. Each waveform is tranformed into a sequence of modulated symbols representing a codeword of a predetermined codeset. Additionally, with the (8,4) code, each transmitted waveform is a member of a biorthogonal waveform set that includes each transmitted waveform that may be generated from the codeset.
Yet another embodiment of the invention is a satellite communication system. The satellite communication system generally includes one or more ground stations in communication with at least one satellite. The ground station includes an outer coder which is adapted to apply an outer code to an information block, thereby forming an outer coded block. An inner coder is also provided and is adapted to encode the outer coded block by applying a short block code, thereby producing a concatenated coded block. The ground station also includes a transmitter that
Linsky Stuart T.
Wilcoxson Donald C.
Wright David A.
Yee-Madera Gefferie H.
De'cady Albert
Keller Robert W.
Ton David
TRW Inc.
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