Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
2000-05-19
2001-09-11
Chin, Wellington (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C375S346000
Reexamination Certificate
active
06289000
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to OFDM (Orthogonal Frequency Division Multiplexing) data transmission systems.
In OFDM data transmission systems, available transmission channel bandwidth is subdivided into a number of discrete channels or carriers that are overlapping and orthogonal to each other. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of carrier frequencies. The data transmitted over these OFDM symbol carriers may be encoded and modulated in amplitude and/or phase, using conventional schemes such as Binary Phase Shift Key (BPSK) or Quadrature Phase Shift Key (QPSK).
A well-known problem in the art of OFDM data transmission systems is that of impulse noise, which can produce bursts of error on transmission channels, and delay spread, which often causes frequency selective fading. To address these problems, prior systems have utilized forward error correction (FEC) coding in conjunction with interleaving techniques. FEC coding adds parity data that enables one or more errors in a code word to be detected and corrected. Interleaving reorders the code word bits in a block of code word data prior to transmission to achieve time and frequency diversity.
Although the prior interleaving techniques can minimize some of the effects of impulse noise and delay spread on OFDM data transmission, they cannot mitigate the collective impact of impulse noise and frequency nulls, which may result in lengthy noise events. Additionally, the same FEC encoding and interleaving are used for all data to be transmitted, thus providing the same level of error protection irrespectively of the importance of the data and how the data are to be used.
SUMMARY OF THE INVENTION
The present invention features a mechanism for encoding and decoding of frame data for robust OFDM transmissions of the frame data, in particular, frame control information.
In one aspect of the invention, encoding frame data for an OFDM frame transmission includes producing a code block of elements from frame data to be modulated onto carriers of OFDM symbols in an OFDM frame, and interleaving the elements so that the elements are modulated onto the carriers in groupings along diagonals were the elements to be organized as a matrix.
Embodiments of the invention may include one or more of the following features.
The frame data can include PHY layer frame control information for supporting a medium access control protocol. The OFDM frame can include at least one delimiter and the PHY layer frame control information can be located in the at least one delimiter. The OFDM frame can include a body and the at least one delimiter can include a start delimiter that precedes the body. The at least one delimiter can further include an end delimiter than follows the body. The at least one delimiter can be a response or a Request-to-Send (RTS) type of delimiter. The medium access control protocol can be a Carrier Sense Multiple Access type. The medium access control protocol can be a Time Division Multiple Access protocol and the at least one delimiter can include beacon information used by such protocol. The medium access control protocol can be token passing type.
Interleaving can include selecting from the elements along the diagonals to produce diagonal sequences. Collectively, the diagonal sequences can form a vector of vector elements. Interleaving can further include selecting consecutive vector elements from the vector for modulation on carriers in a succession of symbols so that the elements in the diagonals appear on adjacent carriers and across adjacent symbols in the succession of symbols to produce the groupings along diagonals. Selecting the vector elements from the vector for modulation on carriers in the successive symbols can result in a level of redundancy.
For a number of OFDM symbols equal to three, selecting can include selecting a first element in a main diagonal and then selecting every third element from among consecutive elements in the diagonals and placing the selected first element and every third element in the vector in the order of selection. Interleaving can further include selecting consecutive vector elements from the vector for modulation on carriers in a succession of symbols so that the consecutive elements in the diagonals appear on adjacent carriers across adjacent symbols in the succession of symbols. Selecting consecutive vector elements from the vector for modulation on carriers in the successive symbols can result in a level of redundancy.
For a number of OFDM symbols equal to four and the vector arranged as an array of four columns of rows, selecting includes selecting consecutive elements along each of the diagonals and placing the selected elements in the vector in groups of adjacent rows.
The code block can be a product code block and producing can include deriving the product code block from a shortened extended hamming code code word set. Producing can further include selecting a generator matrix to achieve symmetric properties of the code word set.
The OFDM frame can include a body and the frame data can include frame control information that precedes the body. Alternatively, the OFDM frame can be an acknowledgement frame and the frame data can include frame control information.
In another aspect of the invention, decoding encoded frame control information includes producing soft decision values from frame control information encoded in code words having information bits, and modulated in an interleaved order onto carriers in OFDM symbols, the code words belonging to a set of code words having properties of symmetry, de-interleaving the soft decision values to produce sets of soft decision values, each set associated with one of the code words, and performing on each set of soft decision values a decoding procedure according to the properties of symmetry to reduce the complexity of the decoding procedure.
Embodiments of the invention may include one or more of the following features.
The decoding procedure can be a turbo decoding procedure and performing the turbo decoding procedure can include performing each of i iterations. Performing each of i iterations can include the following: determining a new set of soft decision values from the set of decision values; determining difference values for a difference between the set of soft decision values and the new set of soft decision values; weighting the difference values; and updating the set of soft decision values with the sum of the set of soft decision values and the weighted difference values.
Determining a new set of soft decision values can include generating from the set of soft decision values correlation values corresponding to a subset of the code word set and generating from correlation values corresponding to the subset correlation values corresponding to a remainder of the code word set based on the properties of symmetry of the code word set.
Determining a new set of soft decision values can further include using the properties of symmetry of the code word set to select a best correlation value for each of the soft decision values.
Performing can further include producing a hard decision value from each set of soft decision values for each of the information bits in the one of the code words with which the set of soft decision values is associated.
The code words can be produced by a product code and the turbo decoding procedure can be a turbo product decoding procedure.
The code words can be in the form of a product code block of elements, the interleaved order can include copies of at least some of the elements and decoding the encoded frame control information can further include combining the copies. Combining can include producing phase noise values for the copies, weighting the copies according to the phase noise values and summing the weighted copies.
Among the advantages of the present invention are the following. The interleaving technique provides a level of redundancy and combines that level of redundancy with diversity in the frequency and
Chin Wellington
Fish & Richardson P.C.
Intellon Corporation
Tran Maikhanh
LandOfFree
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