The invention relates to OFDM data transmission systems.
OFDM is a spread spectrum technology wherein the 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 impact of a combination of impulse noise and frequency nulls, which may result in lengthy noise events.
The present invention features a mechanism for adapting a forward error correction encoder (including an associated interleaver) to a channel.
In one aspect of the invention, a forward error correction (FEC) encoder is adapted to a channel over which data encoded by the FEC encoder and modulated onto OFDM symbols is to be transmitted in a data transmission to a receiving network node. Channel information specifying OFDM symbol block sizes associated with the data transmission are received. The channel information is based on a prior data transmission to the receiving network node. FEC encoder configuration values are computed based on the received channel information. The FEC encoder is then configured to operate on the data according to the configuration information.
Embodiments of the invention may include one or more of the following features.
The received channel information can be based on characteristics of the channel. It can include a modulation type and carriers capable of supporting the modulation type for the channel.
The OFDM symbol block sizes may include a fixed size and a variable size. The FEC encoder configuration values may be computed by determining from the channel information a number of OFDM symbol blocks of the fixed size and a number of remaining data bytes to be included in a last OFDM symbol block of the variable size, and computing a number of symbols for transmitting the remaining data bytes from the number of remaining data bytes, the channel information and a code block size associated with the FEC encoder.
The FEC encoder can include an interleaver configured to store the last variable size OFDM symbol block of encoded data based on the computed number of symbols and the channel information.
The FEC encoder can include a Reed-Solomon encoder. A maximum number of Reed-Solomon bytes in the last variable size OFDM symbol block can be computed. A Reed-Solomon block size can be computed from the maximum number of Reed-Solomon bytes in the last variable size block.
Among the advantages of the present invention are the following. In network having a transmitting network node communicating with a receiving network node over a data channel, and each of the nodes having a transmit portion and receive portion, the transmit portion in the transmitting network node can take advantage of information regarding recent channel conditions as reflected in the most up-to-date channel map for the channel produced by the receive portion of the receiving network node based on a prior data transmission to the receiving network node. The FEC encoder configuration may be adjusted as necessary to accommodate variable OFDM symbol block sizes dictated by the channel map updates.
Other features and advantages of the invention will be apparent from the following detailed description and from the claims.