Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
1999-08-19
2001-08-21
Chin, Wellington (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C370S208000, C375S346000, C375S348000, C714S746000
Reexamination Certificate
active
06278685
ABSTRACT:
BACKGROUND OF THE INVENTION
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, on transmitted OFDM data symbols.
SUMMARY OF THE INVENTION
In one aspect of the invention, encoded data to be modulated onto carriers of OFDM symbols in a packet of consecutive OFDM symbols for transmission over a transmission channel is interleaved to produce copies of the encoded data which are spread in time on non-consecutive OFDM symbols in the packet of consecutive OFDM symbols and in frequency on nonadjacent carriers.
In another aspect of the invention, OFDM data received from a transmission channel are processed for a more robust data transmission. Multiple copies of the OFDM data are received from the transmission channel, the multiple copies being spread in time and frequency. Phase angle information for the multiple copies is combined to produce a single metric value to be used in decoding the OFDM data.
Embodiments of the invention may include one or more of the following features.
Interleaving can include storing the encoded data in an interleaver memory by row and reading the encoded data from the interleaver memory by column, the encoded data stored in the interleaver memory being read n consecutive times.
The encoded data reads can include an offset to all but the first of the column reads of each of the n consecutive reads and different additional offsets to all but the first of the n consecutive reads.
The phase angle information can include a metric value for each of the four copies. Alternatively, the phase angle information can include phase angle representation values.
The phase angle representation values for the data copies can be combined in the following manner. Phase noise values are computed from the phase angle representations for the data copies. A weighting is applied to the phase angle representation values based on the computed phase noise values. The weighted phase angle representation values are summed and converted to a single metric value.
If metric value copies are used, then they may be combined in the following manner. Phase angles of the multiple copies are converted to metric values. Phase noise values are computed from the phase angles for the data copies. A weighting is selected and applied to the metric values based on the computed phase noise values and the weighted metric values are summed.
Alternatively, the metric value copies can be combined by summing the metric values to produce a sum and using the sum to compute an average metric value as a single metric value.
In yet another alternative, combining the metric value copies can include selecting one of the metric values.
In either of the combination processes, amplitudes of the copies may be compared to a jammer detection threshold and the results of the comparison used to override the selected weighting so that a minimal weighting is applied to the metric values or phase representation values for the copies.
The technique of the invention offers several advantages. It provides a level of redundancy and combines that level of redundancy with frequency and time diversity. Consequently, because each data bit is evenly distributed across the frequency band in each symbol and across the transmitted symbols in time, there is a greater likelihood of recovering data lost as a result of a noise event or destructive canceling (caused by delay spread), since the best copies of the redundant data can be used. The technique also uses phase noise to weight the copies differently prior to combining the copies into a single copy. Strong carriers with low phase noise are weighted more heavily. Thus, the transmission as a whole is more reliable, even in extremely noisy environments.
Other features and advantages of the invention will be apparent from the following detailed description and from the claims.
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Blanchard Bart W.
Mashburn Harper Brent
Yonge, III Lawrence W.
Chin Wellington
Fish & Richardson P.C.
Intellon Corporation
Tran Maikhanh
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