diff --git a/lib/ftrsd/ftrsd_paper/ftrsd.lyx b/lib/ftrsd/ftrsd_paper/ftrsd.lyx index 28ce5564d..ab7e219a7 100644 --- a/lib/ftrsd/ftrsd_paper/ftrsd.lyx +++ b/lib/ftrsd/ftrsd_paper/ftrsd.lyx @@ -138,7 +138,7 @@ key "kv2001" Since 2001 the KV decoder has been considered the best available soft-decision decoder for Reed Solomon codes. We describe here a new open-source alternative called the Franke-Taylor - (FT, or K9AN-K1JT) algorithm. + (FT, or K9AN-K1JT) soft-decision decoding algorithm. It is conceptually simple, built around the well-known Berlekamp-Massey errors-and-erasures algorithm, and in this application it performs even better than the KV decoder. @@ -298,7 +298,15 @@ d=n-k+1.\label{eq:minimum_distance} \end_inset -The minimum Hamming distance of the JT65 code is +With +\begin_inset Formula $n=63$ +\end_inset + + and +\begin_inset Formula $k=12$ +\end_inset + + the minimum Hamming distance of the JT65 code is \begin_inset Formula $d=52$ \end_inset @@ -361,8 +369,11 @@ erasures. \begin_inset Quotes erd \end_inset - With perfect erasure information up to 51 incorrect symbols can be corrected - for the JT65 code. + With perfect erasure information up to +\begin_inset Formula $n-k=51$ +\end_inset + + incorrect symbols can be corrected for the JT65 code. Imperfect erasure information means that some erased symbols may be correct, and some other symbols in error. If @@ -701,8 +712,31 @@ How might we best choose the number of symbols to erase, in order to maximize \end_inset symbols. - Decoding will then be assured if the set of erased symbols contains at - least 37 errors. + According to equation +\begin_inset CommandInset ref +LatexCommand ref +reference "eq:erasures_and_errors" + +\end_inset + +, with +\begin_inset Formula $s=45$ +\end_inset + + and +\begin_inset Formula $d=52$ +\end_inset + + then +\begin_inset Formula $e\le3$ +\end_inset + +, so decoding will be assured if the set of erased symbols contains at least + +\begin_inset Formula $40-3=37$ +\end_inset + + errors. With \begin_inset Formula $N=63$ \end_inset @@ -826,19 +860,32 @@ The Franke-Taylor Decoding Algorithm \begin_layout Standard Example 3 shows how statistical information about symbol quality should make it possible to decode received frames having a large number of errors. - In practice the number of errors in the received word is unknown, so we - use a stochastic algorithm to assign high erasure probability to low-quality - symbols and relatively low probability to high-quality symbols. + In practice the number of errors in the received word is unknown, so our + algorithm simply assigns a high erasure probability to low-quality symbols + and relatively low probability to high-quality symbols. As illustrated by Example 3, a good choice of erasure probabilities can increase by many orders of magnitude the chance of producing a codeword. - Note that at this stage we must treat any codeword obtained by errors-and-erasu -res decoding as no more than a + Once erasure probabilities have been assigned to each of the 63 received + symbols, the FT algorithm uses a random number generator to decide whether + or not to erase each symbol according to its assigned erasure probability. + The list of erased symbols is then submitted to the BM decoder which either + produces a codeword or fails to decode. + +\end_layout + +\begin_layout Standard +The process of selecting the list of symbols to erase and calling the BM + decoder comprises one cycle of the FT algorithm. + The next cycle proceeds with a new selection of erased symbols. + At this stage we must treat any codeword obtained by errors-and-erasures + decoding as no more than a \emph on candidate \emph default . Our next task is to find a metric that can reliably select one of many proffered candidates as the codeword actually transmitted. + \end_layout \begin_layout Standard @@ -898,7 +945,7 @@ The FT algorithm uses quality indices made available by a noncoherent 64-FSK \end_inset of the symbol's fractional power -\begin_inset Formula $p_{1,\,j}$ +\begin_inset Formula $p_{1,\, j}$ \end_inset in a sorted list of @@ -923,7 +970,8 @@ The FT algorithm uses quality indices made available by a noncoherent 64-FSK \end_layout \begin_layout Standard -We use an empirical table of symbol error probabilities derived from a large +We use a 3-bit quantization of these two metrics to index the entries in + an 8x8 table of symbol error probabilities derived empirically from a large dataset of received words that were successfully decoded. The table provides an estimate of the \emph on @@ -938,10 +986,22 @@ a priori \end_inset metrics. - These probabilities are close to 1 for low-quality symbols and close to - 0 for high-quality symbols. + This table is a key element of the algorithm, as it will define which symbols + are effectively +\begin_inset Quotes eld +\end_inset + +protected +\begin_inset Quotes erd +\end_inset + + from erasure. + The a priori symbol error probabilities are close to 1 for low-quality + symbols and close to 0 for high-quality symbols. Recall from Examples 2 and 3 that candidate codewords are produced with - higher probability when + higher probability when the number of erased symbols is larger than the + number of symbols that are in error, i.e. + when \begin_inset Formula $s>X$ \end_inset @@ -968,7 +1028,7 @@ t educated guesses to select symbols for erasure. , the soft distance between the received word and the codeword: \begin_inset Formula \begin{equation} -d_{s}=\sum_{j=1}^{n}\alpha_{j}\,(1+p_{1,\,j}).\label{eq:soft_distance} +d_{s}=\sum_{j=1}^{n}\alpha_{j}\,(1+p_{1,\, j}).\label{eq:soft_distance} \end{equation} \end_inset @@ -986,7 +1046,7 @@ Here \end_inset if the received symbol and codeword symbol are different, and -\begin_inset Formula $p_{1,\,j}$ +\begin_inset Formula $p_{1,\, j}$ \end_inset is the fractional power associated with received symbol @@ -1030,7 +1090,7 @@ In practice we find that \begin_layout Standard \begin_inset Formula \begin{equation} -u=\frac{1}{n}\sum_{j=1}^{n}S(c_{j},\,j).\label{eq:u-metric} +u=\frac{1}{n}\sum_{j=1}^{n}S(c_{j},\, j).\label{eq:u-metric} \end{equation} \end_inset @@ -1063,7 +1123,7 @@ The correct JT65 codeword produces a value for bins containing noise only. Thus, if the spectral array -\begin_inset Formula $S(i,\,j)$ +\begin_inset Formula $S(i,\, j)$ \end_inset has been normalized so that the average value of the noise-only bins is @@ -1078,7 +1138,7 @@ The correct JT65 codeword produces a value for \begin_layout Standard \begin_inset Formula \begin{equation} -\bar{u}_{1}=1+y,\label{eq:u1-exp} +\bar{u}_{c}=1+y,\label{eq:u1-exp} \end{equation} \end_inset @@ -1090,7 +1150,7 @@ where is the signal-to-noise ratio in linear power units. If we assume Gaussian statistics and a large number of trials, the standard deviation of measured values of -\begin_inset Formula $u_{1}$ +\begin_inset Formula $u$ \end_inset is @@ -1099,7 +1159,7 @@ where \begin_layout Standard \begin_inset Formula \begin{equation} -\sigma_{1}=\left(\frac{1+2y}{n}\right)^{1/2}.\label{eq:sigma1} +\sigma_{c}=\left(\frac{1+2y}{n}\right)^{1/2}.\label{eq:sigma1} \end{equation} \end_inset @@ -1143,12 +1203,28 @@ i.e. \begin_layout Standard \begin_inset Formula \begin{equation} -\sigma_{i}=\frac{1}{n}\left[n+2y(k-1)\right]^{1/2}.\label{eq:sigma2} +\sigma_{i}=\frac{1}{n}\left[n+2y(k-1)\right]^{1/2},\label{eq:sigma2} \end{equation} \end_inset +where the subscript +\begin_inset Quotes eld +\end_inset +i +\begin_inset Quotes erd +\end_inset + + is an abbreviation for +\begin_inset Quotes eld +\end_inset + +incorrect +\begin_inset Quotes erd +\end_inset + +. \end_layout \begin_layout Standard @@ -1162,11 +1238,11 @@ If \end_inset to be drawn from a population with statistics described by -\begin_inset Formula $\bar{u}_{1}$ +\begin_inset Formula $\bar{u}_{c}$ \end_inset and -\begin_inset Formula $\sigma_{1}.$ +\begin_inset Formula $\sigma_{c}.$ \end_inset If no tested codeword is correct, @@ -1235,14 +1311,8 @@ reference "sec:Theory,-Simulation,-and" \end_layout \begin_layout Standard -Technically the FT algorithm is a list decoder. - Among the list of candidate codewords found by the stochastic search algorithm, - only the one with the largest -\begin_inset Formula $u$ -\end_inset - - is retained. - As with all such algorithms, a stopping criterion is necessary. +As with all decoding algorithms that generate a list of possible codewords, + a stopping criterion is necessary. FT accepts a codeword unconditionally if the Hamming distance \begin_inset Formula $X$ \end_inset @@ -1312,7 +1382,7 @@ For each received symbol, define the erasure probability as 1.3 times the a priori \emph default symbol-error probability determined from soft-symbol information -\begin_inset Formula $\{p_{1}\textrm{-rank},\,p_{2}/p_{1}\}$ +\begin_inset Formula $\{p_{1}\textrm{-rank},\, p_{2}/p_{1}\}$ \end_inset .