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2028 lines
44 KiB
Plaintext
2028 lines
44 KiB
Plaintext
#LyX 2.1 created this file. For more info see http://www.lyx.org/
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\lyxformat 474
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\begin_document
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\begin_header
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\textclass paper
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\index Index
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\shortcut idx
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\end_header
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\begin_body
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\begin_layout Title
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Open Source Soft-Decision Decoder for the JT65 (63,12) Reed-Solomon code
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\end_layout
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\begin_layout Author
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Steven J.
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Franke, K9AN and Joseph H.
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Taylor, K1JT
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\end_layout
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\begin_layout Standard
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\begin_inset CommandInset toc
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LatexCommand tableofcontents
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\end_inset
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\end_layout
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\begin_layout Abstract
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The JT65 protocol has revolutionized amateur-radio weak-signal communication
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by enabling amateur radio operators with small antennas and relatively
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low-power transmitters to communicate over propagation paths not usable
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with traditional technologies.
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A major reason for the success and popularity of JT65 is its use of a strong
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error-correction code: a short block-length, low-rate Reed-Solomon code
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based on a 64-symbol alphabet.
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Since 2004, most programs implementing JT65 have used the patented Koetter-Vard
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y (KV) algebraic soft-decision decoder, licensed to K1JT and implemented
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in a closed-source program for use in amateur radio applications.
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We describe here a new open-source alternative called the Franke-Taylor
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(FT, or K9AN-K1JT) algorithm.
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It is conceptually simple, built around the well-known Berlekamp-Massey
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errors-and-erasures algorithm, and in this application it performs even
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better than the KV decoder.
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\end_layout
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\begin_layout Section
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Introduction-and-Motivation"
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\end_inset
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Introduction and Motivation
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\end_layout
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\begin_layout Standard
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The following paragraph may not belong here - feel free to get rid of it,
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change it, whatever.
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\end_layout
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\begin_layout Standard
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The Franke-Taylor (FT) decoder is a probabilistic list-decoder that we have
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developed for use in the short block-length, low-rate Reed-Solomon code
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used in JT65.
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JT65 provides a unique sandbox for playing with decoding algorithms.
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Several seconds are available for decoding a single 63-symbol message.
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This is a long time! The luxury of essentially unlimited time allows us
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to experiment with decoders that have high computational complexity.
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The payoff is that we can extend the decoding threshold by many dB over
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the hard-decision, Berlekamp-Massey decoder on a typical fading channel,
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and by a meaningful amount over the KV decoder, long considered to be the
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best available soft-decision decoder.
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In addition to its excellent performance, the FT algorithm has other desirable
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properties, not the least of which is its conceptual simplicity.
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Decoding performance and complexity scale in a useful way, providing steadily
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increasing soft-decision decoding gain as a tunable computational complexity
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parameter is increased over more than 5 orders of magnitude.
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This means that appreciable gain should be available from our decoder even
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on very simple (and slow) computers.
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On the other hand, because the algorithm requires a large number of independent
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decoding trials, it should be possible to obtain significant performance
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gains through parallelization on high-performance computers.
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\end_layout
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\begin_layout Section
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:JT65-messages-and"
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\end_inset
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JT65 messages and Reed Solomon Codes
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\end_layout
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\begin_layout Standard
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JT65 message frames consist of a short compressed message encoded for transmissi
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on with a Reed-Solomon code.
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Reed-Solomon codes are block codes characterized by
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\begin_inset Formula $n$
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\end_inset
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, the length of their codewords,
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\begin_inset Formula $k$
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\end_inset
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, the number of message symbols conveyed by the codeword, and the number
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of possible values for each symbol in the codewords.
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The codeword length and the number of message symbols are specified with
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the notation
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\begin_inset Formula $(n,k)$
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\end_inset
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.
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JT65 uses a (63,12) Reed-Solomon code with 64 possible values for each
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symbol.
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Each of the 12 message symbols represents
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\begin_inset Formula $\log_{2}64=6$
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\end_inset
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message bits.
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The source-encoded messages conveyed by a 63-symbol JT65 frame thus consist
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of 72 information bits.
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The JT65 code is systematic, which means that the 12 message symbols are
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embedded in the codeword without modification and another 51 parity symbols
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derived from the message symbols are added to form a codeword of 63 symbols.
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\end_layout
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\begin_layout Standard
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The concept of Hamming distance is used as a measure of
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\begin_inset Quotes eld
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\end_inset
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distance
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\begin_inset Quotes erd
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\end_inset
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between different codewords, or between a received word and a codeword.
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Hamming distance is the number of code symbols that differ in two words
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being compared.
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Reed-Solomon codes have minimum Hamming distance
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\begin_inset Formula $d$
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\end_inset
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, where
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\begin_inset Formula
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\begin{equation}
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d=n-k+1.\label{eq:minimum_distance}
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\end{equation}
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\end_inset
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The minimum Hamming distance of the JT65 code is
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\begin_inset Formula $d=52$
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\end_inset
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, which means that any particular codeword differs from all other codewords
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in at least 52 symbol positions.
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\end_layout
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\begin_layout Standard
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Given a received word containing some incorrect symbols (errors), the received
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word can be decoded into the correct codeword using a deterministic, algebraic
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algorithm provided that no more than
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\begin_inset Formula $t$
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\end_inset
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symbols were received incorrectly, where
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\begin_inset Formula
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\begin{equation}
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t=\left\lfloor \frac{n-k}{2}\right\rfloor .\label{eq:t}
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\end{equation}
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\end_inset
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For the JT65 code
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\begin_inset Formula $t=25$
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\end_inset
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, so it is always possible to decode a received word having 25 or fewer
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symbol errors.
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Any one of several well-known algebraic algorithms, such as the widely
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used Berlekamp-Massey (BM) algorithm, can carry out the decoding.
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Two steps are necessarily involved in this process.
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We must (1) determine which symbols were received incorrectly, and (2)
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find the correct value of the incorrect symbols.
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If we somehow know that certain symbols are incorrect, that information
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can be used to reduce the work involved in step 1 and allow step 2 to correct
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more than
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\begin_inset Formula $t$
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\end_inset
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errors.
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In the unlikely event that the location of every error is known and if
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no correct symbols are accidentally labeled as errors, the BM algorithm
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can correct up to
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\begin_inset Formula $d-1=n-k$
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\end_inset
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errors.
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\end_layout
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\begin_layout Standard
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The FT algorithm creates lists of symbols suspected of being incorrect and
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sends them to the BM decoder.
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Symbols flagged in this way are called
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\begin_inset Quotes eld
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\end_inset
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erasures,
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\begin_inset Quotes erd
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\end_inset
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while other incorrect symbols will be called
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\begin_inset Quotes eld
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\end_inset
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errors.
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\begin_inset Quotes erd
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\end_inset
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With perfect erasure information up to 51 incorrect symbols can be corrected
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for the JT65 code.
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Imperfect erasure information means that some erased symbols may be correct,
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and some other symbols in error.
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If
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\begin_inset Formula $s$
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\end_inset
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symbols are erased and the remaining
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\begin_inset Formula $n-s$
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\end_inset
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symbols contain
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\begin_inset Formula $e$
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\end_inset
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errors, the BM algorithm can find the correct codeword as long as
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\begin_inset Formula
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\begin{equation}
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s+2e\le d-1.\label{eq:erasures_and_errors}
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\end{equation}
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\end_inset
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If
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\begin_inset Formula $s=0$
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\end_inset
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, the decoder is said to be an
|
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\begin_inset Quotes eld
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\end_inset
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errors-only
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\begin_inset Quotes erd
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\end_inset
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decoder.
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If
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\begin_inset Formula $0<s\le d-1$
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\end_inset
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, the decoder is called an
|
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\begin_inset Quotes eld
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\end_inset
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errors-and-erasures
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\begin_inset Quotes erd
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\end_inset
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decoder.
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The possibility of doing errors-and-erasures decoding lies at the heart
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of the FT algorithm.
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On that foundation we have built a capability for using
|
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\begin_inset Quotes eld
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\end_inset
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|
soft
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\begin_inset Quotes erd
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\end_inset
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information on the reliability of individual symbols, thereby producing
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a soft-decision decoder.
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\end_layout
|
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|
\begin_layout Section
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Statistical Framework"
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\end_inset
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|
Statistical Framework
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\end_layout
|
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|
|
\begin_layout Standard
|
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The FT algorithm uses the estimated quality of received symbols to generate
|
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lists of symbols considered likely to be in error, thus enabling decoding
|
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of received words with more than 25 errors using the errors-and-erasures
|
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capability of the BM decoder.
|
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Algorithms of this type are generally called
|
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\begin_inset Quotes eld
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\end_inset
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reliability based
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\begin_inset Quotes erd
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\end_inset
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or
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\begin_inset Quotes eld
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\end_inset
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probabilistic
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\begin_inset Quotes erd
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\end_inset
|
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decoding methods
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\begin_inset CommandInset citation
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LatexCommand cite
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after "Chapter 10"
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key "key-1"
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\end_inset
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.
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Such algorithms involve some amount of educating guessing about which received
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symbols are in error or, alternatively, about which received symbols are
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correct.
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The guesses are informed by
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\begin_inset Quotes eld
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\end_inset
|
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|
soft-symbol
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\begin_inset Quotes erd
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\end_inset
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quality metrics associated with the received symbols.
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To illustrate why it is absolutely essential to use such soft-symbol informatio
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n in these algorithms it helps to consider what would happen if we tried
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to use completely random guesses, ignoring any available soft-symbol informatio
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n.
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\end_layout
|
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\begin_layout Standard
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As a specific example, we will consider a received JT65 word with 23 correct
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symbols and 40 errors.
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We do not know which symbols are in error.
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Suppose that the decoder randomly selects
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\begin_inset Formula $s=40$
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\end_inset
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symbols for erasure, leaving 23 unerased symbols.
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|
According to Eq.
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(
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\begin_inset CommandInset ref
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LatexCommand ref
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|
reference "eq:erasures_and_errors"
|
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|
|
\end_inset
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|
), the BM decoder can successfully decode this word as long as
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\begin_inset Formula $e$
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\end_inset
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, the number of errors present in the 23 unerased symbols, is 5 or less.
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The number of errors captured in the set of 40 erased symbols must therefore
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be at least 35.
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|
\end_layout
|
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|
\begin_layout Standard
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|
The probability of selecting some particular number of incorrect symbols
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|
in a randomly selected subset of received symbols is governed by the hypergeome
|
|
tric probability distribution.
|
|
Let us define
|
|
\begin_inset Formula $N$
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|
\end_inset
|
|
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|
as the number of symbols from which erasures will be selected,
|
|
\begin_inset Formula $X$
|
|
\end_inset
|
|
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|
as the number of incorrect symbols in the set of
|
|
\begin_inset Formula $N$
|
|
\end_inset
|
|
|
|
symbols, and
|
|
\begin_inset Formula $x$
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|
\end_inset
|
|
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|
as the number of errors in the symbols actually erased.
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|
In an ensemble of many received words
|
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\begin_inset Formula $X$
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|
\end_inset
|
|
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|
and
|
|
\begin_inset Formula $x$
|
|
\end_inset
|
|
|
|
will be random variables but for this example we will assume that
|
|
\begin_inset Formula $X$
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|
\end_inset
|
|
|
|
is known and that only
|
|
\begin_inset Formula $x$
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|
\end_inset
|
|
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|
is random.
|
|
The conditional probability mass function for
|
|
\begin_inset Formula $x$
|
|
\end_inset
|
|
|
|
with stated values of
|
|
\begin_inset Formula $N$
|
|
\end_inset
|
|
|
|
,
|
|
\begin_inset Formula $X$
|
|
\end_inset
|
|
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|
, and
|
|
\begin_inset Formula $s$
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|
\end_inset
|
|
|
|
may be written as
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Formula
|
|
\begin{equation}
|
|
P(x=\epsilon|N,X,s)=\frac{\binom{X}{\epsilon}\binom{N-X}{s-\epsilon}}{\binom{N}{s}}\label{eq:hypergeometric_pdf}
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|
\end{equation}
|
|
|
|
\end_inset
|
|
|
|
where
|
|
\begin_inset Formula $\binom{n}{k}=\frac{n!}{k!(n-k)!}$
|
|
\end_inset
|
|
|
|
is the binomial coefficient.
|
|
The binomial coefficient can be calculated using the function
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
nchoosek(
|
|
\begin_inset Formula $n,k$
|
|
\end_inset
|
|
|
|
)
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
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|
in the interpreted language GNU Octave, or with one of many free online
|
|
calculators.
|
|
The hypergeometric probability mass function defined in Eq.
|
|
(
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "eq:hypergeometric_pdf"
|
|
|
|
\end_inset
|
|
|
|
) is available in GNU Octave as function
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
hygepdf(
|
|
\begin_inset Formula $x,N,X,s$
|
|
\end_inset
|
|
|
|
)
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
.
|
|
The cumulative probability that at least
|
|
\begin_inset Formula $\epsilon$
|
|
\end_inset
|
|
|
|
errors are captured in a subset of
|
|
\begin_inset Formula $s$
|
|
\end_inset
|
|
|
|
erased symbols selected from a group of
|
|
\begin_inset Formula $N$
|
|
\end_inset
|
|
|
|
symbols containing
|
|
\begin_inset Formula $X$
|
|
\end_inset
|
|
|
|
errors is
|
|
\begin_inset Formula
|
|
\begin{equation}
|
|
P(x\ge\epsilon|N,X,s)=\sum_{j=\epsilon}^{s}P(x=j|N,X,s).\label{eq:cumulative_prob}
|
|
\end{equation}
|
|
|
|
\end_inset
|
|
|
|
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|
\end_layout
|
|
|
|
\begin_layout Paragraph
|
|
Example 1:
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Suppose a received word contains
|
|
\begin_inset Formula $X=40$
|
|
\end_inset
|
|
|
|
incorrect symbols.
|
|
In an attempt to decode using an errors-and-erasures decoder,
|
|
\begin_inset Formula $s=40$
|
|
\end_inset
|
|
|
|
symbols are randomly selected for erasure from the full set of
|
|
\begin_inset Formula $N=n=63$
|
|
\end_inset
|
|
|
|
symbols.
|
|
The probability that
|
|
\begin_inset Formula $x=35$
|
|
\end_inset
|
|
|
|
of the erased symbols are actually incorrect is then
|
|
\begin_inset Formula
|
|
\[
|
|
P(x=35)=\frac{\binom{40}{35}\binom{63-40}{40-35}}{\binom{63}{40}}\simeq2.4\times10^{-7}.
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
Similarly, the probability that
|
|
\begin_inset Formula $x=36$
|
|
\end_inset
|
|
|
|
of the erased symbols are incorrect is
|
|
\begin_inset Formula
|
|
\[
|
|
P(x=36)\simeq8.6\times10^{-9}.
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
Since the probability of erasing 36 errors is so much smaller than that
|
|
for erasing 35 errors, we may safely conclude that the probability of randomly
|
|
choosing an erasure vector that can decode the received word is approximately
|
|
|
|
\begin_inset Formula $P(x=35)\simeq2.4\times10^{-7}$
|
|
\end_inset
|
|
|
|
.
|
|
The odds of producing a valid codeword on the first try are very poor,
|
|
about 1 in 4 million.
|
|
\end_layout
|
|
|
|
\begin_layout Paragraph
|
|
Example 2:
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
How might we best choose the number of symbols to erase, in order to maximize
|
|
the probability of successful decoding? By exhaustive search over all possible
|
|
values up to
|
|
\begin_inset Formula $s=51$
|
|
\end_inset
|
|
|
|
, it turns out that for
|
|
\begin_inset Formula $X=40$
|
|
\end_inset
|
|
|
|
the best strategy is to erase
|
|
\begin_inset Formula $s=45$
|
|
\end_inset
|
|
|
|
symbols.
|
|
Decoding will then be assured if the set of erased symbols contains at
|
|
least 37 errors.
|
|
With
|
|
\begin_inset Formula $N=63$
|
|
\end_inset
|
|
|
|
,
|
|
\begin_inset Formula $X=40$
|
|
\end_inset
|
|
|
|
, and
|
|
\begin_inset Formula $s=45$
|
|
\end_inset
|
|
|
|
, the probability of successful decode in a single try is
|
|
\begin_inset Formula
|
|
\[
|
|
P(x\ge37)\simeq1.9\times10^{-6}.
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
This probability is about 8 times higher than the probability of success
|
|
when only 40 symbols were erased.
|
|
Nevertheless, the odds of successfully decoding on the first try are still
|
|
only about 1 in 500,000.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Paragraph
|
|
Example 3:
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Examples 1 and 2 show that a random strategy for selecting symbols to erase
|
|
is unlikely to be successful unless we are prepared to wait a long time
|
|
for an answer.
|
|
So let's modify the strategy to tip the odds in our favor.
|
|
Let the received word contain
|
|
\begin_inset Formula $X=40$
|
|
\end_inset
|
|
|
|
incorrect symbols, as before, but suppose we know that 10 received symbols
|
|
are significantly more reliable than the other 53.
|
|
We might therefore protect the 10 most reliable symbols from erasure, selecting
|
|
erasures from the smaller set of
|
|
\begin_inset Formula $N=53$
|
|
\end_inset
|
|
|
|
less reliable symbols.
|
|
If
|
|
\begin_inset Formula $s=45$
|
|
\end_inset
|
|
|
|
symbols are chosen randomly for erasure in this way, it is still necessary
|
|
for the erased symbols to include at least 37 errors, as in Example 2.
|
|
However, the probabilities are now much more favorable: with
|
|
\begin_inset Formula $N=53$
|
|
\end_inset
|
|
|
|
,
|
|
\begin_inset Formula $X=40$
|
|
\end_inset
|
|
|
|
, and
|
|
\begin_inset Formula $s=45$
|
|
\end_inset
|
|
|
|
, Eq.
|
|
(
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "eq:hypergeometric_pdf"
|
|
|
|
\end_inset
|
|
|
|
) yields
|
|
\begin_inset Formula $P(x\ge37)=0.016$
|
|
\end_inset
|
|
|
|
.
|
|
Even better odds are obtained by choosing
|
|
\begin_inset Formula $s=47$
|
|
\end_inset
|
|
|
|
, which requires
|
|
\begin_inset Formula $x\ge38$
|
|
\end_inset
|
|
|
|
.
|
|
With
|
|
\begin_inset Formula $N=53$
|
|
\end_inset
|
|
|
|
,
|
|
\begin_inset Formula $X=40$
|
|
\end_inset
|
|
|
|
, and
|
|
\begin_inset Formula $s=47$
|
|
\end_inset
|
|
|
|
,
|
|
\begin_inset Formula $P(x\ge38)=0.027$
|
|
\end_inset
|
|
|
|
.
|
|
The odds for producing a codeword on the first try are now about 1 in 38.
|
|
A few hundred independently randomized tries would be enough to all-but-guarant
|
|
ee production of a valid codeword by the BM decoder.
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "sec:The-decoding-algorithm"
|
|
|
|
\end_inset
|
|
|
|
The Franke-Taylor decoding algorithm
|
|
\end_layout
|
|
|
|
\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.
|
|
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
|
|
\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
|
|
The FT algorithm uses quality indices made available by a noncoherent 64-FSK
|
|
demodulator.
|
|
The demodulator computes the power spectrum
|
|
\begin_inset Formula $S(i,j)$
|
|
\end_inset
|
|
|
|
for each signalling interval; for the JT65 protocol
|
|
\begin_inset Formula $i=1,64$
|
|
\end_inset
|
|
|
|
is the frequency index and
|
|
\begin_inset Formula $j=1,63$
|
|
\end_inset
|
|
|
|
the symbol index.
|
|
The most likely value for symbol
|
|
\begin_inset Formula $j$
|
|
\end_inset
|
|
|
|
is taken as the frequency bin with largest signal-plus-noise power over
|
|
all values of
|
|
\begin_inset Formula $i$
|
|
\end_inset
|
|
|
|
.
|
|
The fractions of total power in the two bins containing the largest and
|
|
second-largest powers, denoted respectively by
|
|
\begin_inset Formula $p_{1}$
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset Formula $p_{2}$
|
|
\end_inset
|
|
|
|
, are passed from demodulator to decoder as soft-symbol information.
|
|
The FT decoder derives two metrics from
|
|
\begin_inset Formula $p_{1}$
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset Formula $p_{2}$
|
|
\end_inset
|
|
|
|
, namely
|
|
\end_layout
|
|
|
|
\begin_layout Itemize
|
|
\begin_inset Formula $p_{1}$
|
|
\end_inset
|
|
|
|
-rank: the rank
|
|
\begin_inset Formula $\{1,2,\ldots,63\}$
|
|
\end_inset
|
|
|
|
of the symbol's fractional power
|
|
\begin_inset Formula $p_{1,\, j}$
|
|
\end_inset
|
|
|
|
in a sorted list of
|
|
\begin_inset Formula $p_{1}$
|
|
\end_inset
|
|
|
|
values.
|
|
High ranking symbols have larger signal-to-noise ratio than those with
|
|
lower rank.
|
|
\end_layout
|
|
|
|
\begin_layout Itemize
|
|
\begin_inset Formula $p_{2}/p_{1}$
|
|
\end_inset
|
|
|
|
: when
|
|
\begin_inset Formula $p_{2}/p_{1}$
|
|
\end_inset
|
|
|
|
is not small compared to 1, the most likely symbol value is only slightly
|
|
more reliable than the second most likely one.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
We use an empirical table of symbol error probabilities derived from a large
|
|
dataset of received words that were successfully decoded.
|
|
The table provides an estimate of the
|
|
\emph on
|
|
a priori
|
|
\emph default
|
|
probability of symbol error based on the
|
|
\begin_inset Formula $p_{1}$
|
|
\end_inset
|
|
|
|
-rank and
|
|
\begin_inset Formula $p_{2}/p_{1}$
|
|
\end_inset
|
|
|
|
metrics.
|
|
These 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
|
|
\begin_inset Formula $s>X$
|
|
\end_inset
|
|
|
|
.
|
|
Correspondingly, the FT algorithm works best when the probability of erasing
|
|
a symbol is somewhat larger than the probability that the symbol is incorrect.
|
|
We found empirically that good decoding performance is obtained when the
|
|
symbol erasure probability is about 1.3 times the symbol error probability.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
The FT algorithm tries successively to decode the received word using independen
|
|
t
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
educated guesses
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
to select symbols for erasure.
|
|
For each iteration a stochastic erasure vector is generated based on the
|
|
symbol erasure probabilities.
|
|
The erasure vector is sent to the BM decoder along with the full set of
|
|
63 hard-decision symbol values.
|
|
When the BM decoder finds a candidate codeword it is assigned a quality
|
|
metric
|
|
\begin_inset Formula $d_{s}$
|
|
\end_inset
|
|
|
|
, 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}
|
|
\end{equation}
|
|
|
|
\end_inset
|
|
|
|
Here
|
|
\begin_inset Formula $\alpha_{j}=0$
|
|
\end_inset
|
|
|
|
if received symbol
|
|
\begin_inset Formula $j$
|
|
\end_inset
|
|
|
|
is the same as the corresponding symbol in the codeword,
|
|
\begin_inset Formula $\alpha_{j}=1$
|
|
\end_inset
|
|
|
|
if the received symbol and codeword symbol are different, and
|
|
\begin_inset Formula $p_{1,j}$
|
|
\end_inset
|
|
|
|
is the fractional power associated with received symbol
|
|
\begin_inset Formula $j$
|
|
\end_inset
|
|
|
|
.
|
|
Think of the soft distance as made up of two terms: the first is the Hamming
|
|
distance between the received word and the codeword, and the second ensures
|
|
that if two candidate codewords have the same Hamming distance from the
|
|
received word, a smaller soft distance will be assigned to the one where
|
|
differences occur in symbols of lower estimated reliability.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
In practice we find that
|
|
\begin_inset Formula $d_{s}$
|
|
\end_inset
|
|
|
|
can reliably indentify the correct codeword if the signal-to-noise ratio
|
|
for individual symbols is greater than about 4 in power units, or
|
|
\begin_inset Formula $E_{s}/N_{0}\apprge6$
|
|
\end_inset
|
|
|
|
dB.
|
|
We also find that weaker signals frequently can be decoded by using soft-symbol
|
|
information beyond that contained in
|
|
\begin_inset Formula $p_{1}$
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset Formula $p_{2}$
|
|
\end_inset
|
|
|
|
.
|
|
To this end we define an additional metric
|
|
\begin_inset Formula $u$
|
|
\end_inset
|
|
|
|
, the average signal-plus-noise power in all symbols, according to a candidate
|
|
codeword's symbol values:
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Formula
|
|
\[
|
|
u=\frac{1}{n}\sum_{j=1}^{n}S(c_{j},\, j).
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
Here the
|
|
\begin_inset Formula $c_{j}$
|
|
\end_inset
|
|
|
|
's are the symbol values for the candidate codeword being tested.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
The correct JT65 codeword produces a value for
|
|
\begin_inset Formula $u$
|
|
\end_inset
|
|
|
|
equal to average of
|
|
\begin_inset Formula $n=63$
|
|
\end_inset
|
|
|
|
bins containing both signal and noise power.
|
|
Incorrect codewords have at most
|
|
\begin_inset Formula $k-1=11$
|
|
\end_inset
|
|
|
|
such bins and at least
|
|
\begin_inset Formula $n-k+1=52$
|
|
\end_inset
|
|
|
|
bins containing noise only.
|
|
Thus, if the spectral array
|
|
\begin_inset Formula $S(i,\, j)$
|
|
\end_inset
|
|
|
|
has been normalized so that its median value (essentially the average noise
|
|
level) is unity, the correct codeword is expected to yield the metric value
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Formula
|
|
\[
|
|
u=(1\pm n^{-\frac{1}{2}})(1+y)\approx(1.0\pm0.13)(1+y),
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
where
|
|
\begin_inset Formula $y$
|
|
\end_inset
|
|
|
|
is the signal-to-noise ratio (in linear power units) and the quoted one-standar
|
|
d-deviation uncertainty range assumes Gaussian statistics.
|
|
Incorrect codewords will yield metric values no larger than
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Formula
|
|
\[
|
|
u=\frac{n-k+1\pm\sqrt{n-k+1}}{n}+\frac{k-1\pm\sqrt{k-1}}{n}(1+y).
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
For JT65 this expression evaluates to
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Formula
|
|
\[
|
|
u\approx1\pm0.11+(0.17\pm0.05)\, y.
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
As a specific example, consider signal strength
|
|
\begin_inset Formula $y=4$
|
|
\end_inset
|
|
|
|
, corresponding to
|
|
\begin_inset Formula $E_{s}/N_{0}=6$
|
|
\end_inset
|
|
|
|
dB.
|
|
For JT65, the corresponding SNR in 2500 Hz bandwidth is
|
|
\begin_inset Formula $-23.7$
|
|
\end_inset
|
|
|
|
dB.
|
|
The correct codeword is then expected to yield
|
|
\begin_inset Formula $u\approx5.0\pm$
|
|
\end_inset
|
|
|
|
0.6, while incorrect codewords will give
|
|
\begin_inset Formula $u\approx1.7\pm0.3$
|
|
\end_inset
|
|
|
|
or less.
|
|
We find that a threshold set at
|
|
\begin_inset Formula $u_{0}=4.4$
|
|
\end_inset
|
|
|
|
(about 8 standard deviations above the expected maximum for incorrect codewords
|
|
) reliably serves to distinguish correct codewords from all other candidates,
|
|
while ensuring a very small probability of false decodes.
|
|
\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.
|
|
FT accepts a codeword unconditionally if
|
|
\begin_inset Formula $u>u_{0}$
|
|
\end_inset
|
|
|
|
.
|
|
A timeout is used to limit the algorithm's execution time if no acceptable
|
|
codeword is found in a reasonable number of trials,
|
|
\begin_inset Formula $T$
|
|
\end_inset
|
|
|
|
.
|
|
Today's personal computers are fast enough that
|
|
\begin_inset Formula $T$
|
|
\end_inset
|
|
|
|
can be set as large as
|
|
\begin_inset Formula $10^{5},$
|
|
\end_inset
|
|
|
|
or even higher.
|
|
\end_layout
|
|
|
|
\begin_layout Paragraph
|
|
Algorithm pseudo-code:
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
For each received symbol, define the erasure probability as 1.3 times the
|
|
|
|
\emph on
|
|
a priori
|
|
\emph default
|
|
symbol-error probability determined from soft-symbol information
|
|
\begin_inset Formula $\{p_{1}\textrm{-rank},\, p_{2}/p_{1}\}$
|
|
\end_inset
|
|
|
|
.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
Make independent stochastic decisions about whether to erase each symbol
|
|
by using the symbol's erasure probability, allowing a maximum of 51 erasures.
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
Attempt errors-and-erasures decoding by using the BM algorithm and the set
|
|
of erasures determined in step 2.
|
|
If the BM decoder produces a candidate codeword, go to step 5.
|
|
\begin_inset Foot
|
|
status open
|
|
|
|
\begin_layout Plain Layout
|
|
Our implementation of the FT-algorithm is based on the excellent open-source
|
|
BM decoder written by Phil Karn, KA9Q.
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
If BM decoding was not successful, go to step 2.
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
Calculate the hard-decision Hamming distance between the candidate codeword
|
|
and the received symbols, the corresponding soft distance
|
|
\begin_inset Formula $d_{s}$
|
|
\end_inset
|
|
|
|
, and the quality metric
|
|
\begin_inset Formula $u$
|
|
\end_inset
|
|
|
|
.
|
|
If
|
|
\begin_inset Formula $u$
|
|
\end_inset
|
|
|
|
is the largest one encountered so far, set
|
|
\begin_inset Formula $u_{max}=u$
|
|
\end_inset
|
|
|
|
.
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
If
|
|
\begin_inset Formula $u_{max}>u_{0}$
|
|
\end_inset
|
|
|
|
, go to step 8.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
If the number of trials is less than the timeout limit
|
|
\begin_inset Formula $T,$
|
|
\end_inset
|
|
|
|
go to 2.
|
|
Otherwise, declare decoding failure and exit.
|
|
\end_layout
|
|
|
|
\begin_layout Enumerate
|
|
An acceptable codeword with
|
|
\begin_inset Formula $u_{max}>u_{0}$
|
|
\end_inset
|
|
|
|
has been found.
|
|
Declare a successful decode and return this codeword.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
The inspiration for the FT decoding algorithm came from a number of sources,
|
|
particularly references
|
|
\begin_inset CommandInset citation
|
|
LatexCommand cite
|
|
key "key-2"
|
|
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset CommandInset citation
|
|
LatexCommand cite
|
|
key "key-3"
|
|
|
|
\end_inset
|
|
|
|
and the textbook by Lin and Costello
|
|
\begin_inset CommandInset citation
|
|
LatexCommand cite
|
|
key "key-1"
|
|
|
|
\end_inset
|
|
|
|
.
|
|
After developing this algorithm, we became aware that our approach is conceptua
|
|
lly similar to a
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
stochastic erasures-only list decoding algorithm
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
, described in reference
|
|
\begin_inset CommandInset citation
|
|
LatexCommand cite
|
|
key "key-4"
|
|
|
|
\end_inset
|
|
|
|
.
|
|
The algorithm in
|
|
\begin_inset CommandInset citation
|
|
LatexCommand cite
|
|
key "key-4"
|
|
|
|
\end_inset
|
|
|
|
is applied to higher-rate Reed-Solomon codes on a binary-input channel
|
|
over which BPSK-modulated symbols are transmitted.
|
|
Our 64-ary input channel with 64-FSK modulation required us to develop
|
|
our own unique methods for assigning erasure probabilities and for defining
|
|
an acceptance criteria to select the best codeword from the list of candidates.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "sec:Hinted-Decoding"
|
|
|
|
\end_inset
|
|
|
|
Hinted Decoding
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
To be written...
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "sec:Implementation-in-WSJT-X"
|
|
|
|
\end_inset
|
|
|
|
Implementation in WSJT-X
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
To be written...
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "sec:Theory,-Simulation,-and"
|
|
|
|
\end_inset
|
|
|
|
Decoder Performance Evaluation
|
|
\end_layout
|
|
|
|
\begin_layout Subsection
|
|
Simulated results on the AWGN channel
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Comparisons of decoding performance are usually presented in the professional
|
|
literature as plots of word error-rate versus
|
|
\begin_inset Formula $E_{b}/N_{0}$
|
|
\end_inset
|
|
|
|
, the ratio of the energy collected per information bit to the one-sided
|
|
noise power spectral density,
|
|
\begin_inset Formula $N_{0}$
|
|
\end_inset
|
|
|
|
.
|
|
In amateur radio circles performance is usually plotted as the probability
|
|
of successfully decoding a received word vs signal-to-noise ratio in a
|
|
2.5 kHz reference bandwidth,
|
|
\begin_inset Formula $\mathrm{SNR}{}_{2.5\,\mathrm{kHz}}$
|
|
\end_inset
|
|
|
|
.
|
|
The relationship between
|
|
\begin_inset Formula $E_{b}/N_{o}$
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset Formula $\mathrm{SNR}{}_{2.5\,\mathrm{kHz}}$
|
|
\end_inset
|
|
|
|
is described in Appendix
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "sec:Appendix:SNR"
|
|
|
|
\end_inset
|
|
|
|
.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Results of simulations using the BM, FT, and KV decoding algorithms on the
|
|
JT65 (63,12) code are presented in terms of word error-rate vs
|
|
\begin_inset Formula $E_{b}/N_{o}$
|
|
\end_inset
|
|
|
|
in Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:bodide"
|
|
|
|
\end_inset
|
|
|
|
.
|
|
For these tests we generated at least 1000 signals at each signal-to-noise
|
|
ratio, assuming the additive white gaussian noise (AWGN) channel, and processed
|
|
the data using each algorithm.
|
|
For word error-rates less than 0.1 it was necessary to process 10,000 or
|
|
even 100,000 simulated signals in order to capture enough errors to make
|
|
the estimates of word-error-rate statistically meaningful.
|
|
As a test of the fidelity of our numerical simulations, Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:bodide"
|
|
|
|
\end_inset
|
|
|
|
also shows theoretical results (filled squares) for comparison with the
|
|
BM results.
|
|
The simulated BM results agree with theory to within about 0.1 dB.
|
|
This difference between simulated BM results and theory is caused by small
|
|
errors in the estimates of time- and frequency-offset of the received signal
|
|
in the simulated results.
|
|
Such
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
sync losses
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
are not accounted for in the idealized theoretical results.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
As expected, the soft-decision algorithms, FT and KV, are about 2 dB better
|
|
than the hard-decision BM algorithm.
|
|
In addition, FT has a slight edge (about 0.2 dB) over KV.
|
|
On the other hand, the execution time for FT with
|
|
\begin_inset Formula $T=10^{5}$
|
|
\end_inset
|
|
|
|
is longer than the execution time for the KV algorithm.
|
|
Nevertheless, the execution time required for the FT algorithm with
|
|
\begin_inset Formula $T=10^{5}$
|
|
\end_inset
|
|
|
|
is small enough to be practical on most computers.
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Float figure
|
|
wide false
|
|
sideways false
|
|
status open
|
|
|
|
\begin_layout Plain Layout
|
|
\align center
|
|
\begin_inset Graphics
|
|
filename fig_bodide.pdf
|
|
|
|
\end_inset
|
|
|
|
|
|
\begin_inset Caption Standard
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "fig:bodide"
|
|
|
|
\end_inset
|
|
|
|
Word error rates as a function of
|
|
\begin_inset Formula $E_{b}/N_{0},$
|
|
\end_inset
|
|
|
|
the signal-to-noise ratio per bit.
|
|
The single curve marked with filled squares shows a theoretical prediction
|
|
for the BM decoder.
|
|
Open squares illustrate simulation results for an AWGN channel with the
|
|
BM, FT (
|
|
\begin_inset Formula $T=10^{5}$
|
|
\end_inset
|
|
|
|
) and KV (
|
|
\begin_inset Formula $\lambda=15$
|
|
\end_inset
|
|
|
|
) decoders used in program
|
|
\emph on
|
|
WSJT-X
|
|
\emph default
|
|
.
|
|
The KV results are for decoding complexity coefficient
|
|
\begin_inset Formula $\lambda=15$
|
|
\end_inset
|
|
|
|
, the most aggressive setting that has historically been used in earlier
|
|
versions of the WSJT programs.
|
|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Because of the importance of error-free transmission in commercial applications,
|
|
plots like that in Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:bodide"
|
|
|
|
\end_inset
|
|
|
|
often extend downward to much smaller error rates, say
|
|
\begin_inset Formula $10^{-6}$
|
|
\end_inset
|
|
|
|
or less.
|
|
The circumstances for minimal amateur-radio QSOs are very different, however.
|
|
Error rates of order 0.1 or higher may be acceptable.
|
|
In this case the essential information is better presented in a plot showing
|
|
the percentage of transmissions copied correctly as a function of signal-to-noi
|
|
se ratio.
|
|
Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:WER2"
|
|
|
|
\end_inset
|
|
|
|
shows the FT results for
|
|
\begin_inset Formula $T=10^{5}$
|
|
\end_inset
|
|
|
|
and the KV results that were shown in Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:bodide"
|
|
|
|
\end_inset
|
|
|
|
in this format along with additional FT results for
|
|
\begin_inset Formula $T=10^{4},10^{3},10^{2}$
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset Formula $10^{1}$
|
|
\end_inset
|
|
|
|
.
|
|
The KV results are plotted with open triangles.
|
|
It is apparent that the FT decoder produces more decodes than KV when
|
|
\begin_inset Formula $T=10^{4}$
|
|
\end_inset
|
|
|
|
or larger.
|
|
It also provides a very significant gain over the hard-decision BM decoder
|
|
even when limited to at most 10 trials.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Float figure
|
|
wide false
|
|
sideways false
|
|
status open
|
|
|
|
\begin_layout Plain Layout
|
|
\align center
|
|
\begin_inset Graphics
|
|
filename fig_wer2.pdf
|
|
lyxscale 120
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset Caption Standard
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "fig:WER2"
|
|
|
|
\end_inset
|
|
|
|
Percent of JT65 messages copied as a function of SNR in 2.5 kHz bandwidth.
|
|
Solid lines with filled round circles are results from the FT decoder with
|
|
|
|
\begin_inset Formula $T=10^{5},10^{4},10^{3},10^{2}$
|
|
\end_inset
|
|
|
|
and
|
|
\begin_inset Formula $10$
|
|
\end_inset
|
|
|
|
, respectively, from left to right.
|
|
The dashed line with open triangles is the KV decoder with complexity coefficie
|
|
nt
|
|
\begin_inset Formula $\lambda=15$
|
|
\end_inset
|
|
|
|
.
|
|
Results from the BM algorithm are also shown with filled triangles.
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
The timeout parameter
|
|
\begin_inset Formula $T$
|
|
\end_inset
|
|
|
|
employed in the FT algorithm is the maximum number of symbol-erasure trials
|
|
allowed for a particular attempt at decoding a received word.
|
|
Most successful decodes take only a small fraction of the maximum allowed
|
|
number of trials.
|
|
Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:N_vs_X"
|
|
|
|
\end_inset
|
|
|
|
shows the number of stochastic erasure trials required to find the correct
|
|
codeword versus the number of hard-decision errors in the received word
|
|
for a run with 1000 simulated transmissions at
|
|
\begin_inset Formula $\mathrm{SNR}=-24$
|
|
\end_inset
|
|
|
|
dB, just slightly above the decoding threshold.
|
|
The timeout parameter was
|
|
\begin_inset Formula $T=10^{5}$
|
|
\end_inset
|
|
|
|
for this run.
|
|
No points are shown for
|
|
\begin_inset Formula $X\le25$
|
|
\end_inset
|
|
|
|
because all such words were successfully decoded by the BM algorithm.
|
|
Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:N_vs_X"
|
|
|
|
\end_inset
|
|
|
|
shows that the FT algorithm decoded received words with as many as
|
|
\begin_inset Formula $X=43$
|
|
\end_inset
|
|
|
|
symbol errors.
|
|
The results also show that, on average, the number of trials increases
|
|
with the number of errors in the received word.
|
|
The variability of the decoding time also increases dramatically with the
|
|
number of errors in the received word.
|
|
These results also provide insight into the mean and variance of the execution
|
|
time for the FT algorithm, as execution time will be roughly proportional
|
|
to the number of required trials.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Float figure
|
|
wide false
|
|
sideways false
|
|
status open
|
|
|
|
\begin_layout Plain Layout
|
|
\align center
|
|
\begin_inset Graphics
|
|
filename fig_ntrials_vs_nhard.pdf
|
|
lyxscale 120
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset Caption Standard
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "fig:N_vs_X"
|
|
|
|
\end_inset
|
|
|
|
Number of trials needed to decode a received word versus Hamming distance
|
|
between the received word and the decoded codeword, for 1000 simulated
|
|
frames on an AWGN channel with no fading.
|
|
The SNR in 2500 Hz bandwidth is -24 dB (
|
|
\begin_inset Formula $E_{b}/N_{o}=5.1$
|
|
\end_inset
|
|
|
|
dB).
|
|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Subsection
|
|
Simulated results for hinted decoding and Rayleigh fading
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
Figure
|
|
\begin_inset CommandInset ref
|
|
LatexCommand ref
|
|
reference "fig:Psuccess"
|
|
|
|
\end_inset
|
|
|
|
presents the results of simulations for signal-to-noise ratios ranging
|
|
from
|
|
\begin_inset Formula $-18$
|
|
\end_inset
|
|
|
|
to
|
|
\begin_inset Formula $-30$
|
|
\end_inset
|
|
|
|
dB, again using 1000 simulated signals for each plotted point.
|
|
We include three curves for each decoding algorithm: one for the AWGN channel
|
|
and no fading, and two more for simulated Doppler spreads of 0.2 and 1.0
|
|
Hz.
|
|
For reference, we note that the JT65 symbol rate is about 2.69 Hz.
|
|
The simulated Doppler spreads are comparable to those encountered on HF
|
|
ionospheric paths and for EME at VHF and lower UHF bands.
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Float figure
|
|
wide false
|
|
sideways false
|
|
status open
|
|
|
|
\begin_layout Plain Layout
|
|
\align center
|
|
\begin_inset Graphics
|
|
filename fig_psuccess.pdf
|
|
lyxscale 90
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset Caption Standard
|
|
|
|
\begin_layout Plain Layout
|
|
\begin_inset CommandInset label
|
|
LatexCommand label
|
|
name "fig:Psuccess"
|
|
|
|
\end_inset
|
|
|
|
Percentage of JT65 messages successfully decoded as a function of SNR in
|
|
2500 Hz bandwidth.
|
|
Results are shown for the hard-decision Berlekamp-Massey (BM) and soft-decision
|
|
Franke-Taylor (FT) decoding algorithms.
|
|
Curves labeled DS correspond to the hinted-decode (
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
Deep Search
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
) matched-filter algorithm.
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\begin_layout Section
|
|
Summary
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
...
|
|
Still to come ...
|
|
\end_layout
|
|
|
|
\begin_layout Bibliography
|
|
\begin_inset CommandInset bibitem
|
|
LatexCommand bibitem
|
|
label "1"
|
|
key "key-1"
|
|
|
|
\end_inset
|
|
|
|
Error Control Coding, 2nd edition, Shu Lin and Daniel J.
|
|
Costello, Pearson-Prentice Hall, 2004.
|
|
\end_layout
|
|
|
|
\begin_layout Bibliography
|
|
\begin_inset CommandInset bibitem
|
|
LatexCommand bibitem
|
|
label "2"
|
|
key "key-2"
|
|
|
|
\end_inset
|
|
|
|
"Stochastic Chase Decoding of Reed-Solomon Codes", Camille Leroux, Saied
|
|
Hemati, Shie Mannor, Warren J.
|
|
Gross, IEEE Communications Letters, Vol.
|
|
14, No.
|
|
9, September 2010.
|
|
\end_layout
|
|
|
|
\begin_layout Bibliography
|
|
\begin_inset CommandInset bibitem
|
|
LatexCommand bibitem
|
|
label "3"
|
|
key "key-3"
|
|
|
|
\end_inset
|
|
|
|
"Soft-Decision Decoding of Reed-Solomon Codes Using Successive Error-and-Erasure
|
|
Decoding," Soo-Woong Lee and B.
|
|
V.
|
|
K.
|
|
Vijaya Kumar, IEEE
|
|
\begin_inset Quotes eld
|
|
\end_inset
|
|
|
|
GLOBECOM
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
2008 proceedings.
|
|
\end_layout
|
|
|
|
\begin_layout Bibliography
|
|
\begin_inset CommandInset bibitem
|
|
LatexCommand bibitem
|
|
label "4"
|
|
key "key-4"
|
|
|
|
\end_inset
|
|
|
|
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
Stochastic Erasure-Only List Decoding Algorithms for Reed-Solomon Codes,
|
|
\begin_inset Quotes erd
|
|
\end_inset
|
|
|
|
Chang-Ming Lee and Yu T.
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Su, IEEE Signal Processing Letters, Vol.
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16, No.
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8, August 2009.
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\end_layout
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\begin_layout Bibliography
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\begin_inset CommandInset bibitem
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LatexCommand bibitem
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label "5"
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key "key-5"
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\end_inset
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“Algebraic soft-decision decoding of Reed-Solomon codes,” R.
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Köetter and A.
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Vardy, IEEE Trans.
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Inform.
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Theory, Vol.
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|
49, Nov.
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|
2003.
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\end_layout
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\begin_layout Bibliography
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\begin_inset CommandInset bibitem
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LatexCommand bibitem
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label "6"
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key "key-6"
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\end_inset
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Berlekamp-Massey decoder written by Phil Karn, http://www.ka9q.net/code/fec/
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\end_layout
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|
\begin_layout Section
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\start_of_appendix
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Appendix:SNR"
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\end_inset
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Appendix: Signal to Noise Ratios
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\end_layout
|
|
|
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\begin_layout Standard
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|
The signal to noise ratio in a bandwidth,
|
|
\begin_inset Formula $B$
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\end_inset
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|
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|
, that is at least as large as the bandwidth occupied by the signal is:
|
|
\begin_inset Formula
|
|
\begin{equation}
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\mathrm{SNR}_{B}=\frac{P_{s}}{N_{o}B}\label{eq:SNR}
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\end{equation}
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|
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|
\end_inset
|
|
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|
where
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|
\begin_inset Formula $P_{s}$
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\end_inset
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|
is the signal power (W),
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\begin_inset Formula $N_{o}$
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\end_inset
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is one-sided noise power spectral density (W/Hz), and
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\begin_inset Formula $B$
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\end_inset
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|
is the bandwidth in Hz.
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|
In amateur radio applications, digital modes are often compared based on
|
|
the SNR defined in a 2.5 kHz reference bandwidth,
|
|
\begin_inset Formula $\mathrm{SNR}_{2.5\,\mathrm{kHz}}$
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|
\end_inset
|
|
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|
.
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|
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\end_layout
|
|
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|
\begin_layout Standard
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|
In the professional literature, decoder performance is characterized in
|
|
terms of
|
|
\begin_inset Formula $E_{b}/N_{o}$
|
|
\end_inset
|
|
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|
, the ratio of the energy collected per information bit,
|
|
\begin_inset Formula $E_{b}$
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|
\end_inset
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|
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|
, to the one-sided noise power spectral density,
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|
\begin_inset Formula $N_{o}$
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|
\end_inset
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|
.
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Denote the duration of a channel symbol by
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|
\begin_inset Formula $\tau_{s}$
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|
\end_inset
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|
(for JT65,
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|
\begin_inset Formula $\tau_{s}=0.3715\,\mathrm{s}$
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|
\end_inset
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|
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|
).
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|
Signal power is related to the energy per symbol by
|
|
\begin_inset Formula
|
|
\begin{equation}
|
|
P_{s}=E_{s}/\tau_{s}.\label{eq:signal_power}
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|
\end{equation}
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|
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|
\end_inset
|
|
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|
The total energy in a received JT65 message consisting of
|
|
\begin_inset Formula $n=63$
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|
\end_inset
|
|
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|
channel symbols is
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|
\begin_inset Formula $63E_{s}$
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|
\end_inset
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|
.
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|
The energy collected for each of the 72 bits of information conveyed by
|
|
the message is then
|
|
\begin_inset Formula
|
|
\begin{equation}
|
|
E_{b}=\frac{63E_{s}}{72}=0.875E_{s.}\label{eq:Eb_Es}
|
|
\end{equation}
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|
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|
\end_inset
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|
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|
Using equations (
|
|
\begin_inset CommandInset ref
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|
LatexCommand ref
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reference "eq:SNR"
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|
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|
\end_inset
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|
|
)-(
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|
\begin_inset CommandInset ref
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|
LatexCommand ref
|
|
reference "eq:Eb_Es"
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|
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|
\end_inset
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|
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|
),
|
|
\begin_inset Formula $\mathrm{SNR}_{2.5\,\mathrm{kHz}}$
|
|
\end_inset
|
|
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|
can be written in terms of
|
|
\begin_inset Formula $E_{b}/N_{o}$
|
|
\end_inset
|
|
|
|
:
|
|
\begin_inset Formula
|
|
\[
|
|
\mathrm{SNR}_{2.5\,\mathrm{kHz}}=1.23\times10^{-3}\frac{E_{b}}{N_{o}}.
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
If all quantities are expressed in dB, then:
|
|
\end_layout
|
|
|
|
\begin_layout Standard
|
|
\begin_inset Formula
|
|
\[
|
|
SNR_{2.5\,\mathrm{kHz}}=(E_{b}/N_{o})_{\mathrm{dB}}-29.1\,\mathrm{dB}.
|
|
\]
|
|
|
|
\end_inset
|
|
|
|
|
|
\end_layout
|
|
|
|
\end_body
|
|
\end_document
|