the Beauty of error-
by Daniel A. Spielman
eRRoR-CoRReCtinG CoDeS aRe
means by which we compensate for
interference in communication, and
are essential for the accurate transmission and storage of digital data.
All communication mechanisms and
storage devices are subject to interference, typically called “noise,” which
corrupts communicated messages and
stored data. Thus, for a communication system to faithfully transmit data,
it must build redundancy into its transmissions in such a way that even if a
transmission is partially corrupted, the
intended message may be reconstructed. Error-correcting codes provide the
mapping from messages to redundant
For example, a message is usually a
string of zeros and ones. A redundant
encoding of a message may be obtained by appending a few parity bits
to the original message, to form a
codeword. The rate of a code is the ratio of
the length of a message to the length
of a codeword, and equals the reciprocal of the redundancy. A communication medium, called a channel, might
transmit bits, and noise could flip bits
from zero to one or one to zero. For example, the Binary Symmetric Channel
with crossover probability p transmits
bits, and flips each bit with probability
p, independently. An error-correcting
code is designed with an abstract model of the target communication channel in mind.
Given a model of a channel, one
should design a code that maximizes the rate while minimizing some
tradeoff of error-probability, delay, and
the computational complexity of encoding and decoding. While the goal
of achieving low probability of error
in a communication system is fundamentally probabilistic, major advances
in the field have been made through a
worst-case, deterministic, approach.
The paper here by Guruswami and
Rudra surveys developments in the
worst-case approach to the coding
problem, and explains their own recent contributions. They build on the
classical Reed-Solomon codes.
Reed-Solomon codes employ a signaling alphabet containing more elements than just zero and one: each
symbol is an element of a finite field,
such as the integers modulo a prime. In
a Reed-Solomon code of rate R, classic
decoding algorithms can efficiently reconstruct a message so long as at most
a ( 1−R)/2 fraction of the symbols in the
transmitted codeword are corrupted.
This is exactly the fraction of errors up
to which the problem is guaranteed to
have a unique solution: there exist rare
patterns containing just one more error for which two codewords are equally close to the corrupted transmission.
A major advance in the decoding
of Reed-Solomon codes was Sudan’s
algorithm for list decoding Reed-Solomon codes. A list-decoding decoder
returns the list of all codewords within
some distance of a corrupted transmission. While the closest codeword is
usually unique, the algorithmic task is
simplified by the option of returning a
list. Guruswami and Sudan’s1 improvement of Sudan’s list decoder efficiently
returns the list of all codewords that
differ from a corrupted transmission
in at most a 1 − R fraction of symbols,
and the list is guaranteed to be short.
This was a big improvement over
previous decoding algorithms, but
made little difference at the desirable
high rates (near 1), where 1 − R is approximately the same as ( 1−R)/2. Guruswami and Rudra’s advance exploits
an idea of Parvaresh and Vardy3 for
bundling Reed-Solomon alphabet symbols together. This makes the signaling alphabet slightly larger, but greatly
increases the fraction of errors under
which efficient list decoding is possible. They obtain codes of rate R from
which one can efficiently produce the
list of all codewords that differ from a
corrupted transmission in a fraction of
symbols approaching 1 − R. For high-
rate codes, this is almost twice as many
errors as previous schemes could correct. Moreover, we know that one cannot hope to do better.
While a tremendous theoretical advance, more work is required before
these codes can be used in practical
communication systems. The decoding algorithms run in polynomial time,
but need to be faster before they can
be applied in practice. They also need
to be extended to incorporate information from lower levels of the communication system. Few communication
media naturally transmit finite field elements, or even zeros and ones. These
symbols are usually converted into analog waveforms. Receivers of partially
corrupted waveforms can do more than
just report which valid waveform is
closest: they can return the likelihood
of each valid waveform. A soft-decision
decoder incorporates this information
into the decoding process.
Koetter and Vardy2 figured out how
to incorporate such information in the
Guruswami-Sudan algorithm, and an
analogous discovery may be required
before we communicate using Gurus-wami-Rudra codes.
1. guruswami, V. and sudan, m. improved decoding of
reed-solomon and algebraic-geometric codes. IEEE
Trans. on Info. Theory, 45 (1999), 1757–1767.
2. koetter, r. and Vardy, a. algebraic soft-decision
decoding of reed-solomon codes. IEEE Trans. on
Info. Theory 49, 112 (2003), 2809–2825.
3. Parvaresh, f. and Vardy, a. correcting errors beyond
the guruswami-sudan radius in polynomial time.
in Proceedings of the 46th IEEE Symposium on
Foundations of Computer Science (2005), 285–294.
4. sudan, m. decoding the reed-solomon codes beyond
the error-correction bound. Journal of Complexity 13,
1 (1997), 180–193.
Daniel A. Spielman ( email@example.com) is a
professor of applied mathematics and computer science
at yale university, new haven, ct.