So far we have addressed the computation of various generating
mappings. In this and subsequent sections we will complete our
analysis by addressing the theoretical mechanism which supports
the
-construction.
Specifically, the
-construction establishes an important
relationship between the coset leaders of the codes
produced at successive iterations. This relationship
allows us to efficiently compute the coset leaders of many
-codes (and hence their generator
matrices) as well as to determine bounds on their code parameters.
We describe this relationship by first associating a companion
with each coset leader. Recall that a coset leader is
a designated vector of minimum weight in a given coset.
We show that one iteration of the lexicographic construction produces
a code whose coset leaders are either
a| u
or
|
(u)
(depending on
which has lower Hamming weight),
for each vector a of appropriate
size and coset leader u in the original code. Here
refers to the complement of a.
Proof: We introduce the following notation to simplify the proof:
g(a, l ) + h(a, l ) | = ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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= ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The second observation of this proof is that
g(a, l ) and
g(a', l') are generally not in the same coset of
.
More specifically, these two vectors are in the same coset precisely
when l and l' are in distinct cosets of
and
a'.
To see this we analyze the two possibilities for the sum
g(a, l )+ g(a', l').
In the first case, its leading bits are neither 0
nor 1
.
Alternatively, its leading bits are 0
but its trailing bits are the sum of distinct coset leaders
.
Both possibilities preclude
g(a, l )+ g(a', l') from being a codeword of
, proving
the observation.
Based on the above two observations,
g( . , . ) and
h( . , . ) represent
the same two-to-one correspondence between (vector, coset-leader)
pairs (a, l ) and the cosets of
.
In fact, for each
a
2n and coset leader
l of
, the coset of
g(a, l ) contains
only the vectors:
In addition, for any
,
g(a, l ) cannot have weight
greater than the weight of
a|(l + c)
, because l is
a coset leader, and
h(a, l ) cannot have weight greater than
|(l +
+ c)
. Therefore, all the vectors
in the
coset containing
g(a, l ) will have weight not less than
[3]
, so that either
g(a, l ) or
h(a, l ) must be a coset leader in
. Since
g(a, l ) and
h(a, l ) are correspondences,
is
the complete set of coset leaders of
.
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Table 3 demonstrates the use of the correspondence
of Theorem 3 to generate the
coset leaders of the (7, 3, 4) binary lexicode
from the
coset leaders of the (6, 2, 4) binary lexicode
. In essence,
the
correspondence indicates that the coset leaders of a
-code are
augmented permutations of the coset leaders of its predecessor in the
-construction. One such permutation produces the lexicodes, whereas
other permutations produce the various other codes described in this paper.
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