reflection group codes and their decodingmath.hawaii.edu/~jb/gpcodes-hand.pdf · marc fossorier,...
TRANSCRIPT
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Reflection Group Codes and their Decoding
Marc Fossorier, J.B. Nation and Wes Peterson
University of Hawai‘i at Manoa
Ashikaga, August 2010
Fossorier, Nation, Peterson Reflection Group Codes
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Background
History.zipBasic idea due to David Slepian in 1950s and 1960s:choose a group of orthogonal matrices and a point on asphere, and use the orbit of that point by that group as aset of signals for communication.Refinement by Mittelholzer and Lahtonen in 1996.Further refinement by F, N, P in 2004–2009.Extension to complex groups by Kim, Shepler, N in2008-2010.
For comparisonModulation via nPSK or QAM
First taskRecall a lot of group theory you never knew!
Fossorier, Nation, Peterson Reflection Group Codes
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Linear Algebra Review
Inner products and transposes
(x,y) = xT y(Mx,y) = (x,MT y) = xT MT y‖x‖ =
√(x,x) and d(a,b) = ‖a− b‖
Orthogonal matrices
M is orthogonal if MMT = I, i.e., MT = M−1
If M is orthogonal, then
(Mx,y) = (x,M−1y)
(Mx,My) = (x,y)
‖Mx− y‖ = ‖x−M−1y‖
Fossorier, Nation, Peterson Reflection Group Codes
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Groups of orthogonal matrices
All n × n orthogonal matrices form a group O(n).
I is orthogonal.The inverse of an orthogonal matrix is orthogonal.Products of orthogonal matrices are orthogonal.
Examples
Dihedral groups Hk2 generated by the matrices[
c −ss c
]and
[1 00 −1
]where c = cos 2π
k and s = sin 2πk
The group An−1 of all n × n permutation matricesThe group Bn of all n × n permutation matrices with entries±1
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Basic idea
Let G be a finite group of isometries on Rn.G acts on the sphere of radius 1.Choose an initial vector x0 ∈ V with ‖x0‖ = 1.Associate a group element g with each message m.m −→ x = g−1x0 −→ r = x + n −→ g′r ≈ x0 −→ m′
G acts faithfully and transitively on Gx0 = {gx0 : g ∈ G}(or if not, factor out the isotropy group).
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Points vs. bitstrings as signals
2n-PSK: With dihedral groups, use points on the circle totransmit bit-strings of length blog 2nc.With the group E6, use points on the sphere in R6 torepresent bit-strings of length 15.With the group B32, use points on the sphere in R32 torepresent bit-strings of length 148.More generally, we can identify the group elementsthemselves with the (not necessarily binary) message.
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Reflection groups
Reflection group
...a group G generated by a set of reflections on Rn.
ReflectionFor α a unit vector, Sα(x) = x− 2(x, α)α.
Reflection planes
Nα = {x : (x, α) = 0} is the set of vectors fixed by Sα.
Roots∆(G) = {α : Sα ∈ G}
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Example - A2
Notation on whiteboard
A = Sα
B = Sβ
G = {I,A,B,AB,BA,ABA}
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Regions
FundamentalsThe reflection planes divide the space into regions.Pick one region to be the fundamental region FR(G).Choose an initial vector x0 in the fundamental region.A root β is positive if (β,x0) > 0.The positive roots whose reflection planes form the walls ofthe fundamental region are the fundamental roots.FR(G) = {y : (α,y) > 0 for every fundamental root α}
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Reflection groups (cont.)
Generators and length
G is generated by the reflections Sα with α a fundamentalroot.The length `(g) is the minimum number k such that g is aproduct of k fundamental reflections.Roughly speaking, ‖gx0 − x0‖ increases with `(g): if`(Sαg) = `(g) + 1, then ‖Sαgx0 − x0‖ > ‖gx0 − x0‖.
Classification of irreducible reflection groups
An, Bn, Dn, E6, E7, E8, F4, Hn2 , I3, I4
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The action of G on roots
Technical definitionLet ∆G(g) denote the set of positive roots for which gα is anegative root.
TheoremIf α is a positive root and v is not on the reflecting plane Nα,then Nα is between v and gv if and only if α ∈ ∆G(g−1).
TheoremIf Sα is a fundamental reflection, then Sαα = −α and Sα
permutes all the other positive roots.
TheoremIf α is any root and u and v are on the same side of thereflection plane Nα, then v is closer than Sαv to u. If u and vare on opposite sides of Nα, then Sαv is closer than v to u.
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Action and length
TheoremTFAE.
`(g) = k .For any vector v that is not in any reflecting plane, thenumber of reflecting planes that separate v and gv is k .|∆G(g)| = k .
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Subgroups
Types of subgroups
parabolic - generated by fundamental reflectionsreflection - generated by reflectionsother
DefinitionIf H is a reflection subgroup, the fundamental region FR(H) isthe region containing x0 and bounded by the reflection planesof H.
TheoremIf H is a reflection subgroup of G, then FR(G) ⊆ FR(H).
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Coset leaders
Let H be a reflection subgroup of G.
LemmaEach left coset gH contains a unique element c of minimallength.Choose c as the coset leader.
TheoremIf x ∈ FR(H), then there is a unique coset leader c such thatcx ∈ FR(G) and we have an efficient algorithm to find it.
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Summary of group theory background
group G of orthogonal matricesreflection, reflection group, reflection subgroupinitial vector x0
fundamental regions FR(H) ⊇ FR(G)
coset leader
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The algorithm
SetupChoose a chain of reflection subgroups
{I} = G0 < G1 < · · · < Gk = G
The fundamental regions are nested in reverse order:
Rn = FR(I) ⊇ FR(G1) ⊇ · · · ⊇ FR(G)
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The algorithm
EncodeGiven a message m,take the corresponding group element g.Write it as a product of coset leaders g = ck . . . c1.Transmit x = g−1x0 = c−1
1 . . . c−1k x0.
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The algorithm
Noiser = x + n
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The algorithm
Decoder ∈ FR(G0)
Find the coset leader d1 with d1r ∈ FR(G1)
Find the coset leader d2 with d2d1r ∈ FR(G2)
etc.to obtain dk . . . d1r ∈ FR(G)
Decode g′ = dk . . . d1 −→ m′.
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Notation for the examples
Coset leadersThe coset leaders are arranged into graphs.
Matrices
τ12 =
0 1 01 0 00 0 1
σ1 =
−1 0 00 1 00 0 1
etc.
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Decoding A4
τ12
τ12
τ12
τ12
τ23
τ23
τ23
τ34
τ34
τ45
x0 = 〈−2,−1, 0, 1, 2〉
1
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Decoding A6 bent
τ17 τ12 τ27 τ26
τ12
τ67
τ23
τ12
τ36
τ67
τ35
τ23
τ12
τ56
τ67
τ34
τ23
τ12
τ45
τ56
τ67
1
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Decoding B4
σ1 σ4 τ14 σ2 τ12 τ24 σ3 τ34τ23
τ12
x0 = 〈0.707, 1.707, 2.707, 3.707〉
1
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Average number of comparisons to decode An
n an a′n log2(n + 1)!
4 7.7 7.1 6.98 24.2 19.5 18.4
16 81.6 57.4 48.332 292.9 182.2 122.7
wherean is the average number of comparisons needed todecode An using straight coset leader graphs (parabolicsubgroups)a′n is the average number of comparisons needed todecode An using bent coset leader graphs (a differentsubgroup sequence)log2(n + 1)! is the theoretical minimum average number ofcomparisons
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Average number of comparisons to decode Bn
n bn b′n b
′′n n + log2 n!
4 11.0 8.9 8.7 8.68 38.6 27.3 24.0 23.3
16 142.3 88.6 67.7 60.332 542.0 307.9 204.5 149.7
wherebn is the average number of comparisons to decode Bnusing parabolic subgroups
b′n is the average number of comparisons to decode Bn
using intermediate subgroups with straight CL graphs
b′′n is the average number of comparisons to decode Bn
using intermediate subgroups with bent CL graphsn + log2 n! is the theoretical minimum average number ofcomparisons
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Refinements
Error controlReceived vectors from neighboring regions decode togroup elements with only one coset leader different.A linear block code can be super-imposed.
EfficiencyChoosing the right subgroup sequence can doubledecoding efficiency.Complex permutation groups G(r , k ,n) work similarly withlarge minimum distance.
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Conclusions
Use of subgroup decoding may make group codingpractical.Group codes with nontrivial isotropy subgroups can bedecoded with this algorithm (though some care isrequired).Further testing and analysis is required.Other groups and other decoding methods should beconsidered.Thank you!
Fossorier, Nation, Peterson Reflection Group Codes