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15-853:Algorithms in the Real World Block Codes Linear Codes

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15-853:Algorithms in the Real World Block Codes Linear Codes Block Codes 15-853:Algorithms in the Real World message (m) Each message and codeword is of fixed size Error Correcting Codes II coder ∑∑∑ = codeword alphabet k n q ∑ –Reed-Solomon Codes codeword (c) =|m| = |c| = | | –Concatenated Codes C ⊆ Σn (codewords) noisy –Overview of some topics in coding channel ∆∆∆(x,y) = number of positions s.t. x ≠ y –Low Density Parity Check Codes codeword’ (c’) i i (aka Expander Codes) d = min{ ∆(x,y) : x,y ∈ C, x ≠ y} -Network Coding decoder s = max{ ∆(c,c’)} that the code can correct -Compressive Sensing Code described as: (n,k,d) –List Decoding message or error q 15-853 Page1 15-853 Page2 Linear Codes Generator and Parity Check Matrices Generator Matrix : If ∑ is a field, then ∑n is a vector space G ∈ ∑k Definition : C is a linear code if it is a linear subspace of A k x n matrix such that: C = { xG | x } ∑n of dimension k. Made from stacking the spanning vectors This means that there is a set of k independent vectors Parity Check Matrix : n n T vi ∈ ∑ (1 ≤ i ≤ k) that span the subspace. An (n – k) x n matrix H such that: C = {y ∈ ∑ | Hy = 0} i.e. every codeword can be written as: (Codewords are the null space of H.) c = a 1 v1 + a 2 v2 + … + ak vk where ai ∈ ∑ These always exist for linear codes The sum of two codewords is a codeword. Minimum distance = weight of least-weight codeword 15-853 Page3 15-853 Page4 1 k mesg Relationship of G and H n For linear codes, if G is in standard form [I k A] n T mesg then H = [ -A In-k] G = codeword Example of (7,4,3) Hamming code: n-k transpose recv’d word n-k 1 0 0 0 1 1 1 1 1 1 0 1 0 0 = 0 1 0 0 1 1 0 H syndrome G = H = 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1 0 1 1 0 0 1 0 0 0 1 0 1 1 if syndrome = 0, received word = codeword else have to use syndrome to get back codeword 15-853 Page5 15-853 Page6 Two Codes Hamming codes are binary (2 r-1–1, 2 r-1-r, 3) codes. Basically (n, n – log n, 3) Reed-Solomon Codes Hadamard codes are binary (2 r-1, r, 2 r-1). Basically (n, log n, n/2) The first has great rate, small distance. The second has poor rate, great distance. Can we get Ω(n) rate, Ω(n) distance? Yes. One way is to use a random linear code. Irving S. Reed and Gustave Solomon Let’s see some direct, intuitive ways. 15-853 Page7 15-853 Page8 2 Reed-Solomon Codes in the Real World 2 (204,188,17) 256 : ITU J.83(A) (128,122,7) 256 : ITU J.83(B) PDF-417 (255,223,33) 256 : Common in Practice QR code – Note that they are all byte based (i.e., symbols are from GF(2 8)). Decoding rate on 1.8GHz Pentium 4: – (255,251) = 89Mbps – (255,223) = 18Mbps Aztec code Dozens of companies sell hardware cores that DataMatrix code operate 10x faster (or more) All 2-dimensional Reed-Solomon bar codes – (204,188) = 320Mbps (Altera decoder) 15-853 Page9 15-853 Page10 images: wikipedia Applications of Reed-Solomon Codes Viewing Messages as Polynomials • Storage : CDs, DVDs, “hard drives”, A (n, k, n-k+1) code: • Wireless : Cell phones, wireless links Consider the polynomial of degree k-1 k-1 • Sateline and Space : TV, Mars rover, Voyager, p(x) = a k-1 x + L + a 1 x + a 0 • Digital Television : DVD, MPEG2 layover Message : (a k-1, …, a 1, a 0) Codeword : (p(1), p(2), …, p(n)) High Speed Modems • : ADSL, DSL, .. r To keep the p(i) fixed size, we use a i ∈ GF(Q ) To make the i distinct, n ≤ Q r Good at handling burst errors. Other codes are better for random errors. For simplicity, imagine that n = Q r. So we have a – e.g., Gallager codes, Turbo codes (n, k, n-k+1) log n code. Alphebet size increasing with the codeword length. A little awkward. (But can be fixed.) 15-853 Page11 15-853 Page12 3 Viewing Messages as Polynomials Polynomial-Based Code A (n, k, n-k+1) code: A (n, k, 2s +1) code: Consider the polynomial of degree k-1 k 2s k-1 p(x) = a k-1 x + L + a 1 x + a 0 Message : (a , …, a , a ) k-1 1 0 n Codeword : (p(1), p(2), …, p(n)) r Can detect 2s errors To keep the p(i) fixed size, we use ai ∈ GF(Q ) To make the i distinct, n ≤ Qr Can correct s errors Generally can correct α erasures and β errors if Unisolvence Theorem : Any subset of size k of α + 2 β ≤ 2s (p(1), p(2), …, p(n)) is enough to (uniQuely) reconstruct p(x) using polynomial interpolation, e.g., Lagrange interpolation formula. 15-853 Page13 15-853 Page14 Correcting Errors RS and “burst” errors Correcting s errors : Let’s compare to Hamming Codes (which are “optimal”). 1. Find k + s symbols that agree on a polynomial p(x). code bits check bits These must exist since originally k + 2s symbols RS (255, 253, 3) 256 2040 16 agreed and only s are in error 11 11 Hamming (2 -1, 2 -11-1, 3) 2 2047 11 2. There are no k + s symbols that agree on the wrong polynomial p’(x) They can both correct 1 error, but not 2 random errors. - Any subset of k symbols will define p’(x) – The Hamming code does this with fewer check bits - Since at most s out of the k+s symbols are in However, RS can fix 8 contiguous bit errors in one byte error, p’(x) = p(x) – Much better than lower bound for 8 arbitrary errors n n log1+ + + > 8log(n − )7 ≈ 88 check bits A brute-force approach. L 1 8 Better algos exist (maybe next lecture). 15-853 Page15 15-853 Page16 4 Concatenated Codes Concatenated Codes Take a RS code (n,k,n-k+1) Q = n code. Take a RS code (n,k,n-k+1) Q = n code. David Forney Can encode each alphabet symbol of k’ = log Q = log n Can encode each alphabet symbol using another code. bits using another code. E.g., use ((k’ + log k’), k’, 3) 2-Hamming code. Now we can correct one error per alphabet symbol with little rate loss. (Good for sparse periodic errors.) k’ k’-1 Or (2 , k’, 2 )2 Hadamard code. (Say k = n/2.) Then 2 2 get (n , (n/2) log n, n /4) 2 code. Much better than plain Hadamard code in rate, distance worse only by factor of 2. 15-853 Page17 15-853 Page18 Wikipedia Concatenated Codes Error Correcting Codes Outline Introduction Take a RS code (n,k,n-k+1) Q = n code. Can encode each alphabet symbol of k’ = log Q = log n Linear codes bits using another code. Reed Solomon Codes Expander Based Codes Or, since k’ is O(log n), could choose a code that – Expander Graphs reQuires exhaustive search but is good. – Low Density Parity Check (LDPC) codes δδδ δδδ Random linear codes give ((1+f( ))k’, k’, k’)) 2 codes. – Tornado Codes Composing with RS (with k = n/2), we get δδδ δδδ ( (1+f( ))n log n, (n/2) log n, (n/2)log n ) 2 Gives constant rate and constant distance ! And poly-time encoding and15-853 decoding. Page19 15-853 Page20 5 Why Expander Based Codes? (α, β ) Expander Graphs (non-bipartite) These are linear codes like RS & random linear codes RS/random linear codes give good rates but are slow: k ≤ αn ≥ βk Code Encoding Decoding G Random Linear O(n 2) O(n 3) RS O(n log n) O(n 2) Properties LDPC O(n 2) or better O(n) – Expansion: every small subset ( k ≤αn) has many Tornado O(n log 1/ ε) O(n log 1/ ε) (≥ βk) neighbors – Low degree – not technically part of the Assuming an (n, (1-p)n, (1-ε)pn+1) 2 tornado code definition, but typically assumed 15-853 Page21 15-853 Page22 (α, β ) Expander Graphs (bipartite) Expander Graphs Useful properties: k bits – Every set of vertices has many neighbors (k ≤ αn) at least βk bits – Every balanced cut has many edges crossing it – A random walk will Quickly converge to the Properties stationary distribution (rapid mixing) – Expansion: every small subset ( k ≤αn) on left has many ( ≥βk) neighbors on right – Expansion is related to the eigenvalues of the – Low degree – not technically part of the adjacency matrix definition, but typically assumed 15-853 Page23 15-853 Page24 6 Expander Graphs: Applications d-regular graphs Pseudo-randomness : implement randomized An undirected graph is d-regular if every vertex has algorithms with few random bits d neighbors. Cryptography : strong one-way functions from weak ones. A bipartite graph is d-regular if every vertex on the Hashing: efficient n-wise independent hash left has d neighbors on the right. functions Random walks: Quickly spreading probability as you We consider only d-regular constructions. walk through a graph Error Correcting Codes: several constructions Communication networks: fault tolerance, gossip- based protocols, peer-to-peer networks 15-853 Page25 15-853 Page26 Expander Graphs: Constructions Expander Graphs: Constructions Theorem: for every constant 0 < c < 1, can construct Important parameters: size (n) , degree (d) , expansion ( β) bipartite graphs with n nodes on left, Randomized constructions cn on right, – A random d-regular graph is an expander with a high d-regular, probability ααα ααα – Construct by choosing d random perfect matchings that are ( , 3d/4) expanders, for constants and – Time consuming and cannot be stored compactly d that are functions of c alone.
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