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- 1. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection . Nonlinear communications: achievable rates, encryption, estimation and decoding . N. Kalouptsidis Dept. of Informatics & Telecommunications, University of Athens Second Greek Signal Processing Jam Coworkers: B. Babadi, A. Katsiotis, N. Kolokotronis, G. Mileounis, I. Sason, V. Tarokh, K. XenoulisN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 1 / 46
- 2. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Outline Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 2 / 46
- 3. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Requirements: MIMO and Nonlinearities Time varying channel Reliability Data integrity and conﬁdentiality ComplexityN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 3 / 46
- 4. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Requirements: Approach: MIMO and Nonlinearities Sieve structures and ﬁnite memory Time varying channel Adaptive methods Reliability Capacity approaching codes Data integrity and Secrecy codes, encryption conﬁdentiality Complexity Simpliﬁcations (EM, relaxation, sparse models and sparsity aware schemes)N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 3 / 46
- 5. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Secrecy Codes Encryption v m Encoder Encryption key k symmetric key Public key cryptography Channel cryptography Decryption key Decryption ˆ m Decoder Eavesdropper y ˆ y Symmetric key cryptography: a common secret key is shared by encoder/decoder Public key cryptography: Each user has a public key and private key. Sender encrypts with the public key of receiver. The receiver decrypts with its own private keyN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 4 / 46
- 6. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Information theoretic secrecy Symmetric key cryptography: (2nR , 2nRk , n) randomized encoder: generates codewords v(m, k) ∼ P (v|m, k) for each message-key pair (m, k) ∈ [1 : 2nR ] × [1 : 2nRk ] Decoder: assigns a message m(y, k) to each received vector y and ˆ key k Decoding rule: joint input-output typicality Performance characteristics: Probability of error for the secrecy code n Pe = P [m(y, k) = m] ˆ n 1 Information leakage rate Rl = n I (M ; Y ) N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 5 / 46
- 7. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Information theoretic secrecy A rate R is achievable at key rate Rk if there is a sequence of secrecy codes with Pe → 0 and Rl → 0. n n Secrecy capacity for the DMC channel C(Rk ): supremum of achievable rates at key rate Rk . Theorem. . { } CRk = min Rk , max I(V ; Y ) . P (v) Secure communication is limited by the key rate until saturated by the channel capacity N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 6 / 46
- 8. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. The wiretap channel y ˆ m Decoder v Channel m Encoder P (y, y |v) ˆ ˜ y Eavesdropper ( ) n 1 Information leakage rate: Rl = n I M ; Y ˜ If the channel to the eavesdropper is a physically degraded version of the channel to the receiver P (y, y |v) = P (y|v)P (˜|y) ˜ y then the secrecy capacity is: ( ) Cs = max I(V ; Y ) − I(V ; Y ) ˜ P (v)N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 7 / 46
- 9. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Encryption u Channel v m Encoder Encoder Encryption key k symmetric key Public key cryptography cryptography Channel Decryption key ˆ m Decryption Channel Eavesdropper Decoder ˆ u Decoder y ˜ yN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 8 / 46
- 10. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Encryption Encoder: one way function u = E(m, k) Channel Encoder: (typically linear) adds redundancy to combat the channel noise. Channel Decoder: u = maxu P (u|y, k) ˆ Decryption Decoder: m = E −1 (ˆ, k) ˆ u Both must computationally tractable Eavesdropper (symmetric key): max P (k|˜), y max P (m|˜) y k m must be computationally hardN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 9 / 46
- 11. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Public key cryptography Encryption encoder: u = G(m, kpub ) Decryption decoder: m = G−1 (ˆ, ksec ) ˆ u Eavesdropper: max P (ksec |˜, kpub ) y ksec max P (m|˜, kpub ) y m N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 10 / 46
- 12. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. McEliece public key encryption scheme Key generation k, n, t ﬁxed common integers choose k × n matrix G which can correct t errors and for which an eﬃcient decoding algorithm is known (RS codes + BM decoding, LDPC + sum product) Draw k × k non-singular S. Draw n × n permutation P . ˆ Compute G = SGP ˆ Public key (G, t) Private key (S, G, P ) N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 11 / 46
- 13. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Encryption ˆ Use the key of the intended recipient k = (G, t) Represent the message m as a binary vector of length k Draw a random binary vector z of weight t ˆ Compute u = E(m, k) = mG + z Decryption Compute uP −1 = mSG + zP −1 . Note w(zP −1 ) = w(z) because P permutation. Use the decoding algorithm to determine mS Compute mSS −1 = mN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 12 / 46
- 14. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Fast decoding of regular LDPC codes (n, k) linear block code with parity check matrix H Codewords: n dimensional binary vectors: vH T = 0 Syndrome s = rH T , r received vector error e = r − v satisﬁes s = eH T Number of errors: = e 0 << n Minimum distance decoding: min e 0 subject to s = eH t min e 1 subject to s = eH t “Kalouptsidis, Kolokotronis. Fast decoding of regular LDPC codes using greedy approximation algorithms.” N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 13 / 46
- 15. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection The error determines an epsilon sparse representation of the syndrome vector in the dictionary generated by the columns of H. The proposed algorithm is motivated by Matching Pursuit but operates mostly over ﬁnite ﬁelds Basic idea: Select columns of H mostly correlated with residual: ⊕ v ˆ sv = hλi ∈ F2 m i=1 Performance guarantees for regular (γ, ρ) LDPC codes H is sparse Each column contains γ ones Each row contains ρ ones The minimum distance of the code satisﬁes dmin ≥ γ + 1 The code can correct ≤ |γ/2|N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 14 / 46
- 16. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection input: parity–check matrix H, received word r, maximum number ν of iterations initialization: Λ = ∅, i = 0 1. s = rH t mod 2 syndrome 2. While (s = 0) ∧ (i < ν) 3. λ ∈ arg max{ s, hω : ω ∈ Λ} / choose randomly 4. s = s ⊕ hλ 5. Λ = Λ ∪ {λ} 6. i=i+1 7. End output: residual s, error locations ΛN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 15 / 46
- 17. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection . Theorem. . Let C be a (γ, ρ)–regular LDPC (n, k) code. The proposed algorithm is capable of correcting all error patterns e satisfying γ ≤ 2 where . = e 1.N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 16 / 46
- 18. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detectionN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 17 / 46
- 19. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. NL AWGN Multi Input Multi Output nonlinear channel with Additive White Gaussian Noise y(t) =D[v](t) + ξ(t) ξ(t) : i.i.d ∼ N (0, Q) , Q>0 Channel Operator D: shift invariant, causal, BIBO stable with fading memory Then D can be approximated by a ﬁnite memory architectureN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 18 / 46
- 20. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. NL AWGN v1 (t) Shift Register 1 . . ··· . v2 (t) Shift Register 2 . yi (t) . . hi ··· . . . . . . . . . vT (t) Shift Register T ··· Canonical ﬁnite memory formN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 19 / 46
- 21. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Modeling options: nonparametric, semi–nonparametric, parametric Focus of this presentation: parametric forms Each function hi (·) is a polynomial in several variables More generally, hi (·) is approximated by a member of a sieve family Examples of linear sieves: Tensor products of Fourier series, splines, wavelets Consequence: Models linear in the parameters Examples of nonlinear sieves: Neural Networks, Radial BasisN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 20 / 46
- 22. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Polynomial AWGN Multi–index i = (i1 , . . . , i ) ir ∈ N and xi = xi1 xi2 · · · xik Then ∑ h(xi1 , xi2 , . . . , xik ) = hi xi i∈I Each output: ∑∑∑ L (j,k) yj (t) = hi vk1 (t − i1 )vk2 (t − i2 ) · · · vk (t − i ) =1 k∈K i∈I ∑T ∑q (j,k) Examples: Linear Systems yj (t) = k=1 i=0 hi vk (t − i) Quadratic ∑∑ T q (j,k) ∑ ∑ ∑ ∑ T T q q (j,k ,k2 ) yj (t) = hi uk (t−i)+ hi1 i21 vk1 (t−i1 )vk2 (t−i2 ) k=1 i=0 k1 =1 k2 =1 i1 =0 i2 =0 (j,k) Sparsity: Most of the coeﬃcients hi in each hj (·) are zero.N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 21 / 46
- 23. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Achievable rates for SISO polynomial channels SISO polynomial channel: yt = D[v]t + ξt ∑∑ L q ∑ q D[v]t = h0 + ... hj (i1 , . . . , ij )vt−i1 · · · vt−ij j=1 i1 =0 ij =0 ξt ∼ N (0, σ 2 ) Let vm denote the transmitted codeword and y the received vector. A maximum likelihood error occurs if P (y|vm ) ≥ P (y|vm ), for some m = m N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 22 / 46
- 24. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Since noise is Gaussian, this is equivalent to Dvm − Dvm + ξ 2 2 ≤ ξ 2 2 Chernoﬀ’s bound and Gallager’s upper bound imply for a speciﬁc (N, R) code C ∑ ( ) Dvm − Dvm 2 Pe (m|C) ≤ exp −ρ 2 ,0 ≤ ρ ≤ 1 8σ 2 m =m Using a random coding argument, the average error probability over an ensemble of codes C we obtain ( ) [ ρ ∑N ] N R− ρ2 N Dv (Q) Pe ≤ e 4σ E e 8σ2 i=1 Zi , 0 ≤ ρ ≤ 1N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 23 / 46
- 25. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Ft the minimal σ-algebra generated by V1 , V1 , . . . , Vt , Vt ˜ ˜ (F0 {∅, Ω}). (Zt , Ft ) martingale diﬀerence sequence: ∑t+q ∑ t+q Zt −E wj Ft + E 2 wj Ft−1 , wj [Dv]j − [D˜]j 2 v j=t j=t 1 ∑( [ ] ) n Output covariance: Dv (Q) E ([Dv]j )2 − (E[Dv]j )2 n j=1 Martingales enable the development of several concentration inequalities. For instance Bennett’s inequality: [ ( )] ( )N ρ ∑ ρd ρd γ2 e 8σ2 + e−γ2 8σ2 N E exp Zi ≤ 8σ 2 1 + γ2 i=1N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 24 / 46
- 26. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Suppose [ ] µ2 max |Zi | ≤ d, max E Zi2 |Fi−1 ≤ µ2 , γ2 vi ,˜i ,j≤i v vj ,˜j ,j≤i−1 v d2 Then, the bound on average error probability becomes: ( [ ]) P e ≤ exp −N R2 (σ 2 ) − R ( 1+γ ) ( ) d 2 γ2 e 8σ2 −1 D γ2 2Dv (Q) 1 2Dv (Q) 2 KL 1+γ2 + d(1+γ2 ) 1+γ2 , d < 1+γ2 R2 (σ ) = max d 8σ 2 Q 1+γ2 e Dv (Q) d 8σ 2 +e −γ2 d2 − ln γ2 e 1+γ2 8σ 4σ 2 , otherwise DKL the binary Kullback-Leibler divergence DKL (p||q) = p log p/q + (1 − p) log(1 − p)/(1 − q) “Xenoulis,Kalouptsidis,Sason. New achievable rates for nonlinear Volterra channels via martingale inequalities.”N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 25 / 46
- 27. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Example: Discrete Memoryless binary input AWGN channel (input: u ∈ {−A, A} with Q(u = A) = α, SNR σ2 ) A ( ) R2 (SNR) = ln 2 − ln 1 + e− 2 SNR in nats per channel use 0.7 Achievable rates in nats per channel use 0.6 0.5 0.4 0.3 Capacity 0.2 R2 SNR 0.1 0 1 2 3 4 5 6 7 8 9 10 SNRN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 26 / 46
- 28. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Sparse joint channel state and parameter estimation Joint state and parameter estimation Blind estimation via EM and smoothing algorithms N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 27 / 46
- 29. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Joint detection and estimation Alternating state estimation and training based parameter estimation Channel parameter: θ = [h, Q] Channel input output form: y t = h(xt ) + ξt xt = [vt , . . . , vt−q ] Maximum Likelihood max log P (y 1:n |x1:n ; θ) v∈C,θ = max max log P (y 1:n |x1:n ; θ) θ v∈C = max max log P (y 1:n |x1:n ; θ) θ x1:n ∈S N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 28 / 46
- 30. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Stage 1: State Estimation Parameter estimate at step i : θ (i) = [h(i) , Q(i) ] State estimate: v = arg max log P (y 1:n |x1:n ; θ (i) ) ˆ x1:n ∈S Convolutional codes of memory < q lead to a Hidden Markov Process (HMP) (y 1:n , x1:n ) The HMP framework implies ∑ n ˆ xi = arg max log P (y t |xt ) x1:n ∈S t=1 Optimization can be carried out by dynamic programming and the Viterbi algorithm. Several relaxations over the real numbers are available for special cases (decoding by linear programming, semideﬁnite relaxation)N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 29 / 46
- 31. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Example: Binary Input Memoryless channel ∑ n ∑ log P (y t |xt ) = vt γt t=1 P (y t |1) γt = P (y t |0) Relax the constraints by replacing the convex hull of the codebook by the intersection of the convex hulls of the parity check equations. The problem is converted to a linear programming problem Decoding by the interior point algorithmN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 30 / 46
- 32. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection STEP 2: Parameter Estimation Given the transmitted message estimate, the decoder updates channel parameters by the rule (i+1) θ (i+1) = arg max P (y 1:n |x1:n ; θ) θN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 31 / 46
- 33. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection The solutions are given as follows: . Table look up 1 ∑n t=1 yt δ(xt , x) h(x) = ∑n ˆ t δ(xt , x) ∑( n )( )H ˆ = 1 Q ˆ ˆ y(t) − h(x) y(t) − h(x) n t=1 (∑n ) ∑n . 2 h linear: H ˆ t=1 xt xt h = t=1 yt xt (∑n ) . 3 h polynomial: ˆ ∑n yt φ(xt ) H h= t=1 φ(xt )φ(xt ) t=1 where φ(xt ) = [xt , ⊗2 xt , . . . , ⊗L xt ] with xt = [v1 (t), . . . , v1 (t − q), · · · , vT (t), . . . , vT (t − q)]N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 32 / 46
- 34. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Adding a sparsity term in the likelihood θ (i+1) = arg max P (y 1:n |x1:n ; θ) + γ h 1 θ a convex program results that can be solved by CS methods.N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 33 / 46
- 35. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Greedy algorithms: CoSaMP/SP The main ingredients of the CoSaMP/SP algorithms are outlined below: . 1 locate the largest components of the proxy . 2 form a union of two sets of indices . 3 estimation via LS on the merged set . 4 prune the LS estimates to s largest components . 5 updates the error residual The proposed algorithm modiﬁes the identiﬁcation, estimation and error residual step. In order to: sequentially track system variations reduce the computational complexity while maintaining the superior performance of CoSaMP/SPN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 34 / 46
- 36. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. The SpAdOMP Algorithm Algorithm description Complexity h(0) = 0, w(0) = 0, p(0) = 0 r(0) = y(0) 0<λ≤1 0 < µ < 2λ−1 max For n := 1, 2, . . . do 1: p(n) = λp(n − 1) + v ∗ (n − 1)r(n − 1) q 2: Ω = supp(p2s (n)) q 3: Λ = Ω ∪ supp(h(n − 1)) s 4: ε(n) = y(n) − v T (n)w|Λ (n − 1) |Λ s 5: w|Λ (n) = w|Λ (n − 1) + µv ∗ (n)ε(n) |Λ s 6: Λs = max(|w|Λ (n)|, s) s 7: h|Λs (n) = w|Λs (n), h|Λc (n) = 0 s 8: r(n) = y(n) − v T (n)h(n) s end For O(q) “Mileounis,Babadi,Kalouptsidis,Tarokh. An Adaptive Greedy Algorithm With Application to Nonlinear Communications.”N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 35 / 46
- 37. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Steady-State MSE of SpAdOMP . Theorem. . The SpAdOMP algorithm produces an s-sparse approximation h(n) that satisﬁes the following steady-state bound h − h(n) 2 C1 (n) ξ(n) 2 + C2 (n) v |Λ (n) 2 |eo (n)|, where eo (n) is the estimation error of the optimum Wiener ﬁlter C1 (n), C2 (n) are constants independent of h . The ﬁrst term is analogous to the SS error of the CoSaMP/SP algorithm The second term is induced by performing a single LMS iteration (instead of using the LS estimate)N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 36 / 46
- 38. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Simulations on sparse ARMA channels ARMA channel: yn = a1 yn−6 + a2 yn−48 + vn + b1 vn−13 + b2 vn−34 + ξn , and 500 samples from CN (0, 1/5). 0 0.2 LMS −5 0.1 LOG−LMS SpAdOMP −10 0 NMSE (dB) NMSE (dB) −15 −0.1 −20 −0.2 −25 −0.3 LMS −30 −0.4 LOG−LMS SpAdOMP −35 −0.5 0 200 400 600 800 1000 250 300 350 400 450 500 Iterations Iterations a. Learning curve (SNR=23dB) b. Time evolution of (a1 ) ( ) NMSE = 10 log10 E{ h(n) − h 22 }/E{ h 22 } SpAdOMP converges very fast SpAdOMP achieves an average gain of nearly 19dBN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 37 / 46
- 39. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Blind estimation via EM and smoothing algorithms Transmitted sequence unknown Likelihood maximization is intractable The Expectation Maximization (EM) method and the underlying iterative algorithm provides an option that inherently addresses symbol detection Augmented likelihood function formed by the state and received sequence. Given Q(θ, θ ) the expectation step forms Q(θ, θ ) = Eθ {log P (x1:n , y 1:n ; θ)|y 1:n } Expectation over the state sequence given the received sequence. N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 38 / 46
- 40. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Marginal output likelihood Ln (θ) = log P (y 1:n ; θ) Jensens inequality implies Ln (θ) − Ln (θ ) ≥ Q(θ, θ ) − Q(θ, θ) This suggests the second step Let θ (i) denote an estimate at step i. Then θ (i+1) = arg max Q(θ i , θ) and Ln (θ (i+1) ) ≥ Ln (θ i ) θ For Gaussian noise, P (y(t)|x(t)) is log concave, the minimizer of Q is unique and the sequence θ i converges to a stationary point of the likelihoodN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 39 / 46
- 41. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection The EM method leads to the following estimates ∑n ˆ (i+1) (x) = ∑ P (xt = x; θ |y 1:n )yt (i) t=1 h n t=1 P (xt = x; θ |y 1:n ) (i) ∑ |M | ( )( )H q+1 n ∑ ˆ (i+1) = 1 Q ˆ ˆ P (xt = x; θ (i) |y 1:n ) y t − h(xl ) y t − h(xl ) n t=1 l=1 Determination of the smoothing probabilities P (xt |y 1:n ) by the forward backward recursions (Chang and Hancock) P (xt , y 1:n ) = α(xt , y 1:t )β(y t+1:n |xt ) ∑ M α(xt , y 1:t ) = b(yt |xt ) α(xt−1 , y 1:t−1 )αxt−1 xt xt−1 =1 ∑ M β(y t+1:n |xt ) = αxt xt+1 β(y t+2:n |xt+1 )b(yt+1 |xt+1 ) xt+1 =1N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 40 / 46
- 42. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection normalized stable versions (for instance Lindgren) P (xt |y 1:t−1 )b(yt |xt ) α(xt |y 1:t ) = ∑M xt =1 P (xt |y 1:t−1 )b(yt |xt ) ∑ M P (xt |y 1:t−1 ) = αxt−1 xt α(xt−1 |y1:t−1 )) xt−1 =1 ∑ αxt xt+1 P (xt+1 |y1:n ) M P (xt |y1:n ) = α(xt |y1:t ) P (xt+1 |y1:t ) xt+1 =1 Sparsity can be incorporated in the maximization stepN. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 41 / 46
- 43. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. The Sparse BW Algorithm Algorithmic description For := 0, 1, . . . , do 1: {r ( ) , R( ) }:=Run {Forward/Backward recursions} {symbol detector} sgn(r i ) [ ( ) ] ( ) ( +1) 2: hi = ( ) |r i | − γ 2 {channel estimator} Ri,i + 2 ( +1) ∑ n 2 3: σq =n 1 yn − x( +1)T h( +1) n {noise variance estima n=1 end For ∑ n ( ) ∑ n ( ) ∗ ˆ ˆ r( ) = yi E{xi |y n ; h }, R( ) = E{xi xH |y n ; h } i i=1 i=1 “Mileounis,Kalouptsidis,Babadi,Tarokh. Blind identiﬁcation of sparse channels and symbol detection via the EM algorithm.”N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 42 / 46
- 44. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection Adaptive Channel Coding Based on Flexible Trellis. (Convolutional) Codes Popular adaptive coding schemes → variable rate punctured convolutional codes IEEE 802.22 standard for cognitive WRAN uses a rate 1/2 convolutional code and a set of puncturing matrices that lead to rates 2/3, 3/4 and 5/6. Flexible Convolutional Codes They can vary both their rate and the decoding complexity → eﬃcient management of the system resources. Constructed by combining the techniques of path pruning and puncturing. Varying quantities associated with the complexity proﬁle of the trellis diagram→ Varying decoding complexity. N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 43 / 46
- 45. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Flexible Convolutional Codes Consider an (n, 1, m) mother convolutional code. Let ut be the information bit and ut the input bit of the ˆ mother encoder at time instant t. every Tpr time units the single input bit of the encoder is not an information bit, rather it is computed as a linear combination of bits of the current state St = {ˆt−1 , · · · , ut−m }. u ˆ { ut1 (Tpr −1)+t2 , if t2 = 0 ut = ˆ ∑d ˆ i=1 ci ut1 Tpr −i , if t2 = 0 ˆ ⌊ ⌋ ˆ where t1 = Tt , t2 = t mod Tpr , t = 1, 2, . . . , and d is the pr ∑m degree of the polynomial c(X) = i=1 ci X i . “Katsiotis,Rizomiliotis,Kalouptsidis. Flexible Convolutional Codes: Variable Rate and Complexity.” N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 44 / 46
- 46. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Flexible Convolutional Codes The ﬁnal step involves periodic puncturing of the encoded bits with period Tpu = pTpr , in order to adjust the rate. The complexity proﬁle of the resulting trellis depends solely on ˆ the parameters m, Tpr , d and the puncturing matrix. Large families of high-performance codes of various rates and values of decoding complexity are constructed. N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 45 / 46
- 47. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Extending the Constructions Flexible Turbo Codes Extending the analysis in the case where recursive mother encoders are used. The goal is to construct ﬂexible parallel concatenated powerful coding schemes. Preliminary results indicate that varying the complexity proﬁle of the trellis can be more eﬃcient than simply varying the number of decoding iterations. Flexible Secret Codes Embedding secret keys in the procedures of pruning and puncturing can result to robust and ﬂexible secret encoders. N. Kalouptsidis SP JAM 2012 Nonlinear communications: achievable rates, encryption, estimation and decoding 46 / 46
- 48. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Publications G. Mileounis, N. Kalouptsidis, A sparsity driven approach to cumulant based identiﬁcation, in IEEE Proc. SPAWC 2012, Turkey. K. Xenoulis, N. Kalouptsidis, I. Sason, New achievable rates for nonlinear Volterra channels via martingale inequalities, in IEEE proc. ISIT 2012. A. Katsiotis, P. Rizomiliotis, and N. Kalouptsidis, ”Flexible Convolutional Codes: Variable Rate and Complexity,” IEEE Trans. Commun., vol. 60, no. 3, pp. 608-613, March 2012. N. Kalouptsidis, G. Mileounis, B.Babadi, and V. Tarokh, ”Adaptive Algorithms for Sparse System Identiﬁcation,” Signal Process., vol. 91, no. 8, pp. 1910-1919, Aug. 2011. K. Xenoulis and N. Kalouptsidis, ”Tight performance bounds for permutation invariant binary linear block codes over symmetric channels, IEEE Trans. Inf. Theory, vol. 57, pp. 6015-6024, Sep. 2011. K. Limniotis, N. Kolokotronis, and N. Kalouptsidis, ”Constructing Boolean functions in odd number of variables with maximum algebraic immunity,” in proc. 2011 IEEE ISIT, pp. 2662-2666, 2011.
- 49. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Publications (contd.) N. Kalouptsidis and N. Kolokotronis, ”Fast decoding of regular LDPC codes using greedy approximation algorithms,” in proc. 2011 IEEE ISIT, pp. 2011-2015, 2011. A. Katsiotis and N. Kalouptsidis, ”On (n, n-1) punctured convolutional codes and their trellis modules,” IEEE Trans. Commun., vol. 59, pp. 1213-1217, 2011. K. Xenoulis and N. Kalouptsidis, ”Achievable rates for nonlinear Volterra channels,” IEEE Trans. Inform. Theory, vol. 57, pp. 1237-1248, 2011. A. Katsiotis, P. Rizomiliotis, and N. Kalouptsidis, ”New constructions of high-performance low-complexity convolutional codes,” IEEE Trans. Commun., vol. 58, pp.1950-1961, 2010. G. Mileounis, B. Babadi, N. Kalouptsidis, and V. Tarokh, ”An Adaptive Greedy Algorithm with Application to Nonlinear Communications,” IEEE Trans. Signal Proc., vol. 58, No. 6, June 2010. B. Babadi, N. Kalouptsidis, and V. Tarokh, ”SPARLS: The Sparse RLS Algorithm,” IEEE Trans. Signal Proc., vol. 58, no. 8, August 2010.
- 50. Encryption encoding and secrecy codes Channel encoding Channel modelling and achievable rates Channel estimation and symbol detection. Publications (contd.) N. Kolokotronis, K. Limniotis, and N. Kalouptsidis, ”Best aﬃne and quadratic approximations of particular classes of Boolean functions, IEEE Trans. Inform. Theory, vol. 55, pp. 5211-5222, 2009. T. Etzion, N. Kalouptsidis, N. Kolokotronis, K. Limniotis and K. G. Paterson, ”Properties of the error linear complexity spectrum,” IEEE Trans. Inform. Theory, pp. 4681-4686, vol. 55, 2009. B. Babadi, N. Kalouptsidis, and V. Tarokh, ”Asymptotic Achievability of the Cramer-Rao Bound for Noisy Compressive Sampling,” IEEE Trans. Signal Proc., vol. 57, no. 3, March 2009. G. Mileounis, P. Koukoulas, N. Kalouptsidis, ”Input-output identiﬁcation of nonlinear channels using PSK, QAM and OFDM inputs, Signal Process., vol. 89, no. 7, pp. 1359-1369, Jul. 2009. K. Xenoulis and N. Kalouptsidis, ”Improvement of Gallager upper bound and its variations for discrete channels,” IEEE Trans. Inform. Theory, vol. 55, pp. 4204-4210, 2009.

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