ECEN 5682 Theory and Practice of Error Control Codes

Transcription

1 ECEN 5682 Theory and Practice of Error Control Codes Convolutional Code Performance University of Colorado Spring 2007

2 Definition: A convolutional encoder which maps one or more data sequences of infinite weight into code sequences of finite weight is called a catastrophic encoder. Example: Encoder #5. The binary R = 1/2, K = 3 convolutional encoder with transfer function matrix G(D) = [ 1 + D 1 + D 2], has the encoder state diagram shown in Figure 15, with states S 0 = 00, S 1 = 10, S 2 = 01, and S 3 = 11.

3 S 1 1/11 1/01 0/00 S 0 S 3 1/00 1/10 0/10 0/01 0/11 S 2 Fig.15 Encoder State Diagram for Catastrophic R = 1/2, K = 3 Encoder

4 S S 2 S 1 S Fig.16 A Detour of Weight w =7andi= 3, Starting at Time t =0

5 Definition: The complete weight distribution {A(w, i, l)} of a convolutional code is defined as the number of detours (or codewords), beginning at time 0 in the all-zero state S 0 of the encoder, returning again for the first time to S 0 after l time units, and having code (Hamming) weight w and data (Hamming) weight i. Definition: The extended weight distribution {A(w, i)} of a convolutional code is defined by A(w, i) = A(w, i, l). l=1 That is, {A(w, i)} is the number of detours (starting at time 0) from the all-zero path with code sequence (Hamming) weight w and corresponding data sequence (Hamming) weight i.

6 Definition: The weight distribution {A w } of a convolutional code is defined by A w = A(w, i). i=1 That is, {A w } is the number of detours (starting at time 0) from the all-zero path with code sequence (Hamming) weight w. Theorem: The probability of an error event (or decoding error) P E for a convolutional code with weight distribution {A w }, decoded by a ML decoder, at any given time t (measured in frames) is upper bounded by P E A w P w (E), w=d free where P w (E) = P{ML decoder makes detour with weight w}.

7 Theorem: On a memoryless BSC with transition probability ɛ < 0.5, the probability of error P d (E) between two detours or codewords distance d apart is given by 8 >< P d (E) = >: dx e=(d+1)/2 1 2 d d/2!! d e ɛ e (1 ɛ) d e, d odd, ɛ d/2 (1 ɛ) d/2 + dx e=d/2+1! d ɛ e (1 ɛ) d e, d even. e Proof: Under the Hamming distance measure, an error between two binary codewords distance d apart is made if more than d/2 of the bits in which the codewords differ are in error. If d is even and exactly d/2 bits are in error, then an error is made with probability 1/2. QED

8 Note: A somewhat simpler but less tight bound is obtained by dropping the factor of 1/2 in the first term for d even as follows P d (E) d e= d/2 ( ) d ɛ e (1 ɛ) d e. e A much simpler, but often also much more loose bound is the Bhattacharyya bound P d (E) 1 2 [ 4ɛ (1 ɛ) ] d/2.

9 Probability of Symbol Error. Suppose now that A w = P i=1 A(w, i) is substituted in the bound for P E. Then X X P E A(w, i) P w (E). w=d i=1 free Multiplying A(w, i) by i and summing over all i then yields the total number of data symbol errors that result from all detours of weight w as P i=1 i A(w, i). Dividing by k, the number of data symbols per frame, thus leads to the following theorem. Theorem: The probability of a symbol error P s (E) at any given time t (measured in frames) for a convolutional code with rate R = k/n and extended weight distribution {A(w, i)}, when decoded by a ML decoder, is upper bounded by P s (E) 1 i A(w, i) P w (E), k w=d free i=1 where P w (E) is the probability of error between the all-zero path and a detour of weight w.

10 The graph on the next slide shows different bounds for the probability of a bit error on a BSC for a binary rate R = 1/2, K = 3 convolutional encoder with transfer function matrix G(D) = [ 1 + D D + D 2].

11 10 0 Binary R=1/2, K=3, d free =5, Convolutional Code, Bit Error Probability P b (E) P b (E) BSC P b (E) BSC Bhattcharyya P b (E) AWGN soft log 10 (ε)

12 Upper Bounds on P b (E) for Convolutional Codes on BSC (Hard Decisions) 10 0 R=1/2,K=3,d free = R=2/3,K=3,d free =5 R=3/4,K=3,d free =5 R=1/2,K=5,d free = R=1/2,K=7,d free = P b (E) log 10 (ε) for BSC

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14 Transmission Over AWGN Channel The following figure shows a one-shot model for transmitting a data symbol with value a 0 over an additive Gaussian noise (AGN) waveform channel using pulse amplitude modulation (PAM) of a pulse p(t) and a matched filter (MF) receiver. The main reason for using a one-shot model for performance evaluation with respect to channel noise is that it avoids intersymbol interference (ISI). Noise n(t), S n (f) s(t) =a 0 p(t) + r(t) Filter b(t) h R (t) b 0 t =0 } {{ } } {{ } Channel Receiver

15 If the noise is white with power spectral density (PSD) S n (f ) = N 0 /2 for all f, the channel model is called additive white Gaussian noise (AWGN) model. In this case the matched filter (which maximizes the SNR at its output at t = 0) is h R (t) = p ( t) p(µ) 2 dµ H R (f ) = P (f ) P(ν) 2 dν, where denotes complex conjugation. If the PAM pulse p(t) is normalized so that E p = p(µ) 2 dµ = 1 then the symbol energy at the input of the MF is E s = E [ s(µ) 2 dµ ] = E [ a 0 2], where the expectation is necessary since a 0 is a random variable.

16 When the AWGN model with S n (f ) = N 0 /2 is used and a 0 = α is transmitted, the received symbol b 0 at the sampler after the output of the MF is a Gaussian random variable with mean α and variance σ 2 b = N 0/2. For antipodal binary signaling (e.g., using BPSK) a 0 { E s, + E s } where E s is the (average) energy per symbol. Thus, b 0 is characterized by the conditional pdf s f b0 (β a 0 = E s ) = e (β+ Es) 2 /N 0, πn0 and f b0 (β a 0 =+ E s ) = e (β Es) 2 /N 0. πn0 These pdf s are shown graphically on the following slide.

17 â 0 = E s â 0 =+ E s f b0 (β a 0 = E s ) f b0 (β a 0 =+ E s ) E s 0 + E s β 2 E s If the two values of a 0 are equally likely or if a ML decoding rule is used, then the (hard) decision threshold per symbol is to decide a 0 = + E s if β > 0 and a 0 = E s otherwise.

18 The probability of a symbol error when hard decisions are used is P(E A 0 = E s ) = 1 πn0 0 e (β+ E s) 2 /N 0 dβ = 1 2 erfc ( E s N 0 ), where erfc(x) = 2 x e γ2 dγ e x2. Because of the symmetry of antipodal signaling, the same result is obtained for P(E a 0 = + E s ) and thus a BSC derived from an AWGN channel used with antipodal signaling has transition probability π ɛ = 1 2 erfc ( E s N 0 ), where E s is the energy received per transmitted symbol.

19 To make a fair comparison in terms of signal-to-noise ratio (SNR) of the transmitted information symbols between coded and uncoded systems, the energy per code symbol of the coded system needs to be scaled by the rate R of the code. Thus, when hard decisions and coding are used in a binary system, the transition probability of the BSC model becomes ɛ c = 1 2 erfc ( R E s N 0 ), where R = k/n is the rate of the code. The figure on the next slide compares P b (E) versus E b /N 0 for an uncoded and a coded binary system. The coded system uses a R = 1/2 K = 3 convolutional encoder with G(D) = [ 1 + D D + D 2].

20 10 2 Binary R=1/2, K=3, d free =5, Convolutional Code, Hard decisions AWGN channel 10 0 P b (E) uncoded P b (E) union bound P b (E) Bhattacharyya P b (E) E /N [db], E : info bit energy b 0 b

21 Definition: Coding Gain. Coding gain is defined as the reduction in E s /N 0 permissible for a coded communication system to obtain the same probability of error (P s (E) or P B (E) as an uncoded system, both using the same average energy per transmitted information symbol. Definition: Coding Threshold. The value of E s /N 0 (where E s is the energy per transmitted information symbol) for which the coding gain becomes zero is called the coding threshold. The graphs on the following slide show P b (E) (computed using the union bound) versus E b /N 0 for a number of different binary convolutional encoders.

22 10 0 Upper Bounds on P b (E) for Convolutional Codes on AWGN Channel, Hard Decisions P b (E) Uncoded R=1/2,K=3,d free =5 R=2/3,K=3,d free =5 R=3/4,K=3,d free =5 R=1/2,K=5,d free =7 R=1/2,K=7,d free = E /N [db], E : info bit energy b 0 b

23 Soft Decisions and AWGN Channel Assuming a memoryless channel model used without feedback, the ML decoding rule after the MF and the sampler is: Output code sequence estimate ĉ = c i iff i maximizes f b (β a=c i ) = N 1 j=0 f bj (β j a j =c ij ), over all code sequences c i = (c i0, c i1, c i2,...) for i = 0, 1, 2,.... If the mapping 0 1 and 1 +1 is used so that c ij { 1, +1} then f bj (β j a j =c ij ) can be written as f bj (β j a j =c ij ) = e (β j c ij Es) 2 /N 0. πn0

24 Taking (natural) logarithms and defining v j = β j / E s yields N 1 ln f b (β a=c i ) = ln j=0 N 1 f bj (β j a j =c ij ) = N 1 j=0 ln f bj (β j a j =c ij ) (β j c ij Es ) 2 = N N 0 2 ln(πn 0) = E s N 0 = 2E s N 0 j=0 N 1 j=0 N 1 (v 2 j 2 v j c ij + c 2 ij) N 2 ln(πn 0) v j c ij j=0 N 1 = K 1 v j c ij K 2, j=0 ( β 2 + NE s + N ) N 0 2 ln(πn 0) where K 1 and K 2 are constants independent of the codeword c i and thus irrelevant for ML decoding.

25 Example: Suppose the convolutional encoder with G(D) = [ D ] is used and the received data is v = -0.4, -1.7, 0.1, 0.3, -1.1, 1.2, 1.2, 0.0, 0.3, 0.2, -0.2, 0.7,...

26 Soft Decisions versus Hard Decisions To compare the performance of coded binary systems on a AWGN channel when the decoder performs either hard or soft decisions, the energy E c per coded bit is fixed and P b (E) is plotted versus ɛ of the hard decision BSC model where ɛ = 1 2 erfc( E c /N 0 ) as before. For soft decisions the expression P w (E) = 1 2 erfc ( w E c N 0 ) is used for the probability that the ML decoder makes a detour with weight w from the correct path. Thus, for soft decisions with fixed SNR per code symbol P b (E) 1 2k Examples are shown on the next slide. ( w E ) c D w erfc. N 0 w=d free

27 Upper Bounds on P b (E) for Convolutional Codes with Soft Decisons (Dashed: Hard Decisions) 10 0 R=1/2,K=3,d free =5 R=2/3,K=3,d free =5 R=3/4,K=3,d free =5 R=1/2,K=5,d free =7 R=1/2,K=7,d free = P b (E) log 10 (ε) for BSC

28 Coding Gain for Soft Decisions To compare the performance of uncoded and coded binary systems with soft decisions on a AWGN channel, the energy E b per information bit is fixed and P b (E) is plotted versus the signal-to-noise ratio (SNR) E b /N 0. For an uncoded system P b (E) = 1 2 erfc ( E b N 0 ), (uncoded). For a coded system with soft decision ML decoding on a AWGN channel P b (E) 1 ( wr E ) b D w erfc, 2k N 0 w=d free where R = k/n is the rate of the code. Examples are shown in the graph on the next slide.

29 10 0 Upper Bounds on P b (E) for Convolutional Codes on AWGN Channel, Soft Decisions P b (E) Uncoded R=1/2,K=3,d free =5 R=2/3,K=3,d free =5 R=3/4,K=3,d free =5 R=1/2,K=5,d free =7 R=1/2,K=7,d free = E /N [db], E : info bit energy b 0 b