1. A computer implemented method of producing a soft symbol estimate of a pulse amplitude modulation (PAM) symbol comprising the steps of: determining from one PAM symbol, s N′ the following relationship Pr ( x = s N ′ ) = ∏ i P b i ( 1 ) = ∏ i 1  λ i + 1. ; determining, for n=N′−1,. .. , 1, (Pr(x=s n ) sequentially from the following relationship Pr ( x = s n ) = Pr ( x = s n + 1 ) π i ⋓ n + 1 , s n + 1 , i ⋓ n + 1 , n  s n , i ⋓ n + 1 , n , ; wherein {hacek over (i)} n+1,n denotes the index of the only different bit between a labeling bit sequence of S n+1 and s n ; determining, after determining Pr(x=s n ) for all n, the soft PAM estimation according to the following x ~ = ∑ s ∈  PAM s Pr ( x = s ) = ∑ n = 1 N ′ s n ∏ i = 1 Q P b i ( s n , i ) , where s n,i is the ith bit value mapped to the PAM symbol s n and P bi (s n,i ) is determined according to P b i ( 0 ) = Δ Pr ( b i = 0 ) = 1  - λ i + 1 , P b i ( 1 ) = Δ Pr ( b i = 1 ) = 1  λ i + 1 .