1. A computer implemented method of producing a soft symbol estimate of a pulse amplitude modulation (PAM) symbol, the method comprising: determining from one PAM symbol, s N′ the following relationship: Pr ⁡ ( x = s N ′ ) = ∏ i ⁢ P b i ( 1 ) = ∏ i ⁢ 1 ⅇ λ i + 1 , where λ i denotes a log-likelihood ratio (LLR) for the i-th bit b i ; 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 , where {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 ; and determining, after determining Pr(x=s n ) for all n, the soft PAM estimation according to the following: x ~ = ∑ s ∈ 𝒮 PAM ⁢ s ⁢ ⁢ Pr ⁡ ( x = s ) = ∑ n = 1 N ′ ⁢ s n ⁢ ∏ i = 1 Q ⁢ P b i ( s n , i ) , where Q is a length of a bit sequence, N′=2 Q , 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 ⅇ - λ i + 1 , ⁢ P b i ( 1 ) ⁢ = Δ ⁢ Pr ⁡ ( b i = 1 ) = 1 ⅇ λ i + 1 .