国家重点实验室 Symmetry assumption and Capacity o Assume that the channel has "input-output symmetry. (Example:BSC,BEC) BEC(c) 0 1-9 0 (Symmetric)Capacity 1 1-c For channels with I/O symmetry,the capacity is given by C(W)≌I(X;Y) with X unif.(0,1) =∑∑)W0y川x)log W(yx) eeX2 W(y川0)+，W0y1D) 2 Use base-2 logarithms: 0≤C(W)≤1 6Symmetry assumption and Capacity Assume that the channel has “input-output symmetry.” (Example: BSC, BEC) (Symmetric) Capacity For channels with I/O symmetry, the capacity is given by Use base-2 logarithms: 6 ( ) ( ; ) with unif. {0,1} 1 (|) ( | )log 2 1 1 ( | 0) ( |1) 2 2 y Yx X CW I X Y X Wyx Wyx W y W y ∈ ∈ = +    0 ()1 ≤ ≤ C W