其中D二1+ep化w为常量. Substituting (4.32)into(4.21).the branch transition probability can be written as ri(s',s)=AD.expil (u)+Lyci+Lyfc P(S =sIS=s',u =i) For a systematic encoder,==i.Thus (s',s)=K expiL(u)+Lviuexp Leyfcf P(S.=s1S=s'.u =i) (4.33) where a constant that is independent of the value of Let rh(s',s)=exp(S:=sS=s'.u=i) Then we have (s',s)expi(u)+ir(s'.s) (4.34) wheredenotes equality up to a scale factor. Combining (4.34)and(4.27)yields ，a-(s')(s',s月(sxp{L(u)+L} L(4:)=lnie ∑d(s')-(s',-B,(s) ∑a(s')Ys',月(s) =L+L(u)+In a)y2(s's月(s =Ly+L(4)+L.(u) (4.35) The first term in the R.H.S.of(4.35)is the weighted channel measurement (which is sometimes called the channel value),the second term represents any a priori information about provided by a previous decoder,and the third term represents extrinsic information that can be passed on to a subsequent decoder.Notice that only extrinsic information contains new information produced by the current decoder.从.(s',s)的定义可以看出，外信息只 包含当前分量编码器校验位的软输出信息，也只有这一部分信息对其它分量码译码器来 说是不具有的新信息，因此可被用作先验信息。 由(4.35)我们可以得到Tubo译码器的另一种实现结构，如图4.14所示。 4-264-26 其中 1 1 exp{ ( )} k a k D L u   为常量。 Substituting (4.32) into (4.21), the branch transition probability can be written as ( ', ) exp ( )   i ss pp k k k a k c kk c k k  s s A D iL u L y c L y c   1 ( | ', ) PS s S s u i kk k     For a systematic encoder, s k k cui   . Thus ( ', ) exp ( ) exp     i s pp k a k ckk ckk  s s K iL u L y u L y c   1 ( | ', ) PS s S s u i kk k     (4.33) where K = AkDk is a constant that is independent of the value of uk. Let , 1 ( ', ) exp ( | ', )   i pp ke c k k k k k  s s Lyc PS s S s u i     Then we have   , ( ', ) exp ( ) ( ', ) i si k a k c k ke   s s iL u iL y s s   (4.34) where “” denotes equality up to a scale factor. Combining (4.34) and (4.27) yields 1 0 1 1 , ( ', ) 0 1 , ( ' , ) ( ' ) ( ' , ) ( ) exp{ ( ) } ( ) ln ( ' ) ( ' , ) ( ) k k s k ke k a k c k ss B k k ke k ss B s s s s L u Ly L u s ss s                       1 0 1 1 , ( ', ) 0 1 , ( ' , ) ( ' ) ( ' , ) ( ) ( ) ln ( ' ) ( ', ) ( ) k k k ke k s ss B ck a k k ke k ss B s ss s Ly L u s ss s                       () () s   Ly L u L u ck a k e k (4.35) The first term in the R.H.S. of (4.35) is the weighted channel measurement (which is sometimes called the channel value), the second term represents any a priori information about uk provided by a previous decoder，and the third term represents extrinsic information that can be passed on to a subsequent decoder. Notice that only extrinsic information contains new information produced by the current decoder. 从 , ( ', ) i k e  s s 的定义可以看出，外信息只 包含当前分量编码器校验位的软输出信息，也只有这一部分信息对其它分量码译码器来 说是不具有的新信息，因此可被用作先验信息。 由(4.35)我们可以得到 Turbo 译码器的另一种实现结构，如图 4.14 所示