[求助]关于Turbo编码器的尾比特处理

moliyady

在3GPP的官方资料里，关于WCDMA系统Turbo编码原理的尾比特的处理是这样说的：
4.2.3.2.2        Trellis termination for Turbo coder
Trellis termination is performed by taking the tail bits from the shift register feedback after all information bits are encoded. Tail bits are padded after the encoding of information bits.
The first three tail bits shall be used to terminate the first constituent encoder (upper switch of figure 4 in lower position) while the second constituent encoder is disabled. The last three tail bits shall be used to terminate the second constituent encoder (lower switch of figure 4 in lower position) while the first constituent encoder is disabled.
The transmitted bits for trellis termination shall then be:
xK+1, zK+1, xK+2, zK+2, xK+3, zK+3, x'K+1, z'K+1, x'K+2, z'K+2, x'K+3, z'K+3.
其中figure 4 就是WCDMA系统Turbo编码器框图。

请教高手：
关于这一段所谓前三比特和最后三比特是指什么？
是不是信息编码完成后最后有这固定的12比特输出呢？