On an upper bound of the secrecy capacity for a general wiretap channel

This paper is concerned with coding theorems for a generalized wiretap channel. In the problem of the wiretap channel it is important to characterize the secrecy capacity, i.e., the maximum achievable rate of information securely transmitted from a sender to a legitimate receiver in the presence of a wiretapper. In our model, channels are not restricted to memoryless channels. We first define the secrecy capacity C
_s
and evaluate C
_s
from information-spectrum approach. We give a new upper bound of C
_s
not including auxiliary random variables and explore conditions under which the obtained upper bound becomes tight for the cascaded wiretap channel