  It is the first scheme which allows the detection apparatus to achieve both
the photon number of arriving signals and quantum bit error rate of the
multiphoton pulses precisely. We show that the upper bound of the fraction of
the tagged multiphoton pulses counts is $\mu $, which is independent of the
channel loss and the intensity of the decoy source. Such upper bound is
$inherent$ and cannot be reduced any longer as long as the weak coherent
scouces and high lossy channel are used. We show that our scheme can be
implemented even if the channel loss is very high. A stronger intensity of the
pulse source is allowable to improve the rate of quantum key distribution. Both
the signal pulses and decoy pulses can be used to generate the raw key after
verified the security of the communication. We analyze that our scheme is
optimal under today's technology.
  PACS: 03.67.Dd
