  The binary confusion noise spectrum at LISA band depends strongly on
observational period and abundance of Galactic close white dwarf binaries
(CWDBs). We have investigated how the number of the resolved Galactic CWDBs
varies with operation period of LISA, and found that the resolved number would
typically grows by a factor of 5 when the operation period increases from 1yr
to 10yr. We have also made a similar estimation for number of CWDBs whose chirp
signal can be measured in matched filtering analysis.
