  We propose an unconditionally secure quantum key distribution (QKD) protocol,
which uses a relatively strong signal pulse. While our protocol shares similar
security bases as the Bennett 1992 protocol with a strong reference pulse
(B92), our scheme uses a smaller number of detectors and it is robust against
Rayleigh scattering in an optical fibre. We derive a lower bound of secret key
generation rate of our protocol and show that our protocol can cover relatively
long distances, assuming precise phase modulations and stable interferometers.
