  We present a discretely modulated continuous-variable quantum key
distribution system in free space by using strong coherent states. The
amplitude noise in the laser source is suppressed to the shot-noise limit by
using a mode cleaner combined with a frequency shift technique. Also, it is
proven that the phase noise in the source has no impact on the final secret key
rate. In order to increase the encoding rate, we use broadband homodyne
detectors and the no-switching protocol. In a realistic model, we establish a
secret key rate of 46.8 kbits/s against collective attacks at an encoding rate
of 10 MHz for a 90% channel loss when the modulation variance is optimal.
