  Without actively aligning the reference frames, the
reference-frame-independent quantum key distribution (RFI-QKD) can generate
secure keys even when the reference frames drift slowly. Here, we propose a new
scheme on the decoy-state RFI-QKD, where the basis assignments of two
legitimate parties are biased, reducing the fraction of mismatched-basis data.
When accounting for statistical fluctuations under different misalignments of
reference frames, we investigate the performance of biased decoy-state RFI-QKD
with full parameter optimization. We also make a clear comparison between the
new scheme with other practical schemes, e.g., unbiased decoy-state RFI-QKD and
biased decoy-state BB84-QKD. Simulation results demonstrate that the new
proposed biased decoy-state RFI-QKD can significantly improve the performance
(secret key rate and secure transmission distance) of practical QKD systems.
