  Quantum secure direct communication (QSDC) guarantees both the security and
reliability of information transmission using quantum states.
One-photon-interference QSDC (OPI-QSDC) is a technique that enhances the
transmission distance and ensures secure point-to-point information
transmission, but it requires complex phase locking technology. This paper
proposes a mixed-encoding one-photon-interference QSDC (MO-QSDC) protocol that
removes the need for phase locking technology. Numerical simulations
demonstrate that the MO-QSDC protocol could also beat the PLOB bound.
