  Higher-order mode (HOM) based intra-cavity beam diagnostics has been proved
effectively and conveniently in superconducting radio-frequency (SRF)
accelerators. Our recent research shows that the beam harmonics in the bunch
train excited HOM spectrum, which have much higher signal-to-noise ratio than
the intrinsic HOM peaks, may also be useful for beam diagnostics. In this
paper, we will present our study on bunch train excited HOMs, including the
theoretic model and recent experiments carried out based on the DC-SRF
photoinjector and SRF linac at Peking University.
