Abstract: Based on the introduction of the space-time time-frequency-shift keying (ST-TFSK) systems, the paper deduces the noncoherent ML decision metric at the receiver with the knowledge of relative delay. The performance of the ST-TFSK systems in the presence of distinct normalized relative delay is discussed for the different SNRs, and the relative delay sensitivity between ST-TFSK systems and quaternary ST-FSK systems is compored. Both theoretical analysis and simulation results show that, (1) lower SNR lessens the normalized relative delay sensitivity of ST-TFSK systems; (2) the sensitivity of ST-TFSK systems is less than that of quaternary ST-FSK systems.