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In the current communication standards, like WiMAX and LTE, the modulation system has usually fixed symbol block length or fixed subcarrier number. In this study, the authors introduce a new concept, namely time-varying symbol block or time-varying subcarrier number into the modulation systems to improve the system performance. The authors call the new modulation system time-varying orthogonal frequency-division multiplexing, time-varying orthogonal frequency-division multiple access for multicarrier modulation and time-varying single-carrier frequency-division multiple access for single-carrier modulation. Here, peak-to-average power ratio (PAPR) is used as a criteria to change the symbol block length timely. The algorithm for determination of the symbol block length is presented. The simulation results show that the time-varying modulation schemes can clearly reduce the PAPR values for both multicarrier and single carrier modulation systems.