This letter provides experimental evidences that, in a complex environment, a time reversal processing of electromagnetic waves can increase the energy efficiency of the reader to tag communication and, hence, can increase the activation range of a passive UHF RFID system. A first experiment performed in a free space environment shows that in addition to the classical continuous wave (CW) mode, a type AD222 passive UHF RFID tag can successfully operate in a pulsed wave (PW) mode using pulses with half-width 5 ns and period 35 ns. In a second experiment performed in a complex multipath environment, it is shown that the energy efficiency of the RFID system in PW mode can be increased by applying a TR processing strategy. Further, it is shown that both PW and TR strategies involve less energy to activate the tag than does the classical CW mode. As a consequence, given a fixed amount of energy, TR processing can increase the activation range of the RFID system.