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Passive RF identification (RFID) tags are traditionally assumed to be downlink limited since typical tag sensitivity is considerably poorer than reader sensitivity, due to stringent power limitations. On the other hand, semipassive tags are generally uplink limited because of improvement in tag sensitivity for battery-assisted tags. In this paper, it is demonstrated that judicious choice and use of integrated circuit impedance for backscatter modulation will be needed to simultaneously maximize tag uplink and downlink ranges as passive tag designs improve. Optimal backscatter modulation indices for amplitude shift keying are derived for range maximization of next-generation low-power RFID tags.