Wireless communications systems, radio frequency identification devices, wireless communications methods, and radio frequency identification device communications methods are described. In one aspect, a wireless communications system includes a reader configured to output a plurality of wireless signals including different modulation frequencies and to receive a plurality of second wireless signals, a plurality of communication devices configured to receive the first wireless signals and to output the second wireless signals using backscatter modulation and wherein the communication devices are individually configured to respond to one of the first wireless signals having a first modulation frequency at a first moment in time and to respond to one of the first wireless signals having a second modulation frequency different than the first modulation frequency at a second moment in time.