A mobile device comprising: an antenna configured to receive electromagnetic energy from an RFID base station; modulation circuitry coupled to modulate an impedance of the antenna between a plurality of impedance values to change a radar cross-section of the mobile device for communicating with the RFID base station via backscattering of the electromagnetic energy; and an encoding module coupled to encode first data at a higher frequency by directing the modulation circuitry to select between the plurality of impedance values and further coupled to encode second data at a lower frequency by directing the modulation circuitry to select among either a first subset or a second subset of the plurality of impedance values for a length of time, wherein a first data state of the second data is represented by selecting among the first subset during the length of time, and wherein a second data state of the second data is represented by selecting among the second subset during the length of time; wherein the first subset of the plurality of impedance values includes a first plurality of impedance values and wherein the second subset of the plurality of impedance values includes a second plurality of impedance values, wherein the first subset of the plurality of impedance values includes impedance values that are common to the second subset of impedance values and also includes impedance values that are different from the second subset of impedance values, or wherein the first subset of the plurality of impedance values does not share impedance values with the second subset of the plurality of impedance values. an antenna configured to receive electromagnetic energy from an RFID base station; modulation circuitry coupled to modulate an impedance of the antenna between a plurality of impedance values to change a radar cross-section of the mobile device for communicating with the RFID base station via backscattering of the electromagnetic energy; and an encoding module coupled to encode first data at a higher frequency by directing the modulation circuitry to select between the plurality of impedance values and further coupled to encode second data at a lower frequency by directing the modulation circuitry to select among either a first subset or a second subset of the plurality of impedance values for a length of time, wherein a first data state of the second data is represented by selecting among the first subset during the length of time, and wherein a second data state of the second data is represented by selecting among the second subset during the length of time; wherein the first subset of the plurality of impedance values includes a first plurality of impedance values and wherein the second subset of the plurality of impedance values includes a second plurality of impedance values, wherein the first subset of the plurality of impedance values includes impedance values that are common to the second subset of impedance values and also includes impedance values that are different from the second subset of impedance values, or wherein the first subset of the plurality of impedance values does not share impedance values with the second subset of the plurality of impedance values.