Low-frequency interference components produced by a time division multiple access (TDMA) portable radio transmitter that might potentially interfere with devices sensitive to low frequencies, such as cardiac pacemakers, are substantially produced. In those TDMA frames in which a protocol calls for either a burst of an information-bearing signal to not be transmitted in its designated time slot, or transmitted at a reduced power level, or for a shorter duration, an interference-compensating signal at a compensation frequency other than the transmitter's carrier frequency is transmitted. The interference-compensating signal is transmitted in the time slot in which the transmitter normally transmits the burst at a power level equal to the power level of the burst. If the protocol calls for a reduced power burst, the interference-compensating signal is transmitted within an adjacent time slot at a power level that compensates for the reduced burst power level. If the protocol calls for a shorter burst length, the interference-compensating signal is transmitted within the same time slot to extend the burst to the length of the time slot. The interference caused by the compensating signal is substantially the same as would have been caused by the information-bearing signal, so that the interference pattern remains unchanged. This eliminates low-frequency variations in the interference pattern.

A modulated backscatter communication system (e.g., an Electronic Shelf Label (ESL) system) includes a central node (e.g., a ceiling node) and one or more remote nodes (e.g., ESL tags). The central node transmits a downlink radio signal which is reflectively modulated at a remote node to produce a modulated reflected radio signal. At the remote node, a precise-frequency subcarrier signal is modulated with an uplink information signal to produce a modulated signal which is used to produce the modulated reflected radio signal. At the central node, the modulated reflected radio signal is received, detected and narrowband filtered to obtain the modulated subcarrier signal which is then demodulated to obtain the uplink information signal, with great immunity to close to carrier downlink radio noise and fluorescent light backscatter interference.

A modulated backscatter communication system (e.g., an Electronic Shelf Label (ESL) system) includes a central node (e.g., a ceiling node) and one or more remote nodes (e.g., ESL tags). The central node transmits a downlink radio signal which is reflectively modulated at a remote node to produce a modulated reflected radio signal. At the remote node, a precise-frequency subcarrier signal is modulated with an uplink information signal to produce a modulated signal which is used to produce the modulated reflected radio signal. At the central node, the modulated reflected radio signal is received, detected and narrowband filtered to obtain the modulated subcarrier signal which is then demodulated to obtain the uplink information signal, with great immunity to close to carrier downlink radio noise and fluorescent light backscatter interference.

In accordance with the teachings described herein, systems and methods are provided for approximating the location of an RFID tag in an RFID system. The RFID system may include a plurality of receivers that receive RF signals from the RFID tag. Tag read count data for two or more of the plurality of receivers may be used to identify a number of times that each of the two or more receivers has received an RF signal from the RFID tag over a period of time. Signal strength data may be used to identify the strength of the RF signals received from the RFID tag. A combination metric for each of the two or more receivers may be determined as a function of the tag read count data and the signal strength data. The combination metrics may be compared to identify one of the receivers that is closest in proximity to the RFID tag.

Radio communication apparatus for transmitting control data, comprising output power amplifying means responsive to a control signal from an output power control means for varying a predetermined output power level from the output power amplifying means in accordance with a sequence of power levels indicative of a control message. The predetermined output power level is selectable from a range of predetermined power levels and may be variable between power levels of adjacent predetermined power levels for the range of predetermined power levels. Typically, the control message is a request to vary the output power level for inhibiting radio signal fading.