Wireless power transfer

Abstract

A power receiver (105) comprises a receiver coil (107) extracting power from an electromagnetic power transfer signal generated by a power transmitter (101). A variable load (511) applies modulation loading to the receiver coil (107). A data transmitter (509) transmits data symbols to the power transmitter (101) by controlling the variable load (511) to load modulate the power transfer signal during communication time intervals that are interspersed by non-communication time intervals during which no data is transmitted to the power transmitter (101). The data transmitter (509) varies the variable load (511) to apply a modulation loading pattern for each data symbol where the modulation loading pattern for each possible data symbol value is different. The data transmitter controls the variable load (511) to apply a repeating loading variation pattern during non-communication time intervals. The repeating loading variation pattern has a power spectral density differing less than 10dB from that of the modulation loading patterns and thus the spectral shapes are closely matching.