Multitone FSK Modulation for SWIPT

Simultaneous wireless information and power transfer (SWIPT) is gaining interest as it enables receivers with smaller batteries or none at all. However, the information and power transfer subsystems influence each other's performance, resulting in a tradeoff. SWIPT receivers should be low power and compact, ideally resulting in an energy-efficient design where hardware is maximally shared between information and power reception blocks. We propose a novel frequency-shift keying (FSK) modulation scheme that uses waveforms that are known to improve wireless power transfer efficiency and can be decoded by relying on the nonlinearity of the energy harvesting hardware. This hardware sharing saves area and avoids using a local oscillator (LO). We analyze two novel multitone FSK schemes while presenting measurement results proving the feasibility of the proposed schemes. Information transfer is based on the relationship between input frequency spacings and rectifier output intermodulation product frequencies. We show that contrary to the existing amplitude variation-based modulation schemes for SWIPT, the symbol rate for uniform multitone FSK in the noiseless case is not limited by the rectifier low-pass filter. For our proposed nonuniform multitone FSK scheme, the symbol rate is limited but spectral efficiency is much higher. We show by measurements that information transfer of at least 18 Mb/s using the rectifier as a receiver is feasible, which is a large increase compared to other solutions. A proposed optimization increases feasible symbol rate for the nonuniform FSK scheme by at least a factor of four.