Trade-Offs Between Data and Power Transfer in Near-Field Inductive Links

This paper studies the fundamental trade-off between power transfer efficiency (PTE) and spectral efficiency for simultaneous power and data transfer through near-field inductive links. In particular, it shows how the highest bit rate achievable for a given bit error rate (BER), which is a measure of spectral efficiency, varies versus PTE for several common modulation schemes. The schemes studied include binary, quadrature, and 8-phase shift keying (BPSK, QPSK, and 8PSK), binary frequency shift keying (FSK), and amplitude shift keying (ASK) with various modulation depths (30% and 50%). Circuit simulations of an optimized inductive link in air, saline, and tissue are used to estimate PTE for each modulation scheme. Also, a theoretical model is used to simulate the received BER for the same set of schemes. The simulations are repeated for different values of coupling coefficient, corresponding to varying separation between the coils. The results show that both PTE and spectral efficiency in (bits/sec)/Hz increases for all schemes as the coupling factor increases. They also show that ASK at low modulation depths has the highest PTE, while the PSK schemes have the highest spectral efficiency.