Abstract
The inductive link is widely used in implantable biomedical sensor systems to achieve near-field communication (NFC) and wireless power transfer (WPT). However, it is tough to achieve reliable NFC on an inductive WPT link when the coupling coefficient is ultra-low (0.01 typically), since the NFC signal (especially for the uplink from the in-body part to the out-body part) could be too weak to be detected. Traditional load shift keying (LSK) requires strong coupling to pass the load modulation information to the power source. Instead of using LSK, we propose a dual-carrier NFC scheme for the weak-coupled inductive link; using binary phase shift keying (BPSK) modulation, its downlink data are modulated on the power carrier (2 MHz), while its uplink data are modulated on another carrier (125 kHz). The two carriers are transferred through the same coil pair. To overcome the strong interference of the power carrier, dedicated circuits are introduced. In addition, to minimize the power transfer efficiency decrease caused by adding NFC, we optimize the inductive link circuit parameters and approach the receiver sensitivity limit. In the prototype experiments, even though the coupling coefficient is as low as 0.008, the in-body transmitter costs only 0.61 mW power carrying 10 kbps of data, and achieves a 1 × 10 bit error rate under the strong interference of WPT. This dual-carrier NFC scheme could be useful for small-sized implantable biomedical sensor applications.
Highlights
Implantable battery-less biomedical sensor systems will play an important role in future medical diagnose and treatment [1,2]
We evaluate the impact of the power carrier interference through calculating the signal-to-interference ratio (SIR) at Vd
We firstly explore the circuit parameters that satisfy the constraint of avoiding saturation and keep the received uplink signal power to be sufficient for reliable demodulation
Summary
Implantable battery-less biomedical sensor systems will play an important role in future medical diagnose and treatment [1,2]. The inductive link consisting of a magnetic-coupled coil pair is a viable method to transmit wireless power [3] and data [4]. The ultra-weak coupling challenges the traditional inductive link in transmitting stable power and reliable data. RFID could transmit power and uplink data simultaneously through only one coil pair and one carrier. We propose a near-field communication (NFC) attached to an inductive WPT link, tailored to an implantable sensor system for glaucoma treatment. BPSK modulation is used in the downlink to minimize the amplitude ripple of the power carrier. Our proposed NFC overcomes the strong interference from the power carrier and achieves reliable communication under ultra-weak coupling. This paper focuses on the design and optimization of the physical link layer, as it is the key to achieve the simultaneous power and data transmission.
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