192 IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 4, NO. 3, JUNE 2010 High-Speed OQPSK and Efficient Power Transfer Through Inductive Link for Biomedical Implants Guillaume Simard, Student Member, IEEE, Mohamad Sawan, Fellow, IEEE, and Daniel Massicotte, Senior Member, IEEE Abstract—Biomedical implants require wireless power and bidi- rectional data transfer. We pursue our previous work on a novel topology for a multiple carrier inductive link by presenting the fabricated coils. We show that the coplanar geometry approach is better suited for displacement tolerance. We provide a theoretical analysis of the efficiency of power transfer and phase-shift-keying communications through an inductive link. An efficiency of up to 61% has been achieved experimentally for power transfer and a data rate of 4.16 Mb/s with a bit-error rate of less than 2 10 has been obtained with our fabricated offset quadrature phase-shift keying modules due to the inductive link optimization presented in this paper. Index Terms—Biomedical communication, biomedical implant, inductive link, power transfer, quadrature amplitude modulation. I. INTRODUCTION C LASSICALLY, biomedical implants depending on induc- tive links for power and bidirectionnal data transfer have been designed with only one link. This link had to be shared to meet often contradictory requirements and a general compro- mise had to be made. A typical example is bandwidth which needs to be widened for data transfer but narrowed for efficient power delivery. Back telemetry could also be achieved using load-shift keying (LSK), which allows the external controller to sense variations of its coil current. This modulation scheme is slow (100 kb/s) and problematic with complex implants where the overall current is not constant. Within recent years, people have begun to push the down- link data rates above the 2.5-Mb/s [1] threshold. Back telemetry, however, attracted much less attention from researchers so far, until they introduced the possibility of using RF for this pur- pose [1], making their system a hybrid RF and dual-band induc- tive link. Others are proposing dual-band inductive links [2], [3], keeping load-shift keying (LSK) as a possible feedback path. We follow this avenue and separate power from data, the later being itself split into two more links to allow for a good crosstalk-free Manuscript received September 16, 2009; revised November 17, 2009. First published February 05, 2010; current version published May 26, 2010. This work was supported in part by the Canada Research Chair on Smart Medical De- vices, in part by the NSERC research grants, in part by an NSERC ES M Grad- uate Scholarships Award, and in part by an ReSMiQ Scholarship. This paper was recommended by Associate Editor T. Constandinou. G. Simard and M. Sawan are with the Polystim Neurotechnologies Labora- tory, Ecole Polytechnique de Montreal, Montreal, QC H3T IJ4, Canada (e-mail: guillaume.simard@polymtl.ca). D. Massicotte is with the Department of Electrical Engineering, University of Quebec at Trois-Rivières, Trois-Rivières, QC G9A 5H7, Canada. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBCAS.2009.2039212 full-duplex bidirectional transfer. The novelty is that these three links are inductive in nature and independent from each other. Although [1]–[3] present the orthogonal geometry for their dual-band inductive link, none have made an attempt at back- telemetry through a third inductive link. The coaxial geometry, which is a special case of the coplanar geometry where coils share a common centroid, is also presented by [2]. In the next section, we compare the orthogonal geometry with our proposed coplanar geometry and show the latter is better for cortical im- plants. Polystim Neurotechnologies Laboratory’s work has pro- duced a high-speed bidirectional transceiver using the offset quadrature phase-shift keying (OQPSK) modulation scheme that can be used for bidirectional transmission of information [4], [5]. The chip has been successfully tested experimentally through our new multiple carrier inductive link presented in [6]. See (1) at the bottom of the next page. Fig. 1 shows the block diagram of our system. The double layer between each pair’s coils represents skin and is not to be confused with the symbol for an iron-core transformer. Coils 1–2 form the power transfer pair, coils 3–4 form the downlink data path (outside to inside), and coils 5–6 form the uplink data path (inside to outside). The external controller is responsible for processing data from the implant, supplying power and pre- processed data from the outside world (i.e., sound, video, etc.). phase-shift keying (PSK)-based modulators and demodulators allow the internal control unit to communicate sensor readings and activate electrical stimulators on demand, giving extra flex- ibility to the system. Our main contributions to this paper are as follows. 1) We propose an efficient power transfer and higher full-du- plex data rate allowed by the minimization of crosstalk, a problem present in most multiple carrier-based systems. 2) We compare the orthogonal geometry with our proposed coplanar geometry and show the latter is better for cortical implants. 3) We report the characterization of our fabricated coils, the efficiency of the inductive link for power transfer, and a strong improvement in the data rate of our OQPSK module allowed by our theoretical analysis of the propagation of phase shifts through an inductive link. We first present the topology of our inductive link and the- oretical predictions on its sensitivity to lateral displacements in Section III. Section II develops the theory behind inductive links in general and then targets the efficiency of power transfer and presents guidance to increased speed in the PSK data links. Fi- nally, we present experimental results comparing theory to mea- surements in Section V. 1932-4545/$26.00 © 2010 IEEE