## 1 Introduction
### Preliminary Work  
Up to now, standardized devices in radio frequency identification (RFID) and near-field communication (NFC) are limited to data rates of up to 848 kBit/s when using load modulation. Commercial devices implementing custom protocols with active signal generation achieve up to 3.4 MBit/s, but at the cost of extended energy consumption (approx. 20 mW in receive mode) [11]. Due to the lack of commercial NFC technologies complementing the presented biomedical sensor systems with high data throughput, a series of research efforts have been taken to yield solutions with both high communication speed and low energy consumption:  
[PERSON] et al. were among the first to increase the data rate of load modulation systems: a near-field interface without simultaneous power transfer capabilities was implemented by custom CMOS circuits operating at a carrier frequency of 25 MHz. The implant circuits incorporated a simple switch-based transmitter for impedance modulation with a power consumption of 0.1 mW. At the reader side, a custom PLL-based clock and data recovery unit extracted the transmitted data. With these characteristics, an uplink data rate (from implant to reader unit) of 2.8 MBit/s at a bit error rate (BER) of smaller than \(10^{-6}\) was demonstrated [12]. Remaining limitations are the relatively small communication distance as a consequence of the load modulation principle, as well as the susceptibility to interference from an additional energy link. Active signal generation concepts were improved by [PERSON] et al., who used custom CMOS circuits with a modified Costas loop receiver to implement a 4-PSK modulation on the inductive link and an arrangement of six coils in total to allow for simultaneous energy and data transfer. The data link was capable of achieving 4.18 MBit/s with a BER smaller than \(10^{-5}\) at a carrier frequency of 13.56 MHz [13]. Complementing the energy interface, which was realized in the form of two planar spiral coils, separated data coils are available for both uplink and downlink. The presented data coil topologies either imply a large spatial separation from the energy coil or require the coil axes to be tilted by \(90^{\circ}\), in order to minimize interference of energy and data link. Still, the decoupling of energy and data interface is only valid for perfect alignment of implant and reader. Along with a proposed very high bit rate (VHBR) extension to the ISO-14443 standard, [PERSON] et al. from _NXP Semiconductors_ developed a 16-PSK receiver based on a time-to-digital converter, supporting a data rate of up to 13.56 MBit/s with a BER below \(2.4\times 10^{-4}\)[14]. The analog front-end, including a phased-locked loop and a time-to-digital converter, consumes 0.1 mW;  
Figure 1: Physical and electrical representations of a near-field system. this excludes the data processing and symbol recovery, which was outsourced to an FPGA circuit of unknown energy consumption. Another concept frequently applied in literature is that of pulse-harmonic modulation (PHM). Here, a primary LC resonator is excited by pulses encoding a digital '1', causing a decaying oscillation in both the primary and secondary resonator. The concept was adapted e.g., by [PERSON] et al., who established a 20 MBit/s inductive link at an LC resonance frequency of 66 MHz [15]. To adapt PHM for simultaneous wireless power transfer, the authors investigated a 4-coil system in which the pulses evoked by data communication circuit shift the zero-crossings of the 13.56 MHz energy carrier [16]. With this so-called pulse delay modulation scheme, a data rate of 13.56 MBit/s at a BER of \(4.7\times 10^{-7}\) was reported, which is valid for the downlink from reader to implant and only for certain coil alignments and power levels of the wireless energy transmission.