An overview of the recent wideband transcutaneous wireless communication techniques <s> C. Carrier Based vs. Pulse Based <s> A high data-rate frequency-shift keying (FSK) modulation protocol, a wideband inductive link, and three demodulator circuits have been developed with a data-rate-to-carrier-frequency ratio of up to 67%. The primary application of this novel FSK modulation/demodulation technique is to send data to inductively powered wireless biomedical implants at data rates in excess of 1 Mbps, using comparable carrier frequencies. This method can also be used in other applications such as radio-frequency identification tags and contactless smartcards by adding a back telemetry link. The inductive link utilizes a series-parallel inductive-capacitance tank combination on the transmitter side to provide more than 5 MHz of bandwidth. The demodulator circuits detect data bits by directly measuring the duration of each received FSK carrier cycle, as well as derive a constant frequency clock, which is used to sample the data bits. One of the demodulator circuits, digital FSK, occupies 0.29 mm/sup 2/ in the AMI 1.5-/spl mu/m, 2M/2P, standard CMOS process, and consumes 0.38 mW at 5 V. This circuit is simulated up to 4 Mbps, and experimentally tested up to 2.5 Mbps with a bit error rate of 10/sup -5/, while receiving a 5/10-MHz FSK carrier signal. It is also used in a wireless implantable neural microstimulation system. <s> BIB001 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> C. Carrier Based vs. Pulse Based <s> A tri-state FSK modulation protocol and demodulator circuit have been developed and explained in this paper for wireless data rates as high as the carrier frequency. This method is used for asynchronous timing of high-rate stimulation pulses in wireless implantable microstimulators to improve the timing resolution of the stimulation pulses from one data-frame period to only one carrier cycle period. The demodulator circuit is used in a 16-site wireless active stimulating microprobe fabricated in the University of Michigan 3-mum, 1-metal, 2-poly, N-epi, BiCMOS process, occupying 1.17 mm2 of the probe active area. The FSK demodulator circuit is simulated up to 2.5 Mega bits per second (b/s) and tested up to 300 kb/s <s> BIB002 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> C. Carrier Based vs. Pulse Based <s> As a component of the RP2009 project, the IMES system has emerged as a strong candidate for extracting naturally-occurring control signals to be used for providing functional control of an upper body artificial limb. In earlier publications, we described various elements of this system as they were being researched and developed. Presently, the system has matured to a level for which it is now appropriate to consider application-specific-integrated circuits (ASIC) that are of a standardized form, and are suitable for clinical deployment of the IMES system. Here we describe one of our emerging ASIC designs that addresses the design challenges of the extracoporal transmitter controller. Although this ASIC is used in the IMES system, it may also be used for any command protocol that requires FSK modulation of a Class E converter. <s> BIB003 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> C. Carrier Based vs. Pulse Based <s> We analyze the performance of wireless data telemetry links for implanted biomedical systems. An experimental realization of a bidirectional half-duplex link that uses near-field inductive coupling between the implanted system and an external transceiver is described. Our system minimizes power consumption in the implanted system by using impedance modulation to transmit high-bandwidth information in the uplink direction, i.e., from the implanted to the external system. We measured a data rate of 2.8 Mbps at a bit error rate (BER) of <10-6 (we could not measure error rates below 10-6 ) and a data rate of 4.0 Mbps at a BER of 10-3. Experimental results also demonstrate data transfer rates up to 300 kbps in the opposite, i.e., downlink direction. We also perform a theoretical analysis of the bit error rate performance. An important effect regarding the asymmetry of rising and falling edges that is inherent to impedance modulation is predicted by theory and confirmed by experiment. The link dissipates 2.5 mW in the external system and only 100 muW in the implanted system, making it among the most power-efficient inductive data links reported. Our link is compatible with FCC regulations on radiated emissions. <s> BIB004 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> C. Carrier Based vs. Pulse Based <s> This paper introduces a new modulation technique, called pulse harmonic modulation (PHM), for wideband, low power data transmission across inductive telemetry links that operate in the near-field domain. The use of sharp and narrow pulses, similar to impulse-radio ultrawideband (IR-UWB) in the far-field domain, leads to significant reduction in the transmitter power consumption. However unlike IR-UWB, where all pulses are the same, in PHM each bit consists of a pattern of pulses with specific time delays and amplitudes, which minimize the intersymbol interference (ISI) across the receiver coil. This helps achieve a high data rate without reducing the inductive link quality factor and selectivity, which are necessary to block interferers. The received signal consists of an oscillation pattern that is amplitude modulated by the amplitude and timing of the successively transmitted pulses to facilitate data demodulation with low bit-error rate (BER). The main application of the PHM is expected to be in the neuroprostheses, such as brain-computer interfaces (BCIs) or cochlear/retinal implants, which need to transfer large volumes of data across the skin. It may also be used in short-range proximity-based digital communications with high-throughput wireless devices. This paper describes the PHM theoretical foundation and demonstrates its operation with a proof-of-concept prototype setup, which achieves a data rate of 5.2 Mbps at 1 cm coil separation with a BER of 10- 6 . <s> BIB005 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> C. Carrier Based vs. Pulse Based <s> A low power wireless transceiver has been presented for near-field data transmission across inductive telemetry links, which operates based on pulse harmonic modulation (PHM). This PHM transceiver uses on-off keying (OOK) of a pattern of pulses to suppress inter-symbol interference (ISI), and its characteristics are suitable for low-power high-bandwidth telemetry in implantable neuroprosthetic devices. To transmit each bit across a pair of high-Q LC-tank circuits, the PHM transmitter generates a string of narrow pulses with specific amplitudes and timing. Each pulse generates a decaying oscillation at the harmonic frequency that the receiver LC-tank is tuned at, which is then superimposed with other oscillations across the receiver at the same frequency, to minimize the ISI. This allows for reaching high data rates without reducing the inductive link quality factor (to extend its bandwidth), which significantly improves the range and selectivity of the link. The PHM receiver architecture is based on non-coherent energy detection with programmable bandwidth and adjustable gain. The PHM transceiver was fabricated in a 0.5- μm standard CMOS process, occupying 1.8 mm2. The transceiver achieved a measured 10.2 Mbps data rate with a bit error rate (BER) of 6.3×10-8 at 1 cm distance using planar implant sized (1 cm2) figure-8 coils. The PHM transmitter power consumption was 345 pJ/bit and 8.85 pJ/bit at 1 cm and zero link distances, respectively. The receiver dissipates 3 mW at 3.3 V supply voltage. <s> BIB006
Designers have devised a wide variety of modulation techniques for transcutaneous data telemetry, many of which are variations of the same methods used in free space communications. Attractive features in modulation methods chosen for this application are robustness against electromagnetic interference and coil misalignments, simple and low power modulation and demodulation circuitry, and high information content per carrier cycle (data rate to carrier frequency ratio, DRCF). For instance, in a method known as phase-coherent Frequency Shift Keying (pcFSK), every bit "1" is transmitted by a carrier cycle at f 1 and every bit "0" is sent by two carrier cycles at f 0 = 2f 1 BIB001 - BIB002 . The result is an FSK carrier with high DRCF ratio, while maintaining the same bit period to facilitate Tx-Rx synchronization. On the Tx side, a multi-resonant class-E converter can switch the carrier frequency based on the serial data bit stream at zero crossings BIB003 . The demodulator circuits on the Rx side detects incoming data bits by directly measuring the duration of each received FSK carrier cycle, and also generates a constant frequency clock, which can be used to sample the recovered data bits BIB001 . Even though modulating a carrier signal provides a robust mean to transfer data across the skin barrier, generation of the carrier signal at a power level that ensures sufficient signal to noise ratio (SNR) at the Rx involves consuming a considerable amount of power at the Tx, which is scarce on the implantable side. Therefore, it is better to keep the source of carrier signal outside of the body if at all possible. This implies that carrier based modulation techniques are more suitable for the downlink, and establishing a wideband transcutaneous data link for applications such as iBCIs that have a heavier load for the uplink is more challenging. One possible solution, which was mentioned earlier, is the LSK. Even though data rates as high as 4 Mb/s have been reported via LSK BIB004 , since this method relies on the power transfer link, its bandwidth is inherently limited for the reasons mentioned in section II.B. Moreover, even though LSK consumes very little power and its implementation is as simple as adding a switch across the Rx power coil  , it blocks the implant received power for the total period of time that the switch is shorted. While reducing the duty cycle can be helpful down to a certain level, there will still be a considerable reduction in the amount of power that can be delivered to the implant at high uplink data rates. An alternative solution, recently proposed in BIB005 and BIB006 , substitutes the carrier signal with a series of sharp and narrow pulses, requiring much less power to generate, which timing and amplitude are carefully selected to reduce the inter-symbol interference (ISI) at the Rx and make it easier to detect the serial data bit stream. This method, which is called Pulse Harmonic Modulation (PHM), takes advantage of the residual oscillation in high quality factor (Q) LCtanks. To transmit each bit "1", the PHM transmitter generates a sharp pulse at the onset of the bit period to initiate an oscillatory response in the Rx high-Q LC-tank, as shown in Fig. 3 . A second pulse is then generated with specific amplitude (P<1) and delay (t D ) with respect to the initial pulse that suppresses the oscillation across the Rx LCtank well before the end of the bit period. No pulses are transmitted for bit "0". This method allows for reaching high data rates without reducing the inductive link quality factor, thus significantly improving the transmission range and selectivity of the data link in rejecting out of band interferes, such as the power carrier.