An overview of the recent wideband transcutaneous wireless communication techniques <s> B. Single Carrier vs. Multi-Carrier <s> This paper presents a novel inductive link for wireless transmission of power and data to biomedical implantable microelectronic devices using multiple carrier frequencies. Achieving higher data bandwidth without compromising the power efficiency is the driving force to use two separate carriers. Two separate pairs of coils have been utilized for inductive power and forward data transmission. One major challenge, however, is to minimize the interference among these carriers especially on the implantable side, where size and power are highly limited. Planar power coils with spiral shape are optimized in geometry to provide maximum coupling coefficient, k. The data coils are designed rectangular in shape and wound across the power coils diameter to be oriented perpendicular to the power coil planes. The goal is to maximize data coils direct coupling, while minimize their cross-coupling with the power coils. The effects of coils geometry, relative distance, and misalignments on the coupling coefficients have been modeled and experimentally measured. <s> BIB001 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> B. Single Carrier vs. Multi-Carrier <s> A 64-site wireless current microstimulator chip (Interestim-2B) and a prototype implant based on the same chip have been developed for neural prosthetic applications. Modular standalone architecture allows up to 32 chips to be individually addressed and operated in parallel to drive up to 2048 stimulating sites. The only off-chip components are a receiver inductive-capac- itive (LC) tank, a capacitive low-pass filter for ripple rejection, and arrays of microelectrodes for interfacing with the neural tissue. The implant receives inductive power up to 50 mW and data at 2.5 Mb/s from a frequency shift keyed (FSK) 5/10 MHZ carrier to generate up to 65 800 stimulus pulses/s. Each Interestim-2B chip contains 16 current drivers with 270 muA full-scale current, 5-bit (32-steps) digital-to-analog converter (DAC) resolution, 100 MOmega output impedance, and a voltage compliance that extends within 150 and 250 mV of the 5 V supply and ground rails, respectively. It can generate any arbitrary current waveform and supports a variety of monopolar and bipolar stimulation protocols. A common analog line provides access to each site potential, and exhausts residual stimulus charges for charge balancing. The chip has site potential measurement and in situ site impedance measurement capabilities, which help its users indicate defective sites or characteristic shifts in chronic stimulations. Interestim-2B chip is fabricated in the AMI 1.5 mum standard complementary metal-oxide-semiconductor (CMOS) process and measures 4.6 x 4.6 x 0.5 mm. The prototype implant size including test connectors is 19 X 14 x 6 mm, which can be shrunk down to <0.5 CC. This paper also summarizes some of the in vitro and in vivo experiments performed using the Interestim-2B prototype implant. <s> BIB002 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> B. Single Carrier vs. Multi-Carrier <s> The next generation of implantable high-power neuroprosthetic devices such as visual prostheses and brain computer interfaces are going to be powered by transcutaneous inductive power links formed between a pair of printed spiral coils (PSC) that are batch-fabricated using micromachining technology. Optimizing the power efficiency of the wireless link is imperative to minimize the size of the external energy source, heating dissipation in the tissue, and interference with other devices. Previous design methodologies for coils made of 1-D filaments are not comprehensive and accurate enough to consider all geometrical aspects of PSCs with planar 3-D conductors as well as design constraints imposed by implantable device application and fabrication technology. We have outlined the theoretical foundation of optimal power transmission efficiency in an inductive link, and combined it with semi-empirical models to predict parasitic components in PSCs. We have used this foundation to devise an iterative PSC design methodology that starts with a set of realistic design constraints and ends with the optimal PSC pair geometries. We have executed this procedure on two design examples at 1 and 5 MHz achieving power transmission efficiencies of 41.2% and 85.8%, respectively, at 10-mm spacing. All results are verified with simulations using a commercial field solver (HFSS) as well as measurements using PSCs fabricated on printed circuit boards. <s> BIB003 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> B. Single Carrier vs. Multi-Carrier <s> A dual-band telemetry, which has different carrier frequencies for power and data signals, is used to maximize both power transfer efficiency and data rate for transcutaneous implants. However, in such a system, the power signal interferes with the data transmission due to the multiple magnetic couplings paths within the inductive coils. Since the power level of the transmitted power signal is significantly larger than that of the data signal, it usually requires a high-order filter to suppress the interference. This paper presents a non-coherent DPSK receiver without a high-order filter that is robust to the interference caused by the power carrier signal. The proposed scheme uses differential demodulation in the analog domain to cancel the interference signal for a dual-band configuration. The data demodulation also uses subsampling to avoid carrier synchronization circuits such as PLLs. The experimental results show that the demodulator can recover 1 and 2 Mb/s data rates at a 20 MHz carrier frequency, and it is able to cancel an interference signal that is 12 dB larger than the data signal without using complex filters. The demodulator is fabricated in a 0.35 mum CMOS process, with a power consumption of 6.2 mW and an active die area of 2.6times1.7mm2. <s> BIB004 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> B. Single Carrier vs. Multi-Carrier <s> We have presented the design methodology along with detailed simulation and measurement results for optimizing a multiband transcutaneous wireless link for high-performance implantable neuroprosthetic devices. We have utilized three individual carrier signals and coil/antenna pairs for power transmission, forward data transmission from outside into the body, and back telemetry in the opposite direction. Power is transmitted at 13.56 MHz through a pair of printed spiral coils (PSCs) facing each other. Two different designs have been evaluated for forward data coils, both of which help to minimize power carrier interference in the received data carrier. One is a pair of perpendicular coils that are wound across the diameter of the power PSCs. The other design is a pair of planar figure-8 coils that are in the same plane as the power PSCs. We have compared the robustness of each design against horizontal misalignments and rotations in different directions. Simulation and measurements are also conducted on a miniature spiral antenna, designed to operate with impulse-radio ultra-wideband (IR-UWB) circuitry for back telemetry. <s> BIB005 </s> An overview of the recent wideband transcutaneous wireless communication techniques <s> B. Single Carrier vs. Multi-Carrier <s> Biomedical implants require wireless power and bidirectional 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 × 10-6 has been obtained with our fabricated offset quadrature phase-shift keying modules due to the inductive link optimization presented in this paper. <s> BIB006
Unlike pacemakers, which can run for years on a primary battery, the power consumption of most neuroprosthetic devices is too high for primary or even rechargeable batteries. The high power consumption is mainly due to large number of stimulation sites and high stimulus rates, regardless of the internal electronics' power consumption. Extreme space limitation at the site of implantation is yet another reason that has so far rendered battery powered cochlear or retinal implants unfeasible. iBCIs which are expected to be implanted under the scalp also face the same limitation. These implantable devices need to be powered via a transcutaneous inductive link, and once the power transfer link is in place, it is quite reasonable to use the same power carrier signal for downlink data transfer  - BIB002 . In some cases the same carrier has also been used for the uplink through impedance modulation, also known as Load Shift Keying (LSK)  . This is the same mechanism that has been implemented in radio frequency identification (RFID) systems for not only powering the passive RFID tags but also establishing a bidirectional data link between the tag and the reader  ,  . The main advantage of using a single carrier for both power and data transmission is the relatively robust coupling between power coils, which can lead to more reliable data transfer. Another advantage is saving space by using the power coils for multiple purposes. On the down side, modulating the power carrier in any form or direction complicates the power Tx circuitry and reduces the power transmission efficiency, which eventually leads to more heat dissipation for the same amount of power delivered to the implant. A more important issue is the low power carrier frequency, which has a strict safety limitation because of its large amplitude. This can further limit the data transfer bandwidth in either direction to levels that are not sufficient for advanced neuroprosthetic devices. As a result, the use of two or three carrier signals for power, downlink, and uplink has been proposed with each carrier having its own pair of coils or antennas in order to decouple the data transfer link bandwidth from the power transmission efficiency BIB001 -  . Aside from the size overhead, the use of multiple carrier signals within a space as small as an implant introduces new challenges, the most important of which is the strong power carrier interference with much weaker data carriers. Several researchers have offered solutions such as using orthogonal symmetrical coils BIB005 , BIB001 , coaxial coils with differential phase shift keying (DPSK) BIB004 ,  , and shifted coplanar coils with offset quadrature phase-shift keying (OQPSK) BIB006 . Nonetheless, the most effective way to reduce interference is to separate out the carrier frequencies and take advantage of the bandpass filtering effect of the LC-tanks at resonance. In the case of the orthogonal coils, a pair of planar spiral coils (PSC), shown in Fig. 2a , which geometries have been optimized based on the power carrier frequency and tissue volume conductor, are used for transcutaneous power transfer BIB003 . A second pair of coils is wound symmetrically across the PSC pair to establish the data transfer link. Orthogonal orientation and symmetry lowers the undesired mutual coupling between the two pairs without affecting the desired coupling within the pairs BIB005 . This will minimize the power carrier interference on the data carrier, which can benefit from any robust modulation technique. There are also other geometries, such as figure-8 shown in Fig. 2b , which can attenuate the effects of external common mode magnetic fields and reduce cross coupling from power coils.