Amplitude-Engraving Modulation (AEM) Scheme for Simultaneous Power and High-Rate Data Telemetry to

Biomedical ImplantsReza Erfani1, and Amir M. Sodagar1,2

1 Research Laboratory for Integrated Circuits and Systems (ICAS), EE Department, K.N. Toosi University of Technology, Tehran, Iran

2 Polytechnique Montréal, Montréal, Quebec, Canada Erfani@ee.kntu.ac.ir, amsodagar@ieee.org

Abstract—This paper proposes pulse-polarity encoding (PPE) followed by a new modulation technique, called Amplitude-Engraving Modulation (AEM), for short-range data and power telemetry to biomedical implants. The proposed approach is used to simultaneously transfer both power and high-rate data through a 3-contact capacitive link. Key advantage of the proposed modulation scheme lies in the fact that the rate of the data being telemetered is independent from the power carrier frequency, which makes it a proper candidate for high-density micro-stimulation biomedical implants. Simple circuit implementation of the power, data, and clock retrieval circuitry on the implant side is another major advantage for the proposed approach, which leads to extremely low power consumption on the implant. A proof-of-concept prototype setup was developed to verify the idea presented in this paper and carry out preliminary experimental results.

I. INTRODUCTION Ever increasing demand for the development of

implantable biomedical devices continually pushes forward the edge of knowledge and technology for various aspects of such devices. Power telemetry to biomedical implants as well as bidirectional data exchange between these devices and the external world are of interest to researchers in this field (Fig. 1).

Complying with the constraints enforced by the standards developed for the safety of living tissue being exposed to radio frequency (RF) electromagnetic energy (such as [1]), design of wireless links for the transfer of power and high-rate data to implantable systems aiming at high-density stimulation (e.g., visual prostheses [2]-[2]) will be extremely challenging. In such applications, while high carrier frequencies are preferred for high-speed data transfer, maximum permissible safe power transfer occurs at low carrier frequencies. To overcome this conflict, a variety of approaches has been proposed. There have been some efforts in trying to keep the carrier frequency reasonably low and at the same time modulating data with the highest possible rate. From among the traditional digital modulation schemes, wide-band frequency-shift-keying (FSK) [3] and phase-shift-keying (PSK) [5]-[5] have been successful

in providing data-rate-to-carrier-frequency (DRCF) ratios of up to 67% and 100%, respectively.

Pulse-based approaches such as impulse radio ultra-wideband (IR-UWB) techniques have been introduced as successful candidates for high-rate data transfer from biomedical implants as well as in wireless body area networks (WBAN) [7]-[8]. For the UWB technique, simple and all-digital circuit implementation and also ultra-low power consumption of the transmitter is achieved at the expense of the complexity and rather high power consumption of the receiver [9]-[10]. Recently pulse-based techniques such as pulse harmonic modulation (PHM) [11] and pulse delay modulation (PDM) [12] have been reported for forward telemetry to biomedical implants. Major weakness of pulse-based modulation techniques for power telemetry to implantable microsystems lies in the fact that they are either carrier-less or have carrier but with a very low duty cycle. As a result, these approaches do not have the potential of carrying enough electric energy to power up the implant.

This paper proposes a novel modulation technique, which allows for both power transfer using a rather low-frequency sinusoidal carrier and data transfer with pulse-based UWB scheme. From among the key advantages of the proposed technique one can name the simple, reliable and ultra-low-

Fig.1: Wireless interfacing in biomedical implants

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power data demodulation, clock recovery, and power retrieval on the implant side.

II. PROPOSED IDEA The modulation technique proposed in this work combines

the traditional sinusoidal-based modulation for RF wireless energy transfer with the pulse-based UWB modulation techniques for low-energy wireless data transfer.

A. Pulse-Polarity Encoding (PPE) To realize the proposed idea, as illustrated in Fig. 2, first,

pulse-polarity encoding (PPE)1 is used here as a kind of return-to-zero (RZ) encoding. In the PPE scheme, each data bit is represented by a narrow pulse (referred to as a spike), whose polarity is determined according to the bit value: a negative pulse for a ‘0’ and a positive pulse for a ‘1’. This way, not only a synchronized clock is embedded in the data stream, but also each data bit is represented by a pulse which is short enough to allow for high-rate data transfer.

B. Amplitude-Engraving Modulation (AEM) The PPE data (VD) is then superposed on a differential

1 This method of encoding is also called Bipolar Encoding in the literature

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sinusoidal carrier pair (+VC/2 and –VC/2) to realize a new modulation scheme proposed in this work, named: Amplitude-Engraving Modulation (AEM), hereafter. The AEM signal pairs (shown in Fig.2 as X1 and X2) can therefore be written as:

Where AM and ωC are the amplitude and frequency of the differential power carrier signals, respectively. The power carrier sinusoids can be thought of as the differential-mode components and the PPE data pulse stream can be considered as the common-mode component of the AEM signals.

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Fig.2: Basic idea of AEM in time domain

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General frequency spectrum of a typical AEM signal is shown in Fig. 3 for the case of a 20-MHz data pulse stream with a pulse width of 1/80MHz = 12.5ns.

A major advantage of the AEM scheme is that, unlike the traditional carrier-based digital modulation approaches, bit rate of the data can theoretically go far beyond the carrier frequency, resulting in extremely high DRCF ratios. Therefore, the AEM modulation technique allows for tissue-safe power telemetry at a rather low carrier frequency and at the same time extremely high-rate data telemetry.

C. The Wireless Link As illustrated in Fig. 4, a 3-contact capacitive link is used

for the wireless transcutaneous transfer of the AEM signals, X1 and X2. One of the key reasons for using this type of wireless link is that it allows for transferring the two carrier signals (which are of the same frequency but the different phase shift) with no interference, which is never possible using the inductive coupling approach. The high-pass nature of the capacitive link also makes it a more appropriate medium (compared with its inductive counterpart) for the transfer of data ‘spikes’ of the AEM signals.

D. Power, Data, and Clock Retrieval Retrieval of the power, data, and clock that are

concurrently conveyed to the implant side across the capacitive link using the proposed AEM approach is both simple and easy to implement. Referenced to an internal ground, two signals ( and ) are received on the implant

side, based on which (according to eq.(2)), the data and power can be retrieved by addition and subtraction operations, respectively:

Fig. 5 shows the retrieval of power using a full-wave

bridge rectifier during the first and second half-cycles of the power carrier sinewave, separately. The received AEM signals, and are applied to the rectifier as the input. Delivering these signals in a differential form to the rectifier indeed realizes the subtraction operation in eq. (2.b). Simulated operation of the power retrival block is shown in Fig. 6, exhibiting complete rejection of the data spikes, full-wave rectification, and the generation of a DC supply voltage.

The circuit designed for data detection and clock recovery is shown in Fig. 7. To both reject the sinusoidal carriers and extract the stream of bipolar data pulses, V’D, from the

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received AEM signals, a simple passive common-mode sensing circuit, comprising the impedances Z1 and Z2, is used. A Symbol Detection block is then used to generate two streams of binary pulses with logically recognizable HIGH/LOW levels (Symb+ and Symb-) out of the detected bipolar pulse stream. As the simulated waveforms in Fig. 8 show, for each positive spike on the bipolar data pulse stream, V’D, a HIGH pulse appears on Symb+, and like-wise, each negative data spike causes a HIGH pulse on Symb-. By XORing Symb+ and Symb- a periodic clock signal is recovered. Applying the recovered clock signal and Symb+ to a D-type flip flop as shown in Fig. 7, the telemetered data bits are simply recovered (Fig. 8).

III. EXPERIMENTAL RESULTS To verify and evaluate the operation of the idea presented

in this paper, a proof-of-concept prototype was developed using off-the-shelf components. Two 200-kHz, 10-V differential sine waves were used to carry electric power, and a 2-Mbps train of bipolar 100-ns data spikes was considered as the data stream. Fig. 9 shows the oscilloscope screen shots exhibiting the original data and clock in TX side, differential AEM signal pair and detection of the PPE data spikes train on the receiver side followed by the retrieval of the data and recovery of a synchronized clock.

IV. CONCLUSION A new modulation technique, called Amplitude-Engraving

Modulation (AEM), for short-range data and power telemetry to biomedical implants is presented. AEM combines the

traditional sinusoidal-based modulation for RF wireless energy transfer with the pulse-based UWB modulation techniques for low-energy wireless data transfer. By using pulse-polarity encoding (PPE) here as a kind of return-to-zero (RZ) encoding, not only a synchronized clock is embedded in the PPE data stream, but also each data bit is represented by a pulse which is short enough to allow for high-rate data transfer. The PPE data is then superposed on a differential sinusoidal carrier pair to realize AEM scheme. Simple circuit implementation of the power, data, and clock retrieval circuitry on the implant side and independent data-rate from the power carrier frequency make it a proper candidate for high-density micro-stimulation biomedical implants. A proof-of-concept prototype setup was developed to verify the idea and carry out preliminary experimental results.

V. ACKNOWLEDGMENT The authors would like to thank Ms. Marefat of the

Research Laboratory for Integrated Circuits and Systems (ICAS), for her helpful comments.

REFERENCES [1] IEEE standards for safety levels with respect to human exposure to

radio frequency electromagnetic fields, 3 kHz to 300 GHz. IEEE Standard C95.1,2005.

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[3] M. Monge, et al., "A fully intraocular 0.0169mm2/pixel 512-channel self-calibrating epiretinal prosthesis in 65nm CMOS," in IEEE Int. Solid-State Circuits Conf. (ISSCC) Dig. of Tech. Papers, 2013, pp.296-297.

[4] M. Ghovanloo, and K. Najafi, “A wideband frequency-shift keying wireless link for inductively powered biomedical implants,” IEEE Trans. on Circuits and Systems I: vol. 51, pp. 2374-2383, Dec. 2004.

[5] F. Asgarian, and A. M. Sodagar, "A carrier-frequency-independent BPSK demodulator with 100% data-rate-to-carrier-frequency ratio," in IEEE Biomed. Circuits and Syst. Conf. (BioCAS), 2010, pp.29-32.

[6] F. Asgarian, and A.M. Sodagar, “A high-data-rate low-power BPSK demodulator and clock recovery circuit for implantable biomedical devices,” in Proc. 4th Int. IEEE/EMBS Conf. Neural Eng., 2009, pp. 407-410.

[7] M. S. Chae, et al., “A 128-channel 6 mW wireless neural recording IC with spike feature extraction and UWB transmitter,” IEEE Trans. on Neural Syst. Rehabil. Eng., vol. 17, no. 4, pp. 312–321, Aug. 2009.

[8] Y. M. Rasit, H. C. Keong, and M. S. Chae, "Wideband communication for implantable and wearable systems," IEEE Trans. on Microw. Theory Tech., vol. 57, no.10, pp. 2597 – 2604, Oct. 2009.

[9] D. D. Wentzloff, and A. P. Chandrakasan, “A 47 pJ/pulse 3.1 to 5 GHz all-digital UWB transmitter in 90 nm CMOS,” in IEEE Int. Solid-State Circuits Conf. (ISSCC) Dig. of Tech. Papers, 2007, pp. 118–119.

[10] A. P. Chandrakasan, et al., "Low-power impulse UWB architectures and circuits," Proc. of the IEEE, vol. 97, no. 2, pp. 332-352, Feb. 2009

[11] F. Inanlou, M. Kiani, and M. Ghovanloo, “A 10.2 Mbps pulse harmonic modulation based transceiver for implantable medical devices,” IEEE J. Solid State Cir. vol. 46, pp.1296-1306, June 2011.

[12] M. Kiani, and M. Ghovanloo, "Pulse delay modulation (PDM) a new wideband data transmission method to implantable medical devices in presence of a power link," In IEEE Biomedical Circuits and Systems Conf. (BioCAS), 2012, pp. 256-259.

[13] V. S. Bagad, and I. A. Dhotre, Data Communication, 1st ed., Technical Publications,2008.

Fig.9: Experimental results of the proof-of-concept prototype

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