U.S. patent application number 13/734772 was filed with the patent office on 2013-08-22 for method and apparatus for efficient communication with implantable devices.
This patent application is currently assigned to The Board of Trustees of The Leland Stanford Junior University. The applicant listed for this patent is The Board of Trustees of The Leland Stanford Junior University. Invention is credited to Teresa H. Meng, Daniel Michael Pivonka, Ada Shuk Yan Poon, Anatoly Anatolievich Yakovlev.
Application Number | 20130215979 13/734772 |
Document ID | / |
Family ID | 48982248 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130215979 |
Kind Code |
A1 |
Yakovlev; Anatoly Anatolievich ;
et al. |
August 22, 2013 |
Method and Apparatus for Efficient Communication with Implantable
Devices
Abstract
Described herein are methods of making and using and apparatus
for wirelessly communicating data and providing power, particularly
from a location exterior to a body and to an implantable device
disposed within a body with tissue. The described embodiments
provide apparatus and methods for efficiently transfer data and
power between an external transceiver and an (implanted) biomedical
device. The method is to modulate power carrier, which wirelessly
powers the device, using an asynchronous modulation scheme, such as
amplitude shift keying (ASK) modulation, with minimal modulation
depth in order to not disrupt the power flow. The digital data is
encoded in the pulse width, eliminating the need for
synchronization to the power carrier signal and further minimizing
the power consumption necessary for data transfer. Additionally, a
reverse backscatter method for obtaining data from the implant is
described that has flexible, low power operation.
Inventors: |
Yakovlev; Anatoly Anatolievich;
(Mountain View, CA) ; Pivonka; Daniel Michael;
(Palo Alto, CA) ; Poon; Ada Shuk Yan; (Redwood
City, CA) ; Meng; Teresa H.; (Saratoga, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stanford Junior University; The Board of Trustees of The
Leland |
|
|
US |
|
|
Assignee: |
The Board of Trustees of The Leland
Stanford Junior University
Stanford
CA
|
Family ID: |
48982248 |
Appl. No.: |
13/734772 |
Filed: |
January 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61582980 |
Jan 4, 2012 |
|
|
|
Current U.S.
Class: |
375/256 ;
375/300 |
Current CPC
Class: |
H04B 5/0031 20130101;
H04B 5/0037 20130101; Y02D 30/70 20200801 |
Class at
Publication: |
375/256 ;
375/300 |
International
Class: |
H04B 5/00 20060101
H04B005/00 |
Claims
1. A method for wireless transmission of data and power to an
implantable device disposed within a body that causes varying
transmission characteristics, the method comprising the step of:
providing, from a forward link transmitter exterior of the body, a
power and data signal, the power and data signal including a RF
carrier signal and data encoded on the RF carrier signal; directing
the power and data signal toward the implantable device disposed
within the body; receiving, at the implantable device or devices,
the power and data signal; and processing within the implantable
device or devices the received power and data signal, the
processing including: collecting power required for operation of
the implantable device from the RF carrier signal of the power and
data signal; and decoding the data encoded on the RF carrier
signal, wherein the decoding occurs without synchronizing to the RF
carrier signal.
2. The method according to claim 1 wherein: the step of collecting
power required for operation uses a rectifier; and the step of
decoding uses a decoder to asynchronously decode the data encoded
on the RF carrier signal.
3. The method according to claim 1 wherein the implantable devices
are individually addressable.
4. The method according to claim 2 wherein the step of decoding
uses a detected envelope of the RF carrier signal to asynchronously
decode the data encoded on the RF carrier signal.
5. The method according to claim 4 wherein the data encoded on the
RF carrier signal is encoded during the step of providing using
amplitude shift keying modulation, with the data being encoded in a
pulse width, amplitude, and/or timing.
6. The method according to claim 5 wherein the data encoded on the
RF carrier signal is also encoded with minimal modulation
depth.
7. The method according to claim 5 wherein the data encoded on the
RF carrier signal is encoded as either a digital "0" or a digital
"1",
8. The method according to claim 5 wherein the amplitude shift
keying modulation includes multi-level encoding.
9. The method according to claim 2 wherein the data encoded on the
RF carrier signal is encoded as a symbol.
10. The method according to claim 1 wherein the data provided in
the step of providing includes clock data and other circuit data,
wherein the implantable device further includes a controller that
received the other circuit data and a PLL coupled to the
controller, and further including the steps of: training the PLL
using the clock data; and using the other circuit data in the
controller.
11. The method according to claim 1 wherein the step of decoding
uses a dynamically generated reference level.
12. The method according to claim 10 wherein adjustable reference
level is adjusted continuously or periodically.
13. The method according to claim 2 wherein the data encoded on the
RF carrier signal is encoded during the step of providing using
frequency modulation.
14. The method according to claim 13 wherein the decoding of the
encoded data uses selective filtering of the transmitted
frequencies.
15. The method according to claim 5 wherein the step of providing
changes the pulse width, amplitude, and/or timing to accommodate a
configuration of the decoder.
16. The method according to claim 5 wherein the step of providing
changes a data rate to accommodate a configuration of the
decoder.
17. The method according to claim 5 wherein the step of providing
reduces a data rate in response to the step of collecting obtaining
less power over a period of time.
18. The method according to claim 5 wherein the step of providing
changes a data rate to accommodate the intended purpose of the
device or devices.
19. The method according to claim 1, further including the step of:
providing, from a reverse link transmitter within the implantable
device, a reverse link data signal, encoding the reverse link data
signal by adjusting a load on an antenna that receives the RF
carrier signal, thereby causing a reflected RF carrier signal that
as the reverse link data encoded thereon; and decoding the
reflected RF carrier signal at a location exterior of the body to
asynchronously receive and reconstruct the reverse link data
signal.
20. The method according to claim 19 wherein the device or devices
configure the reflected signal pulse width, amplitude, and/or
timing to accommodate its purpose.
21. The method according to claim 19 wherein the device or devices
configure a data rate to accommodate their intended purpose.
22. A method for amplitude modulation of a high frequency carrier
signal comprising the step of: Switching a variable impedance into
a signal path of the high frequency carrier signal to either divert
energy from a transmitter and/or reflect energy back to the
transmitter, allowing modulation depths of the frequency carrier
signal from 0-100%.
23. The method according to claim 22 wherein the step of switching
uses a transistor to act as a switch and cause an impedance to
vary, with the impedance being adjusted by a voltage applied to one
of the dependent terminals of the transistor.
24. The method according to claim 22, wherein the step of switching
occurs in the transmitter and the energy is diverted from the
transmitter.
25. The method according to claim 22, wherein the step of switching
occurs in a receiver and the energy is reflected back to the
transmitter.
26. The method according to claim 1 wherein, during the step of
directing, the distance between the forward link transmitter and
the body being in the range of carrier wavelength/100 to carrier
wavelength*100.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/582,980 filed Jan. 4, 2012, and is
hereby incorporated by reference.
FIELD OF THE ART
[0002] The embodiments described herein relate to methods of making
and using and apparatus for wirelessly communicating data and
providing power, particularly from a location exterior to a body
and to an implantable device disposed within a body with
tissue.
BACKGROUND
[0003] Implantable devices are known. Most use a battery internally
disposed within the device from which to obtain power. Such devices
requiring a battery that cannot draw additional power, however, are
bulky and as such have limitations associated with them.
[0004] There are also known methods for wirelessly delivering power
to an implantable device that have been proposed. One such
discussion is provided in the patent application entitled "Method
of Making and Using and Apparatus for a Locomotive Micro-Implant
Using Active Electromagnetic Propulsion" filed as U.S. patent
application Ser. No. 12/485,654 on Jun. 16, 2009, which application
is expressly incorporated by reference herein, and priority claimed
thereto,
[0005] There are also communication systems known to allow
transmission of data from the exterior of a body into the
implantable device, as well as allow transmission of data along the
reverse link from the implantable device to the exterior of the
body, which systems all have limitations.
SUMMARY
[0006] Described herein are methods of making and using and
apparatus for wirelessly communicating data and providing power,
particularly from a location exterior to a body and to an
implantable device disposed within a body with tissue.
[0007] The described embodiments provide apparatus and methods for
efficiently transfer data and power between an external transceiver
and an (implanted) biomedical device. The method is to modulate
power carrier, which wirelessly powers the device, using an
asynchronous modulation scheme, such as amplitude shift keying
(ASK) modulation, with minimal modulation depth in order to not
disrupt the power flow. The digital data is encoded in the pulse
width (PW), eliminating the need for synchronization to the carrier
signal and further minimizing the power consumption necessary for
data transfer.
[0008] The combination of data and power in a single signal with
ASK+PW modulation for data transfer to biomedical implantable
devices also allows for an adjustable data rate by changing
integrator gain or time constant. Additionally, the methods and
apparatus described provide a large amount of flexibility for data
transfer, and can operate under a variety of conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] These and other aspects and features will become apparent to
those of ordinary skill in the art upon review of the following
description of specific embodiments of the invention in conjunction
with the accompanying figures, wherein:
[0010] FIG. 1a illustrates a high level diagram of an external
transmitter according to one embodiment;
[0011] FIG. 1b illustrates a high level diagram of an external
transmitter that incorporates a reader for the receiving data from
the device
[0012] FIG. 2a illustrates a high level diagram of a receiver
disposed within the implantable device according to one
embodiment;
[0013] FIG. 2b illustrates a receiver disposed within the
implantable device with an ID block for multi-access when several
devices are used
[0014] FIG. 3a illustrates the usage of amplitude shift keying with
information encoded using pulse width modulation according to an
embodiment;
[0015] FIG. 3b illustrates an embodiment of a high frequency ASK
modulator with variable modulation depth using a bipolar
transistor
[0016] FIG. 4a illustrates an envelope detector and pulse detector
according to one embodiment;
[0017] FIG. 4b illustrates a timing diagram showing operation of
the envelope detector and pulse detector shown in FIG. 4a;
[0018] FIGS. 5(a)-(c) illustrate specific implementation of a
receiver, resulting waveforms, and the general form of amplitude
modulation equations and definition of modulation depth;
[0019] FIG. 6 illustrates the operation of the dynamic reference
generator;
[0020] FIG. 7 illustrates one implementation of a multi-level
decoding circuit;
[0021] FIG. 8. illustrates another implementation of a a
multi-level decoding circuit;
[0022] FIG. 9 illustrates a sample high-level diagram of load
modulation for the reverse data link according to an
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The available power is very limited in autonomous
implantable devices. This imposes strict limits on power budget of
the transceiver components. Decoding data without synchronizing to
the carrier signal could greatly reduce the power requirements of
the transceiver components. Amplitude shift keying (ASK) modulation
in conjunction with pulse width (PW) modulation encoding can be
decoded without carrier synchronization and allows for simple, yet
efficient and robust way to transfer data to an implantable device
by directly modulating the carrier used to power the device. The
demodulation of the data stream is simple and efficient compared to
ASK or other modulation techniques that require synchronization,
primarily because synchronization circuitry usually consists of
phase or delay lock-loops, which can be complex and consume much
power. By encoding the digital "0" and "1" bits with different
pulse width, the demodulator is able to discriminate by comparing
the transmitted energy in the bit. Furthermore, each bit contains
both a transition from high to low and low to high in amplitude,
which allows for digital clock recovery without the need for
synchronization. This clock can be used to latch-in the received
data and drive digital circuitry on chip.
[0024] The key advantage of this modulation technique is the
efficiency of the data receiver and elimination of the need for
synchronization circuitry. The power flow can also have minimum
perturbations by choosing low modulation depth, as opposed to
on-off keying (OOK) modulation. It has a very flexible data rate
defined by the transmitter, and operates robustly as link gain and
other environmental factors change, making it ideal for biomedical
systems.
[0025] High-level diagrams of the transmitter 100 and the receiving
device 200 are shown in FIGS. 1 and 2, respectively. The
transmitter 100 operates with a carrier generated at the desired
frequency by the carrier generator 110, which is then modulated
with data using the modulator 120 and the methods described herein,
and is finally amplified by amplifier 130 to the desired level
before being transmitted via the transmit (Tx) antenna 140. The
receiving device 200 shown in FIG. 2, preferably batteryless (or
synonymously referred to from hereon as passive devices), collects
the power required for operation from the carrier signal using
power receiver 210, and it receives data encoded in the envelope of
the carrier using envelope detector 250 and pulse decoder 260. This
data can be used to directly control the functional elements 220 or
it can be stored in memory 270, for later use by the functional
elements 220. In the case of biomedical implants, the functional
components of the implanted device can range from a drug delivery
system to diagnostic sensors, and these elements can, if desired,
send information back to the transmitter through the optional
reverse link 280, which is described in more detail in the
following sections. For the optional reverse link, the external
reader would need to be more similar to FIG. 1b, wherein
demodulator is included for reverse data link receiving. For other
applications, these functional elements could include RFID
components or any other passive system that makes use of wireless
powering and data transfer. Other systems that utilize some form of
energy storage could also utilize these methods of communication or
some a modified form of them.
[0026] At the transmitter 100, the carrier can be generated with
any number of techniques, and many conventional components can be
used for most frequency ranges of interest. Amplitude shift keying
with information encoded using pulse width modulation also has many
possible implementations, and one potential method is shown in FIG.
3a. With this implementation, the amplitude is modulated by
switching in a variable impedance that creates mismatch, which
reduces the amplitude of the transmitted signal. By adjusting this
impedance, the resulting amplitude can be controlled. The width of
the pulses is controlled by the length of time that the impedance
is switched on. At frequencies in the GHz range, it can be
challenging to precisely control the load impedance. A specific
embodiment of this method is shown in FIG. 3b. The bipolar
transistor in this circuit simultaneously acts as the switch and
the adjustable impedance. The impedance is controlled by adjusting
the voltage at the collector of the BJT, the modulation signal is
fed to the base, and the emitter is tied to ground. This
implementation allows for high-speed modulation of high frequency
signals with a precisely controlled modulation depth. Following
this modulator, the signal is amplified to desired power level for
the intended application. A discussion of choice of carrier
frequency and power levels for different ranges and devices is
given in later sections.
[0027] On the preferably passive device, the incoming signal is
passed to a power receiver, which creates a stable power supply for
the device. In one embodiment, this power received consists of
rectification, reference generation, and regulation circuitry, all
of which have many implementations. The envelope of the incoming
signal is also recovered and passed to a pulse detector which
decodes the data.
[0028] An implementation of the envelope detector 250 and pulse
detector 260 is shown in FIG. 4a, with a timing diagram showing
operation in FIG. 4b. In this method, the envelope is recovered
with rectification circuitry and converted to a digital signal.
This digital signal is then integrated, and the integrated result
is compared to a threshold. For long pulses, the integrated result
will cross the threshold, and for short pulses the integrated
result will not cross the threshold. In this way, the data is
recovered and can be stored in some form of memory if necessary or
processed as need may be.
[0029] With specific reference to FIG. 4b, the envelope and its
average, are fed to the comparator which also acts as an amplifier
that outputs full-swing digital levels. First comparator output is
buffered to produce CLK signal for the rest of the circuitry. Since
data is encoded in pulse duration, first comparator output is
integrated and long pulse durations cause integrator output to
cross the fixed threshold and the second comparator outputs a
digital "1". Short pulses do not provide enough time for integrator
to cross the threshold and thus results in a digital "0", as can be
seen in the diagram. The data can be latched by the falling edge of
the clock.
[0030] This method can operate with minimal modulation depth if the
threshold is set as an average of the envelope. This modulation
depth can be arbitrarily low as long as the envelope can be
extracted, and the preferred-reduction of the transmitted envelope
is in the range of 15-20% during periods when forward link data is
being transmitted. Since the periods during which forward link data
is being transmitted will vary depending on application, in many
instances the overall power loss will be minimal. With this
averaging circuit, which in one embodiment is an RC low pass filter
attached to an envelope detector, the detector could operate at
arbitrary modulation depths. Also, the full-swing amplifier in FIG.
4 could be removed, but there would be additional constraints on
the maximum and minimum envelope amplitudes, digital logic
thresholds, and supply voltage. Alternatively, the RF envelope
could be amplified by low noise amplifier (LNA), implementation of
which should be known to one familiar with this art, to increase
the sensitivity prior to integration and clock recovery.
[0031] With respect to this specific implementation, an envelope
detector 410 is directly connected to a receiving antenna, not
shown, and the impedance matching network, which could be optional,
(shown in FIG. 5 below). In addition to extracting the envelope of
the radio frequency (RF) carrier, the envelope detector 410 may
also detect the average value with a dynamic reference generator as
shown in FIG. 6. In this embodiment of FIG. 6, the envelope signal
and reference signal are generated with RC filters of different
time constants. This particular envelope detector has a faster time
constant to track variations in the RF amplitude, and the dynamic
reference has a slower time constant to average the envelope.
[0032] The same RF carrier signal is passed to the power receiver
210 shown in FIG. 1 and the envelope detector 410 shown in FIG. 4,
and they essentially operate independently. However, the matching
network at the receive antenna has to be designed for the total
load impedance, to which both of these systems contribute. For most
practical applications, this impedance is dominated by the power
receiver 210 shown in FIG. 1, and so very little power is wasted on
envelope detector 410 shown in FIG. 4, though it is important to
consider this in the design and minimize wasted power.
[0033] The envelope is compared to its average and the resulting
waveform is amplified, using a full-swing amplifier 420, to a
full-swing digital signal. The transitions (zero-crossings) of the
envelope with its average are the basis for the obtained clock that
will be used to latch the data in. The resulting waveform is fed to
a resettable integrator 430. The integrator 430 starts integrating
when the envelope rises above its average and stops integrating
when the envelope falls below the average. The output of the
integrator 430 is fed to a comparator 440 which compares the
integrated value to a reference voltage. The duration of
integration determines whether a logical "0" or "1" will be
received. Longer pulses will result in an output of logical "1"
whereas shorter pulses will be interpreted as logical "0". A more
detailed implementation of a receiver is shown in FIGS. 5(a) and
(b) and resulting waveforms are shown in FIG. 5(c). Additionally,
FIG. 5(c) shows general form of amplitude modulation equations and
definition of modulation depth.
[0034] The modulation depth can be chosen arbitrarily low and can
be as high as 100%, as in the case of on-off keying (OOK). The
pulse width encoding can also be an arbitrarily long duration for
logical "1" and short duration for logical "0" or vice versa. Also,
the actual duration of high and low amplitudes is arbitrary and
sets the data rate of the data transmission. The receiver can
accommodate different data rates chosen by configuring its
integration time constants (integrator gain) and thus by changing
the slope of the integrated waveform, or by changing the threshold
voltage to the following comparator. Thus, the data rate can be
made variable and adjustable depending on the required application,
and the implantable device can accommodate such different data
rates. In a preferred embodiment, the implantable device has
several different RC time constants from which one can be selected
and/or changed, thereby allowing modification after the implantable
device is in the body. This selection can be made by appropriately
programming a controller within the functional elements of the
device block 220 shown in FIG. 2, when appropriate data is
received, to make the change to the time constant. The controller
switches in or out additional capacitive loading for the
integrator, which is implemented using a skewed inverter, thereby
changing the time constant or gain of the integrator. To ensure
correct data rate after power on reset (POR) of the controller, the
default data rate is selected, which is known by the external
transmitter. The derived clock can also be either constant period
by adjusting the duty cycle of the PW modulated data or
non-constant period by modulating only the high or low RF amplitude
and not keeping the total pulse width constant. The receiver
circuitry, including the envelope detector and its averaging
circuitry, the comparators and integrator can be implemented in
many different ways.
[0035] With respect to the forward data link created by the
transmitter 100 shown in FIG. 1 and received at the receiving
device 200, other modifications can also be advantageous.
[0036] Firstly, multi-level encoding, which allows for the transfer
of bits or symbols representing multiple bits of information, can
be provided. Symbols can be encoded in pulse durations, pulse
amplitude, pulse timings, or any combination of these. In one
embodiment, instead of encoding digital "0" and "1" into pulse
width duration followed by negative transition, digital "0" and
digital "1" can be encoded into negative and positive envelope
transitions of equal pulse duration. This allows for constant
period clock transmission and thus training of a PLL if one is
required for an on-chip clock without use of preamble (which clock
is being used to control and/or synchronize with circuits and
signals other than the asynchronous data that is decoded from the
RF carrier signal. With this multi-level encoding, each bit has
equal duration resulting in a constant data rate. Pulses for each
bit can be arbitrarily short and both "0" and "1" takes equal
amount of time to transmit, and this eases decoding because the
dynamic reference setting becomes a simple averaging of the
envelope and thus simplifies threshold detection. An additional
advantage is that average envelope becomes constant for equal
probability of "0"s and "1"s transmitted and thus average power
transferred is also constant, making it easier to estimate how much
power is being transmitted for SAR or other regulations
compliance
[0037] A specific implementation of a multi-level decoding circuit
is illustrated in FIG. 7. RC filter can be used to average the
envelope and obtain reference voltage V.sub.refm. A resistive
divider can then be used to shift the reference voltage down
compared to original signal and obtain V.sub.reflow. Furthermore,
another resistive divider can be used to shift down the envelope
waveform and obtain V.sub.envlow. These signals can then be used to
recover clock and data as described earlier.
[0038] Another implementation of a multi-level decoding circuit is
illustrated in FIG. 8. Another way to generate necessary voltages
from the envelope V.sub.env is to use RC low pass filter again to
obtain average of the envelope V.sub.ref. Further, two resistors
can be tied in series to a higher potential than V.sub.ref, such as
supply voltage V.sub.dd and output taken from the middle of the two
as shown in figure to obtain V.sub.ref+d voltage. Similarly, in
order to obtain V.sub.ref-d voltage, two resistors in series can be
tied to a lower potential than V.sub.ref, such as chip ground and
output can be taken from the middle to obtain V.sub.ref-d voltage,
as can be seen in figure. These signals can then be used to recover
clock and data as per earlier description and as can be seen in the
figures.
[0039] Another embodiment uses frequency modulation rather than
amplitude phase shift keying. In this embodiment frequency shifting
can be used, which advantageously does not perturb the amplitude of
the RF carrier signal, but which does require additional matching
and/or filtering circuitry and the like in order to decode the
frequency encoded data signal, which circuitry can be complex and
consume substantial power. For frequency modulation on the
transmitter side, two frequency generators which represent
different bits can be switched in and out as power carriers. On the
demodulator side, a filter that is selective of only one of the
transmitted frequencies can be employed to recover either the
presence or lack of carrier and thus decoding either a digital "1"
or a "0".
[0040] Configurability of the previously described embodiments of
the forward data link is a significant benefit achieved by the
embodiments described. Some aspects that are configurable include
the carrier frequency, the modulation depth and frequency, the
pulse width, the pulse amplitude, and the pulse timing. Adjusting
these parameters allows for variable data rates and operation with
lower or higher available power as dictated by the environment and
application. Another parameter that can either be fixed or
adjustable is device identification (ID) number for individual
addressability, similar to device shown in FIG. 2b, wherein the
device includes some form of ID block for multi-access for
accommodation of multiple devices. In one embodiment, the ID can be
implemented using re-writable non-volatile memory. In a different
embodiment, the ID can be programmed using wirebonding or flip-chip
configuration by utilizing different connection of ID inputs to
either supply voltage (VDD) or ground.
[0041] The reverse data link 280 from the implantable device to the
external reader, if used, can be implemented in many different ways
ranging from complex and power intensive methods to simple and
low-power solutions. A brute force solution would be a dedicated
transmitter with a local oscillator that transmits data to the
external reader. This approach allows for full-duplex communication
at the cost of high complexity and power consumption. A lower
power, more simple solution uses load modulation, which is
modulation of the load impedance as seen by the antenna. Modulating
this load reflects energy that can be detected by the transmit
antenna or by a different receive antenna or multiple antennas
disposed exterior to the body. For power-limited devices, load and
backscatter modulation for the reverse link are more practical
solutions. The load can be modulated by changing either the
resistivity, reactivity, or some combination of the two. Depending
on how the load is varied and the link transfer function between
the antennas, the phase and/or amplitude of the carrier will be
modulated. In particular with respect to modulation of the load, as
shown in FIG. 2, the reverse data transfer block 280 may include a
reverse link modulator and pulse generator, which preferably
receives an encoded reverse link data stream and an internal clock
of the implantable device. The reverse link modulator and pulse
generator will cause variations to the load on the antenna while
the RF carrier is being received without any forward link data
encoded thereon. These load variations can be used to to provide
load shift keying backscattered modulation of the RF carrier.
Depending on the particular application, how the load is modulated
can be varied in dependence thereon, including prior to any device
being used, or even after a device is in use. Once a device is in
use, modification can be made in a similar manner to that described
previously with respect to modification of the RC time constant for
an implantable device previously. In one embodiment, the preferred
load can be selected using a multiplexor implemented using
passgates and the modulation of the load that is seen by the
antenna can be done using another passgate that is connected
between selected load and antenna. In one embodiment with a
particular antenna, implanted devices generally performed best with
capacitive loading, while devices in air generally performed better
with inductive loading. Therefore, the possibility of having
multiple different loads, either on-chip or off-chip, for load
modulation can be beneficial for accommodation of different tissue
conditions (due to different permittivity), different antennas,
different encapsulations, or overall for varying environmental
conditions. These different loads can also be selected to trade off
between reflected power (which perturbs power delivery to the
implant), and data signal quality which is transmitted to the
external reader to ensure its readability. One such case could be
to compensate for motion artifacts while a person is breathing and
separation between implantable device and external antenna is
changing. Also, these different loads can be used for multi-level
encoding because on the external receiver side different loads
would correspond to different pulses due to variation of amplitude
and phase by different loads.
[0042] A sample high-level diagram of load modulation for the
reverse data link is shown in FIG. 9. In the figure, variable load
impedance is switched on and off by the data stream on the device
and produces different antenna termination for the different bits.
In connection with the variable load impedance, it is advantageous
to include within the reverse link data stream a preamble that the
external receiver can use to allow the external receiver to decode
the reverse link data stream, which, similar to the forward link,
is preferably transmitted asynchronously. It is generally not
optimal, however, to reflect large amounts of energy back to the
external reader because it interferes with the power delivered to
the device, and so in certain applications phase modulation may be
advantageous, as was described in the paragraph above.
Additionally, load modulation can be easily combined with
continuous-time sensing and processing that was recently described
by Schell and Tsividis in "A Continuous-Time ADC/DSP/DAC System
With No Clock and With Activity-Dependent Power Dissipation,"
Solid-State Circuits, IEEE Journal of, vol. 43, no. 11, pp.
2472-2481, November 2008. They demonstrated a system that converts
an analog waveform into a digital representation without a constant
sampling interval. Not only does this eliminate the need for clock
generation, but it also saves power because it does not need to
sample the input signal when there is no activity. This is
perfectly suitable for biological waveforms as they tend to have
long periods of little or no activity. The modulator controlled by
the event-driven sampling only transmits data during physiological
activity and will save energy during idle periods. Since the
forward link is asynchronous and does not require a clock, the
event-driven sampling with load modulation is well-suited for the
reverse data link as it also does not require a clock.
[0043] The range of communication with the device depends on the
carrier frequency, the transmitted power, and the intended
application of the device. These ranges typically vary from a
millimeter to a meter, though a preferred set of ranges is carrier
wavelength/100 to carrier wavelength*100. For biomedical implants,
there are safety considerations with transmitting RF power into the
body, and these requirements are associated with the amount of
energy absorbed by the tissue. This absorption varies with
frequency, and so the frequency must be carefully chosen to suit
the application. Lower frequencies tend to have greater penetration
through the body with less absorption, but also tend to be
inefficient as implantable devices become very small due to antenna
inefficiencies. High frequencies are absorbed more strongly by the
tissue, but for small implants they are necessary because both the
receive antennas are smaller and power transfer efficiency is
higher. Different applications will also have different power
requirements, and so the carrier must be chosen to accommodate the
power budget for the intended purpose, which specifically include
transcutaneous powering and data transfer for implantable
biomedical devices body area network (BAN). For other passive
devices that are not intended to be implanted such as near field
communications (NFC) and radio frequency identification (RFID)
tags, there is different set of considerations and limitations. The
frequency and power levels can be chosen to be compatible with
existing standards and regulations. Also, different privacy
measures and encoding schemes can be implemented with the existing
data link to make the link more robust and secure. However, the
modulation and data transfer method described can operate over a
wide range of frequencies and power levels, and so it should be
able to robustly accommodate the different needs of these types of
devices.
[0044] The preferred embodiment operates at 2 GHz (typical for
small implantable devices), and RFID frequencies tend to be near
900 MHz. However, modulation at the impedance at any frequency, and
recovery is straightforward, and as such, the present embodiments
can operate from the kHz range to the mid-GHz range, and this
choice would be made based on the application. It is noted,
however, that there is a limitation at the rectifier by the
switching speed of the transistors, but this is inherent to the
technology. Also, the carrier frequency should be higher than the
modulation frequency in order to properly receive data. This sets
the lower bound for the carrier frequency if the application
requires certain data rate. Alternatively, lower carrier
frequencies limit the maximum data rate if the application requires
a certain carrier frequency.
[0045] The typical lengths/powers are also set by the specific
application, and this data transmission method could be applied for
these applications with minimal impact on power transfer.
Implantable devices tend to be shorter range because the tissue
absorbs so much power, while RFID systems can be longer range
because they transmit through air. The exact choice of frequency
depends on many things and influences the size of the device,
distance (depending on transmission medium), and the resulting
efficiency of the transfer. It also must comply with the
regulations associated with the frequency band, which can force
devices to operate in certain ranges. These can be determined based
upon the teachings described herein.
[0046] In a specific configuration that uses as the functional
device elements 220 illustrated in FIG. 2 a controller to receive
the forward link data, auxiliary circuits controlled by the
controller, and also a PLL that provides a clock used by the
controller and other auxiliary circuits, and in which the
controller can also optionally configure the reverse link data
stream, the forward link data provided can preferably include both
clock data and other circuit data. In this configuration, training
the PLL can occur using the clock data that is transmitted, and
other circuit data can then be provided for use by the controller,
which controller is then more functional given the PLL having been
trained. Training of the PLL can be done once or periodically as
the delivered power to the implant, aging of the device, drift, and
other conditions can cause the on-chip clock to deviate from its
trained clock period. Therefore, each forward data stream can be
potentially used to retrain the PLL if there may be a need for
this. This also allows for the implantable device not to have a
crystal oscillator or any other sort of precision clock generation
which can be expensive, power hungry, or bulky. The external
transceiver can have these components due to fewer restrictions on
power budget, size, weight, and price.
[0047] In use, the implantable device can be packaged with epoxies,
plastics, or other materials that cover the receive antenna and
isolate circuitry both physically and electrically, though it
should be transparent at the frequency of operation for the best
performance. These epoxies are readily available and there are even
bio-compatible versions. While there will be some minimal loss
through this material, for most applications it is insignificant,
especially when transmitting through tissue.
[0048] Although the embodiments have been particularly described
with reference to embodiments thereof, it should be readily
apparent to those of ordinary skill in the art that various
changes, modifications and substitutes are intended within the form
and details thereof, without departing from the spirit and scope
intended. Accordingly, it will be appreciated that in numerous
instances some features will be employed without a corresponding
use of other features. Further, those skilled in the art will
understand that variations can be made in the number and
arrangement of components illustrated in the above figures.
* * * * *