Designing Next-generation Implantable Wireless Telemetry
Deyasini Majumdar
1
, Christian Schlegel
1
, Navid Rezaei
2
and Bruce Cockburn
2
1
Dept. of Electrical and Computer Engineering, Dalhousie University, Halifax, Nova Scotia, B3H 4R2 Canada
2
Dept. of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, T6G2E8 Canada
Keywords:
Low Power Budget, In-vivo Wireless Communications, Body Area Networks, Implantable Telemetry.
Abstract:
Biomedical applications in general, and health monitoring in particular, extensively involve on-body as well
as implantable wireless communications devices to enable viable end-user solutions. While technologies to
wirelessly transmit data from implanted devices have already been reported, they fall short of being able to
support the needs of emerging next-generation biomedical applications. In order to translate state-of-the-
art wireless technologies into solutions fitting body area network applications (BANs), a key challenge to
overcome is the strictly limited power budget. This paper attempts to review design challenges and proposes
a viable solution for wireless telemetry to meet the targets for next-generation BANs.
1 INTRODUCTION
A key to designing and implementing novel next-
generation health care technologies lies in being able
to seamlessly tap vital body parameters that help iden-
tify the cause of physiological anomalies. Mature
state-of-the-art biomedical technologies utilize teth-
ered devices. While these wired systems potentially
enable interfaces between a living organism and an
external machine for data analysis, wired transmis-
sion significantly hinders mobility and ease of use.
The possibility of monitoring undistorted biological
signals using an autonomous device with a low degree
of invasivity forms the basis for exploring wireless
communications for biomedical applications. Some
of the specific applications for which these systems
are targeted include brain-machine interfaces, cortical
and retinal prostheses, amongst others.
Implantable electronics forms a vital component
of the envisioned next-generation BANs using multi-
implant communications. Such networks (Figure-1)
would require implantable, multi-nodal communica-
tion systems that can offer safe as well as reliable
communication between implantable nodes and exter-
nal devices, enabling interoperability between differ-
ent systems. Device autonomy, form factor and ex-
tended lifetime form key features of these potentially
data-intensive communication applications. The de-
sign of a viable implant telemetry mandates intelli-
gent use of the available resources, primarily power.
This paper reviews the current state-of-the-art
Figure 1: Envisioned next-generation Body Area Networks.
technologies and limitations that pose challenges to
designing viable solutions for next-generation wire-
less telemetry, together with the proposed method-
ology to devise a solution that can effectively con-
form to the power budgets for next-generation BANs.
Section-II presents a brief review of state-of-the-
art technologies used to design wireless telemetry
for BANs and their shortfalls. Section-III elabo-
rates the design challenges in devising solutions for
next-generation BANs. Sections IV and V present
our proposed methodology and approach to meet
the projected power budgets for next-generation sys-
tems, respectively. A comparison of the projections
based on our approach with existing wireless solu-
271
Majumdar D., Schlegel C., Rezaei N. and Cockburn B..
Designing Next-generation Implantable Wireless Telemetry.
DOI: 10.5220/0004914802710277
In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2014), pages 271-277
ISBN: 978-989-758-013-0
Copyright
c
2014 SCITEPRESS (Science and Technology Publications, Lda.)
tions for next-generation appears BANs in Section-IV
followed by a conclusion in Section-VII.
2 STATE-OF-THE-ART
Implantable transmitters (TXs) are widely used for
mature biomedical applications such as cardiac pace-
makers, retinal and cortical prostheses, etc. However,
the key differences between these applications and
next-generation devices lie in the application require-
ments. While former applications require data rates
in the order of kbps, future applications need data
rates at least in the range of Mbps. Further, these ap-
plications address point-to-point communications in-
volving single-channel links, where interference from
other operational devices is low in a strictly interfer-
ence limited regime. Novel systems and design ap-
proaches must support high data rate implant commu-
nications within the already stringent power budgets.
Existing technologies (Bluetooth (Group, 2002),
Zigbee (Group, 2011), etc.) fail to provide a platform
for reliable low-power communications between mul-
tiple implant sites, primarily due to the fact that power
requirements for BAN devices can be no higher than
a few tens of mW, and must support data rates on the
order of kb/s to 100 Mb/s (Drude, 2007), while state-
of-the-art technologies require power levels on the or-
der of 20 mW to support data rates of a few tens of
kb/s (one to two orders of magnitude too high). Thus,
in order to translate state-of-the-art wireless technolo-
gies into modern BAN applications, a key challenge
is to address these limited power budgets.
3 DESIGN CHALLENGES
The fundamental design challenge on the amount of
available power is primarily limited by the safe tem-
perature rise in tissue induced by the heat generated
by an implantable device. Pennes bio-heat equation
(Pennes, 1948) stipulates that the total heat accumu-
lation in tissue is a cumulative effect of the thermal
conduction, blood perfusion, radiation, circuit power
consumption and metabolic heating. The allowable
limit should accommodate both the required transmit
power level and the total power consumption in the
circuit, thereby implying that it is imperative to de-
sign highly efficient circuit components to target next-
generation BAN applications, especially those with
high data rate requirements. This upper bound on the
total heat accumulation contributed by circuit power
consumption is also critical in dictating the choice
of the wireless communication scheme adopted for
any given application. Typically to optimize power
expenditure at the system level, higher modulation
schemes such as Quadrature Amplitude Modulation
(QAM) are generally avoided since their required cir-
cuit complexity deters an optimum implantable solu-
tion (J. Abouei and Pasupathy, 2011). Conventionally,
modulation schemes such as Binary Phase-Shift Key-
ing (BPSK), On-Off Keying (OOK) and Frequency-
Shift Keying (FSK) have found widespread use in
wireless implantable solutions requiring data rates up
to a few Mbps, primarily due to their lower power re-
quirements. The need for a more linear Power Ampli-
fier (PA) pulls BPSK back in terms of power require-
ments, although BPSK is 3 dB better than OOK and
FSK. Thus, since the choice of modulation scheme
can be powerful in dictating the final architecture,
and thus the power requirements, transceiver designs
for implantable medical devices generally trade off
spectral-efficiency for energy-efficiency.
Attempting to devise energy-efficient solutions
close to upper bounds stipulated for BAN technolo-
gies at 500 pJ/bits (Drude, 2007) becomes ever more
challenging due to device miniaturization towards de-
sired rice-grain sized form factors. Fully implantable
system for wireless telemetry additionally requires a
signal acquisition unit, power source, implantable an-
tenna as well as increased device functionalities. Fur-
ther, link losses need to be minimized with the choice
of frequency of transmission assuming significant im-
portance. While conventional understanding teaches
that transmission loss in tissue is minimized at lower
frequencies, theoretical and experimental results in
the 1-3 GHz band suggest that this band is a better
(Poon, 2009), due to the fact that magnetic coupling
for short distances of 1-5 cm for small antennas is
strong with negligible dielectric polarization losses;
contrary to E-M transmission modes.
To extend the life of an implanted device, it is
important that a suitable mode of powering be se-
lected based on the power requirements of the de-
vised implant. The power source should be able to
support the power requirements of the complete de-
vice, while ensuring an active device life of at least
a few years. Implantable batteries are usually hard-
wired and sealed hermetically into the final device
during the manufacturing process. The limitations
of currently available battery technologies provide
only limited amounts of power, while being large and
bulky. Additionally, although a number of existing
methods for energy harvesting can provide supple-
mental energy to the implantable batteries, thereby
enhancing the useful life of the implant, given the
power density offered by current technologies and the
fact that most next-generation applications have sig-
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272
nificant power requirements, current harvesting tech-
nologies can only aid in relaxing a power require-
ments of the primary powering source rather than of-
fering an autonomous solution (J. Olivo and Micheli,
2011). Inductive power links can be used, and most of
the existing implant devices use inductive links with
frequencies on the order of a few MHz or less, primar-
ily due to the low absorption rates in tissues at these
frequencies. The primary limitations of these conven-
tional wireless powering technologies, however, are
low efficiency and large form factor. Use of high-
frequency inductive links allows a 100-fold reduction
in the size of the implanted coil (Poon, 2009). How-
ever, high-frequency inductive links also suffer from
low efficiency, in addition to high transmit power
(A. S. Y. Poon and Meng, 2007). A critical issue in
high-frequency inductive coils is the amount of power
received at the implanted secondary coil. It has been
shown that power on the order of a couple hundred
µW can be provided ((Poon, 2009), (A. Yakovlev and
Poon, 2012)), which can possibly be extended to sub-
mW or mW ranges. Such power levels are safe for op-
eration in tissue and and obey specific absorption rate
(SAR) requirements. To power an implanted device
with state-of-the-art high-frequency inductive links it
is imperative to optimize the link. Designing an im-
plantable wireless system within the limits of current
technology therefore mandates a careful review of a
number of factors: link power requirements, modula-
tion, system and circuit design, and system operation.
4 PROPOSED METHODOLOGY
Transmission scenarios primarily dictate the topol-
ogy of the complete link and the architectural design
choices of required electronics and thus the design
methodology. BAN applications target two primary
transmission scenarios: (i) body surface, extending
up to 2 cm above the skin, and (ii) external- up to
5 m (Chavez-Santiago et al., 2013). An evaluation
of approximate link budgets for these topologies re-
veals a need for a two-pronged approach, wherein the
first system would use magnetically coupled antennas
while the second uses RF-based transmission. A key
design objective is thus to enable reliable transmis-
sion with minimum energy requirements.
Use of spread spectrum signaling can not only
aid to enhancing the reliability of the proposed im-
plantable telemetry but can also help in supporting
communication between multiple implantable nodes.
Also, since transmissions in these applications are in-
termittent and erratic, a preamble-based detection sys-
tem suits the application scenario. A low-complexity
transmission architecture can further be supported
by using the proposed asynchronous packet-based
transmission system based on direct-sequence spread-
spectrum modulation (S. Nagaraj and Burnashev,
2009) due to its inherent ability to reliably support
uncoordinated data transmission with low power con-
sumption. The basic modulation scheme is differ-
ential binary phase shift keying (DBPSK) in addi-
tion to spread spectrum. While DBPSK allows for a
simple modulation with superior bit-error-rate (BER)
performance as compared to low-complexity modu-
lation schemes typically used in biomedical applica-
tions (such as on-off keying, frequency shift keying
etc.), the use of spread spectrum signaling offers sys-
tem reliability due to inherent robustness to interfer-
ence and multipath propagation, in addition to en-
abling scalability to support multi-implant communi-
cations. The proposed scheme embeds the transmit
data into a packet structure marked with a preamble
sequence, which when detected by the receiver can
be used to extract timing information for synchro-
nization. This allows for reduced power consumption
since it alleviates the need for continuously operat-
ing a synchronization stage, otherwise typically used
in conventional spread spectrum systems. Addition-
ally, it has been shown in (S. Nagaraj and Burnashev,
2009) that due to differential encoding, the proposed
preamble detection method does not require phase
and frequency offset estimates for channel coherence
time larger than the block duration, thereby, contribut-
ing to a significant reduction in system complexity.
Also, without the need for exact timing information,
a detector operating with two samples per chip essen-
tially performs close to that of a synchronous system
and is robust in frequency-selective channels.
The design methodology is dictated primarily by
the application scenario or link topology. In the case
of body surface transmission, data transfer can typ-
ically be enabled without the need for high transmit
power levels since the amount of loss encountered in
the channel is low for small transmission distances. A
first estimate of link losses at an implantation depth of
2 cm of tissue reveals that losses are about 20-30 dB
less in the 2.4 GHz ISM transmission band as com-
pared to the conventional understanding (N. R. Sar-
choghaei and Schlegel, 2013). It is observed that
to be able to support acceptable transmission using
the proposed communication scheme, the energy re-
quired to transmit every data bit over 2 cm of tissue
is about 64 fJ. Table-1 lists first hand worst-case es-
timates of energy requirements (using upper-bounds
on loss margins) for data transmission through a pro-
posed link for body surface transmission at an im-
plantation depth of 2 cm at low kbps data rates. With
DesigningNext-generationImplantableWirelessTelemetry
273
such link topologies, energy requirements imply that
optimization of the system design is dominated by
the power consumption of the implantable electron-
ics, and hence the need to aggressively reduce system
complexity. A pair of inductive coils can be used to
enable data transfer through magnetic coupling with-
out overly burdening the power consumption of the
embedded electronics. Furthermore, subthreshold de-
sign techniques can be applied to baseband electron-
ics to significantly lower power consumption levels.
Table 1: Approximate link budgets for proposed transmis-
sion scheme to achieve P(miss)=10
6
+ P(false)=10
3
in
the 2.4 GHz ISM band; GA - Antenna Gain, RX - Receiver.
Link Parameters MI RF
Required
E
b
N
o
(dB) 12 12
Receiver Noise Figure (dB) 3 3
Noise Power (dBm/Hz) -140 -108
TX and RX GAs (dBi) - 0
RX Sensitivity (dBm) -128 -96
Implant depth (cm) 2 0.2-0.7
Tissue Path Loss (dB) -30 -(7-17)
Free Space Path (m) - 1-5
Free Space Path Loss (dB) - -(40-53)
Fade Margin + Excess Loss -25 -2
(in dB) (Merli, 2011)
Man-made Noise Margin (dB) - -25
Transmit Power (dBm) -73 -22/1
Data Rate (kbps) 1.25 2000
Energy per bit (pJ/bit) 0.1 3-600
Data transmission to nodes external to implants
located at 2 cm beyond skin surface is estimated to re-
quire significant transmit power levels to address the
increased path loss encountered in such topologies.
Hence, the proposal to use RF transmission, which re-
quires antennas that generate both electrical and mag-
netic field components. A first estimate of link losses
at an implantation depth of 0.2-0.7 cm of tissue and
an over-the-air transmission up to 5 m shows that to
be able to support acceptable transmission (Table-1)
the energy required to transmit a data bit from the
proposed implanted TX is 3-600 pJ and the required
transmit power level is -22 to 1 dBm at 2 Mbps.
With the given definition of the BB section, up-
conversion of the BB signal in the RF TX calls for
a suitable architecture requiring a phase-locked loop
(PLL), voltage-controlled oscillator (VCO), mixer
and power amplifier (PA). The primary design con-
straint stems from the efficiency of the PA. A linear
PA theoretically offers an efficiency of 50%, while
realistically achievable efficiencies are about 20-40%.
Use of a non-linear PA is possible only when the sig-
nal has a constant envelope. This again points to a
careful selection of the modulation scheme. Antenna
efficiency and thus the choice of antenna also impacts
the total power that can be transmitted. Thus, a criti-
cal design issue for the RF section is to balance device
size, circuit power consumption and link efficiency.
While the basic system definition and adopted
implementation architecture contributes to significant
power-savings, implementation using shorter gate
lengths with low-power techniques can further con-
tribute to shrinkage in power consumption. Energy-
efficiency (EE) can be further enhanced by using
duty-cycled operation, and such systems can then be
effectively operated near their high-efficiency modes.
5 FITTING POWER BUDGETS
The envisioned design roadmap towards viable low-
power implantable wireless telemetry primarily re-
quires adaptation of the basic design to shrink the fun-
damental power requirements, such that the device
can be deemed fit for next-generation BANs. In an
effort to design a viable energy-efficient implantable
wireless telemetry system using our methodology, a
previous first-generation prototype (GEN-I) included
the implementation of the baseband section in IBMs
130 nm CMOS process with an EE of 1.2 nJ/bit. Gen-
II implementation involves fabrication of both the
baseband and the RF TX sections in Taiwan Semicon-
ductor Microelectronic Corporation
´
s (TSMC
´
s) 65-nm
technology. Experimental evaluation of fabricated
Gen-II ICs is currently underway and is expected see
completion by end-2013.
The estimated energy/bit requirements of the TX
core in the BB section of the proposed wireless com-
munications device for data transmission to 2 cm be-
yond body surface is about 118 pJ/bit for operation
at nominal voltages in TSMCs 65nm CMOS technol-
ogy. A comparison of the link budget with the en-
ergy requirements of the GEN-II prototype TX core,
shows that the design is dominated by the circuit
power consumption. However, it is observed from
preliminary estimations of the proposed subthreshold
implementation of the current TX design that in body
surface transmission scenarios with low data rate re-
quirements, circuit power would not form the primary
bottleneck. However, a key design challenge to ad-
dress is enhancement of the supportable data rate at
a minimum threshold voltage. As shown in (Mein-
erzhagen et al., 2011), a suitable combination of im-
plementation technology from among low-V
T
, high-
V
T
and standard-V
T
technologies together with a care-
ful architectural revision can definitely help in achiev-
ing the targets. Therefore, with the target to devise a
high-efficiency BB TX prototype at higher data rates,
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274
we plan to further explore subthreshold implemen-
tation for the next-generation prototype development
for application in body surface transmission scenarios
(Gen-III (a)) with a revision in the BB architecture.
For the case of an RF link for transmission to ex-
ternal nodes, the first-order estimates project that the
power requirements of the baseband packet assembly
and RF sections in the proposed implantable wireless
telemetry in a standard TSMC 65 nm implementation
(Gen-II) are about 1.5 mW each (energy efficiency
1.5 nJ/bit approx.) at 50MCps (data rate 3.125 Mbps)
and the maximum transmit power rating of 0.2 dBm.
The estimated energy/bit requirements of the pro-
posed wireless communications device for data trans-
mission to external nodes projects that power con-
sumption can be progressively reduced over the next-
generation prototypes. While a significant amount of
energy reduction is achieved with implementation of
the modified baseband TX in a smaller technology
node, with the power requirements lowering to 88%
of that for Gen-I (65% is attributed to the reduction of
gate length and supply voltage), power requirements
need to be aggressively scaled down further to make
it comparable to the link budget of the given applica-
tion. The proposed implementation roadmap targets
design optimization as well as burst-mode operation
to achieve the targeted allowable power consumption
metrics for next-generation BANs. The energy sav-
ings in Gen-II can be further reduced in Gen-III(b)
(to 1.2 nJ/bit) with architectural revision (control unit
of BB) and optimization (PA efficiency 50%). To
enable higher power savings, we propose to imple-
ment duty-cycled (50 %) in Gen-IV while progressing
towards the final target stipulated for next-genartion
BAN devices (Gen-V). Figure-2 presents the esti-
mated energy/bit requirements of the proposed wire-
less communications device, projecting that power
consumption can be progressively reduced over the
next-generation prototypes using revised design ap-
proaches at both the gate and system levels.
Proposed targets for the BB design aim to achieve
10pJ/bit or less for all digital processing in final proto-
types, implying that power budget of the implantable
electronics will be dominated by the RF section, with
system operation at the minimum required data rate.
Thus, the RF circuitry is the next limiting factor af-
ter link requirements (Table-2). However, operation
at higher transmit rates can significantly reduce the
EE of RF TX. An increase in required transmit power
levels (P
T X
) increases with data rate but results in in-
creasing overall EE of the telemetry system, thus im-
plying that a fundamental increase in efficiency can be
achieved by running the device at higher data rates.
Hence, operation in burst-mode would enable two-
Figure 2: Proposed evolution of next-generation wireless
communications IC energy consumption with respect to the
allowable targets for BANs, with 0.2 dBm transmit power;
Blue - Energy consumption of BB; Red - RF.
fold energy savings conservation during sleep times
and operation at high-efficiency transmission modes.
To better understand the advantage of supporting
higher P
T X
at high data rates, we also evaluated the
power requirements and energy-efficiencies of the BB
and RF TXs at a rate higher than allowable in the
chosen band and at full P
T X
of the RF TX (Table-2);
a specific transmission scenario with the base station
located at 2m from body surface is considered.
Table 2: Estimated energy requirements of the baseband
and RF TXs (Gen-II) with different transmit rates to a dis-
tance of 2m in free-space.
Chip P
T X
Link Gen-II Gen-II
(Rate) (dBm) Budget EE-BB EE-RF
(MCps) (pJ/bit) (pJ/bit) (pJ/bit)
132 0.2 127 110 545
83.5 -2 127 115 632
50 -4 127 119 844
41.8 -5 127 121 920
32 -6 127 124 1108
At a 132 MCps chip rate (full P
T X
), EE of the RF
section is significantly higher than that at the mini-
mum required transmission rate. Thus, at the max-
imum rating, the transmit rates that can actually be
supported would be higher, thereby reinforcing the
proposed methodology that a burst-mode operation at
a higher efficiency operating node would be the op-
timal solution to address the limitations posed by RF
design. The next challenge is then to reduce the power
requirements of both the BB and RF sections with de-
tailed evaluation of both system-level architectures as
well as gate-level implementations. It is observed that
there is room for significant improvement in the EE
of the BB section with a careful review of system ar-
chitecture and intelligent delegation of system blocks
that can remain external to the implantable device. In
addition, our first-order estimates clearly suggest that
burst-mode operation can further help improve EE.
DesigningNext-generationImplantableWirelessTelemetry
275
6 DISCUSSION
In comparison with existing implementations of im-
plantable wireless telemetry systems with comparable
link topologies in the literature (Maush and Delgado-
Rstituto, 2013), we conclude that our current (Gen-II)
projections are competitive (Table-3). In comparison
to recent TX prototypes with supportable data rates
greater than 1 Mbps, our proposed TX design has a
higher efficiency. A careful review of the existing sys-
tems also shows that although the proposed TX falls
short of some existing TXs in terms of the efficiency,
the former fall behind in terms of supportable data
rates and have low link budgets.
Table 3: Comparison of energy per bit requirements of pro-
posed implantable wireless device with recent implantable
solutions; η - TX efficiency and R
b
-Data rate (Mbps).
Link Energy TX
Design Budget R
b
per bit η
(pJ/bit) (pJ/bit) (%)
Solda11 14 5 200 7.1
Gambini12 3.4 1 30 11.3
Vidojkov11 68 10 400 17.1
Ayers10 191 1 950 20.1
D’Fabbro10 1050 1 10
4
10.5
Mausch13 1470 1 6000 24.5
This Work 127 3.125 963 13.2
Thus the next step is to optimize the PA effi-
ciency as well as reduce the power consumption of
the mixer+VCO unit together with optimization of
the BB architecture such that the EE at high trans-
mit rates do not remain bottlenecks in the realization
of the next-generation implantable wireless teleme-
try solutions. Once the basic design of a single-
terminal communication system is established based
on asynchronous packet transmission, the inherent
advantage of the proposed system to support multi-
terminal communications using signature spreading
sequences will be explored. The envisioned system
design issues, primarily those regarding link topol-
ogy and channel distortions, need to be further investi-
gated in-depth, both theoretically and experimentally.
7 CONCLUSIONS
Power budgets stipulated for safe operation of highly
miniaturized implantable devices to be used in next-
generation BANs are stringent. In addition, the chal-
lenges to target high data rates and longer operational
life using devices with small form factors contribute
to the need for novel system architectures and design
approaches. While the proposed packet-based trans-
mission system can offer a solution to effectively ad-
dress power usage limitations, the reliability of such
devices needs to be further investigated.
ACKNOWLEDGEMENTS
The authors would like to thank Alberta Innovates
Health Solutions (AIHS) and Project SMART for
their financial support.
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