IEEE Solid-State Circuits Magazine - Spring 2014 - 50

s p r i n g 2 0 14

IEEE SOLID-STATE CIRCUITS MAGAZINE

10
10
0
1,
00
0

0.
1

1E
-6
1E
-5
1E
-4
1E
-3
0.
01

MPE W/cm2

e.g., tissue heating by absorpfor a single transmit chantion or electrostimulation.
nel, but we are only allowed
100,000
10,000
These regulations or parto use 300 kHz. Thus, even
1,000
tially only recommendations
with sophisticated modu100
are enacted by national or
lation schemes, data rates
10
266 nm
international
standardizaexceeding 1 MB/s are hardly
1
2.9-10 µm
1,064 nm
tion agencies such as the
achieved [9]-[11].
0.1
355 nm
IEEE, International TelecomIn 2009, the MICS band
0.01
800 nm
1E-3
munication Union, American
was
expanded to 401-406
400-700 nm
1E-4
National Standards Institute
MHz and called MedRadio as a
1E-5
(ANSI), International Electroresponse to the emerging field
1E-6
technical Commission (IEC),
of -neurorecording, neurostimuor the U.S. Federal Communilation, and neuromodulation
Exposure Time (s)
cations Commission (FCC).
devices with their significantly
Figure 4 illustrates a simincreased data rates. In 2011,
plified radio spectrum fre- Figure 6: The maximum permissible exposure for the eye
the Alfred Mann Foundation
based on IEC 60825.
quency allocations chart,
filed a petition to expand the
which is of interest for IMDs.
frequency range further, and
FCC 15, if they are used unlicensed,
Thereby, equivalent isotropically
this is currently installed roughly
as well as in FCC Part 18 if licensed.
radiated power (EIRP) is the peak
between 410 and 450 MHz, where the
In a licensed application, one is
power density in the direction of
usable bandwidth will been extended
allowed unlimited radiated energy in
the maximum radiation referred to
to 6 MHz, allowing a data rate of more
the specified bands. If not licensed,
an isotropic radiator, meaning that,
than 10 MB/s. Note that this band is
the transmit power is restricted
in case we employ antennas, we also
currently not international.
but still at a very high level, e.g.,
need to take the antenna main lobe
As a final standard, UWB has
in the commonly used 13.56-MHz
and its gain into consideration.
become prominent also for IMDs and
ISM band, the EIRPmax c 5 dBm,
There are industrial, scientific,
short-range, high-speed communicawhich is almost 50 dBm larger than
and medical (ISM) bands with their
tion. The standard requires at least
for arbitrary transmitting frequenlarge emission power; then we have
500-MHz signal bandwidth, which
cies. Thus, attention must be paid
the MICS band at about 400 MHz,
is usually achieved by using short
to possibly very strong interferers,
the WMTS band for externally worn
pulses in the time domain for the
which originate from other radiamedical devices, and ultrawide band
transmission, translating to large
tors. This might not be the case in
(UWB) standard in the gigahertz
spectral bandwidth in the frequency
a nicely controlled laboratory envirange. On top of all, the regulations
domain. The standard then defines
ronment but of course may be in the
for "arbitrary frequency transmission
emission masks, meaning that the
real world! Another important fact in
limits": in the United States, the FCC
bandwidth indeed must be wide, but
ISM bands is the regulated limitation
rules and regulations are codified in
the signal is spectrally limited in
in channel bandwidth. For example,
Title 47 of the Code of Federal Reguoutput power density.
in the 13.56-MHz band, the maxilations. Part 15 of this code applies
It is important to note two things:
mum bandwidth Tf is 14 kHz. This
to radio frequency devices operatFirst, that the spectral emission
becomes important when the ISM
ing at unlicensed frequencies and is
masks are different in different
carrier is used for data telemetry
often colloquially referred to as FCC
regions as shown, e.g., in Figure 5.
because any spurious emission outPart 15. Outside the restricted bands,
Second, as we have discussed in the
side the ISM band must comply with
e.g., for MICS and ISM, one can transtissue properties, we face very large
the FCC 15 arbitrary transmitting
mit at any frequency as long as the
attenuation for UWB frequencies in
restrictions and its much lower EIRP.
radiated output power is below certhe gigahertz range. Thus, it is difThe MICS band, installed in 1999,
tain limits for the spurious emission.
ficult to achieve deep implantation
mostly spans around 402-405 MHz.
In the frequency range of interest, an
together with high data rate.
EIRP = - 50, f, - 45 dBm is always
It is important to note that we are
After having discussed standards
possible [6], [7], which is quite feaonly allowed quite limited transand regulations concerning interfersible if the distance between the
mit power ^EIRP = - 16 dBmh, but,
ence, the second issue for IMDs are
when compared to ISM, interference
transmitter and the receiver is short
recommendations for safe exposure
becomes much less of an issue.
and the required transmit power is
by questioning how much optical or
Another drawback of the MICS
thus small.
EM power can we emit onto the perband is that the 3-MHz frequency
The ISM bands are a special case
son wearing our IMD to avoid possiband cannot be used in full extent
because they are regulated both in
bly harmful adverse effects, such as

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Spring 2014