IEEE Solid-States Circuits Magazine - Fall 2022 - 57

On the other end of the spectrum
is a trend toward applications requiring
dual source energy harvesting.
In these cases, the motivation is not
lower cost but, rather, lower environmental
impact combined with energy
requirements that are too high
to support purely wireless power
transfer. An example is BLE voice
remote controls with a single AAA
battery that lasts four to eight years.
To enable this, the battery is supplemented
with both a solar cell and the
harvesting of Wi-Fi signals. Combining
these three power sources, the
battery can last for up to eight years
to power the radio, LEDs, inter-IC
sound, audio processing, and audible
remote finder. Two recent product
examples were introduced at CES in
2021 and 2022 [10], [11].
IoT Wireless Node Performance
So far, reducing power consumption
and system cost have been explored,
including adding energy harvesting
to remove the cost of the battery.
Although energy harvesting helps
to avoid environmental aspects of
requiring so many batteries, more
innovations are still needed in the
long term to support the IoT market
growth. This leads to wireless node
performance as the final area that
needs optimization. To support energy
harvesting more easily, lower current
consumption is desired. Relaxing
performance requirements for wireless
standards can, in some cases,
allow the system to be designed to operate
while consuming less power. To
understand how much benefit is possible,
BLE will again be used as an example
for this analysis, although the
concepts apply to other standards.
The BLE standard requires a PA
output power of at least -20 dBm, an
Rx sensitivity of better than -70 dBm
at 1 Mb/s, and a connection interval
between 7.5 ms and 4 s [12]. First,
the impact of lower PA output power
on the BLE 1-s connection interval
average current is considered. Assuming
an efficiency of 50%, the PA
uses <1 mA of current into a 50-Ω
load when transmitting at -20 dBm.
Although energy harvesting helps to avoid
environmental aspects of requiring so many
batteries, more innovations are still needed in
the long term to support the IoT market growth.
For typical connection events, this is
a small amount of energy compared
to that used in other power states.
This can also be seen in data sheets
showing Tx current versus output
power [13], where the Tx current has
reached the minimum level as the
output power is reduced to -20 dBm.
Therefore, there is little to be gained
by altering the standard to transmit
at lower than -20 dBm for a traditional
radio architecture with an RF
synthesizer and a saturated PA.
The second parameter to consider
relaxing is the Rx sensitivity. Figure
8 shows the sensitivity versus
energy per bit for Rxs from published
papers and data sheets over approximately
20 years (adapted from [14]).
These Rxs were for carrier frequencies
between 10 MHz and 61 GHz and
in technology nodes between 180
and 14 nm. The modulation formats
vary as well, from simple OOK to
higher order more complex modulation
formats, and finally the data rate
is anywhere from bits per second to
gigabits per second. Yet even with
this wide range, the data are remarkably
clustered and follow the trend of
a 10× Rx power reduction for a 20-dB
sensitivity reduction. Most commercially
available BLE radios achieve
sensitivities in the range of -96 dBm
with the average current drawn from
the battery shown in Figure 2. A sensitivity
of -70 dBm is still compliant
with the BLE standard, which allows
the Rx power consumption to be reduced
enough that it is no longer a
dominant contributor to the average
current drawn from the battery. The
third parameter to be adjusted is the
connection interval. By operating the
radio only every 4 s, the minimum
duty cycle allowed by the standard
100,000
10,000
1,000
100
10
1
Process Node
14
22
28
40
55
65
90
0.1
0.01
0.001
110
130
140
180
Unknown
10× Power/20-dB Sensitivity
-130 -110 -90 -70
-50 -30 -10
Sensitivity (dBm)
FIGURE 8: The sensitivity versus the energy per bit for Rxs from published papers or data
sheets over the past two decades (adapted from [14]).
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2022
57
Energy per Bit (nJ/b)
IEEE Solid-States Circuits Magazine - Fall 2022

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