IEEE Solid-States Circuits Magazine - Summer 2021 - 35

implies that receiver power and
range scale together, assuming a
path loss coefficient of 2; e.g., a 10×
increase in power results in a 10×
increase in range. In Figure 2, the
sensitivity is normalized to a 1-kb/s
data rate using (1), which reduces the
spread in points, particularly for
nanowatt receivers since they have
relatively low data rates. These
normalized points are compared to
two groups with a constant figure
of merit: 10× power/20-dB sensitivity
and 10× power/10-dB sensitivity.
The latter is more commonly
used for energy detection front
ends [3].
ULP Receiver Architectures
Several receiver architectures have
been published in the literature;
however, most ULP receivers leverage
some variation of a passive
envelope detection radio-frequency
(RF) front end, eliminating powerhungry
RF blocks, such as low-noise
amplifiers (LNAs) and RF local oscillators
(LOs), as shown in Figure 3.
Hybrid architectures have been
demonstrated that, for example,
add back an LNA for improved sensitivity
and that include a passive
mixer-first architecture incorporating
an RF LO. The power of these
RF components is >20 µW and often
>100 µW; therefore, we are not considering
them ULP. Exploring the
architecture in Figure 3 further,
passive transformers and matching
networks are added in front of the
envelope detector (ED) to reduce the
noise bandwidth and improve the
sensitivity by up to 20 dB, extending
the wireless range [4]. This passive
voltage boosting performs better
with a high RF ED input impedance,
which is easier to achieve at lower
frequencies; therefore, <10-nW rece ivers
tend to be subgigahertz (Figure 4).
However, ULP receivers at <100 µW
have been demonstrated across a
wide range of frequencies, up to millimeter-wave
bands.
Following the transformer, a passive
envelope detector is used for
downconversion, which has a wide
This calls for better networking solutions to
enable massive scales of devices and ultralowpower
radios to enable self-powered operation.
bandwidth; therefore, the amount of
added noise can be high. This limits
sensitivity to around -50 dB referenced
to 1 mW. Baseband gain and filtering
stages operate in the subthreshold,
with a low bandwidth to keep the
power minimal, resulting in a typical
minimum detectable voltage in the
1-10-mV range. Finally, digital baseband
processing typically consists
of correlators to identify an on-off
keying (OOK) wake-up sequence,
cutting down on false detections
and adding 5-15 dB of processing
gain. Data rates for these receivers
are less than 1 kb/s (Figure 5), limited
by the speed and bandwidth of
the subthreshold analog and digital
baseband circuits.
Improving Selectivity
Many ULP receivers suffer from
poor performance in the presence
of in-band interferers. This is highlighted
in Figure 6, which plots
the SIR for all 191 receiver publications.
Note that, for ULP receivers, the
SIR is either poor or not reported.
The ED-first architecture is inherently
susceptible to interference
because of its wideband response.
Recently reported ED-first receivers
have addressed this with Manchester
encoding [5] and two-tone
20-40 dB S = -50 dBm
Passive
Xform
Passive
ED
Vmin = 1-10 mV
5-15 dB
A
C
BB
Proc Gain
FIGURE 3: An energy detection receiver architecture with passive voltage boosting at RF and
digital processing gain for improving sensitivity. Xform: transformer; ED: envelope detector; BB:
baseband; Proc: processing.
100,000
10,000
1,000
100
10
1
0.1
0.01
0.001
0.0001
<-100 dBm
Subgigahertz
Subgigahertz
1-3 GHz
3-10 GHz
>10 GHz
-140 -120 -100 -80
-60
Sensitivity (dBm)
FIGURE 4: A comparison of power consumption and operating frequency.
IEEE SOLID-STATE CIRCUITS MAGAZINE
SUMMER 2021
35
< 10 nW
Subgigahertz
-40
-20
Power (µW)

IEEE Solid-States Circuits Magazine - Summer 2021

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