IEEE Solid-States Circuits Magazine - Fall 2023 - 25

in power consumption. The medical
body area network band (MBAN)
was added to the MICS band, mainly
including the frequency band of
2.36-2.4 GHz. The interference from
ISM bands is avoided in the MBAN,
making it suitable for stable neural
recording in a larger experimental
space. An emerging research scenario
nowadays is recording neural signals
from multiple freely moving subjects
simultaneously, in order to study the
physiological explanation of social interaction
behaviors such as cooperation
[62] and competition [63]. A larger
working distance is usually required.
The interference among node devices
should also be considered. The IEEE
802.15.6 protocol was published in
2012, enabling the deployment of a
network topology of a coordinator
and several node devices [64]. Several
transmitters have been proposed
under this protocol [65], [66], [67].
More efficient modulation schemes
and transmitter architectures can be
proposed to achieve a higher energy
efficiency [68].
In addition, multiple
access methods that avoid the interference
among node devices, while
maintaining a low power consumption
for the nodes, can be further optimized
based on these protocols.
While narrowband RF technology
can provide a data rate of up to tens
of Mb/s, the latest high-density electrodes
equipped with several thousand
channels generate data with
a throughput in the range of Gb/s.
Although some lossy compression
technologies have been proposed to
reduce the workload [69], [70], a consensus
on the specifications of compressed
neural data has not yet been
reached [71]. Consequently, the lossless
raw data are sometimes more preferred
in current scientific research.
For such needs, impulse radio (IR)UWB
emerges as a potential solution
in recent studies. Due to the >500 MHz
available bandwidth, >1 Gb/s throughput
is enabled with a relatively low
complexity and low power consumption
design. However, IR-UWB faces a
strict design challenge as the transmit
PSD is limited to −41.3 dBm/MHz by
Therefore, an open loop VCO-ADC can realize
a low-overhead closed loop neuromodulation
solution with a 100-mV input range.
most of the telecommunications organizations
around the world, leading
to a conflict between the data rate
and the transmission range. Several
strategies including increasing the
bandwidth [72], [73], [74] and increasing
the modulation order [75],
[76], [77], [78] have been proposed
to relieve this conflict. To maximize
the modulation order while simultaneously
reducing the requirements for
the SNR, hybrid modulation schemes,
which modulate an IR-UWB pulse with
several modulation methods, have
been proposed [77], [78]. The IR-UWB
transmitters in [77], [78] achieved data
rates of 1.66 Gb/s and 1.8 Gb/s, with
power efficiencies of 5.8 pJ/bit and
2.3 pJ/bit, respectively. The transmission
range was more than 10 cm even
with 21 cm pork tissue applied. However,
the receiver design for hybrid demodulation
is also very complicated.
To the best of our knowledge, there
is no compact IR-UWB transceiver design
that fully implements the hybrid
modulation and demodulation.
System volume is also a crucial
concern for neural interfaces, especially
for implant devices. The wireless
module usually occupies most of
the volume due to the use of the crystal
oscillator, and the antenna with
the matching network is very hard to
eliminate. The crystal oscillator with
a PLL offers a precise clock for signal
modulation. The antenna is essential
for the radiation efficiency. Many explorations
have been conducted to address
these volume-related concerns,
including attempts to eliminate crystal
oscillators [78], [79], eliminate or
employ on-chip matching networks
[68], [77], [79], and utilize on-chip antennas
[80], [81]. Most of the existing
works only solve part of the problem,
or the transmission distance and data
rate are limited. Therefore, further
efforts are necessary to achieve a
reliable and compact design, particularly
in scenarios that require high
data rates.
In addition to the RF technologies
mentioned above, there are also other
wireless technologies that have been
explored for neural interfaces, including
inductive coupling [82], ultrasonic
communication [83], optical
communication [84], and human body
communication (HBC) [85]. These
technologies offer a wide range of
data rates from several kb/s to approximately
300 Mb/s. The inductive
and ultrasonic methods are also used
for wireless power delivery. These
technologies are usually for data communication
from inside the body to
outside the body. A secondary data
forwarding is needed for the free moving
subjects. Among these technologies,
HBC adheres to the regulations
defined by IEEE 802.15.6, whereas the
remaining technologies lack specific
regulations for clinical or commercial
use. Nevertheless, these wireless
technologies hold great potential for
neural interfaces and continue to be
subjects of active exploration.
Wireless Power Transfer
To extend the battery life of biomedical
devices, wireless power transfer
(WPT) has been introduced as a key
technique [86], [87], [88]. Existing solutions
reported in the literature include
encompassing inductive power transfer
(IPT) [89], capacitive power transfer,
ultrasonic power transfer (UPT)
[90], [91], RF-based radiative power
transfer, body-coupled energy harvesting,
and optical techniques, etc.
In the implant applications, IPT and
UPT are commonly employed due to
their compact volume size, high power
delivery density, and safety [92].
IPT is a method of powering devices
using a magnetic field generated
by RF signals in the near-field
region, operating from a few kHz to a
few GHz. IPT can provide up to a few
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2023
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IEEE Solid-States Circuits Magazine - Fall 2023

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