News
Sub-terahertz receiver array chip enables autonomous
vehicles ‘see’ through fog and dust
A sub-terahertz receiving system that
could help steer driverless cars see
through blinding conditions, such as
fog and dust has been developed by
researchers at MIT.
Sub-terahertz wavelengths can be
detected through fog and dust clouds
with ease, whereas the infrared-based
LiDAR imaging systems used in autonomous
vehicles today struggle. To
detect objects, a sub-terahertz imaging
system sends an initial signal through a
transmitter – a receiver then measures
the absorption and reflection of the
rebounding sub-terahertz wavelengths.
That sends a signal to a processor that
recreates an image of the object.
But implementing sub-terahertz sensors
into driverless cars is challenging.
Sensitive, accurate object-recognition
requires a strong output baseband
signal from receiver to processor. Traditional
systems, made of discrete components
that produce such signals, are
large and expensive. Smaller, on-chip
sensor arrays exist, but they produce
weak signals.
In a paper published online by the
IEEE Journal of Solid-State Circuits, the
researchers describe a two-dimensional,
sub-terahertz receiving array on a
chip that is orders of magnitude more
sensitive, meaning it can better capture
and interpret sub-terahertz wavelengths
in the presence of a lot of signal noise.
To achieve this, they implemented a
scheme of independent signal-mixing
pixels – called «heterodyne detectors» –
that are usually very difficult to densely
integrate into chips. The researchers
drastically shrank the size of the
heterodyne detectors so that many of
them can fit into a chip. The trick was
to create a compact, multipurpose
component that can simultaneously
down-mix input signals, synchronize the
pixel array, and produce strong output
baseband signals.
The researchers built a prototype,
which has a 32-pixel array integrated
on a 1.2-square-millimeter device. The
pixels are approximately 4,300 times
more sensitive than the pixels in today’s
best on-chip sub-terahertz array sensors.
With a little more development,
the chip could potentially be used in
driverless cars and autonomous robots.
DECENTRALIZED DESIGN
The key to the design is what the researchers
call «decentralization.» In this
design, a single pixel – called a «heterodyne
» pixel – generates the frequency
beat (the frequency difference between
two incoming sub-terahertz signals)
and the «local oscillation,» an electrical
signal that changes the frequency
of an input frequency. This «downmixing
» process produces a signal in
the megahertz range that can be easily
interpreted by a baseband processor.
The output signal can be used to
calculate the distance of objects,
similar to how LiDAR calculates the
time it takes a laser to hit an object and
rebound. In addition, combining the
output signals of an array of pixels, and
steering the pixels in a certain direction,
can enable high-resolution images
of a scene. This allows for not only the
detection but also the recognition of
objects, which is critical in autonomous
vehicles and robots.
Heterodyne pixel arrays work only
when the local oscillation signals from
all pixels are synchronized, meaning
that a signal-synchronizing technique is
needed. Centralized designs include a
single hub that shares local oscillation
signals to all pixels.
These designs are usually used by
receivers of lower frequencies, and can
cause issues at sub-terahertz frequency
bands, where generating a high-power
signal from a single hub is notoriously
difficult. As the array scales up, the
power shared by each pixel decreases,
reducing the output baseband signal
strength, which is highly dependent on
the power of local oscillation signal.
As a result, a signal generated by each
pixel can be very weak, leading to low
sensitivity. Some on-chip sensors have
started using this design, but are limited
to eight pixels.
The researchers’ decentralized
design tackles this scale-sensitivity
trade-off. Each pixel generates its own
local oscillation signal, used
for receiving and downmixing
the incoming signal.
In addition, an integrated
coupler synchronizes its local
oscillation signal with that of
its neighbor. This gives each
pixel more output power,
since the local oscillation
signal does not flow from a
global hub.
The new architecture,
however, potentially makes
the footprint of each pixel
much larger, which poses a great
challenge to the large-scale, high-density
integration in an array fashion. In
their design, the researchers combined
various functions of four traditionally
separate components – antenna, downmixer,
oscillator, and coupler – into a
single «multitasking» component given
to each pixel. This allows for a decentralized
design of 32 pixels – enabling a
large-scale dense array.
GUIDED BY FREQUENCIES
In order for the system to gauge an
object’s distance, the frequency of the
local oscillation signal must be stable.
To that end, the researchers incorporated
into their chip a component
called a phase-locked loop, that locks
the sub-terahertz frequency of all 32
local oscillation signals to a stable,
low-frequency reference. Because the
pixels are coupled, their local oscillation
signals all share identical, high-stability
phase and frequency. This ensures that
meaningful information can be extracted
from the output baseband signals.
This entire architecture minimizes signal
loss and maximizes control.
In summary, the researchers developed
a coherent array with very high
local oscillation power for each pixel, so
each pixel achieves high sensitivity.
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