ESP-CSI: Transforming Wi-Fi into a Sensing Platform

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Even beyond the camera's view, the drone

can be located using Wi-Fi alone.

We use one ESP chip as the transmitter,

eight ESP chips as the receiving array.

The two sides exchange only standard

Wi-Fi packets. The receiver extracts CSI

from the packets. This allows the Wi-Fi

chip to sensing the environment and

perform positioning. Next, we attach the

transmitter to the drone and place the

receiving array on the ground. Test

positioning performance at different

altitudes.

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CSI based sensing does not require open

space and is not limited by line of

sight, allowing for accurate detection

even through walls.

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During transmission, Wi-Fi signals are

affected by wall reflections and by

obstructions such as obstacles, which

reduces their accuracy. To solve this

problem, the Wi-Fi protocol itself

includes a mechanism. When transmitting

data, Wi-Fi signals embed reference

information known to both sides called

pilot signals. The receiver compares the

original pilot with the received one to

determine how the environment has

affected the signal. These changes are

used to reconstruct the original data

and form the CSI. CSI can be seen as an

enhanced version of signal strength

information, offering more expanded and

detailed channel state information in

the frequency domain for wireless

sensing. Since it is highly sensitive to

environmental changes, even the slight

movement of a finger can cause a change

in CSI. Because of this, CSI can also be

used for environmental sensing, turning

Wi-Fi into more than just a data

transmission tool. It can act as a radar

to detect spatial changes. ESP chips

with Wi-Fi capability require no

additional sensors. They can gain

sensing capability through a simple

software upgrade. For example, a light

bulb equipped with Wi-Fi CSI can detect

human presence or movement.

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Expressive's new generation chips offer

further optimizations in RF circuitry.

We conducted comparative tests on the

CSI performance of different chips in a

dedicated lab. One ESP chip serves as

the transmitter, another serves as the

receiver. observing the CSI response as

a metal plate moves along a sliding

rail. The new Wi-Fi CSI offers higher

receiving sensitivity and lower static

noise levels. The ESP 32C5 also supports

higher sensitivity at 5.8 GHz. It can

even detect tiny vibrations after the

sliding rail stops. This means chest

movements caused by breathing can also

be detected.

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Different movements affect the Wi-Fi

transmission path in different ways.

Such effects can be sensitively captured

by CSI. Actions like nodding, shaking

the head or raising a hand can be

clearly distinguished in the raw data

waveform. With the edge processing

capabilities of ESP chips, neural

networks can be deployed on device to

detect actions.

Compared to sensing technologies like

millimeter wave radar and UWB, Wi-Fi CSI

offers lower cost and power consumption

as well as broader sensing range and

stronger interference resistance. It has

already been applied in commercial

products across various fields. Existing

products can quickly add wireless

sensing functionality to accelerate the

deployment of wireless sensing

capabilities. Expressive provides a

complete solution. Wi-Fi's default

automatic gain control may mask

environmental changes in CSI. ESP chips

offer fixed gain options at the physical

layer, which can improve sensing

accuracy. In multi- antenna solutions,

we use a multi-chip co- crystal

oscillator design to achieve precise

clock synchronization across multiple

chips, enabling millimeter level

detection of disturbances and RF signal

paths. Espressive provides use cases for

motion and presence detection. The

repository also includes open- source

data processing tools. In addition to

Espressive's official documentation,

many third-party developers have also

contributed a large number of

open-source projects. Espressive also

actively collaborates with various

universities. Scholars have published

numerous papers based on Expressive

chips, providing a solid foundation for

the exploration and application of CSI.