Tx
(Source)
Rx
(Observer)
Target
(Reflector)
Towards link
Positive Doppler shifts
Away from link
Negative Doppler shifts
(a) Illustration of Doppler effect (b) Spectrogram of Doppler effect
Tx
A
1
A
2
x
y
f
a
o
Static
Target
Cross-term
Rx
(c) Doppler effect with multiple antennas
Figure 3. Modeling Doppler effect in multipath scenarios.
phase offsets, burst noise and interferences using a series of
signal processing techniques including antenna selection, data
sanitization and time-frequency analysis. The second step is
to recognize player reactions from the spectrogram of Doppler
frequency shifts. The challenge for this step is to robustly
recognize each individual player reaction from continuous
Doppler frequency shifts that may consist of multiple reaction-
s. Operations adopted in this step include movement detection,
trace segmentation and motion classification. The output of
this step is reaction series recognized by WiDance.
DOPPLER EFFECT IN WI-FI
WiDance extracts Doppler effect from Wi-Fi signals to rec-
ognize dancing actions of players. This section provides the
technical preliminaries, fundamental model and practical is-
sues of identifying Doppler frequency shifts from noisy Wi-Fi
signals on commercial devices.
Doppler Effect
Doppler shift is the change in the frequency of a wave for
observers. It is caused by change in relative locations of
sources, observers and reflectors. In the context of contactless
sensing, both transmitters (sources) and receivers (observers)
are statically deployed, while target objects (reflectors) move
and alter the wireless transmission. As shown in Figure 3a,
when the target object moves towards the transmitter and the
receiver, the crests and troughs of the reflected signals arrive
at the receiver at a faster rate. Conversely, when an object
moves away from the receiver, the crests and troughs arrive
at a slower rate. In general, for a point object, the Doppler
frequency shift of the signal reflected off the object is:
f
D
= −
1
λ
d
dt
d(t), (1)
where
λ
is the wavelength of the signal and
d(t)
is the length
of the reflected path.
As an illustrative example, we prototype a wireless transceiver
system using two USRPs synchronized by an external clock.
The two USRPs are placed together near the ground, and a
participant strides with his right leg at moderate rate, at the
direction orthogonal to the link, as in Figure 3a. The Doppler
effect caused by striding is obtained by tracking the phase
of the received signal. Figure 3b shows the spectrogram of
Doppler effect of striding. Clearly, positive Doppler shifts
appear as the user strides towards the link, while negative
Doppler shifts appear as the user strides away from the link.
Thus, it is possible to track target motion (both speed and
direction) by exploiting Doppler effect.
Doppler Effect in CSI
In reality, instead of single path as the reflected path in Fig-
ure 3a, there are multiple paths where signal propagates from
the transmitter to the receiver. The phenomenon is known as
multipath. As a result, the response of the wireless channel at
frequency
f
and time
t
is the superimposition of responses of
each individual path [21]:
H( f ,t)=
K
∑
k=1
α
k
(t)e
− j2π f τ
k
(t)
, (2)
where
K
is the total number of multipath, and
α
k
(t)
and
τ
k
(t)
are the complex attenuation factor and time of flight for the
k-th path, respectively.
For the
k
-th path, the time of flight
τ
k
(t)
is the time for light to
travel at a distance of path length of
d
k
(t)
, i.e.
d
k
(t)=cτ
k
(t)
,
where
c
is the speed of light. Thus, according to Equation 1,
the channel response can be represented by Doppler frequency
shift on each path and further divided into two categories:
H( f ,t)=H
s
( f )+
∑
k∈P
d
α
k
(t)e
j2π
t
−∞
f
D
k
(u)du
,
(3)
where
H
s
( f )
is the sum of responses of all static path (
f
D
= 0
),
and P
d
is the set of dynamic path ( f
D
0).
Assuming that
α
k
(t)
and
f
D
k
(t)
are nearly constant during
short time interval, Doppler frequency shifts can be obtained
from spectrogram with time-frequency analysis:
H ( f ,t) ≈ H
s
( f )+
∑
k∈P
d
α
k
(t)B( f
D
k
(t)), (4)
where
B(·)
is the window function for cutting out the signal
segment of interest.
CSI is the sampled version of the channel response in Equa-
tion 2 and 3. It is available from upper layers on off-the-shelf
Human Performance Gaming
CHI 2017, May 6–11, 2017, Denver, CO, USA
1963
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