Instrumentation & Measurement Magazine 24-6 - 51

Fig. 4. (a) Ultrasonic sensor with more coverage area. (b) Corresponding sensing unit with four HC-SR04 and a LV-MaxSonar-EZ0 [12] (©2017 IEEE, used with
permission).
multipath, and as a result, signals from different paths experience
phase shifts and amplitude attenuations and the reader
receives the superposition of signals with different delays, amplitudes
and phases. Consequently, environmental dynamics
such as humans and moving objects could affect a portion of
the multipath component. The received signal from different
paths could result in destructive or constructive phases,
so RSSI decreases or increases, respectively. These changes in
RSSI can be measured for movement detection as done in [5],
for example.
One of the problems of RSSI is that it fails to capture the multipath
effects and analyzes paths individually. Furthermore, a
slight change in certain multipath components may add up
to significant constructive or destructive relative phases of
the delayed signals; consequently, considerable fluctuations
could appear in RSSI [5]. The RSSI of multipath signals are depicted
in Fig. 5.
In [13], an RFID-based device-free posture recognition system
is proposed. For recognizing human activities, it deploys
passive RFID tags as an array in the environment. If the tags are
not placed properly, some unpredictable effects may happen.
For instance, if two tags are put at a certain distance, significant
signal loss or fading can occur. For this reason, and to achieve
the best performance, an investigation is performed in [13] for
Fig. 5. Multipath signals propagation with RSSI and CSI. Figures reused from [22].
September 2021
IEEE Instrumentation & Measurement Magazine
51

Instrumentation & Measurement Magazine 24-6

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