Mobile Netw Appl
Table 1 A glance of unpacked frames
Record time(HH:MM:SS) Frequency RSS BSSID Source address Destination address
06:53:31.148619 5500 MHz -−57 dB 30:5a:3a:cb:16:a0 00:16:ea:12:34:56 74:e5:43:5e:fd:a9
06:53:31.153621 5500 MHz −57 dB 30:5a:3a:cb:16:a0 00:16:ea:12:34:56 74:e5:43:5e:fd:a9
06:53:31.158509 5500 MHz −-57 dB 30:5a:3a:cb:16:a0 00:16:ea:12:34:56 74:e5:43:5e:fd:a9
··· ··· ··· ··· ··· ···
06:53:38.141203 5500 MHz −58 dB 30:5a:3a:cb:16:a0 00:16:ea:12:34:56 74:e5:43:5e:fd:a9
··· ··· ··· ··· ··· ···
06:53:45.722784 5500 MHz −57 dB 30:5a:3a:cb:16:a0 00:16:ea:12:34:56 74:e5:43:5e:fd:a9
that have been added to the kernel recently. For instance,
we can use command iw phy phy0 interface add
mon0 type monitor to create a new interface mon0
to set phy0 into monitor mode. In addition, we can use
command iw dev mon0 set freq f
c
to switch the
center frequency of mon0 to f
c
MHz. These operations will
only spend several milliseconds. In motion detection phase
(see Fig. 2), the client keeps normal communication to APs
and obtain CSIs from preambles. When the system detects
object’s abnormal moving (see details in Section 4.2),
the client’s NIC card will change to monitor mode and
record all the packets heard from ambient APs. Table 1
lists the unpacked packets information using single channel
(Channel 100 whose f
c
= 5500 MHz). The table lists
the packets information as follow, time stamp indicated
by the Timing synchronization function,
1
channel center
frequency, RSS value, the identifier of basic service set,
2
transmitter’s MAC (source) address, and receiver’s MAC
(destination) address, respectively.
As shown in Table 1, SDF-based solutions expose two
main drawbacks: 1) In training phase, we have no need to
record such amount of RSS values in a single channel,
whereas we should leverage this opportunity to obtain
more abundant, and meaningful fingerprints; 2) In matching
phase, since users cannot endure long waiting cycles to
obtain a stable RSS numerical characteristics, e.g., mean or
median, calculating the distance between collected samples
average or median and the mean or median in fingerprint
database would result some localization errors. Hence, we,
in this paper, first propose to leverage multiple channel
fingerprint to improve location accuracy (detailed in
Section 4.1) and exploit sample hypothesis testing to verify
1
Timing synchronization function (TSF) is specified in IEEE 802.11
wireless local area network (WLAN) standard which is based on a
1-MHz clock and “tick” in microseconds [49].
2
In IEEE 802.11 wireless local area networking standards, basic
service sets (BSS) are units of devices operating with the same medium
access characteristics.
users’ location. In this way, we can not merely leverage all
the sampled RSS values, but also obtain the confidence of
localization precision (detailed in Section 4.6).
3.2Resolutiondiversity
Wireless devices, e.g., APs, have a discrimination capability
when fingerprinting a specific location. This capability,
however, is subject to inherent constraints of radio signal
propagation.
Discrimination capability is referred to as the ability
that an AP distinguishes a specific location when using
its RSS observations as fingerprints. Ideally, following the
propagation law of wireless signals, the RSS of a certain
wireless signal decays logarithmically with its propaga-
tion distance d. In other words, an identical ΔRSS can
imply a smaller distance change Δd at the location closer
to the antenna of AP, or a larger Δd at the location far
away from the AP’s antenna. As shown in Fig. 1,theRSS
variance shows vast difference in different physical posi-
tions, depending on the specific distance d. Hence, using
the RSS reading from a long-distance AP may cause a large
error in the location estimation, while using those from near
APs can conversely mitigate the error. In fact, such a RSS
variance also indicates a diversity of AP resolution across
different transmitter-receiver distances.
Fig. 1 AP resolution