Based on the effective models, we consider to tackle the
following problem in a typical scenario of RFID applica-
tions, i.e., using a mobile reader to identify a large volume
of tags deployed over a wide area. We seek to execute con-
tinuous scanning over the tags along a certain direction,
while respectively considering a situation where the tags are
continuously placed with a uniform/nonuniform density.
We focus on several critical metrics like time-efficiency,
energy-efficiency and coverage ratio. We design efficient
and practical algorithms for continuous scanning, by skill-
fully adjusting the reader’s power and moving speed, which
can dramatically improve the performance, as shown in our
real experiments. By exploring the inherent regularities in
continuous scanning, we aim to give some fundamental
guidance for future RFID system design towards more com-
plicated realistic settings. We make the following contribu-
tions in this paper (a preliminary version of this work
appeared in [4]). 1) We are the first to conduct an extensive
experimental study and performance evaluation over a rela-
tively large number of tags (up to 160 tags for experimental
study and up to 480 tags for performance evaluation) and a
rather high tag density (up to 90 tags per square meter) in
realistic settings. To the best of our knowledge, this is the
first work to propose a model for investigating how the
important parameters including reader’s power, moving
speed and tag density jointly affect the reading performance.
2) This is also the first work to give a framework of optimiz-
ing reading performance based on experimental study. We
apply our model to solve the problem of continuous scan-
ning with mobile reader. By carefully adjusting the power
and moving speed, we design efficient algorithms to opti-
mize time-efficiency and energy-efficiency. We have a num-
ber of novel techniques in making our algorithms practical.
3) Being compatible with RFID standard (with no changes to
the C1G2 protocols or low-level parameters for commercial
RFID readers), our solutions can deliver significant perfor-
mance gain. Experiment results indicate that, while achiev-
ing the same coverage ratio, our practical solutions
respectively reduce scanning time by 50 percent and energy
consumption by 83 percent compared to the prior solutions.
2RELATED WORK
In RFID systems, a reader needs to receive data from multi-
ple tags. These tags are unable to self-regulate their radio
transmissions to avoid collisions. In light of this, a series of
slotted ALOHA-based anti-collision protocols [5], [6], [7], as
well as tree-based anti-collision protocols [8], [9], [10], [11],
are designed to resolve collisions in RFID systems. In order
to deal with the collision problems in multi-reader RFID
systems, scheduling protocols for reader activation are
explored in [12], [13]. Recently, a number of polling-based
protocols [14], [15], [16], [17] are proposed, aiming to collect
information from RFID tags in a time/energy efficient
approach. In order to estimate the number of tags without
collecting tag IDs, a number of protocols are proposed [18],
[19], [20], [21], [22], [23], [24], [25], [26] to leverage the infor-
mation gathered in slotted ALOHA protocol for fast estima-
tion of tag size. In regard to tag identification with the
mobile reader, Sheng et al. develop efficient schemes for
continuous scanning operations [27], aiming to utilize the
information gathered in the previous scanning operations
to reduce the scanning time of the succeeding ones.
In order to verify the impact of the physical layer’s unre-
liability, a number of researchers conduct experimental
studies in realistic settings, while trying to explore the gap
between the ideal situation and the realistic situation for
RFID systems. Buettner and Wetherall [1] examine the per-
formance of the C1G2 RFID system in a realistic setting.
They identify factors that degrade overall performance and
reliability with a focus on the physical layer. Jeffery et al. [3]
conduct experiments in realistic settings and find that
within each reader’s detection range, a large difference
exists in reading performance. Zheng and Li investigate
into the physical layer information of tag responses for
missing tag identification [28]. Realizing that the reader’s
transmission power actually has a significant impact on the
reading performance of the RFID system, Xu et al. investi-
gate the impact of transmission power on reading perfor-
mance through extensive empirical study on passive tags
[29], [30]. Su et al. find that, when the transmission power is
set to a reasonable range, the “capture effect” can be used to
resolve the collision slots into singleton slots [31]. Therefore,
they propose a progressing scanning algorithm to improve
the reading throughput.
3PROBLEM F ORMULATION
We consider a typical scenario of continuous scanning in
realistic settings, i.e., using a mobile reader to identify a
large volume of tags deployed over a wide area. We respec-
tively consider a situation where the tags are continuously
placed with a uniform/nonuniform density, we seek to exe-
cute continuous scanning over the tags along a certain direc-
tion. The performance metrics in our consideration are as
follows: 1) Time-efficiency: considering it is time-consuming
to identify a large volume of tags in realistic settings, the
overall scanning time should be as small as possible. 2)
Energy-efficiency: considering the mobile reader is conven-
tionally battery powered, e.g., a typical battery for the
mobile reader has a capacity of 3,200 mAh with output volt-
age 3.7 v, if we scan the tags with a maximum radiation
power 36 dBm, the mobile reader can execute continuous
scanning for only 3 hours, therefore, the overall energy
used should be as small as possible. 3) Coverage ratio: due to
various issues like path loss in realistic settings, it is difficult
to identify all tags with a high probability for one single
scanning cycle, therefore, the coverage ratio, i.e., the ratio of
the number of identified tags to the total number of tags,
should be guaranteed, while each tag should have a uni-
form probability to be identified.
In regard to the continuous scanning, we define the scan-
ning time as T , the overall energy used as E, and the cover-
age ratio as C. Assuming the tag density is r and the length
of the scanning area is l, then the total tag size is n ¼ l r,
we denote the overall tag set as S. We assume that each tag
t
j
2 S is successfully identified with probability of p
j
after
the continuous scanning. The reader’s antenna is deployed
towards the tags with a distance of d. We can adjust the
parameters including the reader’s power p
w
and the moving
speed v to improve the reading performance. Therefore,
during the continuous scanning, the problem is how to
XIE ET AL.: EXPLORING THE GAP BETWEEN IDEAL AND REALITY: AN EXPERIMENTAL STUDY ON CONTINUOUS SCANNING WITH... 2273