efficiently set the parameters p
w
and v such that the follow-
ing objectives can be achieved:
Time-efficiency:
minimize T (1)
subject to
E a energy constraint
(2)
Pr½C ub coverage constraint (3)
8t
j
2 Sp
j
¼ p coverage constraint (4)
Energy-efficiency:
minimize E (5)
subject to
T g time constraint
(6)
Pr½C ub coverage constraint (7)
8t
j
2 Sp
j
¼ p coverage constraint: (8)
According to the above formulation, in regard to the
time-efficiency, the objective is to minimize the overall
scanning time T while the energy constraint and the cover-
age constraint should be satisfied. T he energy constrai nt
requires the energy used should be no greater than a certain
threshold a. In regard to the coverage constraint, due to the
random factors in the anti-collision scheme and the com-
munication envi ron ment, the coverage rati o C cannot guar-
antee to be deterministically equal or greater than a
threshold u, hence we use the probabilistic approach to
denote the requirement. The probability for the coverage
ratio C to be equal or greater than u should be no less than
b. Moreover, there co uld exist mul ti ple f e asible sol ut ions to
guarantee the coverage constraint, in some of the solutions
the tags are detected with nonuniform probabilities. In fair-
ness, we require that each tag t
j
in the set S should be
detected with a uniform probability p, i.e., the detection
probability p
j
should be equal to p. Similarly, in regard to
the energy-efficiency, the objective is to minim iz e the over-
all energy E,whilethetimeconstraintandthecoverage
constraint should be satisfied. The time constraint requires
that the scanning t ime should be no greater than a certain
threshold, g.
4DERIVING A MODEL FROM REALISTIC
EXPERIMENTS
In order to understand how the reader’s power and tag den-
sity affect the reading performance, while dealing with
issues like the path loss, energy absorption, and mutual
interference, we illustrate several original findings from our
realistic experiments. In our experiments, we use the Alien-
9900 reader and Alien-9611 linear antenna with a directional
gain of 6 dB. The 3 dB beamwidth is 40 degrees. The RFID
tags used are Alien 9640 general-purpose tags which sup-
port the EPC C1G2 standards. We attach the RFID tags onto
the books which are placed in a large bookshelf. Each tag is
attached onto a distinct book with a unique ID. The
bookshelf is composed of 16 grids with four columns and
four rows, the height and width of each grid are respec-
tively 60 cm and 75 cm. In the experiments we only consider
the grids in the three rows of upper layers, since the grids in
the bottom layer may be greatly affected by the multi-path
effect. Therefore, we choose to deploy the tags in the 12 grids
with four columns and three rows. The RFID reader is stati-
cally deployed by facing its antenna towards the book shelf.
Note that in order to set an appropriate value for the dis-
tance between the reader and the bookshelf, it is difficult to
directly derive the optimal distance from geometry accord-
ing to the beamwidth, due to issues like the multi-path
effect. Therefore, we vary the distance from 0.5 to 3 m and
measure the number of effectively identified tags while
scanning 160 tags uniformly distributed on the shelf.
As shown in Fig. 1, we find that the reader achieves the
maximum coverage when the distance is 1.5 m. Thus, we
set the distance to 1.5 m to guarantee the reading perfor-
mance. This setting is close to a typical noisy condition,
which is distinct from the free space condition, since the
issues in the realistic applications like the path loss, multi-
path effect and energy absorption all exist. Considering that
we deploy a relatively large number of tags (up to 160 tags
in experimental study and 480 tags in performance evalua-
tion) and a rather high tag density (up to 90 tags per square
meter) in realistic settings, the experimental findings from
the high tag density deployment can be highly scalable and
generalized to rather large scale settings. Specifically, we
attach each tag to a book and put these books back-to-back
in a very dense approach. We believe this tag density (up to
90 tags per m
2
) should be close to extreme case in scale for
conventional RFID applications. Since we use the mobile
RFID reader to scan the tags within its limited scanning
range, hence, after the whole process of continuous scan-
ning, all tags can be effectively identified. Therefore, as long
as we can tackle the problem in this situation, it can be
guaranteed that our solution is scalable to any large scale
during the continuous scanning.
On the whole, it took us over 300 hours to conduct an
extensive experimental study of up to 160 tags in realistic
settings. In order to sufficiently understand how the param-
eters separately/jointly affect the actual reading perfor-
mance, we conduct up to 100 various experiments, carrying
out lots of experimental comparisons and analysis on
the obtained results. In the following experiments, we
vary the tag density, r, from 10 to 40 tags/grid, while
adjusting the reader’s power from 20.7 dBm to 30.7 dBm for
0.5 1 1.5 2 2.5 3
20
30
40
50
60
70
80
Distance(m)
Effective tag size
Fig. 1. The number of tags read for various distances.
2274 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 14, NO. 11, NOVEMBER 2015