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threshold) in default, therefore these threshold values
are used in our experiments.
4.2 Evaluation and Discussion
Evaluation of the proposed methods are performed
using different measures. Manual placement of gate-
ways is used as a baseline during the comparison
of the proposed approaches. Firstly, the number of
gateways is compared. Secondly, a measure called
Failed Packet Attempt (FPA) ratio (as percentage to
all packet transmissions attempts of a sensor) of each
gateway placement scenario is extracted from the re-
sults of ns-3. Thirdly, by using LoRaPlan, RSSI
(dBm) value of each reachable sensor is calculated
for different coverage distances ranging from 1000
meters to 10000 meters. Finally, in order to see the
differences between proposed approaches, gateway
placements are shown on map.
Figure 3a shows how many gateways are used in
order to cover all the sensors located in Ergene River.
In all methods, the number of gateways decreases as
the range increases. Specially in MIP formulation the
decline of gateway count is immense and constant.
This is because the main objective of the optimization
model is to minimize the number of gateways and the
formulation in Equation 5. Also in coverage intersec-
tion method, gateway count declined in short and long
ranges. However, it did not change when the coverage
distance is 6000 to 8000 meters. Also as expected,
the comparison results shows that placing gateways
manually is inefficient in term of the total of number
of gateways, since it can be seen that even when the
range is 10000 meters, 40 gateway locations are se-
lected in manual placement, whereas 27 gateways are
used in coverage intersection method and 14 gateway
location is proposed in MIP formulation.
Figure 3b indicates the probability of a packet loss
during a transmission considering the proposed gate-
way placement. FPA percentage for every method
increase, while the coverage distance increases. As
expected, the manual placement is the best approach
to have a network with less transmission failures.
Nonetheless, it is important to remember that within
the manual placement, the number of gateways is the
highest among proposed approaches. In small ranges
such as 1000 to 3000 meters; MIP method ends up
with finer FPA percentages than coverage intersection
method. For an example in the scenario, in which the
range is 2000 meters, FPA percentage of MIP formu-
lation is approximately 25,63%, whereas the FPA per-
centage of coverage intersection methods is 30,83%.
In medium ranges, FPA percentage of Coverage Inter-
section method is less than the proposed optimization
model. For long ranges; gateway placement proposal
of the MIP method performs better than coverage in-
tersection approach.
In figures 4a and 4b; distribution of RSSI values
of the reachable sensors for each gateway placement
is compared. Comparisons are done for coverage dis-
tances of 2000 meters and 8000 meters, respectively.
In other words, behaviour of the introduced methods
and the base case are analyzed in terms of short and
long ranges of coverage. In all comparisons -100 dBm
is selected as an upper threshold, whereas the -120
dBm is selected as a lower threshold as they set as
default in simulation software, LoRaPlan.
Figure 4a shows the distributions of RSSI values
for reachable sensors in a short range of coverage,
where the coverage is 2000 meters. All the distri-
butions indicate that after each gateway placement
provided by the proposed methods, strength of the
sensors are above the upper threshold. Moreover, it
is noticeable that coverage intersection method ends
up with RSSI values which are similar to the manual
placement, whereas the number of gateways for cov-
erage intersection much less than the manual place-
ment for 2000 meters as shown in Figure 3a.
Distributions of RSSI values in long range sce-
nario, where the coverage distance is 8000 meters, are
presented in Figure 4b. In terms of MIP formulation,
results show that signals strengths are between upper
and lower thresholds. Also in this scenario, MIP for-
mulation ends up with signals that their strengths are
below the lower threshold which means there are no
signal. As it mentioned previously, this is because
that the number of gateways is reduced significantly
in MIP formulation, specially in long range coverage
cases. Figure3a shows that for 8000 meters range,
the number of gateways used in the MIP formulation
is less than half the gateways used in the coverage
intersection method and approximately one third the
gateways used in manual deployment. Therefore the
distances between gateway and sensors are greater in
MIP formulation, thus signal strengths are low.
On the other hand, distribution of RSSI values
with gateway deployment by coverage intersection
differ from the one with manual placement. In cov-
erage intersection method, RSSI values are mostly
between lower threshold and a bit above the upper
threshold. Also there is a group of sensors receiving
signals that their strengths are very high, in coverage
intersection method. However, in manual placement
of gateways, some of the signals are below the lower
threshold. Also in manual placement, there is a bigger
group of sensors that their signal strengths are very
high like in coverage intersection method. In overall,
coverage intersection method protects its stability in
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