# Underwater Communications for Video Surveillance Systems at 2.4 GHz
## 3 Scenario and Test Bench
### Scenario Description for Underwater Measurements  
The measurements are performed in a fresh water swimming pool. The chemical parameters of water have been measured. It contains 0.3 mg/L of dissolved chlorine and bromine and the water pH is 7.2. The swimming pool is 8 m length and 4 m wide with a depth that ranges between 1.5 m. and 1.80 m. The size of pool allows us avoiding any reflection on the walls, ground and water surface. The swimming pool has been made with bricks covered by small tiles of mosaic. To carry out the tests, it is configured an ad hoc wireless connection between two laptops that have two vertical monopole antennae connected to their wireless cards. The laptops are maintained outside the water. We have used two HP pavilion dv6-6c13 ss Intel Core i7 2670 QM with 4 GB RAM memory. The antennae, that have 2 dBi of gain, are placed inside the water. They are connected to each laptop using a 3 m pigtail. The goal of these tests is the design of a system composed by an antenna and a small electronic board placed inside a watertight plastic box. Then, we evaluate if the antenna is able to transmit to the water when the medium changes. To do it, each antenna is put in a sealed plastic box (of 23.5 cm height, 8.7 cm width and 4.2 cm thickness) to make it watertight. The antennae are placed in the middle of the plastic box. Both antennae are faced by the widest part of the plastic box, so that the space between the antenna and the water is 2.1 cm. On the other hand, our system does not require the adaptation that a reviewer appoints because the antenna is surrounded by an air gap, as it is put inside a plastic and sealed box/container. For this reason the antenna starts to emit inside a small air gap created before the signal is introduced into the water. However and in order to be sure about this fact (because there is not previous work about it), we performed a preliminary test to ensure that the space between the antennae and the water does not influence over the maximum distance between devices. Because we are working with antennae prepared to transmit in air, we had to base our test in the wave's behavior in the medium where our antennae started to transmit. To perform these tests, we have used commercial 2.4 GHz wireless devices with a power transmission 100 mW. In [30], we tested different values of power from 100 mW to 800 mW. We observed that the transmission behavior worsens. In fact, the maximum distance between devices did not increase due to probably to the big strength of reflections and refractions. For this reason, we finally decided to work at 100 mW. As we  
Figure 5: Maximum and average data transfer rate for each video at 30 fps and 60 fps. stated in one of our previous work [31], the theoretical transmitted power for an IEEE 802.11 WLAN device is -40.2 dBm at 1 m. This is the maximum power it is possible to reach. For this reason, we designed a container that allowed having 1 m of space between the antenna and the water. Theoretically, we had to have longer distances because we allowed to the antenna to transmit with its maximum power transmission before transmitting through the water. However, the maximum distances obtained in both cases were the same, i.e., 27 cm (see Figure 6). For this reason, we finally decided to the use the small container. The goal of these tests is the design of a system, composed by an antenna and a small electronic board placed inside a watertight plastic box. Then, we evaluate if the antenna is able to transmit to the water when the medium changes. As Figure 7 shows, both antennae should be placed under the water. They were placed at enough depth to avoid any transmission from the open air and at enough distance to the pool ground, walls and surface to avoid reflections. Figure 6: Preliminary test to check the effect of air gap. (**a**) distance between antenna and water: 100 cm; (**b**) distance between antenna and water: 2.1 cm. Figure 7: Swimming pool where measures have been taken.