Chitode Signals And Systems Pdf 17
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When higher data rates are needed, we should make use of radio frequency (RF) methods, which are able to reach communication data rates of up to 100 Mb/s in very short distances, apart from presenting substantial immunity from the environmental features. Electromagnetic (EM) waves, in the RF range, can also be a good option for underwater wireless communication systems. EM waves are less sensitive to reflection and refraction effects in shallow water than acoustic waves. In addition, suspended particles have very little effect on them. The speed of EM waves is higher (150,000 times greater) than that of acoustic ones The speed of an EM wave mainly depends on permeability (μ), permittivity (ε), conductivity (σ) and volume charge density (ρ) [7]. These parameters change with the type of water and the electrical conductivity value associated with the medium often varies, thus the wave propagation speed and absorption coefficient, which are directly related to the working frequency, also vary. Conductivity presents different values for each case, seawater has a high conductivity average value, which is around 4 S/m (obviously it changes with the salinity and physical properties of each kind of sea water), but in fresh water the typical value is 0.01 S/m and drinking water presents a conductivity between 0.005 and 0.05 S/m. Moreover, the permittivity of seawater changes as a function of the frequency, the temperature and the salinity. In [8,9], authors provided a relationship model of this dependency in the water. Thus, the main problem for underwater communications based on EM waves is the high attenuation due to the conductivity of the water. This attenuation increases when the EM wave frequency increases. Hence, the higher frequencies will register greater signal losses.
In this paper, we perform a practical study of the behavior of EM signals (in the 2.4 GHz ISM frequency band) in underwater environments, using devices compatible with the IEEE 802.11 standard [10]. We have analyzed other technologies that also work on this frequency. This is the IEEE 802.15.4 standard [11]. A priori, we think that, due to the low-power consumption of IEEE 802.15.4, it would be better to use these devices as sensor nodes. However, our application needs data transfer rates higher than the ones offered by IEEE 802.14.5. For this reason, we should sacrifice a little power consumption in favor of improved data transfer rates. Table 1 shows a comparison of the maximum data transfer rates of both wireless technologies.
Anguita et al. dismissed the RF method for underwater communication because they reported that it is strongly attenuated [17]. Thus they took as invalid the 2.4 GHz frequency. However EM signals offer higher throughputs than acoustic signals by up to an order of magnitude. For example, Nowsheen et al. [18,19] developed an FPGA-based modem, that used frequencies ranging from 100 kHz to 1 MHz and BPSK modulation, with shipments of data packets with a duration of 1 ms with a wait time of 20 ms. They stated that it is an appropriate interval to avoid the effects of reflections in the tank.
where σ is the electrical conductivity, Xe the electric susceptibility of the medium and μr is the magnetic permeability of medium (in this case, it is the water). Equations (1) and (2) show that the wave propagation and absorption coefficients in freshwater are independent of the working frequency of the transmitted signals.
Although our proposal provides short communication distances in underwater wireless sensor networks, we can use it for precision monitoring such as ecosystems contaminated by invasive plants (especially in ponds where there are some poisonous plants that can contaminate the water) or hazardous waste (e.g. in swamps, the quality of the water is different depending on the season because the water may contain some organic material that may be affected when it is warmer because the pH is different). In both cases the water cannot be used for human consumption, but, in some cases, it can be used by industries to run their plants and supply the water cooling system. Moreover, our proposal can be used to control the pollution of the water, which may come from industries and nearby roads, accurately. 153554b96e
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