The increasing exploitation of natural resources under water, particularly in the sea, has ignited the development of many technological advances in the domains of environmental monitoring, oil and gas exploration, warfare, among others. In all these domains, underwater wireless communications play an important role, where the technologies available rely on radiofrequency, optical, and acoustic transmissions. This paper surveys key features inherent to these communication technologies, putting into perspective their technical aspects, current research challenges, and to-be-explored potential. I. INTRODUCTION Underwater wireless communications present new and distinct challenges when compared to wired and wireless communications through the atmosphere, requiring sophisticated communication devices to achieve relatively low transmission rates, even over short distances. Indeed, the underwater environment possesses a number of distinguishing features that make it unique and rather different from terrestrial radio propagation where traditional communication systems are deployed. Under water, several phenomena may influence communications, such as salt concentration, pressure, temperature, amount of light, winds and their effects on waves, just to mention a few [1], [2]. Despite all challenges, wireless communications certainly play an important role in practical underwater systems. Monitoring different phenomena in underwater environment is relevant in many different applications, such as oil and gas exploration, coastal security, environmental impact surveillance, navigation, and ocean-pollution control [3]. Specific examples include transmission of data among devices, such as AUV (autonomous underwater vehicle) to AUV and buoy to AUV, particularly those employing wireless links. In fact, an entire underwater wireless communication network could

We study the energy consumption of individual links in wireless sensor networks (WSN). Three widely used digital modulation schemes, i.e. MQAM, MPSK, and MFSK, are analyzed and compared. Both transmitted signal power and circuit power consumptions are considered. For modeling the signal power, we use the relation between channel capacity or cutoff rate and signal to noise ratio (SNR). By numerical calculation, we find the optimized parameters for minimizing the energy consumption per information bit in a point-to-point wireless link.

We study the energy consumption of wireless sensor network links applying three widely used digital modulation schemes, i.e. MQAM, MPSK, and noncoherent MFSK. For MQAM and MPSK, pulse shaping is considered. Both transmitted signal power and circuit power are taken into account where the former is modeled using the relationship between cutoff rate and signal-to-noise ratio (SNR) and the latter is modeled considering the power consumption of typical hardware used in wireless sensor nodes. For each modulation type, the optimum parameters for minimizing the energy per information bit are derived. Moreover, a comparison among the three modulation types is presented. I. INTRODUCTIONEnergy efficient schemes are of paramount importance for wireless sensor networks (WSN) composed of battery-powered nodes, where energy efficiency directly affects the lifetime of the network.Energy consumption in energy-constrained networks has already been addressed in the literature. For example, Cui et al.[1] analyzed both transmission time and constellation size optimization for MQAM and MFSK modulations (both coded and uncoded), considering both transmission and circuit energy consumption. It has been shown that optimizing transmission time or, equivalently, constellation size, minimizes the total energy used for transmission of information.In [2], the energy performances of uncoded MPSK, MQAM, and MFSK modulations in both AWGN and Rayleigh fading channels were evaluated and compared for very short-range (less than 10 meters) communication. It has been concluded that MQAM is more energy-efficient than the other modulation schemes in this case.More recently, the combination of Reed-Solomon codes with MPSK, MQAM, and MFSK modulations has been studied in [3], where an optimum pair of coding and modulation were derived by computing required energy per bit.In our previous work [4,5], based on the relations between capacity (and cutoff rate) and SNR of MQAM, MPSK, and noncoherent MFSK, the optimum transmission scheme (constellation size and information rate) that results in minimum energy was derived for distances up to one hundred meters between WSN nodes. In [5], a comparison among the three cited modulation schemes was performed. A fixed circuit power consumption model developed in [1] was used.

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