Underwater acoustic sensor networks are a promising technology that allow real-time data collection in seas and oceans for a wide variety of applications. Smaller size and weight sensors can be achieved with working frequencies shifted from audio to the ultrasonic band. At these frequencies, the fading phenomena has a significant presence in the channel behavior, and the design of a reliable communication link between the network sensors will require a precise characterization of it. Fading in underwater channels has been previously measured and modeled in the audio band. However, there have been few attempts to study it at ultrasonic frequencies. In this paper, a campaign of measurements of ultrasonic underwater acoustic channels in Mediterranean shallow waters conducted by the authors is presented. These measurements are used to determine the parameters of the so-called κ-μ shadowed distribution, a fading model with a direct connection to the underlying physical mechanisms. The model is then used to evaluate the capacity of the measured channels with a closed-form expression.

We introduce the fluctuating double-Rayleigh with line-of-sight (fdRLoS) fading model as a natural generalization of the double-Rayleigh with line-of-sight fading model, on which the constant-amplitude line-of-sight component is now allowed to randomly fluctuate. We discuss the key benefits of the fdRLoS fading model here formulated over the state of the art, and provide an analytical characterization of its chief probability functions. We analyze the effect of the fading parameters that define the model, and discuss their impact on the performance of wireless communication systems.

In this paper we present a bivariate Rician shadowed fading model where the shadowing is assumed to follow a Nakagami-m distribution. We derive exact expressions involving a single integral for both the joint probability density function (PDF) and the joint cumulative distribution function and we also derive an exact closed-form expression for the moment generating function. As a direct consequence we obtain a closed-form expression for the power correlation coefficient between Rician shadowed variables as a function of the correlation coefficient between the underlying variables of the model. Additionally, in the particular case of integer m we show that the PDF can be expressed in closed form in terms of a sum of m Meijer G-functions of two variables. Results are applied to analyze the outage probability of a dual-branch selection combiner in correlated Rician shadowed fading and the evaluation of the level crossing rate and average fade duration of a sampled Rician shadowed fading envelope.

In this work we present the results of a measurement campaign carried out in the Mediterranean sea aimed at characterizing the underwater acoustic channel in a wideband at ultrasonic frequencies centered at 80 kHz with a width of 96 kHz, covering two octaves from 32 to 128 kHz. So far, these type of wideband measurements are not found in the literature. Periodic orthogonal frequency division multiplexing (OFMD) sounding signals using Zadoff-Chu sequences have been specially designed for this purpose. The collected data has been post-processed to estimate the time-variant impulse and frequency responses and relevant parameters for system design like the time coherence, bandwidth coherence, delay spread and Doppler bandwidth. The statistical behavior of the channel gain random fluctuation has also been analyzed. This information has been extracted for both the global channel and each path separately. The wide bandwidth of the measurements have allowed the characterization of the channel in a scarcely explored ultrasonic band with an accuracy that is far beyond what is reported in previous works.

The second order scattering fading (SOSF) channel is a versatile generalization of the Rician fading model. Besides a line of sight (LOS) component and a Rayleigh-diffused scattering component, it also incorporates a double-Rayleigh scattering component. In this letter, we present an exact closed-form formula for the generalized moment generating function (MGF) of the signal-to-noise ratio in SOSF channels, as well as for its moments. This new result allows for the evaluation of important performance metrics in wireless communication systems, such as the amount of fading and energy detection, for which results in the literature are unavailable because of the complicated nature of this fading model. Strikingly, we observe that whereas the amount of fading is monotonic with respect to the fading parameters α and β, this is not the case for the energy detection performance under strong LOS conditions.

We present a new approach to the statistical characterization of the second order scattering fading (SOSF) channel model, which greatly simplifies its analysis. Exploiting the unadvertised fact that the SOSF channel can be seen as a continuous mixture of Rician fading channels, we obtain expressions for its probability density function and cumulative density function that are numerically better-behaved than those available in the literature. Our approach allows for obtaining new results for the SOSF model, such as a closed-form expression for its moment-generating function, as well as the characterization of the average channel capacity. Relevantly, and somehow counterintuitively, we observe that in the presence of a strong line-of-sight (LOS) component, the channel capacity of a LOS plus double-Rayleigh scattered diffuse component is larger than its LOS plus Rayleigh (i.e Rician-like) counterpart.