Unai Fernandez-Plazaola

In this work, we present a transmission system that uses single-carrier modulation MQAM for UAC in the ultrasonic band. It exploits the diversity that four hydrophones provide at reception in a SIMO 1×4 configuration and also includes, for each stream, a PLL for synchronization and a turbo-equalization subsystem, with FSE and DFE filters, adaptive by means of LMS algorithm, both of which are driven by an LDPC decoder decisions. The paper describes the system design and analyze its performances on trials carried out on shallow waters of the Mediterranean sea. Achievable data rates exceeding 200 kb/s between links of transmission distances longer than 200 m are obtained with a satisfactory low BER.

The understanding of the video encoding/decoding processes is a key skill for students of computer science, engineering, or telecommunications, especially those specialized in multimedia. The special characteristics of the video signals, and how such features are leveraged to compress the video frames using coding theory, are hard to understand for engineering students. In this paper, we describe MPEG‐2 analyzer, an educational tool designed to provide a comprehensive and interactive way to understand MPEG‐2 video compression and encoding techniques. We have used our tool to evaluate its usefulness in reinforcing the knowledge about the MPEG‐2 video (de)coding process to Telecommunication Engineering students. Results show that the evaluation of the tool is very positive, and it has helped them to understand many of the complex concepts covered in the course.

The purpose of this work is to present a flexible system that supports the study of wideband underwater acoustic communications (UAC). It has been developed both to measure channels and to test transmission techniques under realistic conditions in the ultrasonic band. This platform consists of a hardware (HW) part that includes multiple hydrophones, projectors, analog front-ends, acquisition boards, and computers, and a software (SW) part for the generation, reception, and management of acoustic sounding signals and noise. UAC channels are among the most hostile ones and exhibit an important attenuation and distortion, essentially due to both multipath propagation, which results in a very long channel impulse response, and time-varying behavior, which produces a notable Doppler spread. To cope with this challenging medium, sophisticated transmission techniques must be employed. In this sense, adequate signal processing algorithms have been designed aiming not only at the analysis and characterization of underwater communication channels but also at the evaluation of diverse modulation, detection, and coding schemes, from Orthogonal Frequency Division Multiplexing (OFDM) to single-carrier digital modulations with a single-input multiple-output (SIMO) configuration that takes advantage of diversity techniques. Wideband sounding signals, to be injected into the sea from the transmitter side, are created with patterns that allow multiple tests on a batch. With offline processing of the captured data at the receiver side, different trials can be carried out in a very flexible manner. The different aspects of the platform are described in detail: the HW equipment used, the SW interface to control acquisition boards, and the signal processing algorithms to estimate the UAC channel response. The platform allows the analysis and design of new proposals for underwater communications systems that improve the performance of the current ones.

In this paper, we present the development of a multicarrier modulation system of low complexity for broadband underwater acoustic communications (UAC), whose frequency band is located in the ultrasonic range, specifically between 32 kHz and 128 kHz. Underwater acoustic channels are recognized among the most hostile communication channels due to their strong time and frequency selectivity and, hence, the design of high-performance systems is a challenge that is difficult to resolve at the present time with state-of-art technology. The aim of the proposed system is to reach a reasonable bit rate, between 40 and 50 Kbps, over these channels that allows, for instance, the transmission of video signals of limited quality. We describe an orthogonal frequency division multiplexing (OFDM) modem prototype with a parameter setting and design specifically adapted to the channel nature. For this purpose, actual measurements carried out at the Mediterranean sea, on shallow waters, have been used to evaluate the system performance and to optimize the design. A discussion on several modulations and OFDM configurations is presented that leads to the selection of differential and non-differential quadri-phase shift keying (QPSK) as good candidates depending on synchronization capabilities.

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.

We present a general and tractable fading model for line-of-sight (LOS) scenarios, which is based on the product of two independent and non-identically distributed κ-μ shadowed power envelopes. Simple closed-form expressions for the probability density function, cumulative distribution function and moment-generating function are derived, which are as tractable as the corresponding expressions derived from a product of Nakagami-m random variables. This model simplifies the challenging characterization of LOS product channels, as well as combinations of LOS channels with non-LOS ones. We leverage these results to analyze performance measures of interest in the contexts of wireless powered and backscatter communications, where both forward and reverse links are inherently of LOS nature, as well as in device-to-device communications subject to composite fading. In these contexts, the proposed model shows a higher flexibility when fitting field measurements with respect to conventional approaches based on product distributions with deterministic LOS, together with a more complete physical interpretation of the underlying propagation characteristics.

We here present a mobile-to-mobile shallow water acoustic (SWA) channel simulator based on the proper construction of a linear time-variant (LTV) channel impulse response (CIR) from a set of linear time-invariant (LTI) CIRs. These LTI CIRs are obtained from a deterministic static SWA channel model, which we also describe here. We validate the results of our simulator by comparing them in statistics terms with data collected in a measuring campaign.

The significant attenuation experienced by electromagnetic waves in sea water is the main reason why acoustic waves are generally preferred in underwater communication. Nevertheless, acoustic waves have various drawbacks. For example, they are negatively affected by factors such as mechanical noise, slow propagation speed, and, particularly, low bandwidth, which leads to digital links at a lower bit rate. However, in short-range links, these problems can be overcome by reconsidering the use of electric current communications. For instance, data collected by remote-control vehicles in offshore oil and gas and renewable energy plants can be transmitted at distances of even 1m or less. This study uses previous frequency response measurements taken in deep water to explore the capacity of a short-range electromagnetic underwater channel. Because of water movement, the nonstatic position of the vehicle when the transmission occurs means that the channel is regarded as randomly time-variant. A statistical model is proposed and the ergodic capacity is calculated for a 7MHz bandwidth channel at distances ranging from 0.5m to 5m as well as for different values of transmitter power. The results of this study reflect capacity values of tens of kbps at distances of approximately 5m to several Mbps at distances of less than 1.5m.

Because it achieves similar performance to that of orthogonal frequency-division multiple access (OFDMA) with much more favorable envelope characteristics and lower peak-to-average power ratio (PAPR), single-carrier frequency-division multiple access (SC-FDMA) was chosen as the uplink transmission technology for Long Term Evolution (LTE) and LTE-Advanced systems. This choice has provoked a growing interest in this transmission technology; therefore, there is extensive literature on its performance in terms of bit error rate (BER) and PAPR. However, research on its spectral efficiency has been scarce until now. This paper aims to help fill this gap with an analytical study of the spectral efficiency in SC-FDMA when adaptive modulation and coding (AMC) and linear frequency-domain equalization are applied. This paper therefore computes the spectral efficiency for zero-forcing (ZF) and minimum-mean-square-error frequency-domain equalization (MMSE-FDE) for different Nakagami-m fading channels.

In this paper, we present a novel analytical framework for the calculation of the level crossing rate (LCR) and the average fade duration (AFD) of fading channels sampled at a certain sampling period TS. These expressions are valid for arbitrary fading distributions with arbitrary correlation and can be easily computed in terms of the cumulative distribution function (cdf) of the fading envelope and its bivariate cdf. This approach yields interesting insights into the effect of finite sampling in the higher order statistics of fading processes. We also demonstrate that the proposed expressions for sampled fading process converge with the existing expressions for continuous fading processes as the sampling period tends to zero. As a direct application, exact closed-form expressions are given for the LCR and AFD of sampled Rayleigh fading processes, which are suitable to characterize the higher order statistics of the equivalent frequency-domain fading process in multipath Rayleigh fading.