Bruno S. Chang

Optimizing the Number of Hops and Retransmissions for Energy Efficient Multi-Hop Underwater Acoustic Communications

Energy efficient underwater communications systems design is a crucial task, once battery replacement may be barely feasible in this scenario. As acoustic underwater channels are usually harsher than terrestrial wireless channels, decreasing the transmission distance through a multi-hop approach is very appealing. In this sense, in this paper, we analyze how much energy is required to successfully transfer an information bit over a multi-hop underwater acoustic network while considering the optimum number of hops, retransmissions, code rate, and signal-to-noise ratio. We also investigate the impact of delay constraints in the total energy consumption, by allowing limited or unlimited retransmissions. Our results show that great energy savings can be obtained when the above-mentioned parameters are jointly optimized for a given link distance. Moreover, even though the impact of multiple hops is more beneficial, allowing for a small number of retransmissions lead to important energy savings. Finally, we compare the obtained results with a terrestrial wireless communications case, showing that an adequate modeling of the underwater scenario is imperative for an energy efficient design, once the optimal number of hops for terrestrial wireless may considerably differ from that for underwater communications.

Widely linear MMSE precoding and equalization techniques for SC-FDE systems

Single-carrier systems using frequency-domain equalization (SC-FDE) systems were proposed to overcome the low robustness to carrier frequency offset (CFO) and high peak-to-average-power ratio (PAPR) inherent to regular orthogonal frequency-division multiplexing (OFDM) systems. Usually, linear minimum mean square error (MMSE) equalization is used to compensate the channel effect, since maximum likelihood (ML) detection is computationally impractical. However, if the transmitted signal comes from an improper constellation, widely linear processing can be used to take advantage of all the available second-order statistics from this transmitted signal, obtaining this way a performance gain when compared to the strictly linear case. In this paper, a SC-FDE system employing widely linear MMSE equalization is proposed in its regular and decision-feedback (DFE) versions. A SC-FDE system employing widely linear MMSE Tomlinson-Harashima precoding (THP) and equalization is also proposed. With Tomlinson-Harashima precoding, the error propagation problem observed in systems using a decision-feedback equalizer vanishes, because the feedback processing is done at the transmitter. Simulation results show that together with the error performance gain, these systems have lower sensibility to the feedback filter length in systems using decision-feedback equalizers. In Tomlinson-Harashima precoded systems, the performance gain is observed even with channel estimation/channel state information errors.

On the distribution of the SINR in precoded multicarrier systems using linear MMSE equalization

To calculate the unconditional bit error probability directly, the Signal to Interference-plus-Noise Ratio (SINR) distribution of a communications system must be known. However, for precoded systems employing Minimum Mean Square Error (MMSE) equalization, a suitable distribution has not been found yet. In this paper, we propose, as an approximation for the SINR of a precoded multicarrier system employing linear MMSE equalization, the use of the lognormal distribution with the smallest Kullback-Leibler distance to the observed SINR distribution. Simulation results show that the lognormal distribution matches accurately the SINR distribution. The Bit Error Rate (BER) performances with analytical computations using the lognormal distribution are very close to Monte Carlo simulations, even for high SNR values.

On the effect of channel estimation errors in SC-FDE systems using widely linear iterative equalization

In this paper the effect of channel estimation errors in single carrier systems using frequency domain widely linear iterative equalization is investigated. It was found out that when compared to their strictly linear versions their error performance is more robust with respect to these errors. They do not have the high error floor present in the error performance of strictly linear systems that do not take into account the channel estimation error into account; even when this error is considered their performance is slightly better.

Widely linear equalizer performance with multiple independent interferences

When non second order circular (NSOC) signals are transmitted over time/frequency varying channels, classical linear equalization (LE) suffers from performance loss due to the signals impropriety. For such signals, and signals corrupted by one single improper interference, it has been demonstrated that widely linear (WL) processing can enhance equalization performance. In this paper, we will study the limit of WL processing as a function of the number of interferers. We will carry simulations corroborated by analytical analysis and show that the performance of WL processing, in terms of bit error rate, decreases as the number of improper interferences increases.

Energy-Efficient Channel Coding Strategy for Underwater Acoustic Networks

Underwater acoustic networks (UAN) allow for efficiently exploiting and monitoring the sub-aquatic environment. These networks are characterized by long propagation delays, error-prone channels and half-duplex communication. In this paper, we address the problem of energy-efficient communication through the use of optimized channel coding parameters. We consider a two-layer encoding scheme employing forward error correction (FEC) codes and fountain codes (FC) for UAN scenarios without feedback channels. We model and evaluate the energy consumption of different channel coding schemes for a K-distributed multipath channel. The parameters of the FEC encoding layer are optimized by selecting the optimal error correction capability and the code block size. The results show the best parameter choice as a function of the link distance and received signal-to-noise ratio.

A simplified widely linear iterative equalizer for SC-FDE systems

A new simplified widely linear iterative equalizer for SC-FDE systems is presented in this paper. The proposed equalizer does not require the computation of the correlation coefficient of the symbol decisions after the first iteration of the equalizer, reducing its complexity. It also has almost the same error performance of its more complex version.

Energy consumption analysis of underwater acoustic networks using fountain codes

Underwater sensor networks have a wide range of applications, but face great technical challenges due to the harsh nature of the acoustic channel. Moreover, most sensors are battery operated, making energy saving essential in network design. In this paper we analyze the energy consumption of an underwater network employing fountain codes, whose main idea is to transmit an undetermined amount of coded bits as to ensure a given target frame error rate (FER) at the receiver without the need for retransmissions. In addition we optimize the carrier frequency, signal-to-noise ratio (SNR) and the code rate for such target FER. Then, we analyze the amount of extra redundant frames needed by the fountain code as a function of the transmission distance. Our main contribution is to show that the use of fountain codes can decrease up to 30% the energy consumption, especially in long-range scenarios.

Widely linear SC-FDE systems for underwater acoustic communication: Experimental results

This work presents experimental results for single carrier underwater communication systems using widely linear frequency domain equalization. The underwater channel models were obtained from a stochastic replay-based channel simulator, which was fed with data from channel measurements conducted in the harbor of Brest (France). It was seen that when symbols from improper constellations are considered it is possible to obtain a performance gain in this scenario for different conditions if widely linear equalization is used.

Modulation order optimization for energy efficient underwater acoustic communications

In this paper an optimization process to obtain the optimal code rate, SNR and modulation order for energy efficient underwater communications is presented. It was seen that it is possible to further decrease the consumption when compared to the binary case considering code rate optimization in conjunction with the selection of the optimum modulation order, while achieving higher data transmission rates.