Doctor of Philosophy

dissertationThe use of multicarrier techniques has allowed the rapid expansion of broadband wireless communications. Orthogonal frequency division multiplexing (OFDM) has been the most dominant technology in the past decade. It has been deployed in both indoor Wi-Fi and cellular environments, and has been researched for use in underwater acoustic channels. Recent works in wireless communications include the extension of OFDM to multiple access applications. Multiple access OFDM, or orthogonal frequency division multiple access (OFDMA), has been implemented in the third generation partnership project (3GPP) long- term evolution (LTE) downlink. In order to reduce the intercarrier interference (ICI) when user's synchronization is relaxed, filterbank multicarrier communication (FBMC) systems have been proposed. The first contribution made in this dissertation is a novel study of the classical FBMC systems that were presented in 1960s. We note that two distinct methods were presented then. We show that these methods are closely related through a modulation and a time/frequency scaling step. For cellular channels, OFDM also has the weakness of relatively large peak-to-average power ratios (PAPR). A special form of OFDM for the uplink of multiple access networks, called single carrier frequency division multiple access (SC-FDMA), has been developed to mitigate this issue. In this regard, this dissertation makes two contributions. First, we develop an optimization method for designing an effective precoding method for SC-FDMA systems. Second, we show how an equivalent to SC-FDMA can be developed for systems that are based on FBMC. In underwater acoustic communications applications, researchers are investigating the use of multicarrier communication systems like OFDM in underwater channels. The movement of the communicating vehicles scales the received signal along the time axis, which is often referred to as Doppler scaling. To undo the signal degradation, researchers have investigated methods to estimate the Doppler scaling factor and restore the original signal using resampling. We investigate a method called nonuniform fast Fourier transform (NUFFT) and apply that to increase the precision in the detection and correction of the Doppler scaling factor. NUFFT is applied to both OFDM and FBMC and its performance over the experimental data obtained from at sea experiments is investigated

A State of Art Concept in Contriving of Underwater Networks

the underwater ocean environment is widely considered as one of the most difficult communications channels. Underwater acoustic networks have recently emerged as a new area of research in wireless networking. Underwater networks are generally formed by acoustically connected ocean - bottom sensors, underwater gateways and a surfa ce station, which provides a link to an on - shore control center. In recent years, there has been substantial work on protocol design for these networks with most efforts focusing on MAC and network layer protocols. Low communication bandwidth, large propag ation delay, floating node mobility, and high error probability are the challenges of building mobile underwater wireless sensor networks (WSN) for aquatic applications. Underwater sensor networks (WSNs) are the enabling technology for wide range of appl ications like monitoring the strong influences and impact of climate regulation, nutrient production, oil retrieval and transportation, man y scientific, environmental, commercial, safety, and military applications. This paper first introduces the concept o f UWSN, operation, applications and then reviews some recent developments within this research area and proposes an adaptive push system for dissemination of data in underwater wireless sensor networks. The goal of this paper is to survey the existing net w ork technology and its applicability to underwater acoustic channels. In this paper we provide an overview of recent medium acces s control, routing, transport, and cross - layer networking protocols. It examines the main approaches and challenges in the desi gn and implementation of underwater wireless sensor networks. Finally, some suggestions and promising solutions are given for th ese issues

An efficient scalable scheduling mac protocol for underwater sensor networks

Underwater Sensor Networks (UWSNs) utilise acoustic waves with comparatively lower loss and longer range than those of electromagnetic waves. However, energy remains a challenging issue in addition to long latency, high bit error rate, and limited bandwidth. Thus, collision and retransmission should be efficiently handled at Medium Access Control (MAC) layer in order to reduce the energy cost and also to improve the throughput and fairness across the network. In this paper, we propose a new reservation-based distributed MAC protocol called ED-MAC, which employs a duty cycle mechanism to address the spatial-temporal uncertainty and the hidden node problem to effectively avoid collisions and retransmissions. ED-MAC is a conflict-free protocol, where each sensor schedules itself independently using local information. Hence, ED-MAC can guarantee conflict-free transmissions and receptions of data packets. Compared with other conflict-free MAC protocols, ED-MAC is distributed and more reliable, i.e., it schedules according to the priority of sensor nodes which based on their depth in the network. We then evaluate design choices and protocol performance through extensive simulation to study the load effects and network scalability in each protocol. The results show that ED-MAC outperforms the contention-based MAC protocols and achieves a significant improvement in terms of successful delivery ratio, throughput, energy consumption, and fairness under varying offered traffic and number of nodes

Underwater acoustic networks

Charla invitada.Wireless underwater acoustic communication systems have become one of the most promising technologies for the development and deployment of future ocean observation and sensor networks. Applications range from oil prospecting and transportation to aquaculture, and include pollution control, climate recording, prediction of natural disturbances, search and survey missions, etc. The high attenuation of electromagnetic waves in underwater medium precludes them from being used as the information vehicle. Therefore, acoustic waves are the most viable alternative for the transmission. Nevertheless, the underwater acoustic channel is not free of drawbacks. On the contrary, it exhibits important obstacles that must be overcome if communication networks are to be implemented in the future. In addition, underwater systems have inherently serious problems of power supply and batteries duration. At the same time, network protocols must take into account the specific characteristics of the transmission medium, not dismissing the slow propagation speed. This talk will present an overall view of the acoustic channel characteristics and their impact on network protocols.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

PERFORMANCE EVALUATION OF A MULTICARRIER MIMO SYSTEM BASED ON DFT-PRECODING AND SUBCARRIER MAPPING

The ever-increasing end user demands are instigating the development of innovative methods targeting not only data rate enhancement but additionally better service quality in each subsequent wireless communication standard. This quest to achieve higher data rates has compelled the next generation communication technologies to use multicarrier systems e.g. orthogonal frequency division multiplexing (OFDM), while also relying on the multiple-input multiple-output (MIMO) technology. This paper is focused on implementing a MIMO-OFDM system and on using various techniques to optimize it in terms of the bit-error rate performance. The test case considered is a system implementation constituting the enabling technologies for 4G and beyond communication systems. The bit-error rate optimizations considered are based on preceding the OFDM modulation step by Discrete Fourier Transform (DFT) while also considering various subcarrier mapping schemes. MATLAB-based simulation of a 2 × 2 MIMO-OFDM system exhibits a maximum of 2 to 5 orders of magnitude reduction in bit-error rate due to DFT-precoding and subcarrier mapping respectively at high signal-to-noise ratio values in various environments. A 2-3dBs reduction in peak-to-average power ratio due to DFT-precoding in different environments is also exhibited in the various simulations

An Underwater Acoustic Implementation of DFT-Spread OFDM

Nature Science Foundation of China [60672046]; Natural Science Foundation of Fujian Province, China [2009J05155]; 985 innovation project on information technology of Xiamen UniversityThis paper presents a design of DFT-spread OFDM system applied to an underwater acoustic channel. It does not only combine all the advantages of a conventional OFDM system but also reduces the peak-to-average power ratio of the transmit signal. Besides, the scheme spreads the information over several subcarriers as a result of the application of an additional DFT operation and leads to a diversity gain in a frequency-selective fading channel, which is one of the many challenges of communicating data through an underwater acoustic channel. Simulation results show that our proposal possesses good bit-error-rate performance. The system has been tested in a real underwater acoustic channel-the experimental pool in Xiamen University. The experimental results show that the DFT-spread OFDM system can achieve better results than a simple OFDM system in a benign underwater channel