Performance evaluation of T-transform based OFDM in underwater acoustic channels

PhD ThesisRecently there has been an increasing trend towards the implementation of orthogonal frequency division multiplexing (OFDM) based multicarrier communication systems in underwater acoustic communications. By dividing the available bandwidth into multiple sub-bands, OFDM systems enable reliable transmission over long range dispersive channels. However OFDM is prone to impairments such as severe frequency selective fading channels, motioned induced Doppler shift and high peak-to-average-power ratio (PAPR). In order to fully exploit the potential of OFDM in UWA channels, those issues have received a great deal of attention in recent research. With the aim of improving OFDM's performance in UWA channels, a T-transformed based OFDM system is introduced using a low computational complexity T-transform that combines the Walsh-Hadamard transform (WHT) and the discrete Fourier transform (DFT) into a single fast orthonormal unitary transform. Through real-world experiment, performance comparison between the proposed T-OFDM system and conventional OFDM system revealed that T-OFDM performs better than OFDM with high code rate in frequency selective fading channels. Furthermore, investigation of different equalizer techniques have shown that the limitation of ZF equalizers affect the T-OFDM more (one bad equalizer coefficient affects all symbols) and so developed a modified ZF equalizer with outlier detection which provides major performance gain without excessive computation load. Lastly, investigation of PAPR reduction methods delineated that T-OFDM has inherently lower PAPR and it is also far more tolerant of distortions introduced by the simple clipping method. As a result, lower PAPR can be achieved with minimal overhead and so outperforming OFDM for a given power limit at the transmitter

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

Ciphered BCH Codes for PAPR Reduction in the OFDM in Underwater Acoustic Channels

We propose an effective, low complexity and multifaceted scheme for peak-to-average power ratio (PAPR) reduction in the orthogonal frequency division multiplexing (OFDM) system for underwater acoustic (UWA) channels. In UWA OFDM systems, PAPR reduction is a challenging task due to low bandwidth availability along with computational and power limitations. The proposed scheme takes advantage of XOR ciphering and generates ciphered Bose–Chaudhuri–Hocquenghem (BCH) codes that have low PAPR. This scheme is based upon an algorithm that computes several keys offline, such that when the BCH codes are XOR-ciphered with these keys, it lowers the PAPR of BCH-encoded signals. The subsequent low PAPR modified BCH codes produced using the chosen keys are used in transmission. This technique is ideal for UWA systems as it does not require additional computational power at the transceiver during live transmission. The advantage of the proposed scheme is threefold. First, it reduces the PAPR; second, since it uses BCH codes, the bit error rate (BER) of the system improves; and third, a level of encryption is introduced via XOR ciphering, enabling secure communication. Simulations were performed in a realistic UWA channel, and the results demonstrated that the proposed scheme could indeed achieve all three objectives with minimum computational powerThis research was funded by a grant from the Spanish Ministry of Science and Innovation in the framework of the project “NAUTILUS: Swarms of underwater autonomous vehicles guided by artificial intelligence: its time has come” (PID2020-112502RB / AEI / 10.13039/501100011033). Partial funding for open access charge: Universidad de Málag

Enhanced Artificial Bee Colony, Square Root Raised Cosine Precoding, and Mu law Compandor for Optimization of MIMO-OFDM System

The efficiency and high-speed data transfer rate of the communication system are increased based on Orthogonal Frequency Division Multiplexing (OFDM). The existing research in OFDM involves applying optimization methods to improve the system's efficiency. The high Peak Average Power Ratio (PAPR) value is a major limitation in the OFDM system, and this provides distortion due to the non-linear High-Power Amplifier (HPA). Local optima trap and lower convergence are two main limitations in existing optimization methods. This research proposes Enhanced Artificial Bee Colony (ABC) optimization method with a precoding-compandor technique to increase the efficiency of the OFDM system. Enhanced ABC method is applied with Boltzmann search to increase the exploitation capacity of the optimization efficiency. The selective mapping technique is applied to transform the candidate signal in the system. The ABC method increases exploration, and Boltzmann search increases exploitation. The enhanced ABC method increases the exploitation process that helps to overcome local optima traps and lower convergence. Square Root Raised Cosine (SRRC) precoding and Mu law compandor techniques were applied to reduce the PAPR. The Discrete Cosine Transform (DCT) technique is applied for domain conversion in the OFDM system. The proposed method has a convergence rate of 6.4069, and the existing one has a 6.4033 convergence rate. The enhanced ABC method provides higher efficiency in the MIMO-OFDM system regarding Symbol Error Rate (SER), PAPR, and Bit Error Rate (BER)

Estimation of Symmetric Channels for Discrete Cosine Transform Type-I Multicarrier Systems: A Compressed Sensing Approach

The problem of channel estimation for multicarrier communications is addressed. We focus on systems employing the Discrete Cosine Transform Type-I (DCT1) even at both the transmitter and the receiver, presenting an algorithm which achieves an accurate estimation of symmetric channel filters using only a small number of training symbols. The solution is obtained by using either matrix inversion or compressed sensing algorithms. We provide the theoretical results which guarantee the validity of the proposed technique for the DCT1. Numerical simulations illustrate the good behaviour of the proposed algorithm

Digital signal processing for fiber-optic communication systems

As the available bandwidth of optical fibers has been almost fully exploited, Digital Signal Processing (DSP) comes to rescue and is a critical technology underpinning the next generation advanced fiber-optic systems. Literally, it contributes two principal enforcements with respect to information communication. One is the implementation of spectrally-efficient modulation schemes, and the other is the guarantee of the recovery of information from the spectrally-efficient optical signals after channel transmission. The dissertation is dedicated to DSP techniques for the advanced fiber-optic systems. It consists of two main research topics. The first topic is about Fast-orthogonal frequency-division multiplexing (OFDM) — a variant OFDM scheme whose subcarrier spacing is half of that of conventional OFDM. The second one is about Fresnel transform with the derivation of an interesting discrete Fresnel transform (DFnT), and the proposal of orthogonal chirp-division multiplexing (OCDM), which is fundamentally underlain by the Fresnel transform. In the first part, equalization and signal recovery problems result from the halved subcarrier spacing in both double-sideband (DSB) and single-sideband (SSB) modulated Fast-OFDM systems are studied, respectively. By exploiting the relation between the multiplexing kernels of Fast-OFDM systems and Fourier transform, equalization algorithms are proposed for respective Fast-OFDM systems for information recovery. Detailed analysis is also provided. With the proposed algorithms, the DSB Fast-OFDM was experimentally implemented by intensity-modulation and direct detection in the conventional 1.55-μm and the emerging 2-μm fiber-optic systems, and the SSB Fast-OFDM was first implemented in coherent fiber-optic system with a spectral efficiency of 6 bit/s/Hz at 36 Gbps, for the first time. In the second part, Fresnel transform from optical Fresnel diffraction is studied. The discrete Fresnel transform (DFnT) is derived, as an interesting transformation that would be potentially useful for DSP. Its properties are proved. One of the attractive properties, the convolution-preservation property states that the DFnT of a circular convolution of two sequences is equal to the DFnT of either one convolving with the other. One application of DFnT is practically utilized in the proposal of OCDM. In the OCDM system, a large number of orthogonal chirped waveforms are multiplexed for high-speed communication, achieving the maximum spectral efficiency of chirp spread spectrum systems, in the same way as OFDM attains the maximum spectral efficiency of frequency-division multiplexing. Owing to the unique time-frequency properties of chirped waveforms, OCDM outperforms OFDM and single-carrier systems, and is more resilient against the noise effect, especially, when time-domain and frequency-domain distortions are severe. Experiments were carried out to validate the feasibility and advantages of the proposed OCDM systems

The Discrete Linear Chirp Transform and its Applications

In many applications in signal processing, the discrete Fourier transform (DFT) plays a significant role in analyzing characteristics of stationary signals in the frequency domain. The DFT can be implemented in a very efficient way using the fast Fourier transform (FFT) algorithm. However, many actual signals by their nature are non--stationary signals which make the choice of the DFT to deal with such signals not appropriate. Alternative tools for analyzing non--stationary signals come with the development of time--frequency distributions (TFD). The Wigner--Ville distribution is a time--frequency distribution that represents linear chirps in an ideal way, but it has the problem of cross--terms which makes the analysis of such tools unacceptable for multi--component signals. In this dissertation, we develop three definitions of linear chirp transforms which are: the continuous linear chirp transform (CLCT), the discrete linear chirp transform (DLCT), and the discrete cosine chirp transform (DCCT). Most of this work focuses on the discrete linear chirp transform (DLCT) which can be considered a generalization of the DFT to analyze non--stationary signals. The DLCT is a joint frequency chirp--rate transformation, capable of locally representing signals in terms of linear chirps. Important properties of this transform are discussed and explored. The efficient implementation of the DLCT is given by taking advantage of the FFT algorithm. Since this novel transform can be implemented in a fast and efficient way, this would make the proposed transform a candidate to be used for many applications, including chirp rate estimation, signal compression, filtering, signal separation, elimination of the cross--terms in the Wigner--Ville distribution, and in communication systems. In this dissertation, we will explore some of these applications