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

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

Peak to average power ratio reduction and error control in MIMO-OFDM HARQ System

Currently, multiple-input multiple-output orthogonal frequency division multiplexing (MIMOOFDM) systems underlie crucial wireless communication systems such as commercial 4G and 5G networks, tactical communication, and interoperable Public Safety communications. However, one drawback arising from OFDM modulation is its resulting high peak-to-average power ratio (PAPR). This problem increases with an increase in the number of transmit antennas. In this work, a new hybrid PAPR reduction technique is proposed for space-time block coding (STBC) MIMO-OFDM systems that combine the coding capabilities to PAPR reduction methods, while leveraging the new degree of freedom provided by the presence of multiple transmit chairs (MIMO). In the first part, we presented an extensive literature review of PAPR reduction techniques for OFDM and MIMO-OFDM systems. The work developed a PAPR reduction technique taxonomy, and analyzed the motivations for reducing the PAPR in current communication systems, emphasizing two important motivations such as power savings and coverage gain. In the tax onomy presented here, we include a new category, namely, hybrid techniques. Additionally, we drew a conclusion regarding the importance of hybrid PAPR reduction techniques. In the second part, we studied the effect of forward error correction (FEC) codes on the PAPR for the coded OFDM (COFDM) system. We simulated and compared the CCDF of the PAPR and its relationship with the autocorrelation of the COFDM signal before the inverse fast Fourier transform (IFFT) block. This allows to conclude on the main characteristics of the codes that generate high peaks in the COFDM signal, and therefore, the optimal parameters in order to reduce PAPR. We emphasize our study in FEC codes as linear block codes, and convolutional codes. Finally, we proposed a new hybrid PAPR reduction technique for an STBC MIMO-OFDM system, in which the convolutional code is optimized to avoid PAPR degradation, which also combines successive suboptimal cross-antenna rotation and inversion (SS-CARI) and iterative modified companding and filtering schemes. The new method permits to obtain a significant net gain for the system, i.e., considerable PAPR reduction, bit error rate (BER) gain as compared to the basic MIMO-OFDM system, low complexity, and reduced spectral splatter. The new hybrid technique was extensively evaluated by simulation, and the complementary cumulative distribution function (CCDF), the BER, and the power spectral density (PSD) were compared to the original STBC MIMO-OFDM signal

OFDM base T-transform for wireless communication networks

The prominent features associated with orthogonal frequency division multiplexing (OFDM) have been exploited in the area of high-speed communication networks. However, OFDM is prone to impairments such as frequency selective fading channel, high peak-to-average power ratio (PAPR) and heavy-tailed distributed impulsive noise, all of which can have negative impacts on its performance. These issues have received a great deal of attention in recent research. To compensate for these transmission impairments, a T-OFDM based 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. The key contribution in this thesis is on the use of the T-transform along with three novel receiver designs. Additionally, new theoretical bit error rate (BER) formulae for the T-OFDM system are derived over communications channels using zero forcing (ZF) and minimum mean square error (MMSE) detectors, that are validated via simulation and shown to have close performance with the obtained performance results. It has been found that the T-OFDM outperformed the conventional OFDM based systems in the investigated channel models by achieving a signal-to-noise ratio (SNR) gain range of between 9dB and 16dB measured at 10−4 BER. In addition, the sparsity and block diagonal structure of the T-transform, along with its lower summation processes are exploited in this study to reduce the superposition of the subcarriers, leading to reduce the peak of the transmitted signals by a range of 0.75 to 1.2 dB with preserved average power. Furthermore, these attractive features of T-transform are employed with the conventional selective mapping (SLM) and partial transmitted sequences (PTS) schemes to propose three low cost novel techniques; T-SLM, T-PTS-I, and T-PTS-II. Compared to the conventional schemes, the T-SLM and T-PTS-I schemes have achieved a considerable reduction in both computational complexity and in PAPR, further increasing multipath resilience, even in the presence of high power amplifier (HPA). Whereas using the T-PTS-II scheme, the complexity ratio has been significantly reduced by approximately 80%, as well as reducing the SI bits further by two, with negligible PAPR degradation. Moreover, the effect of the independent sections of T-transform on the performance of T-OFDM system over the impulsive channel is addressed in this work, by deriving a new theoretical BER formula over such a transmission media. Furthermore, two novel II schemes WHT-MI-OFDM and WHT-MI-OFDM incorporating nonlinear blanking, both of which utilise the WHT and a matrix interleaver (MI) with the OFDM system, are proposed to suppress the deleterious effects of a severe impulsive noise burst on the T-OFDM system performance. Comparing with the traditional MI-OFDM system, the proposed schemes are much more robust to disturbances arising from the impulsive channel.EThOS - Electronic Theses Online ServiceMinistry of Higher Education and Scientific ResearchIraqGBUnited Kingdo