Desimir Vucic

The core objective is to securely transmit digital data over widely accessible voice channels like VoIP or GSM networks. Encryption at the highest security level is applied digitally, but a challenge lies in selecting a modulation technique that effectively converts encrypted digital data into analogue signals capable of navigating the obstacles inherent in voice channels. These channels are lossy and optimized for speech transmission, employing sophisticated signal processing techniques such as Voice Activity Detection (VAD) to maintain speech quality and suppress noise. This paper introduces a novel system design and approach for transmitting Data over Voice (DoV) channels using Offset Quadrature Amplitude Modulation (OQAM) combined with Filter Bank Multicarrier (FBMC) modulation. FBMC/OQAM generates speech-like waveforms that can evade detection by non-speech signal detection mechanisms inherent in voice channels, marking its debut in this domain. In order to demonstrate the feasibility of the proposed system, we have simulated the distortion of FBMC/OQAM signals when processed by an AMR-NB codec using MATLAB®. Subsequently, the performance of the proposed modulation technique is evaluated across various real voice channels, considering actual time and phase deformations, real-time voice transmission bit-rate constraints and possible bit errors, which all demand specific waveform structures for synchronization and algorithms for correct demodulation and decoding at the receiver. Evaluation is done for waveforms that were sent through GSM and VoIP channels. The obtained results demonstrate a favorable bit error rate (BER) on the 3G channel compared to existing methods, such as conventional Orthogonal Frequency Division Modulation (OFDM), thereby proving that FBMC/OQAM is suitable for secure DoV transmission. Finally, the decoded speech from all channels is assessed using Perceptual Evaluation of Speech Quality (PESQ) metric.

An extended frequency spread (FS) multicarrier system based on the filter bank and offset quadrature amplitude modulation (FS-FBMC/OQAM) approach applied to a critically sampled nonuniform FBMC/OQAM (Nu-FBMC/OQAM) transmission, using subchannel aggregation of uniform filter bank, is analyzed and described. The new adapted structure for this Nu-FBMC/OQAM transmission and corresponding orthogonality conditions are analyzed and their MATLAB implementation and verification are performed. Some novel results related to the cyclic features of this Nu-FBMC/OQAM signal, estimated by the fast Fourier transform accumulation method (FAM), are presented.

In this paper, we propose two spectral cyclic MUSIC DOA estimation algorithms for wideband cyclostationary signals: the wideband spectral cyclic conjugateMUSIC (WSCCM) algorithm and the extended wideband spectral cyclicMUSIC (EWSCM) algorithm. The proposed algorithms can be applied to estimate the spectral cyclic conjugate correlation matrix and the extended spectral cyclic correlation matrix of wideband cyclostationary signals. The presented algorithms do not require any knowledge of the optimal time lag parameter, which appears in the similar algorithms in time domain. In the proposed algorithms, the DOAs are estimated at each spectral component of the selected frequency band for a cyclic frequency of interest. This can be used to form the azimuth cyclic frequency diagram, similar to the azimuth frequency diagram in wideband direction-finding systems. Some simulation results are provided to confirm the theoretical findings.

A subspace-based method for direct position estimation of impulse UWB transmitters in multipath conditions is proposed. Performances of a proposed method are illustrated by the results of simulations.

This paper presents an algorithm for DOA estimation in frequency domain by exploitation of cyclostationarity (cyclic features) of signals. Signal selective estimation of direction of arrival at an antenna array can be performed by using the proposed algorithm. The problem of optimal time lag tau knowledge requirement, which appears in the cyclic MUSIC algorithm in time domain can be solved by using the proposed algorithm. Cyclic spectral density estimation at selected cycle frequency a is obtained by employing the digital frequency smoothing method