Showing papers by "Romano Fantacci published in 1989"

A new speech signal scrambling method for secure communications: theory, implementation, and security evaluation

Abstract: A two-dimensional signal-scrambling method implemented by digital signal-processing techniques that eliminate the need for frame synchronization without impairing security is presented. Such techniques include short time Fourier analysis and the filter bank concept. The use of special digital finite-impulse-response filters, which make it possible to implement the system algorithm completely via commercial processor software, is discussed. As a result, the system can be configured with very little hardware. Methods for determining available keyspace and selecting keys are also presented. >

Joint carrier and clock recovery for QPSK and MSK digital communications

Abstract: A new digital implementation of a receiver for digital communications is presented. The receiver structure performs the integration of a maximum a posteriori probability (MAP) carrier and clock synchronisation with the maximum likelihood (ML) demodulation. It is shown that the key feature of the receiver structure is that the same hardware is able to perform both operations; thus the receiver implementation complexity is greatly reduced. This scheme can be adapted to different modulation techniques suitable for digital communications, such as QPSK-O-QPSK and MSK. As examples, the application to QPSK and MSK signals is considered, owing to the interest of these modulation schemes for satellite communications.

Multicarrier demodulator for digital satellite communication systems

Abstract: A multicarrier demodulator (MCD) suitable for advanced digital satellite communications is presented. This system permits the direct interfacing of FDMA and TDM communication links by using digital signal processing techniques. Two main functions are implemented by an MCD: demultiplexing and demodulation. We focus here only on a digital implementation of an MCD with a view to achieving flexibility, better performance and suitability for VLSI.The demultiplexer is implemented according to a per-channel structure based on an analytic signal method that allows a highly modular and flexible implementation to be achieved. This approach permits a certain degree of integration of the demultiplexer and demodulator functions. In the proposed MCD scheme the receiver pulse-shaping filter can be integrated in the demultiplexer structure, thus lowering the overall implementation complexity. Coherent demodulation is used to reduce the signal-to-noise ratio required to achieve a specified bit error rate and is carried out using the maximum likelihood (ML) estimation method. A maximum a posteriori probability (MAP) method is used to jointly estimate the carrier phase and bit timing of the received signal. The digital architecture of the proposed MCD can be adapted to different digital modulation techniques. However, we focus here on the application for QPSK signals, since this modulation scheme is of interest in digital satellite communications. A theoretical analysis and computer simulations are performed in order to evaluate the performance degradation of the proposed MCD, including the finite arithmetic implementation.

Efficient ARQ schemes for error control of satellite channels

Abstract: In this paper, the performance of some efficient ARQ schemes which make use of repeated transmissions of the same data block and compound detection are analysed. The repeated transmission of the same data block involves repeated transmission of each symbol of the data block. At the receiving end, compound detection formed of a soft detection jointly with a classical hard detection is performed by using all the received copies of the same symbol. The throughput efficiencies for the proposed ARQ schemes are analytically derived in closed form and optimized with respect to the number of copies to be transmitted for each data block. It will be shown that the optimized ARQ schemes proposed in this paper achieve better performance than classical ARQ schemes, in particular under high error rate conditions.

A simplified I-Q digital multicarrier demodulator

Abstract: An on-board (satellite) processing system which receives an input frequency-division multiple-access signal and supplies an output to interface the time-division multiplexed links is considered. The system separates each individual radio channel, its demodulation, and its correct switching to the appropriate downlink channel. An appropriate name for the on-board processing system performing the first two operations is the multicarrier demodulator (MCD). Two main functions are implemented by an MCD: demultiplexing and demodulation. The focus is on a digital implementation of the MCD, because it offers several advantages such as flexibility, VLSI integrability, and better efficiency. >