Emmanuel Lemois

Description of a complete multi-carrier spread spectrum transmission chain for robust and discrete tactical communications

Multi-carrier spread spectrum (MC-SS) techniques have been widely studied for the last few years. This paper describes a complete transmission chain based on an MC-SS technique for tactical communications. The constraints on the waveform are discreteness and robustness to frequency selective channels and jammers. Therefore, polyphase filtering is used in order to meet these requirements. Besides, the system must be able to provide a high data rate for different tactical configurations. The paper presents the features of these configurations (filtering, modulation, coding, spreading), and the algorithms used for time synchronization and for the phase and frequency estimations. Then performance obtained over an AWGN channel (with-and without a power amplifier), over a Rummler channel or with jammers is presented. Finally, complexity estimation reveals that the implementation of such a modem is technically achievable. The study shows that the modem is able to resist to multipath channels, and that encoding and filtering allows it to resist a powerful jammer.

New advances in multi-carrier spread spectrum techniques for tactical communications

Spread spectrum techniques are classically used in tactical communications, in order to mitigate interference due to jamming, while providing a low probability of intercept. This paper addresses a new SS technique, referred to as multi-carrier spread spectrum (MC-SS). An original MC-SS scheme is proposed using polyphase filtering. As for tactical communications, dramatic performance improvements are achieved, especially for partial-band jammers. Moreover, classical problems of synchronization encountered in multi-carrier modulation systems (MCM) are resolved by the proposed waveform. This provides an extremely good safety level. The MC-SS technique consists in a combination of standard DS/SS with a MCM scheme. Such a combination is motivated by the advantages of the MCM, which provides robustness in the case of frequency selective channels, as well as an efficient digital implementation in its well known OFDM form (implemented in DAB, DVB, ...). The paper highlights that this new MC-SS method fits tactical communications constraints. It shows that this new modem proposes an active and reliable method to combat agile jammers. Special attention is paid to synchronization issues. For tactical communications, this leads to a new protection feature.