QPSK Transmission System for Machine to Machine Communication

In this paper are presented the results regarding the design and the implementation of an enhanced digital transmission system dedicated especially for applications in the field of machine to machine communications. The improved performances of the system, regarding the transmission speed, spectral efficiency and the resistance to noises, are ensured by the very efficient and reliable implementation based on quadrature phase-shift keying (QPSK) modulation technique. For obtaining a bidirectional communication, the architecture of the proposed design relies on a cascaded configuration. For each direction of communication are used pairs of AD 633 multipliers connected in symmetrical structures that forms a set of balanced quadrature modulators and demodulators. The synchronization of the emission and reception modules that compose the system is realized with a custom scheme that use specialized circuits based on phase-locked-loop principle. Compared with other implementations, the proposed transmission system represents a very suitable solution for direct communication and extended data exchange between various devices, including industrial instrumentation and other equipments that can be integrated into larger networks for increased efficiency.