Mobile communication systems must support multiple users achieving at same time privacy. Thus security is a crucial requirement to assure privacy of the data transmitted from and for each user. Several solutions can be adopted to implement security. Commonly they are based on encrypted algorithms implemented by higher layers, such as private and public encrypted keys. Also security can be implemented at physical layer and complemented with other security schemes from higher layers. Directivity introduced at the transmitted constellation can be also used to assure some kind of physical layer security. This is the case of power efficient transmission schemes where constellations are decomposed into several BPSK (Bi-Phase Shift Keying), being each one amplified and transmitted independently by an antenna. It can be shown that privacy among user contents is achieved under this approach, since each user must know the transmitter parameters associated to the constellation shaping, i.e., the direction in which the constellation is optimized, otherwise receives a meaningless data. If transmitter parameters are unknown, mutual information will be null which means a secrecy level of 100%. Simulation results show the effectiveness of the proposed approach.
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