  The Reconfigurable Intelligent Surface (RIS) constitutes one of the prominent
technologies for the next generation of wireless communications. It is
envisioned to enhance the signal coverage in cases when the direct link of the
communication is weak. Recently, beam training based on codebook selection is
proposed to obtain the optimized phase configuration of the RIS. After that,
the data is transmitted and received by using the classical coherent
demodulation scheme (CDS). This training approach is able to avoid the large
overhead required by the channel sounding process, and it also circumvents
complex optimization problems. However, the beam training still requires the
transmission of some reference signals to test the different phase
configurations of the codebook, and the best codeword is chosen according to
the measurement of the received energy of the reference signals. Then, the
overhead due to the transmission of reference signals reduces the spectral
efficiency. In this paper, a zero overhead beam training for RIS is proposed,
relying on data transmission and reception based on non-CDS (NCDS). At the BS,
the received differential data can also be used for the determination of the
best beam for the RIS. Therefore, the efficiency of the system is significantly
enhanced since reference signals are fully avoided. After choosing the best
codebook, NCDS is still more suitable to transmit information for high mobility
scenarios as compared to the classical CDS. Analytical expressions for the
Signal-to-Interference and Noise Ratio (SINR) for the non-coherent
RIS-empowered system are presented. Moreover, a detailed comparison between the
NCDS and CDS in terms of efficiency and complexity is also given. The extensive
computer simulation results verify the accuracy of the presented analysis and
showcase that the proposed system outperforms the existing solutions.
