  In this paper, we propose two reconfigurable intelligent surface-aided
$M$-ary frequency-modulated differential chaos shift keying (RIS-$M$-FM-DCSK)
schemes. In scheme I, the RIS is regarded as a transmitter at the source to
incorporate the $M$-ary phase-shift-keying ($M$-PSK) symbols into the FM
chaotic signal and to reflect the resultant $M$-ary FM chaotic signal toward
the destination. The information bits of the source are carried by both the
positive/negative state of the FM chaotic signal and the $M$-PSK symbols. In
scheme II, the RIS is treated as a relay so that both the source and relay can
simultaneously transmit their information bits to the destination. The
information bits of the source and relay are carried by the positive/negative
state of the FM chaotic signal and $M$-PSK symbols generated by the RIS,
respectively. The proposed RIS-$M$-FM-DCSK system has an attractive advantage
that it does not require channel state information for detection, thus avoiding
complex channel estimation. Moreover, we derive the theoretical expressions for
bit error rates (BERs) of the proposed RIS-$M$-FM-DCSK system with both scheme
I and scheme II over multipath Rayleigh fading channels. Simulations results
not only verify the accuracy of the theoretical derivations, but also
demonstrate the superiority of the proposed system. The proposed
RIS-$M$-FM-DCSK system is a promising low-cost, low-power, and high-reliability
alternative for wireless communication networks.
