  Reconfigurable intelligent surface (RIS) has recently emerged as a promising
technology enabling next-generation wireless networks. In this letter, we
develop an improved index modulation (IM) scheme by utilizing RIS to convey
information. Specifically, we study an RIS-aided multiple-input single-output
(MISO) system, in which the information bits are conveyed by reflection
patterns of RIS rather than the conventional amplitude-phase constellation.
Furthermore, the K-means algorithm is employed to optimize the reflection
constellation to improve the error performance. Also, we propose a generalized
Gray coding method for mapping information bits to an appropriate reflection
constellation and analytically evaluate the error performance of the proposed
scheme by deriving a tight upper bound of the average bit error rate (BER).
Finally, numerical results verify the accuracy of our theoretical analysis as
well as the substantially improved BER performance of the proposed RIS-based IM
transmission scheme.
