  Orbital angular momentum (OAM) and rate splitting (RS) are the potential key
techniques for the future wireless communications. As a new orthogonal
resource, OAM can achieve the multifold increase of spectrum efficiency to
relieve the scarcity of the spectrum resource, but how to enhance the privacy
performance imposes crucial challenge for OAM communications. RS technique
divides the information into private and common parts, which can guarantee the
privacies for all users. In this paper, we integrate the RS technique into
downlink OAM-MIMO communications, and study the precoding optimization to
maximize the sum capacity. First, the concentric uniform circular arrays (UCAs)
are utilized to construct the downlink transmission framework of OAM-MIMO
communications with RS. Particularly, users in the same user pair utilize RS
technique to obtain the information and different user pairs use different OAM
modes. Then, we derive the OAM-MIMO channel model, and formulate the sum
capacity maximization problem. Finally, based on the fractional programming,
the optimal precoding matrix is obtained to maximize the sum capacity by using
quadratic transformation. Extensive simulation results show that by using the
proposed precoding optimization algorithm, OAM-MIMO communications with RS can
achieve higher sum capacity than the traditional communication schemes.
