  With the goal of improving spectral efficiency, complex rotation-based
precoding and power allocation schemes are developed for two multiple-input
multiple-output (MIMO) communication systems, namely, simultaneous wireless
information and power transfer (SWIPT) and physical layer multicasting. While
the state-of-the-art solutions for these problems use very different
approaches, the proposed approach treats them similarly using a general tool
and works efficiently for any number of antennas at each node. Through modeling
the precoder using complex rotation matrices, objective functions (transmission
rates) of the above systems can be formulated and solved in a similar
structure. Hence, this approach simplifies signaling design for MIMO systems
and can reduce the hardware complexity by having one set of parameters to
optimize. Extensive numerical results show that the proposed approach
outperforms state-of-the-art solutions for both problems. It increases
transmission rates for multicasting and achieves higher rate-energy regions in
the SWIPT case. In both cases, the improvement is significant (20%-30%) in
practically important settings where the users have one or two antennas.
Furthermore, the new precoders are less time-consuming than the existing
solutions.
