  Rate-splitting (RS) has recently been recognized as a promising
physical-layer technique for multi-antenna broadcast channels (BC). Due to its
ability to partially decode the interference and partially treat the remaining
interference as noise, RS is an enabler for a powerful multiple access, namely
rate-splitting multiple access (RSMA), that has been shown to achieve higher
spectral efficiency (SE) and energy efficiency (EE) than both space division
multiple access (SDMA) and non-orthogonal multiple access (NOMA) in a wide
range of user deployments and network loads. As SE maximization and EE
maximization are two conflicting objectives, the study of the tradeoff between
the two criteria is of particular interest. In this work, we address the SE-EE
tradeoff by studying the joint SE and EE maximization problem of RSMA in
multiple input single output (MISO) BC with rate-dependent circuit power
consumption at the transmitter. To tackle the challenges coming from multiple
objective functions and rate-dependent circuit power consumption, we first
propose two methods to transform the original problem into a single-objective
problem, namely, weighted-sum method and weighted-power method. A successive
convex approximation (SCA)-based algorithm is then proposed to jointly optimize
the precoders and RS message split of the transformed problem. Numerical
results show that our algorithm converges much faster than existing algorithms.
In addition, the performance of RS is superior to or equal to non-RS strategy
in terms of both SE and EE and their tradeoff.
