  In a multiuser MIMO broadcast channel, the rate performance is affected by
the multiuser interference when the Channel State Information at the
Transmitter (CSIT) is imperfect. To tackle the interference problem, a
Rate-Splitting (RS) approach has been proposed recently, which splits one
user's message into a common and a private part, and superimposes the common
message on top of the private messages. The common message is drawn from a
public codebook and should be decoded by all users. In this paper, we propose a
novel and general framework, denoted as Hierarchical Rate Splitting (HRS), that
is particularly suited to FDD massive MIMO systems. HRS simultaneously
transmits private messages intended to each user and two kinds of common
messages that can be decoded by all users and by a subset of users,
respectively. We analyse the asymptotic sum rate of HRS under imperfect CSIT. A
closed-form power allocation is derived which provides insights into the
effects of system parameters. Finally, simulation results validate the
significant sum rate gain of HRS over various baselines.
