  Non-orthogonal multiple access (NOMA) is considered a key technology for
improving the spectral efficiency of fifth-generation (5G) and beyond 5G
cellular networks. NOMA is beneficial when the channel vectors of the users are
in the same direction, which is not always possible in conventional wireless
systems. With the help of a reconfigurable intelligent surface (RIS), the base
station can control the directions of the channel vectors of the users. Thus,
by combining both technologies, the RIS-assisted NOMA systems are expected to
achieve greater improvements in the network throughput. However, ideal phase
control at the RIS is unrealizable in practice because of the imperfections in
the channel estimations and the hardware limitations. This imperfection in
phase control can have a significant impact on the system performance.
Motivated by this, in this paper, we consider an RIS-assisted uplink NOMA
system in the presence of imperfect phase compensation. We formulate the
criterion for pairing the users that achieves minimum required data rates. We
propose adaptive user pairing algorithms that maximize spectral or energy
efficiency. We then derive various bounds on power allocation factors for the
paired users. Through extensive simulation results, we show that the proposed
algorithms significantly outperform the state-of-the-art algorithms in terms of
spectral and energy efficiency.
