  Reconfigurable intelligent surface (RIS) is a promising candidate technology
of the upcoming Sixth Generation (6G) communication system for its ability to
provide unprecedented spectral and energy efficiency increment through passive
beamforming. However, it is challenging to obtain instantaneous channel state
information (I-CSI) for RIS, which obliges us to use statistical channel state
information (S-CSI) to achieve passive beamforming. In this paper, RIS-aided
multiple-input single-output (MISO) multi-user downlink communication system
with correlated channels is investigated. Then, we formulate the problem of
joint beamforming design at the AP and RIS to maximize the sum ergodic spectral
efficiency (ESE) of all users to improve the network capacity. Since it is too
hard to compute sum ESE, an ESE approximation is adopted to reformulate the
problem into a more tractable form. Then, we present two joint beamforming
algorithms, namely the singular value decomposition-gradient descent (SVD-GD)
algorithm and the fractional programming-gradient descent (FP-GD) algorithm.
Simulation results show the effectiveness of our proposed algorithms and
validate that 2-bits quantizer is enough for RIS phase shifts implementation.
