Multi-User Active RIS Communications: Joint Power, Gain and Phase Shifts Optimization

In this paper, we address the problem of optimizing the users' transmit power jointly with the amplification factors and phase shifts of an active reconfigurable intelligent surface (RIS) where each element is optimized individually. We consider a multi-user uplink scenario with inter-user interference. The non-convex problem of maximizing the sum-rate is transformed into a convex one using the multi-variate Taylor expansion and solved using the Lagrangian and gradient descent methods to provide a suboptimal solution in return for a feasible complexity. Numerical results show that active RISs with each amplification factor optimized individually effectively outperform passive ones in fading and highly interfering situations.