  Integrating the reconfigurable intelligent surface in a cell-free (RIS-CF)
network is an effective solution to improve the capacity and coverage of future
wireless systems with low cost and power consumption. The reflecting
coefficients of RISs can be programmed to enhance signals received at users.
This letter addresses a joint design of transmit beamformers at access points
and reflecting coefficients at RISs to maximize the energy efficiency (EE) of
RIS-CF networks, taking into account the limited backhaul capacity constraints.
Due to a very computationally challenging nonconvex problem, we develop a
simple yet efficient alternating descent algorithm for its solution. Numerical
results verify that the EE of RIS-CF networks is greatly improved, showing the
benefit of using RISs.
