Abstract: A method and a system for downlink pre-coding in cloud radio access networks uses a baseband unit, a number of remote radio heads, and a number of user equipments. The baseband unit determines a received signal expression for each user equipment based on system parameters and channel state information, and determines a sum rate expression which may be achievable for downlink transmission based on the received signal expression. The baseband unit then sets up a constrained optimal problem expression subject to a maximum transmitting power of each remote radio head with an objective function of sum rate maximization based on the achievable sum rate expression, and finally determines optimal solutions for the constrained optimal problem where the optimal solutions comprise downlink pre-coding used for downlink transmission.