In this paper, we consider a MIMO relaying model for cellular networks, in which the base station communicates with far mobile users via the support of a relay. Both base station and relay are equipped with multiple antennas. Two optimization problems are investigated: maximizing the sum rate of all far users subject to the power constraints and minimizing the transmission power subject to the QoS requirement of each user. For the sum rate maximization, we propose a practical zero-forcing design scheme and optimize the power allocation to each users. For the power minimization problem, we design a zero-forcing precoders to help convert the optimization problem to a convex problem and solve it using the dual decomposition method. An iterative algorithm to obtain the optimal solution as well as the numerical experiments to support our method are also provided in this paper.