The problem of resource allocation for the downlink of wireless systems operating over a frequency-selective channel is investigated. It is assumed that the BS (Base Station) uses many antennas and many antennas are available for each user too (Multiple Input Multiple Output - MIMO case). To relieve heavy computational burden, a suboptimal, but efficient algorithm is devised that maximizes the sum of the users' data rates subject to total available power constraint while guaranteeing minimum resources for each user. Simulation results are provided to prove our statements. Thus, the algorithm can be applied to latest-generation wireless systems that provide Quality-of-Service (QoS) guarantees.