In this paper we propose a novel MAC scheduling mechanism for the downlink of LTE systems based on a channel and QoS aware algorithm which performs joint time and frequency scheduling. The proposed scheduler aims at maximizing system radio resource utilization while providing QoS requirements to classes of traffic that have very tight QoS requirements in the terms of bit rate and delay, e.g. VoIP, online gaming and video conferencing.The performance of the proposed scheme is evaluated on voice traffic by using the E-Model to measure the perceived QoS. We carried out performance evaluation by simulation to compare the behavior of the proposed scheduler with state of the art schedulers in different network conditions. Results show that, in realistic scenarios in which the channel quality varies over time and frequency, the proposed scheduler significantly outperforms the state of the art solutions in terms of provided QoS and system capacity.