A device is provided. The device includes: at least one antenna, a plurality of buffer sets, and a circuit. Each buffer set includes a plurality of queue buffers, and each queue buffer is dedicated for a respective application category. The circuit communicates with one or more stations through the at least one antenna. In response to the circuit determining that a plurality of incoming frames or packets are classified into the same application category, the circuit performs an intra-Access Category (intra-AC) scheduling mechanism to determine priorities of data stored in the queue buffers in the same application category according to QoS (quality-of-service) requirements of each station so as to arrange for an MU (multi-user) PPDU (physical-layer protocol data unit) to be transmitted to at least a portion of the stations.