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IEEE 802.11ac standard has newly adopted a downlink multi-user multiple-input and multiple-output (DL-MU-MIMO) scheme. For user multiplexing in downlink WLAN, we can also use a frame aggregation scheme for multiplexing multiple users' data with space-time block coding (STBC) for achieving spatial diversity. We compare the performance of the two downlink user multiplexing schemes: multi-user MIMO and frame aggregation in IEEE 802.11ac. If each user's encoded data stream has a similar length, the multi-user MIMO scheme yields better average throughput than the frame aggregation scheme. On the other hand, if each user's encoded data stream has a different length, the frame aggregation scheme outperforms the multi-user MIMO scheme in terms of average throughput. In a fast-varying channel, the multi-user MIMO scheme yields worse throughput due to the channel feedback overhead, compared to that with the frame aggregation scheme. We also observe that the multi-user frame aggregation scheme with STBC always outperforms a single-user transmission scheme with STBC in terms of average throughput due to enhanced MAC layer efficiency through frame aggregation.