High-Density and De-Densified Smart Campus Communications. Daniel Minoli

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FIGURE 2.8 SU‐MIMO versus MU‐MIMO.

      As noted, in 802.11ac, only a single‐user WN is allowed to transmit (in the UL direction) at a point in time; multiuser DL transmission from an AP to non‐AP WNs is supported through DL‐MU‐MIMO beamforming. The more WNs active in the network, the longer the stations may need to wait before they are allowed to transmit UL a buffered frame. The issue is improved in the 802.11ax specification.

      2.5.2 Beamforming

      Beamforming is a methodology that focuses the AP's transmit energy of the spatial stream toward the targeted WN. Channel estimation is employed to introduce a small difference in the phase and amplitude in the transmitted signal (a process called precoding) to enable the AP to focus the signal in the direction of the receiving WN. 802.11n had previously defined a number of methods of beamforming, and consequently, chipset vendors implemented various non‐interoperable techniques, keeping beamforming from general acceptance. To address the issue, the 802.11ac specification defined a single closed‐loop SU/MU Transmit Beamforming (TxBF) method where the AP transmits a “special sounding signal” to all WNs – each WN estimates the channel and reports its channel feedback information back to the AP. In the sounding mechanisms, each WN provides channel feedback, which the AP uses to give its spatial streams the necessary mobility. Once channel probing request to the WN results in the WN providing the AP with a characterization of its environment, the AP uses MU‐MIMO beam‐shaping capabilities to maximize signal in the desired direction and squelch the signal in the undesired direction. MU‐MIMO capitalizes on the transmit beamforming capabilities to establish up to four simultaneous directional RF links: this technique provides each of the four users with its own dedicated full‐bandwidth channel. In practice, however, the beamforming process is imperfect, and some of the energy of a spatial stream appears in sidelobes for several degrees off‐axis. Two adjacent MU‐MIMO streams start to interfere with each other as soon as their sidelobes begin to overlap. The presence of this interference adds to the overall noise floor of the channel at the AP. Analysis shows that adding additional MU spatial stream adds intra‐stream interference but increases the number of usable spatial streams; this requires a design tradeoff analysis for specific environments and applications [21].

      2.5.3 Dynamic Frequency Selection

The 802.11ac system throughput is at, or greater than 1 Gbps and single‐link throughput of at least 0.5 Gbps; 800 ns guard intervals are supported. Figure 2.9 depicts available frequencies for the 802.11ac LAN environment. Dynamic Frequency Selection (DFS) is a Wi‐Fi function that enables WLANs to use 5 GHz frequencies that are generally reserved for radars; these are less‐crowded Wi‐Fi bands and can be utilized to increase the number of available Wi‐Fi channels, especially in (residential) multi‐dwelling units. When support for DFS is enabled, it will be necessary for the AP to verify that any radar in proximity is not using DFS frequencies; this is done by a process called Channel Availability Check, which is executed during the boot process of the AP and also as during its normal operations. See Table 2.5 [22].