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× 3/3‐stream: 1.300 Gbps max rate

       2‐stream 802.11ac: 0.867 Gbps max rate

       1‐stream 802.11ac: 0.433 Gbps max rate

      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].

FIGURE 2.9 5GHz spectrum usability for IEEE 802.11ac LANs.

TABLE 2.5 5 GHz Wi‐Fi Frequencies

Band	Channel	Frequency (MHz)
U‐NII‐1	36–48	5170–5250
U‐NII‐2A/DFS	52–64	5250–5330
U‐NII‐2C/DFS	100–140	5490–5710
U‐NII‐3	149–165	5735–5835

      2.5.4 Space–Time Block Coding

      In addition to the standard WLAN mechanisms at the MAC and PHY layers, IEEE 802.11ac incorporates STBC. Space–time Codes (STCs) involve the transmission of multiple redundant copies of the information to deal with fading and thermal noise with the expectation that some copies may arrive at the receiver in a better condition than other copies; this is known as diversity reception. In the particular case of STBC, the data stream to be transmitted is encoded in blocks, which are distributed among spaced antennas and across time [23–26]. While one must have multiple transmit antennas, it is not always necessary to have multiple receive antennas, although having multiple receive antennas improves performance.

      STBC improves data transfer reliability in wireless systems by transmitting a data stream and variations of the data stream across multiple antennas. STBC is a method to transmit multiple copies of a data stream across a number of antennas and to utilize the various received versions of the data to endeavor to improve the quality and assurance of the information transfer. An STBC receiver combines all the copies of the received signal to extract as much usable information from each copy as possible. In general, scattering, absorption, reflection, multipath, refraction, and receive‐point amplifier thermal noise typically result in (some) corruption of the signal, such that some of the received copies of the information may be more faithful to the original signal than other copies. The redundancy achieved by STBC implies that there is an opportunity to use one or more of the received copies to correctly decode the received signal. An STBC is usually represented by a matrix where each row represents a time slot, and each column represents an antenna's transmissions over time.

The environment of the WLAN often distorts both the transmitted data stream and the transmitted variations of the data stream. Typically, the distortion of the transmitted data stream is different from the distortions of the transmitted variations of the data stream. A receiver receives the distorted data stream and the distorted variations of the data stream. STBC combines the distorted data stream and the distorted variations of the data stream to extract as much information from each of them as possible [2].

FIGURE 2.10 Space–time block coding [2].

In IEEE 802.11ac, STBC is used to expand the spatial streams into twice as many space–time streams; that is, 1, 2, 3, and 4 spatial streams may be expanded into 2, 4, 6, and 8 space–time streams, respectively. Alamouti's scheme is used to provide full transmit diversity gain with low complexity for a system with two antennas [20]. Each spatial stream is expanded separately using Alamouti's code as follows: for first and second symbols x₁ and x₂ (in a time domain), a first spatial stream transmits the symbols x₁ and x₂ in their original order, and a second spatial stream transmits symbols and having values corresponding

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