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

FIGURE 2.3 A general MIMO system (after [12]).

In 802.11ac standards, MU‐MIMO is employed for the downlink (DL). MU‐MIMO is a technology that enables a plurality of signals to be transmitted in the same time slot by space division multiplexing; with this technology, it is possible to improve utilization efficiency of the spectrum [13]. MU‐MIMO is used in WLANs to support congested environments where many users are trying to access the wireless network at the same time. When multiple users (e.g. smartphones, tablets, computers) begin accessing the AP simultaneously, or practically simultaneously, congestion can arise as the AP services the first user's request, while the second (and subsequent) users are forced to wait. MU‐MIMO mitigates this situation by allowing multiple users to access AP functions, thus reducing congestion. MU‐MIMO systems segment the available bandwidth into separate, discrete streams that share the connection equally. An MU‐MIMO AP may have 2 × 2, 3 × 3, 4 × 4, or 8 × 8 variations, where the designation refers to the number of streams (two, three, four, or eight) that are created by the AP. To obtain the desired improvements, the AP must enable MU‐MIMO and beamforming functionality. The streams are spatial in nature; while interference is minimized if two devices are in proximity to each other, they still share the same stream. Prior to 802.11ax, the MU‐MIMO procedure only applies to DL connections; this may be fine for home users that need faster downloading speeds for 4K video content, but it is less useful for business users who need faster uploads for two‐way high‐quality video conferencing applications [14]. IEEE 802.11ax supports bidirectional MU‐MIMO. In addition to allowing 8 × 8 arrays, the 802.11ax standard addresses the use of uplink (UL) MU‐MIMO.

      The PHY entity is based on OFDM or OFDMA. OFDM is a type of digital modulation that uses frequency‐division multiplexing (FDM) principles. The method subdivides an RF channel into a large number of contiguous subchannels to provide reliable high‐speed communications. All subcarrier signals within a subchannel are orthogonal to one another. The subcarrier frequency signals that are being modulated are selected such that the subcarriers are orthogonal to each other whereby cross‐talk between the subchannels is minimized or eliminated; note that inter‐carrier guard bands are not needed. OFDM transmitters and receivers are relatively simple; in particular, a separate filter for each of the subchannel is not required. Modulation is achieved by encoding signals on multiple carrier frequencies. In this scheme, multiple closely spaced orthogonal subcarrier signals with minimally overlapping spectra are transmitted such that they can carry information in parallel. Each individual subcarrier signal can be modulated with a traditional modulation scheme at a low symbol rate, for example, using Quadrature Amplitude Modulation (QAM). Demodulation utilizes Fast Fourier Transform (FFT) methods. While the total data rates in an OFDM scheme are generally similar to conventional single‐carrier modulation in the same aggregate bandwidth, the key advantage of OFDM over single‐carrier schemes is its ability to function in environments with challenging channel conditions, for example, with attenuation of high frequencies components in a cable, and with channel interference including fading due to multipath reflections. OFDM is broadly deployed for wideband digital communication, including digital television, Digital Subscriber Line (DSL) internet access, wireless networks, and 4G/5G mobile communications.

      OFDM, in its basic form, is a digital modulation technique being employed for transferring a data stream from a single user over an aggregate communication channel, utilizing a sequence of OFDM symbols. Nonetheless, OFDM can be combined with multiple access techniques to support multiple users utilizing time, frequency, or coding separation of the various users. In OFDMA, Frequency‐Division Multiple Access (FDMA) is achieved by assigning different OFDM subchannels to different users. IEEE 802.11ax WLANs utilize OFDMA for high‐efficiency and simultaneous communication; OFDMA is also used in wide‐area applications including but not limited to WiMAX, 3GPP LTE 4G mobile broadband standard DL, and the 3GPP 5G NR (New Radio) fifth‐generation mobile network standard for the DL and for the UL.

In either OFDM or OFDMA PHY layers, a station is capable of transmitting and receiving PPDUs that are compliant with the mandatory PHY specifications. A PHY specification defines a set of MCSs and a maximum number of spatial streams. Some PHY entities define DL and UL Multi‐User (MU) transmissions having a maximum number of STSs per user and employing up to a predetermined total number of STSs. A PHY entity may provide support for 20, 40, 80, and 160 MHz contiguous channel widths and support for an 80 + 80 MHz noncontiguous channel width. Each channel includes a plurality of subcarriers, which may also be referred to as tones. A PHY entity may define fields denoted as Legacy Signal (L‐SIG), Signal A (SIG‐A), and Signal B (SIG‐B) within which some necessary information about PHY Service Data Unit (PSDU) attributes are communicated. For example, a High‐Efficiency (HE) PHY entity may define an L‐SIG field, a HE Signal A (HE‐SIG‐A) field, and an HE Signal B (HE‐SIG‐B) field. In the IEEE Std 802.11ac, SIG‐A and SIG‐B fields are called VHT SIG‐A and VHT SIG‐B fields. In IEEE Std 802.11ax, SIG‐A and SIG‐B fields are, respectively referred to as HE‐SIG‐A and HE‐SIG‐B fields. See Figure 2.4.