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Traditionally, at the PHY level, the 802.11 protocol uses a Carrier Sense Multiple Access (CSMA)/Collision Avoidance (CA) channel management method: WNs first sense the channel and endeavor to avoid collisions by transmitting a packet only when they sense the channel to be idle; if the WN detects the transmission of another node, it waits for a random amount of time for that other WN to stop transmitting before sensing again to assess if the channel is free. The process is based on the AP or the WN establishing signal detection energy on a given channel; specifically, the Received Signal Strength Index (RSSI) of the received PLCP (Physical Layer Convergence Protocol) Protocol Data Unit (PPDU)⁴: if the signal detection energy is less than a Clear Channel Assessment (CCA) threshold, the AP or the WN then contends for the channel and transmits its data.

As described, a terminal in a WLAN checks whether a channel is busy or not by performing carrier/channel sensing before transmitting data. Such a process is referred to as CCA, and a signal level used to decide whether the corresponding signal is sensed, is referred to as a CCA threshold. When a radio signal is received by a terminal, it is processed to determine if it has a value exceeding the CCA threshold. When a radio signal having a predetermined or higher‐strength value is sensed, it is determined that the channel under consideration is physically busy, and the terminal delays its access to that channel. When a radio signal is not sensed in the channel under consideration or a radio signal is sensed having a strength smaller than the CCA, then the terminal determines that the channel is idle.

At the PHY layer, the data frame exchanges in WLANs could be performed with a single‐antenna transmission or multiple‐antenna transmission (MIMO techniques). In general, a MIMO communication system employs multiple (N_T) transmit antennas and multiple (N_R) receive antennas. A MIMO channel formed by the N_T transmit and N_R receive antennas may be decomposed into N_S independent channels, with N_S ≤ min {N_T,N_R}. Each of the N_S independent channels is also referred to as a spatial subchannel or eigenmode of the MIMO channel. In some environments, MIMO exploits multipath propagation. MU‐MIMO is an evolution from the single‐user MIMO technology. In WLAN applications, MIMO methods allow the APs and STAs to increase the number of antennas for both transmitting and receiving, thus improving the system capacity for wireless connections. MIMO methods are utilized in IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ax; they are also used in Evolved High Speed Packet Access (HSPA+)/3G cellular, Long Term Evolution (LTE)/4G cellular, 5G cellular, and WiMAX. Older WLAN standards, such as 802.11b, 802.11g, and 802.11n, do not support MU‐MIMO. Initially, only routers and APs supported the technology; now, many endpoint devices support MU‐MIMO (including smartphones) support 802.11ac MU‐MIMO technology. Figure 2.3 depicts a general MU‐MIMO system.

In the case of a multiple‐antenna, or MIMO transmission, multiple spatial streams (SS) are sent within the same frame from one station or AP, which usually is called a beamformer (BFer), to another station or AP, which is usually called a beamformee (BFee); this type of transmission is called beamforming (BF) transmission. BF and MIMO transmissions are usually enhanced by some initial frame exchanges so that the BFer knows about the MIMO channel conditions. This initial exchange of frames before the actual data frame exchange is called a sounding procedure. The frames that might be used in a sounding procedure are (i) the HT and very high throughput (VHT) Null Data Packet (NDP) frames, (ii) the VHT MIMO Compressed Beamforming Report frame, (iii) the VHT NDP Announcement (NDPA) frame, and (iv) the VHT Beamforming Report Poll frame. Each of these frames may have various fields and subfields such as: VHT MIMO Control, VHT Compressed Beamforming Report, MU Exclusive Beamforming Report, Sounding Dialog Token, STA Info, and related fields that are utilized for exchanging information relevant to beamforming [4].

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.

FIGURE 2.4 Example of PHY transmit procedure [2].

In some of the 802.11 standards, such as 802.11ah and beyond, the identity of the BSS (e.g. as managed by an AP of the BSS) is indicated in a PPDU by a set of bits that described the “color” of the BSS. The color of a BSS corresponds to an identifier (ID) of the BSS that is shorter than the Basic Service Set Identifier (BSSID) defined by 802.11. The BSS color may be contained in the PHY Signal (SIG) field in a PHY header of a PPDU, whereas the BSSID is typically included in a MAC portion of PPDUs. A device (e.g. an AP or client) in a BSS can determine whether a PPDU is from the BSS to which the device belongs (the “same‐BSS”) or some other BSS (e.g. an overlapping BSS [OBSS]), by decoding the SIG field and interpreting BSS color bits included therein [6].

Machine To Machine (M2M) communication technology has been discussed as a next‐generation communication technology. The technological standard for supporting M2M communication in an IEEE 802.11 WLAN system has been developed as IEEE 802.11ah (other standards or recommendations have been advanced by ETSI). Regarding M2M communication, a scenario of occasionally communicating a small amount of data at low speed in an environment in which numerous devices are present is considered. Communication in a WLAN system is performed in a medium shared by all devices. When the number of devices is increased like M2M communication, there is a need to enhance a channel access mechanism more effectively to reduce unnecessary power consumption and interference [15].