# A Survey of Wi-Fi 6: Technologies, Advances, and Challenges
## 2 Wi-Fi Basics
### MAC in Wi-Fi
#### 2.2.1 Distributed Coordination Function (DCF)  
Wi-Fi nodes utilize Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to share the wireless medium. Before any transmission, a node has to verify that the medium is free for the duration of a DIFS. Then, it engages a backoff phase: the transmitter picks a random value that represents the number of _slots_ that a transmitter has to wait for before transmitting its packet. After each idle slot, the backoff is decremented. Conversely, Wi-Fi implements a residual backoff: the backoff is paused if the medium is busy to maximize short-term fairness. The transmitter has to wait until the medium is free for the period of a DIFS before resuming the countdown. The transmission starts when the backoff is null.  
The receiver may acknowledge the correct reception of a unicast packet. More precisely, the ACK is transmitted after waiting for a Short InterFrame Space (SIFS) duration after the reception of the data frame. The transmitter considers a transmission failure if an ACK is not received. It retransmits the frame with the exponential backoff algorithm to progressively reduce the collision probability. This means that a random backoff time is chosen from within an initial contention window, doubling after every unsuccessful transmission attempt.  
In Wi-Fi, the hidden node problem may occur, which may severely impact network performance [18]. Consider the topology illustrated in Figure 5. Nodes A and B are within each other's transmission and interference range; the same holds for nodes B and C but nodes A and C cannot hear each other. As a result, node C may start a transmission, although transmission from nodes A to B is ongoing so B's reception is impaired.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**MCS Index** & **Modulation** & **Coding** & **Data Rate (Mb/s)** \\ \hline
0 & BPSK & 1/2 & 72 \\
1 & QPSK & 1/2 & 144 \\
2 & QPSK & 3/4 & 216 \\
3 & 16-QAM & 1/2 & 282 \\
4 & 16-QAM & 3/4 & 432 \\
5 & 64-QAM & 2/3 & 576 \\
6 & 64-QAM & 3/4 & 649 \\
7 & 64-QAM & 5/6 & 721 \\
8 & 256-QAM & 3/4 & 865 \\
9 & 256-QAM & 5/6 & 961 \\
10 & 1024-QAM & 3/4 & 1081 \\
11 & 1024-QAM & 5/6 & 1201 \\ \hline \hline \end{tabular}
\end{table}
Table 4: MCS maximum data rates in a 160 MHz channel for one spatial stream. Payload symbol duration and guard interval are 12.8 \(\upmu\)s and 0.8 \(\upmu\)s, respectively.  
Figure 5: Hidden node problem.  
The Request-To-Send (RTS)/Clear-To-Send (CTS) mechanism has been introduced to cope with the hidden node problem. Before the transmission of a large frame, the sender sends a small RTS packet to the receiver to reserve the medium and the receiver responds with a CTS packet after the duration of a SIFS. The RTS/CTS mechanism ensures that all nodes within the transmission range are informed about an upcoming transmission and defer potential transmission attempts. This prevents C from jamming A's transmission to B in Figure 5. The duration for RTS/CTS is much shorter than the one for data and ACK together, which drastically reduces the likelihood of collisions.  
Each node maintains a Network Allocation Vector (NAV), which represents a record of ongoing transmissions. Since the RTS (resp. CTS) frame contains the duration of the whole frame exchange after the reception of the RTS (resp. CTS), any neighbor of the transmitter and the receiver can update its NAV (Figure 6). In particular, a node cannot transmit any frames if its NAV value is not null. The receiver blocks all its radio neighbors with a CTS until the end of the whole frame exchange.