of the next frame. Various types of waiting times Distributed Coordination Function (DCF) scheme are listed below (these values typically depend on the standard, e.g. 802.11n, 802.11 ac).
FIGURE 2.5 Inter‐frame Space relationships [2].
A WLAN device that supports a Quality of Service (QoS) functionality may transmit the frame after performing backoff if an AIFS for an associated Access Category (AC) (AIFS[AC]), has elapsed. When transmitted by the QoS station, any of the data frame, the management frame, and the control frame, which is not the response frame, may use the AIFS[AC] of the AC of the transmitted frame.
Figure 2.6 illustrates a CSMA/CA‐based frame transmission procedure for avoiding collision between frames in a channel. The figure shows a first station, STA1 transmitting data, a second station, STA2 receiving the data, and a third station, STA3 that may be located in an area where a frame transmitted from the STA1, a frame transmitted from the second station STA2, or both can be received. STA1 may determine whether the channel is busy by carrier sensing. The STA1 may determine the channel occupation based on an energy level in the channel or autocorrelation of signals in the channel or may determine the channel occupation by using a Network Allocation Vector (NAV) timer. After determining that the channel is not used by other devices (that is, that the channel is IDLE) during a DIFS (and performing backoff if required), STA1 may transmit an RTS frame to STA2. Upon receiving the RTS frame, after a SIFS, STA2 may transmit a CTS frame as a response of the RTS frame. If Dual‐CTS is enabled and the second station STA2 is an AP, the AP may send two CTS frames in response to the RTS frame: a first CTS frame in the legacy non‐HT format and a second CTS frame in the HT format.
FIGURE 2.6 Carrier Sense Multiple Access/Collision Avoidance‐based frame transmission procedure.
When the third station STA3 receives the RTS frame, it may set a Network Allocation Vector (NAV) timer of the third station, STA3 for a transmission duration of subsequently transmitted frames (for example, a duration of SIFS+CTS frame duration+SIFS+data frame duration+SIFS+ACK frame duration) using duration information included in the RTS frame. When the third station STA3 receives the CTS frame, it may set the NAV timer of the third station STA3 for a transmission duration of subsequently transmitted frames using duration information included in the CTS frame. Upon receiving a new frame before the NAV timer expires, the third station STA3 may update the NAV timer of the third station STA3 by using duration information included in the new frame. The third station STA3 does not attempt to access the channel until the NAV timer expires. When STA1 receives the CTS frame from the second station STA2, it may transmit a data frame to the second station STA2 after SIFS elapses from a time when the CTS frame has been completely received. Upon successfully receiving the data frame, the second station STA2 may transmit an ACK frame as a response of the data frame after SIFS elapses. When the NAV timer expires, the third station STA3 may determine whether the channel is busy using carrier sensing. Upon determining that the channel is not used by other devices during a DIFS after the NAV timer has expired, the third station STA3 may attempt to access the channel after a CW according to a backoff process elapses [2].
When Dual‐CTS is enabled, a station that has obtained a transmission opportunity (TXOP) and that has no data to transmit may transmit a CF‐End frame to cut short the TXOP. An AP receiving a CF‐End frame having a BSSID of the AP as a destination address may respond by transmitting two more CF‐End frames: a first CF‐End frame using STBC and a second CF‐End frame using non‐STBC. A station receiving a CF‐End frame resets its NAV timer to 0 at the end of the PPDU containing the CF‐End frame.
Figure 2.6 also shows the second station STA2 transmitting an ACK frame to acknowledge the successful reception of a frame by the recipient.
2.4 HARDWARE ELEMENTS
Figure 2.2, called out above, further illustrates components of a wireless device configured to transmit data, including a Transmission (Tx) Signal Processing Unit (TxSP), an RF transceiver, an antenna unit, and four illustrative antennas. The TxSP, RF transmitter, and antenna unit may be components of the transmitting signal processing unit, RF transmitter, and antenna unit of the WLAN device. Each spatial stream needs its own dedicated transmit/receive chain; for example, 802.11ac 8 × 8 AP capable of supporting all eight spatial streams needs eight independent radio chains and antennas.
The RF transceiver includes an RF transmitter and an RF receiver. The RF transceiver is configured to transmit information received from the baseband processor to the WLAN, and provide information received from the WLAN to the baseband processor. The antenna unit includes one or more antennas; when MIMO or MU‐MIMO is used, the antenna unit may include a plurality of antennas [2].
The
