# IEEE 802.11ax
**Topic: Random Access Procedure**
Bariq Sufi Firmansyah
Institut Teknologi Bandung
[e-mail]
###### tags: `NTUST-RESEARCH`
---
## Introduction
- the legacy IEEE 802.11 medium access control (MAC) protocol, distributed coordination function (DCF), is unsuitable for dense scenario.
- In dense Wi-Fi environments, high network throughput does not translate to sufficient bandwidth and thus satisfactory delay/latency since
-- severe collisions from channel contention and
-- increased interference from neighboring devices
- We need new metrics that reflect user experience such as
-- delay,
-- latency, and
-- average per user throughput,
**instead of** the peak link throughput of a single user
- IEEE 802.11ax amendment
-- **scope**: define standardized modifications to both the IEEE 802.11 physical layer (PHY) and the medium access control (MAC) sublayer for high efficiency operation in frequency bands between 1 GHz and 6 GHz,
-- **goal**: provide a better user experience by improving at least four times of average throughput per user in densely deployed environments
## State of the Art Technology
- **General 802.11**
power management is based on alternating between awake state (STA can transmit and receive frames) and doze state (its radio is switched off). There are two modes, active mode and power-saving mode
-- active mode: STA is always awake
-- power-saving mode: STA alternates between awake and doze states. The AP buffers data destined for PS STAs until the STA wakes up and retrieves it.
- **802.11a/b/g**
new modulation and coding schemes from the original 2 Mb/s IEEE 802.11-1997 "legacy" up to 54 Mb/s in both 2.4GHz (802.11g) and 5GHz (802.11a)
- **802.11n**
- PHY layer improvements:
-- widen the channels up to 40 MHz which is twice larger than those of previous 802.11 PHY.
-- Usage of 5/6 coding rates.
-- Transition toward MIMO technology (4 spatial streams)
- MAC layer improvements
-- new Reduced InterFrame Space (RIFS) of 2 µs
-- two aggregation methods, the A-MSDU (Aggregated MAC Service Data Unit) and the A-MPDU (Aggregated MAC Protocol Data Unit).
- **802.11e**
-- introduces Enhanced Distributed Channel Access (EDCA) and HCCA which distinguish voice, video, best effort and background traffic and serve them differently.
-- EDCA assigns different priorities to these types of traffic,
-- HCCA allows an AP to schedule transmissions by taking into account specific QoS requirements, like the delay bound, the packet loss ratio, or the required bandwidth.
- **802.11ac**
-- increasing the data rate of a 10x factor with respect to 802.11n, up to 7 Gb/s
-- increase spatial streams up to 8
-- introduces the DL MU-MIMO, which allows an AP to assign various DL spatial streams to different STAs
-- widens the transmission bands up to 160 MHz (also exploting non-contiguous 80+80Mhz channels)
-- increases the maximal length of a frame from 65 535 (802.11n) to 4 692 480 octets.
## 802.11ax new Features
- features:
-- orthogonal frequency division multiple access (OFDMA) PHY
-- preamble puncturing
-- UL MU MIMO
-- spatial reuse
-- trigger frame
-- OFDMA random access
-- power-saving with target wake time (TWT)
-- station-tostation (S2S) operation
-- backwards compatibility with legacy IEEE 802.11 devices
- The **key feature** of 802.11ax is the adoption of an **OFDMA** approach. very wide channels suffer from frequency selective interference. With OFDMA, adjacent subcarriers are grouped into a resource unit (RU) and a sender can choose the best RU for each particular receiver.
- OFDMA makes Wi-Fi radio access closer to the LTE. However in contrast to LTE, OFDMA works on top of the legacy DCF and is coordinated by the AP.
- Having accessed the channel, the AP
-- start a usual DL transmission,
-- start a DL MU transmission (using OFDMA, MIMO or both), or
-- allocate RUs for UL MU transmission
- For a DL MU transmission, a PHY preamble specifies the duration of the frame and the tone mapping between STAs.
- For an UL MU transmission, a schedule is specified in the preceding frame which can be either trigger frame, new control frame which allocates the channel for UL MU transmission, or data frame
- UL MU transmission starts exactly one SIFS after the DL frame containing a schedule to synchronize the STAs
- Introducing OFDMA in Wi-Fi affects the other MAC and PHY functionality.
-- **First**, TGax changes the OFDM parameters to improve the flexibility and the efficiency of the OFDMA operations.
-- **Second**, TGax changes the PHY frame format to include OFDMA-related information in the PHY preamble. Moreover, TGax continues moving MAC-layer information to the PHY preamble, since sometimes the preamble can be decoded even if the entire frame is corrupted.
-- **Third**, OFDMA causes numerous MAC changes related to the MU operation and the fairness between the devices of different generations
- Apart from OFDMA, the new features include:
-- BSS coloring: inherited (and extended) from 802.11ac and 802.11ah, allows to distinguish inter- and intraBSS frames based on their preambles even if the frame payloads are corrupted by collisions
-- Network Allocation Vector (NAV), modifications of the legacy virtual carrier sense
-- Virtualization
-- Microsleep operation, which enables a STA to switch off its radio just for the duration of an alien frame
-- Target Wake Time amendment
-- opportunistic **power save**
-- changing the **sensitivity threshold** and the transmission power to improve spatial reuse in a dense deployment
-- Periodic channel reservations (namely, the **Quiet Time periods**) during which only predefined STA(s) can transmit, can be used to protect direct link communications. The mechanism can also be applied for time division between BSSs in dense deployment
## PHY: modulation and frame format
### Modulation
- Similar to 802.11ac, it is based on Orthogonal Frequency-Division Multiplexing (OFDM) and supports operations in 20 MHz, 40 MHz, 80 MHz, 80+80 MHz$\ ^1$ and 160 MHz channels.
- The number of tones increased because the duration of the OFDM symbols used for the PHY payload is made longer (up to 12.8 µs).
- Benefits of long OFDM symbols are
-- more resilient to the inter-user jitter inherent in outdoor scenarios, which is very important for the UL MU transmission which may be simultaneously performed by several users.
-- longer symbols permit to reduce the overhead due to Guard Intervals (GI).
- new modulation techniques in addition to
BPSK, 16-QAM, 64-QAM, and 256-QAM.
-- The **first** one is an optional 1024-QAM (in high SINR condition) together with forward error correction codes (convolutional or low-density paritycheck) — which have code rates of 1/2, 2/3, 3/4 and 5/6.
-- **Second**, optional Dual Carrier Modulation (DCM). DCM enhances transmission robustness by allocating the same signal on a pair of tones, which are separated far apart in the frequency domain. The usage of DCM reduces the data rate twice
- The maximum 9.6 Gbps data rate is achieved when data is transmitted at the highest HE-MCS11 with a code rate of 5/6 in a 160 MHz or 80+80 MHz channel with 8 spatial streams and a GI of 0.8 µs
### Frame Format
- 4 types of PHY frames (referred to as PPDU, PHY Protocol Data Unit):
-- for the Single User (SU) transmission,
-- for the extended range SU transmission$\ ^2$,
-- for the DL MU transmission, and
-- for the UL MU transmission
- each different frame type = baseline frame + selected fields specialized for the different frame types
- The main feature of DL MU transmission is the frame contains a common preamble describing which tones a particular receiver shall decode to obtain its part of the Data field
- For the UL MU transmission, the preamble is common and it is emitted by all the STAs. Then, each STA sends its own part of the Data field using a **predefined set of tones**
- For all the frame types, the **preamble** is duplicated in every 20 MHz subchannel within the transmission band and consists of two parts:
-- the **legacy** part: for backward compatibility
-- the **HE** (High Efficiency): provides signaling for the new 802.11ax and can only be decoded by 802.11ax devices
- The legacy part contains
-- **training fields**, which synchronize the transmitter and the receiver, and
-- the **legacy signal field** (L-SIG), which describes the parameters of the rest of the frame, specifically allows the calculation of the frame duration
- The HE part of the preamble starts with
-- a repetition of the **L-SIG** field,
-- which is followed by the mandatory **HE-SIG-A** field,
-- an optional **HE-SIG-B** field and
-- training fields (**HE-STF and HE-LTF**) needed for tuning MIMO.
- **HE-SIG-A** provides information about
-- MCS,
-- bandwidth,
-- a number of spatial streams (NSTS)
-- some other parameters that are needed to correctly decode the rest of the frame
-- BSS Color
-- remaining Transmission Opportunity (TXOP) duration, whether the frame is sent in UL or DL, etc
-- spatial reuse parameter (SRP) which is used to signal the sum of **transmission power** and an **acceptable level of interference** to allow for the spatial reuse operation
- TGax continues moving some MAC signaling to the PHY preamble. Since the preamble has rigid structure and it is transmitted at the lowest MCS, but the cost of such additional information is high. However, it is advantageous, since
-- the preamble is transmitted with the most robust MCS and
-- it can be decoded before the PHY payload is fully received and its checksum is calculated
- In case of both UL and DL SU transmission, as well as in a UL MU transmission, all the necessary information can be fitted into HE-SIG-A which consists of **two legacy OFDM symbols**
- In case of a DL MU transmission, the information for various users may differ and shall be specified for each of them separately. In this case, an **additional HE-SIG-B** field of variable length is included in the frame preamble. Specifically, the field contains two blocks:
-- one with common: describes the OFDMA resource allocation
-- one with per-user information: consists of several subfields defining for each resource unit its MCS, the number of spatial streams, etc
- the **HE-STF** and **HE-LTF** fields are used for MIMO.
-- Specifically, the main purpose of the HE-STF field is to improve the automatic gain control estimation in a MIMO transmission,
-- while the HE-LTF fields provide a tool for the receiver to estimate the MIMO channel between the set of constellation mapper outputs and the receive chains.
- Similar to the legacy PPDU, the **Data field** contains
-- the SIGNAL subfield needed to initialize the encoder/decoder scrambler
-- and the encoded MAC frame.
The Data field is transmitted with 4 times longer OFDM symbols
- Quadrupling the symbol duration = 4 times more calculations at the receiver side, while the time is limited by the SIFS
- A straightforward solution — increasing SIFS — was not approved because of backward compatibility as well because it would have decreased the channel usage efficiency.
- Rather, TGax provides the possibility to **extend the tail** of a frame with an **extension**
### Open Issues
- Rate control
- PHY preamble is longer than the legacy one. Thus it should be used only for long transmissions which benefit from the new 802.11ax features
## Multi-User Transmissions & Channel Access
### Fundamentals of 802.11ax's OFDMA
- in [1] has been proposed a novel OFDMA-based MAC protocol called OMAX, the authors consider only **random** access. TGax has designed a much more flexible and powerful framework, can be used for both **deterministic** and **random** access
- OFDMA transmission is organized on a per-frame basis.
- a frame can carry information from or to multiple STAs
- Various tones are assigned to different STAs.
- The duration of all the RUs within a frame is the same
- An RU can contain 26, 52, 106, 242, 484, 996 or 2x996 tones (including service ones)
- Because of various problems with binary convolutional codes, multiple RU allocations for a STA are forbidden.
- MU-MIMO and OFDMA can be used together in both UL and DL MU-MIMO only if there are ≥106-tone RUs
- Thanks to MU-MIMO, up to eight users can be assigned to an RU (due to eight spatial streams)
- In the case of the **DL OFDMA** transmission, the **HE-SIG-B** field contains
-- the common preamble contains an RU allocation map
-- per-user content fields indicating the RUs assigned to an STA and the transmission parameters to be used by the STA (NSTS, MCS, coding, etc)
--- *Note that* an RU can represent either an SU or an MU-MIMO allocation. In MU-MIMO, spatial configuration shall be also signaled to the STA
- Organizing the **UL MU** transmission is a **more challenging** task. MU transmissions in Wi-Fi shall be synchronized in the time domain. it is difficult to maintain strict time synchronization because of clock drifting. AP coordinates the UL MU transmission as follows
- The AP transmits a new type of a control frame — **Trigger frame** — in which
--- it specifies the common parameters of the upcoming UL MU transmission (duration, GI which shall be the same for all the STAs participating in the UL MU transmission),
--- allocates RUs for the STAs, and
--- defines transmission parameters for each particular STA (MCS, coding, etc.)
- To achieve synchronization, the MU transmission is performed immediately, i.e., a SIFS after the Trigger frame, see figure below
- For **UL MU OFDMA** transmissions, the AP shall receive signals from different STAs at almost the same power level
- Therefore, 802.11ax defines a **power pre-correction mechanism**
-- **1.** AP indicates in the Trigger frame its current transmit power
-- **2.** AP indicates in the Trigger frame the target signal strength that the AP is expected to receive from a STA in the following UL transmission
-- **3.** So, STA knows the AP’s transmit power and the signal strength of the received Trigger frame,
-- **4.** Now, the STA can estimate the path loss to the AP and it can calculate an appropriate transmit power for the following UL transmission
- In order to be efficient, the AP shall allocate RUs **only to** STAs which have data to transmit. For that, STAs report to the AP the amount of buffered data they have. Such reports may be
-- requested by the AP or
-- sent by STAs on their own
- **Another challenge** arises because the AP does not know whether the **channel** is **idle** from the **STA’s point of view**
- For each STA, the AP specifies in the Trigger frame whether
-- the STA shall perform carrier sensing before an OFDMA transmission
-- or not.
If carrier sensing is required, the STA shall perform both
-- virtual carrier sensing and
-- physical carrier sensing in at least the 20 MHz channel(s) that contain(s) subcarriers allocated for the STA
If physical carrier sensing indicates busy medium, i.e., the STA detects high energy, it **cancels** the UL transmission
- However, in some cases the STA can neglect virtual carrier sensing, i.e., **NAV**,
-- if STA is going to transmit ACK or BlockAck which duration does not exceed some agreed value
-- if it has been set by a frame originating from an intra-BSS neighbor (?)
- the STA always **cancels** the UL transmission if its duration exceeds the UL MU transmission duration indicated in the Trigger frame
### Performance Improvements
- to allow UL MU transmission just after DL MU transmission, the DL MU transmission should contain the Trigger frame describing the UL RU allocations
- another possibility is to include information in the DL PPDU MAC header
- 802.11ax also implements cascading MU transmissions which means that within a TXOP (transmission opportunity), DL MU and UL MU transmissions can alternate
- **Note that within cascading MU transmissions the AP can exchange frames in an MU manner with different sets of STAs**
- if a STA has a short frame to transmit, it either
-- tries to aggregate it with another frame
-- uses padding, if the remaining space is not enough for aggregating the whole frame
- To avoid wasting channel resources, 802.11ax STA is allowed to **fragment frames** in order to fill the remaining **airtime** with user **payload**
- To improve the efficiency even more, the 802.11ax STA can also **aggregate frames** from different **Access Categories** (ACs)
- Since the aggregation of several fragments is complicated, TGax has found a compromise, having defined several optional levels of HE fragmentation.
-- The first level permits to send only one fragment without any aggregation
-- The second level allows a STA to aggregate not more than one fragment per **MSDU** in an **A-MPDU**
-- Finally, the third level allows the aggregation of two or more fragments per **MSDU** in an **A-MPDU**
### Special Trigger Frames
- In addition to the basic Trigger frame for **data and management frames**, 802.11ax has **special Trigger frames** which
-- initiate parallel Request To Send / Clear To Send (**RTS/CTS**) handshakes,
-- request block acknowledgments from a group of STAs, and
-- collect beamforming reports or buffer status reports (BSR)
- To *protect* a DL MU transmission from *hidden nodes*, **MU-RTS/CTS handshake** is introduced. The **CTS** frames can be sent simultaneously, thanks to the UL MU transmissions in 802.11ax
- The channel which shall be used by a particular STA to transmit the CTS is determined in MU-RTS
- The 802.11ax amendment proposes an additional way for acknowledging UL MU transmissions by sending new **MultiSTA Block ACK (BA)** frames.
- Another new frame defined in 802.11ax is the **MU Block ACK Request (BAR)** frame which is a variant of the Trigger frame. It is used to solicit acknowledgements from multiple STAs in the UL MU transmission instead of sending individual BAR frames
- One more variant of the Trigger frame is used to collect BSRs. In each **BSR** (Buffer Status Report), each STA informs the AP about the amount of buffered traffic in a queue of the requested **AC** (AC_BE, AC_BK, AC_VI, or AC_VO) or of a subset of **ACs** (Access Categories)
- Finally, 11ax defines special Trigger frames used to poll beamforming information or to request information about the channel
### UL OFDMA Random Access
- Besides the **scheduled** **UL MU** access described above, TGax has designed an optional mechanism which allows performing **random** **UL OFDMA** transmissions
- this random feature is important when
-- the AP does not know which associated STAs have data to transmit,
-- or when an unassociated STA wants to transmit an association request.
- **DCF/EDCA** is not efficient for **short transmissions** because of the large **overhead** caused by PHY headers and interframe spaces.
- The designed random access is similar to the [multichannel slotted ALOHA](https://hackmd.io/@bariqsufif/multichannelslotted). Trigger frame can allocate some **RUs** for random access
- if the user identifier for some RU is 0 or 2045, the corresponding record in the Trigger frame defines **a group of contiguous RUs for random access** which can be used by associated and unassociated STAs
- To decide whether to transmit and in which RU, STAs use the so-called **OFDMA Back-off (OBO)** procedure.
-- Each STA chooses a random value from $[0, OCW]$, where **OCW** is the OFDMA contention window.
-- If the current OBO value < the number of RUs allocated for random access by a Trigger frame, the STA **randomly selects** an RU from those allocated for random access and transmits a frame in this RU
-- Otherwise, the STA **decreases OBO** by the number of RUs allocated for random access and **waits** for the **next Trigger frame** containing RUs for random access
- If the transmission attempt **fails**, the STA doubles its $OCW$ until it reaches $OCW_{MAX}$ and selects an $OBO$ value from the **new interval [0, 2OCW]**
- If the transmission attempt is **successful**, the STA resets its OCW to the minimum value $OCW_{MIN}$
- $OCW_{MAX}$ and $OCW_{MIN}$ are specified by the AP in **beacons** and in the **probe response** frames
- Random access is less efficient than scheduled access and worth to use only for **short** packet transmissions and for **BSR** (Buffer Status Report).
- a STA having data for transmission can
-- generate a BSR and
-- send it **with random access** to ask the AP for channel resources
### EDCA Improvements
- OFDMA works on top of the legacy CSMA/CA (Carrier Sense Multiple Access With Collision Avoidance) mechanism called **EDCA** or **DCF**
- to transmit a Trigger frame, the AP should contend for the channel with other STAs. the AP rarely wins the contention if the AP uses the same channel access parameters
- OFDMA is much more efficient than EDCA. So, to achieve higher throughput, the STAs should rarely access the channel with EDCA but they should almost always use OFDMA
- **do the AP use OFDMA or EDCA?????**
- the AP can change the EDCA parameters for all the **associated STAs** by broadcasting them in beacons by setting high values for $CW_{min}$ and $CW_{max}$ , the AP can almost forbid EDCA transmissions in the network
- **problem** = if there are legacy STAs in the network that can't use OFDMA transmissions. Setting the same high values of $CW_{min}$ and $CW_{max}$ for both 802.11ax and legacy STAs will block the legacy STAs
- **solution** = second set of EDCA parameters which is used only by those 802.11ax STAs
- RTS/CTS mechanism is also improved
-- **Historically**, the use of the RTS/CTS mechanism is determined by the length of the transmitted data frame. If the frame length exceeds the RTS threshold then the data transmission is preceded by an RTS/CTS handshake
-- **Now**,
--- the use of the RTS/CTS mechanism is determined by the duration of the transmission rather than by the length of the frame **because** with a high MCS even a long frame can be transmitted fast, and the usage of RTS/CTS handshake performed with a slow MCS will cause high overhead
--- the value of the duration-based RTS/CTS threshold is controlled by AP.
---- the AP can **lower** the threshold if interference from hidden nodes is suspected in a dense environment
---- or **increase** the threshold otherwise to reduce the transmission overhead
### Open Issues
- Wi-Fi becomes similar to LTE due to its OFDMA implementation. This makes the channel resource allocation in LTE becomes relevant to channel resource allocation in Wi-Fi. But the channel resource allocation in Wi-Fi is much more difficult, here's why
- **First**, LTE networks operate in license bands, that makes inter-cell interference can be controlled. in Wi-Fi, the interference level can't be guaranteed, and therefore it will need more sophisticated algorithm to reduce interference
- **Second**, in LTE networks the channel is divided into resource blocks of equal size. In Wi-Fi, the restrictions on possible RUs are more sophisticated
- **Third**, for UL, according to the standard the highest MCSs cannot be used with 26-tone RUs. Thus by splitting the channel into too narrow RUs we may obtain a lower throughput
- **Fourth**, the impossibility of splitting some channels into a given number of RUs. For example, in case of three STAs with UL traffic, the AP can divide a 40 MHz channel into two RUs (242-tone + 242- tone) or into four RUs (242-tone + 2x 106-tone + 26-tone), but not into three RUs. This means that a 26-tone RU is wasted
- **Fifth**, a portion of RUs shall be allocated for the RA. Obviously, the number of RUs allocated for the RA affects the latency and the network capacity and shall be selected based on some estimation of the traffic patterns
- **Sixth**, In an 802.11ax network, the channel resources are allocated by the AP. So a misbehaving AP can allocate more channel time to those STAs which are produced by the same vendor
- **Seventh**, how to select an appropriate duration of an MU frame. This may affect the efficiency of the channel usage as well as the fairness and the QoS
## Overlapping BSS Management and Spatial Reuse
there are a lot of debates on how to improve the performance in case of dense networks. Since the launch of TGax, about one hundred submissions on these topics were proposed, most of which were rejected. Here are the accepted ones
### BSS Color
- [BSS](https://hackmd.io/TEyqKf1wRrGQU0HN9BFQ6Q?both#BSS) Color is **non-unique** ID of the BSS which is transmitted in the frame preamble in order to determine which BSS is the originator of a frame without decoding the entire frame
- BSS color is selected randomly by the AP, the colors of two neighboring BSSs may coincide or collide in terms of 802.11ax
- To decrease the BSS color collision probability, TGax has agreed to increase the length of the BSS color field to 6 bits
- If the collision occurs,
-- the STAs associated to an AP can notify to AP, and
-- the AP can start a procedure of changing its BSS color
- the color will be changed by sending special information element in beacons
- The identification of a BSS by the BSS color field is used for determining channel access rules and for power saving mechanisms
### Two NAVs
- The Wi-Fi channel access follows the listen-before-talk principle, i.e., a STA performs carrier sensing before transmitting a frame
- The **channel** is supposed to be **busy** in the following cases:
-- If during carrier sensing a STA detects a frame preamble, it considers the channel as busy for the **frame duration** that is indicated in the preamble
-- If during carrier sensing a STA detects an unknown signal at more than 20 dBm above the minimum sensitivity level
-- If the channel is indicated to be virtually busy
- The virtual carrier sensing in Wi-Fi, called NAV, is as follows:
-- In the MAC header, a STA indicates the NAV value, i.e., for how long the following frame exchange will occupy the channel
-- after decoded the frame, the other STAs set NAV, i.e., they consider the channel to be busy during the indicated time
-- The STA cancels its NAV, if it receives a CF-End frame (contention-free end)
- In the **legacy Wi-Fi**, STAs do not take into account by which frame the NAV value was set and this may lead to the following misbehavior:
-- Suppose a frame from the same BSS sets the NAV value at a STA
-- After that, the STA receives a CF-End frame coming from an Overlapping BSS (OBSS)
-- STA will reset the NAV and will not consider the medium to be virtually busy anymore
-- that STA can start its own transmission which causes a collision
- in the **802.11ax Wi-Fi**, the STAs will support two NAVs:
-- one for their own BSS and
-- the other for all the OBSSs, and they will **modify the NAVs separately**
### Quiet Time Period
- Ad Hoc and direct links$^3$ operation are promising solutions that reduce the channel busy time
- Quiet Time Period (QTP) mechanism is a series of periodic time intervals of equal duration used for ad hoc or direct links operation
- the STA request for QTP to AP by describing
-- the offset of the first reserved interval,
-- the duration and period of the intervals, and
-- the total number of requested intervals
- If the AP satisfies the request, it broadcasts information about the reserved QTP and forbids the other STAs to access the channel during QTP
### Adjustment of Sensitivity Threshold and Transmit Power
- Dynamic Sensitivity Control (DSC) is a carrier sensing mechanism based on the dynamic adjustment of the carrier sensing threshold referred to as the DSC threshold
- DSC Threshold determines when the STA considers the medium to be busy
- to prevent transmissions within a BSS from being blocked by an OBSS, the DSC threshold should be increased
- to allow communication between all devices within a BSS, the DSC threshold shall be small enough not to miss a transmission within this BSS
- Smith and Afaqui et al. propose to set the DSC threshold at the STAs to $$TxPower-\max_{i\in BSS} PassLoss(AP,i)-MRG$$
- TxPower is the AP's transmit power, PassLoss is path loss between AP and STA i, MRG is margin (tunable parameter, recommended between 18 dB and 25 dB)
- The above equation is difficult to implement therefore the authors suggest the DSC threshold to be $$AvgRSSI-MRG$$
- the path loss may increase so the instant RSSI from the AP’s beacon will be less than AvgRSSI−MRG, and the STA will start to ignore beacons
- To prevent such an undesirable behavior, it is proposed to decrement AvgRSSI by RSSIDEC dBs (some constant value) if several beacons are lost in a row and, thus, to automatically decrease the DSC threshold
- The above figure show the increase of the metrics compared to the legacy constant carrier sensing threshold
- the gain in throughput and fairness is achieved at the cost of a higher number of hidden nodes and PER
- Having reduced the number of exposed STAs, DSC increases the number of hidden STAs. To address this issue, RTS/CTS is used together with DSC
- proposed by **TGax**. STAs may dynamically change their $OBSS\_PD$ ( OBSS Preamble Detection threshold) and $TX\_PWR$ (transmit power ) parameters
- During backoff, a STA sets up its $OBSS\_PD$ to some value
- STA suspends its backoff when it senses the start of a packet
- The STA will understand whether the packet belongs to OBSS
- if the signal strength is less than OBSS_PD and no other conditions (e.g., NAV), STA can resume backoff even before the end od the packet
- When the STA obtains channel access, it can start transmission with the power not higher than that corresponding to the used value of $OBSS\_PD$
- By default, an STA transmits signals of power $TX\_PWR$ and considers the medium to be idle if the signal strength is less than $OBSS\_PD = −82 dBm$
- Let the STA receive a signal from an OBSS STA X dB stronger than −82 dBm.
- This means that the attenuation between the STA and the OBSS STA is X dB weaker than necessary for considering the medium idle
- If the STA wants to start a new transmission in this case, it shall first increase its OBSS_PD by X dBm, and
- second, it shall decrease its transmit power also by X dB in order not to produce a huge interference at the location of the OBSS STA
### Channel Bonding and Preamble Puncturing
- in **802.11ac**, a STA can extend the bandwidth by step-by-step concatenation of the secondary channels after obtaining primary 20 MHz channel
-- if the secondary 20 MHz channel is idle, the STA can transmit in 40 MHz bandwidth.
-- If both the secondary 20 MHz and the secondary 40 MHz channels are idle, 80 MHz bandwidth can be used
-- if the secondary 40 MHz channel is idle but the secondary 20 MHz channel is busy, the STA can only transmit in the primary 20 MHz channel
-- in dense networks, the secondary 20 MHz channel of a BSS can be the primary 20 MHz channel of another one (if that happens, then the secondary 40 MHz channel will not be used)
- **802.11ax** introduces a new optional feature called **preamble puncturing** to improve the efficiency of channel bonding in dense environment
- For an MU OFDMA transmission in a channel greater than or equal to 80 MHz, one or more busy 20 MHz subchannels **can be** punctured. It means that frame preamble is not transmitted and RUs are not allocated in these subchannels
- In dense deployment, such a feature allows using channel resources in a much more flexible way, e.g. can use 60 MHz instead of 80 MHz
### Virtualization
- a single physical device can have multiple virtual APs (VAPs) so that it can create multiple independent BSSs
- All the BSSs in the multiple BSSID use the same BSS color
- The frames of BSSs from a Multiple BSSID set are considered as intra-BSS frames
---
#### BSS
- a **service set** is a group of wireless network devices which are identified by the same SSID (service set identifier). They are on the same logical network segment (e.g., IP subnet or VLAN)
- Basic service sets (**BSS**) are a subgroup of devices **within a service set** which are additionally also operating with the same PHY and MAC characteristics (i.e. radio frequency, modulation scheme, security settings etc.) such that they are wirelessly networked.
- aren't all the devices in a service set have the same PHY and MAC characteristics? maybe not all, some have different MCS etc etc
---
$^1$ In contrast to continuous 160 MHz channel, an 80+80 MHz channel is combined from two non-adjacent 80 MHz channels.
$^2$ An extended range PPDU was designed for robust delivery and can only be transmitted in a 20 MHz channel at one of the three lowest MCSs (Modulation and Coding Scheme) without MIMO
$^3$ Direct links allow two STAs associated with the same AP to communicate directly without using the AP as a relay

[1] Q. Qu, B. Li, M. Yang, and Z. Yan, “An OFDMA based concurrent multiuser MAC for upcoming IEEE 802.11ax,” in Proc. Wireless Commun. Netw. Conf. Workshops (WCNCW), New Orleans, LA, USA, 2015, pp. 136–141.