# wifi6 筆記 by Owen

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:::warning
### 個人資訊/Personal Information
#####
>Name: Owen/李冠勳
>Departement: 電機工程系/Electrical Engineering
>Email: [e-mail],[e-mail]
:::

## 802.11ax (wifi6)
### 802.11ax Protocol
1. HEW（High Efficiency WLAN)
2. 802.11ax features
* compatibility:
* compatible with previous 802.11a/b/g/n/ac
* work in both **2.4G** and **5G** frequency bands
* All of transmitting frame has the preamble
* The preamble consists of two parts: Legacy (compatible with traditional 802.11 preamble) and High Efficiency (high efficiency, dedicated to 802.11ax).
* Legacy is for the traditional terminal to be able to identify.
* Better energy saving:
* Increase the endurance of mobile devices
* 802.11ax introduced **Target Wake Time (TWT)** technology. TWT allows the AP to set a series of time intervals. When the terminal waits for a predetermined time interval, it will be “awakened” and exchange data frames.
* Higher transmission rate and coverage:
* Provide higher-level coding combinations (MCS10 and MCS11)
* Introduce 1024QAM(higher transmission rate)
* ac only have 256 QAM
* Introduce UL-MU-MIMO(ac only defined DL MU-MIMO)
* Introduce **OFDMA**
* Refers to the use of OFDMA in LTE, which allows multiple users to access the channel at the same time through different sub-carrier resources, improving the utilization of the channel.

### OFDMA & RU
* In 802.11ax, we can only consider concepts of **RU** (resource unit).
* 
* OFDM:
* User need to occupy whole frequency channel to transfer complete data package, like WLAN packet.
* OFDMA:
* RU is the minimum load user data unit cut out according to the time and frequency domain, so it can be achieved that multiple users use frequency domain resources at the same time
* Divide the resources of the entire channel into small fixed-size time-frequency resource blocks. This time-frequency resource block is also called RU (Resource Unit).
* 
* OFDMA refines the interval between subcarriers. Similarly, Band can have more subcarriers than OFDM.
* RU:
* The smallest time-frequency unit in 802.11ax is RU
* **RU in ax is a relative concept with many different values**
* In frequency, besides RU, there are other types of subcarriers in the frequency domain.
* Guard Sub-carriers: Resist interference from adjacent channels or adjacent sub-channels (resist ICI)
* DC
* NULL Subcarrier: Resist ICI
* Pilot Subcarrier
* In 802.11ax, a specific subcarrier is always used as the Pilot Subcarrier to complete the channel estimation function
* 根據pilot subcarrier，接收方才可估計channel完成解調的工作
### UL Random Access (TF & TF-R)
* The MAC layer mechanism required by 802.11ax and traditional DCF is different.
* Before 802.11ax, **The MAC access mechanism is a typical CSMA mechanism (ie CSMA/CA used in DCF)**
* **802.11ax uses OFDMA technology, The MAC access mechanism that requires multiple users to access the channel at the same time**

* TF（Trigger Frame） access mechanism
* Mainly used in UL MU （Uplink Multi-users）
* 
1. The AP sends a Trigger frame, declaring that this round of access has started.
* TF will contain the RU location corresponding to Multi UserID
* TF中會包含Multi UserID 對應的RU位置
2. According to the instructions in the Trigger frame, the node selects its corresponding RU location for OFDMA access.
* 根據對應的RU位置進行OFDMA存取
3. After the AP receives all the data, it sends back an ACK to end this round of transmission.

* TF-R（Trigger Frame for Random access）
* **TF-R is a further expansion based on TF**, which introduces a competition mechanism in the TF mechanism
* TF-R is **executed before** the TF process we described earlier.
* During each access, the AP first sends a TF-R frame. Some RUs in this frame have corresponding AID=X. This X represents that this RU is for nodes to compete for access.
* After the node(mobile devices) recognizes the TF-R frame, it specifically uses **the OBO (UL-OFDMA Backoff) mechanism to compete**
* 
* TF-R features
* Unlike CSMA/CA, there is LBT
* The node judges whether it can send this round according to the local random number and the number of RUs that can be competed. If it can, then directly select an RU at random to send the competition signal. During this sending process, there is no listening first.

### BSS coloring
* We call that **multiple APs and clients deploy on the same channel** and perform competing transmissions are called OBSS
* Occur CCI
* BSS Coloring is a method used to identify overlapping basic service sets (OBSS).
* BSS Coloring is a field that identifies the ID of the BSS.
* When multiple wireless terminals transmit on the same channel, the 802.11ax radio can use the BSS Coloring field to distinguish BSS.
* The HE Element in the Beacon frame contains the subfield of BSS Coloring. It is also a 6-bit BSS Coloring information field, which can identify 63 BSSs.
* 

----------
### 802.11ax Frame Aggregation Enhancements

* Frame aggregation is to combine multiple radio frames into a single longer radio frame for transmission, and then transmit them at once.
* the content of transmission is increased, and the additional air interface time is reduced.
* Multiple radio frames are combined into a single transmission
* Increase in transmission content
* As long as the competition reaches one time, all can be transmitted, reducing extra air interface time

* MSDU & MPDU
* MAC Service Data Unit , MSDU
* MSDU is only the Payload part, there is no Header.
* MAC Protocol Data Unit ,MPDU
* In addition to Payload, it also contains the Header of the MAC layer
* MDPU contains MAC Header and MSDU
* 
* In 802.11n
* There are 2 kinds of Frame Aggregation
* A-MSDU: Aggregate multiple MSDU and they use same MAC header
* A-MPDU: Aggregate multiple MPDU
* 
* After the node receives the aggregated frame, it will give feedback in the form of **Block ACK (Block ACK)**.
* 802.11ac only use A-MPDU (payload can carry A-MSDU)

* Before 802.11ax:
* **A-MPDU can only hold same QoS level's MPDU**
* High priority MPDUs (such as voice) cannot be combined with other relatively low priority MPDUs (such as Best Effort or Video) to form an aggregate frame for transmission

* In 802.11ax (Multi-TID A-MPDU)
* 802.11ax introduces Multi-Traffic Identifier (Multi-TID) A-MPDU
* A-MPDU can aggregate from different QoS level's MPDU
* A-MPDU allows the aggregation of traffic from **the same or different QoS access categories**, which **is identified by the traffic identifier (TID)**

### CSMA/CA
* CSMA/CA
* CSMA/CA主要使用兩種方法來避免碰撞 :
1. 送出資料前，聆聽網路上的狀態，如果沒有人使用，維持一段時間後，再等待一段隨機的時間後如果還沒有人使用，才送出資料。由於每一個裝置採用的隨機時間不同，可以減少碰撞的機會。
2. 送出資料前，先送一個RTS (Request to Send) 封包給目標端，等待目標端做出回應有就是送出 CTS(Clear to Send) 封包後，才開始傳送。

* 與CSMA/CD的差別
1. CSMA/CD可以檢測碰撞，但無法避免碰撞；CSMA/CA在發送package的同時不能檢測到通道上有無碰撞，只能儘量避免。

### UL-OFDMA Random Access (UORA)
* In general UL-OFDMA access
* **The AP will allocate global resources based on the BSR information.** When the allocation is completed, the AP initiates a transmission process through the trigger frame mechanism
* AP根據BSR直接分配users的RU

There is another way for UL-OFDMA access, which called **UL-OFDMA Random Access (UORA)**

* About UORA (It don't need to know terminals BSR)
* The allocation of RU resources in the uplink UL-OFDMA link is not determined by the AP, RU competes through the random access mechanism UORA.
* This access mechanism does not require real-time feedback of BSR information on the terminal side

### BSR
* BSR: Buffer Status Report
* STA回報給AP不同QoS類型的傳輸封包大小
* Used for UL MU operation
* AP藉由BSR來了解各個STA要傳輸的封包大小並決定如何分配RU
* BSR info:
* BSR如何傳輸
* Solicited BSR(請求式BSR):
* 定義: The non-AP STA can explicitly deliver BSRs in any frame sent to the AP in response to a BSRP Trigger frame (solicited BSR).
* AP主動發BSRP要求各個STA回傳BSR
* 
* PAD為沒有用的封包(為了補齊長度)
* Unsolicited BSR(非請求式BSR):
* 定義: The non-AP STA can implicitly deliver BSRs in the QoS Control field or BSR Control subfield of any frame transmitted to the AP (unsolicited BSR)
* 在任何上行frame中的QoS Control field或是BSR control subfield中直接回報給AP

### Dueling NAVs
* What is NAV?
* NAV is one of the most basic MAC layer elements in 802.11
* Under the advanced protocol, basically all compatibility mechanisms are developed around NAV.
* Carrier sense method to determine if the channel is busy
* NAV is the timer mechanism of Carrier sense method.
* In 802.11ax:
* There are two NAV timers have been defined
* intra-BSS(Basic Service Set) NAV timer
* It can only be set by the terminal from the same BSS, and the value of the NAV timer is set by the Duration/ID field transmitted by the terminal within the BSS
* 同一個BSS內的STA才能設置此NAV: 同一個BSS內的STA用Duration/ID field傳來NAV timer
* basic NAV timer
* It can be set by terminals from different BSS areas, and the value of the NAV timer is also set according to the Duration/ID field transmitted by these terminals.
* 不同BSS區域的STA才能設置此NAV: 不同BSS內的STA用Duration/ID field傳來NAV timer
* **These two NAV timers work at the same time.**
* If any NAV is set to non-zero, then it is considered that the channel is busy and is being occupied.
* Why using 2 NAV?
* Because of the Dense User Environments
* 802.11ax terminals not only need to protect the frames transmitted within the BSS (intra-BSS), but also need to avoid interference with other BSS (ie, inter-BSS) transmissions.

### OMI Mechanism

* In 802.11ax
* When any node wins the competition, it will initiate a **TXOP** transmission (Transmit Opportunity).
* During TXOP, AP will use OFDMA technology to divide the channel into Resource Units (RUs) so that multi-user transmission can be carried out at the same time.

* Then what is OMI?
* Operating Mode Indication
* **Message from STA that Used to indicate to change the sending or receiving mode of the AP**
* The terminal acts as an OMI initiator
* Send the OM control field to AP
* OM Control Subfield, which is usually located in the data or management frame)
* The AP acts as an OMI responder

* 802.11ax terminal can send TOM information (Transmit Operating Mode) to AP
* Switch between 2 modes
* Switching between **single-user or multi-user UL-OFDMA**
* If multi-user mode, UL-OFDMA transmission is performed under AP control (多用戶模式就採正常AP分配RU給大家同時使用channel)
* If single-user mode, then the terminal will compete on its own and perform single-user data transmission (單用戶模式就是STA自己競爭來做單用戶資料傳輸(不去共用頻域來傳送多人訊號))
* 
* 改AP的接收模式、STA的發送模式
* 802.11ax terminal can send ROM information (Receive Operating Mode) to AP
* 改AP的發送模式、STA的接收模式
* OMI Initiator to indicate to AP that it supports the maximum number of spatial streams and maximum channel bandwidth of the downlink
* 

### Resource Units (More info)
* We have already known that:
* OFDMA subdivides the channel into smaller resource units RU.
* Through OFDMA's resource subdivision, multiple clients can perform parallel transmission by occupying different RU resources.
* 
* A 20MHz OFDMA channel consists of 256 subcarriers (Subcarriers, or Tones)
* Of course the OFDMA channel can have 20MHZ 40MHZ 80MHZ 160MHZ
* Here is an example of 20MHZ
* Subcarriers form Sub-Channel (also known as RU
* When the 20 MHz channel is subdivided, 802.11ax terminals can use 4 different sizes of RU units, including 26 subcarriers, 52 subcarriers, 106 subcarriers and 242 subcarriers.
* Remember that the RU in WiFi6 has no fixed size, which is different from that in LTE
* 
* Because of Compatability
* **In 802.11ax, management frames and control frames will still use the OFDM technology of 802.11a/g/n/ac**
* Ensure that other early 802.11 terminals can understand each other.
* At this time, **both the management frame and the control frame use the entire 20MHz channel** (因為不是使用OFDMA技術，OFDM的20MHZ只有64 sub carrier)
* 

* **OFDMA technology is only used for data frame interaction between 802.11ax AP and 802.11ax terminal**
* Management frame和Control frame都還是使用OFDM技術(佔用整個通道)來確保向下相容
* OFDMA技術只用在 802.11ax STA AP間的data frame

### TF and UL-OFDMA Access mechanism
* We talk more detail about UL-OFDMA
* 
* After AP winning competition, during UL TXOP
* There are 3 kinds of TF
1. BSRP & BSR
* BSRP (Buffer Status Report Poll)
* It is kind of TF.
* AP send to terminals
* Buffer Status Report Poll which is used to request the terminal's Buffer information
* BSR (Buffer Status Report)
* Terminals reply back their BSR
* The BSR contains the amount of data buffered by the terminal and **the corresponding QoS category information**.
* This information will help the AP optimize the resource allocation of the RU
* About more details of BSR
* The terminal can directly feed back inactive BSR frames to the AP without BSRP request
* The terminal can use the following fields in any frame to send this inactive BSR
* in QoS Control field (QoS Control field)
* in BSR Control subfield (BSR Control subfield)

2. MU-RTS
* It is kind of TF.
* Don't forget
* MU-RTS uses traditional OFDM technology to transmit on the entire 20MHz channel, and all nodes (including 802.11ax nodes and traditional 802.11 nodes) can receive.
1. For Non-802.11ax terminals
* Non-802.11ax terminals will set the local NAV timer by receiving the Duration/ID field in the MU-RTS frame to ensure that no active competition will be initiated during the remaining UL-OFDMA time
2. For 802.11ax terminals
* **The AP allocates RU resources in the MU-RTS frame**, and the next trigger frame only indicates to start transmission.
* After the AP allocates RU resources, it needs to get feedback from the terminal, that is, the terminal will feed back the CTS frame to inform the AP that it recognizes and knows the current resource allocation.
3. Triiger #3
* Notify the terminal to perform uplink transmission on the corresponding RU resource
* Also indicate the time of this uplink transmission

### 802.11 PHY Comparision
* The most important ideas from this table
* AX's Symbol time(max) is 16 microseconds
* AX's subcarrier spacing is 78.125kHZ
* Subcarriers => tones
* 
* In 11ax:
* Divided subcarriers are called tones
* For example:
* 256 tones is in per 20MHZ channel that can be used in groups of 26, 52, 106, 242 tones.

### What is Multi-User (MU)?
* Definition:
* Transmissions can occur between the AP and multi-clients simultaneously
* 802.11ax supports 2 types of MU
* MU-MIMO (UL and DL)
* 
* It would cause
* High overhead
* High complexity
* OFDMA
* 

### What’s Subcarrier, QAM(Quadrature Amplitude Modulation)
* 
* 

* 
* 
* 

### UORA, OCW(OFDMA contention window), OBO(OFDMA Back-off) counter
* In UORA, a frequency domain back-off technique (ie OFDMA back-off, OBO) is used. In OBO, the terminal will select a random number at the beginning, and then the AP will send a contention-type trigger frame, which also contains the number of available RUs in this round. The terminal will subtract the number of RUs in this round from its random number until it is reduced to 0. If the value of the current round is 0 after the terminal uses random numbers to subtract, it means that the competition is successful, and the terminal will randomly select an RU to occupy. If the current round of subtraction is not 0, it is equivalent to the failure of the current round of competition, then this value will be maintained, and the subtraction will continue in the next round.
* The User Info field in the trigger frame contains the AID field (each RU has an AID indication), the AID field indicates whether the RU can be used for random access
* If AID=0, it means that the RU is used for random access by one or more associated terminals (as shown in the figure above, STAs 1, 2, and 4 are already associated)
* If AID=2045, it means that the RU can be used for random access by one or more unassociated terminals (as shown in the figure above, STA 3 is not associated)

* The range of random values selected by the terminal at the initial moment is specified by the AP, and the AP will use the OFDMA contention window (OCW) field (in the UORA Parameter Set Information field) to specify the range
* In the process of UORA, there may still be conflicts. That is, after the terminal OBO counter=0, it needs to randomly occupy one RU for uplink transmission. If two or more nodes occupy the same RU at this time, a conflict will occur on this one RU. If a collision occurs, the AP cannot successfully receive the corresponding data frame on the RU, and thus will not feed back an ACK. The terminal uses ACK to know whether the current round of transmission is successful. If it fails, then they will increase the size of their OCW to twice to reduce the probability of conflict.

### WiFI6 OFDMA TXOP:
* 和一般WiFi EDCA的TXOP類似，一樣都需要經過競爭成功獲得傳輸時間，但要注意的是由**AP競爭成功後**來啟動OFDMA模式(TF分資源，根據上下行傳送PPDU)
* 簡而言之: AP要先在EDCA競爭到TXOP才可以進入到OFDMA模式達成MU mode傳輸
* DL-OFDMA TXOP: 
* UL-OFDMA TXOP: 
* 無論是UL/DL，在trigger frame之前(TXOP開始前)，都還是要經過傳統EDCA backoff競爭channel的階段

### TWT(Target Wake Time）
* TWT定時喚醒機制，用於支持大規模物聯網環境下的節能工作。
* TWT一共有三種工作模式，分別是：(1)Individual TWT，(2)Broadcast TWT，(3)Opportunistic PS。
1. Individual TWT:該模式下sta會和AP協商特定的TWT時間，每一個sta僅僅知道自己和AP協商的TWT時間，不需要知道其他sta的TWT時間。
2. Broadcast TWT：由AP負責管理的工作機制。在該機制下，TWT時間週期是由AP宣告。
3. Opportunistic PS：和前面兩種工作模式類似，但是沒有AP和節點的協商過程。AP會在每一個Beacon內，公開宣告一個TWT時間。任意終端可以選擇在這個公開TWT時間內進行甦醒，並和AP執行數據幀交換。這個交換可以是單個節點的，也可以是採用OFDMA機制進行交換。

### BSS coloring
構成BSS的元素包含，包括1個AP和多個節點。在多個節點在信道傳輸時，802.11ax通過BSS Coloring字段來區分BSS。若BSS Coloring信息相同，那麼是BSS內的傳輸，若BSS Coloring不同，那麼是BSS間的傳輸

對於802.11ax的AP，其如果檢測到使用相同顏色的OBSS，則它能夠更改變其BSS顏色，減少同頻干擾。若AP與AP間的BSS Coloring一樣，那麼這也是一種BSS Coloring的衝突。

如果終端檢測到顏色衝突，則該終端會向其關聯的AP發送顏色衝突報告。
當AP檢測到顏色衝突後，其可以決定改變其BSS顏色

AP會通過Beacon告知所有關聯在本BSS內部的節點，BSS Coloring的改變。

### EDCA（Enhanced Distributed Channel Access）
1. 支持QoS的DCF就變成了EDCA模式 [DCF](https://zhuanlan.zhihu.com/p/20721272)
2. AIFS（arbitration interframe space）：在EDCA中，為了提供優先級，所以這裡等待幾個Slot時間是可以配置的，這個就是EDCA裡面的AIFSN（arbitration interframe space number）。如下圖所示
PIFS=SIFS+1* Slot，DIFS=SIFS+2* Slot，AIFS=SIFS+n*SIFS。如果n越大，那麼意味著每一次接入信道前需要等待更多的時間，從而優先級更低。

3. CW（Contention Window）：傳統的DCF中，CWmin=15，CWmax=1023，這個是所有的競爭節點都是相同設置的。但是在EDCA中，由於節點的優先級與CW值關聯起來，所以針對不同的AC，其CWmin和CWmax設置會不同。

上圖可以看出來，當經過不同的AIFS後，每一個AC類型的節點，其採用的Backoff隨機數範圍也是不同的，這裡Backoff的隨機數範圍是與AC有關的，比如AC_BK流，也就是背景流，其隨機數範圍比較大，接入延長會多一些，而AC_VO，也就是語音流，其隨機數範圍比較小，接入時延會短一些。
* 高優先級是明顯比低優先級有更高的傳輸效率的。

4. TXOP：還有一個部分就是TXOP傳輸部分了，TXOP也是EDCA的一個重要部分。在EDCA中，當節點獲得採用優先級參數進行競爭並獲得信道後，其可以採用TXOP的形式進行傳輸。