WiFi Sensing Study - 802.11 bf

Sensing systems can be categorized based on the number of devices used to obtain sensing measurements. 
These architectures are naturally supported by Wi-Fi systems:

• Monostatic requires only one station (STA) and at least two antennas. This architecture does not need any communication or coordination between multiple STAs from a sensing point-of-view and therefore does not need to be specifically addressed by a Wi-Fi sensing standard as long as it respects IEEE 802.11 mandatory requirements (e.g., channel access).

• Bistatic requires a communication between two STAs. It can thus be achieved between an AP and a client (a client being typically connected to a single AP).

• Multistatic requires communication involving more than two STAs and can thus be achieved between an AP and multiple clients either by obtaining measurements using signals transmitted by multiple clients or by transmitting signals that are measured by multiple clients.

It is worth mentioning that the set of sensing applications that can be supported with measurements obtained by a client (bistatic) is limited compared to those that rely on measurements obtained by an AP, which is typically connected to multiple clients (multistatic). To address this issue, the IEEE 802.11bf amendment defines features specifically designed to enhance client-based sensing, as discussed later.

The procedure that allows a STA to perform sensing in frequency bands below 7 GHz is the WLAN sensing procedure.

As depicted in Fig. below, the WLAN sensing procedure is organized with roles and phases.

The roles provide a sensing terminology to describe each STA role in the sensing framework, e.g., which STA initiates the sensing procedure, which STA performs measurements, etc.

The phases organize the protocol operations that constitute the WLAN sensing procedure.

The WLAN sensing procedure main goal is to enable STAs to:
1) inform other STAs of their sensing capabilities
2) request and setup transmissions that allow for sensing measurements to be performed
3) perform sensing measurements and exchange sensing measurement results
4) release resources allocated for sensing

Let's take a look at the figure above, the following are the steps of WLAN sensing procedure.

• Sensing Session Setup :
  Prior to any sensing measurements, the sensing initiator and sensing responder(s) must first determine their respective support for the WLAN sensing procedure and the sensing capabilities they implement if any. The sensing session setup relies on procedures commonly used in the IEEE 802.11 standard, such as the association process.
  
  
• Sensing Measurement Setup :
  Each sensing application has unique requirements in terms of sensing configuration and parameters, e.g., which devices perform sensing measurements, for how long, reporting parameters,etc.
  The sensing initiator must configure the sensing itself by exchanging and agreeing with each sensing responder on Operational Parameters (OPs) to use during the sensing.
  OPs include , for example, the role of the sensing initiator and responder(s) (sensing transmitter, receiver, or both roles), or if sensing reporting is needed.
  A sensing initiator can configure sensing measurements to be performed by multiple responders using the same OPs (in this case, multistatic sensing).
  A sensing initiator can also establish multiple sensing setups with the same sensing responder to handle multiple sensing applications with different requirements.
  
  Procedure explanation :
  <1> Initiated when the sensing initiator send Sensing Measurement Request Frame with the OPs to use with a sensing responder. Need to assign a Measurement Setup Identifier (MSID) + sensing initiator's MAC address, so that it can be used to uniquely identify the OPs.
  <2> Sensing responder must send a Sensing Measurement Setup Response frame in return that either accepts/rejects the request or proposes different OPs. 
  
  
• Sensing Measurement Instance :
  It is a packet exchanges between a sensing initiator and one or more sensing responders that allow for sensing measurements to be obtained. It is associated with a single set of OPs.
  
  
• Sensing Measurement Setup Termination :
  It is used wen a sensing application no longer requires sensing measurements between a sensing initiator and a sensing responder. 
  <1> Explicit SMST : a STA (either the sensing initiator or the sensing responder) sends a Sensing Measurement Setup Termination frame (including the MSID) to the peer STA, terminating the measurement setup between these two STAs.
  <2> Implicit variant, the sensing measurement setup is terminated at the expiration of the measurement setup expiry timer (set during the sensing measurement setup).
  
  
• Sensing Session Termination :
  STAs stop performing measurements and terminate the sensing session, releasing any resource associated to sensing (e.g., CSI report buffered).