WiFi 802.11bf - Sensing Meaurement Instance

Sensing measurements are performed during a sensing measurement instance.Each sensing measurement instance is assigned a Measurement Instance Identifier.
Two variants of sensing measurement instances exist:

• Trigger-Based (TB). In this variant, the sensing initiator is an AP, and one or more clients assume the role of sensing responders.
• Non-TB. In this variant, the sensing initiator is a client, and only one STA (an AP) assumes the role of sensing responder.

Both variants allow for measurements to be obtained in the uplink (AP is the sensing receiver), downlink (AP is the sensing transmitter), or both.

Notice that the TB variant is the only one allowing multistatic sensing. Both variants allow for sensing measurements results to be reported if requested by using sensing measurement reports. CSI is the only sensing measurement defined by TGbf, and as such, is the only sensing measurement report available.

To reduce the overhead of the report, sensing measurements are encoded into 8 or 10 bits, and only 1 out of 4, 8, or 16 subcarrier values are reported depending on the measurement configuration.

1.TB sensing measurement instance:
This instance (between an AP and STA(s)) take place during the period of time called the sensing availability window. A sensing availability window is made of one or more TXOPs (Transmission Opportunities) and each TXOP consists of one or more TB sensing measurement instances.
This instance may include the following phases:

• Polling phase

The sensing initiator determines the availability of sensing responder(s) at the start of the instance. The polling phase is typically required to overcome the fact that in the TB case, sensing responder(s) are non-AP STAs which might be in power saving mode even though they have accepted the corresponding sensing measurement setup. 

• NDP Announcement (NDPA) sounding phase
• Trigger Frame (TF) sounding phase

The NDPA sounding phase allows for measurements to be performed in the downlink; and the TF sounding phase, in the uplink.

• Reporting phase

The reporting phase allows to feedback the measurement results obtained in the NDPA sounding phase.

The presence of these phases is defined by the OPs(Operation Parameters) set in the corresponding sensing measurement setup. For example, the reporting phase is only present when a sensing responder is a sensing receiver, and the OP defined in the sensing measurement setup states that the sensing responder must report obtained measurements.

2. Non-TB sensing measurement instance

A client may initiate a non-TB sensing measurement instance whenever it gains channel access (without the need for a polling phase). Indeed, contrary to the TB case, the sensing responder in the non-TB case is an AP and thus never goes to powersaving mode.

As illustrated in Fig. below, a sensing initiator (STA1) initiates a non-TB sensing measurement instance by sending an NDPA frame to the sensing responder (AP), followed by an SI2SR NDP after SIFS. Once the SI2SR NDP is received, the sensing responder transmits an SR2SI NDP within SIFS to the sensing initiator.

SI2SR NDP = Sensing Initiator to Sensing Responder NDP
SR2SI NDP = Sensing Responder to Sensing Initiator NDP

Different from the TB variant, for ease of implementation, the packet exchange defined for non-TB sensing measurement instances is the same, as illustrated in Fig. above, independent of whether measurements are obtained in the uplink only, downlink only, or both uplink and downlink.

During the optional reporting phase, the AP sends a Sensing Measurement Report frame to the client SIFS after transmitting the SR2SI NDP. Non-TB sensing is used when a sensing application is initiated by a client device and is limited to bistatic sensing.