Hey everyone, thanks for coming back to Wi Fi fundamentals with location and analytics. This course will help you to muster the air. We have reached the holy grail of location and raging Wi Fi RTT. Time and the air these two words are the essence of Wi Fi RTT the next generation of Wi Fi location and ranging. Wi Fi RTT is all about flight time. How long does it take for a frame to reach from one side to the other and back.

It will open the door for services and applications never seen before in indoor environment. The type of services and applications that Wi Fi RTT is bringing along was up until a few years ago impossible. Using Wi Fi RTT with more than one receiver can make up for a very high resolution location, but you need both the receiver and the transmitter to support Wi Fi RTT. The main challenges behind Wi Fi RTT is the fact that measuring has to be done at the nanosecond accuracy. A tiny mistake could lead to hundreds of meters. reflections from nearby obstacles lead to multiple signals arriving to the receiver.

We can see in our example that the direct path time from the access point to the station is 10 nanosecond what is being reflected from one side of the wall. It takes up to 80 nurses And when it's being reflected from the other side of the world, it takes 22 nanoseconds. The way RTT works is quite simple. It uses time of flight instead of signal strength, the time it takes to send a frame from our access point to our station and back again, since RF travels at the speed of light, so the equation to get the distance is fairly simple. The total time times the speed of light divided by two delay time between receiving and sending the acknowledgement is also calculated. Our session as always scans the environment listening to beacon frames and Sending probe request if it uses the probe response with FDM capabilities FDM stands for fine timing measurement.

It knows that the access point supports Wi Fi RTT and it's sensing an FTM request frame. An FTM request frame is a specific action frame that is sent to the BSS to trigger an action. It is the first step for an FTM session that will end with a range and location. It allows the station to obtain its range relative to another access point. Ranging is the first step to acquire position, but that needs to have at least another access point. Usually, our access point is the responder and the station Is the initiator for an FTM session to happen both our access point and station must be synchronized to the same clock reference.

So, our session as the access point to make a measurement, or access point sends an FTM frame with the time of departure the time it leaves our access point, which we'll call it timestamp number one, our station receives it, timestamps it and response within acknowledgment or access point receives the acknowledgement frame and timestamp the arrival time T for and now the distance can be measured by subtracting the timestamps. Distance can be calculated in both sides, once the four timestamps are available, the access points since the two remain remaining timestamp T one and T four inside a new measurement frame to the mobile device to make the calculation. So the time of flight is actually the AP is the access point the delivery time, the first frame that hits sense minus the station arrival time. Take that divided into two and you have the time of flight.

The distance is actually C which is the speed of flight times the time of flight. So let's recap the range from a station to the access point is calculated using This properties, the time of departure, which is the timestamp of the sent frame, the time of arrival which is the timestamp of the received frame, the delay time the time that took our station to send the frame back the time of flight, which is the departure time minus the arrival time divided by two and the speed of light. Since mobile devices move a lot, they can schedule periodic measurement and this allow ranging every few seconds FTM request frame can be requested with other access points in different BSS IDs. FTM distance is driven from the time it takes the initiator signal to get to the responder and back that requires a highly accurate clock for time measuring moment in nanosecond.

Remember, light travels at about 30 centimeters per nanosecond. We can get one meter accuracy in Wi Fi RTT but both devices have to be synchronized. Next up Wi Fi RT t location. See you soon