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Supported Targets ESP32-S2 ESP32-C3

FTM Example

(See the README.md file in the upper level 'examples' directory for more information about examples.)

Introduction

One of the ways in which WiFi enabled devices can measure their distance to the Access Point is by measuring Wi-Fi Round Trip Time (Wi-Fi RTT). Wi-Fi RTT is the time a WiFi signal takes to travel from Station to an AP. This time is proportional to the actual distance between them. Given the RTT, the distance can be calculated with below simple formula -

distance = RTT * c / 2 (Where c is the speed of light)

Wi-Fi RTT is calculated using a procedure called Fine Timing Measurement(FTM). During FTM procedure, a burst of Action frames is transmitted by one device(FTM Responder) to another(FTM Initiator) and each of them is ACK'ed. Hardware in both the devices mark time-of-arrival (TOA) and time-of-departure (TOD) of both Action frame and its ACK. In the end, the FTM Initiator collects the data for all pairs of Action frame and ACK and calculates RTT for each pair with below formula -

RTT[i] = (T4[i] - T1[i]) - (T3[i] - T2[i]) Where T1[i] : TOD of i'th Action frame from Responder T2[i] : TOA of i'th Action frame at Initiator T3[i] : TOD of i'th ACK from Initiator T4[i] : TOA of i'th ACK at Responder

Average RTT is calculated over all such pairs to get a more accurate result. Use this example to perform FTM between a Station and a SoftAP or en external AP that supports FTM Responder mode. Both Station and SoftAP need to be run on the supported ESP targets that support FTM and have it enabled.

How to use example

With this example, users can scan for AP's that support FTM Responder role and perform FTM procedure with different configurations. Below steps show how to do this using 2 devices in Station and SoftAP mode. First make sure that FTM Initiator support on Station and FTM Responder support on SoftAP is enabled in the project configuration menu (idf.py menuconfig). These options are located in Component config -> Wi-Fi. Furthermore for getting a per frame detailed report of the FTM procedure on the console, enable FTM Report logging option. Users can also access this report data in the example code. Build and flash the example on a supported device to see below output -

 ==========================================================
 |                      Steps to test FTM                 |
 |                                                        |
 |  1. Use 'help' to gain overview of commands            |
 |  2. Use 'scan' command to search for external AP's     |
 |                          OR                            |
 |  2. Start SoftAP on another device using 'ap' command  |
 |  3. Start FTM with command 'ftm -I -s <SSID>'          |
 |                                                        |
 ==========================================================
ftm>

Use help to get a list of available commands and options. Use scan command to scan for AP's that support FTM Responder mode. Before initiating FTM with an external AP, make sure that FTM Responder is visible in the respective scan result entry.

ftm> scan
I (476765) ftm_station: sta start to scan
ftm> I (478805) ftm_station: [Abeeys Palace][rssi=84]
I (478805) ftm_station: [privateproperty][rssi=76]
I (478805) ftm_station: [C904][rssi=69]
I (478815) ftm_station: [FTM][rssi=-94][FTM Responder]
I (478815) ftm_station: [Velop][rssi=-115]
I (478825) ftm_station: sta scan done

AP's that support FTM Responder mode can be seen in the scan results. Or setup a SoftAP on another device using the ap command -

ftm> ap FTM password
I (91271) ftm_ap: Starting SoftAP with FTM Responder support, SSID - FTM, Password - password
ftm>

Issue ftm -I to initiate a session with default configuration of 32 FTM frames. For more configurations below options are available - ftm [-I] [-c <0/16/24/32/64>] [-p <2-255 (x 100 mSec)>] [-s SSID] Where -

  • -I OR --ftm_initiator: FTM Initiator mode
  • -c OR --frm_count: FTM frames to be exchanged (Valid values: 0=No preference, 16, 24, 32, 64, default: 32)
  • -p OR --burst_period: Periodicity of FTM bursts in 100's of miliseconds (0: No preference, default: 2)
  • -s OR --ssid=SSID: SSID of AP that supports FTM Responder mode

Currently FTM is only supported in below configuration -

  1. Station as Initiator and SoftAP as Responder on supported ESP devices
  2. Station as Initiator and an external AP that supports FTM in Responder mode The first option should be preferred since ESP devices are self calibrated for high resolution measurement. FTM Responder support for external Stations and ASAP mode will follow in future updates.

Example Output

Example output of an FTM Procedure -

ftm> scan
I (356414) ftm_station: sta start to scan
I (358514) ftm_station: [DigitalFortress][rssi=114]
I (358524) ftm_station: [TEST][rssi=-96][FTM Responder]
I (358524) ftm_station: sta scan done
ftm> ftm -I -s TEST
Starting FTM with 18:fe:34:72:50:c9 on channel 1
I (391824) ftm_station: Starting FTM Initiator with Frm Count 32, Burst Period - 200mSec
W (391834) wifi:Starting FTM session in 0.200 Sec
W (393564) wifi:FTM session ends with 26 valid readings out of 31, Avg raw RTT: 49.218 nSec, Avg RSSI: -1
I (393564) ftm_station: Estimated RTT - 33 nSec, Estimated Distance - 5.07 meters

The final statement gives the average calculated RTT along with an estimated distance between the Station and the AP. This distance is measured by first adjusting the RTT with any physical analog delays and a calibration delta. Distances measured using RTT are not perfectly accurate, and are subjected to various errors like RF interference, multi-path, path loss, orientations etc. The design requires line-of-sight with straightforward propagation path with no less than -70dBm RSSI for better results.