JEJeija·Feb 18, 2025
Every WiFi chip needs to be able to synchronize to the received signal down to a few I/Q samples (the remaining synchronization error is accounted for by the cyclic prefix of OFDM). For example, with 40MHz of bandwidth, there is an I/Q sample every 25 nanoseconds (the wave travels 7.5m during that time), so the level of synchronization will be on that order. The ESP32 reports the time of arrival, the code for extracting a nanosecond-precision timestamp is here: https://github.com/ESPARGOS/pyespargos/blob/main/espargos/po...
Additionally, you can use the phase information in the estimated channel coefficients (CSI) to determine the more precise time of arrival. Very briefly, a time delay in time domain will be visible as a frequency(=subcarrier)-dependent phase shift in frequency(=subcarrier)-domain. Now synchronization is only limited by
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impairments like thermal noise and phase noise. In practice in nice lab conditions, the time of arrival accuracy is on the level of <<1m.
On top of phase differences between subcarriers (--> timing information), we also measure phase differences between antennas, which provides angle of arrival and, with multiple arrays, phase of arrival information.
You can go even further by measuring phase differences between subsequent packets, which provides frequency offset and Doppler (--> velocity) information. I have some nice results on that, but that's a topic for the future ;)