Time-of-flight estimation using sampling error values
A circuit includes a first wireless radio frequency (RF) transceiver and a time-of-flight estimator included with or coupled to the first wireless RF transceiver. The time-of-flight estimator estimates a time-of-flight between the first wireless RF transceiver and a second wireless RF transceiver using: a first interval value that indicates an amount of time between when the second wireless RF transceiver received the message and when the second wireless RF transceiver transmitted the response; a first error value that indicates an offset between when the second wireless RF transceiver sampled the message and a target sampling point for the message; a second interval value that indicates an amount of time between when the TX chain sent the message and when the RX chain received the response; and a second error value that indicates an offset between when the RX chain sampled the response and a target sampling point for the response.
1 . A method comprising:
receiving, by a first device, a first message at a first time;
determining, by the first device, a first offset value between the first time and a next clock cycle of a first clock of the first device;
transmitting, by the first device, a second message at a second time that corresponds to another clock cycle of the first clock;
determining a first time interval between the next clock cycle and the another clock cycle; and
transmitting, by the first device, the first time interval and the first offset value.
2 . The method of claim 1 , wherein determining the first offset value between the first time and the next clock cycle comprises determining the first offset value between the first time and a rising edge of the next clock cycle.
3 . The method of claim 1 , wherein determining the first offset value comprises determining the first offset value using a Gardner algorithm.
4 . The method of claim 1 , wherein the first message is a car identification with challenge message, and the second message is a key response message.
5 . The method of claim 1 , wherein the first device comprises a key fob.
6 . The method of claim 1 , wherein transmitting the second message comprises transmitting the second message in accordance with a Bluetooth Low Energy (BLE) protocol.
7 . The method of claim 1 , further comprising:
receiving, by the first device, a wake up message; and
transmitting, by the first device, an acknowledge message responsive to the wake up message, wherein transmitting the acknowledge message comprises transmitting the acknowledge message before receiving the first message.
8 . The method of claim 1 , wherein transmitting the first time interval and the first offset value comprises transmitting the first time interval and the first offset value after transmitting the second message.
9 . The method of claim 1 , wherein receiving, by the first device, the first message comprises receiving the first message from a second device that comprises a second clock that does not track the first clock.
10 . The method of claim 1 , wherein receiving, by the first device, the first message comprises receiving the first message from a second device, the method further comprising determining a time-of-flight time between the first and second device based on the first time interval and the first offset value.
11 . The method of claim 10 , wherein the second device is a vehicle.
12 . The method of claim 10 , further comprising asserting a fault signal when the time-of-flight time is higher than a threshold.
13 . The method of claim 12 , further comprising sounding an alarm, turning on a light or disabling another device in response to the assertion of the fault signal.
14 . The method of claim 12 , further comprising disabling an engine responsive to the assertion of the fault signal.
15 . The method of claim 10 , wherein the time-of-flight time is smaller than a clock period of the first clock.
16 . The method of claim 10 , further comprising determining presence detection based on the time-of-flight time.
17 . The method of claim 10 , further comprising determining spatial positioning based on the time-of-flight time.
18 . The method of claim 10 , further comprising determining relative movement based on the time-of-flight time.
19 . The method of claim 1 , further comprising:
transmitting, by a second device, the first message at a third time that corresponds to a first clock cycle of a second clock of the second device;
receiving, by the second device, the second message at a fourth time;
determining, by the second device, a second offset value between the fourth time and a next clock cycle of the second clock;
determining, by the second device, a second time interval between the first clock cycle and the next clock cycle of the second clock;
receiving, by the second device, the first time interval and the first offset value; and
determining a time-of-flight time between the first and second device based on the first and second time intervals and the first and second offset values.
20 . The method of claim 19 , wherein determining the time-of-flight time comprises performing
ToF
=
0.5
*
(
(
Ta
-
Tb
)
-
(
Da
+
Db
)
)
,
wherein ToF represents the time-of-flight time, Ta represents the second time interval, Tb represents the first time interval, Da represents the second offset value, and Db represents the first offset value.