Calibrating a retro-directive array for an asymmetric wireless link
View Patent ↗The disclosed embodiments relate to a technique for calibrating a retro-directive array. During the calibration process, the system measures a gain g 1 through a first pair of antennas in the retro-directive array. Next, the system measures a gain g 2 through a second pair of antennas in the retro-directive array. The system then simultaneously measures a combined gain G 1,2 through the first and second pairs of antennas in the retro-directive array. If G 1,2 is less than g 1 +g 2 by more than a threshold value, the system calibrates a phase relationship between the first and second pairs of antennas.
1. A method for calibrating a retro-directive array, comprising:
measuring a gain g 1 through a first pair of antennas in the retro-directive array;
measuring a gain g 2 through a second pair of antennas in the retro-directive array;
simultaneously measuring a combined gain G 1,2 of the first and second pairs of antennas in the retro-directive array; and
if G 1,2 is less than g 1 +g 2 by more than a threshold value, calibrating a phase relationship between the first and second pairs of antennas.
2. The method of claim 1 , wherein measuring the gain g 1 through the first pair of antennas comprises:
activating the first pair of antennas while other antennas in the retro-directive array are inactive; and
performing a loopback measurement through the first pair of antennas to determine g 1 .
3. The method of claim 1 , wherein measuring the gains g 1 , g 2 , and G 1,2 comprises measuring the gains g 1 , g 2 , and G 1,2 at more than one frequency.
4. The method of claim 1 , wherein the first pair of antennas includes a first antenna and a second antenna which are coupled together through a loopback mechanism.
5. The method of claim 4 ,
wherein the first antenna comprises a first set of antenna elements; and
wherein the second antenna comprises a second set of antenna elements.
6. The method of claim 1 , wherein calibrating the phase relationship between the first and second pairs of antennas comprises adjusting the phase relationship until G 1,2 moves within the threshold value of g 1 +g 2 .
7. The method of claim 1 , wherein calibrating the phase relationship between the first and second pairs of antennas comprises:
calibrating delay elements associated with the first and second pairs of antennas;
calibrating phase mixers associated with the first and second pairs of antennas; or
calibrating local oscillators associated with the first and second pairs of antennas.
8. The method of claim 1 ,
wherein the first pair of antennas is in a first calibrated group of antennas, wherein antenna pairs in the first calibrated group of antennas are calibrated with each other;
wherein the second pair of antennas is in a second calibrated group of antennas, wherein antenna pairs in the second calibrated group of antennas are calibrated with each other; and
wherein calibrating the phase relationship between the first and second pairs of antennas additionally comprises calibrating a phase relationship between the first and second calibrated groups of antennas to form a combined third calibrated group of antennas.
9. The method of claim 1 , wherein calibrated groups of antennas are combined with each other to form larger calibrated groups of antennas until all antenna pairs in the retro-directive array are all calibrated against a common reference.
10. An apparatus for calibrating a remote retro-directive array, comprising:
a phase array configured to communicate with the remote retro-directive array; and
a controller coupled to the phase array and configured to perform operations to calibrate the remote retro-directive array, wherein the operations include,
measuring a gain g 1 through a first pair of antennas in the remote retro-directive array,
measuring a gain g 2 through a second pair of antennas in the remote retro-directive array,
simultaneously measuring a combined gain G 1,2 through the first and second pairs of antennas in the remote retro-directive array, and
if G 1,2 is less than g 1 +g 2 by more than a threshold value, calibrating a phase relationship between the first and second pairs of antennas in the remote retro-directive array.
11. The apparatus of claim 10 , wherein while measuring the gain g 1 through the first pair of antennas, the apparatus is configured to:
send a command to activate the first pair of antennas while other antennas in the remote retro-directive array are inactive;
transmit a test signal to the remote retro-directive array;
receive an echo signal from the remote retro-directive array; and
measure the echo signal to determine a loopback gain g 1 through the first pair of antennas.
12. The apparatus of claim 10 , wherein measuring the gains g 1 , g 2 , and G 1,2 comprises measuring the gains g 1 , g 2 , and G 1,2 at more than one frequency.
13. The apparatus of claim 10 , wherein calibrating the phase relationship between the first and second pairs of antennas comprises sending commands to the remote retro-directive array to adjust the phase relationship until G 1,2 moves within the threshold value of g 1 +g 2 .
14. The apparatus of claim 10 , wherein calibrating the phase relationship between the first and second pairs of antennas comprises sending commands to the remote retro-directive array to:
calibrate delay elements associated with the first and second pairs of antennas;
calibrate phase mixers associated with the first and second pairs of antennas; or
calibrate local oscillators associated with the first and second pairs of antennas.
15. The apparatus of claim 10 ,
wherein the first pair of antennas is in a first calibrated group of antennas, wherein pairs of antennas in the first calibrated group of antennas are calibrated with each other;
wherein the second pair of antennas is in a second calibrated group of antennas, wherein pairs of antennas in the second calibrated group of antennas are calibrated with each other; and
wherein calibrating the phase relationship between the first and second pairs of antennas additionally comprises calibrating a phase relationship between the first and second calibrated groups of antennas to form a combined third calibrated group of antennas.
16. The apparatus of claim 15 , wherein calibrated groups of antennas are combined with each other to form larger calibrated groups of antennas until all pairs of the antennas in the retro-directive array are calibrated with each other.
17. A retro-directive array, comprising:
an array of antennas, wherein pairs of antennas in the array are coupled together through circuits to form a retro-directive array;
a command interface configured to receive activation commands to activate pairs of antennas in the retro-directive array; and
an activation circuit, wherein in response to receiving one or more activation commands, the activation circuit is configured to activate one or more pairs of antennas in the retro-directive array, while other pairs of antennas in the retro-directive array remain inactive.
18. The retro-directive array of claim 17 , wherein each pair of antennas in the retro-directive array includes a first antenna and a second antenna that are coupled together through a loopback mechanism.
19. The retro-directive array of claim 17 , wherein the each antenna in the retro-directive array comprises a set of antenna elements.