Frequency hopping synchronization

[Swanepoel]

Hi,
In an upcoming project I want to program two radio modules to communicate and also apply frequency hopping. One challenge that I see is how to synchronize the two radio's when powered up?

One option would be to keep listening on one channel while transmitting (a sync packet) on the hopping channels until they find each other. I think this should work but what about instances where the radios are switched on at the exact same time (or at a time so that both are in sync with transmitting and receiving cycles). In this case both will continue to transmit and receive at the same time and never find each other (the links are half-duplex). Since both radios run the same code the logic would be the same for both.

I guess I can give each radio kind of a unique "serial" number which is then also used as a seeding value for a short delay when the devices does not synchronize within a given time...

Has anyone done something like this before? Maybe the odds of powering up the radios in such a way that their transmission and reception are in sync is also extremely rare...

 

[JimB]

Has anyone done something like this before?

I have not done it myself.

Maybe the odds of powering up the radios in such a way that their transmission and reception are in sync is also extremely rare...

Indeed they are, but...
It WILL happen one day.
You need some sort of randomiser in there.

JimB

 

[Swanepoel]

You need some sort of randomiser in there.

I guess so... using the serial number as seeding for a pseudo random generator (which in turn use the generated number for a delay) seems like a option for the moment, although it also does not guarantee it...

 

[Mikebits]

I think cell phones use GPS to synchronize. Not exactly sure how it is done though. I might try to look this up.