Electronic – How do LoRa end-devices configure their upstream and downstream communication

I've tried a similar setup and I've realized that you're using a very small sweep time (500 us). Your transient, that seems so wide in the picture, last only about 30 us. When using a quasi-peak detector it will be certainly removed.

I can show you my pictures. In my setup I've used a 10 dBm signal, 868 MHz central frequency, and an offset of 200 kHz when looking for the transient. In all the figures RBW is 120 kHz and VBW is 100 kHz.

Here it is the zero-span picture, no PA ramp is used:

My sweep time is 5 s. The pulse is then about 0.8 seconds wide, and the transient is less than a few ms (didn't measure it accurately).

When using a Quasi-Peak detector I get this:

If I repeat both measurement after activating PA ramp in the transmitter, the transient peak disappears:

And using a Quasi-Peak detector:

Conclusions:

Your phase noise is in fact fine, and so are the transients. The fact that the transient doesn't change when enabling PA ramp seems to indicate that there is some kind of implementation error with this feature. Anyway your transient will strongly smooth out under a Quasi-Peak detection.

This clock sync doesn't necessarily happen before; it often happens before, and during transmission.

The things you might want to read in the usual literature (especially: Sklar and Proakis, imho) is timing synchronization.

How to implement that depends on a lot of unknowns, so I can't give you one answer. In reality, there's literally hundreds of approaches to synchronize timing, and which one you choose depends, among a lot of other things, on

• physical relation of symbol rate and carrier frequency
• modulation
• signal model
• statistical noise model
• statistical interference model
• receiver imperfection models (nonlinearities, I/Q imbalances, clock drifts, sampling jitter, …)
• length of transmission
• coherency time of your channel (which is closely linked to)
• doppler bandwidth of your channel
• statistical model of the clock offsets
• affordable overhead for CSI acquisition vs. payload data rate
• receiver operation characteristics, ARQ vs robustness trade-off etc
• memory available at receiver
• computing power available at receiver
• power consumption limits and tradeoffs
• MIMO necessity
• receiver cost vs BER tradeoff
• Channel coding used (effect of SER, in the end)
• multi-user capabilities

If you pick any of these books, they'll probably introduce some classical synchronizer – and you can still find those in application very often! But all of these are already relatively application-specific. For example, a control loop that stabilizes a QPSK reception doesn't necessarily (and usually: won't, because a few mathematical tricks don't work here) work for 8PSK.

If you're really just interested in timing estimation happening before actual payload transmission, you're after preambles. Those are known sequences that the receiver looks for to estimate the parameters of the channel and transmission – timing is one of these parameters.