Hi Max, Thanks for the response, I think I found my answer meanwhile, funny enough your interpretation of aligned timing made me read again part 1 of the standard, and I realized that my interpretation of it was actually wrong :)
Aligned timing is used to describe the RF timeslot transmission arrangement between MS transmission and BS transmission. It actually does not involve TDMA frame sample alignment in time between MS and BS. That's sort of implied and the standard does not prescribe how to do that. Personally I use GNU radio timestamps to have a deterministic transmission time for BS so MS can be received at the right time. The code for that is not very complicated for the BS side, if you want I can link here the GNU radio flowgraphs with timestamps that I wrote in case you ever want to implement the MS side in OP25, the code is very arcane but it works. The MS side in SDR is an interesting discussion. The way I would do it for the MS is to keep a SDR-time reference with RX_TIME timestamps, search for BS outbound CACH or VOICE / DATA sync, and update the time reference by adding the number of samples from the center of the sync sequence to the end of the TDMA frame. BS-side is easier since BS sets it's own time reference. I haven't really investigated this since the main problem I see is switching a PA on and off fast enough. With pin diode RF switches, we can achieve tens or hundreds of nanoseconds switch time, but then there's also the PA ramp-up and ramp-down which should take less than tens or hundreds of microseconds. I don't know really how to achieve that, I have a LimeRFE PA board, but I control it via USB so I think it's too slow for that. Coming back to the aligned timing issue, to me it seems the ETSI language is clear as mud. I would have expected aligned timing to mean that the BS is transmitting timeslot 1 while the MS is transmitting also timeslot 1. But no, in ETSI lingo, that is called offset timing. Aligned timing actually means BS is transmitting timeslot 2 while MS is transmitting timeslot 1. It became clear to me after I read you message that actually in aligned timing mode, the MS transmits timeslot 1, then switches to RX during timeslot 2 and hears its own transmission as the BS transmits timeslot 1 during MS RX period. So that also neatly explains why full duplex calls are not possible in aligned timing mode. So problem solved, the Reverse Channel can always be transmitted in the default aligned timing mode, since it is always appended to the active transmission instead of the alternate slot transmission as I believed at first! Thanks and have a great day, Adrian
