Hello Adrian
Sorry to say I'm not particularly conversant with DMR; anyone who can furnish
more detail is encouraged to jump in here... Accordingly I'll have to limit
this to generalities...
> reached a point where I no longer know how to interpret the ETSI standard
Yes, I know the feeling. The TIA standards can induce the same feelings...
As for "aligned timing", I assume this to mean the capability of the MS to
transmit TDMA fragments whose start and end times are in strict alignment with
the slot boundaries demarcated by the outbound data stream transmitted by the
BS. The feasibility of this is more questionable when the MS is implemented in
PC software and separated from the hardware radio by a number of software
layers and drivers each having its own associated latencies / unpredictable
delays.
All of this goes double when the MS also has to retune the radio at precisely
the timing needed to acquire reverse channel bursts sandwiched between its own
transmitted fragments.
For all the above reasons I decided it's not currently feasible for us to try
to implement a TDMA subscriber in terms of GNU Radio blocks. A proper job
would seem to require an implementation at a level much closer to the RF
hardware and/or the FPGAs.
> There are a few scenarios where it seems to me these tier 3
> services (power control, priority interrupt) cannot be
> available at all...
> 1. Alternate timeslot is a control channel
> 2. Alternate timeslot is idle
> 3. Alternate timeslot carries something other than voice payloads
In each of these cases I wouldn't be surprised if there might be room enough to
also sneak a PDU or two into the alt. timeslot somewhere along the line?
> The standard doesn't clarify these cases
And it wouldn't be the first or the last time. In such cases there is always
the option of performing RF captures and decoding them. This situation might
require spacing/separation between BS, MS, and test receiver to enable
everything to fit within the limited dynamic range of the test reeciver (or use
two test receivers).
This also requires access to a vendor system and setup, of course...
hth 73
Max KA1RBI
On Sunday, January 14, 2024, 5:02:11 AM EST, Adrian Musceac
<kanto...@droiddv.org> wrote:
Hello OP25 devs,
I'm an amateur radio operator working on implementing an open source DMR tier
3 trunked radio base station based on a multi-carrier SDR structure. I have
reached a point where I no longer know how to interpret the ETSI standard and
I thought to ask for help here due to your expertise with trunked radio
systems.
The outbound Reverse Channel PDU is used in DMR tier 3 for MS power control
and MS de-key (transmit interrupt). Based on my interpretation of the
standard, a couple of things are clear:
1. Reverse channel is only available if aligned timing is used
2. It is supposed to be embedded in voice burst F of a voice transmission on
the alternate timeslot
3. The MS is supposed to monitor the alternate timeslot during transmit and
derive RC information from it
There are a few scenarios where it seems to me these tier 3 services (power
control, priority interrupt) cannot be available at all...
1. Alternate timeslot is a control channel
2. Alternate timeslot is idle
3. Alternate timeslot carries something other than voice payloads
The standard doesn't clarify these cases and I have doubt that these 2
services can even be reliably implemented.
Are you able to help me understand how the Reverse Channel is used in the real
world by existing industry implementations of DMR tier 3?
Thanks,
Adrian YO8RZZ
