Re: [Discuss-gnuradio] USRP Digital Modulation - Loopeback Constellation

[CEL]

Well, you asked for "and other modulations", so I gave you one where
phase synchronization is possible with simple control loops.

Synchronization of 16QAM is... nontrivial. I've just discussed this
with my favourite officemate.

In practical systems, it's very likely that you'd simply send a
preamble, and use that to estimate your phase. Then, do a decision-
feedback phase tracking, and/or insert pilot symbols periodically to
keep track of your channel. Preambles have the advantage of
singlehandedly also solve the pi/2 ambiguity.

Regarding symbol timing recovery: puuuh, that's where things get
interesting. Gardner might be a viable method, as it's basically
looking at the ACF of your pulse shaping; same goes for the symbol
recovery through polyphase derivative estimation.

I might be seduced to admit for a quick prototype I'd be lazy and treat
16 QAM for all synchronization purposes as a QPSK with an SNR no better
than √5 and deal with that bad SNR through very low loopback filter
bandwidths, but that can only work if your channel isn't changing
faster. But afterwards, even if this does work out (I'm not sure it
would), I'd still have a pi/2 phase ambiguity that I'd have to resolve
using other methods, so:

Probably start with a preamble, and oversample that by 4 to 16. Try to
do a phase and timing recovery based on the known pulse shape.

Best regards,
Marcus

On Thu, 2018-06-14 at 13:09 +0200, Luis Felipe Albarracin Sanchez
wrote:
> Hello Marcus, and all,
> 
> Thanks for the response,
> 
> I am reading the PSK example you told me, nevertheless y am not so sure it 
> applies also for 16QAM...i understand that Costas Loop, does not work with 16 
> QAM.
> 
> Is there any flowgrpah for a complete 16QAM modulator/demodulator?
> 
> Kind regards.
> 
> On Thu, Jun 14, 2018 at 12:46 PM, Müller, Marcus (CEL) <address@hidden> wrote:
> > Hi Luis,
> > On Thu, 2018-06-14 at 12:29 +0200, Luis Felipe Albarracin Sanchez
> > wrote:
> > > Hello Marcus,
> > > 
> > > Thanks for the response,
> > > 
> > > I am not using an attenuator, because i font have one , i am only with my 
> > > cable, but i have meassured the Power before connecting to the RX in my 
> > > USRP, so it wont be bigger than the threshold of the USRP.
> > 
> > That's a dangerous game – you rely on never inadvertedly setting a high
> > RX or TX gain.
> > 
> > > 
> > > Thanks for the info, is there any guide where i can look all of the 
> > > blocks for Sync, phase tracking and shape filtering  implemented in a 
> > > loopback for 16QAM and other digital modulations (Flowgraphs examples)?
> > 
> > https://tutorials.gnuradio.org has a PSK example if you read from
> > chapter 1 to 7 :)
> > 
> > Best regards,
> > Marcus
> 
> 
>

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