Re: [Discuss-gnuradio] Why no phase ambiguity in digital-bert...

[Johnathan Corgan]

On Mon, Apr 12, 2010 at 22:11, Ian Holland <address@hidden> wrote:

> I have been studying up on the Costas loop, and have a couple of queries as
> to the benchmark_tx.py and benchmark_rx.py as a result.
>
> Firstly, for BPSK, there should in theory be a 180 deg. phase ambiguity when
> using a Costas loop. Why does this not seem to occur with the
> benchmark_rx.py example? Is this related somehow to the PN code introduced
> by the scrambler.

The digital-bert example uses a self-synchronizing scrambler to
generate a bit sequence that occupies the full baseband bandwidth of
the channel.  The scrambler/descrambler pair is insensitive to the
phase ambiguity; however, this comes at the price of generating extra
bit errors on the descrambled sequence for every bit error in the
channel.

> Secondly, I came across another post in which it was mentioned the Costas
> loop should only operate on a single sample per symbol. However, as I read
> the source code, it seems as though it is actually passed sps samples per
> symbol, where sps > 1. Have I misread something here?

Not sure what post you are referring to.  While a Costas loop can
indeed operate on a single sample per symbol, it can also operate on
more than that.  Different strategies in a receiver chain places the
frequency/phase synchronization at different places.

Johnathan