No BPSK modulator?

Hi,

Is there a reason why there is no “ordinary” n-PSK modulator and
demodulator available, only the differential one? If not, I’ll modify
the existing one and submit a patch.


Trond D.

demodulator available, only the differential one? If not, I’ll modify
the existing one and submit a patch.

It’s not so much that there isn’t a non-differential BPSK modulator,
it’s
that there is no demod for it. Basically, we have to be able to
determine
the phase orientation of the incoming signal in some way to know what
our
phase is. In BPSK, this is simple, and this was the original reason why
gr_correlate_access_code_bb had a d_flip variable in it, so that when
the
phase inversion was detected, it would decode it properly.

However, this was never a good solution and ended up hurting us because
sometimes we’d trigger this flip with random data, which would screw up
the
packets. This was the reason the SSH sessions would crash and why
Johnathan
made the original fix in the whitener.

The solution to this is to use the correlator to detect the phase, but
let
the next guy down the line understand what to do with it. We probably
want
to send a control signal out to set the phase derotation and trigger the
access code found state at the same time, specifically, in the simple
framer
state machine.

Tom

On Tue, Feb 27, 2007 at 05:07:31PM -0500, Tom R. wrote:

demodulator available, only the differential one? If not, I’ll modify
sometimes we’d trigger this flip with random data, which would screw up the
packets. This was the reason the SSH sessions would crash and why Johnathan
made the original fix in the whitener.

The solution to this is to use the correlator to detect the phase, but let
the next guy down the line understand what to do with it. We probably want
to send a control signal out to set the phase derotation and trigger the
access code found state at the same time, specifically, in the simple framer
state machine.

You could use bit 2 of the output stream of
gr_correlate_access_code_bb to encode the “I’ve seen the inverted
pattern”. Then anybody who cared could look at it and take action if
they wanted to, but it wouldn’t interfere with any existing code.

Eric

(which makes me
curious, does anyone use non-differential 8PSK?).

Yes, satellites often do.

Matt

On Tue, Feb 27, 2007 at 11:01:58PM -0800, Matt E. wrote:

(which makes me
curious, does anyone use non-differential 8PSK?).

Yes, satellites often do.

I might add that non differential PSK/QPSK has a theoretical
noise advantage if some mechanism exists to figure out the correct
initial rotation and not lose it later on. And this is often pretty
easy to do when using convolutional code inner FEC from the FEC
correction which works with few errors for only one of the two or four
possible rotations.

Basicly best case in differential modulation an error in
detecting a symbol causes TWO bit errors in the data while a single
symbol error in absolute phase, non-differential modulation causes only
one bit error in the data before FEC.


Dave Emery N1PRE, [email protected] DIE Consulting, Weston,
Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
‘For Rent’ sign still vainly flapping outside on the weed encrusted pole

  • in
    celebration of what could have been, but wasn’t and is not to be now
    either."

[mailto:discuss-

want
Eric
Yes, I realize that, but I’m not a huge fan of doing it this way, quite
frankly. It’s a bit more inefficient to use multiple output streams for
this, but by putting the flag inside the data stream seems to obfuscate
the
system. I’d much rather open a separate control stream to pass this kind
of
information around. This would, of course, require reworking the block
(or
adding a new one to avoid any conflicts), so if there is a consensus to
add
more information, sure. I would suggest, though, that we might use at
least
3 bits for this to support up to 8 phase rotations for 8PSK (which makes
me
curious, does anyone use non-differential 8PSK?).

Also, is there a clever way of figuring out the phase rotation from the
access code without going through the permutations? For BPSK we only
need to
test two, the access code and its bitwise compliment, but for QPSK we
have
four and 8PSK has eight possible phases. At least with QAM, we should be
able to just use 4, so I think 8 is the maximum we would ever need to
account for.

Tom

2007/2/27, Eric B. [email protected]:

Is there a reason why there is no “ordinary” n-PSK modulator and
However, this was never a good solution and ended up hurting us because
You could use bit 2 of the output stream of
gr_correlate_access_code_bb to encode the “I’ve seen the inverted
pattern”. Then anybody who cared could look at it and take action if
they wanted to, but it wouldn’t interfere with any existing code.

Eric

Thank you all for anwering. What I am really looking for is just BPSK,
so that should simplify things a bit. I intent to demodulate the BPSK
modulated message from GPS satellites, therefore the interest in
not-differential PSK modulation.

To make things a bit clearer: correlate_access_code is in reality a
preamble detector, or am I missing something?


Trond D.

Sent: Tuesday, February 27, 2007 10:46 AM
It’s not so much that there isn’t a non-differential BPSK modulator,

the next guy down the line understand what to do with it. We probably
they wanted to, but it wouldn’t interfere with any existing code.

Eric

Thank you all for anwering. What I am really looking for is just BPSK,
so that should simplify things a bit. I intent to demodulate the BPSK
modulated message from GPS satellites, therefore the interest in
not-differential PSK modulation.

I was under the same opinion, which was why d_flip was in
correlate_access_code originally, but that screwed some stuff up down
the
chain.

I think whatever new solution we use should be general for
non-differential
MPSK and not just BPSK.

To make things a bit clearer: correlate_access_code is in reality a
preamble detector, or am I missing something?

The access code, which is not only used to identify the start of a MAC
frame, but it can also identify the network being used.

Tom

-----Original Message-----
From: Matt E. [mailto:[email protected]]

(which makes me
curious, does anyone use non-differential 8PSK?).

Yes, satellites often do.

Matt

Good enough for me. We should support it, then.

Tom

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