The last thing I wonder is, can a receiver just pick up a DSSS signal and start
applying the despreading code? I watched a YouTube video about this and the
example involved multiplying the spreading code by the voltages of the
composite waveform and averaging them. My system takes 16 chips to express 1
bit. Let’s say my demodulator starts on the 10th chip and goes on for 16 chips,
getting 6 chips from this cycle and 10 from the next. If it keeps on like this,
it will never fall into sync, and without being in sync it can’t get any real
bits to help itself align.
Sent from Windows Mail
From: Richard Bell
Sent: Friday, February 5, 2016 5:59 PM
To: Henry Barton, [email protected]
It will depend on how the rest of the radio is built up. I'm not familiar with
VP9, but can I assume it's a spec on bits in a higher layer then Layer 1?
Another words, you are assuming you have bits to correlate with, as opposed to
wave shapes?
You're getting into the difficulties of radio design now. You need to fully
understand the needs of your system to make decisions like this. You don't have
bits until you've synchronized and demodulated your signal. If you require some
sort of FEC, it will need block alignment before you can decode it, so the
correlator will need to be in the waveshape domain. You can still use the known
VP9 headers to correlate to in this cas
, but you wouldn't correlate to the bit version, you would correlate to the
modulated version of those headers.
P.S. Please reply to the mailing list, so others can see and reply if need be.
Rich
On Fri, Feb 5, 2016 at 2:53 PM, Henry Barton <[email protected]> wrote:
I'm hoping to transmit a VP9 transport stream, so perhaps the predictable
headers will be enough?
Sent from Windows Mail
From: Richard Bell
Sent: Friday, February 5, 2016 5:51 PM
To: Henry Barton, [email protected]
So long as you know what you're looking for in any given scenario, you can use
that to correlate to. It can be data or a preamble. If your receiver knows the
data will always be a certain way ahead of time though, it's hard to call that
data. Semantics at that point.
Rich
On Fri, Feb 5, 2016 at 2:44 PM, Henry Barton <[email protected]> wrote:
That sounds great, Richard. But I wonder, what if the useful payload contains
that sequence by chance?
Sent from Windows Mail
From: Richard Bell
Sent: Friday, February 5, 2016 5:27 PM
To: Henry Barton
Cc: [email protected]
Typically a correlator is used to look for a known sequence of bits, so the
radio can align the rest of the processing from the end of this known sequence.
This is referred to as frame synchronization. You could use the correlation
estimation block to implement something like this. It would place a tag on the
stream when it finds your known sequence and you would then know how everything
is aligned from then on.
Rich
On Fri, Feb 5, 2016 at 1:04 PM, Henry Barton <[email protected]> wrote:
Hi all. I've successfully written a DSSS modulator and demodulator in Windows
with a chip rate of 16x. It writes samples to a file that the demodulator can
read and despread. Before I try any practical implementations, I need to know
how a DSSS stream would be synchronized. Assuming the transmitter and receiver
were perfectly clocked in unison, what stops the receiver from tuning in in the
middle of a byte, thus getting a nibble from the current byte and a nibble from
the next?
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