Re: Collision detection and recovery

Hi Nitin,

did you make your packet detector signal a detection even in the presence of collision? As
said, it's very unlikely you couldn't detect a strong cross-correlation.

Best regards,
Marcus

On 11.07.22 05:44, Nitin Shivaraman wrote:
> Thanks a lot for the response Marcus! As the correlation is done only at the decoding
> level in Gr-802-15-4, the received data is mapped to one of 16 symbols to decode as 0x0 to
> 0xF.
> As seen in the attached file, I'm trying to recreate a collision in the simulation
> environment by feeding two signals into an add block.
>
> However, when the collision happens, I only receive 0x00 as the symbol for the decoder
> which does not correspond to any symbol. My understanding is that the collided packet can
> be obtained if a known original packet is subtracted and decoded from the collided signal.
>
> Since the collided signal received is zero, I'm unable to decode any symbol. Please advise
> if I'm missing anything.
>
> Thanks in advance.
>
> Best Regards,
> Nitin.
>
>
> On Mon, Jun 13, 2022 at 9:19 PM Marcus Müller <mueller@kit.edu <mailto:mueller@kit.edu>>
> wrote:
>
> Hi Nitin,
>
> not involved in gr-ieee802-15-4, I can maybe still comment on the collision detection
> part:
>
> On 12.06.22 09:13, Nitin Shivaraman wrote:
> > Since the preamble detection happens after the clock recovery mechanism, I want to
> know
> > how the collision can be detected using correlation.
>
> Yep!
>
> > To detect collision, I intend to
> > receive the collided packet and subtract part of the correctly received packet to
> recover
> > both packets correctly.
>
> You don't even need to subtract! You're correlating, and correlation is a linear
> operation. When correlating for the preamble of the second packet, which lies within the
> first packet, you simply get as correlation the sum of the autocorrelation at
> timeshift 0,
> i.e. the energy, of the preamble (what you actually want to detect) and its
> autocorrelation with the first packet at the timeshift between the two packets
>
> By design, preambles typically have high autocorrelation at \tau=0 and low for other
> timeshifts \tau. So, this should actually be pretty reliable, even without subtracting
> the
> other packet first! You *will* get some interference, yes, and strictly speaking, this
> should put your detection threshold a bit higher.
>
>
> Best regards,
> Marcus
>
>