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Re: [Discuss-gnuradio] Question regarding correlating output and IQ samp


From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Question regarding correlating output and IQ samples in a reciever
Date: Thu, 28 Jan 2016 10:17:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

Hi Abhinav,

On 01/28/2016 04:32 AM, abhinav narain wrote:
Hi Marcus, 

timing offset estimate) that your symbols have duration of 2.236472 samples and start with a sample offset of 114.060072; I don't see how knowing that would map the end of a preamble to any "exact" sample.
 If I understand this, it is not straightforward to find the end of the preamble, to start the clock for the data transmission part. I can approximately chop the preamble off then.
Well, the preamble is, by definition, not part of your data, so you'd wanna chop it off as soon as you've used it to synchronize/get channel state info, anyway.



I am simply doing OOK.
 
Generally, $\frac{S+N}{N}$ is only really a useful measure if you either have
  1. a constant power modulation (e.g. PSK), or
  2. whiten your over-the-air bits sufficiently (using coding), so that for (stochastically speaking almost any) data sequence, the signal power is the same.

Usually, even when you have 1., you do 2..

so the above formula is what I am using currently over the whole message run, and for similar amount of time when no message was transmitted to get S+N/N .
I have looked through Gnuradio docs and doesn't seem to appeal that there is any of the four would work.
Sorry, I don't understand what you're referring to; could you explain?
Their output varies significantly amongst one another and hard to say which one is correct (as mentioned in the docs, some implementations are to be used cautiously).

If you measure SNR based on a subset of bits that actually have energy, you're biasing your measurement, and it won't have any meaning for your real transmission, unless you've done the math and know that these bits represent a certain percentage of the power of an actual transmission. But as soon as you do the math, and figure out that some bits have higher Energy per bit $E_b$, you'd try to minimize the average $E_b$, so that you can just increase the signal amplitude without breaking your specifications, and get a higher average SNR; this implies spreading the energy over different bits more evenly, and that just leads to the whitening mentioned above.

Okay, then it makes sense to average the energy over the whole trace instead of searching for the isolated pulses and calculating SNR only due those samples.

Best regards,
Marcus

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