Re: [Discuss-gnuradio] Re channel state information on rx_ofdm.grc

[avinash kalyanaraman]

Thanks Marcus.

Following up on the rx_time format, I notice from the link that the timestamp format is the same as uhd::time_spec_t . Therefore the timestamp printed out on a tag is of the form:  full_seconds and a fractional_seconds.

For e.g., I see:  Offset: 667  Source: n/a     Key: rx_time   Value: {2317 0.394114}

2317 is the full second and 0.39 is the fraction of that 2317th second ?

Could you please tell me what these seconds calculated are with respect to - i.e. 2317 seconds since when ?

When does the fraction of second roll over? 

For example, I see:

  Offset: 1034  Source: n/a     Key: rx_time   Value: {2317 1.39162}

  Offset: 1035  Source: n/a     Key: rx_time   Value: {2318 0.393495}

What does it mean to have a fraction of 1.39 of a second ?

Thanks!

On Fri, Jun 17, 2016 at 1:12 PM, Marcus Müller <address@hidden> wrote:

Hi Avinash,

that's pretty much a basic OFDM question:

You take the DFT of the input signal. Hence, the bins of the DFT are f_nyquist/l_fft spaced.

Best regards,

Marcus

On 17.06.2016 17:57, avinash kalyanaraman wrote:

Thanks Marcus - that helps.

Could you please let me know what's the bandwidth of each sub-carrier? How can I calculate that and reconfigure the same?

On Sat, Jun 18, 2016 at 12:50 AM, Marcus Müller <address@hidden> wrote:

Hi Avinash,

On 06/16/2016 03:29 PM, avinash kalyanaraman wrote:

i) These 64 complex values (a + ib) represent the 64 sub-carriers and I can get the amplitude and phase of each sub-carrier as sqrt( a^2 + b^2) and arctan(b/a) respectively ?

Yes.

ii) I see that the CSI calculation is per tag. I understand from your link that the tag represents additional information/metadata attached to a specific item. However, in this case, I am unsure what a tag denotes? In other words, could you please tell me what a “tag” is being attached to here, on which a channel estimate is calculated?

If I remember correctly:
To the first sample of the "chunk" of samples that were used for estimation, that is, on the first sample of the OFDM symbol.

I do think that if you're into channel estimation, reading the source code is very much worth your time, and you'll notice variable and method names were chosen sensibly to make understanding what's happening easier. Go for the get_chan_taps() method in ofdm_chanest_vcvc_impl.cc; you'll find it in your GNU Radio source tree; be sure to use a recent version of GNU Radio (3.7.7 or later).

iii) Please let me know what is the unit of rx_time ? Seconds?

  Offset: 76  Source: n/a     Key: rx_time   Value: {7 0.303}

  Offset: 77  Source: n/a     Key: rx_time   Value: {7 0.399}

http://gnuradio.org/doc/doxygen/page_uhd.html

look for "timestamp" on that page.

Best regards,
Marcus

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Avinash

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Avinash