CSI in gr-ieee80211

Dear Bloessl and gnuradio list,

I am currently working on gr-ieee80211. Now I want to use the long
preamble to estimate the CSI. However, after I apply fft to the long
preamble, I found the power for each channel is not the same. I took an
average over lots of packets. The power near the middle sub-carriers are
always very lower than on the edges. I would like to ask if it is
because the Power spectral density in the transmitter or some other
reasons.

Any feedback would be appreciated.

Best regards

Alex

On 26 Mar 2014, at 15:05, alex [email protected] wrote:

I am currently working on gr-ieee80211. Now I want to use the long preamble to
estimate the CSI. However, after I apply fft to the long preamble, I found the
power for each channel is not the same. I took an average over lots of packets.
The power near the middle sub-carriers are always very lower than on the edges. I
would like to ask if it is because the Power spectral density in the transmitter
or some other reasons.

If your sample rate is 20MHz you will see an uncompensated filter, but
in that case the power of the subcarriers in the center should be
higher.

On 28.03.14 19:40, Surligas Manos wrote:

Which device did you used for receiving samples? I have noticed a same
phenomenon with my device, caused by a coarse implementation of a high
pass filter.

The filter in the Maxim2829 in the XCVR2450 that we used can be
configured to 0, 100Hz and 30kHz and the default is 100Hz I believe? If
you didn’t change anything there this cannot explain your problem. Any
filter than goes above 150kHz would be a problem for the centerish
carriers in 802.11 OFDM (~300kHz carrier spacing).

what happens if you TX a sinusoid and stepwise tune the RX to slightly
off frequencies (take note of the actual RX LO frequency when doing
the sweep - the difference to that one and the TX sinusoid matters).
Does the RX power also start to decrease as soon as you set the TX-RX
offset smaller than 150kHz?

-paul

PS: as Sebastian said, the outer subcarriers being lower can be
explained by odd decimation factors like 5 - which causes the
digital-downconversion filter chain to change to a single CIC stage I
believe.

Which device did you used for receiving samples? I have noticed a same
phenomenon with my device, caused by a coarse implementation of a high
pass filter.
On 03/26/2014 04:08 PM, Bastian B. wrote:

On 26 Mar 2014, at 15:05, alex [email protected] wrote:

I am currently working on gr-ieee80211. Now I want to use the long preamble to
estimate the CSI. However, after I apply fft to the long preamble, I found the
power for each channel is not the same. I took an average over lots of packets.
The power near the middle sub-carriers are always very lower than on the edges. I
would like to ask if it is because the Power spectral density in the transmitter
or some other reasons.

If your sample rate is 20MHz you will see an uncompensated filter, but in that
case the power of the subcarriers in the center should be higher.


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/* Code is the Law! */

I use a card from Per Vices. I do not have the option to change the
cuttoff frequency of the
filter without flashing the firmware. As a workaround, I tuned my device
to a different frequency (+10 MHz) and using an NCO I managed to bypass
the power loss at the center sub-carriers.

On 03/29/2014 03:11 PM, Paul Fuxjaeger wrote:

what happens if you TX a sinusoid and stepwise tune the RX to slightly
believe.

/* Code is the Law! */

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