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#1 2017-Mar-08 10:19:40

_wurzel
Member
Registered: 2017-Mar-08
Posts: 3

802.11 Reference Design - increase the sample rate

Hi,

I am using the WARPv3 boards and the 802.11 Reference Design v1.6.1 for localization experiments. Therefore I am interested in as accurate reception timestamps as possible. This accuracy is mainly limited by the sample rate. So I want to increase the sample rate.
As described in
https://warpproject.org/forums/viewtopic.php?id=3117
the integrated ADC/DAC chip is configured in wlan_mac_low.c : set_phy_samp_rate(). This function is called in line 270 with the argument PHY_20. This coincides with my experiments, where the value of rx_frame_info->timestamp_frac takes 20 different values. I would like to approach to PHY_40. However, if I change the line to set_ph_samp_rate(PHY_40); the WARPs (all implemented as STA) do not connect to the AP anymore. The WARPs should connect to a 2.4 GHz Network prior timestamp collection. If I leave the line at PHY_20, everything works fine.

Any recommendations?

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#2 2017-Mar-08 10:37:13

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 5159

Re: 802.11 Reference Design - increase the sample rate

I can't tell from your post whether your WARP v3 STA nodes are connecting to a WARP v3 AP node or a Wi-Fi AP. In the current ref design (v1.6.2) the 40MSps mode will only work with an all-WARP network. The 40MSps mode does not use a standard waveform - it realizes 40MHz bandwidth by double-clocking the normal 20MHz 802.11 waveforms. We're planning to build proper HT40 (the 40MHz waveform from 802.11n) in a future release.

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#3 2017-Mar-09 11:58:02

_wurzel
Member
Registered: 2017-Mar-08
Posts: 3

Re: 802.11 Reference Design - increase the sample rate

Thanks for your reply!
In fact I was using a Wi-Fi AP. This explaines my problem. I read that the mode uses a non-standard waveform, but I assumed, that it is anyways compatible with of the shelf hardware.

I will test this tomorrow and write here, whether it worked.

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#4 2017-Mar-16 08:23:31

_wurzel
Member
Registered: 2017-Mar-08
Posts: 3

Re: 802.11 Reference Design - increase the sample rate

It did work. However this introduces several problems to my design. I need the WARPs to be able to talk to a Laptop associated with the same Network. If I run a WARP Network with 40 MHz, the WARPs are not able to communicate with the laptop. I do not want to connect them with ethernet.

Is it possible to use the 20 MHz waveforms, such that the WARPs are able to communicate with the laptop, and do an oversampling of 40 MHz (or possibly even 80 MHz?), such that the timestamps are more accurate?

I experimented a little bit with the wlan_mac_low.c : set_phy_samp_rate(), but failed so far.

Any suggestions?

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#5 2017-Mar-16 10:07:14

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 5159

Re: 802.11 Reference Design - increase the sample rate

Is it possible to use the 20 MHz waveforms, such that the WARPs are able to communicate with the laptop, and do an oversampling of 40 MHz (or possibly even 80 MHz?), such that the timestamps are more accurate?

This would require modifying the Rx PHY core to extend the LTF correlator to 128 points (2x 64 20MHz samples) and to add a 2x decimation filter somewhere after the correlator (probably before the FFT, so it could still use 64 points). However I'm not sure this would improve the timestamp resolution as you'd still be processing a 20MHz-bandwidth waveform. With a 20MHz waveform and non-zero carrier and sampling frequency offsets, I think it would be difficult to reliably identify the "right" 40MHz sample as the correlation peak.

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