openwifi/doc/app_notes/packet-iq-self-loopback-test.md

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<!--
Author: Xianjun Jiao
SPDX-FileCopyrightText: 2022 UGent
SPDX-License-Identifier: AGPL-3.0-or-later
-->
One super power of the openwifi platform is "**Full Duplex**" which means that openwifi baseband can receive its own TX signal.
This makes the IQ sample and WiFi packet self loopback test possible. Reading the normal IQ sample capture [app note](iq.md) will help if you have issue or
want to understand openwifi side channel (for IQ and CSI) deeper.
![](./openwifi-loopback-principle.jpg)
[[IQ self loopback quick start](#IQ-self-loopback-quick-start)]
[[Check the packet loopback on board](#Check-the-packet-loopback-on-board)]
[[Self loopback config](#Self-loopback-config)]
## IQ self loopback quick start
(Please replace the IQ length **8187** by **4095** if you use low end FPGA board: zedboard/adrv9464z7020/antsdr/zc702)
- Power on the SDR board.
- Put the Tx and Rx antenna as close as possible.
- Connect a computer to the SDR board via Ethernet cable. The computer should have static IP 192.168.10.1. Open a terminal on the computer, and then in the terminal:
```
ssh root@192.168.10.122
(password: openwifi)
cd openwifi
./wgd.sh
(Bring up the openwifi NIC sdr0)
./monitor_ch.sh sdr0 44
(Setup monitor mode in WiFi channel 44. You should find a channel as clean as possible in your location)
insmod side_ch.ko iq_len_init=8187
./side_ch_ctl wh11d0
(Set 0 to register 11. It means the pre trigger length is 0, so we only capture IQ after trigger condition is met)
./side_ch_ctl wh8d16
(Set 16 to register 8 -- set trigger condition to phy_tx_started signal from openofdm tx core)
./sdrctl dev sdr0 set reg xpu 1 1
(Unmute the baseband self-receiving to receive openwifi own TX signal/packet -- important for self loopback!)
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./side_ch_ctl wh5h0
(Set the loopback mode to over-the-air)
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./side_ch_ctl g0
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(Relay the FPGA IQ capture to the host computer that will show the captured IQ later on)
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```
You should see on outputs like:
```
loop 22848 side info count 0
loop 22912 side info count 0
...
```
Now the count is always 0, because we haven't instructed openwifi to send packet for loopback test.
- Leave above ssh session untouched. Open a new ssh session to the board from your computer. Then run on board:
```
cd openwifi/inject_80211/
make
(Build our example packet injection program)
./inject_80211 -m n -r 5 -n 1 sdr0
(Inject one packet to openwifi sdr0 NIC)
```
Normally in the previous ssh session, the count becomes 1. It means one packet (of IQ sample) is sent and captured via loopback over the air.
- On your computer (NOT ssh onboard!), run:
```
cd openwifi/user_space/side_ch_ctl_src
python3 iq_capture.py 8187
```
You might need to install beforehand: "sudo apt install python3-numpy", and "sudo apt install python3-matplotlib".
- Leave the above host session untouched. Let's go to the second ssh session (packet injection), and do single packet Tx again:
```
./inject_80211 -m n -r 5 -n 1 sdr0
```
Normally in the 1st ssh session, the count becomes 2. You should also see IQ sample capture figures like this:
![](./openwifi-iq-loopback.jpg)
- Stop the python3 script, which plots above, in the host session. A file **iq.txt** is generated. You can use the Matlab script test_iq_file_display.m
to do further offline analysis, or feed the IQ sample to the openwifi receiver simulation, etc.
## Check the packet loopback on board
- While signal/packet is looped back, you can capture it on board via normal sniffer program for further check/analysis on the packet (bit/byte level instead of IQ level), such as tcpdump or tshark.
A new ssh session to the board should be opened to do this before running the packet injection:
```
tcpdump -i sdr0
```
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Run the packet injection "./inject_80211 -m n -r 5 -n 1 sdr0" in another session, you should see the packet information printed by tcpdump from self over-the-air loopback.
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- You can also see the openwifi printk message of Rx packet (self Tx looped back) while the packet comes to the openwifi Rx interrupt.
A new ssh session to the board should be opened to do this before running the packet injection:
```
cd openwifi
./sdrctl dev sdr0 set reg drv_rx 7 7
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./sdrctl dev sdr0 set reg drv_tx 7 7
(Turn on the openwifi Tx/Rx printk logging)
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```
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Stop the "./side_ch_ctl g0" in the very first ssh session. Run the packet injection, then check the printk message:
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```
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./inject_80211/inject_80211 -m n -r 5 -n 1 sdr0
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dmesg
```
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You should see the printk message of packet Tx and Rx from the openwifi driver (sdr.c).
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## Self loopback config
- By default, the loopback is via the air (from Tx antenna to Rx antenna). FPGA inernal loopback option is offered to have IQ sample and packet without
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any interference. To have FPGA internal loopback, replace the "./side_ch_ctl wh5h0" during setup (the very 1st ssh session) by:
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```
./side_ch_ctl wh5h4
```
- Lots of packet injection parameters can be set: number of packet, type (data/control/management), MCS/rate, size, interval, etc. Please run the packet injection
program without any arguments to see the help.
- Besides the packet Tx via injection over monitor mode for loopback test, normal WiFi mode (AP/Client/ad-hoc) can also run together with self loopback.
For instance, run **fosdem.sh** instead of **wgd.sh** to setup an openwifi AP that will transmit beacons. The wgd.sh can also be replaced with other scenario
setup scripts. Please check [Application notes](README.md)
- To understand deeper of all above commands/settings, please refer to [Capture IQ sample, AGC gain, RSSI with many types of trigger condition](iq.md) and
[Capture dual antenna TX/RX IQ for multi-purpose (capture collision)](iq_2ant.md)