Instead of [**normal IQ sample capture**](iq.md), this app note introduces how to enable the I/Q capture for dual antennas. Besides the I/Q from the main antenna (that is selected by baseband), the I/Q samples from the other antenna (monitoring antenna) is captured as well (coherently synchronized) in this dual antenna mode. You are suggested to read the [**normal IQ sample capture**](iq.md) to understand how we use the side channel to capture I/Q samples by different trigger conditions.
The main antenna rx0 (by default selected by baseband if you do not select explicitly by set_ant.sh) is always used for communication and I/Q capture. Meanwhile, the other antenna (rx1 -- monitoring antenna) will be also available for capturing rx I/Q if you are using AD9361 based RF board, such as fmcomms2/3 and adrv9361z7035, by turning on the **dual antenna capture** mode. In this case, you can place the other antenna (rx1) close to the communication peer (for example, the other WiFi node) to capture the potential collision by monitoring rx1 I/Q. The nature of collision is that both sides of a communication link are trying to do transmission at the same time.
In the above example, the upper half shows the signal received from the main antenna (self tx is not seen because of self muting in FPGA), the lower half shows not only the rx signal from the monitoring antenna but also the tx signal from the main antenna due to coupling.
Meanwhile the script also prints the maximum amplitude of the rx0 and rx1 I/Q samples. Check the 3rd column that is displayed by the script: Those small value printing indicate noise (most probably, because the rx1 gain is very low). The big value printing indicates a packet from rx1 (although rx1 has very low gain, rx1 is very close to the peer WiFi node). Go through the noise and the packet max I/Q amplitude numbers from rx1 printing (the 3rd column), and decide a threshold value that is significantly higher than the noise but less than those big values (packets).
- Set trigger condition to 29, which means that rx1 I/Q is found larger than a threshold while SDR is transmitting -- this means a collision condition is captured because rx1 I/Q implies the transmitting from the peer WiFi node. The threshold value is decided in the previous step (2500 is assumed here).
```
(Quit side_ch_ctl by Ctrl+C)
./side_ch_ctl wh8d29
./side_ch_ctl wh9d2500
./side_ch_ctl g
```
- Now the trigger condition can capture the case where both sides happen to transmit in an overlapped duration. If the printed "**side info count**" is increasing, it means the collision happens from time to time.
- You can also see it via iq_capture_2ant.py or do offline analysis by test_iq_2ant_file_display.m
- Check the **iq1** signal in FPGA ILA/probe (triggered by signal "iq_trigger") for further debug if you want to know what exactly happened when collision is captured.
To capture the TX I/Q (baseband loopback), a scenario where openwifi will do TX needs to be set up. Such as beacon TX when openwifi act as AP, or [packet injection](inject_80211.md).
The example command sequence on board and explanations are as follows.
```
cd openwifi
./fosdem.sh
insmod side_ch.ko iq_len_init=511
(511 I/Q samples cover the short, long preamble and some OFDM symbols. Change it according to your case)
./side_ch_ctl wh11d1
(1 sample before the trigger met will be captured. So most of the I/Q will be captured after trigger met)
./side_ch_ctl wh8d16
(trigger condition 16: phy_tx_started signal from openofdm tx core)
