1Instead of [**normal IQ sample capture**](iq.md), this app note introduce how to enable the I/Q capture for dual antenna. In this dual antenna mode, the RSSI and AGC status won't be captured as in the normal mode. Instead, they are replaced by the I/Q samples from the other antenna. But you are suggested to read the [**normal IQ sample capture**](iq.md) to understand how do we use the side channel to capture I/Q sample by different trigger conditions. 2 3In this app note, we show how to use the dual antenna I/Q capture to capture the collision. 4 5## Quick start 6 The currently selected antenna (rx0 by default if you do not select explicitly by set_ant.sh) is always used for communication and I/Q capture. Meanwhile, the other antenna (rx1) will be also avaliable 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. 7 8 The collision capture steps: 9 - Change rx1 AGC to manual mode instead of fast_attack in rf_init.sh by: 10 ``` 11 echo manual > in_voltage1_gain_control_mode 12 ``` 13 - Change rx1 gain to a low level, such as 20, by: 14 ``` 15 echo 20 > in_voltage1_hardwaregain 16 ``` 17 - Use the new rf_init.sh script to boot up the SDR board, and setup the working scenario. 18 - Setup the side channel: 19 ``` 20 insmod side_ch.ko iq_len_init=8187 21 (iq_len_init should be <4096, like 4095, if smaller FPGA, like z7020, is used) 22 ./side_ch_ctl wh11d2000 23 (Set a smaller pre_trigger_len 2000, because we want to see what happens after the trigger instead of long period stored before the trigger) 24 ``` 25 - Put the other antenna (rx1) close to the peer WiFi node, set trigger condition to 23 (baseband tx done) 26 ``` 27 ./side_ch_ctl wh8d23 28 ``` 29 - Enable the **dual antenna capture** mode 30 ``` 31 ./side_ch_ctl wh3h11 32 ``` 33 - Run some traffic between the SDR board and the peer WiFi node, and start the user space I/Q capture program 34 ``` 35 ./side_ch_ctl g 36 ``` 37 If the printed "**side info count**" is increasing, it means the trigger condition is met from time to time. 38 - On remote computer, run 39 ``` 40 python3 iq_capture_2ant.py 41 (if smaller FPGA, like z7020, is used, add a argument that equals to iq_len_init, like 4095) 42 ``` 43 Above script will plot the real-time rx0 and rx1 I/Q captured each time trigger condition met. Meanwhile the script also prints the maximum amplitutde 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 indicate 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). 44 - 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). 45 ``` 46 (Quit side_ch_ctl by Ctrl+C) 47 ./side_ch_ctl wh8d29 48 ./side_ch_ctl wh9d2500 49 ./side_ch_ctl g 50 ``` 51 - 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. 52 - You can also see it via iq_capture_2ant.py or do offline analysis by test_iq_2ant_file_display.m 53 - 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. 54