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Instead of normal IQ sample capture, 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 to understand how we use the side channel to capture I/Q samples by different trigger conditions.

(By default, openwifi Rx baseband is muted during self Tx, to unmute Rx baseband and capture self Tx signal you need to run "./sdrctl dev sdr0 set reg xpu 1 1" after the test running)

This feature also support capturing TX I/Q (loopback) to test the baseband transmitter.

Quick start for collision capture

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.

The collision capture steps:

  • Change rx1 AGC to manual mode instead of fast_attack in rf_init.sh by:
echo manual > in_voltage1_gain_control_mode
  • Change rx1 gain to a low level, such as 20, by:
echo 20 > in_voltage1_hardwaregain
  • Use the new rf_init.sh script to boot up the SDR board, and setup the working scenario.
  • Setup the side channel:
insmod side_ch.ko iq_len_init=8187
(iq_len_init should be <4096, like 4095, if smaller FPGA, like z7020, is used)
./side_ch_ctl wh11d2000
(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)
  • Put the other antenna (rx1) close to the peer WiFi node, set trigger condition to 23 (baseband tx done)
./side_ch_ctl wh8d23
  • Enable the dual antenna capture mode
./side_ch_ctl wh3h11
  • Run some traffic between the SDR board and the peer WiFi node, and start the user space I/Q capture program
./side_ch_ctl g

If the printed "side info count" is increasing, it means the trigger condition is met from time to time.

  • On remote computer, run
python3 iq_capture_2ant.py
(if smaller FPGA, like z7020, is used, add a argument that equals to iq_len_init, like 4095)

Above script will plot the real-time rx0 and rx1 I/Q captured each time the trigger condition is met. . 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.

Quick start for TX IQ capture in trigger mode

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.

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)
./side_ch_ctl wh5h2
(I/Q source selection: 2--openofdm_tx core; 4--tx_intf)
./side_ch_ctl wh3h11
./side_ch_ctl g1

On computer:

openwifi/user_space/side_ch_ctl_src/python3 iq_capture_2ant.py 511

Quick start for TX IQ capture in free running mode

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 wh8d0
(trigger condition 0 is needed for free running mode)
./side_ch_ctl wh5h3
(I/Q source selection: 3--openofdm_tx core; 5--tx_intf)
./side_ch_ctl wh3h11
./side_ch_ctl g1

On computer:

openwifi/user_space/side_ch_ctl_src/python3 iq_capture_2ant.py 511