Ant switch pattern and IQ sampling

Question

Hi, 
 
 I'm working on direction finding with nrf52833dk (my own board latter on) and Nordic antenna matrix. I'm using NCS v1.6.0-rc2 and example Direction fing connectionless locator & bacon. I want to compute aoa and since i've only one antenna matrix i disabled aod of bacon example by adding "SET(OVERLAY_CONFIG "overlay-aoa.conf")" to CMakeLists.txt. 
 
 After few try, my locator found the beacon and print parameter as expected. I want to compute angle, so i print all IQ sample by modifying main.c with 
 static void cte_recv_cb(struct bt_le_per_adv_sync *sync,
 struct bt_df_per_adv_sync_iq_samples_report const *report)
{
	printk("CTE[%u]: samples count %d, cte type %s, slot durations: %u [us], "
 "packet status %s, RSSI %i
",
 bt_le_per_adv_sync_get_index(sync), report->sample_count,
 cte_type2str(report->cte_type), report->slot_durations,
 packet_status2str(report->packet_status), report->rssi);

 printk("IQ sample content : 
");
 for(int i=0; i<report->sample_count; i++){
 printk("sample[%d] = I:%d Q:%d
", i, report->sample[i].i, report->sample[i].q);
 }
} 
 and include bluetooth/hci.h. I got on my output 
 PER_ADV_SYNC[0]: [DEVICE]: 23:13:D2:D8:82:4B (random), tx_power 127, RSSI -63, CTE AOA, data length 0, data: 
CTE[0]: samples count 45, cte type AOA, slot durations: 2 [us], packet status CRC OK, RSSI -630
IQ sample content : 
sample[0] = I:5 Q:0
sample[1] = I:-6 Q:-1
sample[2] = I:5 Q:0
sample[3] = I:-6 Q:0
sample[4] = I:5 Q:-1
sample[5] = I:-6 Q:0
sample[6] = I:5 Q:-1
sample[7] = I:-6 Q:0
sample[8] = I:7 Q:8
sample[9] = I:15 Q:20
sample[10] = I:-17 Q:20
sample[11] = I:-4 Q:-5
sample[12] = I:-6 Q:14
sample[13] = I:-14 Q:-2
sample[14] = I:-7 Q:-6
sample[15] = I:-18 Q:-1
sample[16] = I:-1 Q:4
sample[17] = I:-15 Q:9
sample[18] = I:-7 Q:-32
sample[19] = I:4 Q:-11
sample[20] = I:10 Q:-23
sample[21] = I:26 Q:4
sample[22] = I:-3 Q:4
sample[23] = I:16 Q:-2
sample[24] = I:6 Q:12
sample[25] = I:-2 Q:7
sample[26] = I:9 Q:14
sample[27] = I:4 Q:-2
sample[28] = I:14 Q:8
sample[29] = I:-26 Q:20
sample[30] = I:-11 Q:1
sample[31] = I:-26 Q:3
sample[32] = I:-9 Q:-26
sample[33] = I:5 Q:0
sample[34] = I:-10 Q:-13
sample[35] = I:8 Q:-12
sample[36] = I:8 Q:-2
sample[37] = I:9 Q:-15
sample[38] = I:-3 Q:-4
sample[39] = I:-1 Q:-17
sample[40] = I:30 Q:10
sample[41] = I:6 Q:8
sample[42] = I:14 Q:21
sample[43] = I:-18 Q:21
sample[44] = I:-3 Q:-5 
 
 I have several question about the output : 
 
 Why am i having 45 samples? I know there is a guard period and then as many samples as ant_pattern_length but it is 12 antenna long. 
 Since there are more value than i expect, should i make an algorithm in order to discard the wrong one ? 
 Is the values correct ? i know the swing of IQ value are bigger than those. 
 Last but not least, where can i find in example project all the parameter such as TSAMPLESPACINGREF / TSAMPLESPACING / TSWITCHSPACING ? 
 
 I read very carefully your whitepaper, but i need a little help to put all this fine. 
 
 Regards, 
 Nathan

Saleh · Answer

Hi 
 
 There is a comprehensive conversation available in this thread, just I try to write a fresh answer from my point of view, I hope it can help: 
 
 Nathan_INOSI said: Why am i having 45 samples? I know there is a guard period and then as many samples as ant_pattern_length but it is 12 antenna long. 
 Number of samples doesn't depend on the antenna pattern size, it is dependent on two parameters: 
 a. CTE duration 
 b. TSAMPLESPACING (Corresponds to the 1us or 2us sample slots) 
 The first parameter is chosen by the transmitter and can changed here, the default value is the maximum: 
 https://github.com/nrfconnect/sdk-nrf/blob/master/samples/bluetooth/direction_finding_connectionless_tx/src/main.c#L20 
 
 The second parameter is chosen by the receiver and can be modified here, the default value is 2us sample slot: 
 https://github.com/nrfconnect/sdk-nrf/blob/master/samples/bluetooth/direction_finding_connectionless_rx/src/main.c#L232 
 
 NOTE: At the time of writing this answer, there is a bug that prevents the receiver to work on 1us slot, an issue and fix has provided for this problem: 
 Issue: https://github.com/zephyrproject-rtos/zephyr/issues/37305 
 Fix: https://github.com/zephyrproject-rtos/zephyr/pull/37308 
 
 Nathan_INOSI said: Since there are more value than i expect, should i make an algorithm in order to discard the wrong one ? 
 The numbers of values is correct, you can calculate it as: 
 NumberOfValues = 8 + (CTELength - 12) / (2 * SampleSlot) 
 
 Hint: 12 is total length of "Gaurd Period" and "Refrence Period" and despite of the TSAMPLESPACING, it generates 8 sample per "Refrence Period". 
 e.g. for CTE with maximum length (160 us), and default value for sample slot (2 us), it will be: 
 8 + (160 - 12) / ( 2 * 2) 
 = 8 + 148 / 4 = 8 + 37 = 45 
 
 Nathan_INOSI said: s the values correct ? i know the swing of IQ value are bigger than those. 
 At least I am receiving the same values, but I am not sure about it and should be evaluated with a reference, later. 
 
 Nathan_INOSI said: Last but not least, where can i find in example project all the parameter such as TSAMPLESPACINGREF / TSAMPLESPACING / TSWITCHSPACING ? 
 These parameters are set by lower layers and is not accessible in application layer. Also it is not recommended to modify these values in lower layers, because they are set by experienced developers for each chip-set. 
 but instead you can change the sample slot duration at this line as I mentioned before, it consequently will change those parameters in the lower layers: 
 https://github.com/nrfconnect/sdk-nrf/blob/master/samples/bluetooth/direction_finding_connectionless_rx/src/main.c#L232 
 
 Regards, 
 Saleh