Bluetooth: Direction finding connectionless locator

The direction finding connectionless locator sample application demonstrates Bluetooth® LE direction finding reception.


The sample supports the following development kits:

Hardware platforms


Board name

Build target

nRF5340 DK




nRF52833 DK




nRF52833 DK (emulating nRF52820)




The sample also requires an antenna matrix when operating in angle-of-arrival mode. It can be a Nordic Semiconductor design 12 patch antenna matrix, or any other antenna matrix.


The direction finding connectionless locator sample application uses Constant Tone Extension (CTE), that is received and sampled with periodic advertising PDUs.

The sample supports two direction finding modes:

  • Angle of Arrival (AoA)

  • Angle of Departure (AoD)

By default, both modes are available in the sample.


See Configuring your application for information about how to permanently or temporarily change the configuration.

This sample configuration is split into the following two files:

  • Generic configuration available in the prj.conf file

  • Board-specific configuration available in the boards/<BOARD>.conf file

nRF5340 configuration files

The following additional configuration files are available for the nRF5340 DK:

  • The Bluetooth LE controller is part of a child image meant to run on the network core. The configuration for the child image is stored in the child_image/ subdirectory.

  • The DTS overlay file boards/nrf5340dk_nrf5340_cpuapp.overlay is available for the application core. This file forwards the control over GPIOs to network core, which provides control over GPIOs to the radio peripheral in order to execute antenna switching.

Angle of departure mode

To build this sample with AoD mode only, set OVERLAY_CONFIG to the overlay-aod.conf file.

See Providing CMake options for instructions on how to add this option. For more information about using configuration overlay files, see Important Build System Variables in the Zephyr documentation.

To build this sample for nRF5340 DK, with AoD mode only, add content of overlay-aod.conf file to child_image/hci_rpmsg.conf file.

Antenna matrix configuration for angle of arrival mode

To use this sample when AoA mode is enabled, additional configuration of GPIOs is required to control the antenna array. An example of such configuration is provided in a devicetree overlay file nrf52833dk_nrf52833.overlay.

The overlay file provides the information of which GPIOs should be used by the Radio peripheral to switch between antenna patches during the CTE reception in the AoA mode. At least two GPIOs must be provided to enable antenna switching.

The GPIOs are used by the Radio peripheral in the order provided by the dfegpio#-gpios properties. The order is important, because it has an impact on the mapping of the antenna switching patterns to GPIOs (see Antenna patterns).

To successfully use the direction finding locator when the AoA mode is enabled, provide the following data related to antenna matrix design:

  • The GPIO pins to dfegpio#-gpios properties in the nrf52833dk_nrf52833.overlay file.

  • The default antenna that will be used to receive a PDU dfe-pdu-antenna property in the nrf52833dk_nrf52833.overlay file.

  • Update the antenna switching patterns of the ant_patterns array in the main.c file.

Antenna patterns

The antenna switching pattern is a binary number where each bit is applied to a particular antenna GPIO pin. For example, the pattern 0x3 means that antenna GPIOs at index 0,1 will be set, while the following are left unset.

This also means that, for example, when using four GPIOs, the pattern count cannot be greater than 16 and the maximum allowed value is 15.

If the number of switch-sample periods is greater than the number of stored switching patterns, the radio loops back to the first pattern.

The length of the antenna switching pattern is limited by the CONFIG_BT_CTLR_DF_MAX_ANT_SW_PATTERN_LEN Kconfig option. If the required length of the antenna switching pattern is greater than the default value of that option, set it to the required value in the board configuration file. For example, for the nRF52833 DK, set the option value to the required antenna switching pattern length in the nrf52833dk_nrf52833.conf file.

The following table presents the patterns that you can use to switch antennas on the Nordic-designed antenna matrix:




0 (0b0000)


1 (0b0001)


2 (0b0010)


3 (0b0011)


4 (0b0100)


5 (0b0101)


6 (0b0110)


7 (0b0111)


8 (0b1000)


9 (0b1001)


10 (0b1010)


11 (0b1011)


12 (0b1100)


13 (0b1101)


14 (0b1110)


15 (0b1111)

Constant Tone Extension transmit and receive parameters

Constant Tone Extension works in either AoA or AoD mode. The transmitter is configured with operation mode that is used for CTE transmission. The receiver operating mode depends on the configuration. By default, both AoA and AoD modes are enabled, and the CTE type is received in the CTEInfo field in PDU’s extended advertising header. Depending on the operation mode selected for the CTE reception, the receiver’s radio peripheral will either execute both antenna switching and CTE sampling (AoA), or CTE sampling only (AoD).

The allowed antenna switch slot lengths are 1 µs or 2 µs. The transmitter uses the antenna switch slot length to configure the radio peripheral in AoD mode only. The receiver uses the local setting of the antenna switch slot length in AoA mode only. When the CTE type in the received PDU is AoD, the receiver’s radio peripheral uses the antenna switch slot length appropriate for the AoD type (1 or 2 µs). The antenna switch slot length has an impact on the number of IQ samples provided in the IQ samples report.

Constant Tone Extension length is limited to a range between 16 µs and 160 µs. The value is provided in units of 8 µs. The transmitter is responsible for setting the CTE length. The value is sent to the receiver as part of the CTEInfo field in PDU’s extended advertising header. The receiver’s radio peripheral uses the CTE length, provided in periodic advertising PDU, to execute antenna switching and CTE sampling in AoA mode, or CTE sampling only in AoD mode. The CTE length has an impact on the number of IQ samples provided in the IQ samples report.

Constant Tone Extension consists of the following three periods:

  • Guard period of 4 µs. There is a gap before the actual CTE reception and the adjacent PDU transmission to avoid interference.

  • Reference period where a single antenna is used for sampling. The spacing between the samples is 1 µs and the period duration is 8 µs.

  • Switch-sample period that is split into switch and sample slots. The length of each slot can be either 1 or 2 µs.

The total number of IQ samples provided by the IQ samples report varies. It depends on the CTE length and the antenna switch slot length. There will always be eight samples from the reference period, 1 to 37 samples with 2 µs antenna switching slots, 2 to 74 samples with 1 µs antenna switching slots, totalling in 9 to 82 samples.

For example, the CTE length is 120 µs and the antenna switching slot is 1 µs. This will result in the following number of IQ samples:

  • Eight samples from the reference period.

  • Switch-sample period duration is 120 µs - 12 µs = 108 µs. For the 1 µs antenna switching slot, the sample is taken every 2 µs (1 µs antenna switch slot, 1 µs sample slot). In this case, the number of samples from the switch-sample period is 108 µs / 2 µs = 54.

The total number of samples is 62.

Building and running

This sample can be found under samples/bluetooth/direction_finding_connectionless_rx in the nRF Connect SDK folder structure.

To build the sample with Visual Studio Code, follow the steps listed on the How to build an application page in the nRF Connect for VS Code extension documentation. See Building and programming an application for other building and programming scenarios and Testing and debugging an application for general information about testing and debugging in the nRF Connect SDK.


After programming the sample to your development kit, complete the following steps to test it:

  1. Connect to the kit that runs this sample with a terminal emulator (for example, PuTTY). See How to connect with PuTTY for the required settings.

  2. In the terminal window, check for information similar to the following:

    Starting Connectionless Locator Demo
    Bluetooth initialization...success
    Scan callbacks register...success.
    Periodic Advertising callbacks register...success.
    Start scanning...success
    Waiting for periodic advertising...
    [DEVICE]: XX:XX:XX:XX:XX:XX, AD evt type X, Tx Pwr: XXX, RSSI XXX C:X S:X D:X SR:X E:1 Prim: XXX, Secn: XXX, Interval: XXX (XXX ms), SID: X
    success. Found periodic advertising.
    Creating Periodic Advertising Sync...success.
    Waiting for periodic sync...
    PER_ADV_SYNC[0]: [DEVICE]: XX:XX:XX:XX:XX:XX synced, Interval XXX (XXX ms), PHY XXX
    success. Periodic sync established.
    Enable receiving of CTE...
    success. CTE receive enabled.
    Scan disable...Success.
    Waiting for periodic sync lost...
    PER_ADV_SYNC[X]: [DEVICE]: XX:XX:XX:XX:XX:XX, tx_power XXX, RSSI XX, CTE XXX, data length X, data: XXX
    CTE[X]: samples count XX, cte type XXX, slot durations: X [us], packet status XXX, RSSI XXX


This sample uses the following Zephyr libraries:

  • include/zephyr/types.h

  • lib/libc/minimal/include/errno.h

  • include/sys/printk.h

  • include/sys/byteorder.h

  • include/sys/util.h

  • Bluetooth APIs:

    • include/bluetooth/bluetooth.h

    • include/bluetooth/direction.h