Distance Measurement

The Distance Measurement module provides an integration of Nordic Distance Measurement library into nRF Connect SDK.

Note

The current implementation is experimental.

Overview

The Distance Measurement module is independent of the communication protocol used. The Multiprotocol Service Layer library is used to access the radio module. The distance is measured during the allocated timeslot.

Ranging

When a new ranging request arrives, the module checks if a timeslot for the ranging can be scheduled. It also checks if the request fits in the schedule of rangings. Rangings are not executed immediately, but are scheduled for future execution, which gives additional flexibility. New ranging requests may arrive at any point in time.

If the request can be scheduled, the module performs the ranging after the time set in the CONFIG_DM_RANGING_OFFSET_US option has passed. The ranging is executed within a timeslot. After ranging, a callback is called to store or process the measurement data.

Configuration

To use the Distance Measurement module, complete the following steps:

  1. Enable the CONFIG_NRF_DM Kconfig option.

  2. Include dm.h in your main.c file.

  3. Call dm_init() to initialize the module.

  4. Call dm_request_add() to perform the measurement.

After synchronizing, the nodes that perform the measurement with each other will issue the measurement request. The callback function data_ready() is called when the measurement data is available.

Configuring synchronization

To adjust the synchronization of the nodes, change the values of the following options:

  • CONFIG_DM_INITIATOR_DELAY_US - Extra delay of the start of the initiator role for the distance measurement. Reducing this value decreases the power consumption, but leads to less successful rangings.

  • CONFIG_DM_MIN_TIME_BETWEEN_TIMESLOTS_US - Minimum time between two timeslots. This option should account for processing of the ranging data after the timeslot.

Enabling output pin state change

It is possible to enable an output pin state change when an event related to this module occurs. You can use this functionality to determine the synchronization accuracy. A logic analyzer or oscilloscope are helpful for this purpose.

Enabling the CONFIG_DM_GPIO_DEBUG option changes the state of the pins when a new measurement request is added and the timeslot is assigned. To assign the pin numbers, use the options CONFIG_DM_RANGING_PIN and CONFIG_DM_ADD_REQUEST_PIN.

Configuring timeslot queue

Use the following options to configure the timeslot queue:

For optimal performance and scalability, both peers should come to the same decision to range each other. Otherwise, one of the peers tries to range the other peer that is not listening and therefore wastes power and time during this operation.

If you enable the CONFIG_DM_TIMESLOT_RESCHEDULE option, the device will try to range the same peer again if the previous ranging was successful.

Defining ranging offset

The option CONFIG_DM_RANGING_OFFSET_US defines the time between the synchronization (adding a request) and ranging. Increasing this value allows for more rangings to different nodes, but also increases latency.

API documentation

Header file: include/dm.h
Source files: subsys/dm/
group dm

API for the Distance Measurement (DM).

Enums

enum dm_dev_role

Role definition.

Values:

enumerator DM_ROLE_NONE

Undefined role.

enumerator DM_ROLE_INITIATOR

Act as an initiator.

enumerator DM_ROLE_REFLECTOR

Act as a reflector.

enum dm_ranging_mode

Ranging mode definition.

Values:

enumerator DM_RANGING_MODE_RTT

Round trip timing.

enumerator DM_RANGING_MODE_MCPD

Multi-carrier phase difference.

enum dm_quality

Measurement quality definition.

Values:

enumerator DM_QUALITY_OK

Good measurement quality.

enumerator DM_QUALITY_POOR

Poor measurement quality.

enumerator DM_QUALITY_DO_NOT_USE

Measurements not usable.

enumerator DM_QUALITY_CRC_FAIL

Incorrect measurement CRC.

enumerator DM_QUALITY_NONE

Measurement quality not specified.

Functions

int dm_init(struct dm_init_param *init_param)

Initialize the DM.

Initialize the DM by specifying a list of supported operations.

Parameters

  • init_param – [in] Initialization parameters.

Return values

0 – if the operation was successful. Otherwise, a (negative) error code is returned.

int dm_request_add(struct dm_request *req)

Add measurement request.

Adding a measurement request. This is related to timeslot allocation.

Parameters

  • req – [in] Address of the structure with request parameters.

Return values

0 – if the operation was successful. Otherwise, a (negative) error code is returned.

struct dm_result
#include <dm.h>

Distance Measurement result structure.

Public Members

bool status

Status of the procedure. True if measurement was done correctly, False otherwise.

enum dm_quality quality

Quality indicator

bt_addr_le_t bt_addr

Bluetooth LE device address. The address is used to distinguish peers.

enum dm_ranging_mode ranging_mode

Mode used for ranging.

float ifft

MCPD: Distance estimate based on IFFT of spectrum.

float phase_slope

MCPD: Distance estimate based on average phase slope estimation.

float rssi_openspace

RSSI: Distance estimate based on Friis path loss formula.

float best

Best effort distance estimate.

float rtt

RTT: Distance estimate based on RTT measurement.

union dm_result.[anonymous] dist_estimates

Container of distance estimate results for a number of different methods, in meters.

struct dm_cb
#include <dm.h>

Event callback structure.

Public Members

void (*data_ready)(struct dm_result *result)

Data ready.

Callback that is executed when the result of the Distance Measurement is available for reading.

Param result

[out] Pointer to the variable that is used to store the Distance Measurement result.

struct dm_init_param
#include <dm.h>

DM initialization parameters.

Public Members

struct dm_cb *cb

Event callback structure.

struct dm_request
#include <dm.h>

DM request structure.

Public Members

enum dm_dev_role role

Role of the device.

bt_addr_le_t bt_addr

Bluetooth LE device address.

uint32_t rng_seed

Seed used in pseudo random number generation. Needs to be the same for an initiator and a reflector that will range with each other.

enum dm_ranging_mode ranging_mode

Ranging mode to use in the procedure.

uint32_t start_delay_us

Start delay.

uint32_t extra_window_time_us

Extra time to extend the ranging window.