Can a Peripheral device know the RSSI for a connected Central

[Kristian Sylwander]

Hello fellow Nordic community,

Assume that a given Peripheral (P) is advertising a BLE GAP signal which allows for multiple connections to be made.

In the vicinity to P we have two central devices (C1, C2) that hear the advertisement signal and connect to P.

(P) - - - - - - < 5 m > - - - - - - (C1)
  |
  - - - - - - - - - - <  10m > - - - - - - - - - - - - - - - - - - - - - - - - - - - - (C2)

Is it possible to determine on the Peripheral side which connected Central (C1 or C2) that's signal-wise closest to the Peripheral. Hence in the above example determine that C1 is closer than C2?

Thanks for any help!

Cheers,
Kristian

[AndreasF]

Hi.

You can use the two functions sd_ble_gap_rssi_get() and sd_ble_gap_rssi_start() to get the RSSI value between a given peripheral and a central.

You can find more information about these functions in the infocenter under:

nRF52 Series -> SoftDevices -> Your SoftDevice -> Your SoftDevice API -> Your SoftDevice version API -> API Reference -> Generic Access Profile (GAP)

Here is the link to the infocenter.

With RSSI you can't really state which device is closest in distance, since RSSI is a measurement of signal strength, and lots of factors play a part in that value.

I can give you an example: If there is a wall between P and C1, but P and C2 are in line of sight with no obstacles, you might get a higher RSSI value for C2 then C1 even though C2 is 5 meter further away from P.

- Andreas

[Kristian Sylwander]

Thanks Andreas,

I get that the actual distance is hard to estimate given a bunch of factors that affects the signal strength.

I read from your answer that it is possible on the Peripheral side to know the RSSI to a given connected Central. Is that correct?

Do you know of any examples that show how this could be done?

Thank you so much for all the help!

[AndreasF]

Hi.

I can show you how I did it.

I used the examples\ble_peripheral\ble_app_uart example in SDK 15.0 for this. I did the following in main.c:

To get a continuous RSSI value I created a timer, which I first defined in the header, like this:

(Library and API for the timer)

APP_TIMER_DEF(my_timer_id);

I then created the timer in int main(void) with the following code (ADDED BY ME):

int main(void)
{
    bool erase_bonds;

    // Initialize.
    uart_init();
    log_init();
    timers_init();
    buttons_leds_init(&erase_bonds);
    power_management_init();
    ble_stack_init();
    gap_params_init();
    gatt_init();
    services_init();
    advertising_init();
    conn_params_init();
    uint32_t err_code = app_timer_create(&my_timer_id, APP_TIMER_MODE_REPEATED , timeout_handler); //ADDED BY ME
    APP_ERROR_CHECK(err_code);// ADDED BY ME

    // Start execution.
    printf("\r\nUART started.\r\n");
    NRF_LOG_INFO("Debug logging for UART over RTT started.");
    advertising_start();

    // Enter main loop.
    for (;;)
    {
        
        idle_state_handle();
    }
}

The timer required a timeout handler, which I created above int main(void), which looked like this:

static void timeout_handler(void *timeoutpointer)
{
            uint8_t rssi;
            uint8_t channel;

            uint32_t err_code = sd_ble_gap_rssi_get(m_conn_handle,&rssi,&channel);
            APP_ERROR_CHECK(err_code);
            NRF_LOG_INFO("RSSI: %d", rssi);
}

It was in the timeout_handler I collected and printed the RSSI values.

I started the timer in the case BLE_GAP_EVT_CONNECTED found in static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)

I added the following code before the switch case in the ble_evt_handler:

static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
    uint32_t err_code;
    ble_gap_evt_t const * p_gap_evt = &p_ble_evt->evt.gap_evt; // ADDED BY ME

    switch (p_ble_evt->header.evt_id)
    {
    .....
    ....
    ...
    ..
    .

In the BLE_GAP_EVT_CONNECTED case i added the following code:

        case BLE_GAP_EVT_CONNECTED:
            NRF_LOG_INFO("Connected");
            err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
            APP_ERROR_CHECK(err_code);
            m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
            err_code = nrf_ble_qwr_conn_handle_assign(&m_qwr, m_conn_handle);
            APP_ERROR_CHECK(err_code);
            

            err_code = sd_ble_gap_rssi_start(p_ble_evt->evt.gap_evt.conn_handle,BLE_GAP_RSSI_THRESHOLD_INVALID,0); //ADDED BY ME
            APP_ERROR_CHECK(err_code); //ADDED BY ME

            err_code = app_timer_start(my_timer_id,APP_TIMER_TICKS(5000),0); //ADDED BY ME
            APP_ERROR_CHECK(err_code); //ADDED BY ME

            break;

I then compiled programmed the example and got roughly the same RSSI in dBm as in nRF Connect between the two devices.

Hope this helps.

- Andreas

[Kristian Sylwander]

Awesome, thanks for the examples. I tried it myself and it seems to work fine. 

[Prarthana]

Hello, 

Thank you for this example. It definitely works fine. 

I had another doubt, is it possible to display the RSSI value on the Serial Interface ? I'm using PuTTY, and I wanted to display the RSSI values on it 

[MJ Ma]

Hi ,

Looks like this sd_ble_gap_rssi_get() API not works on sdk 16.0.0, I did the same thing but it returns with  err_code NRF_ERROR_INVALID_STATE

[MJ Ma]