NRF9161 based LTE-M tracker data drops while moving – can it be optimized?

[marsal]

Hi everyone,

I’m using a custom board (a tracker) that can travel at speeds up to 65 km/h and transmits data regularly every 200 ms. I expect, therefore, that the inactivity timer should not be activated. While the device is in motion, I’m seeing intermittent data gaps ranging up to several seconds, which is unacceptable for our application.

Could this be caused by non-optimal cell-handover settings? I’ve attached a few logs below for reference.

Any suggestions would be greatly appreciated.

Thanks,
Martin

 

Three log fragments examples, related to data send drops:

 

17:31:57.179 +CSCON: 0

17:31:57.230 <inf> main: RRC mode: Idle

17:31:57.276 +CEREG: 1,"04D3","0DBC2351",7,,,"11100000","11100000"

17:31:57.298 <inf> main: Cell update, cell tac: 1235, cell id: 230433617

17:31:57.371 +CSCON: 1

17:31:57.429 <inf> main: RRC mode: Connected

17:31:57.539 +CSCON: 0

17:31:57.568 <inf> main: RRC mode: Idle

17:31:57.907 +CSCON: 1

17:31:57.926 <inf> main: RRC mode: Connected

***

17:37:54.095 +CEREG: 1,"04D3","0DC18750",7,,,"11100000","11100000"

17:37:54.154 [00:54:10.770,263] <inf> main: Cell update, cell tac: 1235, cell id: 230786896

***

17:38:05.373 +CEREG: 1,"04D3","0DDD4350",7,,,"11100000","11100000"

17:38:05.434 <inf> main: Cell update, cell tac: 1235, cell id: 232604496

***

 

Modem firmware: mfw_nrf91x1_2.0.2

 

Current 9161 modem initiation sequence is:

 

AT+CFUN=0

AT%XSYSTEMMODE=1,0,0,0

AT+CPSMS=0

AT+CEDRXS=0

AT%RAI=0

AT+CSCON=1

AT+CEREG=5

AT+CFUN=1

[marsal]

Thanks, dejans,
one question regarding the current setup: are you please aware of an implementation with LTE-M NRF9161 where the handover goes smoothly? Which carrier provider?
Thx!

[marsal]

This is really a puzzle for us. We did run other bunch of tests today on both O2 and Tmobile network in the area where is clearly no issue with the signal strength/quality (always better than -90dBm). We were observing dropouts up to 15s, typically related to cell update, but not always. So if I understand well, both Tmobile and O2 networks in the Czech Republic are currently not suitable to run LTE-M devices which are moving for a businss case which needs uninterrupted data delivery?

[marsal]

I'd like just say thanks again with the hope that DECT NR+ will be able to save our business case.

I am not a network expert but looking at the modem trace just confirms for me what you had stated - there seems really to be no on connect handover, all "handovers" are initiated by the modem (even when the transition is "good" or "bad"):



Just FYI, the modem trace is here: https://e.pcloud.link/publink/show?code=XZEMTtZRiah3HaWN7Q0Pifo1kt09HcMUDTX, I have used a tiny app for testing:

/*
 * Copyright (c) 2018 Nordic Semiconductor ASA
 *
 * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 */

#include <modem/nrf_modem_lib.h>
#include <nrf_modem_at.h>
#include <stdio.h>
#include <string.h>
#include <zephyr/drivers/clock_control.h>
#include <zephyr/drivers/clock_control/nrf_clock_control.h>
#include <zephyr/drivers/uart.h>
#include <zephyr/kernel.h>
#include <zephyr/logging/log.h>
LOG_MODULE_REGISTER(main, CONFIG_LOG_DEFAULT_LEVEL);

#include <zephyr/net/net_context.h>
#include <zephyr/net/net_core.h>
#include <zephyr/net/net_if.h>
#include <zephyr/net/net_ip.h>
#include <zephyr/net/socket.h>


/* To strictly comply with UART timing, enable external XTAL oscillator */
void enable_xtal(void) {
    struct onoff_manager *clk_mgr;
    static struct onoff_client cli = {};

    clk_mgr = z_nrf_clock_control_get_onoff(CLOCK_CONTROL_NRF_SUBSYS_HF);
    sys_notify_init_spinwait(&cli.notify);
    (void)onoff_request(clk_mgr, &cli);
}

char *msend;
char atb[500];


int main(void) {
    int err;

    printk("The AT host sample started\n");

    err = nrf_modem_lib_init();
    if (err) {
        printk("Modem library initialization failed, error: %d\n", err);
        return -1;
    }
    enable_xtal();
    printk("Ready\n");


    msend = "AT+CFUN=0";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    msend = "AT+CEMODE=0";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    // syntax: if %, this character must be doubled for nrf_modem_at_cmd!
    // disable GNSS, LTE-M only
    msend = "AT%XSYSTEMMODE=1,0,0,0";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    msend = "AT+CPSMS=0";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    msend = "AT+CEDRXS=0";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    // syntax: if %, this character must be doubled for nrf_modem_at_cmd!
    //  disable RAI
    msend = "AT%RAI=0";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    // if %, this character must be doubled for nrf_modem_at_cmd!
    //  runtime bandlock mask
    //  CAN VERIFY AT%%XBANDLOCK?
    msend = "AT%XBANDLOCK=2,\"10000000000000000000\"";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    msend = "AT+CSCON=1";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    msend = "AT+CEREG=5";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);

    msend = "AT+CFUN=1";
    LOG_INF("Initialization sequence of modem, %s", msend);
    err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
    LOG_INF("at response (%d): %s", err, atb);


reset_socket:

    int sock;
    struct sockaddr_in server_addr;
    const uint8_t keepalive_payload[1] = {0x42};

    /* (Re)create UDP socket ------------------------------------------------ */
    sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock < 0) {
        LOG_ERR("socket() failed: %d", errno);
        k_msleep(500);
        goto reset_socket;
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(12345);

    /* 8.8.8.8 -> binary ---------------------------------------------------- */
    inet_pton(AF_INET, "8.8.8.8", &server_addr.sin_addr);

    LOG_INF("Socket ready, entering keep-alive loop");

    /* Keep-alive loop ------------------------------------------------------ */

#define INTEVALRESET 20

    int intervalreset = INTEVALRESET;  // how many loops to try to reconnect
    for (;;) {
        ssize_t sent = sendto(sock, keepalive_payload, sizeof(keepalive_payload), 0, (struct sockaddr *)&server_addr,
                              sizeof(server_addr));

        if (sent < 0) {
            int err = errno;
            LOG_WRN("sendto() failed: %d, resetting socket", err);
            close(sock);
            k_msleep(100);
            goto reset_socket;
        } else {
            LOG_INF("Keep-alive sent (%zd bytes)", sent);
        }

        // tick wait loop
        k_msleep(500);

        // reconnect trial?
        intervalreset -= 1;
        if (intervalreset <= 0) {
            intervalreset = INTEVALRESET;

            msend = "AT+CGATT=0";
            LOG_INF("Initialization sequence of modem, %s", msend);
            err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
            LOG_INF("at response (%d): %s", err, atb);

            k_msleep(500);

            msend = "AT+CGATT=1";
            LOG_INF("Initialization sequence of modem, %s", msend);
            err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
            LOG_INF("at response (%d): %s", err, atb);

            /*

                        msend = "AT+CFUN=4";
                        LOG_INF("Initialization sequence of modem, %s", msend);
                        err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
                        LOG_INF("at response (%d): %s", err, atb);

                        k_msleep(1);

                        msend = "AT+CFUN=1";
                        LOG_INF("Initialization sequence of modem, %s", msend);
                        err = nrf_modem_at_cmd(atb, sizeof(atb), "%s", msend);
                        LOG_INF("at response (%d): %s", err, atb);

            */
        }
    }
}

We are negotiating with the local O2 MNO, we will see...

Now, I am trying the only option we possibly have, to try to force reconnect when we observe the low quality signal. Our best option found so far is something like (shown in the code): 

"AT+CGATT=0";

wait 500ms

"AT+CGATT=1";

but this situation is not optimal, indeed...

Thanks again!

[Achim Kraus]

marsal said:

but this situation is not optimal, indeed...

Such gaps are more or less the nature of radio transmissions, regardless of cellular or other radio technologies.

In some more expensive implementations, a second antenna/receiver is used to minimize the gap. If that pays-off depends on the use-case.

As I wrote above, for the very most use-cases there is a common practice about that, even if this requires some adaptions in the application logic.

[marsal]

Thanks, Achim, well, if the modem is connected to a cell with strong signal, it behaves perfectly. If there is cell update, our problems begin and without on-connect handover enabled by MNO they will persist (yes, you are true, we can expect issues even if it were enabled, but not so systemic).