Downlink

payload might be an empty string and port null if the downlink is for mac command and/or confirming the previous uplink.
That example has mac commands for ADR and Status request, and it's also confirming previous uplink.

{
  "timestamp": 1680197610131,
  "type": "downlink",
  "meta": {
    "application": "61cc747fdbb07352dbd89e74",
    "gateway": "fedcba9876543210",
    "device": "0123456789abcdef",
    "packet_id": "1301732101",
    "application_name": "FH",
    "organization_name": "Volvo",
    "device_name": "Demo device",
    "device_tags": ["tag-abc", "tag-xyz"],
    "gateway_tags": ["tag-abc", "tag-xyz"],
    "time": 1680197610.131
  },
  "params": {
    "payload": "",
    "counter_down": 3,
    "port": null,
    "radio": {
      "freq": 926.3,
      "datarate": 5,
      "hardware": {
        "power": 27,
        "snr": 9.3,
        "rssi": -39,
        "channel": 5
      },
      "modulation": {
        "bandwidth": 500,
        "spreading": 7,
        "coderate": "CR_4_5",
        "inverted": true
      },
      "datr": "SF7BW500"
    },
    "lora": {
      "header": {
        "confirmed": false,
        "adr": true,
        "ack": true,
        "class_b": false,
        "pending": false
      },
      "mac_commands": [
        {
          "cid": "LinkADRReq",
          "payload": {
            "data_rate": 5,
            "tx_power": 8,
            "ch_mask": [
              true,
              true,
              true,
              true,
              true,
              true,
              true,
              true,
              false,
              false,
              false,
              false,
              false,
              false,
              false,
              false
            ],
            "redundancy": {
              "ch_mask_cntl": 0,
              "nb_rep": 1
            }
          }
        },
        {
          "cid": "DevStatusReq",
          "payload": null
        }
      ]
    }
  }
}