How to build RF device payload

I try to build RF device payload inside my own packet-forwarder according:

Name |  Type  | Function
time | string | UTC time of pkt RX, us precision, ISO 8601 'compact' format
tmms | number | GPS time of pkt RX, number of milliseconds since 06.Jan.1980
tmst | number | Internal timestamp of "RX finished" event (32b unsigned)
freq | number | RX central frequency in MHz (unsigned float, Hz precision)
chan | number | Concentrator "IF" channel used for RX (unsigned integer)
rfch | number | Concentrator "RF chain" used for RX (unsigned integer)
stat | number | CRC status: 1 = OK, -1 = fail, 0 = no CRC
modu | string | Modulation identifier "LORA" or "FSK"
datr | string | LoRa datarate identifier (eg. SF12BW500)
datr | number | FSK datarate (unsigned, in bits per second)
codr | string | LoRa ECC coding rate identifier
rssi | number | RSSI in dBm (signed integer, 1 dB precision)
lsnr | number | Lora SNR ratio in dB (signed float, 0.1 dB precision)
size | number | RF packet payload size in bytes (unsigned integer)
data | string | Base64 encoded RF packet payload, padded

How should field data look? In my understanding data is the json string converted to bytes, isn’t it? But what’s the format of this json?

As far as I know, field ‘data’ contains a string. It’s the raw byte payload (including all LoRaWAN headers) as transferred over the air, encoded using base64. Example: