// GetDataRate returns the band.Datarate for the current Metadata
func (m *Metadata) GetDataRate() (dataRate band.DataRate, err error) {
switch m.Modulation {
case Modulation_LORA:
dataRate.Modulation = band.LoRaModulation
dr, err := types.ParseDataRate(m.DataRate)
if err != nil {
return dataRate, err
}
dataRate.Bandwidth = int(dr.Bandwidth)
dataRate.SpreadFactor = int(dr.SpreadingFactor)
case Modulation_FSK:
dataRate.Modulation = band.FSKModulation
dataRate.BitRate = int(m.BitRate)
}
return
}
// ComputeLoRa computes the time-on-air given a PHY payload size in bytes, a datr
// identifier and LoRa coding rate identifier. Note that this function operates
// on the PHY payload size and does not add the LoRaWAN header.
//
// See http://www.semtech.com/images/datasheet/LoraDesignGuide_STD.pdf, page 7
func ComputeLoRa(payloadSize uint, datr string, codr string) (time.Duration, error) {
// Determine CR
var cr float64
switch codr {
case "4/5":
cr = 1
case "4/6":
cr = 2
case "4/7":
cr = 3
case "4/8":
cr = 4
default:
return 0, errors.New("Invalid Codr")
}
// Determine DR
dr, err := types.ParseDataRate(datr)
if err != nil {
return 0, err
}
// Determine DE
var de float64
if dr.Bandwidth == 125 && (dr.SpreadingFactor == 11 || dr.SpreadingFactor == 12) {
de = 1.0
}
pl := float64(payloadSize)
sf := float64(dr.SpreadingFactor)
bw := float64(dr.Bandwidth)
h := 0.0 // 0 means header is enabled
tSym := math.Pow(2, float64(dr.SpreadingFactor)) / bw
payloadNb := 8.0 + math.Max(0.0, math.Ceil((8.0*pl-4.0*sf+28.0+16.0-20.0*h)/(4.0*(sf-2.0*de)))*(cr+4.0))
timeOnAir := (payloadNb + 12.25) * tSym * 1000000 // in nanoseconds
return time.Duration(timeOnAir), nil
}
