first commit

lora.go

@@ -0,0 +1,130 @@
+package lora
+
+import "time"
+
+type Config struct {
+	// Bandwidth relates to data rate of communication. Double the bandwidth means
+	// double the data rate, which implies faster communications and less energy usage.
+	Bandwidth Frequency
+	Frequency Frequency
+	// Length of the preamble preceding the header. Can be arbitrarily long.
+	// Longer preambles ensure more robust communications but can lead to congestion.
+	//
+	// Note: The preamble length on a receiver should be configured equal-to or greater
+	// than the expected packet preamble length on the SX1278.
+	PreambleLength uint16
+	HeaderType     HeaderType
+	CodingRate     CodingRate
+	SpreadFactor   SpreadFactor
+	SyncWord       uint8
+	TxPower        int8 // Tx Power in dBm.
+	CRC            bool
+	// Low data rate optimisation flag. The use of this flag is mandated when
+	// the symbol duration exceeds 16ms. Increases reliability at high spreading factors.
+	LDRO   bool
+	IQMode uint8
+}
+
+func (cfg *Config) TimeOnAir(payloadLength int) time.Duration {
+	if cfg.Bandwidth == 0 {
+		return 0
+	}
+	crc := int64(b2u8(cfg.CRC))
+	ih := int64(cfg.HeaderType)
+	ldr := int64(b2u8(cfg.LDRO))
+	cr := int64(cfg.CodingRate)
+	spread := int64(cfg.SpreadFactor)
+	// Page 31 SX1276IMLTRT SEMTECH | Alldatasheet.
+	Npayload := 8*int64(payloadLength) - 4*spread + 28 + 16*crc - 20*ih
+	div := 4 * (spread - 2*ldr)
+	// Apply Ceil and max with minimal branching.
+	if Npayload < 0 || div <= 0 {
+		Npayload = 0
+	} else if Npayload%div == 0 {
+		Npayload /= div
+		Npayload *= (cr + 4)
+	} else {
+		Npayload /= div
+		Npayload++
+		Npayload *= (cr + 4)
+	}
+	Npayload += 8 + int64(cfg.PreambleLength) + 5 // Says 4.25 in manual but we round up.
+	// Calculate LoRa Transmission Parameter Relationship page 28.
+	Ts_us := 1000_000 * cfg.SpreadFactor.ChipsPerSymbol() / cfg.Bandwidth.Hertz()
+	return time.Microsecond * (time.Duration(Npayload * Ts_us))
+}
+
+type HeaderType uint8
+
+const (
+	HeaderExplicit HeaderType = 0
+	HeaderImplicit HeaderType = 1
+)
+
+type CodingRate uint8
+
+const (
+	CR4_5 CodingRate = 1
+	CR4_6 CodingRate = 2
+	CR4_7 CodingRate = 3
+	CR4_8 CodingRate = 4
+)
+
+type SpreadFactor uint8
+
+const (
+	SF6 SpreadFactor = iota + 6
+	SF7
+	SF8
+	SF9
+	SF10
+	SF11
+	SF12
+)
+
+func (sf SpreadFactor) ChipsPerSymbol() int64 {
+	return 1 << sf
+}
+
+type Frequency int64
+
+func (f Frequency) Hertz() int64 { return int64(f) }
+
+const (
+	Hertz     Frequency = 1
+	KiloHertz Frequency = 1000 * Hertz
+	MegaHertz Frequency = 1000 * KiloHertz
+)
+
+// Common LoRa bandwidths
+const (
+	BW125k = 125 * KiloHertz
+	BW250k = 250 * KiloHertz
+	BW500k = 500 * KiloHertz
+)
+
+// Common LoRa frequencies
+const (
+	// 433.05MHz  Low limit medical, scientific and industrial band.
+	Freq433_0M = 433050000 * Hertz
+	// 434.8MHz High limit medical, scientific and industrial band.
+	Freq434_8M = 434790000 * Hertz
+	Freq868_1M = 868100000 * Hertz
+	Freq868_5M = 868500000 * Hertz
+	Freq916_8M = 916800000 * Hertz
+	Freq923_3M = 923300000 * Hertz
+)
+
+func max[T ~int | ~int64 | ~uint8](a, b T) T {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func b2u8(b bool) uint8 {
+	if b {
+		return 1
+	}
+	return 0
+}