SX127x

1 Preamble

LoRa 数据包前导码，用于接收机辩识 rx data flow。内容固定，长度可选。

默认 12 个 symbol（Bytes?）, 0x8 (RegPreambleLsb) + 4

可写 RegPreambleMsb 和 RegPreambleLsb 这两个寄存器，将其改为 6 +4 ~ 65535 +4

10 symbol 的短前导码，通信够用但具有更短的接收周期。。。

当前系统一般皆适用 12 (4 + 8) 的长度：

......
^$Get Preamble Length: state 0
^$Preamble Length: 8
......

2 Header

Explicit Header & Implicit Header

可通过寄存器 RegModemConfig1 内的位 ImplicitHeaderModeOn 来控制

Explicit Header:

Reset 默认的模式。其能提供的信息：

• Payload 长度
• The forward error correction code rate
• The presence of an optional 16-bits CRC for the payload

The head is transmitted with maximum error correction code (4/8). It also has its own CRC to allow the receiver to discard invalid headers.

Implicit Header:

Header is removed from the LoRa Packet.

Payload, Coding Rate and CRC presence are fixed.

Be advantageous to reduce transmission time

3 CRC

RegPayloadCrcOn: RegModemConfig2[2:2]

• 0 ---> CRC disable
• 1 ---> CRC enable

In Explicit Header Mode: on the Tx side alone (recovered from the header in Rx side)

In Implicit Header Mode: must be set on Tx and Rx side