Data Sheet

Page 26
RFM95/96/97/98(W)
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WIRELESS & SENSING PRELIMINARY DATASHEET
4.1.1.6. LoRa
TM
Packet Structure
The LoRa
TM
modem employs two types of packet format, explicit and implicit. The explicit packet includes a short header
that contains information about the number of bytes, coding rate and whether a CRC is used in the packet. The packet
format is shown in the following figure.
The LoRa
TM
packet comprises three elements:
 A preamble.
 An optional header.
 The data payload.
Preamble
Figure 5. LoRa
TM
Packet Structure
The preamble is used to synchronize receiver with the incoming data flow. By default the packet is configured with a 12
symbol long sequence. This is a programmable variable so the preamble length may be extended, for example in the
interest of reducing to receiver duty cycle in receive intensive applications. However, the minimum length suffices for all
communication. The transmitted preamble length may be changed by setting the register PreambleLength from 6 to 65535,
yielding total preamble lengths of 6+4 to 65535+4 symbols, once the fixed overhead of the preamble data is considered.
This permits the transmission of a near arbitrarily long preamble sequence.
The receiver undertakes a preamble detection process that periodically restarts. For this reason the preamble length
should be configured identical to the transmitter preamble length. Where the preamble length is not known, or can vary, the
maximum preamble length should be programmed on the receiver side.
Header
Depending upon the chosen mode of operation two types of header are available. The header type is selected by the
ImplictHeaderMode bit found within the
RegSymbTimeoutMsb register.
Explicit Header Mode
This is the default mode of operation. Here the header provides information on the payload, namely:
 The payload length in bytes.
 The forward error correction code rate
 The presence of an optional 16-bits CRC for the payload.