Datasheet

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SX1272/73
WIRELESS, SENSING & TIMING DATASHEET
Rev. 2 - July 2014
©2014 Semtech Corporation
Payload
The packet payload is a variable-length field that contains the actual data coded at the error rate either as specified in the
header in explicit mode or in the register settings in implicit mode. An optional CRC may be appended. For more
information on the payload and how it is loaded from the data buffer FIFO please see Section
4.1.2.3.
4.1.1.7. Time on air
For a given combination of spreading factor (SF), coding rate (CR) and signal bandwidth (BW) the total on-the-air
transmission time of a LoRa
TM
packet can be calculated as follows. From the definition of the symbol rate it is convenient to
define the symbol period:
The LoRa packet duration is the sum of the duration of the preamble and the transmitted packet. The preamble length is calculated as
follows:
where n
preamble
is the programmed preamble length, taken from the registers RegPreambleMsb and RegPreambleLsb. The
payload duration depends upon the header mode that is enabled. The following formula gives the number of payload
symbols.
Where PL is the number of bytes of payload, SF is the spreading factor, IH = 1 when implicit header mode is enabled and IH = 0 when
explicit header mode is used. DE set to 1 indicates the use of the low data rate optimization, 0 when disabled. CRC indicates the
presence of the payload CRC = 1 when on 0 when off. CR is the programmed coding rate from 1 to 4.
The ceil function indicates that the portion of the equation in square brackets should be rounded up to the next integer value. The max
function compares the evaluated ceil function result and returns 0 or the result - whichever is higher.
Addition of the preamble and payload durations gives the total packet time on air.
4.1.1.8. Frequency Hopping with LoRa
TM
Frequency hopping spread spectrum (FHSS) is typically employed when the duration of a single packet could exceed
regulatory requirements relating to the maximum permissible channel dwell time. This is most notably the case in US
operation where the 902 to 928 MHz ISM band which makes provision for frequency hopping operation. To ease the
implementation of FHSS systems the frequency hopping mode of the LoRa
TM
modem can be enabled by setting
FreqHoppingPeriod to a non-zero value in register RegHopPeriod.
Ts
1
Rs
------
=
T
preamble
n
preamble
4.25+()T
sym
=
n
payload
8 max ceil
8PL 4SF–2816CRC 20IH–++()
4 SF 2DE–()
----------------------------------------------------------------------------------------
CR 4+()0,


+=
T
payload
n
payload
T
s
×=
T
packet
T
preamble
T
payload
+=