Data Sheet

Page 27
RFM95/96/97/98(W)
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⎪
p
aylo a
--
d
⎞
⎩
⎛
WIRELESS & SENSING PRELIMINARY DATASHEET
The header 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 Mode
In certain scenarios, where the payload, coding rate and CRC presence are fixed or known in advance, it may be
advantageous to reduce transmission time by invoking implicit header mode. In this mode the header is removed from the
packet. In this case the payload length, error coding rate and presence of the payload CRC must be manually configured
on both sides of the radio link.
Note With SF = 6 selected, implicit header mode is the only mode of operation possible.
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 rate:
Ts =
--
1
---
Rs
The LoRa packet duration is the sum of the duration of the preamble and the transmitted packet. The preamble length is calculated as
follows:
T
pr ea mble
= ( n
pr ea m b l e
+ 4.25 ) T
sy m
where n
preamble
is the programmed preamble length, PreambleLength.The payload duration depends upon the header
mode that is enabled. The following formulae give the payload duration in implicit (headerless) and explicit (with header)
modes.
⎧
⎛
8l
pa yl oad
– 4SF + 24
⎞
⎪
+
⎝
-------------------------------------
(CR + 4 ) where: l
payloa d
> 0 , implicit header
T
sy m
8 ceil
-------------
4SF
⎠
T
payl oad
=
⎨
⎪
8l – 4SF + 44
+
-----------------------------
(CR + 4 ) where: l > 0 , explicit header
⎪
T
sy m
8 cei l
⎝
-------------------
4SF
⎠
Addition of these two durations gives the total packet on -air time.
T
packe t
= T
pr eam b le
+ T
pa yl oad
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 permittable channel dwell time. This is most notably the case in US
operation where the 902 to 928 MHz ISM band can be used ina frequency hopping mode. To ease implementation of
FHSS systems the frequency hopping mode of the LoRa
TM
modem can be enabled (see FhssMode of register RegTxCfg1).