RFM92 Datasheet, PDF (74/126 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

RFM92W/93W V3.0
4.2.14.7. DC-Free Data Mechanisms
The payload to be transmitted may contain long sequences of 1's and 0's, which introduces a DC bias in the transmitted
signal. The radio signal thus produced has a non uniform power distribution over the occupied channel bandwidth. It also
introduces data dependencies in the normal operation of the demodulator. Thus it is useful if the transmitted data is random
and DC free.
For such purposes, two techniques are made available in the packet handler: Manchester encoding and data whitening.
Note Only one of the two methods can be enabled at a time.
Manchester Encoding
Manchester encoding/decoding is enabled if DcFree = 01 and can only be used in Packet mode.
The NRZ data is converted to Manchester code by coding '1' as “10” and '0' as “01”.
In this case, the maximum chip rate is the maximum bit rate given in the specifications section and the actual bit rate is half
the chip rate.
Manchester encoding and decoding is only applied to the payload and CRC checksum while preamble and Sync word are
kept NRZ. However, the chip rate from preamble to CRC is the same and defined by BitRate in RegBitRate (Chip Rate =
Bit Rate NRZ = 2 x Bit Rate Manchester).
Manchester encoding/decoding is thus transparent with NRZ transferred between FIFO and MCU.
1/BR _...Sync
1/BR
Payload...
RF chips @ BR ... 1 1 1 0 1 0 0 1 0 0 1 0 1 1 0 1 0 ...
User/NRZ bits
Manchester OFF
... 1 1 1 0 1 0 0 1
0
0
1
0
1
1
0
1
0 ...
t
User/NRZ bits
Manchester ON
... 1 1 1 0 1 0 0
1
0
0
1
1
...
Figure 36. Manchester Encoding/Decoding
Data Whitening
Another technique called whitening or scrambling is widely used for randomizing the user data before radio transmission.
The data is whitened using a random sequence on the Tx side and de-whitened on the Rx side using the same sequence.
Compared to Manchester coding, whitening has the added advantage that the NRZ data rate is retained i.e. the effective
actual bit rate is not halved.
The whitening/de-whitening process is enabled if DcFree = 10. A 9-bit LFSR is used to generate a random sequence. The
payload and 2-byte CRC checksum is then XORed with this random sequence as shown below. The data is de-whitened
on the receiver side by XORing with the same random sequence.
Payload whitening/de-whitening is thus made transparent to the user who still provides/retrieves NRZ data to/from the
FIFO.
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