RF96 datasheet(73/121 Pages) HOPE

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Page 73

RF96/97/98

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WIRELESS & SENSING

PRELIMINARY

DATASHEET

4.2.13.7. 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 made transparent for the user, who still provides/retrieves NRZ data to/from the

FIFO.

1/BR ...Sync

1/BR

Payload...

RF chips @ BR

...

User/NRZ bits

Manchester OFF

...

User/NRZ bits

Manchester ON

...

Data Whitening

Figure 36. Manchester Encoding/Decoding

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.

Comparing to Manchester technique it has the advantage of keeping NRZ data rate i.e. 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 for the user, who still provides/retrieves NRZ data to/from the

FIFO.

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