CC2500 Datasheet, PDF (33/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
18 Forward Error Correction with Interleaving
18.1 Forward Error Correction (FEC)
CC2500 has built in support for Forward Error
Correction (FEC). To enable this option, set
MDMCFG1.FEC_EN to 1. FEC is employed on
the data field and CRC word in order to reduce
the gross bit error rate when operating near
the sensitivity limit. Redundancy is added to
the transmitted data in such a way that the
receiver can restore the original data in the
presence of some bit errors.
The use of FEC allows correct reception at a
lower SNR, thus extending communication
range. Alternatively, for a given SNR, using
FEC decreases the bit error rate (BER). As the
packet error rate (PER) is related to BER by:
PER = 1 − (1 − BER) ^{packet _ length}
a lower BER can be used to allow longer
packets, or a higher percentage of packets of
a given length, to be transmitted successfully.
Finally, in realistic ISM radio environments,
transient and time-varying phenomena will
produce occasional errors even in otherwise
good reception conditions. FEC will mask such
errors and, combined with interleaving of the
coded data, even correct relatively long
periods of faulty reception (burst errors).
The FEC scheme adopted for CC2500 is
convolutional coding, in which n bits are
generated based on k input bits and the m
most recent input bits, forming a code stream
able to withstand a certain number of bit errors
between each coding state (the m-bit window).
The convolutional coder is a rate 1/2 code with
a constraint length of m=4. The coder codes
one input bit and produces two output bits;
hence, the effective data rate is halved.
18.2 Interleaving
Data received through radio channels will
often experience burst errors due to
interference and time-varying signal strengths.
In order to increase the robustness to errors
spanning multiple bits, interleaving is used
when FEC is enabled. After de-interleaving, a
continuous span of errors in the received
stream will become single errors spread apart.
CC2500 employs matrix interleaving, which is
illustrated in Figure 10. The on-chip
interleaving and de-interleaving buffers are 4 x
4 matrices. In the transmitter, the data bits are
written into the rows of the matrix, whereas the
bit sequence to be transmitted is read from the
columns of the matrix and fed to the rate ½
convolutional coder. Conversely, in the
receiver, the received symbols are written into
the columns of the matrix, whereas the data
passed onto the convolutional decoder is read
from the rows of the matrix.
When FEC and interleaving is used, the
amount of data transmitted over the air must
be a multiple of the size of the interleaver
buffer (two bytes). In addition, at least one
extra byte is required for trellis termination.
The packet control hardware therefore
automatically inserts one or two extra bytes at
the end of the packet, so that the total length
of the data to be interleaved is an even
number. Note that these extra bytes are
invisible to the user, as they are removed
before the received packet enters the RX
FIFO.
Due to the implementation of the FEC and
interleaver, the data to be interleaved must be
at least two bytes. One byte long fixed length
packets without CRC is therefore not
supported when FEC/interleaving is enabled.
1) Storing coded
data
2) Transmitting
interleaved data
3) Receiving
interleaved data
4) Passing on data
to decoder
TX
Data
Transmitter
Receiver
RX
Data
Figure 10: General principle of matrix interleaving
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 33 of 77