CC2550_06 Datasheet, PDF (22/55 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transmitter

CC2550
15 Forward Error Correction with Interleaving
15.1 Forward Error Correction (FEC)
CC2550 has built in support for Forward Error
Correction (FEC) that can be used with CC2500
at the receiver end. 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 CC2550 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.
15.2 Interleaving
Data received through real 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.
CC2550 employs matrix interleaving, which is
illustrated in Figure 12. 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 a CC2500
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 at least
one extra byte is required for trellis
termination. In addition, the amount of data
transmitted over the air must be a multiple of
the size of the interleaver buffer (two bytes).
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
in a CC2500.
When FEC and interleaving is used the
minimum data payload is 2 bytes in fixed and
variable packet length mode.
Note that for the CC2500 transceiver FEC is
only supported in fixed packet length mode
(PKTCTRL0.LENGTH_CONFIG=0).
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 12: General principle of matrix interleaving
PRELIMINARY Data Sheet (Rev.1.2) SWRS039A
Page 22 of 54