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3.14 Forward Error Correction With Interleaving
3.14.1 Forward Error Correction (FEC)
CC11x1-Q1 has built in support for Forward Error Correction (FEC). To enable this option, set
MDMCFG1.FEC_EN
(PKTCTRL0.LENGTH_CONFIG = 0). FEC is employed on the data field and CRC word 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 if the
receiver bandwidth remains constant. 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)
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 produce occasional errors even in otherwise good reception conditions. FEC masks 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 CC11x1-Q1 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. I.e., to transmit at the same
effective data rate when using FEC, it is necessary to use twice as high over-the-air data rate. This
requires a higher receiver bandwidth, and thus reduce sensitivity. In other words the improved reception
by using FEC and the degraded sensitivity from a higher receiver bandwidth are counteracting factors.
3.14.2 Interleaving
Data received through radio channels often experiences burst errors due to interference and time-varying
signal strengths. To increase the robustness to errors spanning multiple bits, interleaving is used when
FEC is enabled. After deinterleaving, a continuous span of errors in the received stream become single
errors spread apart.
CC11x1-Q1 employs matrix interleaving, which is illustrated in
deinterleaving buffers are 4×4 matrices. In the transmitter, the data bits from the rate one-half
convolutional coder are written into the rows of the matrix, whereas the bit sequence to be transmitted is
read from the columns of the matrix. 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 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.
When FEC and interleaving are used the minimum data payload is 2 bytes.
Copyright © 2009–2010, Texas Instruments Incorporated
to
1.
FEC
is
packet_length
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SWRS076B – 11-07-22-013 - APRIL 2009 – REVISED APRIL 2010
supported
only
in
Figure
3-14. The on-chip interleaving and
CC11x1-Q1
fixed
packet
length
Detailed Description
mode
(6)
43
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