CC2550-RTR1 datasheet(22/55 Pages) TI

22 / 55 page

CC2550

PRELIMINARY Data Sheet (Rev.1.2)

SWRS039A

Page 22 of 54

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:

length

packet

BER

PER

)

(

−

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

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

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

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).

Receiver

Transmitter

1) Storing coded

data

2) Transmitting

interleaved data

4) Passing on data

to decoder

3) Receiving

interleaved data

Data

Figure 12: General principle of matrix interleaving

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