CC2500 datasheet(33/76 Pages) TI

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CC2500

Preliminary Data Sheet (rev.1.1.)

SWRS040

Page 33 of 77

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:

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

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

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

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,

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

therefore

not

supported when FEC/interleaving is enabled.

Receiver

Transmitter

1) Storing coded

data

2) Transmitting

interleaved data

4) Passing on data

to decoder

3) Receiving

interleaved data

Data

Figure 10: General principle of matrix interleaving

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