CC2500-RTR1 datasheet(34/84 Pages) TI

34 / 84 page

CC2500

PRELIMINARY Data Sheet (Rev.1.2)

SWRS040A

Page 34 of 83

otherwise remain in RX. This feature is called

TX if CCA.

Four CCA requirements can be programmed:

•

Always (CCA disabled, always goes to TX)

•

If RSSI is below threshold

•

Unless currently receiving a packet

•

Both the above (RSSI below threshold and

not currently receiving a packet)

17.6

Link Quality Indicator (LQI)

The Link Quality Indicator is a metric of the

current quality of the received signal. If

PKTCTRL1.APPEND_STATUS

is enabled, the

value is automatically appended to the end of

each received packet. The value can also be

read from the LQI status register. The LQI is

calculated over the 64 symbols following the

sync word (first 8 packet bytes). LQI is best

used as a relative measurement of the link

quality, since the value is dependent on the

modulation format.

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 only supported

fixed

packet

length

mode

(PKTCTRL0.LENGTH_CONFIG=0).

FEC

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

14.

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 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.

When FEC and interleaving is used the

minimum data payload is 2 bytes.

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