CC2500-RTY1 datasheet(37/99 Pages) TI1

37 / 99 page

CC2500

SWRS040C

Page 37 of 89

17.5

Clear Channel Assessment (CCA)

The Clear Channel Assessment CCA) is used

to indicate if the current channel is free or

busy. The current CCA state is viewable on

any of the GDO pins by setting IOCFGx.GDOx_

CFG

=0x09.

MCSM1.CCA_MODE

selects the mode to use

when determining CCA.

When the STX or SFSTXON command strobe is

given while

CC2500 is in the RX state, the TX

or FSTXON state is only entered if the clear

channel requirements are fulfilled. The chip

will otherwise remain in RX (if the channel

becomes available, the radio will not enter TX

FSTXON

state

before

new

strobe

command is sent on the SPI interface). This

feature

called

TX-if-CCA.

Four

CCA

requirements can be programmed: 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 added to the last byte

appended after the payload. The value can

also be read from the LQI status register. The

LQI gives an estimate of how easily a received

signal can be demodulated by accumulating

the magnitude of the error between ideal

constellations and the received signal over the

64 symbols immediately following the sync

word.

LQI

best

used

relative

measurement of the link quality (a high value

indicates a better link than what a low value

does), 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. 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 will require 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

will be counteracting factors.

18.2

Interleaving

Data received through radio channels will

often

experience

burst

errors

due

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