CC1100-RTR1 datasheet(40/100 Pages) TI1

40 / 100 page

CC1100

SWRS038D

Page 40 of 92

17.4.2 CS Relative Threshold

The relative threshold detects sudden changes

in the measured signal level. This setting is not

dependent on the absolute signal level and is

thus useful to detect signals in environments

with a time varying noise floor. The register

field AGCCTRL1.CARRIER_SENSE_REL_THR

is used to enable/disable relative CS, and to

select threshold of 6 dB, 10 dB, or 14 dB RSSI

change.

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

the

GDO

pins

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

CC1100 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

or FSTXON state before a new strobe

command is sent on the SPI interface). 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 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 is best used as a 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)

CC1100 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 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:

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

CC1100 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 datarate when

using FEC, it is necessary to use twice as high

over-the-air datarate. This will require a higher

receiver

bandwidth,

and

thus

reduce

sensitivity. In other words the improved

CC1100-RTR1 Datasheet(PDF) 40 Page - Texas Instruments

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