CC2550-RTY1 datasheet(23/62 Pages) TI2

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CC2550

SWRS039B

Page 23 of 58

14.1

Frequency Shift Keying

2-FSK can optionally be shaped by a

Gaussian filter with BT=1, producing a GFSK

modulated signal.

The frequency deviation is programmed with

the DEVIATION_M and DEVIATION_E values

in the DEVIATN register. The value has an

exponent/mantissa form, and the resultant

deviation is given by:

DEVIATION

xosc

dev

DEVIATION

)

(

⋅

The symbol encoding is shown in Table 17.

Format

Symbol

Coding

2-FSK\GFSK

‘0’

– Deviation

‘1’

+ Deviation

Table 17: Symbol Encoding for 2-FSK/GFSK

Modulation

14.2

Minimum Shift Keying

When using MSK

1, the complete transmission

(preamble, sync word and payload) will be

MSK modulated.

Phase shifts are performed with a constant

transition time.

The fraction of a symbol period used to

change the phase can be modified with the

DEVIATN.DEVIATION_M

setting.

This

equivalent to changing the shaping of the

symbol.

The MSK modulation format implemented in

CC2550 inverts the sync word and data

compared to e.g. signal generators.

14.3

Amplitude Modulation

The supported amplitude modulation On-Off

Keying (OOK) simply turns on or off the PA to

modulate 1 and 0 respectively.

Identical to offset QPSK with half-sine

shaping (data coding may differ)

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

[9] at the receiver end. To enable this option,

set MDMCFG1.FEC_EN=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

CC2500 [9] 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. 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. 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

CC2500 [9]

receiver

bandwidth,

and

thus

reduced

sensitivity. In other words, the improved

reception by using FEC and the degraded

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