CC2550_11 Datasheet, PDF (23/59 Pages) Texas Instruments – Low-Cost Low-Power 2.4 GHz RF Transmitter

CC2550
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:
f _dev
=
f _xosc
2¹⁷
⋅ (8 + DEVIATION _ M ) ⋅ 2^{DEVIATION _ E}
The symbol encoding is shown in Table 17.
Format
2-FSK\GFSK
Symbol
‘0’
‘1’
Coding
– Deviation
+ Deviation
Table 17: Symbol Encoding for 2-FSK/GFSK
Modulation
14.2 Minimum Shift Keying
When using MSK¹, 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 is
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.
1 _{Identical to offset QPSK with half-sine}
shaping (data coding may differ)
15 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:
PER = 1 − (1 − BER) ^{packet _ length}
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
SWRS039B
Page 23 of 58