CC2420 Datasheet, PDF (24/92 Pages) List of Unclassifed Manufacturers – 2.4 GHz IEEE 802.15.4 / ZigBee-ready RF Transceiver

CC2420
10 IEEE 802.15.4 Modulation Format
This section is meant as an introduction to
the 2.4 GHz direct sequence spread
spectrum (DSSS) RF modulation format
defined in IEEE 802.15.4. For a complete
description, please refer to [1].
The modulation and spreading functions
are illustrated at block level in Figure 6 [1].
Each byte is divided into two symbols, 4
bits each. The least significant symbol is
transmitted first. For multi-byte fields, the
least significant byte is transmitted first,
except for security related fields where the
most significant byte it transmitted first.
Each symbol is mapped to one out of 16
pseudo-random sequences, 32 chips
each. The symbol to chip mapping is
shown in Table 3. The chip sequence is
then transmitted at 2 MChips/s, with the
least significant chip (C₀) transmitted first
for each symbol.
Transmitted
bit-stream
(LSB first)
Bit-to-
Symbol
Symbol-
to-Chip
O-QPSK
Modulator
Modulated
Signal
Figure 6. Modulation and spreading functions [1]
Symbol
0
Chip sequence (C₀, C₁, C₂, … , C₃₁)
11011001110000110101001000101110
1
11101101100111000011010100100010
2
00101110110110011100001101010010
3
00100010111011011001110000110101
4
01010010001011101101100111000011
5
00110101001000101110110110011100
6
11000011010100100010111011011001
7
10011100001101010010001011101101
8
10001100100101100000011101111011
9
10111000110010010110000001110111
10
01111011100011001001011000000111
11
01110111101110001100100101100000
12
00000111011110111000110010010110
13
01100000011101111011100011001001
14
10010110000001110111101110001100
15
11001001011000000111011110111000
Table 3. IEEE 802.15.4 symbol-to-chip mapping [1]
The modulation format is Offset –
Quadrature Phase Shift Keying (O-QPSK)
with half-sine chip shaping. This is
equivalent to MSK modulation. Each chip
is shaped as a half-sine, transmitted
alternately in the I and Q channels with
one half chip period offset. This is
illustrated for the zero-symbol in Figure 7.
SWRS041B
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