Wireless Sensor Networks : Technology, Protocols, and Applications

IEEE 802.15.4 LR-WPANs STANDARD CASE STUDY 177
868 MHz/
915 MHz
PHY
Channel 0 Channels 1-
2
868.3
902 MHz
928
2.4 GHz
PHY
Channels 11-
5 MHz
2.4
Figure 5.13
2.4835
IEEE 802.15.4 PHY-layer operating frequency bands.
These operating frequency bands are depicted in Figure 5.13. The spreading
code of the 868- and 915-MHz PHY layers is a 15-chip m-sequence. Both specifications
use BPSK with a differential encoding data modulation scheme. The data
rate of 868-MHz layer is 20 kbps while the data rat of the 915 MHz specification is
40 kbps. The chip modulation used by both specifications is BPSK with raised
cosine shaping (a ¼ 1.0). The resulting chip rate is 300 kilochips/sec for the 868-
MHz PHY layer and 600 kilochips/sec for the 915-MHz PHY layer.
The data modulation of the 2.4-GHz PHY layer is a 16-ary orthogonal modulation.
Consequently, 16 symbols are an orthogonal set of 32-chip PN codes. The
resulting data rate is 250 kbps (4 bits/symbol, 62.5 kilosymbols/sec). The specification
uses O-QPSK with half-sine pulse shaping, which is equivalent to minimum
shift keying. The resulting chip rate is 2.0 megachips/sec. The packet structure of
the IEEE 802.15.4 PHY layer is depicted in Figure 5.14. The first field of this structure
contains a 32-bit preamble. This field is used for symbol synchronization. The
next field represents the start of a packet delimiter. This field of 8 bits is used for
frame synchronization. The 8-bit PHY header field specifies the length of the PHY
service data unit (PSDU). The PSDU field can carry up to 127 bytes of data.
Figure 5.14
IEEE 802.15.4 PHY-layer packet structure.