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Emerging wireless communication for smart grid applications Chapter 5 181

standard exploit spread spectrum (SS) methods such as direct sequence SS
(DSSS) and parallel sequence SS (PSSS) to provide reliability to interference
and noise effects. In addition, these methods provide some advantages in terms
of interoperability. For example, IEEE 802.11 WLAN systems also serve in the
ISM frequency band similar to the IEEE 802.15.4 systems. Nevertheless, the
effects of WLAN systems on the IEEE 802.15.4 systems are taken into account
as a wideband interference effect because the systems exploit different spread-
ing codes and SS methods. If the Bluetooth (IEEE 802.15.1) systems in the ISM
band are taken into account, the IEEE 802.15.1 systems behave as a narrowband
interference source for other wireless systems because of the narrow-band struc-
ture of the IEEE 802.15.1 systems. Thus, the SS methods employed in the IEEE
802.15.4 based wireless communication systems provide durability to these
destructive effects.
The PHY layer of the standard contains a typical packet scheme to charac-
terize a general MAC interface. This packet is also defined as PHY protocol
data unit (PPDU) that comprises four main sections called preamble, packet
delimiter, PHY header, and PHY service data unit (PSDU). A typical packet
structure exploited in the PHY layer of the standard is depicted in Fig. 5.4.
The PHY layer of the standard supports various significant features such as
activation/deactivation of transceivers, link quality indication (LQI), energy
detection (ED), channel frequency selection and clear channel assessment
(CCA). These characteristics are summarized below.

l Activation/Deactivation of Transceivers: The transceivers compatible with
the IEEE 802.15.4 standard should support different operation modes
including transmitting, receiving and sleeping. The MAC layer controls
activation or deactivation of the transceivers. The turnaround time that
defines the required time among transmission and reception processes is
determined to be less than 12 symbol periods.
l Energy Detection (ED): The ED parameter is related to power level predic-
tion of received signal. In the ED process, there is no requirement for addi-
tional transactions such as demodulation, decoding or signal identification.
A period of eight symbols is specified as the ED duration in the IEEE
802.15.4 standard. The acquired ED result is used on the network layer

FIG. 5.4 PHY packet structure of IEEE 802.15.4 standard.