Wireless Sensor Networks : Technology, Protocols, and Applications

AVAILABLE WIRELESS TECHNOLOGIES 103
digital modulation schemes in terms of their efficiency as measured by the bit rate per
baud (in this context, 1 baud equates to 1 hertz). Typically, in wireless communication
it is desirable to maximize the bandwidth efficiency; in traditional wireless communication,
sophisticated (high-complexity) modulation methods are used to maximize
the link throughput. For example, 64-point quadrature amplitude modulation
(64 QAM) is used in WLANs operating with the IEEE 802.11a to achieve 54 Mbps
throughput in a 20-MHz channel. High efficiency, however, comes with a price: first,
the circuit complexity goes up considerably; second, the power consumption increases
when one targets a high channel throughput. As might be expected, high throughput
and efficiency are also desirable in WSNs; however, a trade-off between efficiency
and power must be accepted: Schemes that support high efficiency require complex
designs (read ‘‘high-count transistor chipsets’’) and fairly high power consumption.
Research has shown that advanced modulation results in degraded energy efficiency
for systems operating with short packets and/or a low duty cycle [4.5].
Spread-spectrum modulation techniques have a higher effective signal-to-noise
ratio than narrowband techniques, but require more channel bandwidth. Directsequence
spread spectrum (DSSS) is one of the two common spread-spectrum techniques
(it being used, for example, in commercial implementation of the WLAN
standards, including ZigBee). Frequency hopping spread spectrum (FHSS) is the other
technique (it is used in the Bluetooth environment for PANs). In DSSS, the incoming
data stream is hashed by a pseudorandom sequence that generates a sequence of output
microbits or chips that are distributed across the underlying broadband channel. To
the casual eye, these distributed microbits appear like noise. Fairly complex digital
signal processing functions are needed to recover the original signal; processing
must occur at the chip rate, and timing synchronization of all the nodes in the system
must be within a fraction of the chip interval (which is the reciprocal of the chip rate).
Compared to DSSS systems, FHSS uses relatively low complexity baseband
hardware. The synchnonization mechanism is also less complex; however, agile
frequency hopping requires fast signaling settling. There are prima facie advantages
in the use of FHSS for WSNs (e.g., improved multipath performance can be
achieved with FHSS); however, the requirement for low-power operation and the
widebandnature of operation gives rise to practical engineering challenges.
Appendix A provides some additional information related to modulation.
4.3 AVAILABLE WIRELESS TECHNOLOGIES
As we noted in passing in Chapter 3, two frequency bands are typically used by
WNs: the ISM band and the U-NII band. As we just described, indoor and outdoor
interference arises from both natural sources and/or phenomena (e.g., loss or
attenuation, absorption, fading, multipath) as well as from other users in proximity
utilizing these ‘‘unprotected bands.’’ A WSN will experience interference whether it
uses one of the IEEE PAN/LAN/MAN technologies or even some other generic
radio technology. For example, as noted above, other devices, such as Bluetoothbased
PDAs and cellular phones, which share operating frequencies with wireless