Low-PowerWireless Sensor Network Platforms - Signal Processing Systems - page 383

Fig. 4 Sensor node hardware architecture

4.1 Communication Subsystem

The communication subsystem consists of a wireless transceiver and an antenna.
A wireless transceiver can be based on acoustic, optical or Radio Frequency (RF)
waves. Acoustic communication is typically used for under water communications
or measuring distances based on time-of-flight measurements [ 3 ] . The disadvan-
tages are long and variable propagation delay, high path loss, noise, and very low
data rate. Optical communication [ 55 ] has low energy consumption especially in
reception mode, and it can utilize very small antenna. However, the alignment
of a transmitter to a receiver is difficult or even impossible in large-scale WSN
applications. RF communication combines the benefits of high data rate, long range
and nearly omnidirectional radiation, making it the most suitable communication
technology for WSNs. Disadvantages are large antenna size and higher energy
consumption compared to the optical technology.
In general, an RF transceiver (radio) has four operation modes: transmit, receive,
idle, and sleep. Radio is active in transmit and receive modes, when power
consumption is also the highest. In idle mode, most of circuitry is shut down, but
the transition to the active mode is fast. The lowest power consumption is achieved
in sleep mode when all circuitry is switched off.
Most short-range radios utilized with WSNs operate in the 433 MHz, 868 MHz,
915 MHz, and 2.4 GHz license-free Industrial Scientific Medical (ISM) frequency
bands. The 2.4 GHz band is the widest providing more channels, while obstacles
have least effect on lower frequency bands. Depending on the frequency band and
antenna type, operating range with 1 mW transmission power is from few meters to
hundreds meters [ 25 ] .
The characteristics of potential commercial low power radios are summarized
in Table 3 [ 25 ] . Microchip, Nordic Semiconductor, and Texas Instruments utilize