RTC Magazine

Installing sensors for data acquisition has traditionally been a labor-intensive process, involving analog or network wiring from sensor locations to aggregation points. The high cost of installation, maintenance and configuration changes limits the use of such sensors to high-value applications.  

Recent progress in wireless technology has made it possible to replace these older wired networks with wireless sensor nodes. In wireless sensor networks, installation labor is limited to the placement of the sensors themselves; node configuration is largely automated and can be managed from a remote network server. Maintenance may consist only of battery replacement once every few years, in the case of battery-powered sensors. Sensors using energy harvesting techniques may require no scheduled maintenance at all.  

To implement such networks requires physically small, energy-efficient, computationally capable wireless devices. Today, such devices can be implemented at low cost using advanced System-on-Chip (SoC) designs, in which advanced microprocessors, SRAM and non-volatile memory, local data interfaces and fully capable wireless modems are combined on a CMOS chip. A complete sensor node can be constructed from the SoC, with the addition of the sensor itself, a battery or other power source, an antenna, and in some cases an optional RF power amplifier. 

As with any system based on digital communications, a protocol is required for such wireless nodes to communicate with back-end devices so that the data can be collected and accessed. Today’s wireless sensor designs use both proprietary and standardized communications protocols. Popular choices for this purpose include IEEE 802.11, often referred to as Wi-Fi, and IEEE 802.15.4, often referred to as Zigbee, though that term should properly be reserved for the Zigbee ad-hoc networking layer that operates over the 802.15.4 physical and link layers.  

Sensor Network Design and Architecture

Wireless links are more flexible but more complicated than wires. The ability of two devices to communicate depends on a number of factors: 

• signal power sent by the transmitter.
• carrier frequency.
• signal power required at the receiver for reliable decoding.
• distance between transmitter and receiver.
• antennas and antenna configuration.
• the nature of any objects or obstructions in the signal path.
• interfering transmissions from collocated wireless devices.
• required instantaneous and average data rates.
• communications protocols in use.

For example, the GS1011 SoC CMOS transmitter (Figure 1) can deliver about 9 mW (9 dBm) of output power in the 2.4 GHz ISM band. In an outdoor line-of-sight environment with a moderate-gain receive antenna, this signal power is sufficient to maintain reliable links at distances of hundreds of meters. In a typical indoor environment, range is extremely dependent on the placement of the node and receiving antenna, and the exact layout of the building walls and interior objects: the same setup that yields a range of 300 meters outdoors may be limited to 30 meters indoors. The expected range determines how far sensor links can be from an aggregation device, such as an access point (Wi-Fi) or full-function device (802.15.4). Wi-Fi devices operate at 2.4 GHz. The 802.15.4 protocol allows for operation in the 868 and 915 MHz bands. Lower fre-quencies suffer less from absorption and diffraction by obstacles, but have correspondingly less bandwidth available for use. It is also more difficult to fabricate efficient small antennas when the wavelength of transmission increases.  

Interfering transmissions on the same or nearby bands may also limit the ability of a sensor node to communicate. Most interferers are time-dependent, so protocol adaptation may be used to communicate successfully in spite of interference. For example, the 802.11 protocol uses a carrier-sense multiple access with collision avoidance (CSMA-CA) approach. The modem tracks other received Wi-Fi transmissions and postpones initiating a transmission until it is expected that other stations have completed their messages, using the network allocation vector (NAV) contained in each packet. When the medium is believed to be clear, the node attempts to send a packet; if it does not receive an acknowledgement (ACK) from the destination, typically an ac-cess point, it waits a random backoff time and tries again. Wi-Fi can provide robust communications, except at very high traffic levels, and requires no central coordination or reservation system.