Electrical/Electronics

While often associated with home automation, the new ZigBee wireless data standard is making fast inroads into industrial, military, and aerospace applications. By supplying highly reliable, wireless mesh networking at very low cost, ZigBee enables improvements to traditional sensing and monitoring applications, and enables new applications that would otherwise be impractical.

Consider two starkly different examples of ZigBee deployments that illustrate its flexibility:

  • In a weapons storage facility management application, ZigBee-enabled sensors attached to missiles could provide close-range readings for status tracking. These systems can run on the same batteries for years without replacement.
  • In a homeland security application, first responders could quickly distribute ZigBee-enabled sensors to detect the presence of NBC (nuclear, biological, chemical) agents. The sensors could transmit data immediately and repeatedly to a command station through a self-forming mesh network.

ZigBee as a “Low-Power” Standard

ZigBee is generally described as being a standard for “low-power” wireless networking. Designed for wireless connections among even the most humble sensors and actuators, including devices that are meant to run on batteries lasting for years, ZigBee must be extremely power-conscious to meet its objectives. However, ZigBee’s low power consumption is rooted not in low RF power, but in a sleep mode specifically designed to accommodate battery power.

ZigBee radios can switch automatically to sleep mode when not transmitting, and remain asleep until they need to communicate again. When in sleep mode, the radio’s RF power rating is irrelevant; it’s only when transmitting that its RF power affects power consumption.

In the case of Cirronet’s ZigBee solutions, a radio with 100-mW RF power typically will consume 150 mA at 3.3 V when transmitting, compared to 25 mA at 3.3 V for a radio with 1-mW RF power. The 100-mW radio consumes six times as much power, but only when actively transmitting. As long as its low-noise amplifier is turned off, the high-power radio’s power consumption while sleeping is roughly equivalent to that of a low-power radio.

If the high RF power radio is awake and transmitting 0.05 percent of the time (once every 10 seconds), which is realistic for most sensing applications, the extra average power consumption is minimal — a 1500 mA/hr cell that would last approximately 3.3 years with a 1-mW RF power radio would last more than two years with a 100-mW RF power radio. As this illustrates, ZigBee radios with higher RF output ratings are still excellent candidates for battery power in many applications.

ZigBee radios with low RF power exist more for cost containment than for battery conservation, because they meet the needs of many applications without the added expense of an amplifier.

Figure 1: Adding RF amplifiers to ZigBee radios dramatically increases transmission range while adhering to the ZigBee specification.

All ZigBee chipsets have an RF power rating of ~1 mW; every 100-mW ZigBee radio features an additional RF power amplifier. While the incremental price increase for a 100-mW module over a 1-mW version is modest — typically about $10 per module at low quantities — it’s an expense that obviously should be avoided unless required by the application.

ZigBee radios with 1-mW RF power have an indoor transmission range of around 100 feet (30 m). This is ample transmission distance for applications where all nodes are in close proximity and transmission is generally unobstructed.

High-Power Zigbee: Range, Coverage, and Multipath

Many applications demand more than the range of a 1-mW RF output radio or require transmission to rise above the noise floor. In these cases, the cost of the RF amplifier is justified (see Figure 1).

The indoor range of 100-mW radios is around 300 feet, roughly three times that of a 1-mW radio. The difference is even more dramatic outdoors, where a 100-mW radio offers a line-of-sight range of around 3,000 feet — roughly 10 times that of a 1-mW radio.

The longer ranges make 100-mW ZigBee radios superior for applications in large indoor settings and in outdoor installations, where nodes are often at intervals that cannot be spanned by unamplified ZigBee radios.

The benefit of increased range in 100-mW radios is cost-significant in applications that rely on ZigBee’s mesh networking to cover a large area. Radio availability, or assurance against outages in mesh coverage, is a function of radio placement relative to each radio’s transmission range. Each radio’s range represents the radius of a circle that defines transmission footprint. For continuous coverage of an area, each radio must be within range of the other radios with which it communicates in the mesh.

Figure 2: The importance of range to coverage in a rectangular mesh.

If the radius is 300 feet (high-RF-power indoor radio), the rectangular area of coverage for four radios is 810,000 square feet. If the radius is 100 feet (low RF power indoor radio), the rectangular area of coverage from four radios is only 90,000 — meaning that it would take nine times as many 1-mW radios to cover the same area as the 100-mW radios (see Figure 2).

For outdoor applications, the difference is extreme. The rectangular area of coverage for four high-power radios is 81 million square feet vs. 810,000 square feet for four low-power radios, meaning that it would take approximately 100 times as many low-power radios to cover the same area.

Multipath fading happens when terrestrial objects reflect a radio signal, causing the signal to reach its destination by multiple paths. When the signals arrive at nearly the same time but out of phase, multipath fading can cause cancellation. Higher RF power can compensate for multipath by ensuring that the most direct path delivers a strong signal that raises the received signal.

ZigBee Options: Sample Scenarios

Each wireless application’s need for range, coverage, and resistance to multipath fading determines whether low or high-RF-power is the better choice.

As mentioned earlier, one application for low-RF-power ZigBee radios is asset tracking at a weapons storage facility. Sensors attached to missiles stored at the facility are embedded with 1-mW ZigBee radios; the sensors identify each missile’s inventory data, status, temperature, etc., along with the status of the sensor/radio battery. At periodic intervals, the sensors are automatically polled with handheld devices that record the data. As all communications are in close proximity, 1-mW RF power is ample.

Figure 3: Multipath fading can impede wireless transmission; higher RF power can help.

Some applications feature a mixture of short-range and longer-range wireless links. When a large number of those links can be satisfied by low RF power radios, it can be beneficial to mix the two radio types. Consider an application for global military supply chains that tracks such assets as containers, air pallets, railcars, trucks, and trailers. Each entity can feature a 1-mW RF power ZigBee radio for relaying sensor data. When assets arrive at mobile choke-points, their radios can communicate with 100-mW ZigBee radios functioning as routers that forward the data over longer distances, typically to a ZigBee Ethernet gateway for delivery to a monitoring program.

Still other applications require all radios in a network to be of high RF power. In the homeland security application mentioned earlier, first responders can quickly populate an incident area by tossing multiple sensors around the area to detect the presence of NBC agents. The ZigBee radios should be all high RF power, so that devices can be placed farther apart and to ensure resistance to multipath fading due to buildings. This application depends heavily on ZigBee’s self-forming mesh networking capability to completely cover and monitor the incident area.

As the examples above illustrate, industrial, military, and aerospace applications benefit greatly from ZigBee high- and low-RF-power options that allow design engineers to tie radio selections directly to requirements for range, coverage, and multipath resistance. Along with ZigBee’s mesh networking and extremely low radio cost, RF power options should go a long way toward helping ZigBee advance as a leading technology for wireless data applications.

This article was written by Tim Cutler, vice president of marketing for Cirronet, Duluth, Georgia. For more information, visit http://info.ims.ca/5655-401.

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