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Extending range of wireless devices in IoT network

Posted: 04 Dec 2014     Print Version  Bookmark and Share

Keywords:Internet of Things  IoT  MCU  Wi-SUN  ZigBee 

While many existing sub-GHz networks use proprietary protocols, the industry is slowly adding standards-based, interoperable systems. For example, the IEEE 802.15.4g standard is gaining popularity worldwide and is being adopted by various industry alliances such as Wi-SUN and ZigBee. As with any standard, there are several specifications, mandatory as well as optional, and identifying the right parameters upfront helps with device selection. Now let's take a closer look at power efficiency and wireless range in the context of sub-GHz wireless networks.

Low power consumption
Engineers designing wireless solutions for power-sensitive and battery-powered applications must consider the standby current, low-power modes, and wakeup times of a wireless IC. Silicon Labs' EZRadio and EZRadioPRO sub-GHz transceivers are designed for such applications. They are energy-friendly wireless devices that consume 40 nA current in standby mode with full memory retention, and require only 440µs to wake up from standby/sleep to receive mode.

Autonomous features such as low duty cycle mode (figure 3) help to further reduce the average receive current consumption, especially in time-slotted systems. In this case, the radio automatically wakes up from sleep and enters receive mode based on a programmable, integrated 32kHz sleep clock.

The radio evaluates the channel and only wakes up the host MCU if a packet is found. The decision is typically made based on preamble detection or receive signal strength indicator. If there is no valid packet, the radio automatically returns to sleep mode without interrupting and activating the host MCU.

 Receiver autonomous duty cycling

Figure 3: Receiver autonomous duty cycling.

Three main factors determine the current consumption of a duty cycling application: the energy required to transition from sleep to receive mode, the time required to evaluate the channel for a valid packet, and the sleep mode current. The preamble sense mode of EZRadio and EZRadioPRO transceivers greatly reduces the channel access time with no degradation in sensitivity, while significantly reducing the average receive current.

These radios require only 8 bits of preamble to detect a valid preamble, compared to 32 bits in other traditional sub-GHz transceivers. The improvement in average receive current is greater for longer preamble lengths and lower data rates.

The power amplifier (PA) consumes the highest current in these sub-GHz transceivers, so an efficient PA design is also critical to achieve long battery life. EZRadio and EZRadioPRO ICs integrate an efficient +20 dBm PA that consumes only 85 mA, which is 40 mA lower than competing solutions. With a +10 dBm output power, the PA consumes only 18 mA, which enables coin cell battery operation.

Wireless range
One of the primary advantages of using sub-GHz wireless technology in any application is the long-range capability of this frequency band, even in obstructed conditions. Long-range systems reduce the cost of deployments as fewer base stations and repeaters are required to serve the same number of devices. Low frequency transmissions can travel longer distances for a given output power. Governed by the laws of physics, this phenomenon can be seen by using the Friis formula for path loss.

Equation

where Pr is the received power, Pt is the transmitted power, Gt and Gr are the antenna gains at the transmitter and receiver, R is the distance between antennas and λ is the wavelength.

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