Michael Armentrout, Product Marketing Manager of Wireless Embedded Systems at Silicon Labs explores the complexities of the worldwide wireless infrastructure and offers a simplified route to greater connectivity
Wireless connectivity continues to expand quickly across all industries as wider varieties of electronic devices and appliances add wireless controls and communications. Consumers are used to the convenience that these RF additions offer, and wireless functionality is becoming a basic requirement for most electronics products. However, implementing wireless connectivity is seldom easy. RF considerations, such as antenna design and modulation settings, can be very different from the requirements of traditional on-board controls. Additionally, regulatory standards and consumer expectations mean it is essential to achieve good wireless performance. Most inexpensive wireless solutions use very basic components that have poor performance and are difficult to implement. Solutions that are simple to implement often don’t have the flexibility needed for good performance, and these seemingly simple solutions can also be costly. Fortunately, it is possible to find wireless solutions that offer high performance, ease of use and low cost, if you know what to look for.
Frequency Usage: Regulatory bodies in each country or region set the most basic performance requirements for wireless systems. These agencies regulate the frequencies on which a given application may operate, the magnitude of radiated emissions and, in many cases, the timing of the transmissions. The specific standards vary widely since each region has allocated the available radio spectrum differently. In the U.S., most applications using an unlicensed band will be operating under FCC Part 15.231 (260-470 MHz), FCC Part 15.240 (433.5-434.5 MHz) or FCC Part 15.247 and 15.249 (902-928 MHz). In Europe, these applications must comply with ETSI EN 300-220 (138.2-138.45 MHz, 433.05-434.45MHz and 863-870 MHz). Elsewhere, different local standards, such as ANATEL in Brazil and ARIB in Japan, will apply.
Unlicensed bands around the world
Transmit Power Limitations: The main requirements of wireless system standards are radiated power versus frequency and the limitation of transmitted power of intended signals and out-of-band spurs and harmonics. The requirement for the main signal is often specified as both a maximum average power level and a maximum peak power level. For example, FCC Part 15.240 specifies maximum average field strength of 11mV/m at 3m but allows up to 55mV/m at 3 m peak field strength. This field strength permits the device to transmit at higher peak power levels if it is duty cycled. Some standards take this requirement even further and set specific guidelines for the periodicity of the transmission or for frequency hopping schemes.
Out-of-Band Restrictions: The out-of-band performance restrictions are much more stringent. Each standards body provides a limit for any transmitted power that goes outside of the designated band. These restrictions can vary by frequency depending on the importance of the potentially blocked application. The FCC provides a list of restricted bands and allows a maximum of 500µV/m field strength at 3m (equivalent to -41dBm EIRP) above 960MHz and 200µV/m at 3m (equivalent to -49dbm EIRP) below 960MHz inside these bands. All other frequencies must maintain at least 20dBc suppression unless using Part 15.249, in which case the -41dBm and -49dBm limit applies to all spurs and harmonics. For any application operating in the 902-928MHz band, this means that any near-band spurs and the third harmonic can be particular areas of concern. In all regions, out-of-band transmitter performance will be one of the major design considerations.
The demands placed on wireless systems by consumers can be even more stringent than the regulatory requirements. Having portable, reliably connected wireless devices is now taken for granted, and consumers assume that any wireless product they buy will continue to operate under any environmental conditions, regardless of RF interference sources or physical barriers. They also expect that each new generation of wireless-capable products has longer range and longer battery life than previous products. The result is an increasing demand for selectivity, range and efficiency.
Receiver Selectivity: The selectivity of the receiver determines how well it can distinguish the desired transmission from other signals and noise in the area. Achieving good performance generally requires either the addition of filtering or the use of a device with high selectivity and blocking specs. Additional filtering is the least desirable of these two options since it adds both cost and loss to the system, reducing the sensitivity of the receiver.
Range: Range is another important parameter. Having sufficient range to effectively pass through walls and other barriers is usually very important, even when having a long line-of-site range is not critical. Achieving good range depends on the antenna gain, receiver sensitivity and transmitter power. The antenna gain is usually limited by cost and device form factor. Most consumer wireless applications use a PCB trace antenna for cost, but these have poor gain (often less than -20dB) compared to other antenna options. Receiver sensitivity is another parameter affecting range and is the best variable to optimise if possible. This sets the lower limit on power that can still be received and understood; thus, finding a receiver with both good sensitivity and selectivity makes it easier to achieve the desired range requirements. This approach also provides more flexibility and design margin for the antenna design and transmitter settings. In addition, range is determined on the transmitter side by the output power level. However, there may be restrictions on how high this can be set. Increasing the transmitter power also increases the current consumption, which can have a negative effect on battery life. Additionally, regulatory standards limit the allowed output power.
Power: Finally, consumers care a great deal about battery life, so the device’s power efficiency is critical. To maximise power efficiency, both active and shutdown current are important considerations since most applications employ some form of duty cycling. In applications that spend the majority of the time off, shutdown current can be even more important than active current.
Implementing a wireless design can be challenging given the requirements of consumers and various regulatory bodies and the complex decisions that must be made regarding hardware design and system configuration. Fortunately, highly-integrated wireless IC products, such as Silicon Labs’ EZRadio devices, can provide inexpensive, easy-to-implement solutions that offer the high performance needed to satisfy regulatory standards and consumer needs while providing on-chip features and development tools that make it easier to add wireless connectivity to virtually any embedded application.