Selecting the appropriate wireless data communication solution for connecting low-bandwidth electronic devices requires several considerations. These considerations may include range related issues, power requirements, 900 MHz versus 2.4 GHz transmission bands, interference immunity and time-to-market issues.
Achieving Long Range
Transmission range in a wireless data communication system is determined by link budget calculations. It is easiest to calculate link budget in dB. For example a 1 Watt transceiver introduced 30 dB into the link budget, which is 10 dB more than a 100 mW transceiver that introduces 20 dB into the system. Link budget provides the sum of dB that is present in the wireless link path, and is dependant on much more than the transmission power output of the module.
The overall link margin of a wireless data communication system includes transmission power output, antenna gain, receiver sensitivity and path loss (due to cable and antenna attenuation, air content and obstacles preventing line-of-sight conditions). Achieving long range with wireless transceiver modules requires an effective combination of output power, antenna gain and receiver sensitivity. Each of these specifications can have dramatic affects on the link budget of a wireless link path.
It would be simple to boost the output power and employ high-gain antennas to acquire the desired range, however, many regulatory agencies in the world place limits on transmission power output and total antenna gain allowed in a wireless link. In addition, many applications require compact size, portability, low power consumption and low cost from their wireless data communication solutions. Improving receiver sensitivity has proven to be a cost effective means for increasing range with out the overhead of high-powered and/or cumbersome antenna solutions.
The more link budget that is available, the more range a designer is able to achieve. As receiver sensitivity becomes more negative it introduces more dB into the link budget. Every -6 dB of receiver sensitivity effectively doubles communication range in line-of-sight conditions (-10dB in urban or indoor environments). The industry's receiver sensitivity average for wireless transceiver modules is -93 dBm. That means a wireless transceiver module with –105 dBm of receiver sensitivity will introduce 12 dB more into the link budget (than the industry's average module) allowing the wireless data communication link to communicate at four times the range in line-of-sight conditions and over twice the range in urban or indoor environments.
Low power consumption can be maintained in a wireless data communication system by employing greater receiver sensitivity. In the example located at the top of this section, a 1 Watt wireless transceiver module introduced 10 dB more into the link budget than a 100 mW module. However, when a 1 Watt wireless transceiver module with –93 dBm receiver sensitivity is range tested against a –109 dBm 100 mW module, the 100 mW module introduces 6 dB more into the link budget than the 1 Watt module, effectively doubling the range while using the current consumption of a 100 mW module.
Where range, low power consumption and low cost is critical to a wireless data communication system, designers are advised to find a transceiver solution that provides greater receiver sensitivity than the industry average of –93 dBm. When range is critical, but low power consumption and low cost is not critical, designers are encouraged to find the most effective and economical means for adding dB into their wireless links with a combination of greater receiver sensitivity, higher output power and high-gain antenna designs.
Radio frequency bandwidths also affect range and the regions of the world where wireless data communication solutions can be legally deployed.
900 MHz vs. 2.4 GHz
Wireless data communication devices typically utilize wireless transceiver modules that operate in the license-free Industrial Scientific and Medical (ISM) radio frequency bandwidths of 900 MHz and 2.4 GHz. Both of these bandwidths can benefit OEMs in different ways.
Wireless transceiver modules operating in the 900 MHz bandwidth offer up to twice the transmission range and penetrate obstacles (i.e. walls, buildings, trees, etc.) better than 2.4 GHz transceivers. While 900 MHz signals outperform 2.4 GHz signals, the 900 MHz band is only available in North America, South America, Australia, New Zealand, and Israel. On the other hand, wireless transceivers operating in the 2.4 GHz band are allowed for license-free communications throughout most of the world.
Designers that want to take advantage of 900 MHz performance, in the approved regions of the world noted above, would best be served by designing a pin-for-pin and software compatible system where 900 MHz and 2.4 GHz transceivers could be swapped based on the country of deployment. Where range is critical, it is disadvantageous to select one transceiver with inferior signal performance (2.4 GHz) for the sake of design consistency. For worldwide deployment of products, OEMs have the option to select manufacturers that offer 900 MHz and 2.4 GHz swappable transceivers.
Where transmission obstructions and interference may be encountered in different environments, some wireless transceivers are designed to penetrate obstructions and block interference to acceptable levels. These abilities allow wireless data communication systems to be more flexible than wired systems when used in portable applications.
Spread spectrum communications by design carry the added benefit of interference immunity. This occurs due to noise resistance capabilities (as in direct sequence spread spectrum - DSSS) or the frequent changes in the hopping sequence as the signal is moved throughout different frequencies (as in frequency hopping spread spectrum - FHSS). Some wireless transceiver modules offer additional interference rejection, or blocking, achieved through the use of proprietary filtering and communication across a more narrow band of hopping frequencies.
Other overlapping issues deserve careful consideration that affect the speed with which products can go through the development phase, onto production and into the marketplace. These issues include module versus chip/chipset design, development costs and regulatory agency approvals in each of the world regions where the wireless data communication products will be deployed.
When time and RF engineering experience is of abundance, a designer's may opt to use RF integrated circuits (chips or chipsets) to save on RF component costs. Using chips/chipsets, the designer actually develops the hardware and software workings of the product. While the individual chips/chipsets offer functionality the designer must dictate how those chips will work in concert with the software the designer will develop. This task is not for the faint of heart, as completed designs must also pass rigid regulatory testing for deployment in the various regions of the world. The regulatory approval process alone can become months or years of a cycle of rejection, followed by a reworking of the product and continued by rejection and reworking until the product is approved.
Wireless transceiver modules offer a faster time-to-market alternative to chips/chipsets that allow designers at all levels of RF experience to integrate a completed wireless system into their products. Many modules are manufactured as a drop-in solution where designers create a compatible pin-out on their processor board and supply serial data to the appropriate pins. Modules offering the easiest integration allow the designer to send raw UART data into the module and expect that same data out on the receiving end of the wireless link. Modules allowing the quickest time-to-market are also FCC and other regulatory agency approved. That means jurisdictions accepting FCC approval allow designers using FCC approved modules to bypass further testing for their wireless products. In Europe and other countries, pre-approved modules by ETSI and other regulatory bodies allow the designer to deploy products in various regions of the world with minimal additional approvals. The completed RF design and agency approval of many wireless transceiver modules make them a popular choice in the fast-paced world of wireless data communication product design.
As an example, Digi has a line of wireless transceiver modules that meet many of the performance and ease of use suggestions discussed. Their modules are all drop-in RS232/485 solutions that are FCC and other agency approved for use throughout the world. Integrating these wireless data communication solutions is as simple as sending serial data from a microcontroller or comm port, while the Digi module handles all of the complexities of spread spectrum transmission and reception. Of particular note is their ability to communicate at short or long range while consuming low power and maintaining high levels of interference rejection over transparent peer-to-peer, point-to-point, point-to-multipoint and multi-drop networks.
The Digi 9XStream 900 MHz Wireless OEM Module is a frequency-hopping spread spectrum transceiver that uses a simple UART interface to communicate with the host system. It has transmission power output of 140 mW and operates at long ranges (up to ¼ mile in urban environments and up to 20 miles in line-of-sight conditions) due to its excellent receiver sensitivity of –110 dBm. Additionally, the 9XStream uses proprietary filtering technology to provide interference immunity, including 70 dB of cell phone and pager rejection (10 million times attenuation). providing OEMs with a robust, reliable wireless method to transmit serial data.