Embedded wireless is simply the combination of an embedded process or system with a wireless interface for communicating. More specifically, these are the burgeoning systems enabling new forms of industrial, commercial and home building automation in addition to new capabilities and feature-rich products in consumer, medical and even agricultural systems. Power consumption, or rather the lack thereof, is a significant and in most cases, the driving requirement of all these low-data rate applications.
The measure of an embedded wireless application's power consumption, however, is not the simple sum of its parts as one might think. While this has been the typical means of comparing and selecting components for a given application, this basic means of obtaining a quantifiable metric to compare fails to adequately capture the true measure of how these components will interact and perform as a system. The focus must be on the wireless system's power consumption and how well a given wireless solution minimizes this costly resource. The often overlooked but equally important system attribute that drives down a wireless system's power consumption is reliability. Reliability, in this sense, is the measure of a system's ability to communicate data from point A to point B in a single attempt. This illustrates how reliability and power consumption in embedded wireless applications are related as well as how to optimize reliability and power efficiency.
Reliability as Related to Power Consumption
The most costly aspect in most embedded wireless applications, in terms of power consumption, is the transmit power of the transceiver components. There are many different transceiver components in the market today that when compared datasheet-to-datasheet offer relatively similar power ratings in the 20-30mA ranges'but if you stopped there and selected the lowest power rating of the components alone you might be overlooking the even more important system reliability attribute. Why does reliability matter? For a low-power application where every uA or mA counts, reliability is the singularly most important attribute in determining how often the application will remain at an active, high-power consuming state versus its extremely low-power, sleep state'the higher the reliability the less power is consumed. The ideal, perfect wireless system will transmit a set of data once and as fast as it possibly can to move data from point A to point B. Of course, no system will perfectly perform to this model all of the time and thus there will be retransmits due to interference or inadequate signal strength to reach remote end points'enter the quest for wireless reliability.
Wireless systems contain specific characteristics that can help qualify how well they will respond reliably in a given system such as RF spectrum usage'where, physically, in the RF spectrum do they communicate; receive sensitivity of the technology'how little do the transceivers need to hear in order to make out the communications, measured as a power ratio of decibels referenced to 1mW (dBm); output power'how loud can the technology communicate, ultimately talking louder than potential interference, measured in terms of dBm; RF agility'the measure of the ability of a technology to move and avoid interference in the RF spectrum, a corollary function of the RF channel size and the number of channels available; and finally, interference immunity'a RF technology's ability to communicate in a given channel despite interference as measured by an increase in receive sensitivity, also known as coding gain (dBm).
RF spectrum usage is a variable in the reliability equation dependent on environment due to the physical nature of RF waves. The lower the frequency, the larger the wavelength and thus the less prone to absorption by typical manufacturing materials such as liquids and reinforced concrete. That said, however, RF spectrum and its usage is a highly, governmentally regulated area of wireless communications to minimize interference with other wireless communications technologies. Only a few areas of the spectrum are reserved either locally or internationally for unlicensed use for these forms of communications, these are known as the Industrial, Scientific and Medical (ISM) band. Within this band, the predominant frequency that's accepted and used is the 2.4-GHz portion of the ISM band. In this frequency, though, the small wavelengths are quickly absorbed by the industrial space's hostile RF environment thus requiring even more focus on the remaining variables for measuring reliability.
Receive sensitivity, output power and even interference immunity can all be quantified together to form a larger, more important variable for defining reliability: link budget. Link budget is defined as the absolute value of the receive sensitivity plus output power and interference immunity. Therefore, the better the receive sensitivity, the larger the output power and the more interference immunity a solution has, the larger the link budget. The larger the link budget, the less prone the wireless solution will be impacted by RF absorption and interference and thus lead to greater potential for reliability.
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