The ISM Revolution: The Next Big Thing

February 09, 2009

By Iboun Taimiya Sylla, Texas Instruments

Editor's Note: This is the first part of a planned three-part series of articles from Iboun Taimiya Sylla. The second part is called: To ZigBee or Not to ZigBee? Factors to consider when selecting ZigBee Technology .

Imagine a farmer in the American Midwest facing the challenges of tracking the temperatures of 1000 head of cattle daily in order to prevent some animal diseases such as foot and mouth disease from decimating his herd. With wireless technology, such challenges can be easily overcome by simply attaching a temperature sensor equipped with a wireless transmitter on each cow, transmitting its reading to a main terminal. Such a method helps save time and the costs of dispatching crews for frequent and more often than not, unnecessary measurements. This illustrates the level of penetration of the low-power wireless devices operating in the ISM frequency band into everyday life (security, medical, industrial, agricultural, etc.). This penetration is being driven by three main factors:

1. The desire to get rid of hardwired communications that are otherwise required for transmitting data over a long range.
2. The allocation of the Industrial Scientific & Medical (ISM) frequency band by the regulatory bodies of various countries.
3. The emergence of different wireless standards to offer interoperability in the ISM band.

Eliminating Wire Wherever and Whenever Possible
For a long time, hardwired communication has been the most reliable way of transmitting or receiving information between two points. Wired communication systems have been able to provide reliable transmission media as well as high speed along with a long life. While presenting many qualities, the wired solution presents limitations that tend to make it obsolete in favor of wireless technologies. Among these limitations are:

• Geographic: Depending on geography and terrain, wire becomes very challenging to install, especially in rural mountainous areas.
• Economic: The cost of the wired system is proportionally related to the length of wire required as in some cases repeaters are needed to compensate for the loss of signal strength. This implies that more cable translates into more costly solution.
• Comfort: When looking at today's consumer desires, dragging wire across certain places is highly undesirable. Therefore, wire systems are being considered as the last choice for consumers.

These three main limitations of wire transmissions explain the momentum gained by the wireless technology.

The Industrial Scientific & Medical (ISM) Band
The ISM band is a general purpose part of the radio spectrum that can be used without a license. The only requirement for developing products in the ISM band is compliance with rules governing this part of the frequency spectrum. These rules vary from country to country. In the US, the Federal Communication Commission (FCC) defines these rules, whereas ETSI is the governing body in Europe. Table 1 illustrated how FCC and ETSI have categorized devices functioning in the ISM band.

Table: 1 FCC and ETSI device Classifications

Systems designed in the ISM band are characterized by their low-power and low data rates. However, in recent years, data rates have been increasingly higher, challenging the designation of low data rates. Mostly used ISM bands are the 2.4GHz band and the sub-1GHz bands. Because of the cluttering in the 2.4GHz bands, some activities have been seen in the 5GHz band, but they remain very limited because of achievable range concerns. While the 2.4GHz is universal, the sub-1GHz bands allocated to the low-power wireless application vary from country to country. In the United States the most popular band remaining is the 902 " 928MHz band, whereas in Europe most activities are in the 868MHz. Understanding the fundamental differences between the 2.4GHz and the sub-1GHz band is an important factor when developing products in the ISM band.

The 2.4GHz band is recommended when interoperability with other systems is required as well as operation in different geographical spaces is a key target. Designing in the 2.4GHz presents two main challenges:

1. Numerous wireless systems such as Bluetooth, Wi-Fi, 802.15.4, Zigbee and Microwave ovens operate in this band. Therefore, high interference levels pose a formidable challenge. The presence of these interference sources requires high frequency selectivity devices to ensure a good wireless link quality. Another efficient way to counter interference is to use techniques such as frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) that provide more significant noise immunities.
2. The second challenge of choosing the 2.4GHz lies in its achievable range. The 2.4GHz frequency tends to be absorbed more readily by the environment and surrounding objects, limiting the range. The rule of thumb is doubling the frequency of operation reduces the range by one half. It is worth noting that the range limitation can be overcome with the use of an external power amplifier (PA).

Choosing to design in the sub-1GHz band helps solve some of the issues faced in the 2.4GHz band; however, the sub-1GHz has its own limitations such as:

• The duty cycle in this band is restricted.
• Impossibility of achieving interoperability with other systems.
• Geographical limitation in the operation, for example: a wireless meter designed in the 902 " 928 MHz band for the US will not be able to operate in Europe.

Different Standards in the ISM Band
The last few years have witnessed the emergence of several wireless standards operating in the ISM band. These standards, along with proprietary solutions provide huge opportunities for developing a wide range of wireless products. These standards differ from each other by their data rates, communication ranges, application domains, as well as the modulation techniques used. Figure 1 illustrates the range versus data rate of several wireless standards.

1. Wireless Standard Operating in the ISM Band

Among the wireless standards cited on Figure 1, Bluetooth, Wi-Fi, Zigbee and IEEE 802.15.4 can be considered as the most prominent today. Most of these standards are operating in the 2.4GHz band.

1. Bluetooth: This technology is based on the IEEE 802.15.1 standard. It is a wireless technology that enables devices to communicate in the 2.400 " 2.4835 GHz band. Bluetooth allows devices such as mobile phones, PDAs, printers, laptops and headsets to exchange data. It uses the Gaussian frequency shift keying (GFSK) type of modulation along with frequency hopping spread spectrum (FHSS). Three output power levels are available in the Bluetooth standards. Classes 1, 2 and 3 devices deliver 20dBm, 4dBm and 0dBm of output powers respectively. Recently, another variant of Bluetooth called Bluetooth Low Energy has been introduced by the Bluetooth SIG. Bluetooth Low Energy targets data exchange using lower power consumption than the earlier Bluetooth versions.
2. Wi-Fi: As of today it represents the most prominent technology for wireless connectivity for computers and internet. Wi-Fi technology integrates most personal computers, PDA and other devices such as gaming and portable audio devices. Wi-Fi term is applicable to wireless devices that utilize the suite of IEEE 802.11 standard. Excepted 802.11b, WI-FI standard operates in the 2.4GHz band (2.4GHz " 2.4835GHz) and use FHSS and DSSS techniques. One area of concern of the 802.11 technology is the security of the network, as WLAN network can be penetrated by a third party.
3. IEEE 802.15.4: Compared with Bluetooth and WI-FI/802.11, IEEE 802.15.4 targets low data rate application within the 868MHz, 915MHz and 2.4GHz bands. The number of channels and the data rates used in this standard vary with the chosen frequency band. The most popular frequency band is the 2.4GHz with 20 available channels with a maximum data rate of 250kbps. The primary target application of this standard is home automation, remote metering, gaming and wireless sensors networks. One key feature of the IEEE 802.15.4 standard is its low-power consumption ability, providing a long battery life (10 to 20 years).
4. Zigbee: Built on top of the IEEE 802.15.4 PHY layer, Zigbee is a standard that utilizes the 802.15.4 standard. The 2.4GHz band remains the most used frequency band for Zigbee. To resolve the range and interference issue faced in the 2.4GHz, some companies are exploring the design of 915MHz Zigbee products. Unlike IEEE 802.15.4, Zigbee allows full mesh network. The announcement by utility companies of the deployment of several millions of Zigbee-based electric and gas meters has built a tremendous momentum for Zigbee and its smart metering applications.

In addition to the wireless standards presented above, the wireless industry is experiencing the emergence of several new standards that are in early development stages. On the other hand, many applications still remain proprietary as companies are concerned about compatibility with legacy products.

The world is experiencing a wireless revolution that is democratizing this very practical technology. Its use in all segments of life is not without problems for governing regulatory bodies that are at the center of more solicitations to make new frequency bands available. A careful and well thought out frequency spectrum management is required.

The second challenge highlighted by this revolution is the need of inventing new techniques to handle more interferers resulting from the crowdedness of the frequency spectrum.

About the Author
Iboun Taimiya Sylla manages business development for Low-Power RF products at Texas Instruments. Prior to his current position, Iboun was a Sr. RF Design Engineer. Iboun received his Bachelor in Telecommunication Engineering at ESPT, University of Tunis, Tunisia). He received his Master and Ph.D in Electrical Engineering from Ecole Polytechnique de Montreal, University of Montreal, Canada. He also holds a Master's in Business Administration with focus on Corporate Finance and Strategic Leadership from the University of Dallas, Texas.

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