Most of us have used short-range-wireless technologies in the form of point-to-point Bluetooth connectivity between mobile phones and headsets or between a PC and a wireless mouse. Short-range-wireless technologies typically have a range of 10 to 50m and data rates of less than 4 Mbps. These technologies enable a new concept, WiEC (Wireless Embedded Control). The philosophy of WiEC calls for the ubiquitous embedding of simple wireless transceivers in host systems beyond PC peripherals and consumer electronics. You can use these transceivers to report data or receive commands, creating networks out of otherwise-stand-alone machines. You can then use these networks to enhance the performance and the efficiency of member nodes.
WiEC-friendly transceivers vary based on data rate, range, occupied bandwidth, collocation ability, and immunity to interference. Local agency regulations for the transceiver band in which the transceiver operates directly impact some of these properties. Therefore, the choice of frequency band affects system performance. A wireless system may operate in a licensed or an unlicensed frequency band. Licensed bands have the advantage of guaranteeing a slice of spectrum dedicated to the wireless system, thus reducing the possibility of interference. However, licensing costs and regulatory certification procedures can significantly increase costs and time to market. Unlicensed frequency bands offer an alternative, but with a caveat. A wireless system can operate in an unlicensed band as long as it complies with restrictions on power output, spectral density, and duty cycle and simultaneously accepts potential interference from other devices in the same band.
But unlicensed bands are not all the same. Typically, the higher the center frequency of the band, the wider the band itself and, thus, the more devices it can accommodate. Conversely, free-space propagation of lower frequencies is typically better than that of higher frequencies, implying that a lower frequency wireless transceiver would have more range for a given RF output power.
The center frequency and bandwidth of unlicensed bands vary by local regulatory agency, which can cause headaches for companies that want to sell a product globally. But the 2.4-GHz ISM (industrial/scientific/medical) band is unique in that most regulatory bodies worldwide have adopted a center frequency of approximately 2450 MHz and bandwidth with sufficient overlap to allow for relatively easy global deployment. This adoption has had the positive effect of the proliferation of consumer and WiEC-type wireless devices but has caused severe spectrum crowding.
Nonetheless, because of international availability, bandwidth, and simplified regulatory requirements, the 2.4-GHz ISM band is possibly the most suited to WiEC applications. But this suitability presents a challenge to WiEC-type transceivers, because they must be able to coexist in a crowded spectrum. Wireless devices that inhabit the same unlicensed bands as WiEC devices provide two challenges. First, they may occupy more bandwidth than a WiEC-type transceiver and thus consume spectrum that WiEC networks can otherwise occupy. For example, an IEEE 802.11g access point occupies 22 MHz of spectrum, compared with a WiEC-type transceiver, which typically occupies 5 MHz or less. Second, these other wireless devices can be of much higher RF output power than a low-power WiEC transceiver, thus interfering with WiEC networks operating in nearby frequencies. For example, IEEE 802.11g access points can have power outputs as much as 100 times those of a typical WiEC transceiver.
Therefore, a WiEC transceiver must employ effective interference avoidance, including the ability to detect RF energy on a frequency, to recover from transmission errors, to provide automatic acknowledgment and retransmission, and to possibly provide spread-spectrum modulation. Furthermore, a WiEC transceiver must occupy minimal spectrum, thus enabling it to find a clear communication channel even in a busy RF environment.
Collocation poses another challenge. For WiEC to realize its full potential, the underlying radio technology must enable a large number of nodes to operate simultaneously. This ability is critical for applications such as environmental control in large buildings, inventory tracking in warehouses, and appliance control in dense housing units. A WiEC transceiver that occupies a small operating bandwidth would be appropriate because it would increase frequency diversity and possibly enable dozens of nodes to collocate in close proximity. A better transceiver would also employ some form of code diversity, as in the case of DSSS (direct-sequence-spread-spectrum) modulation, which can raise the number of collocatable nodes to hundreds instead of dozens.
We all stand to benefit from the added intelligence WiEC will bring to familiar systems. The challenge to system designers lies in selecting an underlying transceiver technology that will be easy to design in and to deploy globally.
WiEC-friendly transceivers vary based on data rate, range, occupied bandwidth, collocation ability, and immunity to interference. Local agency regulations for the transceiver band in which the transceiver operates directly impact some of these properties. Therefore, the choice of frequency band affects system performance. A wireless system may operate in a licensed or an unlicensed frequency band. Licensed bands have the advantage of guaranteeing a slice of spectrum dedicated to the wireless system, thus reducing the possibility of interference. However, licensing costs and regulatory certification procedures can significantly increase costs and time to market. Unlicensed frequency bands offer an alternative, but with a caveat. A wireless system can operate in an unlicensed band as long as it complies with restrictions on power output, spectral density, and duty cycle and simultaneously accepts potential interference from other devices in the same band.
But unlicensed bands are not all the same. Typically, the higher the center frequency of the band, the wider the band itself and, thus, the more devices it can accommodate. Conversely, free-space propagation of lower frequencies is typically better than that of higher frequencies, implying that a lower frequency wireless transceiver would have more range for a given RF output power.
The center frequency and bandwidth of unlicensed bands vary by local regulatory agency, which can cause headaches for companies that want to sell a product globally. But the 2.4-GHz ISM (industrial/scientific/medical) band is unique in that most regulatory bodies worldwide have adopted a center frequency of approximately 2450 MHz and bandwidth with sufficient overlap to allow for relatively easy global deployment. This adoption has had the positive effect of the proliferation of consumer and WiEC-type wireless devices but has caused severe spectrum crowding.
Nonetheless, because of international availability, bandwidth, and simplified regulatory requirements, the 2.4-GHz ISM band is possibly the most suited to WiEC applications. But this suitability presents a challenge to WiEC-type transceivers, because they must be able to coexist in a crowded spectrum. Wireless devices that inhabit the same unlicensed bands as WiEC devices provide two challenges. First, they may occupy more bandwidth than a WiEC-type transceiver and thus consume spectrum that WiEC networks can otherwise occupy. For example, an IEEE 802.11g access point occupies 22 MHz of spectrum, compared with a WiEC-type transceiver, which typically occupies 5 MHz or less. Second, these other wireless devices can be of much higher RF output power than a low-power WiEC transceiver, thus interfering with WiEC networks operating in nearby frequencies. For example, IEEE 802.11g access points can have power outputs as much as 100 times those of a typical WiEC transceiver.
Therefore, a WiEC transceiver must employ effective interference avoidance, including the ability to detect RF energy on a frequency, to recover from transmission errors, to provide automatic acknowledgment and retransmission, and to possibly provide spread-spectrum modulation. Furthermore, a WiEC transceiver must occupy minimal spectrum, thus enabling it to find a clear communication channel even in a busy RF environment.
Collocation poses another challenge. For WiEC to realize its full potential, the underlying radio technology must enable a large number of nodes to operate simultaneously. This ability is critical for applications such as environmental control in large buildings, inventory tracking in warehouses, and appliance control in dense housing units. A WiEC transceiver that occupies a small operating bandwidth would be appropriate because it would increase frequency diversity and possibly enable dozens of nodes to collocate in close proximity. A better transceiver would also employ some form of code diversity, as in the case of DSSS (direct-sequence-spread-spectrum) modulation, which can raise the number of collocatable nodes to hundreds instead of dozens.
We all stand to benefit from the added intelligence WiEC will bring to familiar systems. The challenge to system designers lies in selecting an underlying transceiver technology that will be easy to design in and to deploy globally.
0 Comments:
Post a Comment