MAN/WAN sensor communications can occur over WiMax/hotspots or 3G systems. After a brief discussion of a brand-new (but speculative) technology, cognitive radios (CRs), in the remainder of the section we focus on the evolution of cellular networks in terms of the desire to provide a lateral data channel that supports any number of applications, including WSNs. Cognitive Radios and IEEE 802.22 With the plethora of wireless services that are becoming available, stakeholders believe that the limiting factor at this time is the
scarcity of radio spectrum. Studies have shown that most of this spectrum scarcity is concentrated in the unlicensed bands; this is where the major advancements in spectrum use have taken place (e.g.,Wi-Fi, cordless phones). Licensed bands, however,
typically experience considerable underutilization. CR-based approaches represent a new paradigm in wireless communications that aims at utilizing the large amount of underused spectrum in an intelligent way while not interfering with other incumbent devices in frequency bands already licensed for specific uses [4.43].
The IEEE 802.22 wireless regional area network (WRAN) standard is the first worldwide project to employ CR concepts for dynamically sharing spectrum with television broadcast signals. IEEE 802.22 seeks to develop a standard for a cognitive radio-based PHY–MAC–air interface for use by license-exempt devices on a noninterfering basis in spectrum allocated to the television broadcast service.
This standard specifies the air interface, including the MAC and PHY, of fixed point-to-multipoint wireless regional area networks operating in the VHF–UHF TV broadcast bands between 54 and 862 MHz. This standard is intended to enable deployment of interoperable IEEE 802 multivendor wireless regional area network products, to facilitate competition in broadband access by providing alternatives to
wireline broadband access and extending the deployability of such systems into diverse geographic areas, including sparsely populated rural areas, while preventing harmful interference to incumbent licensed services in the TV broadcast bands [4.44].
There is a large untapped market for broadband wireless access in rural and other unserved or underserved areas where wired infrastructure cannot be deployed economically. Products based on this standard will be able to serve those markets and increase the efficiency of spectrum utilization in spectrum currently allocated
to, but unused by, the TV broadcast service. WRAN supports an approach for operation over large, potentially sparsely populated areas (e.g., rural areas), taking advantage of the favorable propagation characteristics in the VHF and low-UHF TV bands. The unique requirements of operating on a strict noninterference basis in
spectrum assigned to, but unused by, the incumbent licensed services requires a new approach using purpose-designed cognitive radio techniques that will permeate both the PHY and MAC layers [4.44]. In principle, this wireless service can also be used to support metro-area WSNs. Cognitive radio—where a device can sense its environment and location and then alter its power, frequency, modulation, and other parameters so as to dynamically reuse available spectrum—is now just emerging. CR can, in theory, allow multidimensional reuse of spectrum in space, frequency, and time, obliterating the spectrum and bandwidth limitations that have slowed broadband wireless development
in the United States and elsewhere. This new technology is in a way similar to software-defined radio (SDR). With SDR the software embedded in a radio cell phone, for example, can define the parameters under which the phone should operate in real time as its user moves from place to place; traditional cell phone parameters,
by contrast, are relatively fixed in terms of frequency band and protocol. A SDR is a flexible wireless communications device that implements its signal processing entirely in software: Software radios can easily change such features as modulation, bandwidth, and coding, which are fixed in more traditional radios.
The basic technology of software radio is now being deployed in military and commercial applications. CR is even more advanced than SDR: CR, as noted, can sense its environment and learn from it [4.45]. The FCC is currently investigating commercial applications, and the Defense Advanced Research Projects Agency is proposing military applications (under the XG—or next-generation communications—
program). DARPA’s aim is to develop technology that allows multiple
users to share spectrum in a way that coexists with, and complements, sharing protocols included in today’s Wi-Fi technologies.
Work on CR and IEEE 802.22 is currently under way.
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