Why Cognitive Radio?

Why Cognitive Radio?
The discussion on cognitive radio technology can best begin with the remark made by Ed Thomas,
former Chief Engineer of the Federal Communication Commission (FCC). “If you look at the entire
radio frequency (RF) up to 100 GHz, and take a snapshot at any given time, you’ll see that only 5
to 10% of it is being used. So there’s 90 GHz of available bandwidth.” This shows that the usage of
the radio spectrum is severely inefficient, and therefore the cognitive radio can be extremely useful
to exploit the unused spectrum from time to time, as long as the vacancy appears in the spectrum.
The radio spectrum, as regulated by the FCC in the United States (in a similar way in many other
countries also), is divided into channels which are usually licensed by individuals, corporations, and
municipalities as primary users. Most of these channels actively transmit only for the duration of a
small fraction of time. This is an inefficient use of the available spectrum. Clearly, if all those unused
spectra can be utilized, many more radio users can be accommodated without the need to create a
new spectrum.
Radio spectrum is one of the most important natural resources in the world today, and it is
necessary to build up a wireless information infrastructure. Insufficient radio spectrum has always
been a serious bottleneck for the deployment of a wireless information superhighway in the world. For
a long time, we have been resorting to three major strategies to accommodate growing radio/wireless based applications. First of all, we have been trying hard to persuade existing radio spectrum owners
or licencees to vacate their legacy radio applications (for instance, the terrestrial microwave relay
trunk systems) to make way for the deployment of newly emerging wireless services, such as mobile
cellular networks, and so on. Nowadays, almost all (if not all) legacy radio users in most developed
countries who can possibly be reallocated have been moved away from the prime spectrum sectors
(roughly from 800 MHz to 5 GHz bands). Therefore, this strategy for spectrum clearance will be
of little help in solving the problem with severe spectrum shortage. The second traditional way of
accommodating new wireless applications is to move the carrier frequency to new high spectrum
sectors, which have been occupied by very few radio applications. Those new high radio spectra
includes millimeter waves from 10–30 GHz bandwidth. The positive aspect of using a relatively
high frequency spectrum is the ease with which broadband applications where very high data rates
can be implemented are supported. However, the shortcomings of using a very high carrier frequency
are obvious. One of the most problematic issues is that radio propagation properties in very high
frequency spectra are very sensitive to rain, dust, water vapor, and other small particles in the air. In
other words, the radio transmission in very high frequency ranges will no longer be weather-proof.
Therefore, the outage rate will become unacceptably high under rain, snow, and/or other weather
conditions. Finally, the third approach used to support more radio applications in an already crowded
spectrum is to overlay/underlay the new wireless applications on top of existing radio services.
The second generation mobile cellular standard, IS-95A/B, which works on direct-sequence CDMA
technology, was initially proposed for the work on the 900 MHz PCS spectrum in North America to
overlay many existing radio applications. The success of the overlay operation is largely based on
relatively low power spread spectrum transmissions from the CDMA technology. Another example
of such overlay applications is the ultra-wideband (UWB) technology, whose bandwidth overlaps
with those previously allocated for GPS, radar, and satellite services. Therefore, a strict low power
spectral density (PSD) emission mask is necessary to control the UWB transmission power level
below a certain threshold in order to not interfere with them.
It is obvious that all the above three major strategies to introduce new radio applications on top of
an already very crowded spectrum chart cannot solve the problem. Therefore, the need to search for
a more effective solution to solve the problems of severe spectrum shortage has become imperative.
Cognitive radio technology was introduced for this purpose.
To have a real picture of the current radio spectrum allocation situation, the readers may refer to
the US Frequency Allocation Chart [792] (in this chart, all radio spectrum allocations from 3 kHz up
to 300 GHz are shown), which is available from the web site of US National Telecommunications
and Information Administration. Similar situations can be found in many other developed countries,
such as Japan and in Europe. The US Frequency Allocation Chart is shown in Figure 9.1, which is
too large to show all spectrum allocation details clearly within a page. We use it here just to give
readers an idea of what it looks like.
The justification to use cognitive radio technology on top of the existing spectrum licencees to
provide various wireless applications on a licensed exempt basis can be summarized as follows.
Firstly and as discussed above, an unused spectrum is not desirable. Users should not be allowed
to own a spectrum that they do not use. It is also recommended that allocated spectra should not be
underutilized. Whatever the reasons for not fully using the allocated spectrum (economic, historic,
or other systemic reasons), it does not represent the best and highest use of this valuable and scarce
public resource.
Secondly, layering more licensed allocations on top of existing allocations as a solution to the
underutilized spectrum does not, in many people’s view, increase the economic incentives for new
applications in these spectrum slots, since obtaining investor support required to build licensed
services becomes problematic when the economic history of a particular allocation in a particular
geographic area has shown little promise for significant profits. On the other hand, licence exempt
use can support business models which do not require large capital investment to roll out services
because of the low cost of unlicensed equipment and the lack of the high up-front costs of acquiring a spectrum at an auction (especially the case in the United States, Europe, and many other developed
countries). As a result, rural and other low population density areas could obtain services which would
otherwise be unavailable from the business entities which operate on licensed spectra and tend to
focus their investments on the larger, more profitable, urban and suburban marketplaces. For similar
reasons, community based networks and other not-for-profit groups could make use of otherwise
unused spectra to offer their constituencies innovative services and applications that would otherwise
be viewed as uneconomic, and, as a result, ignored by profit-oriented entities.
Thirdly, the assertions made by some people that licence exempt use interferes with business
opportunities flies in the face of the clear evidence that a vast amount of spectrum remains unused
because the high cost of rolling out licensed infrastructure is not justified on investment basis. Without
the opportunity to reclaim this spectrum in the public interest using cognitive radio technology under
licence exempt rules, this fallow spectrum would continue to be underutilized. In the broader context
of licence exempt sharing of licensed spectrum, it is widely believed that opportunities exist to apply
sophisticated cognitive radio technologies to recover otherwise underutilized spectrum for uses which
have significant economic and societal benefits without harming the interests of licensed services.
Finally, the current state-of-the-art radio technology has made it possible to implement a practical
cognitive radio in various wireless applications, such as wireless regional area networks (WRANs),
wireless metropolitan area networks (WMANs), wireless local area networks (WLANs), and wireless
personal area networks (WPANs), and so on, at a reasonable cost. Therefore, the radio terminals can be
given some intelligence to work automatically on the available frequency spectrum at any given time. In fact, a cognitive radio extends the functionality of a software-definable radio (SDR) to permit it to
react and adapt intelligently to its environment. It provides a central nervous system to communications
and computing platforms. This permits intelligent access and configuration by the radio devices.