Digital Signal Processing Reference
In-Depth Information
1.3.1 Spectrum: A Scarce Resource
Like water, air or oil, the wireless spectrum is a shared resource. This, however, does
not imply that it should be free! Wireless spectrum needs to be shared with many
applications, and, hence, it has become expensive. To deliver the required expensive
service, regulatory bodies are using a fixed frequency allocation scheme, out of fear
of harmful interference, which jeopardizes the quality of the delivered service. They
allocate spectrum blocks for multiple years and over large areas. The only exception
to this allocation scheme, is the ISM band, where heterogeneous devices can coexist
using high-level Listen-Before-Talk (LBT) etiquettes.
Due to this fixed frequency allocation scheme, along with the accelerated de-
ployment of broadband communication systems, spectrum is, hence, becoming a
major bottleneck. New applications require more and more spectrum, but no useful
spectrum is apparently left to be allocated (see upper part of Fig.
1.2
).
However, experiments show that up to 85% of the spectrum remains unused at
a given time and location, indicating that a more flexible allocation strategy could
solve the spectrum scarcity problem [8]. We have confirmed this through measure-
ments using the IMEC Scaldio [9] (see bottom part of Fig.
1.2
).
This inefficient use of spectrum and the success of the ISM bands has coerced
regulatory bodies into defining a roadmap towards more local, organic and dynamic
spectrum sharing policies [10-12].
1.3.2 The Opportunistic Spectrum Access Solution
As mentioned above, a need for policies to share spectrum dynamically exists.
A prime example of such a flexible policy is Opportunistic Spectrum Access (OSA),
where users can actively search for unused spectrum in licensed bands and commu-
nicate using these
white holes
. OSA is supported by regulatory bodies, such as the
Federal Communications Commission (FCC) [10] and the European Commission
(EC) [12]. The concept is also often ambiguously referred to as Cognitive Radio
(CR). We discuss the ambiguity of this terminology in Sect.
1.4
.
The ultimate goal of the roadmap for more local and organic spectrum policies,
is called Dynamic Spectrum Access (DSA). In DSA networks licenses and priori-
ties are not fixed at Design Time (DT). DSA networks should allow terminals and
technologies to negotiate the use of wireless spectrum locally for a time window of
hours, minutes or even seconds. Like the SR, which presents the ultimate flexible
radio, DSA is not yet in sight. However, OSA is a first disruptive step towards this
ultimate goal.
