Today’s wireless networks are regulated by a
fixed spectrum assignment policy, i.e. the spectrum
is regulated by governmental agencies and is
assigned to license holders or services on a long term
basis for large geographical regions. In addition, a
large portion of the assigned spectrum is used sporadically
as illustrated in Fig. 1, where the signal
strength distribution over a large portion of the
wireless spectrum is shown.
The spectrum usage is
concentrated on certain portions of the spectrum
while a significant amount of the spectrum remains
unutilized. According to Federal CommunicationsCommission (FCC) , temporal and geographical
variations in the utilization of the assigned spectrum
range from 15% to 85%. Although the fixed spectrum
assignment policy generally served well in the
past, there is a dramatic increase in the access to
the limited spectrum for mobile services in the
recent years. This increase is straining the effectiveness
of the traditional spectrum policies.
The limited available spectrum and the inefficiency
in the spectrum usage necessitate a new communication
paradigm to exploit the existing wireless spectrum
opportunistically. Dynamic spectrum access
is proposed to solve these current spectrum ineffi-
ciency problems. DARPAs approach on Dynamic
Spectrum Access network, the so-called NeXt Generation
(xG) program aims to implement the policy
based intelligent radios known as cognitive radios
.
NeXt Generation (xG) communication networks,
also known as Dynamic Spectrum Access
Networks (DSANs) as well as cognitive radio networks,
will provide high bandwidth to mobile users
via heterogeneous wireless architectures and
dynamic spectrum access techniques. The inefficient
usage of the existing spectrum can be improved
through opportunistic access to the licensed bands
without interfering with the existing users. xG networks,
however, impose several research challenges
due to the broad range of available spectrum as well
as diverse Quality-of-Service (QoS) requirements of
applications. These heterogeneities must be captured
and handled dynamically as mobile terminals
roam between wireless architectures and along the
available spectrum pool.
The key enabling technology of xG networks is
the cognitive radio. Cognitive radio techniques provide
the capability to use or share the spectrum in
an opportunistic manner. Dynamic spectrum access
techniques allow the cognitive radio to operate in
the best available channel. More specifically, the cognitive
radio technology will enable the users to
(1)
determine which portions of the spectrum is available
and detect the presence of licensed users when
a user operates in a licensed band (spectrum sensing),
(2) select the best available channel (spectrum
management),
(3) coordinate access to this channel
with other users (spectrum sharing), and
(4) vacate
the channel when a licensed user is detected (spectrum
mobility).
Once a cognitive radio supports the capability to select the best available channel, the next challenge is to make the network protocols adaptive to the available spectrum. Hence, new functionalities are required in an xG network to support this adaptivity. In summary, the main functions for cognitive radios in xG networks can be summarized as follows:
• Spectrum sensing: Detecting unused spectrum
and sharing the spectrum without harmful interference
with other users.
• Spectrum management: Capturing the best available spectrum to meet user communication requirements.
• Spectrum mobility: Maintaining seamless communication
requirements during the transition
to better spectrum.
• Spectrum sharing: Providing the fair spectrum
scheduling method among coexisting xG users.
These functionalities of xG networks enable spectrum-
aware communication protocols. However,
the dynamic use of the spectrum causes adverse
effects on the performance of conventional communication
protocols, which were developed considering
a fixed frequency band for communication. So
far, networking in xG networks is an unexplored
topic.
The xG network communication components
and their interactions are illustrated in Fig. 2. It is
evident from the significant number of interactions
that the xG network functionalities necessitate a
cross-layer design approach. More specifically, spec-trum sensing and spectrum sharing cooperate with
each other to enhance spectrum efficiency. In spectrum
management and spectrum mobility functions,
application, transport, routing, medium access and
physical layer functionalities are carried out in a
cooperative way, considering the dynamic nature
of the underlying spectrum.
This paper presents a definition, functions and
current research challenges of the xG networks.
