25-08-2017, 09:32 PM
Spectrum Sensing in Wideband OFDM Cognitive Radios
Spectrum Sensing.pdf (Size: 566.49 KB / Downloads: 15)
Abstract
In this paper, spectrum sensing of an orthogonal frequency
division multiplexing (OFDM) based cognitive radio (CR)
is addressed. The goal is to identify the portions of the spectrum
that are unused by primary user systems and other CR systems,
called existing user (EU) systems altogether, with the emphasis on
conquering the challenge imposed by multipath fading channel.
The sensing of EU systems consists of two steps. In the first step,
the maximum likelihood (ML) estimates of the frequency bands
of EU systems are calculated; in the second step, detection is performed
at each suspected band to decide whether an EU system is
truly in operation. The idea is that an EU system appears at a segment
of continuous subcarriers. This fact can be exploited by employing
measurements at a continual subcarriers and executing the
sensing along the frequency domain
INTRODUCTION
RADIO spectrum is the medium for all types of wireless
communications, such as cellular phones, satellite-based
services, wireless low-powered consumer devices, and so on.
Since most of the usable spectrum has been allocated to existing
services, the radio spectrum has become a precious and scarce
resource, and there is an urgent concern about the availability of
spectrum for future needs. The solution to the spectrum scarcity
problem is dynamically looking for the spectrum “white spaces”
and using them opportunistically. Cognitive radio (CR) technology,
defined first by Mitola [1], [2], is thus advocated as a
candidate for implementing opportunistic spectrum sharing.
PROBLEM STATEMENT
Consider a wideband cognitive OFDM system with subcarriers.
It is required that the CR system identifies the portions
of the spectrum that are unused at a specific time. Thus, over
various frequency bands, sensing of primary user systems and
other CR systems in operation is performed regularly. Each of
the systems that have already been functioning within a band
of interest is called an EU system. The CR system performing
sensing is wideband in the sense that its bandwidth can accommodate
more than one EU systems.
When a sensing of EU systems is performed, the CR OFDM
system ceases transmission. The received signal is amplified and
frequency down-converted from the radio frequency (RF) under
sensing to the baseband. After analog-to-digital (A/D) conversion,
cyclic prefix removal and some necessary processing, the
output signal is passed through a -point discrete Fourier transform
(DFT).
CONCLUSION
In this paper, the problem of EU signal sensing in an OFDM
based CR system is addressed, where the number of EU systems
in operation and their frequency bands are all unknown.
Our main idea is that, once an EU system appears, several subcarriers
in a row are interfered. To exploit this fact, observations
along the frequency domain are employed. The emphasis of this
work is on combating the challenge resulting from a severe frequency
selective fading channel. We use an AR model to track
the variation of the received EU signal strength along the frequency
axis. When a deep fade occurs at part of the band of an
EU system, tracking enables the sensing device to identify the
weak signal under deep fade as a part of the entire signal. We
also investigate the scenario of multiple antennas to enhance the
performance of spectrum sensing.