الفهرس 
Fundamentals of OFDM SystemsOnly 14 pages are availabe for public view
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Abstract
Orthogonal frequency division multiplexing (OFDM) has been suggested as a
candidate for multicarrier-based CR systems. One problem that appears in OFDM
system is peak to average power ratio (PAPR). In our research , we focus on this
problem and discussed some techniques to make reduction to PAPR in case of
AWGN in OFDM system .One of these techniques is the clipping and
companding. The rotated companding will be made as proposed technique for
reduction .Evaluation of spectrum sensing will be investigated from through some
parameters such as signal to noise ratio (SNR) and number of cognitive users .The
critical challenging issue in spectrum sensing is the hidden terminal problem. To
address this issue ,cooperative spectrum sensing performance with various fusions
will be discussed .combining cooperative relaying with spectrum sensing in
cognitive radio networks will be illustrated .Where, Evaluation amplify and
forward and decode and forward will be made . The details of these contributions
are listed below:
Proposed scheme will be introduced, combining two techniques which are
companding and rotation .the effects of changing main parameters of
companding and angles of rotation will be studied. This scheme will be
compared with the system that uses other techniques for reduction which are the
clipping technique alone and companding technique alone. By using proposed
scheme, the PAPR of OFDM signal can be reduced by 7.9 dB over the original
system, i.e. without PAPR reduction.
The performance of cooperation based spectrum sensing scheme will be
studied for OFDM based cognitive users using energy detector method. The
effect of variable channel gain on the cooperation between secondary users will
be investigated. Moreover, the effect of fusion rules will be made. . AWGN will
be assumed .The improvement using OR rule with cooperative scheme is better
than using AND rule. This is because OR rule is very conservative for CRs to
access licensed band.
Amplify and forward (AF) will be considered ,which achieves diversity by
using Maximal Ratio Combining (MRC) and Fixed Ratio Combining (FRC).We
can conclude that using both of the direct any relay channels, the transmissions
performance of the secondary system can be improved significantly .Moreover,
exploiting cooperative spectrum sensing technique for applications in a relay
based cognitive radio network, it is seen that detection probability increases by
using relay channel .the best performance was achieved when the relay is at
equal distance from the sender and the destination or slightly closer to the
sender.
Decode and forward (DF) will be discussed, which achieves diversity by
using Convolutional Code (CC) and Low Density Parity Check Code (LDPC).
Comparisons between Convolutional code for different rates will be made under
using AWGN. The effect of the positions of the relay will be studied. The best
performance was achieved when the relay is at equal distance from the sender
and the destination or slightly closer to the sender in the case of MRC and FRC.
BER performance for LDPC is better than CC in the case of decode and forward
relaying mode. Exploiting the cooperative spectrum sensing technique for
applications in relay based cognitive radio network, it is seen that detection
probability increases by using relay channel.