Orthogonal This allows simultaneous transmission of data at

Orthogonal frequency-division
multiple access (OFDMA) is a multi-user
version of the orthogonal frequency-division multiplexing (OFDM) scheme. Multiple access is achieved
in OFDMA by assigning different subcarriers to individual users. This allows
simultaneous transmission of data at a low-data rate from several users.

Cognitive radio (CR) is a form technology in which a transceiver can smartly
detect within the spectrum, which communication channels are being used and which
are free, and instantly use the empty channels. This optimizes the use of
available radio-frequency (RF) spectrum while minimizing interference to other

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In the scheme that uses OFDMA based Cognitive Radio Networks the transmission is performed in 2 consecutive slots,
which give eavesdroppers two opportunities to eavesdrop. In the first time
slot, the Primary Base Station (PBS) transmits to the Primary Users (PUs) and
during this period, the eavesdroppers listen. In the second time slot, SUs
transmit to the secondary base station (SBS), and therefore, the eavesdroppers
listen. In this model, the SUs are allowed to use the additional spectrum of
the PUs as long as they assist the PUs to satisfy their secrecy rate demand.

we propose a secure cooperative
communications scheme for orthogonal frequency division multiple access (OFDMA)
base cognitive radio networks (CRNs) wherever a primary base station (PBS)
needs to transmit information to some distant primary users (PUs) within the
presence of a collection of passive eavesdroppers. The scheme will be simulated
using MATLAB. The proposed scheme will use a ‘decode and forward’ approach
rather than an ‘amplify and forward’ approach which is less secure. The model
will use the maximum-ratio
combining (MRC) technique
which is a method of diversity combining in
which the signals from each channel are added together, the gain of each
channel is made proportional to the RMS signal level and inversely proportional
to the mean square noise level in that channel. This improves the signal
coverage area while providing secure communication.