Orthogonal frequency-divisionmultiple access (OFDMA) is a multi-userversion of the orthogonal frequency-division multiplexing (OFDM) scheme. Multiple access is achievedin OFDMA by assigning different subcarriers to individual users. This allowssimultaneous transmission of data at a low-data rate from several users.
Cognitive radio (CR) is a form technology in which a transceiver can smartlydetect within the spectrum, which communication channels are being used and whichare free, and instantly use the empty channels. This optimizes the use ofavailable radio-frequency (RF) spectrum while minimizing interference to otherusers.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 timeslot, the Primary Base Station (PBS) transmits to the Primary Users (PUs) andduring this period, the eavesdroppers listen. In the second time slot, SUstransmit to the secondary base station (SBS), and therefore, the eavesdropperslisten. In this model, the SUs are allowed to use the additional spectrum ofthe PUs as long as they assist the PUs to satisfy their secrecy rate demand. Therefore,we propose a secure cooperativecommunications 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 thepresence of a collection of passive eavesdroppers. The scheme will be simulatedusing MATLAB. The proposed scheme will use a ‘decode and forward’ approachrather than an ‘amplify and forward’ approach which is less secure.
The modelwill use the maximum-ratiocombining (MRC) techniquewhich is a method of diversity combining inwhich the signals from each channel are added together, the gain of eachchannel is made proportional to the RMS signal level and inversely proportionalto the mean square noise level in that channel. This improves the signalcoverage area while providing secure communication.