In this paper, we present a multi-user resource allocation framework usingfragmented-spectrum synchronous OFDM-CDMA modulation over a frequency-selectivefading channel. In particular, given pre-existing communications in thespectrum where the system is operating, a channel sensing and estimation methodis used to obtain information of subcarrier availability. Given thisinformation, some real-valued multi-level orthogonal codes, which areorthogonal codes with values of $\{\pm1,\pm2,\pm3,\pm4, ... \}$, are providedfor emerging new users, i.e., cognitive radio users. Additionally, we haveobtained a closed form expression for bit error rate of cognitive radioreceivers in terms of detection probability of primary users, CR users' sensingtime and CR users' signal to noise ratio. Moreover, simulation results obtainedin this paper indicate the precision with which the analytical results havebeen obtained in modeling the aforementioned system.
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