Research on the Performance of Overlay / Underlay Cognitive Radio Waveforms in Different Channels

In this paper, we first derived the bit error rates for overlay and underlay waveforms respectively. Then, according to their waveform characteristics, Non-continuous multi-carrier Code Division Multiple Access (NC-MC-CDMA) signal was used to implement the overlay waveforms and exploit spectrum holes; and direct sequence spread spectrum (DSSS) signal, whose power is below the noise’s power spectral density, was used to implement underlay waveforms and exploit underused frequency bands; by adding spread spectrum code sequences of DSSS signals and frequency hopping sequences of NC-MC-CDMA signals, the added signal was used to implement hybrid overlay/underlay waveform. In AWGN, RAYLEIGH and RICEAN channels, bit error rates (BER) of overlay, underlay and hybrid overlay/underlay waveforms were simulated respectively. The results showed that hybrid overlay/underlay waveform performance > overlay waveform performance > underlay waveform performance; and that underlay waveform allows primary users and secondary users to coexist, but when the number of primary users increases, then the performance of underlay waveform decreases.