Performance Degradation Due to MAI in OFDMA-Based Cognitive Radio

We consider a convolutionally coded orthogonal frequency-division multiple access (OFDMA) based cognitive radio system where each user achieves perfect synchronization for its own signal, while different users are asynchronized due to random timing offsets. The presence of asynchronous secondary users introduces potential multiple access interference (MAI) to each primary user´s receiver. The strength of the MAI is determined by the secondary users´ transmission powers, and their distances and frequency separations to the primary user. Expressions for the primary user´s pairwise error probability and average probability of error are derived, and an error floor prediction method is presented. Finally, the trade-off between the MAI to the primary user and the secondary users´ performances are investigated through numerical examples and simulations.