BEP walls for cooperative sensing in cognitive radios using K-out-of-N fusion rules

Cooperative spectrum sensing is used to identify idle spectrum in cognitive radio or co-existing wireless systems. This paper identifies a performance limitation for cooperative sensing (CS) involving distributed secondary users (SUs) and a fusion center (FC). The local binary decisions are fused at the FC using a K-out-of-N fusion rule. The performance limitation presented in this paper for CS is in the form of a bit error probability (BEP) wall and results from imperfect reporting channels between the SUs and the FC. That is, if the BEP of the reporting channel exceeds the BEP wall value, then irrespective of the received signal quality on the listening channel or the sensing time at the SUs, constraints on the detector performance cannot be met at the FC. The BEP wall is an important phenomenon and needs to be taken into account while designing communication protocol between the SUs and the FC. Expressions for the BEP walls are derived in terms of the error probabilities at the FC and the number of users cooperating. Further, important properties of the BEP walls are derived and discussed in detail for the K-out-of-N fusion rules under the assumption of independent and identically distributed reporting channel errors.

► Cooperative sensing performance is shown to be limited by reporting channel errors.
► We propose a bit error probability (BEP) wall for characterizing the limitation.
► We derive the BEP wall for K-out-of-N rules in terms of sensing error probabilities.
► Different choices of K are shown to result in very different BEP wall values.
► The BEP wall values of the OR and AND fusion rules worsen as cooperation increases.