Cluster-based cooperative spectrum sensing is provided for cognitive radio systems. For each cluster of cognitive users, a cluster head is determined. Each cluster head collects energies of a reporting channel measured by the cognitive users within the cluster and decides whether a primary user is absent from a given spectrum. A common receiver then aggregates the cluster-level decisions made by the cluster heads, and makes a decision across multiple, or all of, the clusters whether the primary user is absent based on a fusion function of the cluster-level decisions. If the primary (licensed) user is absent, then secondary (unlicensed) users may utilize the spectrum.