This paper proposes a new hybrid architecture by using multiple unmanned aerial vehicles enabled aerial base stations (ABSs) to offload data traffic for a single overloaded ground base station (GBS). We consider two practical spectrum sharing strategies, i.e., orthogonal spectrum sharing and nonorthogonal spectrum reusing between GBS and ABSs. For the non-orthogonal spectrum reusing case, we aim to maximize the minimum throughput for cell-edge ground users (GUs) by jointly optimizing the coverage radius of ABSs, and the number of ABSs. For the orthogonal spectrum sharing case, we have also optimized an additional variable, i.e., bandwidth allocation ratio. Numerical results indicate that the orthogonal spectrum sharing strategy outperforms the non-orthogonal spectrum reusing strategy and the conventional GBS-only case, thus provides an attractive solution to offload data traffic for a temporary cellular hotspot.