At the same time,it plays an important role in the emerging heterogeneous networks de-pending on its flexibility.The next-generation WLANs are expected to achieve more than 1 Gb/s network coverage.Moreover,the number of short packets grows due to the spread of Machine-to-Machine(M2M)communications.In these emerging scenarios,the conventional random access MAC,e.g.802.11 Distributed Coordination Function(DCF),may cause a drastic drop of channel utilization[1,2].The