The needs for increasing the profits of network operators and protecting our environment have led to great research interest in green communications. In this paper, we propose a novel scheme, called cognitive network cooperation (CNC), to achieve energy-efficient secondary transmissions while ensuring the quality-of-service (QoS) of primary transmissions in heterogeneous cognitive radio networks (HCRNs). In CNC, the secondary users (SUs) are allowed to assist the data transmissions of primary users (PUs) in exchange for some spectrum. Then, the secondary traffic flow is split between the released licensed spectrum and the unlicensed spectrum to reduce the energy consumption. Under a given target secondary data rate, a specific traffic splitting method via network cooperation is developed, aiming at maximizing the amount of the spectrum released from PUs and minimizing the energy consumption of SUs. Extensive simulation studies demonstrate that the proposed CNC scheme can remarkably reduce the energy consumption of secondary transmissions without sacrificing the performance of primary transmissions.
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