The use of unlicensed bands, e.g., ISM (industrial, science and medical) bands is getting more and more attention nowadays due to the high scarcity and high costs of radio spectrum. Due to this, there is a high risk of interference, which degrades the performance and fails to support Quality of Service (QoS) for systems operating in these bands. Therefore, spectrum sharing between coexisting competing wireless systems is an upcoming challenge. In this paper, a 'Scenario Matrix' is drawn to highlight the research directions in context of coexistence scenarios. Then a spectrum sharing concept is proposed, which can be applied generally to all scenario elements of the matrix. The concept has the potential to be realized as policy-based and cognitive spectrum sharing. In the framework of this paper, the proposed concept is adapted for coexisting Wireless Local Area Networks (WLANs) in a Voice over Internet Protocol (VoIP) usage scenario where QoS support is required. In this case, IEEE 802.11e Hybrid Coordination Function (HCF) controlled channel access (HCCA) is used to provide a decision method for sharing the airtime among more than one system. The simulation results show that the proposed algorithm provides fair sharing with an excellent QoS support. It proves that 'unlicensed' and 'unreliability' are not always fully inclusive terms.
