AbstractToday's framework for radio spectrum regulation and spectrum usage is undergoing fundamental changes. In the face of scarce radio resources, regulators, industry and the research community are launching initiatives towards a flexible usage of spectrum. Intelligent radio systems so‐called cognitive radios that autonomously coordinate their spectrum usage are a promising approach towards an opening of spectrum. In this article, we therefore discuss medium access control protocols for cognitive radios operating in parts of the spectrum originally licensed to other radio services. They identify underutilised spectrum, coordinate its usage and release it again when required by licensed radio systems. We apply therefore ‘waterfilling’, a known principle in information theory, in the time‐domain for the distributed coordination of cognitive radios in spectrum sharing scenarios. This application is here referred to as spectrum load smoothing (SLS). Cognitive radios are realised in this article in modifying the medium access of IEEE 802.11e with the SLS. The ability of SLS to support quality‐of‐service in the presence of other, competing cognitive radios, and the prevention of harmful interference to licensed radio systems are evaluated. Copyright © 2006 AEIT