 and the feasible combinations thereof. Likewise, the smallest unit in which radio resources are assigned and distributed among the entities (e.g., power, time slots, frequency bands/carriers, or codes) also varies depending on the fundamentals of the multiple access technology employed on the radio interface [23]. The placement and the distribution of the RRM functions to different net- work entities of the radio access network (RAN), including the functional dis- tribution between the terminal and the network as well as the protocols and interfaces between the different entities, constitute the RAN architecture. Although the required RRM functions determine, to a large extent, the most suitable RAN architecture, it is often an engineering design decision how a particular RRM func- tion should be realized. For example, whether intercell interference coordination or handover control is implemented in a distributed approach (in each base station) or in a centralized fashion both can be viable solutions. We will discuss such design issues throughout this chapter. In LTE, the radio interface is based on the orthogonal frequency division multi- plexing (OFDM) technique. In fact, OFDM serves both as a modulation technique and as a multiple access scheme. Consequently, many of the RRM functions can be derived from the specifics of the OFDM modulation. In the rest of this section we give an overview of the LTE RAN architecture, including an overview of the © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM OFDM-based radio interface. Subsequently, we define and introduce the notion of radio resource in LTE and present the requirements that the 3GPP (3rd Generation Partnership Project) has set on the spectral efficient use of radio resources, which entail the presence of certain RRM functions in the system. For a comprehensive overview and detailed description of the overall LTE sys- tem, the reader is referred to Dahlman et al. [1]. 4.1.1 The LTE Architecture Before discussing the details of