 the LTE architecture, it is worth looking at the gen- eral trends in radio link technology development, which drive many of the architec- tural changes in cellular systems today. The most important challenge in any radio system is to combat the randomly changing radio link conditions by adapting the transmission and reception parameters to the actual link conditions (often referred to as the channel state.) The better the transmitter can follow the fluctuations of the radio link quality and adapt its transmission accordingly (modulation and coding, power allocation, scheduling), the better it will utilize the radio channel capac- ity. The radio link quality can change rapidly and with large variations, which are primarily due to the quickly fading fluctuations on the radio link, although other factors such as mobility and interference fluctuations also contribute to these. Because of this, the various radio resource management functions have to operate on a time scale matching that of the radio link fluctuations. As we will see, the LTE requirements on high (peak and average) data rates, low latency, and high spectrum efficiency are achieved partly due to the radio resource control functions being located close to the radio interface, where such instantaneous radio link quality information is readily available. In addition to the quickly changing radio link quality, the bursty nature of typical packet data traffic imposes a challenge on the radio resource assignment and requires a dynamic and fast resource allocation, taking into account not only the instantaneous radio link quality but also the instantaneous packet arrivals. As a consequence, a general trend in the advances of cellular systems is that the radio- specific functions and protocols get terminated in the base stations, and the rest of the radio access network entities are radio access technology agnostic. Thus, the radio access network exhibits a distributed architecture without a central radio resource control functionality. The LTE architecture is