Autonomous Spectrum Sharing for Mixed LTE Femto and Macro Cells Deployments

We consider the problem of sharing spectrum between different base stations in an OFDM network where some cells have a small radius. Such scenarios will become increasingly common in fourth generation networks where the need for ubiquitous high-speed coverage will lead to an increased use of small cells as well as indoor femtocells. Our aim is to devise autonomous algorithms for small cells and femtocells to choose spectrum so that they can achieve high data rates without causing interference to users in the traditional macro cells. We present a number of algorithms that perform combinations of frequency and time sharing based on the channel conditions reported by the mobile users. Our schemes bear some resemblance to the traditional 802.11 MAC algorithms. However, they differ in the fact that they are able to use better information about channel conditions from the mobiles and they are allowed to adjust the amount of spectrum that they are using. We evaluate our schemes using a platform that combines a physical-layer ray-tracing tool for indoor and outdoor environments with an upper layer OFDM simulation tool. We believe that this type of simulation capability will become increasingly important as cellular networks target the provision of high-speed performance in dense urban environments. Our results suggest that user channel quality measurements can be used to set the level of sharing between femtocells and macrocells and that finding the correct level of sharing is important for optimal network performance.