Interference management techniques are critical to the performance of heterogeneous cellular networks, which will have dense and overlapping coverage areas, and experience high levels of interference. Fractional frequency reuse (FFR) is an attractive interference management technique due to its low complexity and overhead, and significant coverage improvement for low-percentile (cell-edge) users. Instead of relying on system simulations based on deterministic access point locations, this paper instead proposes an analytical model for evaluating Strict FFR and Soft Frequency Reuse (SFR) deployments based on the spatial Poisson point process. Our results both capture the non-uniformity of heterogeneous deployments and produce tractable expressions which can be used for system design with Strict FFR and SFR. We observe that the use of Strict FFR bands reserved for the users of each tier with the lowest average SINR provides the highest gains in terms of coverage and rate, while the use of SFR allows for more efficient use of shared spectrum between the tiers, while still mitigating much of the interference. Additionally, in the context of multi-tier networks with closed access in some tiers, the proposed framework shows the impact of cross-tier interference on closed access FFR, and informs the selection of key FFR parameters in open access.
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.