On fractional frequency reuse in imperfect cellular grids

Current point-to-multipoint systems suffer significant performance losses due to greater attenuation along the signal propagation path at higher frequencies, transmit power constraints of mobile users and base stations, and interference from neighboring cells. Fractional Frequency Reuse (FFR) is a technique to counteract these effects. Typically, the proposed FFR technique partitions a cell into a reuse 1 area, centered near the base-station and a reuse 3 area, located near the edges of the cell, with reuse 3 regions scheduled to minimize interference from neighboring cells. Unfortunately, virtually all analysis of FFR has been done under a perfect hexagonal lattice cellular grid, while no practical deployment has this degree of symmetry. In this paper we revisit the analysis of FFR for non-ideal cellular grids for cases with fading. We find that while for some non-ideal grids, a combination of reuse 1 and 3 is indeed optimal, for many others a combination of reuse 1 and 4 provide better performance. Thus, we conclude that for practical cellular layouts, the optimal re-use pattern for the edge of the cells is not necessarily 3 as commonly assumed, but is topology dependent.