Interference avoidance in OFDMA-based femtocell network

Spectrum splitting is the simplest approach to allocate the spectrum to femtocells overlaid with the existing cellular networks. In orthogonal frequency division multiple access (OFDMA) systems, this can be done by assigning a subset of orthogonal subcarriers to femtocells. To ensure subcarrier orthogonality and thus not interfere with existing macro Mobile Stations (MSs), the signals from the strongest femtocells should be synchronous with the desired macro signal, otherwise inter carrier interference (ICI) and thus multiple access interference (MAI) will influence macro MS performance. This is likely especially in large macro cells when a macro MS visits a private femtocell far from the macro Base Station (BS). This paper proposes a synchronization scheme where each femto BS first determines its distance d from the macro BS through signaling and then delays its downlink transmission by a d/c relative to the macrocell BS downlink transmission time base, where c is the light speed. The detailed illustrations and simulations are given to validate the scheme.