Mitigation of Uplink ICI and IBI in OFDMA Two-Tier Networks

Abstract

For two-tier networks using orthogonal frequency-division multiple access (OFDMA) over frequency-selective channels, macrocell-user (MU) signals generally arrive at small-cell base stations (SBSs) with different delays and can hardly be synchronized in time, causing intercarrier interference (ICI) and interblock interference (IBI) in OFDM demodulation. The effects on small-cell uplink capacity by ICI and IBI and their mitigation have not been well addressed in the literature. In this paper, the ICI and IBI of OFDMA two-tier networks with fractional power control (FPC) in multipath fading channels are formulated, and closed-form expressions of their covariance matrices at the SBS with random MU locations are derived. Simulation results verify the validity of the theoretical analysis. It is shown that small-cell uplink capacity is substantially degraded in the presence of ICI and IBI. A precoding method for small cells for mitigating the effects of ICI and IBI on uplink capacity is proposed. In the method, the received signals over small-cell subcarriers at the SBS are decorrelated jointly by small-cell user (SU) precoders and an equalizer at the SBS. Closed-form design procedures for precoding of different subcarrier sizes to maximize uplink capacity under different information feedback are developed. Simulation results demonstrate that the proposed method for using all the subcarriers for precoding has a large capacity gain over that of no small-cell precoding. Moreover, the method can perform very well, even if MU locations are the only information fed back from the macrocell.