Time-domain enhanced inter-cell interference coordination（eI CIC） is an effective technique to reduce the cross-tier inter-cell interference（ICI） in long term evolution（LTE）-based heterogeneous small cell networks（HetSCNs）. This paper first clarifies two main communication scenarios in HetSCNs, i.e., macrocells deployed with femtocells（macro-femto） and with picocells（macro-pico）. Then, the main challenges in HetSCNs, particularly the severe cross-tier ICI in macro-femto caused by femtocells with closed subscribe group（CSG） access or in macro-pico caused by picocells with range expansion are analyzed. Based on the prominent feature of dominant interference in HetSCNs, the main idea of time-domain interference coordination and two basic schemes in the eI CIC standardization, i.e., almost blank subframe（ABS） and orthogonal frequency division multiplexing symbol shift are presented, with a systematic introduction to the interactions of these techniques with other network functions. Then, given macro-femto and macro-pico HetSCNs, an overview is provided on the advanced designs of ABS-based eI CIC, including self-optimized designs with regard to key parameters such as ABS muting ratio, and joint optimized designs of ABS-based eI CIC and other radio resource management techniques, such as user association and power control. Finally, the open issues and future research directions are discussed.
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