HetNet (heterogeneous networks) is studied intensively in 3GPP to meet with the explosive mobile traffic growth. HetNet is a cell structure in which small cells are deployed under a blanket coverage of macro cells. If macro cells and small cells are working within a common frequency band, eICIC (enhanced inter- cell interference coordination) can be used for mitigating the inter-cell interference between macro cells and small cells. To realize eICIC, signal transmission timing must be synchronized among base stations. We propose a novel base station synchronization scheme in this paper. Then we conduct a field experiment to show the accuracy of the proposed scheme. In the proposed scheme, GPS is used for accurate timing distribution. Then system frames are synchronized among base stations by using a control packet with a timestamp. In addition, we adjust the internal delay of each base station site, i.e. propagation delay of cables or processing delay of equipment. The result of the field experiment shows that it is possible to achieve the synchronization accuracy within 1μs. Also, we confirmed that the measured downlink throughput using eICIC was not degraded at all even under severe inter-cell interference.
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