The throughput (data rate, link reliability) of the wireless cellular system has been dramatically improved in the past decade. However, the major drawback of improving the cell throughput is the interference problem in cell-edge users. As shown in the figure above, for the users located in the cell-edge, the strength of interference signals transmitted from other cells is significant compared to the desired signal. Besides, the area of cell-edge is larger than the cell-center, and hence the throughput of the cell-edge predominates over the throughput of the cell-center.
To overcome this drawback, the concept of “Small Cells” or “Heterogeneous Network (HetNet)” has been studied in the industries or academia. The idea is to employ physically or functionally “small” base stations in the macro cell, thereby achieving uniform throughput in almost all cell region, as shown in figure below. See 3GPP homepage for more comprehensive understanding of Small Cells.
Now, the concept gives rise to a new problem “interference between the macro cell and small cells,” since it is in general allowed to use the same frequency/time resource in both the macro and small cells. Two approaches of handling this interference problem have been studied: Cloud RAN and HetNet, as shown in the figure below. In the Cloud RAN, all the macro and small base stations are connected through wires, and hence interference problem can be minimized at the cost of Gbps backhaul. On the other hand, small cell base stations can simply operate in the wireless mode, and thus the interference should be carefully managed.