Introduction

Motivated by the increase in user demand for high data rates and new service applications due to the fast market penetration of smartphones, a large number of mobile operators in the world are introducing long-term evolution (LTE) into their networks [1]. In accordance with the further growth of mobile data traffic, these operators are deploying, or plan to deploy, their LTE networks with multiple-frequency-band operation in order to provide satisfactory user experience to their customers. Therefore, from the viewpoint of mobile operators, technologies that achieve high capacity LTE networks deployed with multiple-frequency-band operation are essential.

In order to achieve high capacity by utilizing multiple LTE frequency bands, carrier aggregation (CA) was specified as one of the new features for LTE in 3GPP Release 10 (i.e., LTE-advanced) [2]. The CA feature will enable operators to provide improved user throughput in their LTE networks by simultaneously using multiple LTE carriers. It can support large bandwidths (up to 100 MHz) and the flexible use of a fragmented spectrum in different frequency bands, where multiple LTE carriers do not have to be contiguous in a frequency band and can even be located in different frequency bands. The increase in user throughput with CA is achieved by assigning available radio resources over multiple LTE carriers to a single user. However, in a high-load network condition due to a large number of connected users, the increase in user throughput would be limited as the radio resources that could be assigned to a single user would not be changed irrespectively of whether CA is employed. Therefore, the utilization of CA only will not contribute to an increase in network capacity.

One conventional way to increase network capacity is to increase the number of cell sites in a certain area (i.e., to employ a densification of cells). However, the densification way of using macro cell deployment is becoming less efficient especially in dense urban areas since it has become difficult to find sites (a building or tower) in which new macro base stations can be installed. To cope with this problem, the deployment of heterogeneous networks, in which multiple small cells are deployed over a macro-cell area, is considered to be a promising option. In this deployment, the frequency band of the small cells is the same as that of the macro cell.