Bandwidth expansion in Long Term Evolution (LTE)-Advanced is supported via carrier aggregation (CA), which aggregates multiple component carriers (CCs) to accomplish very high data rate communications. Heterogeneous networks (HetNets), which set pico-base stations in macrocells are also a key feature of LTE-Advanced to achieve substantial gains in coverage and capacity compared to macro-only cells. When CA is applied in HetNets, transmission on all CCs may not always be the best solution due to the extremely high levels of inter-cell interference experienced by HetNets. Activated CCs that are used for transmission should be selected depending on inter-cell interference conditions and the traffic offered in the cells. This paper presents a scheme to select CCs through centralized control assuming a centralized baseband unit (C-BBU) configuration. A C-BBU involves pooling tens or hundreds of baseband resources where one baseband resource can be connected to any CC installed in remote radio heads (RRHs) via optical fibers. Fewer baseband resources can be prepared in a C-BBU than those of CCs in RRHs to reduce the cost of equipment. Our proposed scheme selects the activated CCs by considering the user equipment (UE) assigned to CCs under the criterion of maximizing the proportional fairness (PF) utility function. Convex optimization using the Karush-Kuhn-Tucker (KKT) conditions is applied to solve the resource allocation ratio that enables user throughput to be estimated. We present results from system level simulations of the downlink to demonstrate that the proposed algorithm to select CCs can outperform the conventional one that selects activated CCs based on the received signal strength. We also demonstrate that our proposed algorithm to select CCs can provide a good balance in traffic load between CCs and achieve better user throughput with fewer baseband resources.

The fifth generation (5G) mobile communication technologies are attracting a lot of attention in terms of accommodating the huge traffic expected in the future. Millimeter wave communications, which utilize wide frequency bands, are attracting attention for the realization of the high capacity required in the 5G era. In millimeter wave communications, beamforming with massive antennas is expected to play a very important role in compensating the large propagation loss of millimeter waves. Because massive beamforming yields narrow beams, the search for the optimal beam could have considerable impact on the system. In this paper, we propose a new beam search method that can reduce the load of beam search significantly while keeping beamforming gain almost the same as that of the conventional method. The proposed method consists of three stages with the creation of a set of candidate beams in the first stage, selection of an initial beam in the second stage, and refinement of the selected beam in the third stage. In the first stage, the created set of candidate beams contains beams of various widths instead of beams of a uniform width to reduce the number of candidate beams in the set. Here, we leverage the property of millimeter waves according to which the fluctuation of millimeter wave propagation loss is spatially and temporally small because of the fewer multipaths, and therefore, the propagation loss has strong correlation with the user location. By using the decreased set of candidate beams, the beam search time can be reduced in the second stage. Then the beam refinement can increase the beamforming gain to increase user throughput in the third stage. To confirm the effects of the proposed beam search method, we conduct system level simulations by using a propagation model for millimeter wave communications proposed by MiWEBA, which is an international project between Europe and Japan. The results show that the proposed beam search method can reduce the number of candidate beams, and can therefore shorten the beam search time by about 39% without any degradation in outage probability compared with a conventional method.