Heterogeneous networks has recently evolved as one of the key enablers for future wireless technologies. The extremely growing mobile market, together with the arising demand for high data rates, force to off-load the main cellular network traffic. This can be accomplished by deploying small cells within the macro evolved-node base-station (eNodeB) coverage. Thus, more User Equipments (UEs) in the same geographical area can be served simultaneously. To achieve high spectral efficiency in the small-cell deployment, one solution is to reuse the available spectrum and consider interference coordination between the macro and small cells. Another solution is to deploy small cells operating in different frequency band (either licensed or unlicensed) in the Millimeter (mm)Wave range. For this reason, the mmWave band can be considered at the small-cells ensuring no interference while off-loading the macro cells. In this paper, we propose a coexistence approach for enhancing the spectral efficiency in heterogeneous ultra-dense networks (UDN) and demonstrate it with system level analysis. The proposed approach depends on deploying small cells that operate in the mmWave range. Henceforth, avoiding the interference problem between the macro eNodeB and the small cells.
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