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|a Large-scale antenna array aided beamforming is capable of improving both the spectral efficiency (SE) and link reliability as a benefit of its high angular resolution. As far as the dual-beam beamforming is applied in the downlink associated with the train moving towards the cell edge, the inter-beam ambiguity (IBA) will increase as the directional difference between the two beams becomes smaller during the process. In this paper, a novel adaptive antenna-activation (AAA) based beamforming scheme is proposed to mitigate the IBA and hence improve the SE in different geographical districts of the cell. On the one hand, in the district near the base station (BS), all BS antenna elements (AEs) are activated to generate two beams. On the other hand, when the distance from the train to the BS increases, only the minimum number of AEs which can satisfy the angular resolution criterion will be activated in order to reduce the increasing IBA. As a result, when the train moves close to the cell edge, one of the beams is switched off due to the intolerable IBA. Simulation results show that the proposed scheme can achieve 2 bps/Hz SE gain to the non-adaptive dual-beam beamforming scheme when the number of BS AEs is up to 260, and that it is more robust against the direction-of-arrival (DOA) estimation error as it can achieve about 50% SE gain to the latter when the estimation accuracy of the DOA is characterized by (90% ~ 110%).
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