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In Multi-User Orthogonal Frequency Division Multiplexing (MU-OFDM) systems operating in Time Division Duplex (TDD) mode downlink pre-equalization can be applied relying on Channel State Information (CSI) from the uplink direction. Unfortunately, the prerequisite of channel reciprocity is usually not fulfilled due to non-reciprocal transceivers. Hence, this paper deals with a comparison of robust pre-equalization schemes and a calibration approach to enhance the necessary uplink-downlink link equivalence for MU-MISO-OFDM systems applying linear and non-linear pre-equalization techniques and strong Forward Error Correction (FEC). Performance evaluations concerning coded bit error rate results show that robust pre-equalizers exclusively improve the performance in moderate non-reciprocity conditions.
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