A Study on transmit power control per subcarrier in OFDM

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In multiple-input multiple-output systems, most common application is space division multiplexing (SDM) where individual data substreams are transmitted from multiple antennas with the same power and the same rate. SDM applications assume that MIMO channel information in not available at the transmitter. If we know all of MIMO channel information, it is possible to assign matched beam to each substream where the optimum weight for beamforming is obtained by singular value decomposition of MIMO channel matrix. We call SDM with eigenbeam-forming as E-SDM, hereafter. In E-SDM, we have to consider resource (transmit power and data rate) adaptation. In this research, we employed minimum brt error rate (BER) criterion as expansion of transmit power control per sucarrier previously used in OFDM systems. As a result, E-SDM with the resource adaptation obtained higher BER performance in comparison to conventional SDM. However, the MIMO channel information required at the transmitter includes estimation and tracking errors. Under such errors, we also evaluated the E-SDM performance. The results show that degradation due to the channel estimation error is almost the same to E-SDM and SDM applications. Although the channel tracking error caused by a time lag until transmission degrades E-SDM performace severely when the maximum Doppler frequency is very high, we have shown that it can be compensated by employing the first-order extrapolation of estimated channel.