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In this paper, the effect of power and rate adaptation on the spectral efficiency of Orthogonal Frequency Division Multiplexing (OFDM) systems using M-ary Quadrature Amplitude Modulation (MQAM) is investigated in the presence of frequency selective and very rapidly time-varying fading channels, under power and instantaneous Bit Error Rate (BER) constraints. Lower bounds on the maximum spectral efficiency of adaptive OFDM/MQAM systems with perfect and imperfect Channel State Information (CSI) are obtained, together with a closed-form expression for the average spectral efficiency of adaptive OFDM systems. The delay between the channel estimation and the actual transmission is also considered in this paper.
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