Linearly precoded OFDM system with adaptive modulation

We consider the linearly precoded OFDM approach proposed by A.P. Petropulu et al. (see IEEE Trans. Wireless Commun., 2003), where a non-redundant precoding was applied to the symbol blocks before entering the OFDM system. The precoding, while it maintained the transmit power, introduced a structure to the transmitted signal that allowed for blind channel estimation by a simple auto-correlation performed at the receiver. We propose an adaptive modulation based extension of the method of Petropulu et al. in order to combat a channel with deep fading. Bits are allocated on each subcarrier so that the overall transmit power is minimized under a fixed bit error rate (BER). The obtained bit allocation can also be viewed as minimizing BER for the precoded system, under a fixed overall transmit power constraint. The proposed approach provides large performance gains over the uniformly loaded one, especially under deep fading conditions, for the same overall throughput and transmit power.