Blind estimation of channel and modulation scheme in adaptive modulation schemes for OFDM-CDMA based 4G systems

Adaptive modulation is proposed by many researchers for future generation wireless networks based on multi-carrier transmission technologies. In this paper, we propose and investigate some blind mechanisms at the receiver of the adaptive modulation based coded OFDM-CDMA systems in Rayleigh fading channels for the enhancement of the systems in terms of both the practical implementation and data throughput capacity. In this respect, blind techniques to estimate the channel impulse response and modulation scheme are proposed and investigated and their effect on the BER performance of the systems are also analyzed. A subspace based algorithm is proposed to estimate the channel impulse response blindly. This estimate is then used to coherently demodulate the OFDM-CDMA signals and adaptively switches the modulation schemes for varying channel conditions. Furthermore, an algorithm to estimate the modulation scheme at the receiver is also proposed. Results show error free estimation of modulation scheme for a maximum of 7 active users in the system at a channel condition of 20 dB. Simulation results show that in some situations, the systems based on blind channel estimation show little degradation in terms of the BER performance compared to the pilot based scheme. However, the advantages of blind estimation schemes far outweigh this degradation.