This paper presents a bandwidth efficient multilevel concatenated coded modulation scheme for reliable data transmission over the shadowed mobile satellite communication (MSAT) channel. In the heavy shadowed MSAT channel which is statistically close to the Rayleigh fading channel, a coded modulation system suffers very severe distortion due to randomly changing phase and the multipath fading. In this scheme, bandwidth efficient block modulation codes are used as the inner codes and Reed-Solomon codes of various error correcting capabilities are used as the outer codes. The inner and outer codes are concatenated in multiple levels. A systematic method for constructing multilevel concatenated modulation codes is presented and a multistage closest coset decoding for these codes is proposed. Specific multilevel concatenated 8-PSK modulation codes have been constructed. These codes are designed to remove the error floor phenomenon or lower the bit-error rate of the error floor caused by the large Doppler frequency shift due to the motion of vehicles. Simulation results show that these codes perform very well and achieve large coding gains over the uncoded reference modulation systems.
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