Abstract
The problem of automatic modulation identification (AMI) is one of the main challenges that faces orthogonal frequency division multiplexing (OFDM) software-defined radio (SDR) systems. The previously reported studies of this aspect have limited to uncoded transmission; however, practical systems typically employ error-correcting codes. In this work, we exploit the properties of bit-interleaved coded modulation iterative decoding (BICM-ID) schemes, which are widely used in wireless communications systems, to develop a novel AMI algorithm for OFDM-SDR systems. Since the exact maximum-likelihood (ML) solution is intractable in practice, we resort to an expectation-maximization (EM) procedure as a statistical analysis tool. The proposed identification algorithm works in an iterative fashion, in which the soft information provided by the channel decoder of BICM-ID scheme is utilized as a priori information to the proposed identifier. The results indicate that the identification performance of the proposed algorithm achieves a significant improvement with iterations and outperforms that of the conventional uncoded algorithms. These favorable results are achieved with reduced processing time and acceptable sensitivity to transmission impairments, such as clock timing offset, frequency offset, Doppler shift, and phase noise.