In this report we investigate bandwidth-efficient reliable communication over wireless channels. We consider a generalized serial concatenated trellis coded modulation (SCTCM) system consisting of an outer code, one or more inner rate-1 accumulate codes preceded by interleavers, an optional channel interleaver, and a mapping to a higher-order, Gray-labeled signal constellation. The generalized SCTCM system encompasses bit-interleaved coded modulation (BICM) as a special case. We show that under maximum-likelihood (ML) decoding, there exists a system-dependent threshold, such that if the signal-to-noise ratio (SNR) exceeds this threshold, the bit error probability goes to zero as the blocklength goes to infinity. For finite blocklengths we derive a probabilistic performance bound which improves on the conventional union bound. For non-ML, message-passing decoders, we compute tight SNR-thresholds using density evolution, assuming infinite blocklengths. For the particular case of an outer parity check code and one inner accumulate code, we derive a stability condition, which provides a lower bound on the SNR threshold. The spectral efficiency of the proposed SCTCM system is determined by the outer code and the size of the signal constellation, and can easily be tailored to the prevailing channel conditions. The simulated performance in AWGN and i.i.d. Rayleigh fading is equal to or better than the performance of more complex systems suggested in the literature. The iterative decoding thresholds at high rates are within 1 dB of the channel capacity for higher-order constellations.