1. Field of the Invention
The present invention relates to a coded modulation system suited for a poor channel in quality impaired by noise, interferences, and/or distortion such as a mobile, portable, or very small aperture terminal (VSAT) satellite communications channel.
2. Description of the Related Art
The duobinary frequency-modulation (DBFM) system with modulation index h=0.5 proposed by P. J. McLane, "The Viterbi receiver for correlative encoded MSK signals (IEEE Trans. Commun., COM-31, 2, pp. 290-295, Feb. 1983)," is known as a modulation system that possesses the constant envelope, compact spectrum, and almost the same BER performance as the antipodal modulation systems, i.e., BPSK, QPSK, and MSK.
The system combines the duobinary technique with the minimum-shift keying (MSK) and applies a soft-decision Viterbi decoding technique in order to demodulate the signals. However, DBFM has a few disadvantages as follows:
(1) DBFM necessitates block synchronization between the transmit and receive ends because DBFM applies alternatively two different state-transition tables in order to decode the signals.
(2) It is difficult for DBFM to employ bit-interleaving because DBFM utilizes the specific behavior of phasor in two-dimensional signal space.
(3) It is less effective for DBFM to connect tandem in trivial manner such a forward error correction (FEC) codec as a convolutional codec because DBFM already uses up a soft-decision Viterbi decoding of constraint length K=3.