On Improving Bit Error Probability of QPSK and 4-Level Amplitude Modulation Systems by Convolutional Coding

We consider Amplitude Modulation (AM) systems, PhaseShift-Keying (PSK) and Amplitude-Phase-Keying (APK) systems in which each symbol can take 2 n values. After a systematic search of (n, n - 1) binary convolutional codes with large values of free distance and minimal number of code words at that distance, simulations have been performed with an 8-symbol modulation system (n = 3). For a bit error probability of less 2 10^-4, the saving in average power signal-signal-to-noise ratio over that required for an uncoded system with the same channel symbol rate was more than 2 dB with PSK and APK, and 1. 5 dB with AM.