On the calculation of the error probability for a multilevel modulation scheme using QAM-signaling

We analyze the quadrature amplitude modulation (QAM) version of multilevel modulation with multistage decoding using a suboptimal metric, when transmission takes place over a memoryless Gaussian channel. The upper bounds for decoding error probabilities are functions of the Chernoff bounding parameter Z. We argue that the conventional approximation of Z is not adequate; new values of Z that tightens the error bounds without causing them to lose their validity are given. The capacity for this system is also calculated, and we conclude that the use of a suboptimal metric in multistage decoding causes very little degradation in capacity compared to when the optimal metric is used in each decoding stage