Proper selection of a signal-to-noise ratio threshold largely determines the tightness of an approximation to the frame error rate of a system over a quasistatic fading channel. It is demonstrated that the expression for the optimal threshold value, which has been established for singleinput single-output (SISO) channels, remains unchanged for the general case of multiple-input multiple-output (MIMO) channels.

Introduction: In various practical systems, such as fixed wireless access networks, the communication channel experiences extremely slow fading conditions that can be characterised by the quasistatic fading model. In quasistatic fading, the instantaneous signal-to-noise ratio (SNR) at the receiver, denoted as g, remains constant for the duration of a frame but changes independently from frame to frame. El Gamal and Hammons [1] demonstrated that the average error probability of an iteratively decoded scheme over a SISO quasistatic fading channel can be accurately approximated by