Asymptotic CRB on SNR estimation of QPSK signals in low-SNR Nakagami-m fading channels with diversity combining

A simple closed-form expression is derived for the low signal-to-noise ratio (SNR) asymptote of the CramerRao bound (CRB) on the estimation of SNR for quaternary phase-shift-keying signals received over Nakagami-m fading channels with L branch equal gain diversity combining.