The Detection Efficiency of 16-ary QAM

In addition to spectrum efficient angle modulation techniques such as PSK, digital radio transmission may use quadrature amplitude shift keying (QASK, also called digital QAM) or combined amplitude and phase shift keying (CAPSK) when these techniques have bandwidth efficiencies of 3 bits/s/Hz or higher. Because of the relative simplicity of implementation and because of superior immunity to Gaussian noise, QAM and especially 16-ary QAM appears more attractive than 16ary PSK for obtaining high bandwidth efficiencies. The detection efficiency of QAM in a Gaussian noise channel has been determined in Ref. 1. The method of determining the degradation in detection efficiency caused by bandlimiting and the resultant intersymbol interference appears in Refs. 2 and 3. However, since attenuation caused by rain may exceed 10 dB per kilometer at 10 GHz or higher carrier frequencies, repeater spacing must be small and cochannel interference may be the major source of impairment if low noise receivers are used in the system. Hence, it is desirable to compare the cochannel interference immunity of QAM to other modulation systems. 639