Application of real zero concept to coherent detector for quadrature amplitude modulation

This paper proposes a new coherent detector that exploits the real zero (RZ) concept for signal recovery. The detector recovers the complex envelope consisting of in-phase (I) and quadrature (Q) components of linear quadrature amplitude modulation (QAM). The received signal before amplification is processed by RZ conversion, after which the converted signal can be amplified by a severely distorted amplifier. The RZ conversion combines the modulated received signal with a sinusoidal wave whose frequency is slightly outside the modulated frequency band, and the resultant signal is amplified to a saturated level. Next, the IQ detection applies the RZ concept to the IQ signal recovery. It utilizes a time to digital converter (TDC) for RZ timing estimation. A linear interpolation type TDC is introduced. Computer simulations demonstrate the effectiveness of the proposed detector. It is shown that error vector magnitude (EVM) of the recovered complex envelope is around -30 to -40 dB for a narrowband signal.