ACE-QPSK: a new method of coding QPSK for the nonlinear transmitter

Band-limited QPSK signals are prone to a high degree of spectral regrowth when nonlinearity is present in the transmitter amplifier. In a multi-carrier application with close adjacent channels (such as an FDMA satellite system), the spectral regrowth of adjacent channels can result in severe interference with the desired carrier. This may lead to substantial BER performance degradation or make necessary an increase in the spacing between carriers. The transmitted carrier may suffer levels of spurious RF emission which exceed system specifications or spectral mask regulations. This paper proposes a new 4-dimensional trellis coding technique which can be applied to conventional QPSK modulation, and which leads to substantially reduced levels of spectral regrowth. When the coding is applied, the modulated signal exhibits a reduced degree of envelope fluctuation, and hence it is less affected by amplitude nonlinearity. The scheme is termed Almost-Constant-Envelope QPSK (ACE-QPSK).