ACE-8PSK: band-limited 8PSK with an Almost Constant Envelope

8PSK modulation is of increasing interest in satellite communications applications where bandwidth efficiency is a prime concern. However, its adoption in practical systems has been limited, largely due to its increased sensitivity to channel and transmitter imperfections as compared with QPSK and other modulation schemes. In particular, band-limited 8PSK is highly susceptible to the interference associated with adjacent channel spectral regrowth caused by transmitter nonlinearity. Spectral regrowth may lead to substantial 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 new low complexity trellis coding techniques which can be applied to conventional 8PSK modulation, and which lead 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 schemes are termed Almost-Constant-Envelope 8PSK (ACE-8PSK). Combined with outer FEC coding, ACE-8PSK can achieve greater power efficiency than QPSK, and provide significant performance advantages on the nonlinear channel in terms of bit error rate and the spectral purity of transmissions.<
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