In this paper we investigate quaternary combined amplitude- and differential binary phase-shift keying (ASK-DPSK) and 8-ary combined amplitude- and differential quadrature phase-shift keying (ASK-DQPSK) at a bit rate of 40Gbit/s. For both formats, optimal values for amplitude ratios of the signal points and electrical and optical receiver bandwidths are given. Required optical signal-to-noise ratios (OSNR) at the receivers for bit error probabilities of 10^(-9) and chromatic dispersion tolerances are compared to amplitude-shift keying (ASK), binary differential phase-shift keying (DPSK) and quadrature phase-shift keying (DQPSK). For our numerical investigations we use a semi-analytical method for bit error probability computations.
This paper presents a new optical Minimum-Shift Keying (MSK) modulation scheme with high spectral efficiency. The transmitter is realized in an Offset-Quaternary Phase-Shift Keying (QPSK) arrangement. The receiver uses a delay & add filter (DAF) and photodiodes for direct detection. An MSK precoder for the transmitter is derived.