A novel multilevel signalling scheme based on absolute added correlative coding (AACC) is proposed and numerically demonstrated for high-speed fibre optic communication systems. Return-to-zero AACC (RZ-AACC) signal with three different duty cycles of 33, 50 and 67% was created utilising a lithium niobate (LiNbO3) Mach–Zehnder modulator with a sinusoidal drive waveform. The spectral width and receiver sensitivity of 4-instensity level RZ-AACC with 33, 50 and 67% are discussed and compared against conventional AACC and RZ 4-ary pulse-amplitude modulation formats.