T h i s p a p e r considers t h e expansion of a well known four-dimensional Quadrature-Quadrature Phase-ShiR Keying (Q2PSK) signalling scheme by introducing multiple amplitudes and/or phases p e r dimension to increase t h e effective data throughput rate and spectral effix i e n c y of the former system. T h e performance of the n e w multi-level Q'PSK modulation schemes, denoted by Quadrature-Q u a d r a t u r e A m p l i t ude-P hase-S hift Keying ( Q 2 A P S K ) , is compared to the original Q*PSK modulation scheme, as well as to conventional two-dimensional QPSK and QAM modulation strategies under realistic bandlimiting conditions in the presence of additive white Gaussian noise. T h e performance comparisons are made in terms of spectral efficiency (or data throughput rate) versus symbol error probabilities. The results presented exhibit an expected signal to noise penalty for Q2APSK compared t o modulation schemes with two levels p e r dimension, .such as QPSK and Q'PSK, but with a considerable increase in spectral efficiency.