In this paper

we present an amicable-orthogonal-design-based differential space-time block code (ADSTBC) for multiple antennas.Compared with the existing differential space-time modulation designs

our scheme imposes no restrictions on underlying signal constellation

and therefore can improve the spectral efficiency by exploiting efficient modulation techniques such as QAM

APSK etc.We then derive a maximum-likelihood differential decoder (MLDD) with linear complexity for flat Rayleigh fading channels.Particularly

when the QAM constellation is used in ADSTBC

MLDD can be further simplified to independently detect the real and imaginary parts of each data symbol

and accordingly the implementation cost can be greatly reduced;Moreover

the computational amount for detecting single data symbol is kept constant in MLDD while the order of the QAM constellation increases.