A multiplication unit includes first, second, third and fourth receiving terminals, arithmetic units and multiplexers. In complex number mode, the first and second receiving terminals receive a real part value and an imaginary part value of a first complex number, respectively, whereas the third and fourth receiving terminals receive a real part value and an imaginary part value of a second complex number, respectively. In modulus mode, the first and third receiving terminals receive first and second integers, respectively. The multiplexers gate the arithmetic units to perform a complex number multiplication operation according to the first and second complex numbers to generate a third complex number in complex number mode and perform a modulus multiplication operation according to the first and second integers and a predetermined modulus to generate a third integer in modulus mode.