Abstract
In this paper discuss digital communication modulation techniques Binary phase shift keying (BPSK) and Quadrature amplitude modulation (QAM) modulation scheme is widely used to improve spectral efficiency in modern communication. QAM constellation structure (16, 64,128,256, etc. gives high data rate. FFT/IFFT plays a main role in or modern communication both techniques is used to convert DIT/DIF. Hence, as a fact, it does not matter whether you apply FFT (IFFT) at transmitter as long as you apply the reverse transform; IFFT (FFT) at the receiver. The signals need to be modulated by say N-QAM of the orthogonal subcarriers. Using FFT reduces the separation between the orthogonal sub-carriers. Consider a sinusoid of duration T seconds and with frequency k/T where k is a positive integer. It makes two such sinusoids as orthogonal as you want by spacing them far apart in the frequency domain so that their spectra do not overlap much. However, this is a waste of spectrum because ideally two such sinusoids which are spaced 1/T apart in frequency are orthogonal.
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