Orthogonal Frequency-Division Multiplexing :
In OFDM, the subcarrier separation is reduced to a minimum while subcarrier orthogonality is maintained
(The receiver is still able to detect the data that modulates each individual subcarrier without any mutual).
Orthogonality:
Two signals are orthogonal if their inner product is zero:
OFDM Baseband Model:
To preserve subcarriers’ orthogonality we must have:
Rectangular pulse shaping
Linear modulation (BPSK, QPSK, 16QAM, …) on each subcarrier
unmodulated subcarriers in baseband:
Imaginary part (sine)
In baseband:
Subcarriers’ frequencies are harmonics of the lowest-frequency subcarrier (f1=1/T)
Symbol duration T contains an integer number of periods of each subcarrier
OFDM-Symbol Waveform (Baseband):
OFDM Power Spectrum:
OFDM Power Spectrum: 16 subcarriers
Practical OFDM Spectrum:
