Signals are orthogonal if they are mutually independent of each other. Orthogonality is a property that allows multiple information signals to be transmitted perfectly over a common channel and detected, without interference. Loss of orthogonality results in blurring between these information signals and degradation in communications
Another way to view the orthogonality property of OFDM signals is to look at its
spectrum. In the frequency domain each OFDM subcarrier has a sinc, sin(x)/x,
frequency response, as shown in Figure below
This is a result of the symbol time corresponding to the inverse of the carrier spacing. As far as the receiver is concerned each OFDM symbol transmitted for a fixed time (T of FFT) with no tapering at the ends of the symbol. This symbol time corresponds to the inverse of the subcarrier spacing of 1/(T of FFT) Hz 1. This rectangular, boxcar, waveform in the time domain results in a sinc frequency response in the frequency domain. The sinc shape has a narrow main lobe, with many side-lobes that decay slowly with the magnitude of the frequency difference away from the centre. Each carrier has a peak at the centre frequency and nulls evenly spaced with a frequency gap equal to the carrier spacing