Figure 1. Possible multiplexing of sinc-shaped Nyquist pulses.

Periodic sinc-pulse sequences can be split into branches, each of which corresponds to an independent channel. In the nth branch, the periodic sequence is delayed by n times the interval τ=1/(NΔf), with n=0,...,N−1. Each channel can be modulated independently with a modulator M₀,...,M_N−1. These devices can apply any modulation formats to the signal. Then, the N modulated channels are multiplexed. The shown multiplexing is carried out in the time domain at one carrier wavelength. Since the multiplexed channel shows a sharp-edged spectrum, the next wavelength channel can be directly adjacent to the previous with almost no guard band and can be multiplexed in the time domain in the same way, reaching high temporal and spectral densities together.