Feasibility of Interference Alignment in the Time-Frequency Domain

In this paper, we study the feasibility of the interference alignment (IA) scheme for the selective fading interference channel with time-frequency selective links. Unlike the frequency selective channel case, we show here that, for the scalar time-frequency (TF) selective underspread channels, there is no need to scale the bandwidth as the number of users. However, a constraint on the delay-Doppler spread of the channel itself is required. A physical interpretation of this condition in the time-frequency domain is provided and is shown to be easily satisfied in practical system. Our results are based on the polynomial channel decomposition that we derive for the TF channel with large signal duration and bandwidth.