Blind Interference Alignment with ISI: A New Look at OFDM for K-User Interference Channels

This paper considers a K-user single-input-single-output (SISO) interference channel with inter-symbol interference (ISI), in which the channel coefficients are assumed to be linear time-invariant with finite-length impulse response. The key finding of this paper is that, with no channel state information at the transmitter (CSIT), the sum-spectral efficiency can be made to scale linearly with K, provided that the desired links have longer impulse responses than do the interfering links. This linear gain can be achieved (up to the maximum gain of \frac{K}{2} for a particular case) by a novel multi-carrier communication scheme, termed interference-free orthogonal frequency division multiplexing (IF-OFDM). Besides, when a transmitter is able to acquire CSIT from its paired receiver only, i.e., local CSIT, a higher sum-spectral efficiency can be achieved by a two-stage transmission method that concatenates IF-OFDM and vector coding based on singular value decomposition with a water-filling power allocation. A major implication of the derived results is that separate encoding across subcarriers per link is sufficient to linearly increase the sum-spectral efficiency with K in the interference channel with ISI. Furthermore, we discuss several key ideas to facilitate the proposed IF-OFDM from a practical perspective even when the desired links do not have sufficiently long impulse responses. Various numerical results are also provided to support this theoretical analysis.