  This paper considers a $K$-user single-input-single-output 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 primary finding of this paper is that, with no channel state information at
a transmitter (CSIT), the sum-spectral efficiency can be made to scale linearly
with $K$, provided that the desired links have longer impulse response than do
the interfering links. This linear gain is achieved by a novel multi-carrier
communication scheme which we call \textit{interference-free orthogonal
frequency division multiplexing (IF-OFDM)}. Furthermore, when a transmitter is
able to learn CSIT from its paired receiver only, 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
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. Simulation results support this claim.
