  This work addresses the issue of interference generated by co-channel users
in downlink multi-antenna multicarrier systems with frequency-packed
faster-than-Nyquist (FTN) signaling. The resulting interference stems from an
aggressive strategy for enhancing the throughput via frequency reuse across
different users and the squeezing of signals in the time-frequency plane beyond
the Nyquist limit. The error-free spectral efficiency is proved to be
increasing with the frequency packing and FTN acceleration factors. The lower
bound for the FTN sampling period that guarantees information losslesness is
derived as a function of the transmitting-filter roll-off factor, the
frequency-packing factor, and the number of subcarriers. Space-time-frequency
symbol-level precoders (SLPs) that trade off constructive and destructive
interblock interference (IBI) at the single-antenna user terminals are
proposed. Redundant elements are added as guard interval to cope with vestigial
destructive IBI effects. The proposals can handle channels with delay spread
longer than the multicarrier-symbol duration. The receiver architecture is
simple, for it does not require digital multicarrier demodulation. Simulations
indicate that the proposed SLP outperforms zero-forcing precoding and achieves
a target balance between spectral and energy efficiencies by controlling the
amount of added redundancy from zero (full IBI) to half (destructive IBI-free)
the group delay of the equivalent channel.
