Faster-than-Nyquist (FTN) transmission employs non-orthogonal signaling to improve spectral efficiency over conventional orthogonal transmission at the Nyquist rate. However, FTN signaling also introduces inter-symbol interference (ISI), which must be mitigated through additional signal processing.In this paper, we present a scalable FPGA-based architecture for a turbo maximum-a-posteriori (MAP) equalizer based on the Bahl-Cock-Jelinek-Raviv (BCJR) algorithm to mitigate the ISI. In contrast to many existing hardware implementations of BCJR, where the algorithm performs turbo decoding over a binary alphabet for an already-equalized channel, our FPGA design applies BCJR to non-binary signal constellations and for the task of equalization. To the best of our knowledge, this is the first published FPGA-based BCJR equalizer implementation suitable for FTN applications, where a binary forward-error-correction decoder is employed in tandem with the equalizer.Through careful tradeoff selection and optimization of the design space, our implementation achieves a maximum per-PE throughput of 602 Mbps, and a total of 15.4 Gbps within the constraints of a Xilinx UltraScale+(xcvu13p) device.

Faster-than-Nyquist (FTN) transmission employing antenna polarization multiplexing and higher order modulation (HoM) schemes can significantly increase the spectral efficiency (SE) of the existing wireless backhaul links. However, the benefits of each of these SE enhancement techniques come with a price. While FTN introduces inter-symbol interference, a dual-polarized (DP) transmission suffers from cross-polarization interference (XPI), and HoM makes a communication system vulnerable to phase-noise (PN) distortions. In this paper, we investigate, for the first time, a DP-FTN HoM transmission system. We propose a XPI cancellation and PN mitigation structure, coupled with adaptive decision-feedback equalization or linear precoding, to jointly mitigate interference and accomplish carrier-phase tracking. The DP systems combined with the FTN signaling presented in this paper offer more than 150% increase in SE compared with a single-polarized Nyquist transmission. The effectiveness of the proposed algorithms is demonstrated through computer simulations of a coded DP-FTN microwave communication system in the presence of PN. Numerical results suggest that with the proposed interference cancellation methods, a DP-FTN transmission can yield a 3-5.5-dB performance improvement over an equivalent DP-Nyquist system that employs a higher modulation order to achieve the same data rate.

Faster-than-Nyquist (FTN) transmission applies non-orthogonal linear modulation to increase spectral efficiency compared with the well-known orthogonal transmission at Nyquist rate. This comes at a price of inter-symbol interference (ISI), which usually is equalized through receiver processing. In this paper, we investigate the alternative approach of pre-equalization at the transmitter. First, we consider Tomlinson-Harashima precoding (THP) for FTN and propose two novel soft demapping algorithms to generate the soft-input for the error-correction decoder. The developed demappers effectively compensate the modulo-loss associated with conventional THP transmission. Second, we propose a linear pre-filtering strategy to pre-equalize the ISI induced by FTN. We show that the linear pre-equalization approach is equivalent to an orthogonal transmission with a modified pulse shape. It thus yields the optimal error-rate performance while affording higher spectral efficiency. We validate our proposed precoding algorithms through computer simulations of a coded coherent optical communication system as a practical application example for FTN.