Low-Complexity Detection Algorithms Applied to FTN-GFDM Systems

New applications for mobile networks are pushing the data rates to higher numbers, requiring new solutions to support the demand for throughput. In urban and dense areas, the use of massive multiple-input multiple-output, high bands in millimeter waves, and ultra-dense networks based on small cells are being used to provide high data rates. However, these solutions are not applicable to the enhanced remote areas communications scenario, where large cells must be deployed using vacant VHF or UHF bands using an opportunistic spectrum allocation approach. In this case, one reasonable solution to increase the overall system data rate is to enhance the waveform spectrum efficiency. This paper proposes a scheme to improve the generalized frequency division multiplexing spectral efficiency by exploiting the faster-than-Nyquist principles in the time and frequency domains without introducing any penalties in terms of bit error rate. This achievement is obtained by under-sampling the time-frequency grid, which results in transmitting more data symbols per waveform sample, while introducing a controlled amount of interference in the transmitted waveform. On the receiver side, low complexity detectors, such as sphere decoder, successive symbol-by-symbol sequence estimation, successive symbol-by-symbol with go-back K sequence estimator, and frequency-domain equalization, can be used to reconstruct the transmitted data. This paper presents the details for adapting the mentioned detectors for the proposed waveform and it also analyzes their performance in terms of bit error rate under different channel models and implementation complexity. From all the detectors considered in this paper, the sphere decoder stands out because it presents an interesting tradeoff between complexity and bit error rate, even in high interference scenarios. Other detection schemes initially proposed for multiple-input multiple-output systems can be adapted to deal with the interference introduced by the faster-than-Nyquist generalized frequency division multiplexing with affordable complexity and acceptable bit error rate performance. Hence, one may conclude that the faster-than-Nyquist generalized frequency division multiplexing is a feasible candidate waveform for the enhanced remote areas communications scenario when high spectrum efficiency is necessary.