Low-Complexity Detection Algorithms Applied to FTN-GFDM Systems

New applications for mobile networks are driving up data rates, requiring new solutions to meet the demand for throughput. In urban and densely populated areas, massive multiple-input multiple-output, high bands in millimeter waves, and ultra-dense networks based on small cells are being used to achieve high data rates. However, these solutions do not apply to the enhanced remote areas communications scenario, where large cells must be deployed using vacant VHF or UHF bands with an opportunistic spectrum allocation approach. In this case, a reasonable solution to increase the overall system data rate is to enhance the waveform spectral efficiency. This paper proposes a method to improve the spectral efficiency of the generalized frequency division multiplexing scheme by exploiting the faster-than-Nyquist principle 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 more data symbols being transmitted per waveform sample while introducing a controlled amount of interference into 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 of adapting the mentioned detectors to the proposed waveform and analyzes their performance in terms of bit error rate under different channel models and implementation complexity. Among all the detectors considered in this paper, the sphere decoder stands out as it presents an interesting compromise between complexity and bit error rate, even in scenarios with high interference. Other detection techniques, originally proposed for multiple-input multiple-output systems, can be adapted to handle the interference introduced by the faster-than-Nyquist generalized frequency division multiplexing with reasonable complexity and acceptable bit error rate performance. Hence, one may conclude that the faster-than-Nyquist generalized frequency division multiplexing is a suitable candidate waveform for the enhanced remote areas communications scenario when high spectrum efficiency is required.