The achievement of the boundary values of spectral efficiency is associated with the development of methods for generating and receiving signals with a compact spectrum. The reason is a constant increase in the capacity of existing communication channels caused by an increase in the volume of transmitted information. Moreover, the allocated frequency bandwidths have natural limitations, and they are almost all reached. The effective way to approach the boundary values of spectral efficiency is the application of spectrally efficient signals, such as FTN (Faster-Than-Nyquist) signals. This article proposes to synthesize optimal FTN signals that are more compact in the spectrum than RRC (root raised cosine) pulses-based signals. The criterion of maximum energy concentration in the occupied frequency bandwidth and the constraint on the cross-correlation coefficient are used to solve the optimization problem. The contribution of this work is the optimization of FTN signal shape. The obtained optimal FTN signals provide a 24% increase in spectral efficiency compared to the RRC pulses-based signals. At the same time, the energy loss stays almost unchanged. To the best of authors’ knowledge, this is the first case when the frequency bandwidth reduction is achieved at practically no energy loss. The simulation modeling in channels with additive white Gaussian noise and fading channels is done with regard to the FTN-SC-FDE (Faster-Than-Nyquist - Single Carrier system with Frequency Domain Equalization) structure. The optimal FTN signals presented in this work can be used to increase the spectral efficiency of satellite broadcasting systems, such as DVB-S2/S2X.

In this paper the research on realization of transmitter for optimal signals which provide high spectral efficiency without significant energy losses is done. HackRF One is chosen as a transmitter. Experiments showed that peak-to-average power ratio (PAPR) of transmitted sequence is higher than theoretical both in case of “classic” signals with rectangular envelope form and optimal ones (with and without restriction on peak-to-average power ratio). It is caused by parasitic amplitude modulation. However, experimental values of PAPR of optimal signals are less than ones of “classic” signals due to absence of transient processes. In future works BER performance of implemented transmitter will be estimated.

This work is dedicated to researching the influence of the optimization parameter (correlation coefficient) which determines bit-error rate (BER) performance of optimal signals on their spectral and energy efficiency. Optimal envelope shapes, corresponding energy spectra and BER performance curves are presented for different limitations. Occupied frequency bandwidth and energy losses are considered. Values of correlation coefficient providing energy losses no more than 0.5 dB for error probability 10 ^-3 are obtained. The value of correlation coefficient which decrease is not efficient in terms of approaching to the Shannon limit is found.

Numerous works are dedicated to Faster-than-Nyquist (FTN) signaling providing symbol rate higher than “Nyquist limit” without energy losses. It is necessary to use infinite pulses or pulses with rather long duration for FTN signaling. Using limited duration of FTN pulses allows to simplify the structure of transceiver but causes challenges with measurement of spectral efficiency. Finite time optimal pulses with controlled level of intersymbol interference (ISI) may be used for FTN signaling. In this work it is proposed to solve optimization problem taking into account limitation on Euclidean distance between different realizations of two random FTN signals. Obtained optimal FTN signals provide the gain in energy efficiency about 2 dB and 4 times higher spectral efficiency comparing to the known spectrally efficient signals based on RRC pulses.

The increase in the throughput of digital television and radio broadcasting (DVB) channels can be achieved due to application of signals with a compact spectrum and a relatively small peak-to-average power ratio (PAPR). The reason is the usage of traveling wave tubes (TWT) for amplifying and transmitting signals from a satellite repeater in DVB-S2X systems. At the same time, given that the bandwidth allocated for transmission should be used as efficiently as possible, a high reduction rate of out-of-band emissions level is required. The most effective solution in this direction is the transition to spectrum-economic signals, such as optimal Faster-Than-Nyquist (FTN) signals, which can provide a certain reduction rate of the out-of-band emissions level and minimum acceptable PAPR. This article proposes a method for obtaining optimal FTN pulses, which have symmetry in time domain, with specified PAPR and reduction rate of out-of-band emissions for the quadrature phase shift keying (QPSK) and offset quadrature phase shift keying (OQPSK). The possibility of synthesizing signals with OQPSK modulation is presented theoretically for the first time. Optimal FTN signals can provide PAPR reduction by at most 3 dB and outperform known root raised cosine (RRC) pulses. The simulation model adopts an architecture for quadrature generation of optimal FTN signals with OQPSK modulation with blocks for adjustable pre-amplification, clipping, and power amplification. The proposed signals can be used to increase the spectral and energy efficiencies of satellite broadcasting systems, such as DVB-S2/S2X, as well as low-rate return channels of interactive broadcasting systems with a frequency resource shortage.