Reasearch On Single-carrier Faster-than-nyquist Systems For High-throughput Satellites

With the rapid development of mobile communication technologies in recent years,the commercial applications of fifth generation mobile communication(5G)have officially started,and major academic institutions are committing to the research on sixth generation mobile communication(6G),where the high-throughput satellite is the indispensable supplement to the ground mobile network.However,high-throughput satellites are currently in short supply of spectrum resources and have low spectrum efficiencies;therefore,their capacities are far from meeting the requirements of 6G.As a revolutionary technology for high-throughput satellites,the faster-than-Nyquist(FTN)system has been attracting more attention because it can increase the transmission rate and spectrum efficiency without additional bandwidth and antennas;therefore,it can meet the new requirements in the new application scenarios of 6G in the future.Specifically,FTN systems have dual advantages: FTN systems are independent and compatible with existing spectrum efficiency improvement technologies,hence the limit of the spectrum efficiency can be further improved by combining FTN systems with the existing spectrum efficiency improvement technologies;compared with high-order modulation types and low roll-off factors commonly used in Nyquist systems,FTN systems are less sensitive to phase noise and timing errors.Nevertheless,FTN systems violate the Nyquist criterion,which will inevitably introduce inter-symbol interference(ISI)and degrade their performance.Focusing on the satellite-to-ground links of high-throughput satellites,we are committed to researching FTN systems from three aspects: capacity and spectrum efficiency,ISI elimination algorithms in different satellite communication scenarios,distortion.The main innovations of the paper are summarized as follows:1.As a promising spectrum efficiency improvement technology,capacity and spectrum efficiency are the cornerstones of research on FTN systems.At present,there has been relatively sufficient research on the constrained capacities of FTN systems.It shows that capacities of FTN systems are not lower than capacities of Nyquist systems;only when the shaping pulse is a sinc pulse,two of them are equal.On the other hand,as another key performance indicator of FTN systems,the spectrum efficiency improvement rate directly indicates and reflects the superiority of FTN systems,and has great guiding significance for the design of FTN systems;however,the acceleration factor is still regarded as the only evaluation index of the spectrum efficiency improvement rate,and there are few detailed studies on the spectrum efficiency improvement rate.Using the raised cosine roll-off filter as transfer function of the FTN system,we deduce the capacity of the single-carrier FTN system,and compare it with the capacity of the Nyquist system.In this way,we obtain the relationship among the spectrum efficiency improvement rate,the acceleration factor and the roll-off factor,thereby improving the foundation of research on FTN systems.2.For the low symbol estimation accuracy in the satellite gateway scenario,we propose three algorithms to eliminate FTN-induced ISI: multi-layer iterative successive interference cancellation algorithm,cyclic prefix/suffix-based GTMH precoding algorithm and circular convolution-based GTMH precoding algorithm.Existing successive interference cancellation algorithms have low estimation accuracies,and they are only suitable for FTN systems with low-order modulations and very mild ISI.The proposed multi-layer iterative successive interference cancellation algorithm has extremely low complexity,and simultaneously eliminates the ISI introduced by the symbols before and after the current symbol.In mild ISI cases,it can approximate the bit error rate(BER)performance in Nyquist systems.Traditional precoding algorithms ignore interference between transmitted symbol blocks.Using cyclic prefix/suffix and circular convolution,the proposed cyclic prefix/suffix-based GTMH precoding algorithm and circular convolution-based GTMH precoding algorithm construct accurate ISI matries so that they have low error vector magnitudes(EVMs),which makes them able to approximate the BER performance of Nyquist systems in moderate and even serious ISI cases.The precoding and decoding process can be implemented with fast Fourier transform(FFT)and inverse fast Fourier transform(IFFT),so the proposed precoding algorithms have relatively low implementation complexities.In addition,considering that the GTMH precoding algorithm changes the spectral characteristic of the transmitted signal,we derive and reveal the direct relationship among the spectrum broadening,the actual maximum spectrum efficiency improvement,the acceleration factor,and the roll-off factor.It is of great guiding significance for designing FTN systems using the GTMH precoding algorithm.3.For the low channel and symbol estimation accuracy in small and medium satellite receivers,a joint channel estimation and precoding algorithm is proposed to eliminate the ISI caused by multipath and FTN systems.In terms of channel estimation,existing algorithms cannot estimate both forward and backward multipath at the same time due to their pilot design flaws,however,the designed pilot structure enables the proposed channel estimation algorithm to estimate forward and backward multipath simultaneously.The proposed channel estimation algorithm performs precoding processing on the pilots at the transmitter,which relieves the noise amplification phenomenon,significantly improving the channel estimation accuracy.After obtaining the estimated channel information,the transformed channel information can be introduced into the precoding process of the payload symbols,and the ISI introduced by the multipath and FTN system can be eliminated at the same time.The proposed joint channel estimation and precoding algorithm has double advantages over existing algorithms: first,the ISI matrix constructed by the proposed algorithm is a circulant matrix,so the proposed algorithm can be implemented by FFT/IFFT and has lower complexity;second,the proposed algorithm has lower EVMs,higher channel and symbol estimation accuracies,which ensure the extremely low BER loss when improving the spectrum efficiency.4.As for the distortion in FTN systems,since the existing fractionally-spaced equalizers cannot use traditional pilots for training and updating their tap coefficients,a precodingbased fast block fractionally-spaced equalizer is proposed to eliminate the linear distortion caused by the input and output multiplexing filters.Combined with the proposed precoding algorithm,linear distortion and FTN-induced ISI can be simultaneously eliminated.The proposed fast block fractionally-spaced equalizer constructs accurately received pilots by precoding the pilots at the transmitter.In this way,it can perform training and tap updates regardless of ISI,so it has better convergence and equalization performance.Moreover,the proposed precoding-based fast block fractionally-spaced equalizer has a high-throughput parallel structure because its implementation mainly relies on FFT/IFFT.On the other hand,we comprehensively analyze the peak to average power ratio of the FTN signal by drawing the complementary cumulative distribution function curves.On this basis,we study the saturation input/output power backoff of the high-power amplifier and analyze its influence on the power efficiency in FTN systems.Through the above research,we enrich and improve the theoretical framework of capacity and specturm efficiency,ISI elimination,channel estimation,distortion cancellation in FTN systems,providing ideas for the further research of FTN systems and the theoretical foundation for applying FTN systems in high-throughput satellites.