| The integration of low-orbit satellite(LEO)communication systems into terrestrial-based networks to build an integrated Satellite-Terrestrial system is the inevitable trend of nextgeneration(Next-G)communication network.With the increase of communication capacity,the communication carrier frequency is also extended to millimeter wave band,and the transmission rate will reach hundreds of mega-bauds.The relatively high-speed movement of LEO satellites poses serious challenges to the signal synchronization and antenna tracking of the ground receiving terminal.First,broadband signals modulated into the millimeter wave band produce Doppler shifts of several megahertz and ‘compression/expansion’ of the modulated waveform in high-speed mobile channels,which place higher requirements on the frequency synchronization and symbol timing recovery capabilities of terrestrial communications receivers.Second,with the increase of signal bandwidth and antenna size,the conventional phased array system will lead to more serious beam squinting phenomena due to the phase variation in the passband caused by the propagation time delay.Furthermore,with the scale-up of the array size,the problems of frequency selective fading and intersymbol interference caused by the propagation delay are more prominent.Finally,the issue of frequent handovers arising from the short satellite transmission time also impairs communication performance.In order to guarantee the communication service quality,the terrestrial terminal antennas should be equipped with broadband multi-beam tracking capability.However,how to implement an efficient and flexible broadband multibeam forming and tracking framework in the context of large-scale arrays is an urgent problem to be solved.To address the above challenges and issues,this dissertation delves into a digital wideband receiving system based on a Software-Defined-Radio(SDR)platform,deduces a wideband symbol timing recovery architecture using a polyphase signal processing approach,and proposes a high-precision frequency estimation and compensation algorithm for carriersuppressed wideband modulated waveforms while keeping the computational complexity in an acceptable level.Considering the application of wideband multi-beamforming techniques to ground terminal antenna arrays,a True-Time-Delay(TTD)array architecture based on Variable Fractional Delay(VFD)filter is proposed to eliminate the beam squinting,frequency selective fading,and inter-symbol interference introduced by broadband large-scale antenna systems.The main research contents and contributions of this dissertation are listed as follows:1.The Doppler effect in low-orbit satellite broadband communication is studied,and the broadband digital synchronization reception architecture is designed based on the SDR platform.For the ‘compression/expansion’ issue of broadband modulated waveform spectrum caused by the Doppler effect in the high-dynamic scenario,a parallel symbol timing recovery structure based on Gardner’s algorithm is designed considering the polyphase signal processing approaches,and the expressions for polyphase fractional time delay,parallel loop filter,and parallel time adjustment controller are derived to solve the fault of misalignment of feedback delay and sampling pulse.Finally,the BER curves obtained from computer simulations show that the demodulation loss is less than 0.5 dB at low signal-to-noise ratio(SNR).To cope with the large Doppler frequency shift,an iterative window function matching estimation algorithm based on the traditional DFT frequency offset estimation algorithm is proposed,and the mean square error(MSE)of the new algorithm is derived.Then,the problem of SNR loss due to nonlinear transformation is analyzed,and the frequency estimation threshold of the new algorithm is verified based on MATLAB.The simulation shows that the frequency estimation accuracy of the proposed algorithm is improved by 3 dB on average compared with the existing algorithms.Regarding the residual frequency tracking stage,this dissertation proposes a region division rule according to the minimum error probability criterion to optimize the performance of the nonlinear mapping function,which makes the Matsuo-Loop increase the convergence speed as well as reduce the steady-state error.2.To solve the challenges of beam squinting,frequency selective fading and inter-symbol interference in wideband large-scale antenna arrays for phased array technology,a wideband beamforming technique based on TTD array is presented.Firstly,we propose a linear programming-based variable fractional delay(VFD)filter design algorithm to address the requirements of time delay accuracy and time delay range in the TTD array.To cope with the problem of increasing time delay in the high frequency band,a novel algorithm for designing VFD filter coefficients is proposed in conjunction with delay error constraints.The nonlinear constraint of the delay error is converted into an affine function expression by using an approximation method,therefore,the filter coefficient design problem can be converted into a convex optimization problem.Then,the logarithmic barrier method is used to prove the presence of optimal solutions.Compared with existing algorithms,the design examples show the proposed method achieves a 100-fold improvement in delay accuracy while maintaining the frequency response(FR)error at an acceptable level and yields a 5dB reduction in FR error with the same level of delay accuracy.Secondly,to address the resource consumption challenge of VFD filters in polyphase signal processing scenarios,the polyphase Farrow filter architecture is optimized based on the iterative short convolution(ISC)approach.Then,the proposed architecture saves 45% of resources and outperforms the existing algorithms in terms of frequency response error and time delay accuracy by comparative analysis.Finally,in order to deal with the requirement of high accuracy and low complexity of the time delay estimation algorithm in wideband large-scale array systems(LSASs),a new fast polynomial inversion-based time delay estimation(TDE)method is proposed in this dissertation.A convex parabolic pole equation is introduced as the timing error detector(TED),and a polynomial inversion-based function is designed to establish a one-to-one mapping relationship between TTD and TED using the minimum mean square criterion; then,a high precision time delay estimation is obtained by the inversion function.Computer simulation results show that the estimated performance MSE of the new algorithm differs from that of CRLB by less than 1 dB at the cost of adding several multipliers/adders compared to the convex parabolic interpolation method.3.To satisfy the requirement of wideband multi-beam tracking at the receiver end in LEO satellite communication system,a TTD array structure is designed for wideband multi-beam tracking.First,a new spatial filter design method is proposed to optimize the beam response in a specific direction and at a specific frequency by introducing a transformation matrix and implementing a linear constrained minimum variance(LCMV)method.Secondly,we propose a TDE algorithm with a Newton-Raphson iterative update process,which can fast estimate the arrival time delay differences between sensors.Due to the integration of the VFD filter and the fast TDE module,the new architecture can achieve beam steering and tracking in 10 ms.Finally,the simulations show that the proposed architecture can produce accurate beam pointing with demodulation loss less than 0.5 dB in multi-carrier and multibeam scenarios and demonstrate that the new architecture has the capability of multibeam tracking while eliminating narrowband interference. |
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