| Tropospheric scattering communication has received widespread attention at home and abroad due to its strong confidentiality and long transmission distance.It is considered to be one of the important method of communication in military communications.However,tropospheric scattering has serious fading effects,including large-scale fading,frequency selective fading and fast fading.This paper proposes that the SC-FDMA communication system with low peak-to-average ratio can be used for signal processing in a scattering environment,so as to avoid distortion of the output signal of the power amplifier.At any time,synchronization is the first problem that a communication system needs to solve.The path loss of tropospheric scattering communication is large,the channel environment has extremely low signal-to-noise ratio,and the channel has fading phenomena such as fast fading.Therefore,it is necessary to design a low-complexity timing synchronization algorithm and frequency offset estimation algorithm in a low signal-to-noise ratio environment.This article first introduces the principle and fading phenomenon of tropospheric scattering communication,and gives the typical path loss model of tropospheric scattering channel and the channel transmission model of scattering channel.Aiming at the current scattering environment,a SC-FDMA baseband communication system with low peak-to-average ratio is designed.Finally,the physical layer baseband processing flow of the SC-FDMA system,the physical layer frame structure,the division of uplink and downlink channels and the design of system parameters are given.Then the timing synchronization algorithm under extremely low signal-to-noise ratio is studied.First,the optimal value of the root value of the synchronization header sequence used in this paper is given by simulation.Then a piecewise correlation accumulation algorithm based on Barker code weighting is proposed.The algorithm uses Barker code to weight the repetitive ZC sequence.Simulation shows that although the algorithm reduces the computational complexity,the probability of missed detection has a loss of about 1d B compared with time domain correlation algorithms.Therefore,this paper proposes a synchronization algorithm based on Barker code weighted segment correlation combined with RAKE reception.Based on the Barker code weighted segment correlation,the algorithm uses RAKE reception to reduce the impact of noise.The simulation proves that the improved algorithm has a gain of 1-2 d B compared with the traditional time-domain correlation algorithm at-17d B,and the probability of missed detection and false alarms both reach10-4 at-17d B.Finally,the hardware implementation of the improved algorithm proposed in this article is carried out using FPGA.The hardware implementation results show that the resource consumed by the improved algorithm is much less than that of the existing algorithm.Although the improved algorithm increases the complexity,the gain is huge.The algorithm proposed in this paper can achieve the optimal compromise between performance and cost while meeting the detection index.Finally,the frequency offset estimation algorithm of SC-FDMA system downlink under tropospheric scattering environment is studied.The SC-FDMA interleaved mapping and IFFT operation are used to make the SC-FDMA symbols have a natural repetitive structure in the time domain.Based on this,this paper proposes an improved algorithm that replaces the arithmetic average with an approximate Gaussian average in a specific signal-to-noise ratio range.The simulation results show that the improved algorithm has a lower MSE in a specific signal-to-noise ratio range.The improved algorithm can fully estimate the fractional frequency offset.Taking into account the insufficient performance of the algorithm under low signal-to-noise ratio,this paper then proposes an estimation algorithm using the pilot structure of the SC-FDMA system.The estimation algorithm also has a lower MSE under low signal-to-noise ratio. |
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