Research On Calibration Technologies For Spaceborne Large-Scale Multibeam Forming Systems

Spaceborne multibeam forming has emerged as a promising technology of wideband and large-capacity satellite communications due to its wide coverage with high gain and large communication capacity.It utilizes multiple high-gain narrow beams to cover large area.Fre-quency multiplexing is realized by spatial diversity.In practical spaceborne application,the performance of the spaceborne multibeam system depends on accurate amplitude and phase relationships between multiple ports of beam forming networks(BFNs)and large-scale ar-ray antennas.The ground calibration is a necessary step in the manufacturing process and the foundation of the satellite payload.However,the ground calibration before launch can-not guarantee the accurate beam patterns completely.On the one hand,the array antenna distortion and shift are introduced by tremendous impact and collisions with space debris in the satellite launch process.On the other hand,the characteristics of active electronic components are affected by different factors such as dramatic temperature fluctuations,rip-ple of the power supply,and components natural aging.Therefore,in order to maintain the optimal performance of the spaceborne multibeam forming system,it is necessary to com-bine the pre-launch calibration with the in-orbit calibration.Existing pre-launch calibration methods depend mainly on single-input single-output instruments such as the vector network analyzer.For a M-input K-output BFN,connected relations need to be changed at least(M×K)times.And these methods cannot reflect amplitude and phase errors resulting from mutual effects between multibeam signals.In addition,in-orbit calibration technologies are difficult to meet the high precision,complexity,and real-time requirements for future satellite multibeam forming systems because they not only have high numerical complexity and huge hardware resources consumption but also are unable to perform calibration process without occupying extra time slots or frequency slots of the communication traffic.Another disadvan-tage of existing calibration methods is that the calibration performance evaluation depends generally on engineering experience due to the lack of theoretical analysis.Against this back-cloth,the purpose of this paper is to solve the pre-launch and in-orbit calibration technologies for spaceborne multibeam forming systems from three layers:theoretical analysis,technical route,and practical application.It provides the theoretical foundation and technical support for the large-scale application of the future spaceborne multi-beam system.The main research works and novel contributions are summarized as follows:1.An efficient calibration method for beam forming networks based on code division multiple access is proposed to solve the defects in conventional methods such as being unable to calibrate mutual coupling inside the network,low efficiency,and poor anti-noise perfor-mance.Parallel and accurate calibration for BFNs with multi-input and multi-output ports can be achieved.A single calibration operation can be used to obtain all the amplitude and phase transfer relationship of BFN.The amplitude and phase estimation probability is ana-lyzed through theoretical derivation and simulation for a given set of parameters,including the given signal-to-noise ratio,spread spectrum code length,and confidence interval.The the-oretical results are validated by computer simulation.On this basis,in order to mitigate the effects of the channel mismatch in the calibration receiver and the phase noise introduced by oscillators,a virtual multichannel receiver method based on time division multiple switching is proposed.The point of this method is to make quick switching between different channels with a numerical-control electronic switch in the front end of the calibration receiver.Thus the scale and costs of hardware circuits can be effectively reduced.The related results are successfully applied to the ground test of BFN network in China.2.A low-complexity calibration method for spaceborne large-scale array antennas based on non-orthogonal codes is proposed to decrease the complexity of on-board external cali-bration methods.The costs of the calibration signal generator and receiver can be reduced by K-fold with the cyclically shifting an m-sequences as the calibration codes,where K is the number of uncalibrated array antennas.In order to suppress the multiple access interference due to the nonorthogonality of calibration waveforms,an anti-interference method with low complexity based on the decorrelation algorithm is proposed.As the m-sequence has two-value autocorrelation property,its correlation matrix shows a special structure.Thus matrix inversion and multiplication calculations involved in decorrelators are simplified.The com-plexity analysis shows that the complexity of the calibration receiver can be reduced from O(L~2·K)to O(L~2+K),where L is the code length.The relationship between the ampli-tude/phase calibration accuracy and the noise is derived to evaluate the performance of the code division multiple access calibration method.To the author's knowledge,the theoreti-cal calibration accuracy has not been previously derived.The generality of this theoretical analysis framework is that it is not only suitable for the proposed low complexity calibration method based on non-orthogonal waveforms,but also the conventional calibration method based on orthogonal waveforms.The principle prototype and demonstration system are de-veloped to verify the simulation and design results of the above analysis.The theoretical,sim-ulation,and experimental results show that the performance of the proposed non-orthogonal waveform calibration method is equivalent to that of the conventional orthogonal waveform calibration method.The related results are successfully applied to a principle prototype for the national high technology research and development plan major project”863-3G”.3.An in-orbit real-time calibration method for spaceborne array antennas without occu-pying time slots or frequency slots of satellite normal communication traffic is proposed to calibrate fast-changed dynamic errors.The calibration process can be carried out with satel-lite communication traffic simultaneously.Since the power of calibration signals is much lower than that of communication signals,the communication quality is not affected by cal-ibration signals.The calibration performance in the presence of Gaussian white noise and co-channel communication signals is analyzed.In order to suppress the strong disturbance from communication signals,an in-orbit real-time and accurate calibration method based on the decorrelation algorithm is also proposed.The calibration accuracy with weak calibration signals can be improved and the satellite normal communication traffic remains unaffected.The complexity analysis shows that the proposed method without online matrix inversion computation is easy to implement.Moreover,this method is suitable for other communi-cation signals with amplitude-phase modulation constellation.In addition,the relationship between the root mean squared error(RMSE)of the relative amplitude/phase and the signal-to-noise ratio is derived.Simulation results show that the calibration accuracy of the proposed method is the same as that of the method without communication traffic if the total power of calibration signals is 40d B lower than that of the communication signal.