Constant Envelope Multiplexing For Multiple GNSS Signals At Multiple Frequencies

The global navigation satellite system(GNSS)provides all-day,all-weather,global coverage of high-precision positioning,navigation and timing(PNT)services,has become an important infrastructure in the powerful countries.In order to meet the needs of diversified users,there are more signals and more frequencies in the new generation of GNSS.Due to the limited volume and weight of the satellite payload,the multiple signal components at the same frequency or the adjacent frequencies should be multiplexed into a coherent signal to share a common high power amplifier(HPA).In order to maximize the power efficiency of HPA,the composite signal must have the characteristic of constant envelope to make the HPA work at the saturation point without signal distortion.Therefore,the constant envelope multiplexing for multiple GNSS signals is a hot issue in the design of GNSS signals.In this thesis,the constant envelop multiplexing technology for multiple GNSS signals with multiple frequencies is studied,the constant envelope multiplexing method for nonindependent signals,the general time-division Alt BOC technique with adjustable power allocation,the phase aligned dual-frequency constant envelope multiplexing method with symmetrical frequencies and a general constant envelope multiplexing method for multifrequency and multi-value signals are proposed.The main contents of this dissertation are orgainzed as follows:Firstly,the existing envelope multiplexing methods require independent signal components.To address this limitation,a constant envelope multiplexing method for nonindependent signals is proposed.In this section,the complex subcarrier modulated signal is taken as an example,and the influence of the non-independent signal components on the result of POCET is investigated based on analytical expression of POCET.The analysis results show that the waveform of the signal component will be affected by the harmful intermodulation,which is the product of the considered signal component and the two nonindependent signal components.Based on the above analysis result,a phase-optimised constant-envelope transmission method for correlated signals(CS-POCET)is proposed.In this method,the intermodulation terms are treated as a part of the analytic expression of the POCET,and the power constraint of the intermodulation terms is introduced into the optimization progress.By restricting the power of the harmful intermodulation terms to 0,the unwanted influence introduced by those harmful intermodulation terms can be eliminated.CS-POCET method can be used for constant envelope multiplexing of complex subcarrier modulation signals and constant envelope multiplexing of other non-independent signal components.Secondly,aiming at the limitation that TD-Alt BOC requires equal power allocation among the four signal components,a general time division Alt BOC technology(GTDAlt BOC)with adjustable power allocation is proposed.The performance of the proposed method is analyzed and compared with the existing methods in terms of five aspects: the power spectral density,the cross correlation function,the multiplexing efficiency,the code tracking error and the baseband processing complexity.The results show that GTD-Alt BOC has a flexible power allocation,and its performance is equivalent to the existing methods,while the complexity of the receiver baseband processing is lower than the existing methods.Thirdly,aiming at the limitation that the existing dual-frequency constant envelope multiplexing method with symmetrical frequency are mostly focus on the Alt BOC-like model,a phase aligned dual-frequency constant envelope multiplexing(PACEM)method for the general dual-frequency signal model with symmetrical frequency is proposed.Firstly,the signals in each sideband are combined into a constant envelope composite signal using the existing single frequency constant envelope multiplexing technique,and then the two composite signals in two sidebands are combined linearly and amplitude modulated to form a dual-frequency constant envelope composite signal.The power relationship between the linear combination signal and the dual frequency constant envelope signal is derived.According to the power relationship before and after modulation,a constant envelope multiplexing result that satisfying the designed power ratio can be obtained.PACEM method has no special restrictions on the number of signal components,power ratio,phase relationships and sub-carrier sampling rate.It is a general dual-frequency constant envelope multiplexing method with symmetric frequency.Based on the PACEM method,the applications and performance of the PACEM method on two dual-frequency multiplexing scenes with symmetrical frequency are analyzed,which are the Alt BOC-like scene and a general dual-frequency multiplexing scene with symmetrical frequency in Beidou B2 band.The results show that,compared with the existing methods,the PACEM method has higher flexibility and multiplex efficiency.Finally,for the constant envelope multiplexing of multi-frequency and multi-level signals,a phase-optimised constant-envelope transmission method for multi-value signals(MV-POCET)is proposed.In this method,a general GNSS signal is modeled as a multivalue signal model,the signal value can be real or complex.A phase lookup table(LUT)is constructed according to the multi-value signal model.The LUT is composed of the signal value combinations and the corresponding transmition phases.To solve the optimization problem with minimum the envelope value and subject some power constraints,phase constraints and other possible constraints,the required constant envelope multiplexing result for multi-value signals is obtained.For multi-frequency signals,each signal can be moved to its desired frequency from the center frequency by a complex subcarrier modulation.By selecting the appropriate subcarrier sampling rate,the continuous complex subcarrier modulated signal is converted into a complex signal with limited number of values,which conforming to the multi-value signal model,and then a constant envelope composite signal can be obtained through the MV-POCET method.The MV-POCET method has no limitation on number of signal components,number of frequencies,the power ratio,and the phase relationships.It is a general constant envelope multiplexing method for multi-frequency and multi-level signals.Finally,based on the MV-POCET method,three multiplexing cases for multi-level signals,dual-frequency signals,and multi-frequency signals with multi-level are analyzed.The results verify the effectiveness and correctness of the proposed method.Compared with the CEMIC methods,the MV-POCET method is more applicable and more flexible.