先進相列雷達關鍵技術研究

Abstract

本計畫主要是協助中科院發展新一代之陣列雷達系統，研究工作由二大分項所組成，此二大分項又由5個研究工作所組成，其中分項(一)為主動式電子掃描系統前瞻關鍵研究，而分項(二)為被動雷達系統關鍵研究。 在分項研究(一)中，尤其是主動式電子掃描系統，針對RF系統和數位訊號之處理，相關研究構建了前瞻關鍵技術的核心。整體計畫由三大區塊建構而成，包括了陣列天線設計，高頻RF接收機之設計和陣列信號數位波之天線元素。計畫中將利用四個象限之陣列天線來形成Sum及Difference波束，並以主動雷達系統架構實現智慧型天線中switch-beam之概念，以產生具高方向性與低干擾之發射波束，進而優化了回射波在Sum和Difference的特性。在系統之實際構裝上兼顧了系統的擴充性和國內產業的優勢，故在天線設計方面採模組化的方式建構，以2n之元素(n=3)建構於同一模組上，並利用了IC製程之多層模板機構來建置相移器。本計畫為多年期計畫之第二年，將針對前一年計畫執行之成果進行延續性之研究。本年度之工作重點在於前端系統中所有主動元件關鍵零組件之電路設計以及陣列天線單元間耦合效應分析，另外為了考慮未來較多單元系統在電性功能及構裝上的擴充性，有關天線單元以及後端電路間進一步的整合亦將一併探討。有關在高頻操作下電磁波之可能造成耦合與輻射效應，包含天線單元彼此間及天線與後端電路之耦合亦將透過電磁分析模擬技術加以分析，最後將利用分析結果對系統端進行佈局之最佳化設計。 在分項研究(二)中，主要針對被動雷達系統進行關鍵技術之研究，此關鍵技術將包含三個重要關鍵技術(三個工作項目)需進行研究: (2.1)低訊干雜比目標來向(DOA)接收機關鍵技術： 主要是以FPGA設計環狀相列天線多模式數位波束成型處理，應用於被動相列雷達與雙態被動相列雷達系統，實現低訊干雜比(signal-to-interference and noise ratio, SINR)目標來向(Direction of arrival, DOA)估測的功能。配合設計超低訊干雜比尋向估計在角擴散環境之需求，進行全向性環狀相列天線強 健式零化延展之研究。並採用環狀相列天線多模式數位波束成型處理與倍頻多載波直接展頻序列(multiple frequency spreader – multiple carrier – direct sequence spread spectrum, MFS-MC-DSSS)調變波形，設計雙態被動相列雷達之低訊干雜比目標來向估測接收機架構與演算法。 環狀相列天線多模式數位波束成型處理產生雷達之直接路徑(line-of-sight, LOS)接收信號與飛行目標之反射回波信號，並以倍頻多載波直接序列展頻技術來滿足遠距離偵測、高距離解析度與抗干擾之需求，估測低訊干雜比飛行目標反射回波信號之目標來向。 研究項目包含： 環狀相列天線多模式數位波束成型處理FPGA設計。 倍頻多載波直接序列展頻波形設計與混淆函數公式推導。 環狀相列天線雙態被動相列雷達低訊干雜比目標來向估測接收機之抗雜訊性能模擬。 環狀相列天線強健式零化延展以減輕角擴散干擾對尋向估計之影響研究。 (2.2)低訊干雜比之主輔雙陣列天線之快速穩健適應性干擾消除法關鍵技術： 藉由民間廣播電台（類比FM與數位DVB）之發射信號，偵測空中未知目標物之反射信號，藉由天線產生適應性波束指向未知目標物反射之方向，增強未知目標物反射之接收訊號，並針對廣播電台直視波干擾信號與地下電台干擾信號進行干擾消除。主要技術包括智慧型天線波束成型、可適性濾波器。本關鍵技術預計研究項目為： 研究藉由Batch方法產生權重值之系統性能。 研究CLMS（Constraint Least Mean Square Algorithm）進行濾波器之權重值收斂，並探討其收斂速度與系統效能與Batch做比較。 研究收斂更為快速的CRLS（Constraint Recursive Least Square Algorithm）並探討其系統性能與Batch和CLMS做比較。 研究干擾直射波與目標物反射波入射角度偏差之系統容忍度模擬分析。 (2.3)應用於被動相列雷達系統超低訊雜比之直接路徑干擾消除法則： 和主動雷達不同，被動雷達不主動發射訊號即可偵測目標，故擁有不易被攻擊之主要優點而備受重視。在被動雷達系統中，由於直接路徑之接收訊號會把來自目標的回波訊號掩蓋。故必需消除直接路徑干擾，保留未知目標物回波。 在前一個年度的計畫當中，吾人基於直接路徑干擾延遲相對於接收機取樣頻率為整數的假設，設計出直接干擾消除機制及可適性演算法。然而，在更為實際的場景中，直接路徑干擾延遲可能具有分數間距。這將使得前述法則不能將干擾完全消除。因此在本年度的計畫當中，吾人首先將重新推導直接路徑干擾有分數間距時的時域或頻域訊號模型。接著將問題切割成兩個部份：分數間距干擾延遲估測和分數間距插補。首先，吾人將利用LS/WLS配合ANPA演算法進行直接路徑干擾延遲的估測，並且分析其複雜度和估測性能。在插補器的部份，吾人首先分析模擬現有的最小均方誤差(MMSE) FIR 插補器、多項式(Polynomial) FIR插補器、及Farrow架構插補器之插補性能，並進一步開發高精度的插補演算法。 本研究之成果可做為強化被動雷達系統設計之參考依據，所開發之分數間距延遲估測及高精準度插補器等演算法亦可應用於無線通訊領域。

The research works aim to develop state-of-the-art technologies for the applications of radar or smart antenna systems. The research work is consisted of two fundamental topics including(1)Advanced research of active electronic scanning system,and(2) Advanced research on passive radar system. In the topic(1),we intend to develop key elements that can be used in an antenna system consisting of antenna, RF analog T/R modiateule, digital signal processing unit and post adaptive beam forming unit. Since the radiation pattern of such antenna system can easily be adjusted through proper modifications on the weighting functions to respond to environmental changes almost immediately, it has the advantages of interference suppression, communication quality improvement and increase of channel capacity. Therefore, it has been widely applied in advanced phased array radar systems. In general, the complexity of such antenna system depends strongly on system applications. For advanced wireless communication systems, phased array antenna structure is usually adopted with large number of T/R modules involved. The most challenging issue of this arrangement lies in the complexity of system integration caused by the number of T/R modules involved ,which in turn increases drastically the DC power consumption required. The main purpose of this (reseaech topic) is the design and development of Ku-band switched-beam phased array radar systems. This is the second year of the multi-year project with the main focus on the MMIC designs of active components in the T/R and the accurate assessment of all the possible electromagnetic couplings among antenna elements while integrated into compact modules. The main technology development will cover the designs of Ku-band power amplifier, Ku-band low noise amplifier, Ku-band switches, Ku-band phase shifters and the feasibility study of integration of the complete T/R onto single chip. Meanwhile, to guarantee the proper functionality of the whole system, electromagnetic couplings among the antenna elements have to be accurately characterized. Full wave electromagnetic simulation in conjuction with the hybridization method will be applied to resolve this issue. In the topic(2), the key technologies of advanced (passive) phased array radar is developed. This project focuses on passive radar system. It includes three working items: 2.1，DOA Receiver Design for Passive Phase Array Radar System with Low SINR This working item uses a multimode digital beamformer (DBF) of circular array antenna and multiple frequency spreader – multiple carrier – direct sequence spread spectrum (MFS-MC-DSSS) modulation waveform to design a direction of arrival (DOA) estimation receiver and algorithm for a bistatic passive phased-array radar. Specifically, the multimode digital beamformer (DBF) of the circular array antenna can receive both the line-of-sight (LOS) direct path reference (DPR) signal and the low signal-to-interference and noise ratio (SINR) reflected air target signal, simultaneously, on different sub-arrays. The nulling mode of the multimode DBF is used to suppress the direct path interference (DPI) signal and the stationary clutter signal, respectively. In addition, the MFS is applied to the MC-DSSS waveform to provide the spreading gain, which can overcome the very low SINR problem of the reflected air target echo signal. The research items include:  FPGA design of the multimode DBF processor for a circular array passive phased-array radar.  Derivation of the ambiguity functions for MFS-MC-DSSS bistatic passive phased-array radar.  Simulation of the low SINR detection performance for the DOA estimation receiver of a circular array bistatic passive phased-array radar.  Study of the robust null-extension for a circular array antenna to reduce the effect of angular spread on the accuracy of DOA mechanism. 2.2，Research of Fast Adaptive Beamforming Algorithm for Passive Dual Phase Array Radar System with Low SINR This working item design an advanced dual adaptive phase array signal processing algorithm which uses the transmitted signal of the commercial radio stations (FM and DVB) to detect the unknown aircraft targets. By using the robust digital beamforming, we can reserve and enhance the reflect signal of the unknown aircraft targets. Moreover, we can suppress the strong interference arrived from the direct-path LOS signal of commercial radio stations and illegal radio station. The major technologies include the smart antenna system and adaptive filter. The research item is summarized as follows：  Research the system performance by using the Batch method to calculate the dual array weighting values.（midterm report）  Research the Constraint Least Mean Square (CLMS) algorithm which converges the weighting values and is used to compare the performance with the Batch method. （midterm report）  Research the Constraint Recursive Least Square (CRLS) Algorithm which converges much faster and is used to compare the performance with the Batch and CLMS methods.（final report）  Research the arrival angles with point errors of the unknown target and direct-path LOS and analyze the system performance degradation.（final report） 2.3、Direct Path Interference Cancellation for Passive Phase Array Radar System with Ultra Low SINR Differing from the active radar, the passive radar can detect the unknown targets without emitting any electromagnetic wave itself. Hence the passive radar system is harder to be detected, which makes passive radar more attractive as military RADAR. However, the echo signal of the unknown target is always buried by the direct path interference (DPI) from transmitter. Hence an interference cancellation scheme must be adopted to suppress the direct path signal and make the echo signal to be detectable. In the first year, we have designed the direct path interference cancellation schemes and adaptive algorithms based on the integer-delay assumption of DPI. However, the DPI delay may contain fractional part. Then the DPI can not be perfectly eliminated by designed algorithms. Hence we will re-formulate the time domain and frequency domain signal model of the DPI with fractional delays. Then we divide the problem into two parts: fractional interference delay estimation and factional interference delay interpolation. At first, we will combine the LS/WLS and the ANPA algorithm to estimate fractional delays. To design the interpolator, we will first apply the existing algorithms like MMSE FIR interpolator, polynomial FIR interpolator and Farrow interpolators, and simulate their performances. Then we will try to develop high accuracy interpolation algorithms. Through this project, the target detection probability can be improved, and a robust passive radar system can be developed. Besides, the developed estimation and interpolation algorithms can also be applied to wireless communication.