| Secondary radar plays an important role in modern military and civilian fields.With the increasing complexity of detection environment and the emergence of cluster targets,secondary radar will face complex electromagnetic environments such as dense signals,strong countermeasures,and dynamic changes.However,in the current cluster warfare,the reply power of a single small target is usually low,which makes it difficult for the interrogator to receive remotely and makes the traditional secondary radar reply mechanism difficult to meet the detection need of large-scale cluster targets.Aiming at this problem,this thesis draws inspiration from the sparse array of MIMO radar and the Rake receiving of spread spectrum communication,and proposes two signal processing methods to improve the reply performance of secondary radar cluster targets.Both of these two methods can receive the reply signals of multiple targets in the cluster and make cumulative judgments,which greatly improves the receiving efficiency of secondary radar.At the same time,the output signal noise ratio of the received signal is also improved,and the symbol error rate is reduced.This thesis mainly focuses on the problem of secondary radar cluster target reply.The main work can be divided into the following two parts:1.Inspired from the idea of MIMO radar,an inquiry-reply receiving model of MIMO system is established.The cluster targets are regarded as array elements,and each of them directionally synchronous transmits the same reply signals through orthogonal coding modulation,and the multi-antenna receiving synthesis is carried out in the interrogator,so as to achieve high gain.At the same time,since the distance between the reply targets in the cluster will produce the orientation pattern grating lobes,this thesis proposes a uniform circular array null-broadening algorithm to optimize the receiving orientation pattern of the interrogator,and derives the conization transformation formula of the covariance matrix in the case of uniform circular array,which is corresponding to the null-broadening within the range of the grating lobes.Not only can the grating lobes be better suppressed,but also the radar angle resolution can be further improved.The effectiveness of the proposed algorithm is verified by simulation.The algorithm can improve the grating lobe suppression effect of the MIMO radar transceiver under the same conditions,but it has high requirements for target synchronization in the cluster.2.To solve the problem of high synchronization requirement in the above method,this thesis proposes a design idea of merging multiple reply signals in the cluster by using RAKE receiving technology at the interrogator.After receiving the inquiry signal,each target in the cluster can transmit the same reply signal to the interrogator asynchronously,which naturally forms multiple paths with different time delays during the transmission process.Inspired from the idea of RAKE receiving,each path is aligned in time after passing through different time delay lines,and then merged.In this thesis,a RAKE receiver model of BPSK modulated coherent carrier that carries pilot signals is established.The coherent carrier of each path signal is recovered through the pilot signal to achieve carrier separation,and then coherent demodulation is performed.After the delay device to achieve the reply signal synchronization and then stack,and finally complete the despreading.The advantage of this method is to avoid the strict synchronization of reply signals transmitted by each target in the cluster.Several factors that affect the system performance and their performance gain are analysed by simulations in the case of RAKE receiving. |
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