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## Interference Mitigation Using Frequency Agility Techniques

This example shows how to model frequency agility techniques to counter the effects of interference in radar, communications, and EW systems. Using Simulink, a scenario is created with a ground based radar and an approaching aircraft who can also emit jamming signals. A similar example in MATLAB can be found in Frequency Agility in Radar, Communications, and EW Systems.

### Introduction

In this model, a phased array radar is designed to detect an approaching aircraft. The aircraft is equipped with a jammer which can intercept the radar signal and transmit a spoofing signal back to confuse the radar. On the other hand, the radar system is capable of transmitting waveform with different operating frequencies to mitigate the jamming effect. The model includes blocks for waveform generation, signal propagation, and radar signal processing. It shows how the radar and jammer interact and gain advantage over each other.

The radar is operating at 300 MHz with a sampling rate of 2 MHz. The radar is located at the origin and is assumed to be stationary. The target is located about 10 km away and approaching at approximately 100 meters per second.

Waveform Generation

The Waveform Generation subsystem includes a linear FM (LFM) waveform generator and a local oscillator which is used to generate a frequency hopped waveform at a shifted center frequency. Therefore, the radar system is capable of switching transmit waveform either following a fixed schedule or when it detects jamming signals. This example assumes that the waveform can be generated for two different frequencies, referred to as center band and hopped band. The center band is the subband around the carrier frequency and the hopped band is the subband located quarter bandwidth above the carrier.

Propagation Channels

The signal propagation is modeled for both the forward channel and the return channel. Once the transmitted signal hits the target aircraft, the reflected signal travels back to the radar system via the return channel. In addition, the jammer analyzes the incoming signal and send back a jamming signal to confuse the radar system. That jamming signal is also propagated through the return channel. Because different signals may occupy different frequency bands, wideband propagation channels are used.

Signal Processing

The radar receives both the target return and the jamming signal. Upon receiving the signal, a series filters are used to extract the signal from different bands. In this example, there are two of them to extract the signal from the center band and the hopped band. The signal in each band then passes through the corresponding matched filter to improve the SNR and be ready for detection.

### Exploring the Example

Several dialog parameters of the model are calculated by the helper function `helperslexFrequencyAgilityParam`. To open the function from the model, click on Modify Simulation Parameters block. This function is executed once when the model is loaded. It exports to the workspace a structure whose fields are referenced by the dialogs. To modify any parameters, either change the values in the structure at the command prompt or edit the helper function and rerun it to update the parameter structure.

### Results and Displays

First run the model for the case when there is no jamming signal. The scopes shows that there is one strong echo in the center band with a delay of approximately 67 microseconds, which corresponds to a target range of 10 km. Therefore, the target is correctly detected. Meanwhile, there is no return detected in the hopped band.

The spectrum analyzer shows that the received signal occupies the center band.

Now enable jammer by clicking the Jammer Switch block. In this situation, the target intercepts the signal, amplify it, and then sends it back with a delay corresponding to a different range. As a result, the scope now shows two returns in the center band. The true target return is still at the old position, but the ghost return generated by the jammer appears stronger and closer to the radar so the radar is likely to be confused and assign precious resource to track this fake target.

Note that both the jammer signal and the target return are in the center band, as shown in the spectrum analyzer.

If the radar has a pre-scheduled frequency hopping schedule or is smart enough to know that it might have been confused by a jamming signal, it could switch to a different frequency band to operate. Such scenario can be simulated by clicking the Hop Switch block so the radar signal is transmitted in the hopped band.

Because the radar now operates in the hopped band, the target echo is also in the hopped band. From the scope, the target echo is at the appropriate delay in the hopped band. Meanwhile, the jammer hasn't figured out the radar's new operating band yet so the jamming signal still appears in the center band. Yet the jamming signal can no longer fool the radar.

The spectrum analyzer shows that the received signal now occupies two bands.

### Summary

This models a radar system detecting a target equipped with a jammer. It shows how frequency agility techniques can be used to mitigate the jamming effect.

#### Hybrid Beamforming for Massive MIMO Phased Array Systems

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