# Design of Wideband FMCW Radar Transceiver System Using Photonic Elements
## 2 Proposed Radar System Design  
Figure 1 shows the schematic of the proposed microwave photonic FMCW radar transceiver with photonic elements for obtaining wideband characteristics. The proposed radar transceiver consists of a laser diode (LD), RF signal, dual-parallel Mach-Zehnder modulator (DP-MZM), optical coupler, phase modulator (PM), photo detector (PD), transmit and receive antennas, amplifier (AMP), low noise amplifier (LNA), analog-to-digital converter (ADC), radar signal processing (RSP), and a radar control center (RCC). The photonic elements, such as LD, DP-MZM, and PM are implemented in the proposed FMCW radar transceiver to achieve wideband properties, and these allow us to obtain high-resolution ISAR images. A continuous wave light from the LD (model: AA1401, Gooch & Housego, Dowlish Ford, Limister, UK) is modulated by the DP-MZM, which is driven by a continuous wave (CW) linear frequency modulation (LFM) signal in the intermediate frequency band (IF-band). The frequency of the LFM signal in the IF-band can be expressed as:  
\[f_{IF}(t)=f_{0}+kt \tag{1}\]  
where \(f_{0}\) is the initial frequency and \(k\) is the chirp rate. The IF signal passes through an electrical 90\({}^{\circ}\) hybrid coupler, and the two signals with 90\({}^{\circ}\) phase difference are sent to drive the DP-MZM. Bias voltages of the DP-MZM are properly set to obtain second-order modulation, and the output optical signal of the DP-MZM is then split into two branches by an optical coupler. In the transmission path, the optical signal is converted to an electrical signal using a PD, and the obtained electrical signal has a quadrupled LFM signal compared with that of the LFM signal in the IF-band. The frequency of the quadrupled LFM signal can be expressed as follows [9]:  
\[f_{LFM}(t)=4f_{0}+4 kt \tag{2}\]  
In the receiving path, the optical signal split by the optical coupler from transmission is used as a reference for the de-chirping process to obtain beat frequency for the FMCW  
Figure 1: Schematic of the microwave photonic FMCW radar transceiver. radar transceiver system. The beat signal is obtained by combining the transmitted signal and backscattered signal in the receiving path, and the calculated signal can be used for target detection and ISAR image acquisition by using digital signal processing (DSP). The proposed transceiver system with photonic elements does not use various electrical elements such as mixers and filters that are used in conventional radar transceiver systems, and consequently it has some advantages such as low-frequency-dependent loss, wide bandwidth, tunability for operating frequency, and immunity to electromagnetic interference (EMI) [5]. We also observe the optical and the electrical signal of the proposed radar transceiver system using a photonic multiphysics simulation tool (Ansys Lumerical, Vancouver, BC, Canada) to demonstrate the effectiveness of achieving wideband characteristics using photonic elements. Figure 1(a) shows the simulation circuit of the proposed radar transceiver. The laser wavelength is 1550 nm with an optical power of 10 dBm and a linewidth of 100 kHz, and the bias voltage points of the DP-MZM are adjusted to observe the optical power. The property of the PD assumes that responsivity and dark current are 1 A/W and 0.01 A, respectively. The simulation results are measured in the optical domain (before PD) and electrical domain (after PD), respectively. Figure 1(b),\(c\) show the simulation results from the optical spectrum analyzer (OSA) and the electrical spectrum analyzer (ESA), respectively, when an input IF CW signal has a frequency of 2.5 GHz. To obtain the second-order modulation of DP-MZM, the three bias voltages should be controlled, and the sub-MZMs are biased at the full point, and the outer parent of MZM is biased at the null point with a phase shifter when the optical carrier is equally divided into two parallel children sun-MZMs.