# An Ultra-Wideband Microwave Photonic Channelized Receiver with Zero-IF Architecture
## 1 Introduction  
The receiving technology of ultra-wideband (UWB) radio frequency signal has always been a research hotspot in the field of communication. For example, the radar system or electronic warfare needs to process multiple signals of different frequencies arriving at the same time in real time, which can be achieved through a channelized receiver by segmenting the electromagnetic spectrum, so as to realize the full probability interception of signals in the monitoring range of the UWB. Another example is the application in navigation or wireless communications, where it greatly improves the bandwidth efficiency of the wireless communication. So, the channelized receiver can be well applied to photonic technology for 5G. A channelized receiver is usually required to have a broad instantaneous receiving bandwidth, a high resolution, and a high sensitivity, guaranteeing signal processing; it can also process signals at multiple frequency points and various forms of signals at the same time, and has a high resolution and sensitivity [1; 2]. However, the current analog-to-digital converters (ADCs) are limited by sampling rates and bandwidths, which cannot simultaneously process ultra-wideband signals [3]. A channelized receiver can slice the received broadband radio frequency (RF) signal into multiple sub-bands and down-convert each sub-band to the common intermediate frequency(IF) or zero intermediate frequency (zero-IF). The zero-IF receiving structure used in this scheme has the following advantages. It can directly down-convert RF signals to baseband signals, which simplifies the system structure, not only reducing the system volume, but also the cost. In addition, it reduces the bandwidth requirements for ADCs and enhances the digital processing capability of broad instantaneous bandwidth signals. The traditional analog channelized receiver has large power consumption, large insertion loss, high requirements on the electrical filter, and also a large bulk when the number of sub-channels increases, which is gradually replaced by digital channelized receivers. The digital receiver has flexible tunability and high accuracy but is also limited by the sampling rate of the ADC and working bandwidth [4]. In electronic warfare, massive amounts of data also place higher demands on signal processing. So, it is urgent to develop a channelized receiver that can process ultra-wideband signals [5]. In recent years, microwave photonic technology has developed rapidly. Owing to the wide instantaneous bandwidth, broad working frequency range, high isolation, and immunity to electromagnetic interference (EMI), it provides a new solution for the realization of a channelized receiver. A continuous light wave generated by a laser diode (LD) is used as a carrier wave to modulate the RF signals to transmit. The entire transmission link is almost the optical link, and the entire system also uses optical components. For example, optical fibers have replaced cables and optical filters have replaced electrical filters. The optical signals are not transformed to electrical signals before the balanced photodiode (BPD). Since it meets the current demand for large bandwidth in many fields, the channelized receiver based on photonics has become a research hotspot [6; 7; 8; 9; 10; 11]. At present, the reported channelized receivers based on microwave photons can be divided into the following three categories. In the first category, each channel independently filters the light waves and then directly detects [12]. A single-carrier or multi-carrier is used to modulate and split or de-multiplex the RF signal into multiple channels. Each channel passes through a fiber Bragg grating (FBG), Fabry-Perot (F-P) cavity, and other filters for direct intensity detection [13; 14]. However, this method can only perform intensity detection and cannot extract the RF phase information. In the second category, two optical frequency combs (OFCs) are used as the optical carrier and the optical local oscillator (LO) [15; 16], respectively.