# Wideband Mixed Signal Separation Based on Photonic Signal Processing
## 2 Interference Management
### Related Work
#### 2.1.1 Photonic Circuit  
The photonic circuit has two major functions [12; 13]: (1) it adds an arbitrary complex weight to each of the received signals \(x_{i}\); (2) the weighted signal is summed and, by choosing the right weight, the mixed signal is separated as \(S=A^{-1}X\). Figure 2 is the schematic diagram of the photonic circuit and has been demonstrated with both discrete components [14; 15] and integrated chips [16; 17; 18]. In Figure 2, there are two channels, which correspond to two MIMO inputs (\(x_{1}\) and \(x_{2}\)). The number of channels can be scalable to \(n\) (\(n>16\)) by using the WDM technique if the interference source is more than one [18; 19]. The mixed RF signals are modulated on optical carriers with center frequency 193 THz. The bandwidth of the so-called wideband RF signals is extremely narrow, compared with the optical carriers.  
Experimental results show that the systems process the signals in real-time and reach a 30 dB cancellation ratio with bandwidth from 500 MHz to 3.5 GHz [12; 14; 17]. The cancellation ratio describes the capability of the system to separate the mixed signals and is measured by the cancellation ratio between the interference separated from the mixed signal to the interference that remains mixed with the signal. The cancellation ratio is limited by the non-uniform frequency response of RF devices and digitization error in digital systems. When the bandwidth of the mixed signal is beyond 1 GHz, the current state-of-the-art ADC is limited to 4-8 bits resolution, which corresponds to 12-24 dB of cancellation ratio. Using optical carriers to process the mixed-signal and cancel the interference has a unanimous frequency response and as an analog method, the cancellation ratio is not limited by the ADC resolution. Table 1 shows the cancellation ratio at different bandwidths by using the photonics method [12; 14; 17]; all works are summarized in Table 2 in Appendix A.  
Figure 1: System model of BSS.  
Figure 2: Experimental setup of the photonic interference cancellation circuit. (AM: optical amplitude modulator, TD: tunable optical delay, TA: tunable optical attenuator, PD: photon detector).  
The interference source and interference frequency vary in each of the setups. The interference bandwidth varies from 500 MHz to 3.5 GHz. For the optical encryption system [12], the interference noise is generated as an RF sine wave with its frequency changing between 4 GHz to 7 GHz, and applied to a laser frequency of 1550.12 nm, the bandwidth of the interference is tested from 4 GHz to 7 GHz with an average cancellation of 26 dB, which is wide enough to encrypt and protect a signal with the data rate of 10 Gb/s. For radio frequency spectrum control [14], the interference signal is set as an RF sine wave of 1 GHz, applied to a 1544.3 nm laser source. The system is tested to achieve average cancellation of 26 dB from 500 MHz to 5 GHz via a Keysight E5063A Network Analyzer. For optical self-interference cancellation [17], the signal of interest was set as a weak 915 MHz single-tone signal, and the interference was set as a sweeping 0 dBm single-tone interferer across 60 MHz bandwidth centered at 915 MHz, where the 60 MHz bandwidth is greater than most channel bandwidth, used in common LTE or Wi-Fi protocols; a 38 dB cancellation is achieved across the 60 MHz bandwidth, and a 56 dB cancellation is achieved when using a 10 kHz bandwidth interference.