# Wideband Mixed Signal Separation Based on Photonic Signal Processing
## 2 Interference Management
### Challenges  
Blind source separation (BSS) separates interference from SOI without pre-known information of the interference, such as the channel coefficient, modulation format, bandwidth, or frame structure [23; 24; 25; 26; 27; 28]. With the recent deployment of MIMO technique [29; 30; 31; 32; 33; 34], the BSS theory has been implemented in wireless communication systems such as Wi-Fi, LTE, GPS, and others [35; 36; 37; 38]. The existing BSS methods enable the coexistence of multiple wireless systems by separating SOI from interference [39; 40; 41; 42], while with the growing needs of deploying the RF spectrum, the bandwidth and power range of the mixed signals expand beyond the processing capabilities of current BSS methods:  
Figure 3: Mixed signal and second order moment.  
Signal bandwidth: the current trend of both high speed and secure wireless communication spurs the requirement of wide-band operation. The channel bandwidths of wireless communications are moving from 10-100 MHz to 1-3 GHz [43; 44; 45; 46; 47]. The high-speed wireless network functions as the fundamental infrastructure for emerging applications, such as augmented reality, unmanned vehicles, video conferences, etc. Meanwhile, the accessibility of the wideband spectrum is also the cornerstone of the physical layer wireless network security. The effectiveness of physical layer encryption and anti-jamming methods, such as direct-sequence spread spectrum systems, frequency-hopping systems, and Code-Division Multiple Access (CDMA), relies on the bandwidth that the system can access.  
The wideband signal challenges the existing separation methods in two ways: (1) The computation power needed to process the signal and separate the interference increases dramatically with the signal bandwidth, while the wireless nodes, such as personal computers, cell phones, and wireless base stations have limited computational power. Latency in the order of seconds or minutes is introduced when processing wideband signals with wireless nodes. (2) In a wide RF band (beyond 100 Hz), the frequency responses of RF components are inconsistent at different ranges of the band, which require different RF devices for each range of the spectrum. The requirement of different RF devices increases the volume of the hardware system. The inconsistent frequency responses also lead to different channel coefficients, which means, in an orthogonal frequency-division multiplexing (OFDM) system, the optimization of the filter matrix for each channel needs to be calculated separately [48; 49; 50], which further increases the computational complexity.  
Signal power and dynamic range of the wireless system: The coexistence of diverse wireless systems generate signals under different power levels with different transmission distances, which requires the separation system to operate in a large power dynamic range. For example, the interference source can be much closer to the receiver than the SOI transmitter and generates interference with power orders of magnitude larger than the power of SOI. Or as another example, the interference is generated from a cellular station, and the receiver is a radio telescope or a weather radar with an SOI of at least 60 dB lower than the interference. In both cases, the power difference between the interference and SOI requires the separation system to achieve both high resolution and large power ranges. The 60 dB difference corresponds to at least 20-bit ADC, and 20-bit ADC at GHz frequencies is beyond current state-of-the-art ADC techniques [51; 52; 53].  
With the hybrid system and complementary advantages of photonic signal processing and digital edge devices, the wideband mixed signals with orders of magnitudes different from each other can be separated in real-time.