## 3 Circuit Level Perspectives of JCRS RF Hardware
### Phased Arrays and Beam Steering Systems
#### 3.3.2 Beamformer Networks  
The relevant phase for each element in the array is generated through a BF network. The requirements for the BF network can be separate for communication and radar systems, e.g., beam switching angle, switching time, signal processing needs, and reciprocity of the network. Its application in both systems simultaneously depends on the target application. In this paper, we classify the BFs broadly in term of circuit/hardware implementation as active and passive networks. Active BFs, when implemented in the analog domain, have the mostly widely adopted and simple implementation. Each antenna element is fed through a dedicated phase shifter either placed in an RF or local oscillator (LO) path, also known as an RF or LO path BF [69]. On the other hand, active digital beamforming exploits signal processing capabilities to process a unique stream for each antenna element with the required phase shift. It provides maximum control over beam patterns and calibration compared with all of the other implementations. However, it requires an increased number of digital-to-analog converters (DACs) or analog-to-digital converters (ADCs) and RF chains. Another approach is hybrid BF [70], by connecting several antennas to a limited number of ADCs/DACs through analog beamformers. It provides an extended beam control compared with pure analog BF, with reduced digital processing compared with pure digital BF [71]. Full duplex JCRS has been reported using such a BF in [72]. Active BF requires separate phased arrays for the transmitter and receiver because of the unidirectional behavior of active components and DACs/ADCs, which not only increase the size of the antenna system, but also disturb roundtrip channel reciprocity. At miniaturized wavelengths, the transmitter-to-receiver array center-to-center distance becomes greater than half the wavelength. In this case, the transmitting and receiving waves adopt different paths in a multipath environment. This phenomenon is unfavorable for several communication applications, depending on round trip channel reciprocity, e.g., physical layer security [73]. However, it has no operational downside for monostatic radars. On the other hand, passive BFs are relatively large structures implemented off-chip. Lens-based passive BFs are based on optical technologies. A dielectric lens is produced through printed circuit technology (PCT) and the radiating part is connected to the antenna array e.g., Rotman lens [74] and Ruze lens [75]. An alternative implementation is a circuit connecting each input of the network to each antenna element, such that a unique output phase is generated by exciting the network from each input. The popular implementation is Butler matrix, comprising of fixed phase shifters and quadrature hybrid couplers [76; 77]. In addition to PCT, the Butler matrix has been reported in waveguide and CMOS technologies [78]. Passive BFs work on true time delay principles and provide a frequency -dependent phase shift over a bandwidth, unlike active phase shifters. Hence, they do not cause beam squint, which is the frequency dependency of beam angles notable in active analog BFs. Beam squint is not a problem in narrow band communication and has been utilized in wideband communication as a frequency scanning array [79]. However, wideband narrow-beam radars can experience reduced returns due to this phenomenon. Since true time delay is not possible on-chip due to the extended required area, and off-chip passive BF provides a few discrete beam angels, digital BF can be attractive for high resolution radars based on true time delay in the digital domain. Another advantage passive BFs have is their reciprocity. A single antenna system can be used for transmission and reception as phased arrays are already reciprocal. It not only reduces the form factor, but ensures channel reciprocity in mmWave bands. A useful application of the Butler matrix is given in [77] for full-duplex communication with monostatic radars in a JCRS scenario.