## 3 Circuit Level Perspectives of JCRS RF Hardware
### Front-Ends  
As evident from the diversity of applications, no single front-end can fit all applications. Some use cases are limited by the area of the implementation as well as the number of front-ends/antennas that can be used, especially in the case of hand-held devices. Automotive applications use multiple radars to ensure reliability and are more performance oriented, whereas the base stations require optimization based on both power and performance. A comparative analysis of front-ends is given in [8; 23]. Traditionally, front-end communication chips are already available from 24 to 71 GHz ranges thanks to 5G NR frequency range 2 (FR2). The use of mmWave frequencies in both communication and radars can be a key enabler for a combined front-end, even though, traditionally, front-ends are different, as shown in Figure 3 (radar) and Figure 4 (communications). However, it is important to note that many blocks such as mixers, power amplifier (PA), low noise amplifier (LNA), and baseband (BB) variable gain amplifier (VGA) are present in both. The main difference lies in the requirements of these blocks and in ADC (analog-to-digital converter)/receiver digital signal processing (RX DSP) blocks. Radars use mostly stretch processing using FMCW waveforms, which is enabled by analog signal processing and light weight basebands. On the other hand, communication front-ends use digital processing and more spectrally efficient waveforms such as OFDM, as shown in Figure 4. The gaps become thinner when it comes to using an OFDM radar, which uses the same waveform and requires more advanced DSP than the FMCW radar. However, compared with the communication counterparts, the baseband processing differences will remain, as the transmitted signal and timings are known in the radar use case. In communication use cases, the transmitter is located in another device. Figure 4: A representative communication front-end (short forms as defined in Figure 3). Figure 3: A representative radar front-end. DAC = digital-to-analog converter, LPF = low pass filter, Amp. = amplifier, PA = power amplifier, PLL = phase locked loop, TX Ant. = transmitter antenna, RX Ant. = receiver antenna, LNA = low noise amplifier, VGA = variable gain amplifier, ADC = analog-to-digital converter, DSP = digital signal processing. Some of the key performance indicators (KPIs) for a communication front-end are receiver gain and its ranges/controllability, receiver linearity, receiver noise figure, bandwidth of operation for both transmitter and receiver, PLL jitter/phase noise, power amplifier 1-dB and saturated power values, capability of duplex operations (e.g., simplex, half-duplex or in band full duplex), etc. To increase the spectral efficiency or bits/Hz, higher modulations schemes are being used in recent communications systems, thus necessitating the PA to operate at 6 dB or more backoff from the output 1 dB compression point (P1 dB). The noise characteristics of different blocks also play an important role in high data throughput communication systems. On the other hand, radar KPIs primarily include bandwidth, receiver sensitivity, and transmitter saturated output power (PSat). While communications systems may operate at 100 MHz bandwidth, radar systems become severely limited in terms of range resolution for such a limited bandwidth (e.g., 1.5 m for 100 MHz bandwidth). A typical radar transmitter operates at the highest permissible power to increase the detection range in contrast with communication transmitters. However, in terms of LNA design, high gain and low-noise-figure LNAs [26] facilitate both the systems. For a chirp-based radar, PLL is used to create the frequency chirps, whereas in communication systems, after calibration, the frequency of PLL output stays constant as long as the used band is not changed. The authors of [23] outlined further details about these differences and the inherent similarities of different blocks. If communication and radar systems have the same standard specifications in future networks, the gaps will reduce further.