## 2 Proposed STPA Configuration for JCAS, Its Design Pipeline, and Performance
### Intro: JCAS Architecture and Protocol  
Let us consider a BS based on a fully connected analog array, as in Figure 1. A JCAS aperture with \(N_{ini}\) antennas is supposed to communicate with a single user equipment (UE) in the TDD scheme and at the same time illuminate a sensing target by using two beams. The number of RF beams is determined by the number of RF chains, and when accompanied by digital beamforming, provides communication to multiple users. The interference among these users can be mitigated by generating narrow beams with low SLLs, and this demands a careful configuration of antenna arrays. Without loss of generality, we study the performance of only one RF chain and one user, since other RF chains, if needed for multiple users, are also connected to the same antennas. The JCAS aperture serves as the transmitter (Tx) during simultaneous downlink communication and monostatic sensing, and as the receiver (Rx) during simultaneous uplink communication and monostatic sensing. Due to long communication subframes (e.g., 1 ms for 5G NR at FR2) and the resulting radar sensing 'dead zone' of maximum 150 km using TDD, a full-duplex radar sensing functionality is mandatory for perpetual sensing of the surrounding environment. To this end, auxiliary apertures with \(N_{aux}\) antennas are needed to serve as the radar receiver during downlink communication and simultaneous monostatic sensing, and as radar transmitter during uplink communication and simultaneous monostatic sensing. The reason for having transmit antennas at the uplink is to address the limitations of sensing solely by the communication signal, i.e., the power resources of communication users are limited, which affects the SNR for sensing. Additionally, these users often have beamforming toward BS, which means that the environment is not illuminated for sensing. Furthermore the sensing environment depends on the user's location, which may be unknown. Therefore, having transmit antennas at BS during uplink communication are beneficial for sensing [26]. The signal processing during downlink communication while monostatic sensing and during uplink communication while monostatic sensing are different: the backscattered signals from the sensing target and the uplink communication signal interfere at the shared aperture, which operates as a JCAS receiver. In such a case, interference cancellation techniques are needed to separate the signals. Downlink communication with simultaneous monostatic sensing does not encounter such interference, since the auxiliary Rx only receives the backscattered signal from the sensing target. In this paper, we focus on simultaneous downlink communication and monostatic sensing scenarios to demonstrate the performance of the proposed STPA. Nevertheless, the demonstrated performance is representative of the uplink communication and simultaneous monostatic sensing, whereas the prerequisite interference cancellation is beyond the scope of this paper.  
Both a bistatic communication link and a monostatic radar would benefit from narrow transmit and receive beam patterns to increase SNR. The challenging part is rather to situate multiple focusing beams in a single JCAS aperture, and then the auxiliary arrays can be designed accordingly. With a narrow communication and a narrow sensing beam from the shared JCAS aperture, the auxiliary Rx can have fewer antenna numbers to decrease the implementation costs, hence we assume \(N_{ini}\gg N_{aux}\). Therefore, the tiling is targeted only to the \(N_{ini}\) antennas at the JCAS aperture, and we simply choose \(N_{aux}=1\) and focus on the JCAS aperture for the performance study of STPA. We use communication signals with orthogonal frequency division multiplexing (OFDM) waveforms to communicate and sense.