A bidirectional integrated sensing and communication (ISAC) system is proposed, in which a pair of transceivers carry out two-way communication and mutual sensing. Both full-duplex and half-duplex operations in narrowband and wideband systems are conceived for the bidirectional ISAC. 1) For the narrowband system, the conventional full-duplex and half-duplex operations are redesigned to take into account sensing echo signals. Then, the transmit beamforming design of both transceivers is proposed for addressing the sensing and communication (S&C) tradeoff. A one-layer iterative algorithm relying on successive convex approximation (SCA) is proposed to obtain Karush-Kuhn-Tucker (KKT) optimal solutions. 2) For the wideband system, the new full-duplex and half-duplex operations are proposed for the bidirectional ISAC. In particular, the frequency-selective fading channel is tackled by delay pre-compensation and path-based beamforming. By redesigning the proposed SCA-based algorithm, the KKT optimal solutions for path-based beamforming for characterizing the S&C tradeoff are obtained. Finally, the numerical results show that: i) For both bandwidth scenarios, the existence of the interference introduced by sensing results in full-duplex may not always outperform half-duplex, especially in the sensing-prior regime or when the communication channel is line-of-sight-dominated; and ii) For both duplex operations, it is sufficient to reuse communication signals for sensing in the narrowband system, while an additional dedicated sensing signal is required in the wideband system.

翻译：提出了一个双向集成感知与通信（ISAC）系统，其中一对收发器实现双向通信与相互感知。针对双向ISAC，设计了窄带和宽带系统中的全双工与半双工操作模式。1）对于窄带系统，重新设计了传统全双工和半双工操作以考虑感知回波信号。随后提出了两种收发器的发射波束成形设计，以平衡感知与通信（S&C）的权衡。基于逐次凸近似（SCA）的单层迭代算法被提出以获得Karush-Kuhn-Tucker（KKT）最优解。2）对于宽带系统，为双向ISAC提出了新的全双工和半双工操作模式。具体而言，通过延迟预补偿和基于路径的波束成形处理频率选择性衰落信道。通过重新设计基于SCA的算法，获得了表征S&C权衡的基于路径波束成形的KKT最优解。最后，数值结果表明：i）对于两种带宽场景，感知引入的干扰导致全双工并不总是优于半双工，尤其在感知优先区域或通信信道以视距主导时；ii）对于两种双工操作，窄带系统中复用通信信号即可满足感知需求，而宽带系统则需要额外的专用感知信号。