In this paper, an integrated sensing and communication (ISAC) system was proposed under cell-free architecture with different power allocation schemes for both communication and sensing functions. In addition to using uplink (UL) pilot transmission for channel estimation, a group of pre-designed orthogonal downlink (DL) pilot sequences are inserted and transmitted from each cooperative access point (AP) towards the user equipments (UEs). The inserted DL pilots in cell-free ISAC are utilized for both channel estimation in communication and cooperative localization in radar sensing. Based on the ISAC strategy, a power allocation scheme is proposed with the objective of minimizing the maximum channel estimation error for each user, under the power constraints of APs and minimum mean square error constraints of the localization. The full-power scheme which APs consume all the available power during the DL phase is also studied and analyzed. The performance of the proposed ISAC strategy is evaluated under a realistic highway scenario, and compared to the existing non-DL strategies. Additionally, we investigate the impact of the pilot ratio under a constrained signal length, as well as the impact of the pilot proportion that the pilot length to the entire signal frame length. Simulation results show that the performances of both channel estimation and localization can be significantly improved at a relatively low cost of extra power needed for APs. Moreover, the analysis provides insights into optimizing the DL/UL pilot ratio and pilot proportion which can significantly enhance both channel estimation and localization accuracy, while ensuring sufficient resources remain available for data payload.

The integrated sensing and communications (ISAC) system, which integrates radar sensing and communication into the same hardware platform, has been considered as one of the new paradigms for the sixth-generation wireless networks. In order to significantly increase the communication rate and enhance the sensing accuracy with low cost and power consumption, in this paper the hybrid beamforming for millimeter wave (mmWave) ISAC systems is proposed. The hybrid analog and digital beamforming is jointly optimized to maximize the weighted sum rate (WSR) of communication users while satisfying the sensing target angle Cramér-Rao bound (CRB) constraint and transmit power budget. Based on the weighted minimum mean square error (WMMSE), Riemann conjugate gradient under exact penalty method (EPM-RCG), and successive convex approximation (SCA) methods, an efficient alternating algorithm is developed to optimize the WSR of communication users in the ISAC systems. Numerical results demonstrate the effectiveness of the developed hybrid beamforming for WSR maximization in mmWave ISAC systems.