To mitigate the overlapping of the radar and communication frequency bands caused by large-scale devices access, we propose a novel integrated sensing and communication (ISAC) system, where a micro base station (MiBS) simultaneously carries out both target sensing and cooperative communication. Concretely, the MiBS, acting as the sensing equipment, can also serve as a full-duplex decode-and-forward relay to assist end-to-end communication. Moreover, non-orthogonal downlink transmission (NO-DLT) is adopted between the macro base station and the Internet-of-Things devices, so that the spectrum utilization can be further improved. To facilitate the performance evaluation, both the exact and asymptotic outage probabilities, the ergodic rates associated communication, and the probability of successful sensing detection are characterized. Subsequently, a pair of problems of maximizing the receive signal-to-interferenceplus-noise ratio of the sensing signal and maximizing the sum rate of communication are formulated that are solved by the classic Lagrangian method while exploiting the associated function monotonicity. Our simulation results demonstrate that: 1) The proposed ISAC NO-DLT system improves both the communication and sensing performance under the same power consumption as non-cooperative NO-DLT; 2) The proposed power allocation (PA) schemes are superior to the random PA scheme.

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The design options of sensing-communication coexistence (SCC) vs. integrated sensing and communications (ISAC) relying on non-orthogonal downlink transmission (NO-DLT) and transmit antenna selection are investigated. To be practical, both realistic residual hardware impairments (RHI) and imperfect successive interference cancellation are considered. The performance of the SCC and ISAC NO-DLT frameworks is characterized by deriving both the exact and the asymptotic outage probabilities (OPs) as well as the probability of successful target detection (PoD). Our numerical results demonstrate the accuracy of the theoretical analysis and confirm that: 1) The ISAC NO-DLT framework is superior to the SCC NO-DLT framework both in terms of its OP and PoD; 2) The realistic RHI degrades the S&C performance of both the SCC and ISAC NO-DLT frameworks.

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