Can We Achieve Any-Length Cyclic Prefix for Flexible ISAC Coverage?

Future 6G is expected to have integrated sensing and communications (ISAC) as a crucial component. To guarantee the sensing performance, 6G ISAC should implement a specific sensing signal from an industrial standpoint. The cyclic prefix (CP) problem is one of the main obstacles since sensing and communications (S&C) have different CP needs. This paper addresses the problem of whether we can achieve any-length CP(ACP) to allow different CP capabilities for S&C. First, this paper derives the necessary and sufficient conditions for protocol-compatible ACP (PC-ACP) without modifying the 3GPP standard. The conclusion is that certain lengths can make the PC-ACP a reality. The restriction is then loosened in order to create a hardware-compatible ACP (HC-ACP), which reuses hardware structure and sampling interval of orthogonal frequency division multiplexing (OFDM). This work suggests using an OFDM-based chirp design to produce an analogous technical impact of ACP. Accurate dechirp processing is made possible by a resampling and compensating receiver. The proposed schemes can perform better than the current CP scheme, according to the numerical results, and one of them can even handle an ACP longer than half a symbol.