PRACH Sequence Design for NR Unlicensed Spectrum

The employment of additional spectrum is considered the most simple and effective way to boost network capacity for next-generation wireless communications. Towards this end, ongoing research work has been undertaken by the 3rd Generation Partnership Project (3GPP) on enabling 5G new radio (NR) in the unlicensed band, which is called NR-U. In particular, some key physical layer technologies based on the NR air interface must be enhanced, by reason of the different regulatory requirements in the unlicensed band. In this paper, a comprehensive physical random access channel (PRACH) sequence design scheme is presented for the NR-U system, which is characterized by low cubic metric (CM), enhanced coverage, resistance to carrier frequency offset (CFO), and low-complexity detection. For the proposed sequence, a joint timing offset and CFO estimation scheme is proposed, furthermore, the theoretical derivation of CM and maximum coupling loss (MCL) is conducted. Results show that the proposed sequence can maintain a comparatively low CM value with an increment of the number of frequency-domain sequence repetitions while having remarkably improved PRACH coverage and detection performance.