Uplink Transmission Schemes for 5G NR Unlicensed: Design Principles and Achievable Performance

In this paper, we address and analyze different uplink (UL) transmission schemes for the 5G New Radio (NR) deployments at unlicensed spectrum. Specific emphasis is on the new NR-unlicensed (NR-U) wideband physical random access channel (PRACH) under short preamble formats as well as on the base-station receiver sensitivity requirements for the fixed reference channels. In general, in order to comply with ETSI regulations, the UL waveform resource allocation has been revised in NR-U. On one hand, the bandwidth (BW) of the PRACH sequences has been increased according to numerology, which based on the performance analysis presented in this paper provides a substantially improved detection performance while fulfilling the coverage requirements of Rel-15 preambles. On the other hand, the physical uplink shared channel (PUSCH) resource allocation is based on block interlace frequency division multiple access (B-IDFMA). This design is characterized by the number of interlaces allocated within the transmission BW and it should be properly defined for the multiple physical layer numerologies supported in NR systems. The NR-U PUSCH performance results provided in this paper show that the B-IFDMA design yields the best performance for the case of one interlace allocation compared to contiguous resource allocation for all numerologies. Additionally, it is shown that the non-B-IFDMA PUSCH design outperforms the NR-U design for larger number of allocated interlaces. There is thus a trade-off between the frequency diversity gain achieved by the sparse PRB distribution in NR-U and the corresponding channel estimation challenges impacting the demodulation performance.