On Enlarged 5G PRACH Preamble Set Using Alltop Cubic-Phase Sequences

Abstract

5G physical random access channel (PRACH) uses several new preamble formats with less Zadoff-Chu (ZC) sequences available for the network than with formats inherited from LTE. We thus recently proposed an enlarged 5G PRACH preamble construction by covering ZC sequences with cubic-phase Alltop sequences. This construction significantly increases the PRACH sequence set while maintaining a similar intra-cell detection performance from a guaranteed low-correlation zone, and with limited inter-cell interference. However, unlike ZC, the new sequences did not come with a low-complexity PRACH detection which then had to naively rely on an exhaustive search, significantly increasing the PRACH receiver complexity. In this paper, we revisit this extended PRACH sequence construction and modify it to enable a detection of the same complexity-order as current 5G PRACH sequence set, without impacting the detection performance and promised PRACH capacity. Moreover, we describe a symmetrical but slightly non-equivalent PRACH sequence construction that achieves a similar performance and enables the same low-detection capability. Interestingly, this latter construction can be reduced to show that only pure Alltop and ZC sequences may concurrently be used in the network.