Evolution of the 5G New Radio Two-Step Random Access towards 6G Unsourced MAC

TL;DR

This work analyzes the evolution of 5G NR two-step random access (2SRA) through the lens of unsourced multiple access (UMAC), benchmarking against information-theoretic limits to identify energy and spectral efficiency gaps. It finds that the current 2SRA design is suboptimal for massive machine-type communications and proposes two enhancements: expanding the preamble/pilot framework and adopting SB-IDMA, a sparse interleaver-based access scheme that leverages segment-level channel estimation and SIC. SB-IDMA, using either LDPC or polar codes, approaches the finite-length bounds on Gaussian MAC and delivers substantial gains under quasi-static fading, with polar codes offering the largest improvements and enabling support for hundreds of active users in multi-antenna settings. The results indicate a viable path toward a scalable, grant-free 6G RA solution by combining richer access patterns, larger preamble sets, and short-packet coding strategies that preserve legacy protocol compatibility while dramatically increasing efficiency.

Abstract

This report summarizes some considerations on possible evolutions of grant-free random access in the next generation of the 3GPP wireless cellular standard. The analysis is carried out by mapping the problem to the recently-introduced unsourced multiple access channel (UMAC) setup. By doing so, the performance of existing solutions can be benchmarked with information-theoretic bounds, assessing the potential gains that can be achieved over legacy 3GPP schemes. The study focuses on the two-step random access (2SRA) protocol introduced by Release 16 of the 5G New Radio standard, investigating its applicability to support large MTC / IoT terminal populations in a grant-free fashion. The analysis shows that the existing 2SRA scheme may not succeed in providing energy-efficient support to large user populations. Modifications to the protocol are proposed that enable remarkable gains in both energy and spectral efficiency while retaining a strong resemblance to the legacy protocol.

Figures (9)

• Figure 1: Random access procedures employed by LTE/5GNR standards. (a) Four-step random access: the UT transmits a preamble (A). Upon detecting the transmitted preambles, the BS provides a resource allocation to each detected user (B). UTs transmit their data packets in the allocated resources (C). The BS acknowledges the correctly decoded packets (D). The procedure ends when the UT receives the acknowledgment (E). (b) Two-step random access (Release 16 of the 5GNR standard): A UT transmits a preamble, that directly points to the resource that will be used to transmit the data packet. The data packet transmission follows w/o waiting for a resource allocation (A). At the BS, preambles are detected and decoding is attempted in the resources pointed by the preambles. For detected UT transmissions, and acknowledgment is sent to the UTs that are successfully decoded (B). For detected UT transmissions that do not yield to successful decoding, orthogonal resources are allocated for the retransmission of the data packet, resuming the four-step random access procedure.
• Figure 2: Illustration of the two-step random access protocol of the 5GNR standard (first transmission only). The number of PUSCH occasions is $N$.
• Figure 3: Number of supported active users vs. SNR for a target $\mathsf{PUPE} = 5 \times 10^{-2}$. AWGN channel, $n \approx 15000$c.c.u..
• Figure 4: Number of supported active users vs. average SNR for a target $\mathsf{PUPE} = 10^{-1}$. Quasi-static Rayleigh fading channel, $n\approx 20000$ channel uses. Single antenna at the base station. The 2SRA performance is provided for the OTO configuration (Table \ref{['tab:2SRA']}).
• Figure 5: SB-IDMA transmission chain.

Theorems & Definitions (1)

• Definition 1: Access Pattern