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ODMA-Based Cell-Free Unsourced Random Access with Successive Interference Cancellation

Mert Ozates, Mohammad Kazemi, Eduard Jorswieck, Deniz Gunduz

TL;DR

This work tackles unsourced random access (URA) in dense mMTC by introducing a scalable cell-free massive MIMO framework where distributed APs connected to a CPU jointly detect and decode messages. The method combines an ODMA-inspired pilot scheme with polar-coded data, performing pilot detection and channel estimation at the APs via OMP and LMMSE, followed by CPU-level symbol combining and polar SCLD decoding with CRC, and finally SIC at the APs. Key contributions include support for up to 1400 active users, PUPE reductions exceeding two orders of magnitude through AP distribution and cooperation, and notable energy-efficiency gains over centralized schemes. The results demonstrate that combining distributed reception with ODMA-like encoding and SIC yields scalable, energy-efficient URA suitable for future beyond-5G deployments.

Abstract

We consider the unsourced random access problem with multiple receivers and propose a cell-free type solution for that. In our proposed scheme, the active users transmit their signals to the access points (APs) distributed in a geographical area and connected to a central processing unit (CPU). The transmitted signals are composed of a pilot and polar codeword, where the polar codeword bits occupy a small fraction of the data part of the transmission frame. The receiver operations of pilot detection and channel and symbol estimation take place at the APs, while the actual message bits are detected at the CPU by combining the symbol estimates from the APs forwarded over the fronthaul. The effect of the successfully decoded messages is then subtracted at the APs. Numerical examples illustrate that the proposed scheme can support up to 1400 users with a high energy efficiency, and the distributed structure decreases the error probability by more than two orders of magnitude.

ODMA-Based Cell-Free Unsourced Random Access with Successive Interference Cancellation

TL;DR

This work tackles unsourced random access (URA) in dense mMTC by introducing a scalable cell-free massive MIMO framework where distributed APs connected to a CPU jointly detect and decode messages. The method combines an ODMA-inspired pilot scheme with polar-coded data, performing pilot detection and channel estimation at the APs via OMP and LMMSE, followed by CPU-level symbol combining and polar SCLD decoding with CRC, and finally SIC at the APs. Key contributions include support for up to 1400 active users, PUPE reductions exceeding two orders of magnitude through AP distribution and cooperation, and notable energy-efficiency gains over centralized schemes. The results demonstrate that combining distributed reception with ODMA-like encoding and SIC yields scalable, energy-efficient URA suitable for future beyond-5G deployments.

Abstract

We consider the unsourced random access problem with multiple receivers and propose a cell-free type solution for that. In our proposed scheme, the active users transmit their signals to the access points (APs) distributed in a geographical area and connected to a central processing unit (CPU). The transmitted signals are composed of a pilot and polar codeword, where the polar codeword bits occupy a small fraction of the data part of the transmission frame. The receiver operations of pilot detection and channel and symbol estimation take place at the APs, while the actual message bits are detected at the CPU by combining the symbol estimates from the APs forwarded over the fronthaul. The effect of the successfully decoded messages is then subtracted at the APs. Numerical examples illustrate that the proposed scheme can support up to 1400 users with a high energy efficiency, and the distributed structure decreases the error probability by more than two orders of magnitude.
Paper Structure (10 sections, 16 equations, 5 figures)

This paper contains 10 sections, 16 equations, 5 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Proposed Scheme
  4. Encoding
  5. Receiver Operation at AP
  6. Symbol Combining and Decoding at CPU
  7. Successive Interference Cancellation
  8. Complexity Analysis
  9. Numerical Results
  10. Conclusions

Figures (5)

  • Figure 1: Illustration of a cell-free system with 5 APs and 1 CPU. Active and inactive users are shown by green and red triangles, respectively.
  • Figure 2: An illustration of the decoding process of the proposed scheme with 2 APs.
  • Figure 3: Comparison of PUPE versus the number of active users for different AP configurations.
  • Figure 4: PUPE comparison of different cooperation levels.
  • Figure 5: Comparison of the required $E_b/N_0$ versus the number of active users for PUPE $P_e \leq 0.05.$