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RIS-Aided Unsourced Multiple Access (RISUMA): Coding Strategy and Performance Limits

Mohammad Javad Ahmadi, Mohammad Kazemi, Tolga M. Duman

TL;DR

This paper tackles unsourced random access (URA) in environments where direct user-BS links are weak or blocked by introducing RISUMA, a RIS-aided URA scheme. The method uses a slotted frame with a RIS configuration phase (joint pilot detection, CSI estimation, and RIS design) followed by a data phase that employs polar coding, a MMSE-based detector, polar list decoding, and SIC. It introduces a novel joint pilot detection and channel estimation (JDCE) algorithm and two RIS phase-shift design strategies, ASDR and AEVD, along with an approximate achievability bound for RIS-based URA. Numerical results show that RISUMA outperforms the state-of-the-art RIS-URA scheme (CTAD), achieving significant energy savings and maintaining robust performance across blocked and direct-path scenarios, with the AEVD method offering a favorable complexity-performance trade-off.

Abstract

This paper considers an unsourced random access (URA) set-up equipped with a passive reconfigurable intelligent surface (RIS), where a massive number of unidentified users (only a small fraction of them being active at any given time) are connected to the base station (BS). We introduce a slotted coding scheme for which each active user chooses a slot at random for transmitting its signal, consisting of a pilot part and a randomly spread polar codeword. The proposed decoder operates in two phases. In the first phase, called the RIS configuration phase, the BS detects the transmitted pilots. The detected pilots are then utilized to estimate the corresponding users' channel state information, using which the BS suitably selects RIS phase shift employing the proposed RIS design algorithms. The proposed channel estimator offers the capability to obtain the channel coefficients of the users whose pilots interfere with each other without prior access to the list of transmitted pilots or the number of active users. In the second phase, called the data phase, transmitted messages of active users are decoded. Moreover, we establish an approximate achievability bound for the RIS-based URA scheme, providing a valuable benchmark. Computer simulations show that the proposed scheme outperforms the state-of-the-art for RIS-aided URA.

RIS-Aided Unsourced Multiple Access (RISUMA): Coding Strategy and Performance Limits

TL;DR

This paper tackles unsourced random access (URA) in environments where direct user-BS links are weak or blocked by introducing RISUMA, a RIS-aided URA scheme. The method uses a slotted frame with a RIS configuration phase (joint pilot detection, CSI estimation, and RIS design) followed by a data phase that employs polar coding, a MMSE-based detector, polar list decoding, and SIC. It introduces a novel joint pilot detection and channel estimation (JDCE) algorithm and two RIS phase-shift design strategies, ASDR and AEVD, along with an approximate achievability bound for RIS-based URA. Numerical results show that RISUMA outperforms the state-of-the-art RIS-URA scheme (CTAD), achieving significant energy savings and maintaining robust performance across blocked and direct-path scenarios, with the AEVD method offering a favorable complexity-performance trade-off.

Abstract

This paper considers an unsourced random access (URA) set-up equipped with a passive reconfigurable intelligent surface (RIS), where a massive number of unidentified users (only a small fraction of them being active at any given time) are connected to the base station (BS). We introduce a slotted coding scheme for which each active user chooses a slot at random for transmitting its signal, consisting of a pilot part and a randomly spread polar codeword. The proposed decoder operates in two phases. In the first phase, called the RIS configuration phase, the BS detects the transmitted pilots. The detected pilots are then utilized to estimate the corresponding users' channel state information, using which the BS suitably selects RIS phase shift employing the proposed RIS design algorithms. The proposed channel estimator offers the capability to obtain the channel coefficients of the users whose pilots interfere with each other without prior access to the list of transmitted pilots or the number of active users. In the second phase, called the data phase, transmitted messages of active users are decoded. Moreover, we establish an approximate achievability bound for the RIS-based URA scheme, providing a valuable benchmark. Computer simulations show that the proposed scheme outperforms the state-of-the-art for RIS-aided URA.
Paper Structure (20 sections, 1 theorem, 62 equations, 5 figures, 2 algorithms)

This paper contains 20 sections, 1 theorem, 62 equations, 5 figures, 2 algorithms.

Table of Contents

  1. Introduction
  2. System Model
  3. RISUMA with Blocked User-BS links
  4. Encoder
  5. Decoder
  6. Joint Pilot Detection and Channel Estimation (JDCE)
  7. Data Phase
  8. RIS Design
  9. An Approximate Achievability Result
  10. RISUMA with Direct User-BS Link
  11. Decoder Structure
  12. Joint Pilot Detection and Channel Estimation
  13. Data Detection
  14. RIS Design
  15. Computational Complexity
  16. ...and 5 more sections

Key Result

Lemma 1

For $\mathbf{a}\sim \mathcal{CN}\left(\mathbf{0},\mathbf{A}\right)$, $\mathbf{r}\in \mathbb{C}^{1\times K}$, and $\mathbf{A},\mathbf{B}\in \mathbb{C}^{K\times K}$, if $\mathbf{I}_K-\mathbf{A}\mathbf{B}\succcurlyeq 0$, we have where $\mathbf{C}= (\mathbf{I}_K-\mathbf{A}\mathbf{B})$.

Figures (5)

  • Figure 1: Illustration of a RIS-aided URA system.
  • Figure 2: Transmission structure of the proposed URA scheme.
  • Figure 3: The required $E_b/N_0$ to achieve the target PUPE of $0.1$ for different user-BS path-loss exponents.
  • Figure 4: The required $P_c$ to achieve the target PUPE of 0.1 for $n_s=10$, $n_d=256$, and different RIS phase shift strategies.
  • Figure 5: The required $E_b/N_0$ for the proposed RISUMA and CTAD in Shao2022reconf for achieving a target PUPE of $0.1$.

Theorems & Definitions (2)

  • Lemma 1
  • proof