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arxiv: 2512.05748 · v1 · pith:6RQTF2LAnew · submitted 2025-12-05 · 📡 eess.SP

Codebook-based Port Selection and Combining for CSI-Free Uplink Fluid Antenna Multiple Access

Chenguang Rao , Kai-Kit Wong , Sai Xu , Xusheng Zhu , Yangyang Zhang , Chan-Byoung Chae This is my paper

Pith reviewed 2026-05-25 07:52 UTC · model grok-4.3

classification 📡 eess.SP
keywords fluid antenna multiple accesscodebook-based port selectionCSI-free uplinkport combiningmultiuser signal separation6G connectivitylow-complexity scheduling
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The pith

A shared codebook lets each uplink user select its own fluid-antenna ports and weights locally so the base station can separate the signals by projection without global channel state information.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that fluid-antenna multiple access can work in the uplink without the base station knowing any channel state information. A codebook is designed and sent once by the base station; each user equipment measures only its own channel, picks the best-matching codeword, turns on the matching ports, and sets combining weights so that the effective channel aligns with the codeword. The base station then disentangles the overlapping transmissions by projecting onto the known codebook entries. Three simple scheduling rules handle the few cases in which two users pick the same codeword. The result is higher sum rates than fixed-antenna systems at far lower signaling and computation cost.

Core claim

The central claim is that a codebook-based port selection and combining (CPSC) procedure enables CSI-free uplink fluid-antenna multiple access: each UE independently selects a codeword from the broadcast codebook using only local channel knowledge, activates the corresponding ports, and computes combining weights to create a two-way match with the instantaneous effective channel; the base station then separates the superimposed signals through codebook-guided projection operations without global CSI or joint multiuser optimization, while lightweight scheduling resolves occasional codeword collisions.

What carries the argument

The codebook-guided projection at the base station, which uses each UE's locally chosen codeword to isolate its contribution from the received superposition.

If this is right

  • Uplink rates exceed those of fixed-antenna systems while the base station avoids multiuser channel estimation and joint decoding.
  • The optimization burden is shifted from the base station to the individual user equipments, improving scalability with the number of users.
  • Three lightweight scheduling options allow explicit trade-offs between signaling overhead and collision probability.
  • The scheme remains compatible with existing fluid-antenna hardware because port activation and combining occur locally at each user equipment.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same local-matching idea could be tested in downlink scenarios where the base station instead broadcasts the codebook and users still choose independently.
  • If the codebook is made adaptive over longer timescales, the scheme might tolerate slower channel variations without increasing per-slot overhead.
  • Collision probability could be further reduced by letting the base station periodically update only a subset of codewords rather than redesigning the entire book.

Load-bearing premise

A fixed codebook can be designed so that every user's local choice of codeword, ports, and weights produces a sufficiently accurate match to its instantaneous effective channel for the base-station projections to separate the signals.

What would settle it

An experiment in which the base station applies the codebook projections to real measured uplink signals and obtains error rates no better than a conventional fixed-antenna system that uses no channel knowledge at all.

Figures

Figures reproduced from arXiv: 2512.05748 by Chan-Byoung Chae, Chenguang Rao, Kai-Kit Wong, Sai Xu, Xusheng Zhu, Yangyang Zhang.

Figure 1
Figure 1. Figure 1: An uplink communication system, where the BS is equipped with M fixed-antenna, and the UEs are equipped with fluid antennas. of x. (·) T , (·) H denote the transpose and Hermitian transpose, respectively. <{·} represents the real part, and j0(·) denotes the zeroth-order Bessel function of the first kind. II. System Model and The Codebook-based Framework A. System Model Consider an uplink communication syst… view at source ↗
Figure 2
Figure 2. Figure 2: A complete frame, including a channel estimation slot, a downlink transmission slot, and an uplink transmission slot. The uplink slot consists of a reservation phase and a transmission phase. During the transmitting phase, each UE activates Ku multiple ports. Define the port activation matrix Au ∈ {0, 1} N×Ku for UE u, which is given by Au = [au,1, au,2, . . . , au,Ku ] . (5) Each column of Au is a standar… view at source ↗
Figure 3
Figure 3. Figure 3: Average rate per user vs. the number of ports N, when the number of UEs is fixed as U = 16. SVD, where the number of retained singular vectors t is fixed to Ku unless otherwise specified. The results provide insights into how port selection, combining design, and collision control jointly affect the system performance [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Average rate per user vs. the number of UEs U, when the number of activated ports is fixed as K = 16. increasing the number of BS antennas M or the number of available ports N leads to further improvements for both schemes, as a larger M expands the available codebook and a larger N enhances the port-selection flexibility. When considering the sum-rate performance, it can be observed that increasing the nu… view at source ↗
Figure 6
Figure 6. Figure 6: illustrates the average per-user rate versus the number of BS antennas M under different user and port configurations. The number of ports is fixed as N = 100. In addition to showing that the proposed CPSA-FAMA scheme consistently outperforms the fixed￾antenna benchmark, [PITH_FULL_IMAGE:figures/full_fig_p010_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Codeword collision rate vs. the number of BS antennas M. collision rate decreases rapidly when M is small, because a larger number of BS antennas corresponds to a larger codebook and therefore a lower chance that multiple UEs select the same codeword. When M continues to increase, the collision probability quickly approaches a small value and remains almost unchanged. It can also be observed that varying t… view at source ↗
Figure 11
Figure 11. Figure 11: Average rate per user vs. the number of BS antennas M with three different scheduling schemes. based on its local channel condition. Nevertheless, the performance gap among the three schemes narrows as M increases because a larger number of BS antennas provides more available codewords and substantially reduces the collision probability. These results indicate that while the reselection scheme provides th… view at source ↗
Figure 10
Figure 10. Figure 10: Average rate per user vs. SNR with CPSC and exhaustive ports selection methods. and computational efficiency. In [PITH_FULL_IMAGE:figures/full_fig_p012_10.png] view at source ↗
read the original abstract

Fluid antenna multiple access (FAMA) has recently emerged as a simple, promising scheme for large-scale multiuser connectivity, offering strong scalability with low implementation complexity. Nevertheless, most existing FAMA studies focus on downlink transmission under perfect channel state information (CSI) at the receiver side, while the uplink counterpart remains largely unexplored. This paper proposes a novel codebook-based port selection and combining (CPSC) FAMA framework for the uplink communications without CSI at the base station (BS). In the proposed scheme, a predefined codebook is designed and broadcast by the BS. Each user equipment (UE) employs a fluid antenna, acquires its local CSI and independently chooses the most suitable codeword, activates the corresponding fluid antenna ports, and determines the combining weights to achieve a two-way match between the selected codeword and the instantaneous effective channel. The BS then separates the superimposed user signals through codebook-guided projection operations without requiring global CSI or multiuser joint optimization. To handle potential codeword collisions, three lightweight scheduling strategies are introduced, offering flexible trade-offs between signaling overhead and collision avoidance. Simulation results demonstrate that the proposed CPSC-FAMA approach achieves substantially higher rates than fixed-antenna systems while maintaining low complexity. Moreover, the results confirm that amortizing the optimization cost over the UEs effectively reduces the BS processing burden and enhances scalability, making the proposed scheme a strong candidate for future sixth-generation (6G) networks.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 0 minor

Summary. The manuscript proposes a codebook-based port selection and combining (CPSC) framework for CSI-free uplink fluid antenna multiple access (FAMA). A BS broadcasts a predefined codebook; each UE uses only local CSI to independently select a codeword, activate corresponding fluid-antenna ports, and compute combining weights so that the effective uplink channel matches the chosen codeword. The BS then separates superimposed signals via simple codebook-guided projections without global CSI or joint optimization. Three lightweight scheduling strategies address codeword collisions. Simulations are reported to show substantially higher rates than fixed-antenna baselines at low complexity.

Significance. If the two-way match can be reliably achieved and the projections provably limit inter-user interference, the scheme would offer a scalable, low-overhead uplink solution for massive connectivity in 6G by shifting optimization to the UEs and amortizing BS processing. The absence of quantitative metrics, codebook construction details, or mismatch bounds in the abstract, however, leaves the practical significance unassessable from the given text.

major comments (2)
  1. [Abstract] Abstract: the central claim that 'the BS separates the superimposed user signals through codebook-guided projection operations' without residual interference rests on an unproven two-way match between each UE's locally chosen codeword and its instantaneous effective channel. No derivation, bound on mismatch error, or condition on codebook size is supplied to show that finite-codebook projections remain sufficiently orthogonal under continuous fading.
  2. [Abstract] Abstract: the performance assertion that CPSC-FAMA 'achieves substantially higher rates than fixed-antenna systems' is unsupported by any numerical values, baselines, error bars, or verification that the projection step actually separates users; the claim cannot be evaluated.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We provide point-by-point responses below and indicate the revisions we will make to address the concerns raised.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central claim that 'the BS separates the superimposed user signals through codebook-guided projection operations' without residual interference rests on an unproven two-way match between each UE's locally chosen codeword and its instantaneous effective channel. No derivation, bound on mismatch error, or condition on codebook size is supplied to show that finite-codebook projections remain sufficiently orthogonal under continuous fading.

    Authors: The two-way match is achieved by design through the UE's independent port selection and combining weight computation based on local CSI, as detailed in Section III. The codebook-guided projection at the BS is analyzed in Section IV, where we derive the condition for perfect separation when the match holds and provide bounds on the mismatch error for finite codebooks under Rayleigh fading. The codebook size is chosen to ensure sufficient orthogonality with high probability. To make this clearer in the abstract, we will revise it to briefly mention the supporting analysis. revision: yes

  2. Referee: [Abstract] Abstract: the performance assertion that CPSC-FAMA 'achieves substantially higher rates than fixed-antenna systems' is unsupported by any numerical values, baselines, error bars, or verification that the projection step actually separates users; the claim cannot be evaluated.

    Authors: The simulation results in Section V provide quantitative comparisons, including specific rate values, baselines (fixed-antenna systems), and verification of user separation via the projections. We will update the abstract to include key numerical results from the simulations to support the claim. revision: yes

Circularity Check

0 steps flagged

No circularity detected in derivation chain

full rationale

The provided abstract and scheme description outline a CPSC-FAMA framework with codebook broadcast, local UE selection of codeword/ports/weights, and BS projection-based separation, supported by simulation results for rate gains. No equations, derivations, fitted parameters renamed as predictions, or self-citations appear in the text. No load-bearing step reduces by construction to its own inputs or prior author work; the central claims remain independent of such reductions. This is the most common honest finding for descriptive scheme papers without analytical self-reference.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no explicit free parameters, axioms, or invented entities; full text required for ledger population.

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Reference graph

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