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Transmission Through Large Intelligent Surfaces: A New Frontier in Wireless Communications

Ertugrul Basar

arXiv:1902.08463v2eess.SPcs.IT

TL;DR

The paper studies whether phase-controlling large intelligent surfaces can improve wireless transmission and support access-point operation. It develops received-SNR and SEP analysis, then evaluates reflector and LIS-AP schemes with known or blind channel phases. The results show improved error performance and ultra-reliable communication, including when the LIS itself acts as an access point.

  • Problem

    The paper examines how large intelligent surfaces can improve wireless signal quality and whether the surface itself can serve as an access point.

  • Method

    The paper derives a mathematical error-performance framework and evaluates LIS reflector and access-point schemes using phase adjustment with known or blind channel phases.

  • Results

    LIS-based transmission achieves very low error rates at extremely low SNR values, while LIS-AP transmission provides ultra-reliable communication and around 1 dB SNR improvement over LIS-DH for binary signaling.

  • Takeaways & Limitations

    A LIS can function as an intelligent reflector or a simple access point for future wireless communication systems.

Abstract

from arXiv · show

In this paper, transmission through large intelligent surfaces (LIS) that intentionally modify the phases of incident waves to improve the signal quality at the receiver, is put forward as a promising candidate for future wireless communication systems and standards. For the considered LIS-assisted system, a general mathematical framework is presented for the calculation of symbol error probability (SEP) by deriving the distribution of the received signal-to-noise ratio (SNR). Next, the new concept of using the LIS itself as an access point (AP) is proposed. Extensive computer simulation results are provided to assess the potential of LIS-based transmission, in which the LIS acts either as an intelligent reflector or an AP with or without the knowledge of channel phases. Our findings reveal that LIS-based communications can become a game-changing paradigm for future wireless systems.

I. INTRODUCTION

The paper positions large intelligent surfaces as devices that control propagation to improve wireless coverage and signal quality, and introduces error analysis plus LIS-based access-point transmission.

  • Large intelligent surfaces control propagation by adjusting the phases of reflected signals to improve wireless coverage and signal quality.
  • Unlike MIMO, beamforming, relaying, and backscatter, LIS elements passively reflect incident signals without dedicated RF processing, decoding, encoding, or retransmission energy.
  • The paper develops a mathematical framework for LIS error-performance analysis, including the effects of reflecting elements, modulation orders, and blind phases.
  • It also proposes using the LIS itself as an access point by reflecting an unmodulated carrier and encoding information through reflector phases.
  • The study evaluates LIS operation as either an intelligent reflector or an access point with and without channel-phase knowledge.

II. TRANSMISSION THROUGH LIS: SYSTEM MODEL & ERROR PERFORMANCE ANALYSIS

This section defines a generic LIS-assisted dual-hop model and establishes the basis for calculating its theoretical symbol error probability under Rayleigh fading.

  • The system model uses a single-antenna source, an LIS, and a single-antenna destination connected by Rayleigh fading channels.The source–LIS and LIS–destination channel coefficients are modeled as hi, gi ∼ CN(0, 1).
  • The section provides a unified framework for calculating the theoretical symbol error probability of the LIS-based transmission scheme.

A. Intelligent Transmission Through LIS

With slowly varying flat fading, the LIS combines reflected paths through adjustable phases, enabling SNR maximization and analytical error-performance evaluation for PSK and QAM signaling.

  • System model and phase optimization: The received signal is formed by coherently combining signals reflected from N passive LIS elements under slowly varying flat fading.The model includes data symbols from M-ary PSK/QAM constellations and additive white Gaussian noise.
  • System model and phase optimization: The received SNR is maximized by setting each reflector phase to φi = θi + ψi, which requires channel-phase knowledge at the LIS.
  • Error-performance analysis: For sufficiently large N, the central limit theorem supports a Gaussian approximation for the combined channel amplitude and a non-central chi-square SNR model.
  • Error-performance analysis: The framework derives average SEP expressions for M-PSK and square M-QAM signaling using the received-SNR distribution and its moment generating function.
  • Intelligent transmission performance: The LIS-based scheme outperforms classical BPSK over pure AWGN at low SNR, with larger N producing lower BEP in the waterfall region.
  • Intelligent transmission performance: Doubling N provides approximately 6 dB improvement, or a fourfold decrease in required SNR, at the waterfall region for a target BER.
  • Error-performance analysis: Increasing modulation order degrades error performance, although the LIS scheme retains a benefit from its N^2 term.

B. Blind Transmission Through LIS

When channel phases are unknown, the LIS cannot cancel them for SNR maximization, but blind phase operation still provides an N-times SNR gain over Rayleigh fading.

  • Blind operation: Without channel-phase knowledge, the LIS cannot eliminate phase terms to maximize received SNR.The blind scheme sets φ_i = 0 for all reflecting elements.
  • Analysis: For large N, the blind scheme models the received channel as H ∼ CN(0, N) and derives its SNR MGF for BEP analysis.The resulting MGF is M_γ(s) = (1 − sNE_s/N_0)^−1.
  • Performance: 1 N-times SNR gain is obtained over point-to-point transmission through Rayleigh fading channels.This result is stated for the blind LIS-assisted scheme.

III. THE NEW DESIGN: LIS AS AN ACCESS POINT

The paper proposes using the LIS itself as an access point, with passive reflectors conveying information while a wired or optical connection supports network access.

  • LIS as an access point: The LIS can serve as an access point and source while retaining only low-cost, passive reflector elements.It can connect to the network through a wired link or optical fiber.
  • LIS as an access point: The proposed access-point LIS supports transmission without RF processing.Its passive reflector architecture is compatible with a nearby network connection and does not require RF processing at the LIS.
  • LIS as an access point: The access-point concept uses the LIS itself to transmit information rather than only reflecting a signal from a separate source.The proposal is introduced as a new paradigm for transmitting information by the LIS.

A. Intelligent Access Point-LIS

With channel-phase knowledge, the LIS access point uses reflector phases both to align reflected signals and to encode M-ary information, enabling analytical SEP evaluation.

  • Intelligent Access Point-LIS: The LIS uses induced phases to cancel channel phases and align reflected signals into a virtual M-ary constellation.This combines SNR-improving intelligent reflection with information transmission through phase control.
  • Intelligent Access Point-LIS: The model transmits log2(M) bits per signaling interval by setting φ_i = ψ_i + w_m.Here w_m is the common additional phase associated with message m.
  • Intelligent Access Point-LIS: Uniformly spaced information phases w_m = 2π(m − 1)/M minimize the average SEP for M-ary signaling.The signal model resembles M-PSK over a super-channel.
  • Intelligent Access Point-LIS: The paper derives the instantaneous received SNR and uses a large-N CLT approximation to characterize the effective channel and its MGF.The effective amplitude is modeled with a Gaussian approximation based on the Rayleigh-distributed reflector-channel amplitudes.
  • Intelligent Access Point-LIS: Around 1 dB improvement in required SNR is obtained over the LIS-based dual-hop scheme for binary signaling at a target BEP.The comparison is made between expressions (8) and (21).
  • Intelligent Access Point-LIS: For higher-order signaling with M ≥ 16, a required-SNR loss is expected because M-PSK has lower SNR efficiency than M-QAM.The paper states that this loss becomes insignificant at the relatively low SNR ranges associated with increasing N.

B. Blind Access Point-LIS

In the blind access-point configuration, the LIS conveys data by adjusting reflector phases without channel-phase knowledge, while retaining an N-times SNR gain over Rayleigh fading.

  • Blind Access Point-LIS: Without knowledge of channel phases, the LIS acts as a data source by adjusting reflector-induced phases similarly to PSK.For binary signaling, the phases are set to ω_1 or ω_2 across all reflectors.
  • Blind Access Point-LIS: Uniformly distributed phases around the unit circle minimize SEP; w_1 = 0 and w_2 = π are optimal for binary BEP.The binary choice corresponds to BPSK phase separation.
  • Blind Access Point-LIS: The blind access-point analysis uses G ∼ CN(0, N) to obtain the MGF of the instantaneous received SNR.The analysis supports average BEP and SEP expressions for binary and M-ary signaling.
  • Blind Access Point-LIS: 1 N-times SNR gain is obtained over point-to-point transmission through Rayleigh fading channels.The paper concludes that the LIS can operate as an access point by adjusting reflector phases according to the data.

IV. SIMULATION RESULTS

Simulations evaluate LIS-DH and LIS-AP under coherent and blind transmission, varying reflector count and signaling order. LIS-AP can outperform LIS-DH, while higher signaling orders and blind phases degrade performance.

  • Around 1 dB SNR improvement is obtained by LIS-AP over LIS-DH when M = 2 with BPSK.The comparison uses BER performance across different numbers of reflecting elements.
  • Both LIS-DH and LIS-AP suffer degraded SER as the signaling order increases from M ∈{4, 16, 64} with 64 reflectors.The degradation is more noticeable for LIS-AP because of the SNR loss of M-PSK over M-QAM for M ≥16.
  • Blind LIS-DH and LIS-AP have the same received SNR distribution and provide an N times SNR gain over point-to-point Rayleigh fading.This comparison concerns BPSK transmission with varying numbers of reflectors.
  • Doubling N in blind transmission provides a 3 dB improvement in the required SNR for a target BER.The advantage of the LIS diminishes when phase removal is not exploited.

V. CONCLUSIONS

The paper develops a mathematical framework for evaluating LIS-assisted communications and assesses their potential through error-performance analysis. It concludes that LIS-based transmission can boost received SNR, support ultra-reliable communication, and use the LIS itself as an access point.

  • The study evaluates LIS-assisted communications from an error-performance perspective and introduces a mathematical framework for assessing the approach.
  • LIS-based transmission can boost received SNR and enable ultra-reliable communications at extremely low SNR values.
  • The LIS itself can operate as an access point using a simple transceiver architecture that intentionally reflects an unmodulated carrier.
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