Improved Low-Complexity, Pilot-Based Channel Estimation for Large Intelligent Surface Systems
Abstract
:1. Introduction
1.1. Motivation
- Pilot-based techniques: these include methods such as Least Squares (LS) and Minimum Mean Square Error (MMSE).
- Machine learning-based methods: these leverage data-driven approaches for enhanced accuracy.
- Compressed sensing: this approach exploits the channel’s sparsity to estimate its parameters efficiently.
1.2. Pilot-Based Estimation
1.3. Novelty and Contribution
1.3.1. Enhanced Pilot-Based Channel Estimation with Hadamard Sequences
- Unlike traditional pilot sequences used in LIS systems [10], our method employs structured Hadamard sequences that are specifically optimized for low-SNR conditions, reducing cross-interference and improving channel estimation accuracy.
- Unlike conventional pilots, Hadamard sequences enable perfect orthogonality even under multipath fading, leading to more stable channel estimation.
1.3.2. Adaptive Pilot Power Optimization for Dynamic Channel Conditions
- Prior methods such as [11] use fixed pilot power, which does not account for varying noise conditions. Our approach introduces real-time adaptive power scaling, ensuring an optimal SNR while maintaining spectral efficiency.
- Our simulations show that adaptive power optimization reduces the BER more effectively than static power allocation, especially when the signal is weak.
1.3.3. Dynamic Pilot Number Adjustment to Reduce Overhead
- Traditional LIS estimation methods [12] rely on a fixed number of pilots, leading to inefficiencies. Our method uses adaptive pilot clustering, where pilot density increases only when needed.
- Our simulations show that adaptive power optimization helps lower the BER in changing environments better than static power allocation.
1.3.4. Iterative Channel Estimation for Progressive Accuracy Refinement
- Conventional LS estimation [13] relies on single-pass estimation, which can introduce high residual errors. We implement iterative refinement, where channel estimates improve progressively across iterations.
- Our approach gradually improves channel estimation over multiple iterations, making it more effective in noisy LIS environments.
1.3.5. Selective LIS Antenna Activation for Computational Efficiency
- Previous LIS systems [14] activate all antennas, increasing computational complexity. We propose intelligent antenna selection, activating only the most effective elements.
- This technique lowers computational effort by selecting only the most effective antennas while keeping channel estimation accuracy high.
1.3.6. LDPC-Assisted LS Estimation for Noise-Resilient Performance
- While LDPC codes have been used for error correction [15], their integration with LS-based LIS channel estimation has not been fully explored. We introduce a joint LS-LDPC decoding framework, which significantly improves channel robustness.
- Our method enhances channel robustness by combining LS estimation with LDPC decoding, improving the performance in high-noise conditions.
1.4. Characteristics of LIS Systems
- Large surface for signal reflection and transmission: unlike conventional MIMO systems with separate antennas, a LIS is a Large Intelligent Surface with many small antennas that control radio waves.
- More flexibility in selecting the signal path: a LIS improves signal quality and reduces interference, leading to improving spectral efficiency and coverage.
- Reconfigurable electromagnetic properties: unlike traditional base stations, a LIS can change wave properties for better transmission and lower energy consumption.
- Reduced hardware complexity: instead of complex and powerful equipment, a LIS uses simple and low-energy components for sending and receiving signals [14].
- Compatible with 6G networks: a LIS can match with 6G systems easily, providing ultra-reliable, low-latency communication (URLLC) and massive machine-type communication (m-MTC) through intelligent signal manipulation [18].
1.5. Objective and Organization of This Article
2. System Characterization
2.1. Channel Model
2.2. Receiver Algorithms
- ZF: this linear receiver inverts the channel matrix to cancel interference:
- MMSE: balances interference suppression and noise amplification by minimizing the mean square error:
- MRC: this receiver combines the received signals based on the channel gains, improving the SNR:
- EGC: combines signals from multiple antennas with equal weights but adjusts phase to maximize received power:
- Iterative Block Decision Feedback Equalization (IB-DFE) is an advanced equalization technique used in communication systems to improve the performance in multipath or interference-limited environments. It iteratively refines the received signal by leveraging feedback from previously detected data, combining concepts from block-based processing and decision feedback. is a feedback filter matrix, stands for the forward filter matrix, and represents the detected signal from the previous iteration.
2.3. LS Channel Estimation
3. Receiver Design Considerations for LIS: Proposed Improvements in Channel Estimation
3.1. Optimized Pilot Design and Enhancing the Quality of Pilot Data
3.1.1. Orthogonal Pilot Sequences
- A.
- Hadamard as Orthogonal Pilot Sequences
Algorithm 1: Hadamard Orthogonal Pilot-Based Channel Estimation |
Input:
Preprocessing:
Repeat: :
until:
Output:
|
- B.
- Properties of Hadamard Pilots:
- Orthogonality: each pilot sequence is orthogonal to others, allowing easy separation at the receiver [27].
- Low complexity: Hadamard sequences are computationally efficient to generate and process.
- Binary nature: They are simple to implement in hardware or software with entries.
3.1.2. Pilot Power Optimization [19]
- Interpolation process: the estimated pilot-based channel matrix is interpolated across all subcarriers using a suitable interpolation function, which represents the chosen interpolation technique.
- Interpolation methods: commonly used interpolation techniques include the following:
- Linear interpolation: a simple method that connects pilot estimates with straight-line approximations;
- Spline interpolation: a more accurate approach that ensures smooth transitions between estimated points.
- Low-pass filtering: this mitigates noise effects and provides a smoother channel response by eliminating high-frequency artifacts.
- A.
- Defining the Optimization Problem
Algorithm 2: Improved LS Channel Estimation with Increased Pilot Power |
Input:
Pilot symbol insertion:
LS estimation:
Interpolation:
Output:
|
3.1.3. Increasing the Number of Pilots [19]
Algorithm 3: Improved LS Channel Estimation with Increased Pilot Number |
Input:
Pilot symbol insertion:
LS estimation:
Improved interpolation:
Output:
|
- A.
- Optimization Problem for Number of Pilots
3.1.4. Adding Pilot Iteration to Channel Estimation [10]
- A.
- Optimization Problem for Number of Pilot Iterations
Algorithm 4: Improved LS Channel Estimation with Increased Pilot Iteration |
Input:
Initial LS estimation:
Iterative refinement: For :
Improved interpolation:
Output:
|
3.2. Selective LIS Antenna Configurations and Increasing Channel Estimation Accuracy
Optimized Antenna Configurations and LIS Technology for Improved Channel Estimation
- Reduce computational complexity: focusing on fewer, high-impact antennas reduces the size of the matrices involved in channel estimation and subsequent signal processing, leading to faster computations [36].
- Maintain estimation accuracy: selecting the most relevant antennas ensures that the accuracy of channel estimation remains high, as only the antennas that contribute most to the signal quality are used.
- Enhance scalability: this approach is particularly beneficial for LIS systems with hundreds or thousands of antennas, where reducing the number of active antennas directly impacts the system’s computational efficiency.
- Energy efficiency: selective activation conserves power by disabling less effective antennas, improving overall efficiency.
- Interference: by focusing transmission and reception on high-quality antennas, the approach reduces multi-user interference.
3.3. LDPC Codes in Mitigating the Effects of Channel Estimation Errors
3.3.1. Reducing Sensitivity to Channel Estimation Errors
3.3.2. Improving Performance in Sparse Channels
3.3.3. Enhancing Spectral Efficiency and Energy Savings
4. Simulation Performance Results
- LIS channel modeling:
- The receiver is assumed to be in the near field of the LIS.
- The beam focuses on both angle and distance, unlike traditional far-field beamforming, which only considers direction.
- The simulation includes multipath reflections and time delays () to make it more realistic.
- Radio and environmental parameters:
- Carrier frequency: 2G Hz;
- Antenna spacing: ;
- User locations: randomly placed inside a room;
- Number of antennas: 4 × LISs (LIS antenna consist of four panels and each panel has 100 antennas; in total 4 × 100 = 400).
- Practical feasibility of LIS:
- LIS is modeled as an active antenna system, where each element has independent gain and phase control.
- Near-field beamforming is used to focus signals on users.
- Power adjustments are applied to keep the system practical.
- Real-world feasibility depends on hardware challenges, the real-time processing of many antennas, and precise phase/gain calibration.
- LDPC encoding and decoding:
- The LDPC encoder converts input bits into a longer coded sequence to improve error correction.
- The LDPC decoder uses a hard-decision method with a maximum of 20 iterations to decode the received data.
- The decoder stops early if the parity check is satisfied.
- LDPC integration with LIS:
- LDPC is applied to 4PSK modulation before transmission.
- After LIS-based transmission and reception, LDPC decoding is used to recover the original data.
- The simulation includes different equalization and interference cancellation methods (MRC, EGC, ZF, and MMSE).
- Pilot power optimization increases the strength of the pilot signals, improving the channel estimation accuracy but slightly reducing the spectral efficiency because it takes up more transmission power.
- Increasing the number of pilots helps achieve better accuracy but comes with a trade-off: higher complexity and reduced spectral efficiency due to the extra pilot overhead.
- Iterative estimation improves the accuracy over multiple iterations, but it requires more processing power, making it computationally expensive.
- Channel selection focuses only on the most useful LIS antennas, reducing the complexity while still maintaining good accuracy and efficiency.
- Hadamard orthogonal pilot sequences minimize interference between pilots, improving both the accuracy and spectral efficiency while keeping the complexity low.
Improvement Method | Complexity | Energy Efficiency | Interference Reduction | Estimation Accuracy | Spectral Efficiency |
---|---|---|---|---|---|
Pilot power optimization | Moderate | Improves with careful scaling | Minimal impact on interference | Improves SNR for pilots | Moderate (trade-off with data throughput) |
Increasing number of pilots | High (due to increased overhead) | Decreases due to higher overhead | No direct impact | Improves with increased reference data | Decreases due to increased pilot overhead |
Iterative estimation | High (due to iterations) | Improves with better estimation accuracy | Moderate (depends on feedback accuracy) | Significantly improves after iterations | Improves by refining estimates |
Channel selection | Low (reduces processing load) | Improves by focusing resources on high-impact antennas | High (reduces inter-user interference) | Moderate (focuses on relevant antennas) | Improves by reducing unnecessary overhead |
Hadamard orthogonal sequences | Low (efficient generation) | Moderate (no direct impact on energy efficiency) | High (minimizes pilot contamination) | High (ensures minimal cross-correlation) | High (reduces pilot interference) |
- 2.
- Computational Complexity (Figure 6)
- 3.
- Energy Efficiency (Figure 4)
- 4.
- Interference Reduction (Figure 5)
- 5.
- Spectral Efficiency (Figure 7)
5. Conclusions
6. Future Research
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
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Aspect | Previous Methods | Our Improvement |
---|---|---|
Enhanced pilot-based channel estimation | Conventional methods use fixed orthogonal pilots, leading to interference and limited spectral efficiency. | We introduce Hadamard-based dynamic pilot assignment, which significantly reduces interference and improves channel estimation accuracy under varying SNR conditions. |
Pilot power optimization | Previous works rely on fixed pilot power allocation, which does not adapt to fluctuating channel conditions. | Our method dynamically adjusts pilot power based on real-time channel variations, ensuring an optimal SNR while maintaining spectral efficiency. |
Adaptive pilot symbol allocation | Previous works rely on fixed pilot traditional approaches use a fixed number of pilot symbols, leading to inefficient estimation in low-SNR scenarios. | We propose adaptive pilot clustering, where the system selectively increases pilot symbols only when necessary, reducing unnecessary overhead. |
Iterative channel estimation | Conventional LS estimation performs single-pass estimation, often leaving residual errors. | Our iterative refinement process progressively updates channel estimates over multiple iterations, leading to significantly improved accuracy, particularly in noisy environments. |
Selective LIS antenna activation | Previous LIS systems activate all antennas, increasing complexity and computational burden. | We introduce intelligent antenna selection, where only the most effective antennas are used, reducing complexity without sacrificing accuracy. |
LDPC-assisted LS estimation | LDPC codes have been used for error correction but were not integrated with LS estimation in LIS systems. | We develop a joint LS-LDPC decoding framework, which significantly enhances noise resilience and system stability in high-interference conditions. |
Number of Pilots | Channel Estimation Accuracy | Spectral Efficiency | Pilot Overhead | Recommendation |
---|---|---|---|---|
64 | Moderate | Medium | Medium | Best trade-off between accuracy and overhead |
128 | High | Low | High | Suitable for systems with limited antennas |
Receiver Architecture | BER Performance | Complexity | Energy Efficiency | Interference | Spectral Efficiency | Accuracy |
---|---|---|---|---|---|---|
ZF | High | High | Moderate | Low | High | Moderate |
MMSE | High | Moderate | Moderate | Good | Moderate | High |
MRC | Moderate | Moderate | High | Moderate | High | High |
EGC | Moderate | Low | High | Low | Moderate | Moderate |
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Gashtasbi, A.; Marques da Silva, M.; Dinis, R. Improved Low-Complexity, Pilot-Based Channel Estimation for Large Intelligent Surface Systems. Appl. Sci. 2025, 15, 3743. https://doi.org/10.3390/app15073743
Gashtasbi A, Marques da Silva M, Dinis R. Improved Low-Complexity, Pilot-Based Channel Estimation for Large Intelligent Surface Systems. Applied Sciences. 2025; 15(7):3743. https://doi.org/10.3390/app15073743
Chicago/Turabian StyleGashtasbi, Ali, Mário Marques da Silva, and Rui Dinis. 2025. "Improved Low-Complexity, Pilot-Based Channel Estimation for Large Intelligent Surface Systems" Applied Sciences 15, no. 7: 3743. https://doi.org/10.3390/app15073743
APA StyleGashtasbi, A., Marques da Silva, M., & Dinis, R. (2025). Improved Low-Complexity, Pilot-Based Channel Estimation for Large Intelligent Surface Systems. Applied Sciences, 15(7), 3743. https://doi.org/10.3390/app15073743