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Hardware Impaired Ambient Backscatter NOMA Systems: Reliability and Security
Xingwang Li, Mengle Zhao, Ming Zeng, Shahid Mumtaz, Varun G Menon, Zhiguo Ding, Octavia A. Dobre
TL;DR
The paper addresses physical-layer security in ambient backscatter NOMA under residual hardware impairments, channel estimation errors, and imperfect SIC. It introduces source-generated artificial noise, derives OP and IP expressions with asymptotic analyses, and reports reliability-security trade-offs, error floors, and contrasting MER effects across readers and tag.
Problem
The paper asks how residual hardware impairments, channel estimation errors, and imperfect SIC affect reliability and security in ambient backscatter NOMA with an eavesdropper.
Method
The RF source simultaneously transmits NOMA signals and artificial noise, while the paper derives OP and IP expressions and high-SNR and high-MER asymptotics.
Results
RHIs, CEEs, and ipSIC worsen OP but improve IP; RHIs have stronger effects than CEEs, and OP error floors arise from CEEs and the reflection coefficient.
Takeaways & Limitations
Reliability and security trade off, with artificial-noise performance governed by its power coefficient and the readers’ interference factor; MER improves reader security but reduces tag security.
Abstract
from arXiv · showhide
Non-orthogonal multiple access (NOMA) and ambient backscatter communication have been envisioned as two promising technologies for the Internet-of-things due to their high spectral efficiency and energy efficiency. Motivated by this fact, we consider an ambient backscatter NOMA system in the presence of a malicious eavesdropper. Under some realistic assumptions of residual hardware impairments (RHIs), channel estimation errors (CEEs) and imperfect successive interference cancellation (ipSIC), we investigate the physical layer security (PLS) of the ambient backscatter NOMA systems focusing on reliability and security. In order to further improve the security of the considered system, an artificial noise scheme is proposed where the radio frequency (RF) source acts as a jammer that transmits interference signal to the legitimate receivers and eavesdropper. On this basis, the analytical expressions for the outage probability (OP) and the intercept probability (IP) are derived. To gain more insights, the asymptotic analysis and diversity orders for the OP in the high signal-to-noise ratio (SNR) regime are carried out, and the asymptotic behaviors of the IP in the high main-to-eavesdropper ratio (MER) region are explored as well. Numerical results show that: 1) RHIs, CEEs and ipSIC have negative effects on the OP but positive effects on the IP; 2) Compared with CEEs, RHIs have a more serious impact on the reliability and security of the considered system; 3) There exists a trade-off between reliability and security, and this trade-off can be optimized by reducing the power coefficient of the artificial noise or increasing the interfering factor of readers; 4) There are error floors for the OP due to the CEEs and the reflection coefficient; 5) As MER grows large, the security for Rnand Rf is improved, while the security for T is reduced.
I. INTRODUCTION
The introduction positions NOMA and ambient backscatter as efficient IoT technologies, then identifies security and realism gaps caused by eavesdropping threats, imperfect CSI, and nonideal RF hardware.
- NOMA supports massive IoT connections through power multiplexing, high spectral efficiency, low latency, and successive interference cancellation.
- Ambient backscatter enables ultralow-power, low-cost communication by passively reflecting incident RF signals to transmit information.
- Wireless broadcast transmission exposes ambient backscatter NOMA systems to eavesdropping, while conventional encryption can be unsuitable for resource-limited devices.
- Prior studies commonly assume perfect CSI and RF components, whereas channel estimation errors and residual hardware impairments make those assumptions impractical.
- The introduction motivates analyzing realistic NOMA and backscatter systems with imperfect CSI, residual hardware impairments, and physical-layer security.
A. Motivation and Contribution
The paper studies the joint reliability and security effects of residual hardware impairments, channel estimation errors, and imperfect SIC in a secure ambient backscatter NOMA system.
- A. Motivation and Contribution: The paper identifies the joint effects of RHIs, CEEs, and ipSIC on secure ambient backscatter NOMA performance as insufficiently investigated.
- A. Motivation and Contribution: An artificial-noise scheme lets the RF source transmit the desired signal and interference simultaneously to improve security without changing the original system framework.
- A. Motivation and Contribution: Analytical OP and IP expressions are derived for the far reader, near reader, and tag to evaluate reliability and security.
- A. Motivation and Contribution: Asymptotic OP behavior and diversity orders are analyzed at high SNR, while IP behavior is explored at high MER.
- A. Motivation and Contribution: The analysis finds OP error floors caused by channel estimation errors and the reflection coefficient.
B. Organization and Notations
The paper is organized around system modeling, reliability and security analysis, numerical validation, and concluding findings, with standard probability and distribution notation defined.
- Section II introduces the ambient backscatter NOMA model.
- Section III derives analytical and asymptotic outage expressions and intercept-probability expressions for reliability and security analysis.
- Section IV provides numerical results to validate the theoretical analysis, and Section V summarizes the key findings.
- The notation includes expectation, probability, modified Bessel functions, factorials, and probability density and cumulative distribution functions.
II. SYSTEM MODEL
The system model contains an ambient RF source, tag, two readers, and an eavesdropper, incorporating Rayleigh fading, receiver-side RHIs, imperfect CSI, artificial noise, and SIC-based decoding.
- The downlink system comprises one RF source, one tag, far and near readers, and one eavesdropper sharing the same resource block.
- The tag transmits information by reflecting the source signal, while the eavesdropper intercepts signals intended for the readers.
- Channel estimation models each channel as an estimated component plus an error, with the error variance indicating CSI quality.
- The source injects artificial noise alongside NOMA signals; its power is controlled by ϕ_J, and readers and the eavesdropper experience resulting interference.
- The far reader decodes x_2, whereas the near reader and eavesdropper decode x_2, x_1, and the tag signal successively using SIC.
III. PERFORMANCE ANALYSIS
The analysis evaluates reliability and security through outage and intercept probabilities, including high-SNR and high-MER asymptotics.
- Reliability and security are assessed using outage probability (OP) and intercept probability (IP).
- The analysis examines asymptotic OP and diversity orders in the high-SNR regime.
- The analysis also explores asymptotic IP behavior in the high-MER regime.
A. OP Analysis
The OP analysis derives outage expressions for the far reader, near reader, and tag, then characterizes their high-SNR behavior and diversity orders.
- OP for Rf: The far reader’s outage event occurs when it cannot successfully decode x2.
- Asymptotic OP: At high SNRs, asymptotic OP expressions are derived for the far reader, near reader, and tag.
- OP for Rn: The near reader requires successful decoding of both x2 and its own information x1 to avoid outage.
- OP for T: The tag signals are successfully decoded when x2 and x1 are perfectly decoded at the near reader.
- Diversity orders: RHIs, CEEs, and ipSIC worsen reliability, while high-SNR OPs approach constants and produce zero diversity orders.
B. IP Analysis
The IP analysis derives intercept expressions for the readers and tag and studies high-MER security trends under ideal and non-ideal conditions.
- IP formulation: A user is intercepted when the eavesdropper successfully wiretaps that user’s signal.
- IP expressions: Analytical IP expressions are derived for the far reader, near reader, and tag under the considered conditions.
- High-MER asymptotics: High-MER asymptotic IP expressions are obtained for the far reader, near reader, and tag.
- Security trends: RHIs, CEEs, and ipSIC enhance security, while increasing the reflection coefficient decreases reader IP and increases tag IP.
- Security trends: As MER grows, security improves for Rn and Rf but declines for T; increasing ϕJ reduces IP and improves the reliability-security trade-off.
IV. NUMERICAL RESULTS
Numerical results validate the analysis and expose how SNR, artificial-noise parameters, RHIs, CEEs, ipSIC, and MER shape reliability-security trade-offs in the considered system.
- Simulation setup: Monte Carlo simulations with 10^6 trials verify the theoretical results across the evaluated conditions.The simulations use the parameter settings in Table I unless otherwise stated.
- Transmit SNR: The OP approaches a fixed constant at high SNR under fixed estimation error and reflection coefficient, producing zero diversity order.The theoretical results match simulations across the entire SNR region.
- Artificial noise: A smaller artificial-noise power coefficient or larger reader interference factor degrades the reliability-security trade-off by increasing eavesdropper or receiver interference.The recommended design uses a greater artificial-noise power coefficient and smaller reader interference factor.
- RHIs and CEEs: RHIs and CEEs increase OP while decreasing IP, with RHIs causing more obvious reliability and security fluctuations than CEEs.The far reader’s OP changes most drastically with RHIs, whereas T’s OP changes least because T eliminates part of reader interference.
- ipSIC and reflection: At high SNR, OP error floors appear; increasing β improves OP under perfect SIC but reduces T’s security, while ε lowers the reliability of Rn and T.OPs of Rf and Rn are more sensitive to β, whereas T’s IP is more sensitive to β.
- MER: As MER grows, security improves for Rn and Rf but decreases for T; RHIs enhance security under the examined ideal and non-ideal conditions.The asymptotic results closely approximate IP in the high-MER regime.
V. CONCLUSION
The paper studies how RHIs, CEEs, and ipSIC affect reliability and security in ambient backscatter NOMA, using artificial noise to improve security and analyzing OP and IP. Results show opposing effects on reliability and security, with the trade-off adjustable through artificial-noise and reader-interference parameters.
- RHIs, CEEs, and ipSIC significantly worsen OP but improve IP for the far reader, near reader, and tag.
- Increasing the reflection coefficient β reduces reliability and enhances security for the far and near readers.
- The reliability-security trade-off can be optimized by adjusting the artificial-noise power coefficient and reader interference factor.
APPENDIX A: PROOF OF THEOREM 1
Appendix A derives outage-probability expressions for the far reader, near reader, and tag by substituting system equations into the relevant OP formulations.
- The OP of the far reader is obtained by substituting equation (3) into equation (6).
- The integral I1 is evaluated as an intermediate step in deriving the far-reader outage probability.
- The near-reader OP follows by substituting equations (3) and (4) into equation (11).
APPENDIX B: PROOF OF THEOREM 3
Appendix B derives outage and intercept probability expressions under non-ideal and ideal conditions using substitutions, mathematical manipulation, and Gaussian-Chebyshev quadrature.
- Non-ideal conditions: The tag OP is derived by substituting equations (3), (4), and (5) into equation (14).
- Non-ideal conditions: Under non-ideal conditions, I2 is evaluated through intermediate expressions and Gaussian-Chebyshev quadrature.
- Ideal conditions: The ideal-condition tag OP uses I31 and I32, with I32 approximated by Gaussian-Chebyshev quadrature.
- Non-ideal conditions: The far-reader OP and tag IP are obtained from the corresponding intermediate integrals and substitutions into their defining expressions.
- Ideal conditions: Under ideal conditions, setting κ = 0 and σ2_e = 0 yields CE = ME = 0 for the tag intercept-probability derivation.