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Secrecy Analysis of Ambient Backscatter NOMA Systems under I/Q Imbalance
Xingwang Li, Mengle Zhao, Yuanwei Liu, Lihua Li, Zhiguo Ding, Arumugam Nallanathan
TL;DR
The paper studies reliability and security in AmBC-NOMA with an eavesdropper, addressing the unexamined impact of IQI on physical-layer security. It derives closed-form outage and intercept probabilities and analyzes asymptotic outage behavior. The reported results show IQI can reduce reliability while improving security, with outage error floors under the modeled reflection coefficient.
Problem
The impact of IQI on physical-layer security in AmBC-NOMA had not yet been investigated, despite practical RF front-ends being susceptible to IQI.
Source: §I · p. 1Method
The paper derives closed-form outage and intercept probabilities for the far user, near user, and backscatter device, and analyzes high-SNR outage behavior and diversity order.
Source: §I · p. 1Results
IQI reduces reliability but improves security in the considered system, and the outage probability reaches an error floor determined by IQI and reflection coefficient β.
Source: A. Outage Probability Analysis · p. 3, §V · p. 5Takeaways & Limitations
The backscatter device has better secrecy performance in the considered AmBC-NOMA system.
Source: §V · p. 5
Abstract
from arXiv · showhide
We investigate the reliability and security of the ambient backscatter (AmBC) non-orthogonal multiple access (NOMA) systems, where the source aims to communication with two NOMA users in the presence of an eavesdropper. We consider a more practical case that nodes and backscatter device (BD) suffer from in-phase and quadrature-phase imbalance (IQI). More specifically, exact analytical expressions for the outage probability (OP) and the intercept probability (IP) are derived in closedform. Moreover, the asymptotic behaviors and corresponding diversity orders for the OP are discussed. Numerical results show that: 1) Although IQI reduces the reliability, it can enhance the security. 2) Compared with the traditional orthogonal multiple access (OMA) system, the AmBC-NOMA system can obtain better reliability when the signal-to-noise (SNR) ratio is low; 3) There are error floors for the OP because of the reflection coefficient \b{eta} .
I. INTRODUCTION
The introduction motivates studying secrecy in AmBC-NOMA with practical IQ imbalance, a setting not previously investigated to the authors’ knowledge. The work analyzes reliability and security for two NOMA users and a backscatter device in the presence of an eavesdropper.
I. INTRODUCTION
- NOMA multiplexes users on the same time-frequency resources using superposed coding and successive interference cancellation, with more power allocated to weaker users.Source: §I · p. 1
- AmBC uses an ambient RF source, a backscatter device, and a reader, and is presented as a potentially spectrum- and energy-efficient technology for IoT.Source: §I · p. 1
- Prior work proposed an AmBC system in which a reader recovers information from both the backscatter device and RF source, and NOMA-assisted AmBC for massive device connections.Source: §I · p. 1
- Prior AmBC secrecy and NOMA security studies assumed ideal RF front-ends, while practical components can suffer IQI that limits system performance.The paper frames analysis of AmBC-NOMA secrecy under IQI as practically significant.Source: §I · p. 1
- The paper derives closed-form outage and intercept probabilities for the far user, near user, and backscatter device, and analyzes high-SNR outage behavior and diversity order.The eavesdropper can intercept both source signals and signals reflected from the backscatter device.Source: §I · p. 1
- The introduction reports that IQI harms reliability but can improve security, and that the backscatter device achieves highly secure communication in the considered system.Source: §I · p. 1
II. SYSTEM MODEL
The system model considers downlink AmBC-NOMA with a source, backscatter device, far and near users, and an eavesdropper. All nodes’ transceiver RF front-ends are assumed to suffer IQ imbalance.
II. SYSTEM MODEL
- The model includes one source, one backscatter device, a far user, a near user, and an eavesdropper, with single antennas at all nodes.Source: §II · pp. 1–2
- IQI is modeled as phase and/or amplitude imbalance between the in-phase and quadrature-phase branches at the transmitter.Source: §II · p. 2
- The source superposes signals for the far and near users using power coefficients a1 and a2, where a1 + a2 = 1 and a1 < a2.Source: §II · p. 2
- The backscatter device reflects the received signal while embedding its own signal, and the received signal at each node includes source and backscattered components under IQI.Source: §II · p. 2
- The model defines signal-to-interference-plus-noise ratios for decoding, with the near user and eavesdropper using successive interference cancellation to decode signals in turn.Source: §II · p. 2
III. PERFORMANCE ANALYSES
The section evaluates reliability and security through outage and intercept probabilities, then examines high-SNR outage behavior and diversity order.
III. PERFORMANCE ANALYSES
- The analysis derives outage probability and intercept probability expressions to evaluate the AmBC-NOMA system’s reliability and security.Source: §III · p. 2
- It also examines asymptotic outage behavior and diversity order in the high-SNR region.Source: §III · p. 2
A. Outage Probability Analysis
The section derives outage probabilities for the far user, near user, and backscatter device (BD), then examines their high-SNR behavior. It reports an error floor determined by IQI and the reflection coefficient β.
A. Outage Probability Analysis
- The far user's outage event is failure to decode x2, while the near user's outage requires successful decoding of both x2 and its own information x1.The far-user target rate is γth2; the near-user target rate is γth1.Source: A. Outage Probability Analysis · pp. 2–3
- For Rayleigh fading channels, the paper derives analytical outage-probability expressions for the far user, near user, and BD.The results are presented in Theorems 1–3.Source: A. Outage Probability Analysis · pp. 2–3
- The BD signals are successfully decoded only when the near user decodes x2 and x1; the BD decoding target rate is γthc.Source: A. Outage Probability Analysis · p. 3
- At high SNR, the outage probabilities of the far user, near user, and BD approach constants, producing error floors and zero diversity order.The paper attributes the floor to IQI parameters and reflection coefficient β.Source: A. Outage Probability Analysis · p. 3
B. Intercept Probability Analysis
The section defines interception through the eavesdropper's ability to exceed a secrecy SNR threshold and derives intercept-probability expressions. It reports beneficial effects of IQI parameters on intercept probabilities and a reflection-coefficient trend for the far user.
B. Intercept Probability Analysis
- An interception occurs when the eavesdropper's SNR for a user's signal exceeds that user's secrecy SNR threshold.The signals considered are x2, x1, and the BD signal c(t).Source: B. Intercept Probability Analysis · p. 3
- For Rayleigh fading channels, the paper derives analytical intercept-probability expressions for the far user, near user, and BD.Source: B. Intercept Probability Analysis · p. 3
- As reflection coefficient β increases, the far user's intercept probability decreases.Source: B. Intercept Probability Analysis · p. 4
- The paper reports that IQI parameters have a beneficial effect on the intercept probabilities of the far user, near user, and BD.Source: B. Intercept Probability Analysis · p. 4
IV. NUMERICAL RESULTS
The numerical results validate the analysis and show that IQI affects users differently, with reliability–security trade-offs that also depend on the reflection coefficient and power allocation.
IV. NUMERICAL RESULTS
- The theoretical OP and IP results agree well with Monte Carlo simulations.The simulations use 10^6 trials.Source: §IV · p. 4
- As β increases, backscatter-link interference reduces Df and Dn reliability and improves security, while BD outage first decreases and then increases.The optimal reflection coefficient depends on the power allocation coefficient.Source: §IV · pp. 4–5
- IQI reduces system reliability while enhancing security to some extent, with the near user and BD particularly affected through SIC.The BD is more sensitive to IQI at high SNRs; the near user is affected because SIC is impaired.Source: §IV · p. 4, Figure 2 · p. 5
- For a given OP, IQI reduces the corresponding IP, while NOMA’s secure performance versus OMA is better for the near user and worse for the far user.The BD has the smallest IP; the far user’s NOMA/OMA comparison is attributed to the large power allocation parameter for the far user.Source: §IV · p. 4, Figure 3 · p. 5
V. CONCLUSION
The paper derives closed-form OP and IP expressions for AmBC NOMA with IQI and analyzes high-SNR outage behavior and diversity order. Its results indicate reliability losses and security improvements from IQI, while increasing β raises OP and lowers IP for the far and near users.
V. CONCLUSION
- Closed-form OP and IP expressions are derived for AmBC NOMA with IQI, alongside high-SNR outage behavior and diversity-order analysis.Source: §V · p. 5
- IQI reduces reliability for the far user, near user, and BD while improving security for all three devices.Source: §V · p. 5
- Increasing β raises OP and lowers IP for the far and near users, and the BD has the best secrecy performance.Source: §V · p. 5