# A Physical Layer Security Enhancement Scheme under the Ambient Backscatter System

^{*}

## Abstract

**:**

## 1. Introduction

- In the ABC environment, we proposed a scheme IANT that uses physical layer strategies to enhance system security. As far as we know, it is the first time to study the physical layer security scheme from the perspective of artificial noise in thje ABC environment system.
- Different from previous studies, in order to better adapt to the ABC environment, a new method is adopted to dynamically estimate channel parameters, which does not need pilot symbols.
- Compared with the contrast scheme, the IANT scheme achieves a secrecy rate that is approximately twice the contrast scheme under the condition that the bit error rate (BER) is slightly reduced, which effectively enhances the security performance.

## 2. The System Model

## 3. IANT Scheme Design

#### 3.1. Design of Reader Side Scheme

- (1)
- In the first sub-slot, K tags sequentially transmit N
_{0}symbols to reader in the order of index, and the receiving end of reader calculates ${\Phi}_{k}$ according to Formula (4). - (2)
- The reader selects the i-th and j-th tags that satisfy Formula (8). It first sends K symbols to all tags, where the i-th symbol bit is the bit ‘1’, and the rest are ‘0’.
- (3)
- The selected i-th tag responds to the reader with U symbols, and U represents the index of the i-th tag in binary form. After receiving the response symbol, reader converts the binary index to decimal $\widehat{i}$ and checks whether it is equal to the value of i.
- (4)
- Reader sends K symbols to all tags again, the j-th symbol bit is bit ‘1’, and the rest are ‘0’. Repeat step 3) for the j-th tag.
- (5)
- If $\widehat{i}=i,\widehat{j}=j$, reader sends a sine wave, otherwise the reader remains silent.
- (6)
- If the i-th tag and the j-th tag detect the sine wave of the reader’s response, they both send N
_{T}symbols at the same time, corresponding to data and noise signals respectively.

#### 3.1.1. The First Sub-Slot

_{0}bits ‘1’ to reader during (K − 1)N

_{0}to KN

_{0}symbols, while the other (K − 1) tag antennas are in the absorption state and do not backscatter the signal. Therefore, the signal received by the reader can be expressed as:

_{0}bits ‘1’, which can be divided into the following two parts:

#### 3.1.2. The Second Sub-Slot

#### 3.1.3. The Third Sub-Slot

_{t}represents the number of symbol s(n) transmitted by RF signal source during the duration of one tag data symbol. Next, we use the reader to compare the calculated results with the Formulas (5) and (6) and determine whether the symbol backscattered by tag is ‘1’ or ‘0’ according to the following rule:

#### 3.2. The Received Signal on the Eavesdropper Side

_{0}and Θ

_{i}based on the average power of the signal received by its receiver, as follows:

_{B}with Θ

_{0}and Θ

_{i}and then determines whether the symbol received is ‘1’ or ‘0’ according to the following rules:

#### 3.3. BER and Security Performance Analysis

## 4. Numerical Simulation Results

_{0}transmitted by each tag has a strong correlation with the channel estimation error. As shown in Figure 7, the horizontal axis represents the different signal length N

_{0}sent by tag in the first sub-slot. We set the channel signal-to-noise ratio to 0, 5, and 10 respectively, and showed the channel estimation error under different channel conditions. We can conclude that the channel estimation error keeps decreasing as the signal length increases, but when the signal length N

_{0}is greater than 20, the change rate of the channel estimation error slows down significantly. Moreover, when the channel quality is relatively good, a smaller channel estimation error can be obtained.

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 3.**The influence of the channel quality change between tags and reader on the system BER (Bit Error Rate).

**Figure 4.**The influence of the channel quality change between tags and reader on achievable secrecy rate.

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**MDPI and ACS Style**

Hou, P.; Gong, J.; Zhao, J.
A Physical Layer Security Enhancement Scheme under the Ambient Backscatter System. *Symmetry* **2021**, *13*, 5.
https://doi.org/10.3390/sym13010005

**AMA Style**

Hou P, Gong J, Zhao J.
A Physical Layer Security Enhancement Scheme under the Ambient Backscatter System. *Symmetry*. 2021; 13(1):5.
https://doi.org/10.3390/sym13010005

**Chicago/Turabian Style**

Hou, Pengfei, Jianping Gong, and Jumin Zhao.
2021. "A Physical Layer Security Enhancement Scheme under the Ambient Backscatter System" *Symmetry* 13, no. 1: 5.
https://doi.org/10.3390/sym13010005