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Implementation of In-Band Full-Duplex Using Software Defined Radio with Adaptive Filter-Based Self-Interference Cancellation^{ †}

^{*}

^{†}

## Abstract

**:**

## 1. Introduction

- An implementation of an IBFD prototype system is provided in this work. The implemented IBFD system is realized with three-stage SI cancellation, which consists of antenna isolation, analog cancellation, and digital cancellation by adaptive filter.
- By conducting an experimental evaluation, it is known that the implemented three-stage SI cancellation provides a nearly 100 dB SI cancellation effect for the IBFD system implementation.
- With the implemented IBFD system, the resultant bit error rate (BER) can achieve ${10}^{-3}$ order under a moderate signal-to-noise ratio (SNR), which validates the effectiveness and practicality of our proposed IBFD system implementation design.

## 2. System Overview

## 3. Digital SI Cancellation Using Adaptive Filter

Algorithm 1: nLMS adaptive filter algorithm. |

## 4. Experimental Evaluation

- Step 1: Turn on SI signal;
- Step 2: Evaluate SI cancellation of antenna;
- Step 3: Evaluate SI cancellation of AC;
- Step 4: Turn on desired signal and combine desired signal with SI;
- Step 5: Evaluate constellations, EVMs, and BERs before and after turning on AF-based digital SI cancellation.

Parameter | Value |
---|---|

USRP Operating Frequency | $2.15$ GHz |

RF Operating Frequency | $4.7$ GHz |

Signal Bandwidth | 16 MHz |

Antenna Spacing | 15 cm |

Modulation | QPSK |

OFDM FFT Size | 256 |

Operating SNR | 10–25 dB, with $2.5$ dB interval |

AF FIR Taps ${N}_{\mathrm{taps}}$ | 20 |

Step-size $\mu $ | $0.1$ |

#### 4.1. Antenna SI Cancellation

#### 4.2. SI Cancellation by Analog Circuit

#### 4.3. SI Cancellation by Adaptive Filter

#### 4.3.1. Constellation Plot

#### 4.3.2. EVM Evaluation

#### 4.3.3. BER Evaluation

#### 4.4. Discussion

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

Adaptive Filter | AF |

Analog Circuit | AC |

Base Station | BS |

Bit Error Rate | BER |

Coupler | CPL |

Device-to-Device | D2D |

Divider | DIV |

Downlink | DL |

Error Vector Magnitude | EVM |

Fixed Impulse Response | FIR |

Full-duplex | FD |

In-band Full-duplex | IBFD |

Inout-output | I/O |

Intermediate Frequency | IF |

Key Performance Indicator | KPI |

Least Mean Square | LMS |

Mean Square Error | MSE |

Medium Access Control | MAC |

Multiple-input Multiple-output | MIMO |

Multiuser | MU |

Non-orthogonal Multiple Access | NOMA |

Normalized Least Mean Square | nLMS |

Not Available | NA |

Orthogonal Frequency Division Multiplexing | OFDM |

Radio Frequency | RF |

Receiver | RX |

Self-interference | SI |

Signal-to-interference-plus-noise Radio | SINR |

Signal-to-noise Radio | SNR |

Software Defined Radio | SDR |

Successive Interference Cancellation | SIC |

Transmitter | TX |

Universal Software Radio Peripheral | USRP |

Uplink | UL |

Variable Frequency Equalizer | VFE |

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**Figure 3.**The designed prototype of analog circuit proposed in [13].

**Figure 5.**Planar antenna structure: microstrip patch antenna on a dielectric substrate with thickness of $0.5$ mm, and relative dielectric constant of $3.3$. The copper foil has thickness of 18 $\mathsf{\mu}\mathrm{m}$.

**Figure 6.**Parallel (side-by-side) configuration of two antennas (set as TX and RX), integrated on the same substrate with fixed distance (d) between antennas.

**Figure 7.**Measured results of the antennas used in system experiment, return loss (S11) for each antenna, and self-interference (S21) between two antennas. The TX/RX antenna configuration is parallel (side-by-side), distance (d) is 150 mm.

Notation | Description |
---|---|

a | Scalar variable |

A | Scalar constant |

$f\left(x\right)$ | Function of x |

${a}^{*}$ | Complex conjugate of a |

$\mathbf{a}$ | Vector |

$\mathbf{A}$ | Matrix |

${\mathbf{A}}^{\mathrm{T}}$ | Transpose of $\mathbf{A}$ |

${\mathbf{A}}^{\mathrm{H}}$ | Hermitian of $\mathbf{A}$ |

$\left|\left|\mathbf{a}\right|\right|$ | Norm of $\mathbf{a}$ |

$\mathbb{E}\left\{a\right\}$ | Expectation function of random variable a |

${\nabla}_{\mathbf{a}}$ | Derivation with respect to $\mathbf{a}$ |

$\mathrm{tr}\left\{\mathbf{A}\right\}$ | Trace of $\mathbf{A}$ |

SI Cancellation | Proposed | Proposal in [5] | Proposal in [6] | Proposal in [31] |
---|---|---|---|---|

Antenna Type | $46.9$ dB | $30.0$ dB | NA | $31.4$ dB |

Analog Type | $22.0$ dB | $20.0$ dB | $62.0$ dB | $28.2$ dB |

Antenna + Analog | $68.9$ dB | $50.0$ dB | $62.0$ dB | $59.6$ dB |

Digital Type | $30.0$ dB | $10.0$ dB | $48.0$ dB | $24.0$ dB |

Total Cancellation | $98.9$ dB | $60.0$ dB | $110.0$ dB | $83.6$ dB |

BER | ${10}^{-3}$ | NA | NA | ${10}^{-2}$ |

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**MDPI and ACS Style**

Liao, W.-S.; Zhao, O.; Li, K.; Kawasaki, H.; Matsumura, T.
Implementation of In-Band Full-Duplex Using Software Defined Radio with Adaptive Filter-Based Self-Interference Cancellation. *Future Internet* **2023**, *15*, 360.
https://doi.org/10.3390/fi15110360

**AMA Style**

Liao W-S, Zhao O, Li K, Kawasaki H, Matsumura T.
Implementation of In-Band Full-Duplex Using Software Defined Radio with Adaptive Filter-Based Self-Interference Cancellation. *Future Internet*. 2023; 15(11):360.
https://doi.org/10.3390/fi15110360

**Chicago/Turabian Style**

Liao, Wei-Shun, Ou Zhao, Keren Li, Hikaru Kawasaki, and Takeshi Matsumura.
2023. "Implementation of In-Band Full-Duplex Using Software Defined Radio with Adaptive Filter-Based Self-Interference Cancellation" *Future Internet* 15, no. 11: 360.
https://doi.org/10.3390/fi15110360