# A Self-Scrutinized Backoff Mechanism for IEEE 802.11ax in 5G Unlicensed Networks

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## Abstract

**:**

## 1. Introduction

## 2. IEEE 802.11ax Use Cases in Fifth-Generation Radio Access Network on Unlicensed Bands (5G-U)

#### 2.1. Gigabit Ethernet Connection Replacement

#### 2.2. Improved Network Capacity Using Multi-User Multiple-Input and Multiple-Output (MU-MIMO)

#### 2.3. High-Efficiency Wireless (HEW) as a Backhaul for Local Area Network (LAN)

#### 2.4. Support for Highly Dense Scenarios

## 3. A Self-Scrutinized Backoff Mechanism for HEW

#### 3.1. Problem Statement

#### 3.2. Channel Observation-Based Scaled Backoff (COSB)

## 4. Analytical Model

#### 4.1. Recursive Discrete-Time Markove Chain (R-DTMC) Model

^{th}backoff stage and ${W}_{max}={2}^{m}\times {W}_{min}\times {\omega}^{{p}_{obs}}$ for the ${m}^{th}$ backoff stage contention window, where ${W}_{b}$ is the contention window size at b

^{th}backoff stage and ${p}_{obs}$ is the observed channel collision probability. Let us adopt the notation ${W}_{b+1}={2}^{b+1}\times {W}_{min}\times {\omega}^{{p}_{obs}}$, for the adaptively scaled-up contention window for $b+1$ backoff stage, when transmission is failed at the ${b}^{\mathrm{th}}$ backoff stage. Similarly, let ${W}_{b-1}={2}^{b-1}\times {W}_{min}\times {\omega}^{{p}_{obs}}$ be the adaptively scaled-down contention window for the $b-1$ backoff stage, when successfully transmitted at the ${b}^{\mathrm{th}}$ backoff stage.

#### 4.2. Normalized Throughput

#### 4.3. Average Delay

## 5. Performance Evaluation

#### 5.1. Normalized Throughput and Average Delay

#### 5.2. Maximum Approximate Saturation Throughput

#### 5.3. Average Channel Utilization Per Data Frame Transmission

## 6. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 1.**Possible high-efficiency wireless local area network (HEW) use cases in fifth-generation radio access network on unlicensed bands (5G-U) deployments; (

**a**) gigabit ethernet connection replacement; (

**b**) improved network capacity using multi-user multiple-input and multiple-output (MU-MIMO); (

**c**) HEW as a backhaul for local area network (LAN); (

**d**) support for highly dense scenarios.

**Figure 2.**Number of contending Wi-Fi user equipments (WEs) vs. ${p}_{obs}$ for ${W}_{min}=32$, and ${W}_{min}=64$.

**Figure 3.**Channel observation mechanism of channel observation-based scaled backoff (COSB) during the backoff procedure.

**Figure 4.**Backoff stage after collision/successful transmission; (

**a**) backoff stage increment/reset in binary exponential backoff (BEB); and (

**b**) backoff stage increment/decrement in COSB.

**Figure 7.**(

**a**) Average number of idle slot times per successful data frame transmission; (

**b**) average number of transmissions per data frame.

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

Operating Frequency | 5 GHz | Physical rate of channel | 54 Mbps |

Bandwidth | 20 MHz | MAC header | 24 Bytes |

MAC payload | 1024 Bytes | PHY header | 20 $\mathsf{\mu}$s |

Acknowledgment (ACK) | 14 Bytes + PHY header | Transmission range | 10 meters |

${W}_{min}$ | 32 | ${W}_{max}$ | 1024 |

$\omega $ | 32 | $\sigma $ | 9 $\mathsf{\mu}$s |

SIFS | 16 $\mathsf{\mu}$s | DIFS | 60 $\mathsf{\mu}$s |

Propagation delay ($\delta $) | 1 $\mathsf{\mu}$s | Max. backoff stages (m) | 6 |

**Table 2.**Comparison of maximum approximate saturation throughput achieved from ${\gamma}_{opt}$ with throughputs of COSB and BEB.

WEs (n) | Max. Approx. Throughput | COSB | BEB |
---|---|---|---|

5 | 0.510 (${\gamma}_{opt}$ = 0.0573) | 0.493 ($\gamma $ = 0.034) | 0.468 ($\gamma $ = 0.048) |

10 | 0.502 (${\gamma}_{opt}$ = 0.0287) | 0.501 ($\gamma $ = 0.024) | 0.452 ($\gamma $ = 0.037) |

20 | 0.498 (${\gamma}_{opt}$ = 0.0143) | 0.498 ($\gamma $ = 0.016) | 0.421 ($\gamma $ = 0.026) |

30 | 0.497 (${\gamma}_{opt}$ = 0.0095) | 0.493 ($\gamma $ = 0.012) | 0.401 ($\gamma $ = 0.020) |

40 | 0.497 (${\gamma}_{opt}$ = 0.0072) | 0.488 ($\gamma $ = 0.010) | 0.381 ($\gamma $ = 0.017) |

50 | 0.497 (${\gamma}_{opt}$ = 0.0057) | 0.484 ($\gamma $ = 0.008) | 0.365 ($\gamma $ = 0.015) |

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**MDPI and ACS Style**

Ali, R.; Shahin, N.; Bajracharya, R.; Kim, B.-S.; Kim, S.W.
A Self-Scrutinized Backoff Mechanism for IEEE 802.11ax in 5G Unlicensed Networks. *Sustainability* **2018**, *10*, 1201.
https://doi.org/10.3390/su10041201

**AMA Style**

Ali R, Shahin N, Bajracharya R, Kim B-S, Kim SW.
A Self-Scrutinized Backoff Mechanism for IEEE 802.11ax in 5G Unlicensed Networks. *Sustainability*. 2018; 10(4):1201.
https://doi.org/10.3390/su10041201

**Chicago/Turabian Style**

Ali, Rashid, Nurullah Shahin, Rojeena Bajracharya, Byung-Seo Kim, and Sung Won Kim.
2018. "A Self-Scrutinized Backoff Mechanism for IEEE 802.11ax in 5G Unlicensed Networks" *Sustainability* 10, no. 4: 1201.
https://doi.org/10.3390/su10041201