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Theses

Transport of critical services over unlicensed spectrum in 5G networks

Abstract : This thesis studies the transport of critical services in 5G networks, where unlicensed spectrum is advocated to minimize the cost and to cope with the high demand for frequency resources. We first evaluate the performance of Ultra-Reliable Low-Latency Communication (URLLC) which has stringent requirements on reliability and delay, on the order of 99.999% and 1 ms, respectively, transported in unlicensed spectrum. We propose a model based on a Markov chain to quantify the reliability within a delay constraint under Listen-Before-Talk (LBT) medium access procedure, then we deduce the maximum number of stations that can be handled at the same time, while respecting URLLC constraints. This analysis is then used to investigate novel methods for the joint transmission of URLLC over unlicensed and licensed spectrum. We propose three methods for the joint access to available resources, and demonstrate that the optimal method to access the resources is by using licensed ones only when unlicensed transmission fails within a given time budget. This method is then studied in the case of multiple tenants in proximity competing over the same unlicensed channel. If all tenants try to maximize their usage of unlicensed resources then everyone will end up in a tragedy of the commons type of situation. We show that at least one equilibrium point exists for this system which minimizes the cost for all tenants. We study later the coexistence of URLLC with other 5G services, such as enhanced Mobile Broadband (eMBB), in unlicensed spectrum. eMBB has large packets and its multiplexing with URLLC may entail a large degradation in URLLC performance. For that, we propose a new technique to prioritize URLLC packets by transmitting them with higher power. However, high power transmission is not systematically performed to reduce the interference on other users and also to reduce the energy consumption, which is very important for battery-powered devices. In this case, two methods were proposed to transmit with high power, leaving it as a last resort. One is LBT-agnostic and transmits whenever the packet delay approaches time-out, while the other respects LBT and uses high power only when transmission opportunities occur beyond a time threshold. We then propose a decentralized implementation of the time-threshold approach. We formulate the problem as an optimization problem where transmitters are to choose the optimal policy (time threshold) which minimizes the energy consumption while preserving URLLC requirements. We then solve the optimization problem using a learning approach and show a slow convergence to the optimal policy due to the fact that losses are rare events. To remedy to this, we employ the optimization framework and the prior knowledge of the system to accelerate this learning. We finally study the decentralized approach for a different type of critical services which focuses on the freshness of the information, known as the Age of Information (AoI). In this context, instead of guaranteeing a reliability target within a delay, the packet must be delivered as soon as it is generated, or else it loses its value. We demonstrate that optimal policies in the AoI context tend to start aggressively, and reduce the transmission power when the age of the packet increases.
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https://tel.archives-ouvertes.fr/tel-03118924
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Submitted on : Friday, January 22, 2021 - 4:40:08 PM
Last modification on : Tuesday, February 2, 2021 - 2:26:02 PM

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  • HAL Id : tel-03118924, version 1

Citation

Ayat Zaki Hindi. Transport of critical services over unlicensed spectrum in 5G networks. Networking and Internet Architecture [cs.NI]. Institut Polytechnique de Paris, 2020. English. ⟨NNT : 2020IPPAS022⟩. ⟨tel-03118924⟩

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