Employing Novel Coexistence Schemes in LTE-U/Wi-Fi for Efficient Utilization of Unlicensed Spectrum

Baswade, Anand M and Tamma, Bheemarjuna Reddy (2019) Employing Novel Coexistence Schemes in LTE-U/Wi-Fi for Efficient Utilization of Unlicensed Spectrum. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

As the Internet grows aggressively, with the numbers crossing four billion users covering more than half of the world’s population, mobile operators need to augment network coverage and capacity in a significant manner. According to Cisco Visual Networking Index (VNI) forecast, the mobile Internet traffic grows with a Compound Annual Growth Rate (CAGR) of 46% between 2017 and 2022, and reaches 77.5 EB per month by 2022. Furthermore, with the introduction of portable devices like smartphones and tablets, the mobile operators need to boost the network capacity to meet the upsurging traffic demand. Some of the mechanisms to improve the network capacity include densification of cells, employing Multiple Input Multiple Output (MIMO) technology, and addition of more spectrum. Though densification of cells and MIMO increase the capacity, it is still not sufficient to meet the bandwidth requirements. Since the amount of licensed spectrum available is limited, it is expensive proposition to improve the network capacity by increasing the bandwidth. The spectrum crunch and huge cost of licensing the spectrum motivate the mobile industry to make use of the unlicensed spectrum for operation of cellular networks. However, with Wi-Fi being one of the traditional, wide-spread, and far reaching consumers of these unlicensed bands, it has been made incumbent on the Long Term Evolution (LTE) to ensure fair coexistence with Wi-Fi networks, before any such deployment. To ensure fair coexistence with other wireless technologies in the unlicensed spectrum (mainly Wi-Fi), LTE has to go through some modifications in terms of channel access. Two schemes are proposed for LTE operation in the unlicensed spectrum namely LTE in Unlicensed (LTE-U) and Licensed Assisted Access (LAA). LTE-U scheme is restricted to regions where Listen-Before-Talk (LBT) is not mandatory like India, USA, China, and Korea. LAA scheme follows LBT which makes it a global solution because it satisfies all the regional regulatory requirements. In LTE-U scheme, LTE follows discontinuous transmission in the unlicensed spectrum, which is called as duty cycled LTE-U. The discontinuous transmission allows the LTE-U eNB to transmit for some duration in a given duty cycled period called as LTE-U ON period and mute its transmission for the rest of the duty cycled period called as LTE-U OFF period so that Wi-Fi can transmit. LAA is similar to Wi-Fi where it follows LBT using Clear Channel Assessment (CCA) and back-off procedure to access the channel. However, due to its simplicity of requiring minimal changes in the existing LTE protocol, LTE-U is easy and faster to deploy. The main issues and challenges for efficient LTE and Wi-Fi coexistence in the unlicensed spectrum include: (1) Design of LTE-U/LAA channel access schemes ensuring fair coexistence with Wi-Fi, (2) Inter-Radio Access Technology (Inter-RAT) coordination for interference management, (3) Inter-RAT hidden terminal problem, (4) Dynamic channel selection for LTE-U/LAA in the unlicensed spectrum, (5) Dynamic LTE-U/LAA scheduling of user traffic in the licensed and unlicensed spectrum, (6) Guaranteeing Quality of Service (QoS) for real-time traffic in the unlicensed spectrum, and (7) Design of efficient LAA/LTE-U placement techniques for improving network capacity and indoor coverage. In this thesis, some of the issues and fundamental challenges which prevent LTE-U/LAA networks from having fair coexistence with Wi-Fi networks are addressed to achieve efficient sharing of unlicensed spectrum between LTE-U/LAA and Wi-Fi networks. Firstly, we focus on studying the performance of the Wi-Fi network on a real-time testbed setup in the hidden terminal scenario between LTE-U and Wi-Fi networks. We observe the unfairness caused to some of the Wi-Fi users called as victim users in terms of throughput and then study the effect of the presence of these victim users on the Wi-Fi network. This inter-RAT hidden terminal problem is very crucial as it affects the throughput fairness among Wi-Fi users and also decreases the downlink (DL) throughput of Wi-Fi Access Point (AP) drastically thereby leading to an imbalance in the uplink (UL) and DL throughputs of the Wi-Fi network. To address this inter-RAT hidden terminal problem, this thesis proposes one centralized solution and one distributed solution. The proposed centralized and distributed solutions target to meet the following benefits for the Wi-Fi in the presence of LTE-U network: (1) Improve the overall performance of the Wi-Fi network, (2) Achieve UL and DL throughput fairness in Wi-Fi network, and (3) Achieve throughput fairness among Wi-Fi users. In the centralized solution named Coexistence Coordination Function (CCF), an interRAT controller is used to achieve coordination among LTE-U and Wi-Fi networks. In this scheme, Wi-Fi employs Point/Hybrid Coordination Function (PCF/HCF) mode of IEEE 802.11, and achieves fairness among Wi-Fi users with improvement in the channel utilization of the Wi-Fi network. The key idea is that the victim users, who can only be served during the LTE-U OFF period should be served in Contention Free Period (CFP) – so as to improve their throughputs and make them equally competitive with non-victim users. For the considered DL only scenario, the proposed CCF mechanism achieves 35% improvement in throughput at LTE-U ON fraction of 0.5, and the maximum improvement observed is 4X at LTE-U ON fraction of 1 in CCF compared to Distributed Coordination Function (DCF). In the distributed solution, a novel coexistence technique (similar to RTS-CTS mechanism in Wi-Fi) is proposed to solve the hidden terminal problem between LTE-U and Wi-Fi, and subsequently addressed the spectrum underutilization problem caused by hidden terminal collisions. The proposed mechanism does not require any inter-RAT controller as it achieves this by using a modified Clear-To-Send (CTS) frame of Wi-Fi. We have validated the proposed mechanism using a mathematical framework demonstrating its credibility. The next issue addressed in this thesis is the throughput modelling of LTE-U and Wi-Fi networks in the coexistence scenario which helps to study the coexistence in greater detail. For this, an analytical model is proposed for characterization of achievable throughputs of Wi-Fi and LTE-U networks in spatially distributed high-density scenarios. Our extensive simulation results validate it to be a reliable model for estimating throughput of LTE-U and Wi-Fi networks in coexistence scenarios. Our model gives a very good accuracy in throughput estimation i.e., mean normalized error is less than 1% for a 40-node scenario where 50% of nodes are Wi-Fi APs and the rest of them are LTE-U eNBs. Further, considering the 3GPP definition of fairness, we conduct coexistence studies of LTE-U and Wi-Fi, and with proper justification claim that the LTE-U is fair with Wi-Fi in terms of throughput in spatially distributed scenarios. The next issue addressed is modelling throughput of LAA and Wi-Fi with priority classes. For this, a new mathematical model is proposed to investigate the performance of different priority classes in coexisting Wi-Fi and LAA networks. Using Discrete Time Markov Chains (DTMC), the saturation throughput is modelled for all the eight priority classes used by Wi-Fi and LAA. The numerical results conclude that with the 3GPP specified parameters, a fair coexistence between Wi-Fi and LAA cannot be achieved. Wi-Fi users, in particular, suffer significant degradation of their performance due to collisions with LAA transmissions as LAA transmissions have longer duration compared to Wi-Fi transmissions. Therefore, to improve the fairness between Wi-Fi and LAA systems, we propose that LAA transmission opportunity should be reduced. Finally, the thesis addresses the deployment of LTE-U/LAA nodes in the presence of Wi-Fi in indoor environments. The placement of LTE-U/LAA nodes inside a building is done with the following objectives: (1) Minimizing the number of LTE-U/LAA nodes deployed without any coverage holes, (2) Ensuring minimum Signal-to-Interference plus Noise Ratio (SINR) in every sub-region of the building, and (3) Power control to maximize the minimum SINR in the licensed spectrum. To achieve this, we formulate an optimization problem for placing minimum number of LTE-U/LAA nodes with optimal transmission power. The proposed models achieve optimal placement with a maximum energy savings of 88% compared to the model where LAA nodes transmit with maximum power.

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