LAW: A Novel Mechanism for Addressing Hidden Terminal Problem in LTE-U and Wi-Fi Networks

Recently, the use of LTE in unlicensed spectrum (LTE-U) has gained a lot of attention. One of the daunting tasks before any such employment was to ensure the fair sharing of unlicensed spectrum between LTE-U and Wi-Fi radio access technologies (RATs), which now seems to be well addressed in the literature. However, along with fair sharing, the efficient utilization of unlicensed spectrum is also of profound significance, which pushes the need for coordination between LTE-U and Wi-Fi. Hence, this letter proposes a novel LTE-U and Wi-Fi (LAW) inter-RAT coordination mechanism for a more efficient utilization of the unlicensed spectrum. The aim is to address the inter-RAT hidden terminal problem between LTE-U and Wi-Fi and, thereby, offer better spectral efficiency. We modify the regular clear-to-send (CTS)-to-self frame and suggest the transmissions of modified CTS-to-self from LTE-U nodes to solve this hidden terminal issue. Further gains are extracted by allowing the simultaneous transmissions of LTE-U and Wi-Fi whenever possible

On the Impact of Duty Cycled LTE-U on Wi-Fi Users: An Experimental Study

The deployment of LTE in unlicensed spectrum is a plausible solution to meet explosive traffic demand from mobile users. However, fair coexistence with the existing unlicensed technologies, mainly Wi-Fi, needs to be ensured before any such deployment. Duty cycled LTE (LTE-U) is a simple and an easily adaptable scheme which helps in fair coexistence with the Wi-Fi. Nonetheless, the immense deployment of Wi-Fi necessitates a user-oriented study to find the effects of LTE-U operation, primarily in scenarios where the LTE-U eNB remains hidden from Wi-Fi Access Point. To delineate these effects, we perform a user-level Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) throughputs study of Wi-Fi in the presence of LTE-U using a testbed. Since, TCP is a more complicated protocol, we analyzed the Congestion Window and Round Trip Time data to comprehend the throughput results. This further explains the unfairness in throughput distribution among Wi-Fi users. Furthermore, we also notice inability among the disadvantaged users to receive the periodic Wi-Fi beacon frames successfully. The reasons and the subsequent consequences of throughput unfairness and beacon losses, are carefully elaborated. Also, to validate the beacon loss results, we present a beacon loss analysis which provides a mathematical expression to find the beacon loss percentage. Finally, we examine the results and highlight a need for incorporating additional functionalities in either LTE-U or Wi-Fi to overcome the present challenges

A novel coexistence scheme for IEEE 802.11 for user fairness and efficient spectrum utilization in the presence of LTE-U

A promising solution satisfying the industry’s demand to have minimum modification in LTE for its operation in unlicensed spectrum is duty cycled LTE-U scheme, which adopts discontinuous transmission to ensure fair coexistence with 802.11 (Wi-Fi) WLANs. Even though the scheme guarantees to maintain Wi-Fi network performance, the fairness among Wi-Fi users still remains arcane. In this work, we present a practical scenario where LTE-U, despite being discontinuous (by following an ON/OFF cycle), results in not only unfair throughput distribution among Wi-Fi users but also causes degradation in Wi-Fi AP’s downlink performance. This is due to the domination of few Wi-Fi users who harness channel in both ON and OFF durations of LTE-U, namely non-victim users over those who get access only in OFF duration, called victim users. In this paper, we studied the performance of victim and non-victim Wi-Fi users, and Wi-Fi AP while varying LTE-U ON fraction (i.e., duty cycle). A propitious scheme is proposed for WLANs, with regard to ease of implementation, employing Point/Hybrid Coordination Function (PCF/HCF) mode of 802.11, promising fairness among Wi-Fi users with improvement in the channel utilization of 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. Also, we present an analytical model to demonstrate guaranteed improvement and to validate our simulation results

LTE-U and Wi-Fi hidden terminal problem: How serious is it for deployment consideration?

The deployment of LTE in unlicensed spectrum is a plausible solution to meet explosive traffic demand from mobile users. However, fair coexistence with the existing unlicensed technologies, mainly Wi-Fi, needs to be ensured before any such deployment. Duty cycled LTE (LTE-U) is a simple and an easily adaptable scheme which helps in fair coexistence with the Wi-Fi. Nonetheless, the immense deployment of Wi-Fi necessitates a user-oriented study to find the effects of LTE-U operation, primarily in scenarios where the LTE-U eNB remains hidden from Wi-Fi Access Point. To comprehend these effects, we perform a user-level throughput study of Wi-Fi in the presence of LTE-U using a testbed and observe a clear unfairness in throughput distribution among Wi-Fi users. Furthermore, we also notice inability among the disadvantaged users to receive the periodic Wi-Fi beacon frames successfully. The reasons and the subsequent consequences, of throughput unfairness and beacon losses, are carefully elaborated. Also, to validate the beacon loss results, we present a beacon loss analysis which provides a mathematical expression to find the beacon loss percentage. Finally, we examine the results and highlight a need for incorporating additional functionalities in either LTE-U or Wi-Fi to overcome the present challenges

On Placement of LAA / LTE-U Base Stations in the Presence of Wi-Fi

Recently, the use of LTE in unlicensed has gained a lot of attention to improve the data rate and satisfy the increasing user demand. Use of unlicensed spectrum is allowed with restricted transmission power hence mostly LAA/LTE-U nodes are Femto cells and mostly will be used to improve the indoor data rates. In future, the LAA nodes will get deployed inside residential apartment complexes and office buildings to provide high data rates for indoor User Equipment (UEs). The UEs with high Signal-to-Interference plus Noise Ratio (SINR) experience good throughput, but the SINR decreases significantly if building walls and other obstacles are present in the communication path. The coverage difference of licensed and unlicensed spectrum provides different SINR so, efficient placement of LAA Femtos in buildings with consideration of interference from other LAA Femto nodes, Wi-Fi Access Points (APs) and Macro Base Stations (BSs) is very crucial for attaining desirable SINR for the indoor UEs. In this paper, we have considered obstructions (walls, floors) and interference from Macro, Femto BSs, and Wi-Fi APs. Our goal is to find the optimal number of Femtos with minimum SINR requirements in the unlicensed band, and next thing is to achieve good SINR in all sub-regions in licensed band as well. To do this, we formulated an optimization model (MinLF) to find the minimum number of LAA/LTE-U Femtos along with its positions in indoor scenarios which resulted in Mixed Integer Linear Programming (MILP) optimization problem. Further, to maximize minimum SINR in the licensed spectrum, we formulated MinLPow model to find the optimal power of each located Femto inside the building which is Mixed Integer Non-Linear Programming (MINLP). MinLF formulation is solved using GAMS CPLEX optimization solver. After getting positions of Femtos, we solved MinLPow formulation using Genetic Algorithm. Finally, the efficiency of proposed formulation model is shown with sufficient simulation study

A Complete Solution to LTE-U and Wi-Fi Hidden Terminal Problem

With the exponential growth in mobile data traffic, mobile operators are facing the unfortunate limit on the availability of licensed spectrum which has however, led to the popularity of Long Term Evolution (LTE) in unlicensed spectrum (LTE-U). Undeniably, it is expected from LTE-U that it fairly shares the spectrum with Wi-Fi. Along with fair sharing, efficient utilization of the unlicensed spectrum is also equally important, which in some sense requires coordination between the two Radio Access Technologies (RATs) viz., LTE-U and Wi-Fi. In particular, the hidden terminal scenario between LTE-U and Wi-Fi, resulting mainly due to lack of coordination, threatens the spectrum utilization of unlicensed spectrum. Focusing on this hidden terminal problem between LTE-U and Wi-Fi, we highlight the deficiency of existing technologies from the Wi-Fi perspective, both at the user level and at the network level. We then propose a novel coexistence technique (similar to RTS-CTS mechanism in Wi-Fi) that solves the hidden terminal problem between LTE-U and Wi-Fi, and subsequently addresses the spectrum underutilization problem caused by hidden terminal collisions. The proposed mechanism achieves this by using a modified CTS frame of Wi-Fi. We have validated our proposed mechanism using a mathematical framework demonstrating its credibility

On placement and efficient resource allocation of LAA/LTE-U base stations in HetNet

LTE operation in unlicensed spectrum is considered as a promising solution to meet the increase in user data demand. Licensed Assisted Access (LAA), and duty cycled LTE-U are two options for LTE to operate in the unlicensed band for fair sharing of unlicensed spectrum with IEEE 802.11 (Wi-Fi). Due to restriction on the transmission power in the unlicensed band, LAA/LTE-U Base Stations (BSs) will get deployed mostly inside residential and office buildings to provide high data rates for indoor User Equipments (UEs). In an indoor scenario, walls and other obstacles in the communication path along with co-tier and cross-tier interferences decrease the Signal-to-Interference plus Noise Ratio (SINR) significantly which results in throughput decrease. Hence, an optimal placement of LAA BSs is essential. As the available bandwidth in the unlicensed spectrum is more compared to the licensed spectrum, an efficient resource allocation is also necessary for ensuring minimum throughput for the indoor UEs. In this paper, our goal is to find the optimal number of LAA/LTE-U BSs with minimum throughput guarantee inside the building using licensed and unlicensed bands. To do this, we formulate an optimization model (MinLAA) for LAA BSs placement which is Mixed Integer Non-Linear Programming (MINLP) problem. So, we propose a heuristic algorithm to find the minimum number of LAA/LTE-U BSs such that all the users inside the building get minimum guaranteed throughput

Modelling and Analysis of Wi-Fi and LAA Coexistence with Priority Classes

The Licensed Assisted Access (LAA) is shown asa required technology to avoid overcrowding of the licensedbands by the increasing cellular traffic. Proposed by 3GPP,LAA uses a Listen Before Talk (LBT) and backoff mechanismsimilar to Wi-Fi. While many mathematical models have beenproposed to study the problem of the coexistence of LAAand Wi-Fi systems, few have tackled the problem of QoSprovisioning, and in particular analysed the behaviour of thevarious classes of priority available in Wi-Fi and LAA. Thispaper presents a new mathematical model to investigate theperformance of different priority classes in coexisting Wi-Fi andLAA networks. Using Discrete Time Markov Chains, we modelthe saturation throughput of all eight priority classes used byWi-Fi and LAA. The numerical results show that with the 3GPPproposed parameters, a fair coexistence between Wi-Fi and LAAcannot be achieved. Wi-Fi users in particular suffer a significantdegradation of their performance caused by the collision withLAA transmissions which has a longer duration compared toWi-Fi transmissions