Andra M. Voicu

Coexistence of pico- and femto-cellular LTE-unlicensed with legacy indoor Wi-Fi deployments

Due to the high expected increase in mobile data traffic and the scarcity of licensed spectrum for cellular networks, 3GPP has started preliminary work for standardizing LTE operation in the 5 GHz unlicensed band (LTE-U). However, LTE-U would interfere with other legacy technologies operating in the unlicensed band, the most important being contention-based Wi-Fi, which would be blocked by conventional LTE, which is designed for dedicated licensed spectrum. Consequently, some coexistence-enabling mechanisms have been proposed for LTE-U, but their evaluation is still at an early stage. In this paper we present a detailed system-level study on the downlink throughput performance of legacy indoor IEEE 802.11n and LTE-U deployments coexisting in the 5 GHz band. We consider several LTE-U coexistence mechanisms (i.e. listen-before-talk and interference-aware channel selection) in indoor LTE-U femtocell and outdoor LTE-U picocell scenarios with a realistic range of network densities and real outdoor picocell locations. We also study coexistence of LTE-U networks deployed by multiple operators, and evaluate the impact of different LTE-U transmit power levels. Our results show that in general both Wi-Fi and LTE-U benefit from the large number of available channels and isolation provided by building shielding at 5 GHz. Additionally, in typical indoor coexistence scenarios, interference-aware channel selection is more efficient for both Wi-Fi and LTE-U than listen-before-talk mechanisms. For outdoor LTE-U picocells and indoor Wi-Fi deployments, the two networks are isolated from each other, but listen-before-talk can increase LTE-U user throughput when multiple outdoor LTE-U networks deployed by different cellular operators coexist.

Inter-Technology Coexistence in a Spectrum Commons: A Case Study of Wi-Fi and LTE in the 5-GHz Unlicensed Band

Spectrum sharing mechanisms need to be carefully designed to enable inter-technology coexistence in the unlicensed bands, as these bands are an instance of a spectrum commons where highly heterogeneous technologies and deployments must coexist. Unlike in licensed bands, where multiple technologies could coexist only in a primary-secondary dynamic spectrum access mode, a spectrum commons offers competition opportunities between multiple dominant technologies, such as Wi-Fi and the recently proposed LTE in the 5 GHz unlicensed band. In this paper, we systematically study the performance of different spectrum sharing schemes for inter-technology coexistence in a spectrum commons. Our contributions are threefold. First, we propose a general framework for transparent comparative analysis of spectrum sharing mechanisms in time and frequency, by studying the effect of key constituent parameters. Second, we propose a novel throughput and interference model for inter-technology coexistence, integrating per-device specifics of different distributed MAC sharing mechanisms in a unified network-level perspective. Finally, we present a case study of IEEE 802.11n Wi-Fi and LTE in the 5 GHz unlicensed band, in order to obtain generalizable insight into coexistence in a spectrum commons. Our extensive Monte Carlo simulation results show that LTE/Wi-Fi coexistence in the 5 GHz band can be ensured simply through channel selection schemes, such that time-sharing MAC mechanisms are irrelevant. We also show that, in the general co-channel case, the coexistence performance of MAC sharing mechanisms strongly depends on the interference coupling in the network, predominantly determined by building shielding. We thus identify two regimes: (i) low interference coupling, e.g., residential indoor scenarios, where duty cycle mechanisms outperform sensing-based listen-before-talk (LBT) mechanisms and (ii) high interference coupling, e.g., open-plan indoor or outdoor hotspot scenarios, where LBT outperforms duty cycle mechanisms.

LTE in Unlicensed Bands Is Neither Friend nor Foe to Wi-Fi

Proponents of deploying LTE in the 5 GHz band for providing additional cellular network capacity have claimed that LTE would be a better neighbour to Wi-Fi in the unlicensed band, than Wi-Fi is to itself. On the other side of the debate, the Wi-Fi community has objected that LTE would be highly detrimental to Wi-Fi network performance. However, there is a lack of transparent and systematic engineering evidence supporting the contradicting claims of the two camps, which is essential for ascertaining whether regulatory intervention is in fact required to protect the Wi-Fi incumbent from the new LTE entrant. To this end, we present a comprehensive coexistence study of Wi-Fi and LTE-in-unlicensed, surveying a large parameter space of coexistence mechanisms and a range of representative network densities and deployment scenarios. Our results show that, typically, harmonious coexistence between Wi-Fi and LTE is ensured by the large number of 5 GHz channels. For the worst-case scenario of forced co-channel operation, LTE is sometimes a better neighbour to Wi-Fi-when effective node density is low-but sometimes worse-when density is high. We find that distributed interference coordination is only necessary to prevent a “tragedy of the commons” in regimes where interference is very likely. We also show that in practice it does not make a difference to the incumbent what kind of coexistence mechanism is added to LTE-in-unlicensed, as long as one is in place. We therefore conclude that LTE is neither friend nor foe to Wi-Fi in the unlicensed bands in general. We submit that the systematic engineering analysis exemplified by our case study is a best-practice approach for supporting evidence-based rulemaking by the regulator.

Analysing Wi-Fi/LTE coexistence to demonstrate the value of risk-informed interference assessment

Effective interference evaluation methods are crucial when making regulatory decisions about whether new wireless technologies should be allowed to operate. Such decisions are highly relevant for both DSA technologies in licensed bands and for technologies coexisting in unlicensed bands. In this paper we demonstrate the benefit of risk-informed interference assessment as an effective method that aids spectrum regulators in making decisions, and readily conveys engineering insight. We apply risk assessment to a Wi-Fi/LTE coexistence study in the 5 GHz unlicensed band. Our contributions are: (i) we apply, for the first time, risk assessment to a real-life problem of inter-technology spectrum sharing; and (ii) we demonstrate that risk assessment comprehensively quantifies the effect of interference in an intuitive manner. We perform extensive Monte Carlo simulations and we consider throughput degradation and fairness metrics to assess the risk of co- and adjacent channel interference for different network densities, numbers of available channels, and deployment scenarios. Our risk assessment results show that no regulatory intervention is needed to ensure harmonious technical coexistence between Wi-Fi/LTE in the unlicensed band. As an engineering insight, Wi-Fi coexists better with itself in locally dense deployments, but better with LTE in sparse deployments. For the large number of available channels typically expected in practice in the 5 GHz band, the risk of interference for Wi-Fi coexisting with LTE is negligible, rendering policy and engineering concerns largely moot.

Do Unmanned Aerial Vehicles Cause Harmful Interference to Ground Wireless Network Deployments

With the recent proliferation of unmanned aerial vehicles (UAVs), it is unclear what the impact is of interference from UAVs to ground deployments. In this paper we present a study of the interference from UAV transmitters to victim ground receivers operating in LTE and Wi-Fi bands, for a real large-scale scenario in Manhattan. We perform extensive propagation simulations in WinProp for a large number of transmitters and outdoor and indoor receivers at different locations. Our results suggest that outdoor LTE receivers could suffer from harmful interference, which has potential future regulatory implications. By contrast, outdoor Wi-Fi receivers, and indoor LTE and Wi-Fi receivers are less likely to be affected by harmful interference from UAVs.

The Importance of Adjacent Channel Interference: Experimental Validation of ns-3 for Dense Wi-Fi Networks

In its evolution to provide ever higher data rates, the Wi-Fi standard has incorporated sophisticated PHY-layer techniques, which has in turn increased the complexity of network-wide interference relationships. Proper modelling of the resulting inter-device interactions is crucial for accurately estimating Wi-Fi network performance, especially in the contemporary context of traffic and network densification. Event-driven simulators like the open-source ns-3 are in principle able to capture these interactions, however it is imperative to validate, against experimental results, whether their underlying models reflect the network behaviour in practice. In this paper we first perform experiments in a large-scale indoor testbed to validate the IEEE 802.11ac Wi-Fi model in ns-3, for various channel width and allocation configurations. Our results show that ns-3 captures Wi-Fi co-channel interactions with reasonable precision, but fails to model adjacent channel interference (ACI), which our experiments show to be critical in dense networks. We therefore propose and implement an ACI model in ns-3. Importantly, our model successfully captures the qualitative behaviour of the CSMA/CA mechanism when transmissions on adjacent channels occur. Further, our ACI implementation significantly improves the accuracy of both the network and per-device throughput estimates for the considered dense IEEE 802.11ac network compared to the basic ns-3 Wi-Fi model without ACI. For example, without ACI modelling, ns-3 overestimates the aggregate network throughput by up to 230%, whereas with our ACI implementation the aggregate throughput estimate is no more than 65% higher than the experimental results.

Risk-Informed Interference Assessment for Shared Spectrum Bands: A Wi-Fi/LTE Coexistence Case Study

Interference evaluation is crucial when deciding whether and how wireless technologies should operate. In this paper, we demonstrate the benefit of risk-informed interference assessment to aid spectrum regulators in making decisions, and to readily convey engineering insight. We apply, for the first time, risk assessment to an open question of inter-technology spectrum sharing, i.e., a Wi-Fi/LTE coexistence study in the unlicensed band, and we demonstrate that this method comprehensively quantifies the interference impact. We perform simulations with our newly publicly available tool and we consider throughput degradation and fairness as example metrics to assess the risk for different network densities, numbers of channels, and deployments. The risk assessment study shows that no regulatory intervention is needed to ensure harmonious technical Wi-Fi/LTE coexistence: for the typically large number of channels available in the 5 GHz band, the risk for Wi-Fi from LTE is negligible. As an engineering insight, Wi-Fi coexists better with itself in dense deployments, but better with LTE in sparse deployments. Also, both main LTE-in-unlicensed variants coexist well with Wi-Fi in general. For LTE intra-technology inter-operator coexistence, both variants typically coexist well in the 5 GHz band, but for dense deployments, implementing listen-before-talk causes less interference.

Boosting capacity through small cell data offloading: A comparative performance study of LTE femtocells and Wi-Fi

Due to the significant increase in mobile data traffic volume during the last few years, offloading techniques have been considered for alleviating the traffic load from cellular networks. Two principal small cell offloading solutions are LTE femtocells and Wi-Fi. Femtocells are low power user-deployed LTE base stations that overlay the macro-cellular network and share its licensed spectrum, whereas Wi-Fi devices operate in unlicensed bands using the distributed CSMA/CA MAC protocol to coordinate neighbouring transmissions. Given the fundamentally different spectrum use regulations, MAC, and PHY layer capabilities of these two technologies, the resulting interference environments of the respective small cell networks are significantly different. It is thus not trivially obvious which offloading solution realistically provides a better and future-proof capacity extension for service providers. In this paper we present a thorough system-level comparative study of IEEE 802.1 In Wi-Fi against LTE femtocell performance for a range of representative network densities and deployment scenarios, considering realistic propagation effects, multiple interference sources, and several resource allocation schemes. Our results show that in low density suburban and rural scenarios the high spectral efficiency of LTE femtocells yields a higher throughput than Wi-Fi, but that the CSMA/CA MAC protocol enables Wi-Fi to outperform LTE femtocells in dense urban scenarios, where the need for extra capacity is most urgent. We show that future high density heterogeneous networks may be best served by a new hybrid small cell offloading solution, combining the superior PHY of LTE and the distributed co-tier interference coordination afforded by the MAC of Wi-Fi.