Ljiljana Simic

Wi-Fi, but not on Steroids: Performance analysis of a Wi-Fi-like Network operating in TVWS under realistic conditions

The recent decisions by regulators in the USA and UK to open up unused portions of UHF spectrum for secondary use have been met with keen interest in using these TV white spaces (TVWS) for providing broadband services through Wi-Fi-like connectivity. Amid the ensuing media hype about “Wi-Fi on steroids”, there is a widespread perception that Wi-Fi operating in TVWS will provide much longer range, superior speeds, and more reliable connections than traditional Wi-Fi at 2.4 GHz. In this paper, we present a quantitative analysis of the performance of a network of Wi-Fi-like access points (APs) operating in TVWS in order to obtain a realistic estimate of the achievable range and downlink rate of such a secondary system. Unlike previous studies, we explicitly consider the effects of inter-AP interference and congestion and use real TVWS channel availability estimates from an example region of Germany. We confirm the favourable properties of the lower TVWS frequency range, of enabling better propagation through walls and a larger coverage range for the same power budget. Our results show that operating Wi-Fi hotspots in TVWS might be technologically attractive for outdoor rural areas where user demand is low. However, the extended coverage range in TVWS leads to increased congestion which rapidly limits the system capacity for an outdoor urban deployment with high user density. Therefore, a combined technological and economical analysis is essential before any final judgement can be reached about the viability of large-scale Wi-Fi deployments in TVWS.

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.

Survey of IEEE 802.11 Wi-Fi deployments for deriving the spatial structure of opportunistic networks

An understanding of the underlying spatial structure of user-deployed wireless networks is invaluable for the design and optimization of opportunistic small-cell technologies in emerging multi-tier architectures. IEEE 802.11 Wi-Fi networks provide a real-world example of such a large-scale random network structure. In this paper we report on a high-resolution measurement survey of Wi-Fi deployments in Germany as a step towards deriving more sophisticated spatial models for transmitter distributions in emerging small-cell networks, which are the core of offloading strategies for future wireless Internet. Using a custom setup, our measurement campaign covered a range of urbanization and land use scenarios representative of Wi-Fi deployments in developed countries. Our data indicates a 14-fold increase in Wi-Fi density in urban residential areas over the last decade, consistent with increased broadband penetration. This supports our hypothesis that household density has become a strong predictor of access point (AP) density. We infer AP locations using a novel localization technique suitable for large measurement campaigns using off-the-shelf equipment. From this data, we derive pertinent spatial statistics to characterize the topologies of opportunistic networks. We derive nearest-neighbour distributions showing that realistic inter-neighbour distances are higher than predicted by a homogeneous Poisson Point Process, underlining the need for more refined spatial models than the random node location models which are currently prevalent. We also derive the two-pair correlation functions for the AP locations, showing that AP clustering is apparent over a large range of distances in our data set, which has implications for routing and interference mitigation.

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.

Demo: Spectrum-Agile mm-Wave Packet Radio Implementation on USRPs

We present a spectrum-agile mm-wave packet radio communication system and demonstrate it by using multimedia streaming. The developed platform is based on USRP software defined radios (SDRs) executing open-source GNU Radio code and cost-efficient commercial off-the-shelf components. Our system is able to operate in both the V- and E-bands, covering in practice the 60~GHz, 70~GHz and 80~GHz frequency bands. One of the main design goals has been to provide an SDR design that academic groups, particularly protocol designers, could use easily for future mm-wave system development and experimental verification.

Smart mm-Wave Beam Steering Algorithm for Fast Link Re-Establishment under Node Mobility in 60 GHz Indoor WLANs

Millimeter-wave (mm-wave) wireless local area networks (WLANs) are expected to provide multi-Gbps connectivity by exploiting the large amount of unoccupied spectrum in e.g. the unlicensed 60 GHz band. However, to overcome the high path loss inherent at these high frequencies, mm-wave networks must employ highly directional beamforming antennas, which makes link establishment and maintenance much more challenging than in traditional omnidirectional networks. In particular, maintaining connectivity under node mobility necessitates frequent re-steering of the transmit and receive antenna beams to re-establish a directional mm-wave link. A simple exhaustive sequential scanning to search for new feasible antenna sector pairs may introduce excessive delay, potentially disrupting communication and lowering the QoS. In this paper, we propose a smart beam steering algorithm for fast 60 GHz link re-establishment under node mobility, which uses knowledge of previous feasible sector pairs to narrow the sector search space, thereby reducing the associated latency overhead. We evaluate the performance of our algorithm in several representative indoor scenarios, based on detailed simulations of signal propagation in a 60 GHz WLAN in WinProp with realistic building materials. We study the effect of indoor layout, antenna sector beamwidth, node mobility pattern, and device orientation awareness. Our results show that the smart beam steering algorithm achieves a 7-fold reduction of the sector search space on average, which directly translates into lower 60 GHz link re-establishment latency. Our results also show that our fast search algorithm selects the near-optimal antenna sector pair for link re-establishment.

Feasibility of Secondary Networks: Analysis Methodology and Quantitative Study of Cellular and Wi-Fi-Like TVWS Deployments

Recent rulings by US and UK regulators allowing access to unused portions of TV spectrum have elicited high hopes for the practical value of these TV whitespaces (TVWS) for secondary exploitation. However, this optimism has been largely fueled by rather simple early studies, which do not consider all the system aspects in sufficient detail; the few early experimental works reported are proprietary and consider only a small number of nodes, whereas the existing more sophisticated theoretical studies focus on singular aspects of secondary spectrum access in isolation from the overall system interactions. In this paper we study quantitatively the deployment of secondary networks in TVWS by considering two archetypal candidate systems: LTE-like cellular and Wi-Fi-like networks. We develop a systematic framework for the performance evaluation of secondary networks, which we then use to obtain realistic estimates of the performance of our example systems. The secondary network performance assessment methodology demonstrated in this paper can be directly applied for other regions and systems. Our work explicitly takes into account limitations arising from aggregate interference, user density, and specific secondary transceiver characteristics. We argue that a systematic analysis, jointly considering all of these aspects, is key for obtaining realistic and robust results on the estimated value of whitespace spectrum. Our detailed system-level approach reveals a much more conservative picture of the realistic benefit of TVWS deployments than what has been commonly assumed thus far. We find that cellular TVWS networks have limited capabilities, but that a macro-cellular-only network may be a viable option for traffic offloading. Our results also show that Wi-Fi-like secondary deployments in TVWS, although increasing coverage range, lead to increased congestion, which limits the system capacity.

60 GHz outdoor urban measurement study of the feasibility of multi-Gbps mm-wave cellular networks

Future 5G cellular networks are expected to exploit the abundant spectrum resources of the millimeter-wave (mm-wave) bands to satisfy demand for multi-Gbps mobile links anticipated by exponential data traffic growth. However, given the directional nature of mm-wave links, the feasibility of mm-wave mobile networks is critically dependent on efficient antenna beamsteering and a rich inventory of strong LOS (line-of-sight) and NLOS (non line-of-sight) paths from effective reflectors in the urban environment. In this paper we report results from detailed angular measurements of 60 GHz links at an example outdoor pico-cellular site in a mixed-use urban environment typical of European cities. Our work is the first to systematically analyze the beamsteering requirements of future mm-wave cellular networks based on real measurements. Our results reveal that the urban environment provides substantial opportunities for multi-Gbps mm-wave connectivity, but that the availability of strong LOS/NLOS links is highly location and orientation-specific. Our results also show that high speed mm-wave links are very sensitive to beam misalignment. This has important implications for practical mm-wave cellular network design: (i) high-precision beamsteering is required to maintain stable data rates even for quasi-stationary users; and (ii) providing seamless high speed service in mobility scenarios will be extremely challenging. Our results thus cast doubt on whether outdoor mm-wave cellular deployments will be feasible in practice, given the high network control overhead of meeting such stringent beamsteering requirements.

A propagation-centric transmitter localization method for deriving the spatial structure of opportunistic wireless networks

The design of emerging multi-tier dense wireless networks, which integrate opportunistically deployed devices into legacy infrastructure networks, will hugely benefit from a thorough understanding of the spatial structure of existing large-scale unplanned deployments. However, detailed and precise datasets which would enable acquiring such knowledge are unavailable and non-trivial to generate. In this paper we develop a new localization method that focuses on the derivation of transmitter distributions over large areas, such as those of opportunistic Wi-Fi networks. While prior work has emphasized single-node localization through geometrical inference on points of visibility and involved only rudimentary radio propagation modelling, we combine these two approaches in a hybrid technique to improve localization accuracy. Through exploitation of location information for a subset of known transmitters, we derive the parameters of a statistical propagation model that reduces the error induced by shadowing in realistic environments. After deriving theoretical bounds on propagation estimation from measurements, we compare our method to centroid-based localization approaches via simulation. We further assess our algorithm using data from a real Wi-Fi measurement campaign we have carried out in a suburban environment. Our results indicate a significant improvement in localization accuracy using our hybrid approach compared to conventional geometry-focused techniques.

Software tool for assessing secondary system opportunities in spectrum whitespaces

The prospect of increasing wireless capacity via secondary access to spatio-temporally underutilized chunks of spectrum, so-called whitespaces, has been proposed as a central aspect of emerging radio systems in response to the imminent spectrum scarcity problem. In this demonstration we present a novel software tool that helps researchers, industry, and regulators in assessing the feasibility and value of secondary spectrum access beyond simple whitespace availability calculation. Whereas existing software applications merely provide visualization of estimated secondary spectrum over a geographic area, our tool uniquely enables a holistic evaluation of the realistic potential of whitespace technologies, by modelling the performance of entire secondary systems in the envisioned eco-system of dynamic spectrum access policy and technology. Our tool provides a unified and flexible software framework and assessment methodology to conduct such studies, and is composed of an extensive primary spectrum usage database, a graphical interface for user interaction, and an interface to an extensible MATLAB backend for numerical calculations. We showcase the deployment scenarios of cellular and Wi-Fi-like secondary networks in TVWS (TV whitespaces). We also compare the impact of employing FCC-type of regulatory rules (with a fixed power/no-talk distance configuration) against European WG-SE43 regulatory proposals (with probabilistic access and power control). The case studies we will demonstrate are based on real network configuration data of European and US TV networks.