Pavan Nuggehalli

Fundamentals of Heterogeneous Cellular Networks with Energy Harvesting

We develop a new tractable model for K-tier heterogeneous cellular networks (HetNets), where each base station (BS) is powered solely by a self-contained energy harvesting module. The BSs across tiers differ in terms of the energy harvesting rate, energy storage capacity, transmit power and deployment density. Since a BS may not always have enough energy, it may need to be kept OFF and allowed to recharge while nearby users are served by neighboring BSs that are ON. We show that the fraction of time a kth tier BS can be kept ON, termed availability ρk, is a fundamental metric of interest. Using tools from random walk theory, fixed point analysis and stochastic geometry, we characterize the set of K-tuples (ρ1, ρ2, ... ρK), termed the availability region, that is achievable by general uncoordinated operational strategies, where the decision to toggle the current ON/OFF state of a BS is taken independently of the other BSs. If the availability vector corresponding to the optimal system performance, e.g., in terms of rate, lies in this availability region, there is no performance loss due to the presence of unreliable energy sources. As a part of our analysis, we model the temporal dynamics of the energy level at each BS as a birth-death process, derive the energy utilization rate, and use hitting/stopping time analysis to prove that there exists a fundamental limit on ρk that cannot be surpassed by any uncoordinated strategy.

Fundamentals of base station availability in cellular networks with energy harvesting

We develop a new tractable model for K-tier cellular networks, where each base station (BS) is solely powered by a self-contained energy harvesting module instead of a conventional power-line source. The BSs across tiers differ in terms of the energy harvesting rate, energy storage capacity, transmit power and deployment density. Since a BS may not always have enough energy, it may need to be kept OFF and allowed to recharge while its load is served by the neighboring BSs that are ON. Using tools from random walk theory and stochastic geometry, we characterize the fraction of time each type of BS can be kept ON, termed availability, for general uncoordinated strategies, where each BS toggles its ON/OFF state independently of the others. As a part of our analysis, we model the temporal dynamics of the energy level at each BS as a birth-death process, derive energy utilization rate for each BS class, and use hitting/stopping time analysis to study availabilities. We prove that there is a fundamental limit on the availabilities, which cannot be surpassed by any uncoordinated strategy. As a part of the proof, we construct the strategy that achieves this limit.

LTE-WLAN aggregation [Industry Perspectives]

Cellular operators today have to grapple with the dilemma of unprecedented increase in mobile data traffic coupled with the need to maintain low costs. According to one estimate, global mobile data traffic is expected to multiply more than eight-fold over the next five years to around 30.6 exabytes per month. Increasingly, operators are turning to Wi-Fi off-load as a possible solution to reduce the load on their cellular infrastructure, and at the same time provide users with reasonable quality of service (QoS). Apart from increasing network capacity, Wi-Fi offload also holds the promise of providing better coverage, especially when users are near the edge of a cell or in an indoor environment where cellular coverage is weak. The high availability of Wi-Fi networks and relatively low cost of ownership have also contributed to the enthusiastic embrace of Wi-Fi offload by cellular operators. In this column, we begin with a brief discussion of cellular-Wi-Fi integration mechanisms that have been developed in the past, with an emphasis on activities carried out by the Third Generation Partnership Project (3GPP). We then motivate and describe Long Term Evolution-WLAN aggregation (LWA), a Release 13 feature that represents the latest and possibly most ambitious effort to harness the power of Wi-Fi to improve cellular performance. We also discuss the design of a prototype we have built for practical deployment of LWA. The prototype demonstrates that the Release 13 LWA can be implemented with legacy Wi-Fi access points (APs), which minimizes LWA deployment cost. Finally, we provide a brief comparison of LWA with some other competing and complimentary technologies, and describe future advances in LWA.

Stochastic Geometry Perspective of Unlicensed Operator in a CBRS System

In this work, we model and analyze a cellular system that operates in the licensed band of the 3.5 GHz spectrum and consists of a licensed and an unlicensed operator. Using tools from stochastic geometry, we study the co-existence of these networks from the perspective of the unlicensed operator. We model the licensed base station (BS) locations as a homogeneous Poisson point process with protection zones (PZs) around each BS. Since the unlicensed BSs can not operate within PZs, their locations are modeled as a Poisson hole process. In addition, we also consider contention-based channel access mechanism for the unlicensed BSs. We provide useful lower bound for the medium access probability for a typical BS of the unlicensed operator. Further, approximate results for coverage probability are also presented, by effectively handling the correlation in the interference powers induced due to correlation in locations of the licensed and unlicensed BSs. Using the derived expressions, we study the effect of the density of licensed BSs on the average number of active unlicensed BSs per unit area and coverage probability for a typical unlicensed user. To the best of our knowledge, this work presents the first comprehensive stochastic geometry-based analysis of the CBRS system.

Stochastic Geometry-Based Modeling and Analysis of Citizens Broadband Radio Service System

In this paper, we model and analyze a cellular network that operates in the licensed band of the 3.5-GHz spectrum and consists of a licensed and an unlicensed operator. Using tools from stochastic geometry, we concretely characterize the performance of this spectrum sharing system. We model the locations of the licensed base stations (BSs) as a homogeneous Poisson point process with protection zones (PZs) around each BS. Since the unlicensed BSs cannot operate within the PZs, their locations are modeled as a Poisson hole process. In addition, we consider carrier sense multiple access with collision avoidance-type contention-based channel access mechanism for the unlicensed BSs. For this setup, we first derive an approximate expression and useful lower bounds for the medium access probability of the serving unlicensed operator BS. Furthermore, by efficiently handling the correlation in the interference powers induced due to correlation in the locations of the licensed and unlicensed BSs, we provide approximate expressions for the coverage probability of a typical user of each operator. Subsequently, we study the effect of different system parameters on area spectral efficiency of the network. To the best of our knowledge, this is the first attempt toward accurate modeling and analysis of a citizens broadband radio service system using tools from stochastic geometry.