Arvind Merwaday

UAV assisted heterogeneous networks for public safety communications

Communications play an important role during public safety operations. Since the current communication technologies heavily rely on the backbone network, the failure of base stations (BSs) due to natural disasters or malevolent attacks causes communication difficulties for public safety and emergency communications. Recently, the use of unmanned aerial vehicles (UAVs) such as quadcopters and unmanned gliders have gained attention in public safety communications. They can be used as unmanned aerial base stations (UABSs), which can be deployed rapidly as a part of the heterogeneous network architecture. However, due to their mobile characteristics, interference management in the network becomes very challenging. In this paper, we explore the use of UABSs for public safety communications during natural disasters, where part of the communication infrastructure becomes damaged and dysfunctional (e.g., as in the aftermath of the 2011 earthquake and tsunami in Japan). Through simulations, we analyze the throughput gains that can be obtained by exploiting the mobility feature of the UAVs. Our simulation results show that when there is loss of network infrastructure, the deployment of UABSs at optimized locations can improve the throughput coverage and the 5th percentile spectral efficiency of the network. Furthermore, the improvement is observed to be more significant with higher path-loss exponents.

Improved Throughput Coverage in Natural Disasters: Unmanned Aerial Base Stations for Public-Safety Communications

Communications play an important role during public-safety operations. Because the current communication technologies heavily rely on the backbone network, the failure of base stations (BSs) due to natural disasters or malevolent attacks causes communication difficulties for public-safety and emergency communications. Recently, the use of unmanned aerial vehicles (UAVs), such as quadcopters and gliders, has gained attention in publicsafety communications (PSCs). They can be operated as unmanned aerial BSs (UABSs), which can be deployed rapidly as a part of the heterogeneous-network (HetNet) architecture. However, due to their mobile characteristics, interference management in the network becomes challenging.

Capacity analysis of LTE-Advanced HetNets with reduced power subframes and range expansion

The use of reduced power subframes in LTE Rel. 11 can improve the capacity of heterogeneous networks (HetNets) while also providing interference coordination to the picocell-edge users. However, in order to obtain maximum benefits from the reduced power subframes, setting the key system parameters, such as the amount of power reduction, carries critical importance. Using stochastic geometry, this paper lays down a theoretical foundation for the performance evaluation of HetNets with reduced power subframes and range expansion bias. The analytic expressions for average capacity and 5th percentile throughput are derived as a function of transmit powers, node densities, and interference coordination parameters in a two-tier HetNet scenario and are validated through Monte Carlo simulations. Joint optimization of range expansion bias, power reduction factor, scheduling thresholds, and duty cycle of reduced power subframes is performed to study the trade-offs between aggregate capacity of a cell and fairness among the users. To validate our analysis, we also compare the stochastic geometry-based theoretical results with the real macro base station (MBS) deployment (in the city of London) and the hexagonal grid model. Our analysis shows that with optimum parameter settings, the LTE Rel. 11 with reduced power subframes can provide substantially better performance than the LTE Rel. 10 with almost blank subframes, in terms of both aggregate capacity and fairness.

Device-to-device discovery based on 3GPP system level simulations

Device-to-device (D2D) communication as an underlay to cellular networks has gained increasing popularity as a technology component to LTE-Advanced. Through D2D communications, user equipments (UEs) in close proximity to each other, may communicate directly instead of through the eNodeB. This helps to achieve better performance than that offered via eNodeB (two-hops) by offloading eNodeB resources, and enables new types of services. This paper provides an overview of the new agreements in 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Radio Access Networks (RAN) related to evaluation methodology and channel modeling for D2D discovery and communications. We also present a system level simulation environment based on 3GPP assumptions and a performance evaluation of three different D2D discovery algorithms.

Handover Count Based Velocity Estimation and Mobility State Detection in Dense HetNets

In wireless cellular networks with densely deployed base stations, knowing the velocities of mobile devices is key to avoiding call drops and improving the quality of service to the user equipments (UEs). A simple and efficient way to estimate a UEs velocity is by counting the number of handovers made by the UE during a predefined time window. Indeed, handover-count based mobility state detection has been standardized since long term evolution (LTE) Release-8 specifications. The increasing density of small cells in wireless networks can help in accurate estimation of velocity and mobility state of a UE. In this paper, we model densely deployed small cells using stochastic geometry, and then analyze the statistics of the number of handovers as a function of UE velocity, small-cell density, and handover count measurement time window. Using these statistics, we derive approximations to the Cramer-Rao lower bound (CRLB) for the velocity estimate of a UE. Also, we determine a minimum variance unbiased (MVU) velocity estimator whose variance tightly matches with the CRLB. Using this velocity estimator, we formulate the problem of detecting the mobility state of a UE as low, medium, or high-mobility, as in LTE specifications. Subsequently, we derive the probability of correctly detecting the mobility state of a UE. Finally, we evaluate the accuracy of the velocity estimator under more realistic scenarios such as clustered deployment of small cells, random way point (RWP) mobility model for UEs, and variable UE velocity. Our analysis shows that the accuracy of velocity estimation and mobility state detection increases with increasing small cell density and with increasing handover count measurement time window.

On the capacity analysis of HetNets with range expansion and eICIC

Poisson Point Process (PPP) models provide important insights for the analysis of heterogeneous networks (HetNets). Recently, several theoretical results which investigate the outage and the capacity of HetNets have been developed based on PPPs. The goal of the present paper is to provide exact analytical expressions for the capacity of range expanded and interference coordinated HetNets, and validate these theoretical analysis through well-documented computer simulations. Both analytical and simulated capacity results are obtained for various range expansion and interference coordination scenarios, and are shown to have good agreement.

Sojourn Time-Based Velocity Estimation in Small Cell Poisson Networks

Due to the increasing density of small cells, mobility management in heterogeneous networks has become a challenging task. One key challenge facing the development of advanced mobility management techniques is the accurate estimation of the users velocity. One simple way to estimate a users velocity is via the use of sojourn time samples. In this letter, the Cramer-Rao lower bound (CRLB) for the sojourn time-based velocity estimation is analyzed. Stochastic geometry is used for the spatial modeling of small cells, and the CRLB is derived using the tools from estimation theory. An asymptotically unbiased velocity estimator is also derived. Our analysis shows that the sojourn time-based velocity estimation exhibit a lower CRLB compared to the CRLB of classical velocity estimation using handover count.

HetNet capacity with reduced power subframes

The time domain inter-cell interference coordination (eICIC) mechanism specified in LTE Rel. 10 improves the throughput of picocell-edge users by protecting them from macrocell interference. On the other hand, it also degrades the aggregate capacity in macrocell because the macro base station (MBS) does not transmit data during the coordinated subframes (CSFs). The MBS data transmission at a reduced power level during CSFs, which is referred to as FeICIC in LTE Rel. 11, can improve the capacity in macrocell while not causing high interference to the picocell users. Using stochastic geometry, this paper lays a theoretical foundation for analyzing the heterogeneous networks (HetNets) with reduced power subframes and range expansion bias (REB). The analytic expression for average capacity is derived and validated through Monte Carlo simulations. Our analysis shows that with optimum parameter settings FeICIC can provide substantially better performance than eICIC in terms of both aggregate capacity in a cell and fairness among the users.

USRP-based indoor channel sounding for D2D and multi-hop communications

In wireless networks, mobile nodes with limited power budget must rely on multi-hop transmissions so as to achieve reliable communication with a distant peer device. Hence, it is of paramount importance to study the characteristics of wireless transmission over multihop links and its benefits over single-hop communication. In this work, a simulator is developed to assess the bit error rate (BER) performance of multi-hop, multi-carrier communication systems. The channel model used in the simulator is developed based on the channel statistics obtained through indoor channel measurements performed using the universal software radio peripherals (USRPs). As the bandwidth supported by a USRP device is much smaller than the channel bandwidth of interest, we use a frequency domain approach to measure the impulse response of wide band channel. By using the channel measurements, we show the impact of communication distance on the delay spread and coherence bandwidth of the channel. Our simulation results with orthogonal frequency division multiplexing (OFDM) systems show that the BER performance of multi-hop communication is better than the single-hop communication. The BER performance difference between the two cases becomes more significant for wider subcarrier frequency spacing.

Handover Count Based UE Velocity Estimation in Hyper-Dense Heterogeneous Wireless Networks

In wireless cellular networks with densely deployed base stations, knowing the velocities of the user equipments (UEs) is a key for efficient mobility management. A simple and efficient way to estimate a UEs velocity is by counting the number of handovers made by the UE during a predefined time window. Indeed, handover-count based mobility state detection has been standardized since Long Term Evolution (LTE) Release-8 specifications. The increasing density of small cells in wireless networks is advantageous, as it can help in accurate estimation of velocity and mobility state of a UE. In this paper, we model densely deployed small cells using stochastic geometry, and derive an approximation to the probability mass function of handover count as a function of UE velocity, small cell density, and time interval of handover count measurement. Then we derive Cramer-Rao lower bound (CRLB) for the velocity estimate of a UE, and also provide an unbiased estimator for the UEs velocity. Our analysis shows that the accuracy of velocity estimation increases with increasing small cell density and with increasing time interval of handover count measurement.