Vanlin Sathya

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Research Projects



 

Post-Doc Research on 5G Wireless Network

          Sponsored by NSF

    LAA_Wi-Fi Android APP         Chicago: Cellular and Wi-Fi Deployment Data
 

Analytical Modeling of Wi-Fi and LTE-LAA Coexistence: Throughput and Impact of Energy Detection Threshold:

Project Description: With both small-cell LTE and Wi-Fi networks available as alternatives for deployment in unlicensed bands (notably 5 GHz), the investigation into their coexistence is a topic of active interest, primarily driven by industry groups. 3GPP has recently standardized LTE-LAA that seeks to make LTE more co-existence friendly with Wi-Fi by incorporating similar sensing and back-off features. Nonetheless, the results presented by industry groups offer little consensus on important issues like respective network parameter settings that promote “fair access” as required by 3GPP. Answers to such key system deployment aspects, in turn, require credible analytical models, on which there has been a little progress to date. Accordingly, in one of the first works of its kind, we develop a new framework for estimating the throughput of Wi-Fi and LTE-LAA in coexistence scenarios via suitable modifications to the celebrated Bianchi model. The impact of various network parameters such as energy detection threshold on Wi-Fi and LTE-LAA coexistence is explored as a byproduct and corroborated via a National Instrument experimental test bed that validates the results for LTE-LAA access priority classes 1 and 3.

 
 

On the Fairness of Wi-Fi and LTE-LAA Coexistence:

 
Project Description: With both small-cell LTE and 802.11 networks now available as alternatives for deployment in unlicensed bands at 5 GHz, investigation into their coexistence is a topic of great interest. ETSI Rel.14 has standardized LTE LAA that seeks to make LTE more coexistence friendly with Wi-Fi by incorporating listen before talk (LBT). However, the fairness of Wi-Fi and LTE-LAA sharing is a topic that has not been adequately explored. In this work, we first investigate the 3GPP definition of fair coexistence via new analytical models. By tuning the LTE-LAA parameters, we exemplify scenarios when the 3GPP notion of fairness is achieved and conversely, when not achieved. The formal notions of access and proportional fairness is then considered for these scenarios to compare and contrast with the 3GPP definition.
 
 

Energy detection based sensing of multiple Wi-Fi BSSs for LTE-U CSAT:

 
Project Description: In this work, we develop an algorithm, based only on energy detection, to detect the number of Wi-Fi basic service sets (BSSs) operating on the same channel. Such an algorithm can be used by an LTE-U base station to scale back its duty cycle when coexisting with Wi-Fi on the same channel. According to the LTE-U specification, an LTE-U BS scales back its duty cycle from 50% to 33% when it senses that the number of co-channel Wi-Fi BSSs has increased from one to two. There are two ways that this detection can be done: (a) decoding based, where the LTE-U BS decodes the Wi-Fi packet header to determine the unique Wi-Fi basic service set identification (BSSID) and (b) energy based, where only received energy levels are used to determine the number of Wi-Fi BSSs. The former approach requires an LTE-U BS to implement a Wi-Fi decoder and hence increases complexity, whereas the latter is easier to implement, requiring only an energy detector and appropriate detection thresholds to distinguish between one and two Wi-Fi APs. We analyze the latter approach and experimentally verify the feasibility of an energy detector to reliably distinguish between one and two Wi-Fi APs. In order to do so, we first experimentally determine appropriate detection thresholds using comprehensive measurements in realistic environments, both line-of-sight (LOS) and non-LOS (NLOS). These thresholds are then used to perform hypothesis based detection to devise an efficient algorithm that predicts the presence of one or two Wi-Fi BSSs. The performance of the proposed algorithm is evaluated, both theoretically and experimentally, by utilizing two metrics (a) probability of detection (PD ) and (b) probability of false alarm (PFA ). We show that using a threshold of -42 dBm delivers greater than 80% PD and less the 5% PFA which we verify both theoretically and experimentally. Hence, energy based detection is a low-complexity means of determining number of Wi-Fi BSSs to help LTE-U scale back its duty cycle appropriately.
 
 

Auto-correlation based sensing of multiple Wi-FiBSSs for LTE-U CSAT:

 
Project Description: LTE-U (LTE-Unlicensed) is designed to coexist with Wi-Fi in the unlicensed band by balancing its duty cycle according to the number of coexisting Wi-Fi access points (APs) it detects. For example, a LTE-U base-station will reduce its duty cycle from 50% to 33% when it senses an increase in the number of co-channel Wi-Fi basic service sets (BSSs) from one to two. But the problem of detecting how many Wi-Fi BSS’ are operating on the channel in real-time, without decoding the Wi-Fi header, still remains. In this paper, we present a novel algorithm that solves the problem by using an auto-correlation (AC) function on the Wi-Fi preamble and setting appropriate detection thresholds to infer the number of Wi-FiBSSs operating on the channel. Performing auto-correlation on the Wi-Fi preamble is a much simpler operation than decoding the entire Wi-Fi packet, which is what would be needed ifone were to decode the MAC header to identify the BSS. We implement and experimentally validate the proposed AC detector rand demonstrate that there is a differentiable pattern of AC events between one and two Wi-Fi APs. From the collected AC events, we determine a suitable threshold for a reliable detection of Wi-Fi APs. We show that using an AC threshold of NE= 0.8, we can achieve a probability of detection (PD) of 0.9 with a probability of false alarm (PFA) of less than 0.02. Finally, we demonstrate that the performance of the proposed AC detector is superior in terms of PD and PFA compared with the energy detector (ED).
 
 

Impact of changing energy detection thresholds on fair coexistence of Wi-Fi and LTE in the unlicensed spectrum:

 
Project Description: The exponential increase in the number of mobile devices in use today has led to a commensurate increase in the demands on both cellular and Wi-Fi infrastructure, thus requiring that both licensed (cellular) and unlicensed (Wi-Fi) spectrum be utilized as efficiently as possible. One solution being actively pursued by industry is for cellular systems to use the unlicensed spectrum in addition to the licensed spectrum, which would require fair coexistence with Wi-Fi in the unlicensed spectrum. As per the IEEE 802.11 standard, Wi-Fi uses an energy detection (ED) threshold of -62 dBm when Long Term Evolution-Licensed Assisted Access (LTE-LAA) and/or Long Term Evolution Un-Licensed (LTE-U) nodes are deployed close by, whereas the LTE-LAA specification recommends that LTE- LAA detect Wi-Fi at -72 dBm. In our work, we evaluate the effect of this asymmetry in the ED threshold on coexistence between the two systems. We develop a coexistence simulator in ns-3 and vary both the Wi-Fi and LTE energy detection thresholds and demonstrate that lowering the Wi-Fi ED threshold from -62 dBm improves performance for both Wi-Fi and LTE-LAA. Prior work has mostly focused on determining the ED threshold that should be used by LTE-LAA/LTE-U. As far as we are aware, this is the first result that demonstrates that lowering the Wi-Fi ED threshold improves performance for both systems. The conclusion is that if Wi-Fi treats LTE-LAA/LTE-U as it would an overlapping Wi-Fi, coexistence performance improves compared to the current assumption that Wi-Fi treats LTE-LAA/LTE-U as noise.

 

Doctoral Research on 4G Wireless Network

Sponserd by MHRD
 

On Placement and Dynamic Power Control of Femtocells in LTE HetNets:

Project Description: Optimal placement of Femtos ensures good signal strength/throughput. However, placing Femtos inside a building leads to power leakage at the edges/corners of the building. This degrades the performance of High Interference Zone User Equipments (HIZUEs) in HIZone around the building area because both Macros and Femtos typically operate on the same frequency in LTE HetNets. Setting Femto’s transmit power levels at optimum based on occupancy of HIZUEs in HIZone could solve this problem. In this work, we propose an efficient Femto placement and power control algorithm by employing the following two optimization models:
1. Minimize Number of Femtos (MinNF) model.
2. Optimal Femto Power (OptFP) model.

 
 

On improving SINR in LTE HetNets with D2D Relays:

 
Project Description: To guarantee minimum SINR to both Indoor UEs (IUEs) and HIZUEs in the HIZone, we apply the concept of Device-to-Device (D2D) communication where in free/idle IUEs act like UE-relays for serving the downlink traffic of HIZUEs via Femtos deployed inside the buildings. We first formulate a D2D MILP model which establishes D2D pairs between free/idle IUEs and HIZUEs and also guarantees certain SINRTH for both IUEs and HIZUEs. As D2D MILP model takes more computation time, it is not usable in real-world scenarios for establishing D2D pairs on the fly. Hence, we propose a two-step D2D heuristic algorithm for establishing D2D based relay pairs. In step one (called as hDPRA), it efficiently chooses potential D2D based relay pairs and allocates radio resources to them. In step two (called as hDPA), a Linear Programming (LP) model is formulated for power control of D2D links. We have evaluated the performance of the proposed D2D heuristic algorithm for different scenarios (i.e., 500 topologies) by varying densities of IUEs and HIZUEs. From our evaluation, we find that the proposed D2D heuristic algorithm maintains almost the same SINR as that of Full Power Femto Scheme (i.e., each Femto transmits at its peak power) for IUEs and also guarantees a certain minimum SINRTH for HIZUEs. In comparison to the OptFP model, it improves SINR of IUEs by 40%. However, the degradation in SINR of IUEs is only 1.6% when compared to the Full Power Femto Scheme. We also observe that the minimum SINRTH (-2 dB) is maintained for all HIZUEs in the HIZone. We also observe that the running time for D2D heuristic algorithm showed an average decrease of up to 87% when compared to D2D MILP model.
 
 

A Novel Resource Allocation and Power Control Mechanism for Hybrid Access Femtocells (HAFs):

 
Project Description: HAFs are favoured by the operators because they provide the paid subscriber group (SG) users certain QoS and then try to maximize the system capacity by serving near-by non-subscriber group (NSG) users in a best-effort manner. To reap in the benefits of HAFs, the operators need to employ effective resource sharing and scheduling mechanisms to contain co-tier and cross-tier interference arising out of reuse one in the HetNet system. Towards this, we propose the following solutions:
1. An Optimal Placement of hybrid access Femtos (OPF) algorithm which ensures a certain SINRTH inside the building and a certain SINRTH in the HIZone of the building.
2. A decentralized Dynamic Bandwidth Allocation (BWA) mechanism which divides the available HAF bandwidth between the two sets of user groups: SG and NSG.
3. A dynamic Optimal Power Control (OPC) mechanism which reduces the transmit power of HAFs whenever the users in the HIZone cannot be served by the HAFs. In such a case, HIZone users connect to an MBS instead. Since the OPC problem is hard to solve in polynomial time, we also present a Sub-Optimal Power Control (SOPC) mechanism.
4. An Enhanced Priority (EP) scheduling mechanism which employs two schedulers which are based on the Proportional Fair (PF) and the Priority Set (PS) scheduling mechanisms. While one scheduler maintains fairness among the SG users, the other one fairly schedules radio resources among the NSG users. EP scheduling mechanism performs better than the legacy PF and PS scheduling mechanisms by prioritizing the SG users over the NSG users and maintains fairness (Jain’s fairness index of 0.99) within each group.
 
 

Decoupled access for the downlink and uplink in LTE Femtocell Networks:

 
Project Description: Once the placement of Femtos is done optimally in enterprise environments, operators need to ensure that traffic load is evenly distributed among neighboring Femtos for improving QoS of indoor UEs and efficiently utilizing the network resources. In a typical indoor environment, the uplink traffic load would more or less be the same across all Femtos in the entire building, but the downlink traffic load varies from one cell to the other depending on the number of UEs being served by a Femto and their traffic demands. In traditional cellular networks, the uplink access and downlink access of UEs are coupled to the same (Femto) cell. Suppose a Femto is fully loaded when compared to its neighboring Femtos, the traditional offloading or load balancing algorithms will try offloading some of the UEs for both their uplink and downlink access from the loaded cell to one of less loaded neighboring cells (i.e., target cell) provided that these UEs could get connected to the chosen target cell. This type of offloading is a forced handover to reduce traffic imbalance and it is not based on better signal strength from the target cell. But, the offloaded UEs are connected for both their uplink and downlink access to the same target cell. Since UEs are most likely separated by walls and floors from their connected cells in enterprise environments, these offloaded UEs now have to transmit with higher transmit power in the uplink and thereby affects their battery lives. In order to reduce the battery drain for the offloaded UEs while maintaining their QoS, one could use the Decoupled uplink and Downlink (DuD) access method i.e., the uplink of UE is connected to the closest Femto while the downlink is connected to a less loaded neighboring Femto. Our extensive experimentation results in MATLAB based simulator show that decoupled access system achieves 56% energy savings when compared to the traditional coupled access system for LTE Femtocell networks.
 
 

Handover and SINR optimized Femto deployment in enterprise environments:

 
Project Description: Another problem in enterprise buildings having Femtos is frequent handovers, that happen when IUEs move from one room/floor to another room/floor inside the building. This leads to degradation of network performance in terms of increased signaling overhead and low throughputs. In order to reduce this kind of unnecessary handovers in enterprise buildings, Femtos should be placed optimally with handover constraints. Hence, we obtain the optimal coordinates from the joint placement and power control model (OptCTSINR) by adding handover constraints. Such optimized deployment of Femtos reduces the number of handovers while guaranteeing good SINR to all IUEs. Compared to the optimal without handover constraints (OptWHO) model, our proposed OptHO model reduces 30% of the unnecessary handovers in enterprise buildings.

 
 

Enhanced distributed resource allocation and interference management in LTE Femtocell Networks:

 
Project Description: Recently telecom industry is considering dense deployment of small cells in outdoor environments. In these dense deployments, Femtos/Picos are deployed arbitrarily. Though the capacity of network increases through frequency reuse one at Femtos, it could lead to co-tier interference and cause higher interference for cell edge UEs. This problem is more severe due to dense placement of Femtos arbitrarily. Existing co-tier interference management techniques in literature do not solve this problem completely. Hence, in this work, we propose a Variable Radius (VR) algorithm which dynamically increases or decreases the cell edge/non-cell edge region of Femtos and efficiently allocates the radio resources among cell edge/non-cell edge region of Femtos so that the co-tier interference between neighboring Femtos can be avoided. We implemented the proposed VR algorithm on top of the PF scheduling algorithm in NS-3 simulator. In an experiment having 90 UEs, the proposed technique, VR + PF, achieved 29% and 38% improvement in average throughput for static and mobile scenarios, respectively when compared to the classic PF algorithm without any inter-cell interference management.

 
 

Energy-efficient Femto deployment in LTE Networks:

 
Project Description: In this work, we focus on reducing the battery power consumption of indoor UEs (i.e., uplink transmit power) while guaranteeing both the uplink SINR threshold (USINRTH) and the downlink SINR threshold (DSINRTH). We achieve this by placing the Femtos optimally, taking into account wall attenuation factor and co-tier and cross-tier interference among Macros and Femtos. A two-step optimization model has been formulated: in step one, we formulate a Mixed Integer Programming (MIP) model which yields the optimal locations of the Femtos and meets DSINRTH and USNRTH while also minimizing Femto count and the uplink transmission power. In step two, we formulate a Linear Programming (LP) problem with the aim of guaranteeing USINRTH and minimizing the total uplink power, after placing the Femtos in the optimal positions obtained from the step one. When compared to the center K-means placement scheme, the proposed optimal placement scheme obtained by solving the above two-step optimization model registers a significant, 47%, reduction in the uplink energy consumption.

 
 

Phantom Cell Architecture for LTE and its Application in Vehicular IoT Environments:

 
Project Description: Proliferation of Internet of Things (IoT) devices (smart wearables/vehicles, etc.) in the near future, would raise the capacity and bandwidth demands from the cellular infrastructure manifold. Deploying small cells is an effective solution to cope with the problem. This work focuses on a special kind of small eNBs, termed as Phantom eNBs. Phantom eNB acts as a supplement to the current radio access network (RAN) in the LTE infrastructure. It handles the data plane while a Macro eNB holds the control plane. Definitions of control and data plane, along with the modifications needed in the protocol stack are explained in this paper. Communication mechanisms are developed for Phantom and Macro eNB to communicate over the X2 interface between them. NS-3 simulations are performed for handover scenarios of Vehicular IoT environment, consolidating the architecture and network topology designed for Phantom based Heterogeneous Networks (HetNets). Network throughput improvements of 80% and 14% are observed in comparison to the Macro-only RAN and existing small cell solutions (Femto cells), respectively.

 
 

On Improving Capacity of Small Cells with Full-duplex and D2D Communications:

 
Project Description: The recent developments in full duplex (FD) communication promise doubling the capacity of cellular networks using self interference cancellation (SIC) techniques. Dense deployment of FD small cells with device-to-device (D2D) communication links could achieve the expected capacity of the future cellular networks (5G). In this work, we consider joint scheduling and dynamic power control mechanism for FD small cell networks with D2D links. We formulate an optimal user selection and dynamic power control algorithm (O-PCA) as a non-linear programming (NLP) optimization problem.