Arun Ramamurthy

On placement and dynamic power control of femtocells in LTE HetNets

Femto cells a.k.a. Low Power Nodes (LPNs) are used to improve indoor data rates as well as to reduce traffic load on macro Base Stations (BSs) in LTE cellular networks. These LPNs are deployed inside office buildings and residential apartment complexes to provide high data rates to indoor Users. With high SINR (Signal-to-Interference plus Noise Ratio) the users experience good throughput, but the SINR decreases significantly because of interference and obstacles such as building walls, present in the communication path. So, efficient placement of Femtos in buildings while considering Macro-Femto interference is very crucial for attaining desirable SINR. At the same time, minimizing the power leakage in order to improve the signal strength of outdoor users in a high interference (HIZone) around the building area is important. In our work, we have considered obstacles (walls, floors) and interference between Macro and Femto BSs. To be fair to both indoor and outdoor users, we designed an efficient placement and power control SON (Self organizing Network) algorithm which optimally places Femtos and dynamically adjusts the transmission power of Femtos based on the occupancy of Macro users in the HIZone. To do this, we solve two Mixed Integer Programming (MIP) methods namely: Minimize number of Femtos (MinNF) method which guarantees threshold SINR (SINRTh) -2dB for all indoor users and optimal Femto power (OptFP) allocation method which guarantees SINRTh (- 4 dB) for indoor users with the Macro users SINR degradation as lesser than 2dB.

Load-aware dynamic RRH assignment in Cloud Radio Access Networks

Due to spatio-temporal variation of mobile subscribers data traffic requirements, traffic load experienced by base stations present at different cell sites exhibit highly dynamic behavior in traditional cellular systems. This non-uniform and dynamic traffic load leads to under utilization of the base station computing resources at cell sites. Cloud Radio Access Network (C-RAN) is an innovative architecture which addresses this issue and keeps the Total Cost of Ownership (TCO) under safe limit for cellular operators. In C-RAN, the baseband processing units (BBUs) are segregated from cell sites and are pooled in a central cloud data center thereby facilitating shared access for a set of Remote Radio Heads (RRHs) present at cell sites. In order to truly exploit the benefits of C-RAN, the BBU pool deployed in the cloud has to efficiently serve clusters of RRHs (i.e., many-to-one mapping between RRHs and BBUs in the BBU pool) and thereby minimizing the required number of active BBUs. In this work, potential benefits of C-RAN are studied by considering realistic traffic loads of base stations deployed in urban areas by using statistical models. We propose a lightweight and load-aware algorithm, Dynamic RRH Assignment (DRA), which achieves BBU pooling gain close to that of a well known First-Fit Decreasing (FFD) bin packing algorithm. Using extensive simulations, we show that DRA consumes only 25% of time on average compared to FFD for the case of urban cellular deployment of 1000 RRHs. DRA slightly overestimates the required number of active BBUs as compared to FFD by 1.7% and 1.4% for weekdays and weekends, respectively.

On improving SINR in LTE HetNets with D2D relays

Femtos, with frequency reuse one, can be deployed in hotspots, offices and residences alike to provide high indoor data rates and reduce traffic load on Macro. However, arbitrarily deployed Femtos could decrease SINR significantly because of inter-cell interference and obstacles present in the building. Hence, to attain a desirable SINR Femtos have to be placed efficiently. At the same time, minimizing the power leakage from indoor Femtos in order to improve the SINR of outdoor users in the high interference zone (HIZone) around building areas is also important. To guarantee minimum SINR to both indoor UEs (IUEs) and outdoor UEs in HIZone (HIZUEs), we apply the concept of device-to-device (D2D) communication wherein free/idle IUEs act like UE-relays for HIZUEs. We first formulate a D2D MILP model which establishes D2D pairs between free/idle IUEs and HIZUEs and also guarantees certain SINR threshold (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 algorithm maintains almost the same SINR as that of Full Power Femto scheme (i.e., Femto transmits with maximum power) for IUEs and also guarantees certain minimum SINRth for HIZUEs. Our simulation results show that in comparison to the Optimal Femto Power (OptFP; Sathya et?al., 2014) scheme (i.e., Femto transmits with optimal reduced power), 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.

Joint placement and power control of LTE Femto Base Stations in enterprise environments

In order to boost the data rate for indoor users, low power nodes like Femto Base Stations (BSs) are deployed in LTE (long term evolution) networks. The placement of Femtos inside enterprise building environments can significantly affect indoor user performance. We consider a system model that takes into account the following parameters: co-channel interference between Femto BSs and Macro BSs, wall attenuation factor and user density in enterprise building environments. We solve joint placement and power control problem by formulating two Mixed Integer Programming (MIP) optimization models: Optimal Constant Threshold Signal to Interference plus Noise Ratio (OptCTSINR) and Optimal Varying Threshold SINR (OptVTSINR), which optimally tune the power of Femto BS, guarantee a certain minimum SINR to users and also minimize the number of Femtos needed for the coverage of enterprise buildings. We then solve these MIP models by utilizing branch and bound framework of CPLEX solver in General Algebraic Modeling System (GAMS) tool. When compared to center K-Means (CKM) clustering based placement scheme, for a given number of Femtos, proposed scheme OptCTSINR results in average SINR improvement of 39% and proposed OptVTSINR outperforms OptCTSINR by 6.7%, respectively.

Optimal placement of Femto base stations in enterprise femtocell networks

Femto cells a.k.a. Low Power Nodes (LPNs) are deployed to improve indoor data rates as well as reduce traffic load on macro Base Stations (BSs) in 4G/LTE cellular networks. Indoor UEs getting high SNR (Signal-to-Noise Ratio) can experience good throughput, but SNR decreases at faster rate due to obstacles, present along the communication path. Hence, efficient placement of Femtos in enterprise buildings is crucial to attain desirable SNR for indoor users. We consider obstacles and shadowing effects by walls and include them in the system model. We develop a Linear Programming Problem (LPP) model by converting convex constraints into linear ones and solve it using GAMS tool, to place Femtos optimally inside the building. Our extensive experimentation proves the optimal placement of Femtos achieves 14.41% and 35.95% increase in SNR of indoor UEs over random and center placement strategies, respectively.

Energy-efficient femtocell placement in LTE networks

To enhance battery life of mobile handsets and increase the data rates for indoor users in LTE networks, low power nodes like Femtos are deployed in homes and enterprise buildings. But the optimal placement of Femtos is a challenging task due to heterogeneity in building layouts and co-tier intercell interference. In this work, we focus on reducing the battery power consumption (uplink transmit power) while guaranteeing uplink Signal to Interference plus Noise Ratio (SINR) threshold (USINRTh) and downlink SINR threshold (DSINRTh). We achieve this by placing the Femtos optimally, taking into account wall attenuation factor and interference among Macro and Femto base stations. A two-step optimization model has been formulated: in step one, we formulate a Mixed Integer Programming (MIP) problem yields the optimal positions of the Femtos and meets DSINRTh and USNRTh while also minimizing Femto count and 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 step one. When compared to center K-means placement scheme, the proposed optimal placement scheme obtained by solving the two-step model registers a significant, 47%, reduction in uplink energy consumption.

A Novel Resource Allocation and Power Control Mechanism for Hybrid Access Femtocells

LTE Small cells like Femto cells are being deployed in enterprises and residential buildings to improve data rates of indoor users who experience low Signal-to-Interference plus Noise Ratio (SINR) from Macro Base Stations (MBSs). Deployment of Femto cells inside a building can lead to signal leakage at the edges/corners of the buildings. This causes cross-tier interference and degrades the performance of users in High Interference Zone (HIZone) around the building area, who are connected to one of the MBSs in LTE Heterogeneous Networks (HetNets). Hybrid Access Femto cells (HAFs) can ensure QoS for paid Subscriber Group (SG) users by giving them preferential access to resource blocks over non-SG (NSG) users and also improve the throughput of LTE HetNet system by serving nearby NSG users. In this work, we address various challenges involved in deployment and operation of HAFs in indoor environments by proposing an Optimal Placement of Femto cell (OPF) model, a dynamic Bandwidth Allocation (BWA) mechanism for splitting resource blocks between SG and NSG users, a dynamic power control mechanism to mitigate co-tier and cross-tier interference in HetNets and an Enhanced Priority (EP) scheduling mechanism to give more priority to SG users over NSG users. During peak traffic load scenarios, HAFs may not be able to guarantee QoS of both indoor and HIZone users connected to them. As HAFs are primarily meant for indoor users, HAFs employ an Optimal Power Control (OPC) mechanism to tune their transmit powers so that HIZone users are offloaded to near-by MBSs. Since the OPC is a Mixed Integer Non-linear Programming (MINLP) problem, we put forth a Sub-Optimal Power Control (SOPC) mechanism. The SOPC mechanism boosts the throughput of MBSs by almost 62% over the traditional 3GPP proposed enhanced Inter-Cell Interference Co-ordination (eICIC) mechanism for 300 users in the HetNet system. Also, the proposed EP scheduling mechanism maintains Jain’s fairness index of 0.99 for both SG and NSG users while providing a 40% higher per user throughput than that obtained with the legacy proportional fair and Priority Set schedulers.

A novel scheduling algorithm to maximize the D2D spatial reuse in LTE networks

In order to offload base station (BS) traffic and to enhance efficiency of spectrum, operators can activate many Device-to-Device (D2D) pairs or links in LTE networks. This increases the overall spectral efficiency because the same Resource Blocks (RBs) are used across cellular UEs (CUEs) (i.e., all UEs connected to BS for both C-Plane and D-plane communication) and D2D links (i.e., where the UEs are connected to BS only for C-plane communication). However, significant interference problems can be caused by D2D communications as the same RBs are being shared. In our work, we address this problem by proposing a novel scheduling algorithm, Efficient Scheduling and Power control Algorithm for D2Ds (ESPAD), which reuses the same RBs and tries to maximize the overall network throughput without affecting the CUEs throughput. ESPAD algorithm also ensures that Signal to Noise plus Interference Ratio (SINR) for each of the D2D links is maintained above a certain predefined threshold. The aforementioned properties of ESPAD algorithm makes sure that the CUEs do not experience very high interference from the D2Ds. It is observed that even when the SINRdrop (i.e., maximum permissible drop in SINR of CUEs) is as high as 10 dB, there is no drastic decrease in CUEs throughput (only 3.78%). We also compare our algorithm against other algorithms and show that D2D throughput improves drastically without undermining CUEs throughput.

On Femto placement and decoupled access for downlink and uplink in enterprise environments

While it is easier to quench the demand for higher data rates outdoors, it is still a significant challenge when it comes to attaining similar data rates for indoor User Equipments (UEs). Femto cells were introduced for this purpose and also to minimize the traffic load on macro Base Stations (BSs) in 4G/LTE cellular networks. Indoor UEs can achieve good throughput if they get high Signal to Noise Ratio (SNR), but the inherent problem of path loss due to obstacles prevents UEs from receiving good signals. So, the efficient placement of Femtos in enterprise buildings is crucial. For the optimal placement of Femtos, we developed a Mixed Integer Linear Programming (MILP) model and solved it using the GAMS tool. Once the network planning is done, the next problem that has to be addressed is the downlink traffic imbalance which happens due to non-uniform UE traffic distribution. Traditionally load imbalance is addressed by transferring some of the UEs from the highly loaded cell to a less loaded neighboring cell but this could increase the UE uplink transmission power as it now connected to a cell which is not the closest one. To improve UE battery life and to boost the downlink throughput, we decouple the uplink and downlink (DuD) access to UEs by connecting the uplink to the shortest pathloss Femto, and the downlink to one of less loaded neighboring Femtos. Our extensive experimentation in MATLAB shows that on average, the decoupled access system achieves 70% energy savings (i.e., uplink power) when compared to coupled access system. Received on ; accepted on ; published on

Maximizing Dual Cell Connectivity Opportunities in LTE Small Cells Deployment

Most of the LTE (Long Term Evolution) network operators are deploying low power small cells in hotspots like airports, shopping malls and corporate offices to meet increasing data demands. Since users are not deemed to fixed locations in such places, the network experiences uneven distribution of traffic load across the cells which degrades the average user throughput. This problem is even more severe if the deployment of small cells is unplanned. In order to address this, in this work, we propose two variant of small cell placement models: an optimal Femto placement with full power (OPT-FP) model and an opportunistic Femto placement with power control (OPPRPC) model. These models incorporate a constraint which helps small cells providing dual cell connectivity (DCC) for as many number of users as possible and then schedule them jointly for improving their throughputs.