On the Efficiency tradeoffs in User-Centric Cloud RAN

Ambitious targets for aggregate throughput, energy efficiency and ubiquitous user experience are propelling the advent of ultra-dense networks. Intercell interference and high energy consumption in an ultra-dense network are the prime hindering factors in pursuit of these goals. To address the aforementioned challenges, in this paper, we propose a novel user-centric network orchestration solution for Cloud RAN based ultra-dense deployments. In this solution, a cluster (virtual disc) is created around users depending on their service priority. Within the cluster radius, only the best remote radio head (RRH) is activated to serve the user, thereby decreasing interference and saving energy. We use stochastic geometry based approach to quantify the area spectral efficiency (ASE) and RRH power consumption models to quantity energy(EE) efficiency of the proposed user-centric Cloud RAN (UCRAN). Through extensive analysis we observe that the cluster sizes that yield optimal ASE and EE are quite different. We propose a game theoretic self-organizing network (GT-SON) framework that can orchestrate the network between ASE and EE focused operational modes in real-time in response to changes in network conditions and the operator's revenue model, to achieve a Pareto optimal solution. A bargaining game is modeled to investigate the ASE-EE tradeoff through adjustment in the exponential efficiency weightage in the Nash bargaining solution (NBS). Results show that compared to current non-user centric network design, the proposed solution offers the flexibility to operate the network at multiple folds higher ASE or EE along with significant improvement in user experience

Enhancing Downlink QoS and Energy Efficiency through a User-Centric Stienen Cell Architecture for mmWave Networks

This paper presents an analytical framework for performance characterization of a novel Stienen cell based user-centric architecture operating in millimeter wave spectrum. In the proposed architecture, at most one remote radio head (RRH) is activated within non overlapping user equipment (UE)-centric Stienen cells (S-cells) generated within the Voronoi region around each UE. Under the presented framework, we derive analytical models for the three key performance indicators (KPIs): i) SINR distribution (used as an indicator for quality of service (QoS)), ii) area spectral efficiency (ASE), and iii) energy efficiency (EE) as a function of the three major design parameters in the proposed architecture, namely UE service probability, S-cell radius coefficient and RRH deployment density. The analysis is validated through extensive Monte Carlo simulations. The simulation results provide practical design insights into the interplay among the three design parameters, tradeoffs among the three KPIs, sensitivity of each KPI to the design parameters as well as optimal range of the design parameters. Results show that compared to current non user-centric architectures, the proposed architecture not only offers significant SINR gains, but also the flexibility to meet diverse UE specific QoS requirements and trade between EE and ASE by dynamically orchestrating the design parameters

User-Centric Cloud RAN: An Analytical Framework for Optimizing Area Spectral and Energy Efficiency

In this article, we develop a statistical framework to quantify the area spectral efficiency (ASE) and the energy efficiency (EE) performance of a user-centric cloud based radio access network (UC-RAN) downlink. We propose a user-centric remote radio head (RRH) clustering mechanism, which: 1) provides significant improvement in the received signal-to-interference-ratio through selection diversity; 2) enables efficient interference protection by inducing repulsion among scheduled user-centric RRH clusters; and 3) can self-organize the cluster radius to deal with spatio-temporal variations in user densities. It is shown that under the proposed user-centric clustering mechanism, the ASE (bits/s/Hz/m²) maximizes at an optimal cluster size. It is observed that this cluster size is sensitive to changes in both RRH and user densities and, hence, must be adapted with variations in these parameters. Next, we formulate the cost paid for the UC-RAN capacity gains in terms of power consumption, which is then translated into the EE (bits/s/Joule) of the UC-RAN. It is observed that the cluster radius which maximizes the EE of the UC-RAN is relatively larger as compared with that which yields maximum ASE. Consequently, we notice that the tradeoff between the ASE and the EE of UC-RAN manifests itself in terms of cluster radius selection. Such tradeoff can be exploited by leveraging a simple two player cooperative game. Numerical results show that the optimal cluster radius obtained from the Nash bargaining solution of the modeled bargaining problem may be adjusted through an exponential weightage parameter that offers a mechanism to utilize the inherent ASE-EE tradeoff in a UC-RAN. Furthermore, in comparison with existing state-of-the-art non user-centric network models, our proposed scheme, by virtue of selective RRH activation and non overlapping user-centric RRH clusters, offers higher and adjustable system ASE and EE, particularly in dense deployment scenarios

Solution Processable High Performance Multiwall Carbon Nanotube-Si Heterojunctions

10.1002/aelm.202000617ADVANCED ELECTRONIC MATERIALS61

Polyaxial screws for lumbo-iliac fixation after sacral tumor resection: experience with a new technique for an old surgical problem.

Background:Although numerous reports have been published about various methods for reconstruction after sacrectomies, there are still biomechanical and technical dilemmas that are unaddressed. This report describes the experience at authors\u27 institution of five cases in which polyaxial pedicle screws construct has been successfully used for lumbo-iliac fixation after sacral tumor resection. Methods: Five cases of sacral tumors, two of Ewing\u27s sarcoma and three of giant cell tumor (GCT) underwent surgical resection and then reconstruction was done with hardware using vertical rods placed alongside the spine bilaterally, transfixing monoaxial and polyaxial pedicle screws in lower lumbar levels and polyaxial screws into the ilium bilaterally. Cross links were also used to connect the two vertical members, thus enhancing biomechanical stability of the construct. Use of autologous bone grafts was relied upon to fill the gap created by sacral resection. Results: No instrumentation failure was noted and the continuity of the spine and pelvis was well established with the instrumentation and auto grafts. In follow up of these Patients (1-3 years), no complications were seen. Conclusions: Polyaxial pedicle screws fixation is an effective technique to transmit axial load from spine to the appendicular bone and can be used safely in Patients in whom sacral integrity is compromised after surgical resection. However, the long term benefits of this technique need to be evaluated