Eugene Visotsky

Heterogeneous cellular networks: From theory to practice

The proliferation of internet-connected mobile devices will continue to drive growth in data traffic in an exponential fashion, forcing network operators to dramatically increase the capacity of their networks. To do this cost-effectively, a paradigm shift in cellular network infrastructure deployment is occurring away from traditional (expensive) high-power tower-mounted base stations and towards heterogeneous elements. Examples of heterogeneous elements include microcells, picocells, femtocells, and distributed antenna systems (remote radio heads), which are distinguished by their transmit powers/ coverage areas, physical size, backhaul, and propagation characteristics. This shift presents many opportunities for capacity improvement, and many new challenges to co-existence and network management. This article discusses new theoretical models for understanding the heterogeneous cellular networks of tomorrow, and the practical constraints and challenges that operators must tackle in order for these networks to reach their potential.

3D Extension of the 3GPP/ITU Channel Model

Recently there have been proposals to extend MIMO processing to the elevation dimension in addition to the azimuth direction. To accurately assess the promised gains of these 3D-MIMO techniques, a channel model is needed that accurately accounts for the elevation angles of the rays. In addition it would be desirable for the 3D channel model to be a simple extension of an already defined 2D channel model to allow for ease of implementation and to assist the 3GPP standardization effort in the 3D MIMO area. In this paper we propose an extension of the ITU 2D channel model to 3D by adding a distance dependent elevation spread based on observations from ray tracing. Through system-level simulations we observe that the behavior of 3D MIMO is greatly impacted by the modeling of the 3D channel.

3D channel model in 3GPP

Multi-antenna techniques capable of exploiting the elevation dimension are anticipated to be an important air-interface enhancement targeted to handle the expected growth in mobile traffic. In order to enable the development and evaluation of such multi-antenna techniques, the 3rd Generation Partnership Project (3GPP) has recently developed a three-dimensional (3D) channel model. The existing two-dimensional (2D) channel models do not capture the elevation channel characteristics, making them insufficient for such studies. This article describes the main components of the newly developed 3D channel model and the motivations behind introducing them. One key factor is the ability to model channels for users located on different floors of a building (at different heights). This is achieved by capturing a user height dependency in modelling some channel characteristics including pathloss, lineof- sight (LOS) probability, etc. In general, this 3D channel model follows the framework of WINNERII/WINNER+ while also extending the applicability and the accuracy of the model by introducing some height dependent and distance dependent elevation related parameters.

Performance of downlink comp in LTE under practical constraints

It has been recognized that a primary method of further enhancing the downlink spectral efficiency of LTE Release-10 is to introduce support for coordinated multi-point (CoMP) transmission. This paper investigates the performance improvements due to downlink CoMP within the framework of LTE (specifically LTE Release-11). A realistic estimate of achievable CoMP throughput performance is provided with respect to variations in deployment scenario, amount of feedback information and at different traffic loads. It is observed that the performance gains due to CoMP are limited to cell edge UE (user equipment) throughput improvements of up to 30%. Further, we show that dynamic cell selection can realize most of the CoMP gains with less feedback than joint transmission and it is more robust under practical conditions.

Radar inband and out-of-band interference into LTE macro and small cell uplinks in the 3.5 GHz band

National Telecommunications and Information Administration (NTIA) has proposed vast exclusions zones between radar and Worldwide Interoperability for Microwave Access (WiMAX) systems which are also being considered as geographic separations between radars and 3.5 GHz Long Term Evolution (LTE) systems without investigating the changes induced by the distinct nature of the LTE systems as opposed to WiMAX. This paper performs a detailed system-level analysis of the interference effects from shipborne radar systems into LTE macro cells and outdoor small cells. Even though the results reveal impacts of radar interference on LTE systems performance, they provide clear indications of conspicuously narrower exclusion zones for LTE vis-à-vis those proposed for WiMAX and pave the way toward deploying 3.5 GHz LTE within the exclusion zones.

Coordinated scheduling and network architecture for LTE Macro and small cell deployments

Coordinated Multi-Point (CoMP) transmission has been recognized as a primary method of further enhancing the downlink spectral efficiency of LTE Rel-10. A series of air interface enhancements have been made in LTE Rel-11 to support CoMP with Ideal Backhaul (IB), but the support of CoMP using Non-Ideal Backhaul (NIB) was not considered in Rel-11. Recently, 3GPP evaluated CoMP performance gains with non- ideal backhaul and currently considering the necessary signaling to support inter-eNB CoMP. In this study, two types of network architecture are considered, namely i) the current distributed architecture and ii) the centralized architecture with a new node/entity to enable a centralized scheduler. In this paper, a dynamic coordinated muting scheme is proposed using LTE Rel-11 framework for both Macro and HetNet scenarios. Next, the performance of CoMP using non-ideal backhaul is evaluated under various traffic models with distributed and centralized architectures separately. It is shown that dynamic coordinated muting scheme with NIB can achieve a cell edge gain of approximately 40% over a system without coordination for Macro only and ~10% over FeICIC based HetNet scenario. The simulation results also indicate that a centralized architecture is more sensitive to backhaul latency comparing with distributed approach. Finally, it is concluded that the current LTE distributed architecture is well suited to support coordinated scheduling with non-ideal backhaul.

Rate analysis and feasibility of dynamic TDD in 5G cellular systems

In conventional applications of time division duplex (TDD) in cellular systems, the time resource split between uplink (UL) and downlink (DL) is fixed across all base stations (BSs) in the network. This leads to under utilization of BS resources when there is a mismatch between the expected and experienced UL/DL traffic in a given cell. A dynamic split that varies in each cell is desirable, but is challenging due to the high interference experienced by UL receivers in one cell from DL transmissions in adjacent cells. This paper analyzes the performance of UL users in dynamic TDD enabled next generation cellular networks using a stochastic geometry framework. The analysis highlights the trade-off between spectral efficiency and resource utilization for dynamic TDD. With appropriate interference mitigation, dynamic TDD offers a significant gain in data rates as compared to static TDD, which is higher when the BSs are lightly loaded and/or the fraction of UL users is low.

Radar interference into LTE base stations in the 3.5 GHz band

We study the interference from a rotating shipborne radar system that spectrally and spatially coexists with a Long Term Evolution (LTE) cellular communications network in the 3.5 GHz band to investigate the feasibility of LTE deployment in the United States coastal metropolitan cities in that band. First, we simulate the radar systems with realistic operational parameters. Furthermore, we leverage a detailed 3GPP-compliant LTE simulation with a sophisticated air interface modeling and investigate sensitivity of LTE to radar interference in macro cell, outdoor small cell, and indoor small cell scenarios. We simulate the propagation conditions between the radar and LTE system by adopting the Free Space Path Loss and Irregular Terrain Model commonly leveraged by National Telecommunications and Information Administration (NTIA), to account for propagation, diffraction, and troposcatter losses that the radar pulses undergo before they reach the LTE system. As a performance metric, we evaluate the throughput of the LTE system in the uplink direction for various distances between the radar and the cellular system. Our simulation results indicate an LTE link will remain operational even in severe interference conditions. In fact, the LTE system as close as 100 km away from the radar undergoes less than 10 % throughput loss from the LTE total throughput, and the throughput loss is less than 30 % when the radar is only 50 km away from the LTE.

Dynamic cell muting for ultra dense indoor small cell deployment scenario

Ultra dense indoor deployment is widely recognized as one of the predominant network configurations of the future wireless systems. Dense indoor deployments are characterized by severe interference between co-channel access points deployed usually in close proximity of one another. Thus the effectiveness and efficiency of interference management techniques is the key to the success of ultra dense indoor deployments. In this study, a representative indoor layout, “Nokia office” is used as an example of an ultra dense indoor deployment scenario for investigating the proper indoor interference management schemes. In this paper, an enhanced Dynamic Coordinated Muting (DCM) scheme is proposed using LTE Rel-11 CoMP framework for ultra dense indoor deployments. Based on interference profile analysis, DCM performance is characterized with varying degrees of feedback overhead and with different types of traffic distributions (symmetric and asymmetric). It is shown that the dynamic coordinated muting scheme can achieve cell edge gains in the range of 55%-96% and mean user throughput gains in the range of 17-28%. The simulation results demonstrate that in certain radio conditions it is also beneficial to consider muting multiple interference sources in the coordinated scheduling algorithm.

Elevation beamforming with beamspace methods for LTE

Recently, there has been interest in extending MIMO processing techniques to exploit the elevation dimension of the multipath channel in addition to the azimuth dimension. In this paper, we explore the use of beamspace methods for creating virtual antenna ports in the vertical direction to enable a base station antenna array to exploit the elevation dimension of the multipath channel. We consider two ways of configuring the vertical antenna array: vertical sectorization, which creates additional sectors in the vertical domain, and vertical beamforming, which simply leverages the additional vertical antenna ports with existing MIMO processing techniques. For Rel-8/10 LTE, we show that both vertical sectorization and vertical beamforming can provide significant gains in the average and cell-edge throughputs, both in full buffer traffic as well as bursty traffic. We also highlight the sensitivity of the results to the specific assumptions of the underlying multipath channel characteristics and other system parameters.