Bishwarup Mondal

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 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.

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.

Optimization of two layer macro-pico networks using LTE

The exponential increase in data traffic demand has focused attention on two layer network deployment strategies where low-power pico base-stations are placed within a macro cell coverage area targeting specific capacity demands. Two aspects of such a network are different from a macro-only deployment and are studied in this paper - (i) the choice of antenna geometry and polarization for a non-sectorized pico base-station (ii) the necessity for a macro layer to be silenced to protect users served by a pico layer and receiving severe interference from the macro layer. Our results show that orthogonally polarized arrays are markedly better than co-polarized arrays for pico cells. Also, linear arrays (whether cross- or co-polarized) are better matched to the LTE codebook than circular arrays. To manage interference, silencing a macro for an optimized fraction of time-frequency resources is necessary only when a significant fraction of users are off-loaded from a macro layer to a pico layer in an outdoor pico deployment.

Joint Scheduling for CoMP and eICIC in Heterogeneous Network Deployments

In this paper, LTE Rel-10 eICIC, an advanced interference mitigation technique using semi-static coordination and DL CoMP techniques relying on dynamic scheduling are jointly applied in a heterogeneous network comprising of macro and low power nodes. It is shown that these algorithms, when applied jointly and if supported with appropriately designed feedback schemes, can provide complementary benefits and can significantly improve both system capacity and user experience in the network. Using covariance matrix feedback, we observe that the joint application of CoMP and eICIC can provide more than 20% system throughput improvement and more than 40% cell-edge throughput improvement that exceed the gains of either CoMP or eICIC applied individually.

Deployment considerations for 3D-MIMO arrays

In this paper we consider the deployment of 3D-MIMO in traditional LTE bands (~1.8-3.5GHz) though 3D-MIMO is also considered an integral element of 5G systems. We show how the architecture of the 3D-MIMO antenna system (i.e., the connectivity between the transceivers and the physical antenna elements) can significantly impact system performance. At the cost of additional complexity in the array architecture that affects the ability to form wide or narrow RF beams, substantial performance gains can be achieved. We also consider the effect of the deployment scenario on 3D-MIMO performance. Generally environments where users are spatially separated in the elevation domain are better suited for MU-MIMO and consequently 3D-MIMO can provide better system performance. We observe that denser deployments (small inter-site distance) and user distributions offering a wider range of elevation angles of departures are more attractive for 3D-MIMO deployments. We present system simulation results quantifying the achievable gains by doubling the number of transceivers (or antenna ports) compared to a state-of-the-art LTE 8Tx base-station while operating within the existing LTE specifications.

Millimeter Wave System Performance Characterization for 5G Data Access

In this paper, we consider an OFDM(A) based millimeter wave (mmWave) system that is derived from a 5x scaling of the LTE numerology and incorporates analog beamforming-based antenna systems at both the access point (eNB) and user equipment (UE). We characterize the performance of such a system via a single- and multi-user link budget analysis as well as detailed system simulations. The sensitivity of performance to inter-site distance (ISD) and indoor vs. outdoor UEs is evaluated. A key observation is that while mmWave systems can achieve high throughputs, especially for dense deployments, outage is a concern for indoor UEs and non-line-of-sight (NLoS) channels. This motivates the need to build in robustness in mmWave access systems design. An analysis of the throughput impact of in-band backhaul for delivering efficient end-to-end service in ultra-dense mmWave networks is shown. Optimal resource partitioning approaches are derived that yield design guidelines for dense network deployments with in-band backhaul.