Durga Prasad Malladi

A survey on 3GPP heterogeneous networks

As the spectral efficiency of a point-to-point link in cellular networks approaches its theoretical limits, with the forecasted explosion of data traffic, there is a need for an increase in the node density to further improve network capacity. However, in already dense deployments in todays networks, cell splitting gains can be severely limited by high inter-cell interference. Moreover, high capital expenditure cost associated with high power macro nodes further limits viability of such an approach. This article discusses the need for an alternative strategy, where low power nodes are overlaid within a macro network, creating what is referred to as a heterogeneous network. We survey current state of the art in heterogeneous deployments and focus on 3GPP LTE air interface to describe future trends. A high-level overview of the 3GPP LTE air interface, network nodes, and spectrum allocation options is provided, along with the enabling mechanisms for heterogeneous deployments. Interference management techniques that are critical for LTE heterogeneous deployments are discussed in greater detail. Cell range expansion, enabled through cell biasing and adaptive resource partitioning, is seen as an effective method to balance the load among the nodes in the network and improve overall trunking efficiency. An interference cancellation receiver plays a crucial role in ensuring acquisition of weak cells and reliability of control and data reception in the presence of legacy signals.

Network densification: the dominant theme for wireless evolution into 5G

This article explores network densification as the key mechanism for wireless evolution over the next decade. Network densification includes densification over space (e.g, dense deployment of small cells) and frequency (utilizing larger portions of radio spectrum in diverse bands). Large-scale cost-effective spatial densification is facilitated by self-organizing networks and intercell interference management. Full benefits of network densification can be realized only if it is complemented by backhaul densification, and advanced receivers capable of interference cancellation.

LTE Femtocells: System Design and Performance Analysis

In this paper we consider a heterogeneous LTE network where femto cells are randomly deployed in a macro network. Femto cells are modeled as closed cells, namely only group member UEs can be associated with the femto cells. We demonstrate that inter-cell interference may prevent reliable operations for non-member UEs that are in proximity of a closed cell, which thus experience outage. We show how some of the novel features introduced in the Rel-10 specifications of the LTE standard can be leveraged by a suitable inter-cell interference coordination scheme (ICIC), which relies upon resource partitioning among different nodes to reduce the inter-cell interference problem. Additional significant improvements can be achieved when the proposed ICIC scheme is associated to a simple yet effective autonomous power control algorithm, described in detail in the paper, and further gains are demonstrated for UEs employing interference cancellation of broadcast interfering signals. We finally propose an enhanced ICIC method, based on a tighter coordination between macro and femto nodes, whose significant performance improvements advocate for suitable updates to the future LTE specifications.

Coordinated downlink multi-point communications in heterogeneous cellular networks

In this paper we assess how coordination among base stations can be exploited to improve downlink capacity in fourth generation (4G) cellular networks. We focus on heterogeneous networks where low-power pico cells are deployed within the coverage area of an existing macro network with the aim of offloading traffic from the (potentially congested) macro cells to low-power cells. Firstly, we describe an enhanced inter-cell interference coordination scheme which is shown to achieve a significant capacity gain in such deployments by leveraging a loose coordination among neighbor base stations. Secondly, we explore how a tighter coordination among base stations can be exploited to further improve the network capacity. Even though the schemes described in this paper apply to long term evolution (LTE) wireless networks, we point out that most of the findings and conclusions we draw apply to any cellular network.

3GPP LTE Downlink System Performance

In this paper we quantify 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8 downlink system performance for a macro cell hexagonal grid scenario. The system performance is analyzed for a closed loop Single User Multi-Input Multi Output (SU-MIMO) mode and compared with Single Input Multiple Output (SIMO) and Multi-User (MU) MIMO modes, for a full buffer scenario and static users. In addition to the full buffer scenario, traffic models are considered for SIMO mode to evaluate impact of partial loading and handover. Voice over Internet Protocol (VoIP) system performance is quantified for static users. Mobility simulations are performed for Video Telephony (VT) users and compared to the static case.

WLC36-3: Selective Virtual Antenna Permutation for Layered OFDM-MIMO Transmission

In this paper we introduce selective virtual antenna permutation (S-VAP) for layered OFDM-MIMO transmission in downlink cellular environments. In the S-VAP scheme, the base station spatially permutes multiple independently encoded layers and then transmits the permuted layers over selected virtual antennas. The mobile station employs a successive interference cancellation (SIC) based receiver to maximize the spectral efficiency. The layer permutation enables a significant reduction of channel quality feedback information necessary to adjust the data rate of each layer. Transmission over selected virtual antennas instead of physical antennas enables efficient power amplifier utilization and thus brings out a quantized spatial water-filling gain.

WCDMA uplink system performance

In WCDMA, uplink radio resources are shared by multiple users and the radio resource management (RRM) is done at the UTRAN (UMTS Terrestrial Radio Access Network). In this paper, the link and system performance of the WCDMA uplink is evaluated using different techniques such as retransmissions with incremental redundancy (IR), short frame durations and fast scheduling. It is seen that the radio link efficiency can be improved using IR, while the traffic delays can be reduced and cell throughput can be enhanced by using fast scheduling and short frame durations.