Juan Montojo

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.

Overview of 3GPP LTE-advanced carrier aggregation for 4G wireless communications

To satisfy the ever increasing demand for higher throughput and data rates, wireless communication systems need to operate in wider bandwidths. 3GPP LTE-Advanced with carrier aggregation enables operators to maximally and optimally utilize their available spectrum resources for increased data rates and user experience while reducing their incurred OPEX and CAPEX. This article provides a tutorial overview of 3GPP LTE-Advanced with carrier aggregation as specified in Rel-10 including deployment scenarios of interest, main design features, PHY/MAC procedures, and potential enhancements for future standard releases.

Relaying operation in 3GPP LTE: challenges and solutions

With the ever growing demand of data applications, traditional cellular networks face the challenges of providing enhanced system capacity, extended cell coverage, and improved minimum throughput in a cost-effective manner. Wireless relay stations, especially when operating in a halfduplex operation, make it possible without incurring high site acquisition and backhaul costs. Design of wireless relay stations faces the challenges of providing backward compatibility, minimizing complexity, and maximizing efficiency. This article provides an overview of the challenges and solutions in the design of relay stations as one of the salient features for 3GPP LTE advanced.

UE's role in LTE advanced heterogeneous networks

Deployment of low-power nodes such as picocells, femtocells, and relay nodes within macrocell coverage is seen as a cost-effective way to increase system capacity and to equip wireless WANs with the ability to keep up with the increasing demand for data capacity. These new types of deployments are commonly referred to as heterogeneous networks and are currently receiving significant attention in industry. However, simple deployment of low-power nodes can lead to underutilization of air-interface resources due to the relatively small footprint of the lowpower nodes or service outage in the case of femto cells with restricted access. Time-domain interference management techniques by the configuration of almost blank subframes, introduced in LTE Rel-10 standards, allow the removal of most of the interference from the dominant interfering nodes. This mechanism enables cell biasing or cell range extension of weak cells, thereby maximizing the incremental gain provided by the deployment of low-power nodes. The configuration of ABS changes the interference conditions seen by the user equipment and therefore requires corresponding resource-specific measurements and feedback at the UE. In this article, we provide an overview of LTE Rel-10 resource specific radio link monitoring, radio resource management, and channel state information feedback procedures. Also, we provide evaluation results to show that UE receivers, in the detection of weak cells and removal of interference in demodulation of control and data channels, play a critical role in realizing the full potential that the deployment of heterogeneous networks can offer.

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.

Link-Level Analysis of Low Latency Operation in LTE Networks

This paper studies the physical-layer benefits of low latency operation in long-term evolution (LTE) networks. Latency reduction can be achieved by reducing the transmission time interval (TTI) from 1ms to the duration of only a few orthogonal frequency-division multiplexing symbols. The TTI shortening potentially enables faster link adaptation, thereby enhancing system performance. However, enabling low latency operation in a backward compatible manner requires a careful design and performance characterization. This paper conducts a link-level performance analysis of a low latency LTE network in both downlink and uplink with different transmission schemes and under various operating regimes. Our results reveal that a low latency LTE network can provide reasonable link-level performance improvements as compared to a legacy LTE network.

Techniques for Enabling Low Latency Operation in LTE Networks

This paper describes the physical (PHY) layer and medium access control (MAC) layer design updates for Long Term Evolution (LTE) air interface that enable low latency services in LTE cellular networks. In both downlink and uplink, the low latency communication is enabled through the reduction of the transmission time interval (TTI). Reducing the TTI can be viewed as the key enabler for mission critical applications, where system capacity is not of primary importance. This paper, however, shows that enabling low latency operation in LTE networks does not have to come at the expense of capacity. Indeed, we show that the low latency operation can even increase system capacity. In particular, the TTI reduction enables shorter channel state information (CSI) and hybrid automatic repeat request (HARQ) feedback timelines, thereby leading to a more accurate rate control. Hence, a more efficient use of air interface resources becomes possible. This, in turn, translates into a higher perceived user throughput and higher system capacity. In the uplink, TTI reduction leads to a faster user scheduling and a faster access to the medium. Further, it reduces the delay in sending transmission control protocol (TCP) acknowledgments. The benefits are more pronounced during the slow start phase as the reduction in the timeline to access the medium directly translates to a faster download of data.