Elena Virtej

On LTE performance trade-off between connected and idle states with always-on type applications

In cellular networks it is envisioned an explosion of diverse data applications running in smart phones, which could increase significantly not only user plane but also control plane load in the network. Smart phones and their always-on applications introduce several challenges: From the network perspective, an increased amount of UEs/applications can increase the signaling load due to frequent state transitions and handover signaling. From the UE perspective, challenges of power consumption are evident, since UE would increasingly be in connected state. In this article we study the trade-off between releasing the UE into idle state vs. keeping the UE in connected state, in a LTE system. The results indicate that optimum performance can be achieved using UE specific RRC release timer and DRX configurations by taking advantage of traffic characteristics and UE mobility.

Performance of MIMO Aware RRM in Downlink OFDMA

This paper addresses advanced radio resource management (RRM) algorithms for multiple-input multiple-output (MIMO) transmission schemes in downlink OFDMA systems. The analysis covers the main RRM mechanisms including MIMO rank adaptation and time-frequency packet scheduling. The UTRAN Long Term Evolution (LTE) system is used as case study. System-level performance analysis is presented for 2 times 2 and 4 times 4 spatial-multiplexing, single-user MIMO transmission schemes in typical macro- and micro-cell scenarios including the effects of limited and imperfect CQI feedback from the terminals. We show that time-frequency domain RRM schemes can be successfully adapted and optimized for operations with high-rate adaptive MIMO transmission schemes.

System performance of Single-User MIMO in LTE downlink

Orthogonal Frequency-Division Multiplexing is a multi-carrier modulation scheme, where information symbols are transmitted in parallel over the channel by using a set of subcarriers. One of the main advantages is increased robustness against frequency selective fading and narrowband interference. In this paper we study the performance of two dual-codeword Single-User Multiple-Input Multiple-Output (SU-MIMO) schemes, i.e. per antenna rate control (PARC) and Precoded MIMO (PREC), in an OFDM deployment. For PARC we consider 2times2 and 4times4 antenna configurations and for Precoded MIMO 4times4 antenna configuration only. Our study is performed with a downlink Long Term Evolution (LTE) system simulator. The results indicate that in terms of average sector throughput the four-stream schemes outperform the dual-stream scheme by approximately 75-90%. Moreover, the four-stream schemes have significant coverage gain compared to the dual-stream scheme. When comparing the four-stream schemes against each other, the relative gain of the precoded scheme in average sector throughput is about 5-6%. From coverage perspective, PREC is about 13-20% better than PARC.

System Level Analysis of Interference Aware LMMSE Chip Equalization in HSDPA Network

In this paper we present a system level performance analysis of ideally interference aware linear minimum mean squared error (LMMSE) chip-level equalizer capable of both intra-cell and inter-cell interference suppression with high-speed downlink packet access (HSDPA). The evaluation is performed with and without receive diversity, having the intra-cell interference suppressing LMMSE equalizer as a reference. The study is performed with fully dynamic system simulator which includes the modeling of LMMSE equalizer, HSDPA functionality, UE mobility and the most fundamental radio resource management algorithms. The used system scenario is HSDPA network with proportional fair scheduler. The results indicate that having ideal knowledge of interference in LMMSE equalizer would provide benefits for the end user by increasing the achievable data rates at the cell edges but having a minor effect to the average system performance.

Distributed flexible spectrum use for uncoordinated scenarios

Data rates are growing, and limited availability of spectrum drives wireless systems to use higher center frequencies, leading to smaller cell sizes in wireless communication systems. This requires new solutions for dealing with deployments of much larger number of access points, in particular in places with restricted access. Such uncoordinated deployments are likely to consist of separate, overlapping local area networks, creating a demand for a solution enabling networks to co-exist in the same frequency band. Uncoordinated deployments would benefit significantly on self-organizing Flexible Spectrum Use (FSU), in practice providing some degree of automated network optimization for the system. In our paper, we propose a new distributed FSU mechanism for uncoordinated deployments. The concept is based on creating understanding of the local spectrum usage situation, i.e. introducing local awareness to the access point. The mechanism for creating the local awareness is based on new broadcast control messages, FSU beacons, transmitted by access points, and received by user terminals. In order to achieve fairness in the deployed system a model of a priority based fairness is proposed. The performance of the proposed FSU scheme is analyzed using a system simulator. The results indicate that the proposed FSU scheme is beneficial especially when traditional frequency reuse one approach leads to serious coverage problems.

Performance of an Intra- and Inter-Cell Interference Mitigation Algorithm in HSDPA System

Linear equalizer is known to effectively suppress intra-cell interference caused by the multi path propagation in high speed downlink packet access (HSDPA) system. Equalizer algorithms are commonly based on linear minimum mean square error (LMMSE) estimators, which can also take into account the colored interference from other cells. However, several receive antennas are required to efficiently cancel inter-cell interference. On the other hand, non-linear methods are attractive if only one receive antenna is present. However, these algorithms tend to require several auxliary parameters to be known. In this paper, we study a two stage single antenna receiver, combining non-linear and linear processing in HSDPA system. The first stage uses blind interference cancellation (BIC) algorithm to cancel the dominant part of the other cell interference and the second stage uses the LMMSE equalizer to suppress intra-cell interference. The performance of the two stage receiver is compared against conventional LMMSE equalizer. The results show that the studied non-linear algorithm achieves improved performance especially on the cell edge.

HSDPA Link Performance with Multi-Antenna Diversity

This paper presents a comparison of the theoretical outage capacity and link-level simulation results of selected multi antenna techniques for HSDPA. Diversity schemes, covering both transmit and receive diversity methods with and without channel state information at the transmitter were simulated. A moderate difference between the link-level simulation results and the theoretical outage results is observed. An almost optimal increase in diversity order and array gain is achieved by all investigated HSDPA schemes when compared to theoretical channel capacity gains

Coordination between access points in distributed flexible spectrum use

Uncoordinated deployments of high number of access points are likely to consist of separate, overlapping local area networks, creating a demand for solutions enabling networks to co-exist in the same frequency band. Such uncoordinated deployments would benefit significantly on self-organizing Flexible Spectrum Use (FSU), which would in practice provide some degree of automated network optimization for the system. In our paper, we study the distributed FSU in the context of uncoordinated deployment (i.e. randomly placed of APs, where UEs may be connected to clearly suboptimal faraway APs). The goal is to understand if there is any need for FSU coordination in a scenario of completely distributed APs belonging to multiple networks. The results indicate that for the scenarios with several overlapping networks FSU coordination is necessary. Otherwise decent coverage cannot be obtained and thus spectrum resources are not shared fairly among the users.

DRX-Aware Power and Delay Optimized Scheduler for Bursty Traffic Transmission

Discontinuous Reception (DRX) is one of the mechanisms to reduce User Equipment (UE) power consumption in Long Term Evolution (LTE) systems. In this work, a DRX-aware power and delay optimized scheduling scheme is proposed for downlink transmission of bursty packet data traffic. The UE power consumption is minimized by reducing the time that a UE spends receiving data. This is achieved by predominantly scheduling a single UE at a time. The mean packet burst end-to-end delay is minimized by giving priority to packet bursts with relatively small transmission time requirement when compared to a weighted value of the estimated remaining transmission time of the predominantly scheduled packet burst. The weight parameter can be adjusted according to power-delay trade off preference. The DRX timers are further taken into account to avoid wake up delays, by keeping some of the waiting UEs active before the transmission to the currently scheduled packet burst ends. Based on LTE system level simulation results, the UE power consumption could be significantly reduced while improving the mean delay using the scheduler, comparing to conventional proportional fair scheduler.