Riikka Susitaival

LTE Coverage Improvement by TTI Bundling

Compared to WCDMA, the LTE radio access has a significantly shorter Transmission Time Interval (TTI) in order to reduce end-to-end delays. However, if a User Equipment (UE) at the cell edge is limited by its available transmission power, it may not be able to transmit an entire VoIP packet during one TTI, since the instantaneous source data rate is too high. Thus TTI bundling has been recently introduced as a feature of LTE Rel. 8 to improve the uplink coverage. In TTI bundling, a VoIP packet is transmitted as a single PDU during a bundle of subsequent TTIs without waiting for the HARQ feedback. HARQ feedback is only expected for the last transmission of the bundle. This paper studies TTI bundling and compares it to the conventional RLC segmentation. The simulation results indicate that TTI bundling provides a gain of more than 4 dB in terms of the sustainable path loss.

Interference avoidance for in-device coexistence in 3GPP LTE-advanced: challenges and solutions

With the ever growing usage of various wireless technologies and services, more and more handheld devices are equipped with multiple radio transceivers (e.g., LTE, WiFi, Bluetooth, and GNSS). As a result, in-device coexistence interference has become a serious problem due to the extreme proximity of multiple radio transceivers within the same device. It has been shown by the studies of 3GPP that for some specific frequency bands, concurrent operations of LTE and ISM/GNSS radios working in adjacent or sub-harmonic frequencies will result in significant in-device coexistence interference that cannot be completely eliminated by filter technology. This motivates 3GPP to introduce signaling mechanisms and procedures, which enable the devices to effectively solve the indevice coexistence problems with help from the LTE network. This article first discusses the scenarios in which coexistence interference may happen, and then provides an overview of main solutions and related procedures specified in 3GPP Release-11 standards for LTE. In addition, some aspects of device-internal coordination and implementation to facilitate the coexistence scenarios are also discussed.

RAN overload control for Machine Type Communications in LTE

In Radio Access Network (RAN) overload situations, more access attempts are made to the system than the system can handle. In particular, Machine Type Communication (MTC) devices are foreseen as potential source of overload in the Long Term Evolution (LTE) networks. In this paper, we study overload scenarios caused by MTC traffic in LTE and investigate how to solve them. Especially, we study Extended Access Barring (EAB) recently introduced in 3GPP to control the access load. We find that performance of the EAB scheme depends on many factors such as what parameters are used and how the parameter values are updated in the System Information (SI). We find that EAB solution for MTC overload control can solve the overload situation effectively, if configured properly.

Internet Access Performance in LTE TDD

The Time Division Duplex (TDD) uplink-downlink configuration of the 3GPP Long Term Evolution (LTE) determines how the ten subframes in a radio frame are divided between the downlink and the uplink. The specified configurations cover a wide range of allocations from a downlink-heavy resource distribution ratio (9:1) to an uplink-heavy ratio (2:3). In this paper, we compare the performance of Internet access using the TCP protocol in different downlink-heavy asymmetries. We find that the performance depends on many factors such as the transferred file size, the control channel errors and the downlink/uplink traffic mix. When the file size is small, TDD can not fully utilize its potentially higher downlink capacity because of longer uplink access delays as well as shortage of uplink resources in the chosen configurations. With an increased file size, this effect fades away and TDD provides higher download bit rates than FDD. The realized increase in bit rate is however not as high as the calculated increase in available downlink resources.

Protocol Impact of LTE Relays on User Performance

Relays are introduced for LTE Rel-10 in order to improve cell edge bitrates and coverage. 3GPP has specified both inband and outband relays. In this paper, we study the protocol level aspects of relays as well as cell-edge performance of a mobile terminal with system simulations. The results indicate that when a user is performing TCP downloads and uploads, in addition to potential subframe split between the backhaul and access link, also the adopted protocol solution and increased delays have impact on the user performance. Due to this, the bitrates may degrade even with outband relays if the UE having decent macro coverage is connected to a relay. However, when the UE has bad macro coverage, then there are gains from both inband and outband relays.