telecom • networking • design

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LTE S1-interface handover between eNodeBs

LTE networks prefer using the X2 interface for performing inter eNodeB handovers. An S1 handover is a fallback for scenarios where X2 interface is not available.

As the name suggests, S1 handovers take place over the S1-interface. The MME and the SGW are involved during the handover procedure.

An interesting part of LTE S1 handovers is the indirect tunnel that is established to carry the downlink data during the handover process. Refer to the S1 handover call flow for a detailed signaling flow.

Inter eNodeB S1 handover in LTE

 


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OFDM and SC-FDMA lectures

OFDM and SC-FDMA video lectures cover OFDM and SC-FDMA is great detail. The following topics are covered:

  • OFDM and multi-carrier modulation
  • Converting a frequency selective channel to a flat fading channel
  • IFFT based multi-carrier modulator bank
  • FFT based multi-carrier demodulator bank
  • Cyclic prefix and circular convolution
  • MIMO OFDM
  • PAPR in OFDM
  • SC-FDMA reduces PAPR

OFDM transmitter schematic

OFDM receiver schematic

Click here to view the OFDM and SC-FDMA lectures


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Long Term Evolution (LTE) Tutorials

Here are a few hand picked links to LTE tutorials. Click here for the complete list.

LTE video tutorial

LTE video presentations

LTE physical layer

OFDM and SC-FDMA Signal Chains

LTE link layer design

data flow through PDCP, RLC, MAC and PHY layers of LTE

This article describes the LTE link-layer protocols, which abstract the physical layer and adapt its characteristics to match the requirements of higher layer protocols.The LTE link-layer protocols are optimized for low delay and low overhead and are simpler than their counterparts in UTRAN. The state -of-the-art LTE protocol design is the result of a careful crosslayer approach where the protocols interact with each other efficiently. This article provides a thorough overview of this protocol stack, including the sub-layers and corresponding interactions in between them, in a manner that is more intuitive than in the respective 3GPP specifications.

Introduction to LTE Architecture


This article provides an overview of the LTE radio interface, together with a more in-depth description of its features such as spectrum flexibility, multi-antenna transmission, and inter-cell interference control. The performance of LTE and some of its key features is illustrated with simulation results.

This article provides a high-level overview of LTE and some of its key components: spectrum flexibility, multi-antenna transmission, and ICIC. Numerical simulations are used to show the performance of the first release of LTE, as well as assess the benefit of the key features. Indeed these contribute strongly to LTE meeting its performance targets. An outlook of the evolution of LTE toward LTE-Advanced and full IMT-Advanced capabilities complete the article. Clearly, LTE offers highly competitive performance and provides a good foundation for further evolution.

LTE Protocol Stack

Click here for a more LTE tutorials that cover the entire spectrum of LTE development.


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LTE Downlink Frame Structure

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This article will explain the downlink frame structure. For the beginners, there is always a confusion about the frame structure, the physical channels etc, so to make their job easy, I am writing this article.

To start with, what does the downlink frame structure consists of? The answer is simple, it contains the downlink channels and signals. Refer article LTE Physical Downlink Channels for more details. The LTE frame is nothing but a imaginary grid of time vs frequency , where there are placeholders for different channels and signals, for Eg: the dowlink control channels are always restricted to first 3 symbols or less, of the subframe. The frame structure changes depending on the cyclic prefix type, bandwidth and duplexing modes.

Currently let us focus on a 10MHz, FDD, Normal CP type downlink frame structure, with following configuration,

  • Subframe number –…
  • Duplexing mode – FDD
  • System bandwidth – 10Mhz
  • Number…

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LTE X2 Handover Sequence Diagrams

Let’s examine the X2 Handover in detail. We look at the X2 handover signaling procedure through sequence diagrams that focus on different aspects of the procedure.

The sequence diagrams presented here were generated with EventStudio System Designer.

LTE X2 Handover

Full signaling details are presented here.

Overview

Now we examine the same flow at a higher level of abstraction. The diagram focuses on the interactions between the mobile, eNodeBs and the MME/SGW.

UE Interactions

We now explore the signaling procedures that involve the UE.

Source eNodeB Role

Examine the interactions that involve the eNodeB that initiated the handover.

Target eNodeB Role

We now look at the interactions involving the eNodeB that will be serving the UE after the handover.

RRC Signaling the X2 Handover

The Radio Resource Control (RRC) signaling between the UE and the eNodeBs is covered here.

X2AP Signaling Between eNodeBs

X2AP is used for signaling between the eNodeBs. Here we examine the X2AP interactions.

Data Path Changes During an X2 Handover

The data path switching goes through several steps to accomplish a seamless handover.


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LTE X2 Handover Between eNodeBs Served by the same MME

LTE eNodeBs can directly communicate with other eNodeBs on the X2 interface. The X2 interface is used to perform a handover between eNodeB.

The messaging in X2 handover is detailed in LTE X2 Handover Presentation. A few excerpts from the presentation as shown below.

X2 Handover Sequence Diagram

The X2 handover flow is shown in the following sequence diagram:

Figure 1 X2 Handover Sequence Diagram

Handover Preparation

The handover procedure is triggered by the X2AP Handover Request message. The RABs to be handover over are sent from the source eNodeB to the target eNodeB.

Figure 2X2AP Handover Request

The target eNodeB then admits the user and responds with X2AP Handover Request Acknowledge message. This message contains a transparent container that carries the Handover Command message that needs to be sent to the UE.

Figure 3 X2AP Handover Request Acknowledge

The source eNodeB sends the handover command to the UE. It then sends sequence number information to the target eNodeB.

The target eNodeB then requests the MME to switch the path from the source eNodeB to target eNodeB.

Handover Execution

Figure 4 S1AP Path Switch Request

This was an overview of the messaging involved in the X2 handover. For details refer to the LTE X2 Handover Presentation.