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LTE Downlink Transmission Technique Impl

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LTE Downlink Transmission Technique Impl

Text of LTE Downlink Transmission Technique Impl

  • Downlink Transmission Technique

  • Introduction

  • Downlink Transmission TechniqueDownlink?User dataSignaling



  • Scope and ObjectivesDescribe the Downlink Transmission TechniqueExplain the OFDM principleDescribe the use of Reference Signals in DownlinkDescribe the Control Signaling in Downlink


    Downlink Transmission TechniqueOFDMReference SignalsControl Signaling

  • Pre-TestPre-test

  • Pre-Test

  • Click on this button to skip the course and go to EriColl to register your attempt.

    Click this button to go to the first topic in the course.Browse or ProceedMake your choice!

  • Overview

  • LTE Radio InterfaceOFDM/OFDMA

    Time and frequency grid

    L1/L2 Signaling

  • Downlink Transmission Technique

  • LTE Downlink TransmissioneNBOFDM/OFDMA



  • LTE DL Physical Resourcef = 15 kHzScheduling Block (SB) = 2 Resource Blocks, 1 ms

  • Exercise

  • OFDM PropertiesIFFTCP insertion< Coherence timeDelay spread
  • Exercise

  • Reference SignalsOne slot (0.5 ms)Cell-specific reference signalsMBSFN reference signalsUE-specific reference signals

  • Control SignalingReference symbolsDownlink Scheduling AssignmentsUplink Scheduling GrantsHARQ AcknowledgementsControl RegionData Region

  • Channels for L1/L2 ControlLogical ChannelsTransport ChannelsDownlinkPhysical Channels

  • Summary

  • SummaryOFDM transform data into subcarriersOFDMA separate in frequency & timeResource BlockReference SignalsControl Signaling

  • Quiz

  • Quiz

  • Evaluation

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  • More Information

    *In this module, we will focus on the technique used for downlink in LTE, which is based on OFDM and OFDMA.

    *Introduction*What is happening in the Downlink when using LTE? Well, in the downlink we can transmit user data to the different UEs and this will also require some signaling. But how does the UE use the downlink and what kind of transmission technique is used? Maybe you have heard about OFDM, but you do not really know what it is? Those questions and more will be answered during this module. *On completion of this module the students will be able to:Describe the downlink transmission technique Explain the OFDM principle, signal generation and processing Describe the use of reference signals in downlink Describe the control signaling in downlink

    Spend your time well. Answer a few questions to se if you already now master the learning objectives of this module. Based on that you will either be recommended to take this course or not. Good luck! * *Click on the left button to go to the first topic in the course, or Click on the right button to skip the course and go to EriColl to register your attempt.Make your choice!

    * *Overview

    *In the downlink a transmission technique called Orthogonal Frequency Division Multiplex (OFDM) will be used. With Orthogonal Frequency Division Multiple Access (OFDMA) it is also possible to schedule the UEs differently in time and frequency. With these techniques the UEs can be scheduled and receive user data in the downlink. That is not all though, because we also need to inform the UEs about for example where the user data is located, in time and frequency. Layer 1 and Layer 2 signaling will solve that issue.

    *Downlink transmission technique*IPTV Template2008-12-09*The LTE radio interface is based on OFDM (Orthogonal Frequency Division Multiplex) and OFDMA (Orthogonal Frequency Division Multiple Access) is used in the downlink (DL).*IPTV Template2008-12-09*This is the basic principle of OFDM. In the eNodeB, when a data stream has been coded and modulated, it reaches the transmitter (Tx) .Here, the data stream is split up to a number of sub-streams. The number of sub-streams can range from typically 12 and up to 1200 (at 20MHz bandwidth). Each stream is fed into an Inverse Fast Fourier Transform (IFFT) block, with size 2048, and mapped onto subcarriers. The number of subcarriers ranges from 12 to 1200. At the receiver (Rx) a reverse process is performed. The OFDM signal is fed into a Fast Fourier Transform (FFT) block, with the same size as the IFFT block . The output of the FFT block corresponds to the input of the IFFT block at the transmitter side. The parallel data stream output is then converted into a serial stream, demodulated, and decoded.*IPTV Template2008-12-09*OFDMA (Orthogonal Frequency Multiple Access) is the multiple access method that separates the users and channels in frequency and time. The different users and/or different channels are orthogonal within one cell. This means that they do not interfere with each other in the same cell. This is valid for both uplink and downlink in LTE.*IPTV Template2008-12-09*The basic LTE downlink physical resource can be seen as a time-frequency grid where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.A Resource block corresponds to twelve OFDM sub-carriers during one 0.5 ms slot.The smallest unit that can be allocated by the scheduler is two consecutive Resource Blocks (12 sub-carriers during 1ms). This is sometimes referred to as a Scheduling Block (SB) and is equal to the TTI (Transmission Time Interval) of 1 ms.*IPTV Template2008-12-09**IPTV Template2008-12-09*Before a subcarrier is transmitted, a cyclic prefix (CP) is inserted . The CP is a copy of the last part of the symbol in order to preserve the subcarrier orthogonality. This is possible since the FFT is a cyclic operation.The symbol time in LTE is chosen to 66.7s . This choice is based on the average radio channel delay spread (a measure of the radio channel time dispersion) and the coherence time (a measure of how slow the radio channel changes).The symbol time should be much longer than the delay spread in order to keep the ISI (Inter Symbol Interference) low. Also, the cyclic prefix should be longer than the expected delay spread in order to completely remove ISI.However, if the symbol time is too long (i.e. longer than the coherence time), the radio channel will change considerably during one symbol. This would lead to inter carrier interference (ICI).*IPTV Template2008-12-09**IPTV Template2008-12-09*The downlink reference signals consist of so-called reference symbols which are known symbols inserted within in the OFDM time/frequency grid. The reference signals can be used by the UE for downlink channel estimation to enable coherent detection.Three types of reference signals are defined for the LTE downlink: Up to four cell-specific reference signals, each corresponding to one downlink antenna port. MBSFN reference signals within MBSFN subframes UE-specific reference signals, sometimes also referred to as dedicated reference signals. *IPTV Template2008-12-09*Downlink L1/L2 control signaling is used for transmitting downlink scheduling assignments required for the terminal to properly receive, demodulate and decode the DL-SCH, uplink scheduling grants informing the terminal about resources and transport format for UL-SCH transmission, and hybrid-ARQ acknowledgements in response to UL-SCH transmission.The downlink L1/L2 control channels are mapped to the first OFDM symbols within the subframe (in the figure two symbols are used). Thus, each subframe can be said to be divided into a control region , followed by a data region . The size of the control region is always equal to an integer number of OFDM symbols (1, 2, 3 or 4 OFDM symbols can be used for control signaling) and can be varied per subframe. This maximizes the spectral efficiency as the control signaling overhead can be adjusted to match the instantaneous traffic situation.*IPTV Template2008-12-09*The downlink L1/L2 control signaling consists of three different physical channel types:PCFICH (Physical Control Format Indicator Channel), informing the terminal about the number of OFDM symbols (1, 2, 3 or 4) used for L1/l2 control signaling in the current subframe. There is only one PCFICH in a cell.PDCCH (Physical Downlink Control Channel), used to carry downlink scheduling assignments and uplink scheduling grants. In addition, it may also be used for power control of a group of terminals. Typically, there are multiple PDCCHs in a cell.PHICH (Physical Hybrid-ARQ Indicator Channel), used to transmit ACK/NAK in response to reception of UL-SCH transmissions. Typically, there are multiple PHICH in a cell. *Summary*IPTV Template2008-12-09*In summary:OFDM transforms a downlink datastream into a number of subcarriers, depending on the bandwidthOFDMA separates the users and channels in frequency and timeA Resource block corresponds to twelve OFDM subcarriers during one 0.5 ms slot. The reference signals can be used by the UE for downlink channel estimation to enable coherent detection.Downlink control signaling is used for transmitting scheduling assignments, uplink scheduling grants and hybrid-ARQ acknowledgements **IPTV Template2008-12-09*When clicking next you will be directed to an EriColl page. It is important that you fill in the form and click OK. This will register your successful attempt. Finish by clicking on the next button. ** * *

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