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. ** * *
![LTE PHY Layer Measurement Guide...4 LTE PHY Layer Measurement Guide LTE Downlink The LTE downlink can be set on six different frequency profiles, as follows: Channel Bandwidth [MHz]](https://img.pdfslide.net/doc/110x75/5e9903898496907a812cd628/lte-phy-layer-measurement-guide-4-lte-phy-layer-measurement-guide-lte-downlink.jpg)
