User's Manual

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Chapter 2 LTE eNB Overview
LTE eNB System Description v2.0 15
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In addition, when all sub-carriers are divided for multiple users, eNB can select
and assign to each subscriber a sub-carrier with the most appropriate features using
the OFDMA scheme, thus to distribute resources efficiently and increase data
throughput.
In case of uplink SC-FDMA, which is similar to OFDMA modulation and
demodulation, a Discrete Fourier Transform (DFT) is applied to each subscriber in
the modulation at the transmitting side. An inverse Discrete Fourier Transform
(IDFT) is applied for minimizing the Peak to Average Power Ratio (PAPR) at the
transmitting side, which allows continuous allocation of frequency resources
available for individual subscribers. As a result, eNB can reduce the power
consumption of the UE.
Downlink Reference Signal Creation and Transmission
The UE must estimate downlink channel to perform the coherent demodulation on
the physical channel in LTE system. The LTE uses OFDM/OFDMA-based
methods for transmitting and therefore the channel can be estimated by inserting
the reference symbols from the receiving terminal to the grid of each time and
frequency. These reference symbols are called downlink reference signals, and
there are 2 types of reference signal defined in LTE downlink.
 Cell-specific reference signal: The cell-specific reference signal is transmitted
to every subframe across the entire bandwidth of the downlink cell. It is
mainly used for channel estimation, MIMO rank calculation, MIMO precoding
matrix selection and signal strength measurement for handover.
 UE-specific Reference Signal: The UE-specific reference signal is used for
channel estimation for coherent demodulation of DL-SCH transmission where
the beamforming method is used. UE-specific means that the reference signal
is used for channel estimation of a specified UE only. Therefore, the UE-
specific reference signal is used in the resource block allocated for DL-SCH
only, which is transmitted to the specified UE.
Downlink Synchronization Signal Creation and Transmission
The synchronization signal is used for initial synchronization when UE starts to
communicate with eNB.
There are two types of synchronization signals:
 Primary Synchronization Signal (PSS)
 Secondary Synchronization Signal (SSS)
The UE can obtain cell identity through the synchronization signal. It can obtain
other information about cell through the broadcast channel. Since synchronization
signals and broadcast channels are transmitted in 1.08 MHz range, which is right
in the middle of cell’s channel bandwidth, UE can obtain the basic cell information
such as cell ID regardless of the transmission bandwidth of eNB.
MBSFN Reference Signal Creation and Transmission
In the enhanced/evolved Multimedia Broadcast Multicast Services (eMBMS)