User's Manual

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Chapter 2 LTE eNB Overview
LTE eNB System Description v2.0 17
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In particular, Modulation Coding Scheme (MCS) is used for changing the
modulation method and channel coding rate according to the channel status. If
channel environment is good, MCS increases the number of transmission bits per
symbol using a high-order modulation, such as 256 QAM. If channel environment
is bad, it uses a low-order modulation, such as QPSK and a low coding rate to
minimize channel errors.
In addition, in the environment where MIMO mode can be used, eNB operates in
MIMO mode to increase the peak data rate of subscribers and can greatly increase
the cell throughput.
If the channel information obtained is incorrect or modulation method of higher
order or higher coding rate than the given channel environment is used, errors may
occur.
In such cases, errors can be corrected by HARQ function.
H-ARQ
The H-ARQ is a retransmission method in the physical layer, which uses the stop-
and-wait protocol. The eNB provides H-ARQ function to retransmit or combine
frames in the physical layer so that the effects of wireless channel environment
changes or interference signal level changes can be minimized, which results in
throughput improvement.
The LTE uses Incremental Redundancy (IR)-based H-ARQ method and regards
the Chase Combining (CC) method as a special case of the IR method.
The eNB uses asynchronous method for downlink and synchronous method for
uplink.
Power Control
When transmitting a specific data rate, too high power level may result in
unnecessary interferences and too low power level may result in an increased error
rate, causing retransmission or delay. Unlike in other schemes such as CDMA, the
power control is relatively less important in LTE. Nevertheless, adequate power
control can improve performance of LTE system.
In LTE uplink, SC-FDMA is used so that there are no near-far problems that occur
in CDMA. However, the high level of interference from nearby cells can degrade
the uplink performance.
Therefore, UE should use adequate power levels for data transmission in order not
to interfere with nearby cells. Likewise, the power level for each UE could be
controlled for reducing the inter-cell interference level.
In LTE downlink, eNB can reduce inter-cell interference by transmitting data at
adequate power levels according to the location of UE and MCS, which results in
improvement of the entire cell throughput.
Inter-Cell Interference Coordination (ICIC)
Since UEs within a cell in LTE use orthogonal resources with no interference
between UEs, there is no intra-cell interference.