User's Manual

Ver.
CHAPTER 2. System Overview
2600-00FKQFGA2
1.0
H-ARQ
The H-ARQ is a retransmission method in the physical layer, which uses the stop-and-wait
protocol. The eNB provides the 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 the 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 the asynchronous method for downlink and the synchronous method for
uplink.
Power Control
When transmitting a specific data rate, too high a power level may result in unnecessary
interferences and too low a 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 the LTE system.
Therefore, the 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 the LTE downlink, the eNB can reduce inter-cell interference by transmitting data at
adequate power levels according to the location of the UE and the MCS, which results in
improvement of the entire cell throughput.
MIMO
The eNB can support the MIMO by using multiple antennas. For this purpose, the channel
card of the eNB has the baseband part to process the MIMO, and individual RF paths can
be processed separately. The eNB supports various types of the MIMO to provide the high-
performance data service.
The eNB uses multiple antennas to support the MIMO. MIMO has the following schemes.
Direction Item Description
Downlink SFBC (Space Frequency
Block Coding)
This scheme implements the space-time block coding
(STBC) on frequency instead of on time for increased
reliability of the link. The Alamouti codes are used.
SM (Spatial Multiplexing) Different data are divided to multiple antenna paths for
transmission so as to increase the peak data rate.
(Each path uses the same time/frequency resource.)
- Single User (SU)-MIMO: The SM between one eNB and
one UE to increase peak data rate for one UE.
- Open-loop SM: The SM that works without the
Precoding Matrix Indicator (PMI) feedback of the UE
when the UE’s channel is unknown or changes fast due
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