System Level Performance Analysis of Advanced Antenna ... - Centers

Introduction
adaptive variation of the transmission parameters is referred to as link adaptation (LA), and
can be done every 2 ms (i.e. three slots), which is the duration of the radio frame, also referred
to as Transmission Time Interval (TTI). The channel quality estimates for the LA algorithm
can be obtained from the CLPC commands for the associated DL DPCH, the ACK/NACK
ratio or the CQI sent by the UEs through the UL HS-DPCCH [33]. Note that fast power
control is not allowed on the HS-PDSCHs.
A fast physical layer retransmission scheme with chase combining or incremental
redundancy (Hybrid Automatic Repeat reQuest, H-ARQ) is specified, which provides time
diversity and facilitates faster retransmissions than in Release’99, where this process is
handled by the RLC functionality at the RNC. In addition, the PS functionality is also moved
from the RNC to the Node-B, which enables the possibility to consider the fast variations of
the radio channel in the scheduling decisions, which can be made on a TTI basis. The
operational principle of HSDPA is illustrated in Figure 1.7, and Table 1.1 provides a
summary of the key properties of DCH and HS-DSCH [1], [34].
Channel quality feedback
(CQI, Ack/Nack, CLPC
commands)
UE1
UE2
Data
Channel quality feedback
(CQI, Ack/Nack, CLPC
commands)
Data
Figure 1.7: HSDPA operational principle.
1.3 Assessment methodology
Constant transmit power
Adaptive modulation & coding
Fast packet scheduling
Fast layer 1 retransmissions
(HARQ)
Fast scheduling is done directly in
Node-B based on feedback
information from the UE and
knowledge of current traffic state.
As shown in Figure 1.8, there are several approaches to evaluate the performance of
cellular systems. Each of them has advantages and disadvantages. Typically, simple
theoretical studies involve low cost, and can give a reasonable approximation by using simple
mathematical models. However, more detailed studies are problematic due to the high
complexity of UMTS.
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