HSDPA
By Mina Adly
HSDPA new channels
 HS-DSCH and HS-PDSCH
 HS-DSCH is a high-speed downlink shared channel. Its TTI is fixed to 2 ms. It may be
mapped onto one or more HS-PDSCHs.
 HS-PDSCH is a high-speed physical downlink shared channel. Its spreading factor is fixed to
16. According to 3GPP TS 25.433, a maximum of 15 HS-PDSCHs can be used for transmission
at the same time. The number of HS-PDSCHs per cell is configurable.
 The use of 2 ms TTI reduces the round trip time (RTT) on the Uu interface and, together with
AMC, improves the tracking of channel variations. In addition, the use of 2 ms TTI enables
fast scheduling and resource allocation and therefore improves the usage of transmission
resources.
 In each TTI, HSDPA assigns the HS-PDSCHs onto which the HS-DSCH maps. More HS-PDSCHs
can provide higher transmission rates.
 Unlike the DCH, the HS-DSCH cannot support soft handover. The reason is that this type of
handover requires different cells to use the same radio resource for sending the same data
to the UE, but the scheduling function can be performed only within the cell.
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HSDPA new channels
 HS-SCCH
 HS-SCCH is a high-speed shared control channel. It carries the control
information related to the HS-PDSCH. The control information includes the
UE identity, HARQ-related information, and information about transport
format and resource combination (TFRC). For each transmission of the HSDSCH, one HS-SCCH is required to carry the related control information.
One cell can be configured with several HS-SCCHs. The number of HSSCCHs determines the maximum number of UEs that can be scheduled
simultaneously in each TTI.
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HSDPA new channels
 HS-DPCCH
 HS-DPCCH is a high speed dedicated physical control channel. In the
uplink, each HSDPA UE must be configured with an HS-DPCCH. This channel
is mainly used by the UE to report the CQI and whether a transport block is
correctly received. The information about the transport block is used for fast
retransmission at the physical layer. The CQI is used for AMC and
scheduling to allocate Uu resources
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HSDPA new channels
 DPCCH and DPCH/F-DPCH
 DPCCH is a dedicated physical control channel in the uplink. DPCH is a
dedicated physical channel in the downlink. F-DPCH is a fractional dedicated
physical channel in the downlink.
 The HSDPA UE must be configured with dedicated physical control channels in
both the uplink and the downlink.
 The uplink DPCCH is used for closed-loop power control by working with the
DPCH or F-DPCH. In addition, the uplink DPCCH power is used as a reference for
the HS-DPCCH power.
 The downlink DPCH is used for inner-loop power control and as a reference for
the HS-PDSCH power.
 Like the downlink DPCH, the F-DPCH is also used for inner-loop power control.
The difference is that each UE must have a downlink DPCH (SF256) whereas 10
UEs can share an F-DPCH (SF256) to save downlink channel codes.
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HSDPA resource management
 Power resource and HSDPA
 Cell remaining power can not
be less than the power margin
parameter PwrMgn
 The HSDPA power can not be
more than HspaPower
parameter
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HSDPA resource management
 Code resource
 First we need to know the number of consumed codes for HSDPA service
1. HS-SCCH uses an SF128 code, each channel define how many users per one TTI,
normally we set to 4 but in this case but this will take one code SF16 to be closed
which mean we can use only 14 codes maximum for HS-PDSCH
2. If we want to use 15 codes for HS-PDSCH we have to set the codes of HS-SCCH
to 2 maximum
3. Setting for this part is done in the parameter HsScchCodeNum
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HSDPA resource management
 Code resource
 First we need to know the number of consumed codes for HSDPA service
 For the HS-PDCH code allocation we have three modes
1. RNC static mode:

AllocCodeMode is set to static
 Cell fix the HSDPA codes to the value of the parameter HsPdschCodeNum only
with no change
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HSDPA resource management
 Code resource
 First we need to know the number of consumed codes for HSDPA service
 For the HS-PDCH code allocation we have three modes
3. NodeB dynamic mode:

DynCodeSw is set to yes
 In this case the nodeB will assign all free codes to HSDPA and when needed by
other service the codes will be released from HSDPA and used by the other
services
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HSDPA resource management
 Code reshuffling to free codes for HSDPA:
 To enable the function below parameters should be used
1. CodeAdjForHsdpaSwitch (switch function enable)
2. CodeAdjForHsdpaUserNumThd (maximum number of users for code reshuffling)
M.Adly
HSDPA resource management
 IUB BW resource:
 The total Iub bandwidth available for HSDPA depends on the variations in
HSDPA packet delay and the situation of packet loss. HSDPA shares the
bandwidth with the DCH and control signaling, and the DCH and control
signaling has higher priorities than HSDPA. Therefore, when the HSDPA packet
delay or packet loss increases, you can infer that the number of DCHs or the
amount of control signaling increases. In such a case, the bandwidth available
for HSDPA decreases and the bandwidth actually allocated for HSDPA
decreases)
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HSDPA scheduling
 Scheduling is one of the main advantages of HSDPA, so how it works?
 We have 5 different scenarios or methods listed below:
1. MAXCI Algorithm: the user with higher CQI is served, no fairness (jungle life)
2. Round Robin algorithm: first in first out data should reach all, too fair waste of resources
3. Proportional Fair Algorithm: It provides the user with an average throughput that is
proportional to the actual channel quality
throughput corresponding to the CQI reported by the UE/throughput achieved
4. EPF: enhanced way to have more efficiency to assign resources
5. EPF_LOC Algorithm: consider the location of the UE to give higher priority to the near
users
M.Adly
HSDPA scheduling
 Scheduling is one of the main advantages of HSDPA, so how it works?
 How EPF works:
 Different services of the same type are prioritized as follows:
1. Retransmission queues are prioritized over initial transmission queues.
2. Guaranteed bit rate (GBR) queues that have not arrived are prioritized over GBR
queues that have already arrived.
3. Queues with high SPI weights are prioritized over those with low SPI weights.
4. High bit rate (HBR) queues that have not arrived are prioritized over HBR queues that
have already arrived The HBR is specified by the parameter HappyBR
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HSDPA time and codes multiplexing
 The feature of time and HS-PDSCH codes multiplex enables the allocation of different
codes in the same TTI to different users or the time division multiplexing of the same
code in different TTIs for different users to provide the utilization of code resources and
the system throughput.
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HSDPA HARQ
 The HARQ process of HSDPA involves only the NodeB and the UE, without involving the
RNC. After receiving a MAC-hs PDU sent by the NodeB, the UE performs a CRC check
and reports an ACK or NACK on the HS-DPCCH to the NodeB:
 l If the UE reports an ACK, the NodeB transmits the next new data.
 l If the UE reports an NACK, the NodeB retransmits the original data. After receiving the
data, the UE performs soft combining of this data and the data received before,
decodes the combined data, and then reports an ACK or NACK to the NodeB
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TFRC selection
 In case that the TBS is fixed (the UE has reached his needed buffer size) the network
start degrading his resources to have more resource utilization, this can be done
based on three methods:
 Code priority / power priority / power code balance
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CQI Adjustment Based on Dynamic BLER
Target
 The CQI measures the channel conditions of a UE and is reported from the UE to the
NodeB. Without this feature, the NodeB determines an appropriate TBS based on the
reported CQI, system resources, and the TFRC policy. If the reported CQI and related
conditions remain the same, the NodeB does not change the TBS because it does not
consider the ever-changing radio environments
 With the feature CQI adjustment based on dynamic BLER target, the NodeB monitors
the channel quality fluctuations for HSDPA users in a cell in real time and dynamically
selects a proper BLER target based on the monitoring result. The NodeB then uses the
BLER target to adjust the CQI reported by the UE. Based on the adjusted CQI, the
NodeB determines an appropriate TBS to achieve higher downlink throughput for
HSDPA users and higher cell throughput
 Enabled through CQI_ADJ_BY_DYN_BLER
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CQI Adjustment Based on Dynamic BLER
Target
 The feature procedure is as follow:
 Based on the CQI reported by the UE, the NodeB checks the actual radio
environment, which is affected by multipath effects and UE mobility.
 Based on the actual radio environment and channel quality of the UE, the NodeB
obtains an optimum BLER target, which helps to achieve the highest possible
throughput for the UE.
 Based on the ACK, NACK, or DTX indication from the UE in the current TTI and on the
optimum BLER target, the NodeB calculates the CQI offset, which can be a positive or
negative number. The NodeB then uses the CQI offset to adjust the CQI.
 Based on the adjusted CQI, the NodeB selects an appropriate TBS by using the TFRC
algorithm.
M.Adly
Channel modulation of HSDPA
 In HSDPA we have three modulation techniques, each one can provide
different peak rate, but shows lower ability for channel reliability below is
the list :
 QPSK up to 7.2 Mb/s, high channel reliability
 16 QAM up to 14.4 Mb/s, medium channel reliability
 64 QAM up to 21 Mb/s, lower channel reliability
M.Adly