This document provides an overview of HSDPA (High Speed Downlink Packet Access), which was introduced in 3GPP Release 5 to improve downlink packet data performance in WCDMA networks. Key aspects of HSDPA discussed include fast link adaptation, hybrid ARQ with soft combining, channel-dependent scheduling, higher order modulation, connection handling and mobility support, and capacity management functions. HSDPA utilizes a new high-speed downlink shared channel (HS-DSCH) to deliver packet data to user equipment at higher speeds than previous WCDMA networks.

1. HSDPA Overview

2. HS
DP
igh
peed
ownlink
acket
ccess
A

3. HSDPA General Principles
• HSDPA was introduced in 3GPP Release 5
• HSDPA is an optional feature in WCDMA RAN
• HSDPA includes the following key function:
• Fast link adaptation
•Fast hybrid ARQ with soft combining
•Fast channel-dependent scheduling

4. HSDPA General Principles (cont)
3GPP Release 5 extends the specification of WCDMA with a new
downlink transport channel for packet data:
HS-DS
CH
igh
peed
ownlink
hared
hannel
5
mHz
CHA
NNEL
UHNSUDSPEAD
VOICE
TRAFFIC
OVERHEAD

5. Key Functions

6. Fast link adaptation
 Fast adjustment ooff ddaattaa rraattee eevveerryy TTTTII((22mmss))
 TTrraannssmmiissssiioonn ppoowweerr rreemmaaiinnss ccoonnssttaanntt dduurriinngg aa TTTTII
 TToo ffuurrtthheerr iinnccrreeaassee ccaappaacciittyy aanndd ddaattaa rraatteess 1166--
qquuaaddrraattuurree aammpplliittuuddee mmoodduullaattiioonn ((1166QQAAMM)) mmaayy bbee
uusseedd

7. Fast hybrid ARQ with soft combining
 The UE can rapidly rreeqquueesstt rreettrraannssmmiissssiioonn ooff
eerrrroonneeoouussllyy rreecceeiivveedd ddaattaa
 PPrriioorr ttoo ddeeccooddiinngg tthhee UUEE ccoommbbiinneess iinnffoorrmmaattiioonn
ffrroomm tthhee oorriiggiinnaall ttrraannssmmiissssiioonn wwiitthh tthhaatt ooff llaatteerr
rreettrraannssmmiissssiioonnss ((ssoofftt ccoommbbiinniinngg))

8. Fast channel-dependent scheduling
 The scheduler ddeetteerrmmiinneess tthhee UUEE ttoo wwhhiicchh tthhee
sshhaarreedd cchhaannnneell ttrraannssmmiissssiioonn iiss ddiirreecctteedd aatt aannyy
ggiivveenn mmoommeenntt
 CChhaannnneell--ddeeppeennddeenntt sscchheedduulliinngg mmeeaannss tthhaatt tthhee
sscchheedduulleerr mmaayy ccoonnssiiddeerr iinnssttaannttaanneeoouuss rraaddiioo
cchhaannnneell ccoonnddiittiioonnss

9. To enable fast lliinnkk aaddaappttaattiioonn,, ffaasstt hhyybbrriidd
AARRQQ,, aanndd ffaasstt sscchheedduulliinngg,, aa nneeww mmeeddiiuumm--
aacccceessss ccoonnttrrooll ssuubbllaayyeerr ((MMAACC--hhss)) hhaass
bbeeeenn ppllaacceedd iinn tthhee RRBBSS..
TThhee MMAACC--dd aanndd RRLLCC llaayyeerrss iinn tthhee RRNNCC aarree
rreettaaiinneedd

10. Channel Structure

11. The channels introduced with HSDPA
share the same carrier as other channels.
No additional spectrum needs to be
allocated to introduce HSDPA services.
The transport channel HS-DSCH is
mapped on one or several High-Speed
Physical Downlink Shared Channels (HS-PDSCHs)
which are simultaneously
received by the UE.

12. 3GPP standard allows up to 15 HS-PDSCs
per cell with the spreading factor fixed at
16.
In the initial release of UE’s and WCDMA
RAN, each cell has up to 5 HS-PDSCHs.
Channelization codes from the shared
code resource are dynamically allocated
by the RBS every 2 ms.

13. Time multiplexing is the primary means of
sharing common code resources among
users.
Code multiplexing is useful for providing
small payloads or supporting UEs that
cannot despread the full set of codes when
the number of codes is high (>5).

14. The downlink High-Speed Shared Control
Channel (HS-SCCH) carries control
information from the MAC-hs in the RBS to
the scheduled UE.
In WCDMA RAN, the HS-DSCH is shared
in the time domain.
Since only the currently scheduled UE
needs to receive the HS-SCCH, there is
only one such channel configured in each
cell.

15. An uplink channel, the High Speed
Dedicated Physical Control Channel (HS-DPCH)
must be set up for each UE that
uses high-speed services.
Channel quality reported by means of CQI
is used by the scheduling and link
adaptation functions explained further in
HSDPA User Plane.

16. UEs that use high-speed services must
always have a set of dedicated uplink and
downlink channels.
These dedicated channels use soft
handover while HS-PDSCH and HS-SCCH
do not.

17. Physical Channel Structure
DPCCH, DPDCH
DPCH
HS-PDSCH
HS-SCCH
HS-DPCCH
DPCH
DPCCH, DPDCH
UE

18. Iub Flow Control
There is flow control between the RNC and
the RBS, explained further in HSDPA User
Plane.
Iub flow control handles the trade-off
between having enough data in the MAC-hs
buffers to fully utilize the available air
interface resources and keeping the MAC-hs
buffers short to decrease the memory
space.

19. Scheduling
The scheduler is part of the MAC-hs in the
RBS, explained further in HSDPA User
Plane.
An increase in capacity can be obtained if
the scheduler employs channel-dependent
scheduling (proportional fair scheduling)
instead of resources sequentially (round-robin
scheduling).

20. Scheduling
As the load of a cell increases, the number
of UEs queued for scheduling increases.
This in turn raises the probability of
scheduling UEs with good channel quality.
There are two kinds of variations in
channel quality, short-term and long-term.

21. For many packet data applications,
relatively large short-term variation in
service quality are acceptable, while long-term
variations must be restricted.
A practical scheduling strategy utilizes the
short-term variations and maintains some
degree of long-term fairness between the
users.

22. Scheduling comparison from
Parsippany Trial

23. Link Adaptation and
Higher Order Modulation
Link adaptation and higher order
modulation can be combined to maximize
the instantaneous use of the fading
channel
The HS-DSCH adjusts the data rate to
match the instantaneous radio conditions
and the available transmission power in
the RBS.

24. Link Adaptation and
Higher Order Modulation
The system adjusts the data rate by
varying the effective code rate, changing
the modulation scheme, and changing the
number of codes (TFRC selection, see
HSDPA User Plane)
Besides QPSK, the HSDSCH can use
16QAM to provide higher data rates.

25. Hybrid ARQ with
Soft Combining
The hybrid ARQ mechanism allows the UE
to rapidly request retransmissions of
erroneously received transport blocks (see
HSDPA user plane.)
The UE attempts to decode each transport
block reporting to the RBS its success or
failure 5 ms after the reception of the
transport blocks.

26. Hybrid ARQ with
Soft Combining
During retransmission, the UW employs
soft combining.
1st Decoding in UE 2nd Decoding in UE Final Picture

27. Hybrid ARQ with
Soft Combining
The retransmission may be sent by Chase
combining or by incremental redundancy.
Chase combining is used in WCDMA RAN

28. Hybrid ARQ with
Soft Combining
The round-trip time for the MAC-hs
protocol is about 12 ms, which is longer
than TTI (2 ms).
To avoid having to wait for ACK/NACK
indicator from the UE before the next PDU
is transmitted, the RBS supports multiple
hybrid ARQ processes, see HSDPA User
plane.

29. Compared with a 3GPP R99 UE, an HS-DSCH-
enabled UE must contain
processing capacity for the hybrid ARQ
operation, multi-code processing, HS-SCCH
reception, and HS-DPCCH
signaling.
User Equipment

30. Maximum Theoretical Data Rates
HD-DSCH
category
Max #
of
Codes
Min TTI
Interval
Max
Transport
Block
Size
Total # of
soft
channel bits
Max
Modulatio
n Scheme
Max
Frames per
second
Max data
rate (Mbps)
Category 1 5 3 7298 19200 16QAM 167 1.2
Category 2 5 3 7298 28800 16QAM 167 1.2
Category 3 5 2 7298 28800 16QAM 250 1.8
Category 4 5 2 7298 38400 16QAM 250 1.8
Category 5 5 1 7298 57600 16QAM 500 3.6
Category 6 5 1 7298 67200 16QAM 500 3.6
Category 7 10 1 14411 115200 16QAM 500 7.2
Category 8 10 1 14411 134400 16QAM 500 7.2
Category 9 15 1 20251 172800 16QAM 500 10.1
Category 10 15 1 27952 172800 16QAM 500 14.0
Category 11 5 2 3630 14400 QPSK 250 0.9
Category 12 5 1 3630 28800 QPSK 500 1.8

31. Connection Handling and
Mobility
In the network it is possible to have cells
where the HS-DSCH is enabled and other
cells where it is disabled. Furthermore,
there are UEs with and without HSDPA
capability.
At RAB establishment, the UE makes a
HS-DSCH cell selection, (see Connection
Handling)

32. Connection Handling and
Mobility
If the HS-DSCH is disabled in the best cell
but enabled in a cell that has a coverage
relation to the best cell and if the UE is
close enough to the RBS, the connection
is set up in the latter cell.
If this feature is not supported or if the HS-DSCH
is disabled in the best cell, the
connection is established on an interactive
DCH.

33. Connection Handling and
Mobility
When a new cell in the active set becomes
the best cell, or when the current serving
HS-DSCH cell is to be removed from the
active set for some reason, the UE makes
a serving HS-DSCH cell change, (see
Handover)

34. Connection Handling and
Mobility
The bit rate of the downlink user data
depends on the HS-DSCH UE category,
the number of HS=PDSCH codes, and
whether 16QAM is used.
Theoretically, the max bit rate is 2.08
Mbps using QPSK and 4.32 Mbps using
16QAM for the initial release of UEs and
WCDMA RAN, (see HSDPA User Plane)

35. Connection Handling and
Mobility
There are two interactive RABs, with
different uplink rates, that support HS-DSCH.
An uplink rate of 64 kbps or 384 kbps is
selected when the RAB is established,
(see Connection Handling)

36. Connection Handling and
Mobility
There is no channel switching between the
uplink rates.
In case of inactivity in the user data
transmission uplink and downlink, the
connection is released, (see Channel
Switching)

37. Connection Handling and
Mobility
The power of the HS-DSCH is give as an
offset relative the DPCCH; there are
different values depending on whether or
not the UE is in soft handover.
Repetition factors indicate how many
times the CQI and ACK/NACK indicators
are transmitted on the HS-DPCCH, (see
Power Control)

38. Connection Handling and
Mobility
The power of the HS-SCCH is
configurable and given as an offset
relative the P-CPICH.
The ordinary power control of the
dedicated channels is not affected by the
introduction of the HS-DSCH.

39. Capacity Management
The admission control and congestion
control functions control the load in the
WCDMA cell with support from other radio
network functions, (see Capacity
Management)
It is possible to set the load thresholds for
when the non-HS-DSCH traffic should be
rejected.

40. Capacity Management
If the total load of the cell becomes too
high, the congestion control is able to
release radio links in a predetermined
priority order, to decrease load.

41. Load Sharing
The inter-frequency load sharing function
distributes the call accesses between
cells, to even out the traffic load between
cells on different frequencies coverage the
same area, (see Load Sharing)

42. Questions and discussion

43. References
[1] UE Radio Access Capabilities, 3GPP TS 25.306
[2] HSDPA Overview WCDMA RAN User Description, Ericsson 91/1553-HSD 101 02/4 Uen A
[3] Mastering HSDPA, Award Solutions, Inc.

44. Thank you for your participation!