HSDPA Well Explained

1
HSDPA
Robert Bestak
Czech Technical University in Prague
Faculty of Electrical Engineering
2006, January 12 – R. Bestak 2
Overview
• Basic characteristics of HSDPA
• MAC entities
Types
Functions
• Physical channels
Data
Signaling
2
2006, January 12 – R. Bestak 3
HSDPA – introduction
HSDPA ….High Speed Downlink Packet Access
• Provides High Speed (HS) downlink data channel
FDD, TDD mode
• Introduce in Release 5
2006, January 12 – R. Bestak 4
HSDPA - goals
• Increase bit rates in downlink (aggregated data rates up to 14.4 Mbit/s)
…asymmetric capacity
• Efficient user scheduling
• Reduce delay
…In comparison with previous WCDMA performance (Release 4),
packet data throughput is increased 50-100% ([H02])
• HSDPA
should be possible to incorporated to previous Releases (Rel’99, Rel’4)
does not affect the layers above the MAC layer (PDCP, RLC,…)
3
2006, January 12 – R. Bestak 5
What brings HSDPA
• New mechanisms
Hybrid ARQ (Node B-UE)
Fast scheduling
Adaptive Modulation and Coding (AMC)
Fast adaptation …rapid adaptation of transmission parameters to
instantaneous channel conditions
• Modification in
Node B
(UE)
2006, January 12 – R. Bestak 6
HSDPA - new features
- Scheduling
- Retransmission (ARQ
RLC
)
- Scheduling
- Fast Retransmission (HARQ
MAC
)
- Adaptive Modulation and Coding
Node B
RNC
UE
N
e
w

c
h
a
n
n
e
l
s
Fast retransmission: HARQ, Incremental redundancy with Chase combining
4
2006, January 12 – R. Bestak 7
New channels
• One transport channel HS-DSCH (High Speed-Downlink Shared Channel),
• Several physical channels …downlink, uplink
Node B
UE
Downlink signaling
(info how to decode data channels)
D
ata chann
els
(H
S
-D
S
C
H
)
Uplink signaling
(feedback information)
2006, January 12 – R. Bestak 8
HS-DSCH
• Downlink shared channel
Shared between number of HSDPA users
• Short TTI …Transmission Time Interval
TTI = 2 ms (or 3 slots)…for FDD mode
…TTI = 10 ms …for TDD mode
• Fix spreading factor, SF = 16
Note: Compare to “classical” WCDMA, variable SF and fast
power control are disabled in HSDPA
Gives info how often data
arrives from MAC to Phy
5
2006, January 12 – R. Bestak 9
HS-DSCH vs. DSCH
Yes No HARQ
Yes No Adaptive modulation and coding
No Yes Fast power control
No Yes Variable spreading factor
HS-DSCH
(Release 5)
DSCH
(Release 99)
Feature
2006, January 12 – R. Bestak 10
HSDPA resources (1/2)
• Resources
Codes
• several channelisation codes are set aside for the HS-DSCH
(according to UE capabilities →max. per UE - 5, 10, 15 codes)
Power
• Sharing
Several codes / 1 user / 1 TTI
Several users / 1TTI
6
2006, January 12 – R. Bestak 11
HSDPA resources (2/2)
SF = 1
SF = 2
SF = 4
SF = 8
SF = 16
TTI
Codes allocated to
HS-DSCH
Time
S
p
r
e
a
d
i
n
g

f
a
c
t
o
r
(
S
F
)
…
…
…
…
…
…
Codes allocated in a TTI (example)
¤
¤
*
*
*
*
2006, January 12 – R. Bestak 12
New MAC entity
Node B
RNC
UE
MAC
RLC
RRC
Phy
MAC
RLC
RRC
Phy
MAC-hs MAC-hs
MAC-hs controls access to the HS-DSCH
There is one MAC-hs entity (Node-B) per HSDPA cell
7
2006, January 12 – R. Bestak 13
Functions of MAC-hs/Phy
• Adaptive Modulation and Coding
• Hybrid ARQ
• Assembly/Disassembly
• Numbering
• In-sequence data delivery (reordering)
• Flow control (Node B - RNC)
• Transport Format & Resource info.
• Fast scheduling
Lower part of MAC-hs/Phy
Upper part of MAC-hs/Phy
2006, January 12 – R. Bestak 14
Adaptive Modulation and Coding (AMC)
• Provides fast adaptation to the instantaneous channel
condition
Based on fast channel feedback received from the mobile
Short TTI
• Features
Coding rate: 1/4 ÷ 4/4
Modulation: QPSK, 16 QAM (…64QAM under investigation)
8
2006, January 12 – R. Bestak 15
HARQ
HARQ entity
HARQ entity
HARQ
Node B
RNC
UE
HARQ entity
HARQ entity
HARQ entity
HARQ entity
HARQ mechanisms are handled by HARQ entity
There is one HARQ entity
per UE in Node
2006, January 12 – R. Bestak 16
HARQ
• Chase Combining
Simplest form of Hybrid ARQ
Number of repeats of each coded data packet are sent
Decoder combines multiple received copies of the coded packet
weighted by the SNR prior to decoding.
• Incremental redundancy
Instead of sending simple repeats of the entire coded packet,
additional redundant information is incrementally transmitted if the
decoding fails on the previous attempt
9
2006, January 12 – R. Bestak 17
HARQ entity
HARQ entity
HARQ
process
…
HARQ
process HARQ
process
Physical channel(s)
ARQ
Stop and Wait
MAC-hs PDU
• HARQ entity
supports multiple HARQ processes
• 1 process = 1 Stop&Wait protocol
• 1..to 8 active S&W processes per entity
• pot of HARQ processes is common for all
live queues
• HARQ process
manages (re)transmission of MAC-hs PDU
Queue 1 Queue 2 Queue 3
1UE = 1 or several priority queues
2006, January 12 – R. Bestak 18
MAS-hs PDU
Queue Id TSN SID
1
N
1
F
1
. . . MAC-d PDU MAC-d PDU
Padding
(opt)
...
MAC-hs payload
SID
k
N
k
F
k
MAC-hs header
Queue Identifier (3 bits) - identifies the reordering queue
Flag (1 bit) idandifies if the next field is a SID or data
Transmission Sequence Number (6 bits)
Size Index Identifier (3 bits) - identifies the size of a set of
consecutive MAC-d PDUs
Number of MAC-d PDUs (7 bits) - identifies the number
of consecutive MAC-d PDUs with equal size
10
2006, January 12 – R. Bestak 19
TFRC
• TFRC = Transport Format & Resource Combination
• Info about
Applied channelisation codes
( channelisation code = physical channel)
Applied modulation (…QPSK, QAM)
Selected transport bloc size
A block exchanges
between MAC and Phy
2006, January 12 – R. Bestak 20
Function of MAC-hs
Node B
RNC
UE
MAC-hs MAC-hs
• HARQ
• Disassembly
• In-sequence delivery (reordering)
• HARQ
• Assembly
• TFRC Selection
• Flow control (Node B-RNC)
• Scheduling
11
2006, January 12 – R. Bestak 21
Physical channels
Node B
UE
D
o
w
n
lin
k
sig
n
alin
g
(In
fo
h
o
w
to
d
eco
d
e d
a
ta
ch
a
n
n
el)
H
S
-S
C
C
H
D
ata chan
nels (da
ta)
H
S
-P
D
S
C
H
HS-SCCH: HS Shared Control Channel
HS-PDSCH: HS Physical Downlink Shared Channel
HS-DPCCH: A Dedicated Physical Control Channel
U
p
lin
k
sig
n
alin
g
(feed
b
a
ck info
)
H
S
-D
P
C
C
H
2006, January 12 – R. Bestak 22
HS-Physical Downlink Shared Ch.
(HS-PDSCH)
• Carries data of HS-DSCH (…Downlink Shared Ch.)
• 1 HS-PDSCH ⇔1 channelisation code
Fix SF, SF=16
• Transport channel (HS-DSCH) is mapped to {1 or several HS-PDSCH} / per TTI
# simultaneous HS-PDSCH = # codes =1...5, 7, 8, 10, 12,15)
slot 1 slot 2 slot 0 slot 4 slot 5 slot 3 slot 7 slot 8 slot 6 slot 10 slot 11 slot 9 slot 13 slot 14 slot 12
1 radio frame, T
r
= 10 ms
HS-PDSCH frame, T
hs
= TTI = 2 ms
960
480
Channel
bit rate (kbit/s)
640 1920 16 16 QAM
320 960 16 QPSK
Bits/Slot Bits/Frame
HS
SF Modulation
For coding rate 4/4 and 15 channels
⇒14,4 Mbit/s
12
2006, January 12 – R. Bestak 23
HS-Shared Control Ch.
(HS-SCCH)
• Carries information how to decode data on the HS-PDSCH
Modulation and coding, transport format, and HARQ information
• Features of the channel
Is power controlled
Channel rate 60 kbit/s (SF =128, 40 bit/slot, HS-SCCH frame = 3 slots)
Precedes transmission of HS-PDSCH by 2 slots
3 Slots ( = 2 ms)
2 Slots (= 1,33) ms
HS-PDSCH frame
HS-SCCH
HS-PDSCH
HS-SCCH frame
3 Slots ( = 2 ms)
2006, January 12 – R. Bestak 24
HS-Dedicated Physical Control Ch.
(HS-DPCCH)
• Carries uplink feedback signaling related to downlink transmission
Hybrid ARQ Ack (1 bits) and Channel Quality Indication (CQI, 5 bits)
• Features of the channel
Multiplexed with dedicated uplink physical channels (DPCCH, DPDCH)
Channel rate 15 kbit/s (SF =256, 10 bit/slot, HS-DPCCH frame = 3 slots)
τ ≈ 7,5 Slosts
DPCH ↑ Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 8 Slot 9 Slot 10 Slot 11 Slot 12
HS-PDSCH sub-frame
(3 slosts)
HS-PDCCH sub-frame
(3 slosts)
HS-DPCCH ↑
Slot 13
13
2006, January 12 – R. Bestak 25
Physical channels - overview
Node B -Tx
TTI slot
HS-PDSCH
HS-SCCH
• Channelization code set
(i.e. {HS-PDSCHs}), [7 bits]
• Modulation scheme, [1 bit]
• UE identity [16 bits]
• Transport block size [6 bits]
• HARQ id, [3 bits]
• Redundancy&constellation vers. [3 bits]
• New Data Identifier NDI [1 bit]
MAChs header
• Queue Id [3 bits]
• TSN [6 bits]
• n x (Size Index Id. [3 bits], Nb of MAC
d
PDU [7 bit], Flag [1 bit])
UE -Tx
CQI channel qual. Indic. [5 bit]
HS-DPCCH
data
Ack [1 bit]
2006, January 12 – R. Bestak 26
UE categories
*
3630
3630
27952
20251
14411
14411
7298
7298
7298
7298
7298
7298
Max. number of bits of an
HS-DSCH transport block
received within an TTI
HS-DSCH
1
2
1
1
1
1
1
1
2
2
3
3
Minimum
inter-TTI
interval
14,4 15 10
0,9 5 11
1,2 5 2
1,8 5 3
1,8 5 4
3,6 5 5
3,6 5 6
7,2 10 7
7,2 10 8
1.8 5 12
10,2 15 9
1,2 5 1
Transport rate
(Mbit/s)
Max. number of
HS-DSCH codes
Category
*
([3G25306])
11,12 support
QPSK only
Minimum inter-TTI: the minimum distance from the beginning of TTI to the beginning of the next TTI
that can be assigned to the UE
14
2006, January 12 – R. Bestak 27
Example
transmission with several HARQ processes (minimum inter-TTI=3)
slot
HS-PDCCH (Ack)
HARQ Id=1
HS-PDCCH (Ack)
HARQ Id=2
UE -Tx
≈ 7,5 slots
HS-PDCCH (Ack)
HARQ Id=1
HS-PDCCH (Ack)
HARQ Id=2
Node B -Rx
Create a new
HARQ process
with Id =1
Create a new
HARQ process
with Id =2
Terminate the
HARQ process
with Id =1
Create a new
HARQ process
with Id =1
≈ 7,5 slots
HS-PDSCH (data)
HARQ Id=1
HS-PDSCH (data)
HARQ Id=2
HS-PDSCH (data)
HARQ Id=1 UE -Rx
Node B -Tx
HS-PDSCH (data)
HARQ Id=1
HS-PDSCH (data)
HARQ Id=2
HS-PDSCH (data)
HARQ Id=1
TTI
2006, January 12 – R. Bestak 28
HSDPA - overview
Downlink Improve data rates
Release 5 Introduced
Implementation Node B
MAC-hs New MAC entities
Adaptive modulation and coding
Scheduling
TTI = 2 ms
HARQ
Features
Shared
(HS-DSCH)
Transport channel
14,4 Mbit/s Peak data rate
H S D P A
15
2006, January 12 – R. Bestak 29
References
[1] H. Holma, and A. Toskala, WCDMA for UMTS, Radio Access for Third Generation Mobile
Communications,2
nd
edition, John Wiley & Sons, Ltd, England, Aug 2002.
[2] J. Sanchez, and M. Thioune, UMTS: services, architecture et WCDMA, Hermes, Paris 2001.
[3] 3G TS 25.211, Physical channels and mapping of transport channels onto physical channels
(FDD), 3GPP (Release 6), September 2005.
[4] 3G TS 25.212, Multiplexing and channel coding (FDD), 3GPP (Release 6), September 2005.
[5] 3G TS 25.321, MAC protocol specification, 3GPP (Release 6), September 2005.