HSDPA Physical Layer Concepts

1 © Nokia Siemens Networks HSDPA Physical Layer Concepts / Bharat B / April 2007For internal use

HSDPA Physical Layer Concepts

BangaloreApril 2007


Physical Layer Changes introduced for HSDPA

• Shorter Radio Frame(2ms instead of 10ms)

• New Downlink Data and Control Channel

• CDM combined with TDM

• 16 QAM Modulation Type (in addition to QPSK)

• New Uplink Control Channel

• Adaptive Modulation and Coding (AMC)

• Hybrid-ARQ (HARQ)

• New UE Capability Classes


HSDPA Frame Structure

• The HSDPA radio frame is 2 ms in length—equivalent to three of the currently defined W-CDMA slots

• It is actually a sub-frame in the W-CDMA architecture of 10ms

• There are five HSDPA sub-frames in a 10 ms W-CDMA frame, as shown in the Figure.

• The shorter frame size allows the UE to inform the network about failures every 2ms

• The network can respond more quickly to changing channel conditions and re-assign the capacity amongst users


New HSDPA Channels


New HSDPA Channels

DOWNLINK:• New Transport Channel:

– HS-DSCH: High Speed Downlink Shared Channel

• New Physical Channels:

– HS-PDSCH: High Speed Physical Downlink Shared Channel▪ Payload data

– HS-SCCH: High Speed Shared Control Channel▪ UE identity and HS-DSCH coding info

UPLINK:• New Physical Channels:

– HS-DPCCH: High Speed Dedicated Physical Control Channel▪ ACK, NAK and CQI (Channel Quality Indicator)


Downlink Physical Channel: HS-PDSCH

• Used to carry the HS-DSCH transport channel data

• Fixed Spreading Factor of 16 (SF = 16),

• A set of channelization codes (upto 15) are reserved for HS-DSCH transmission

• Turbo Coding (1/3) is used for channel coding

• Uses 16QAM Modulation scheme in addition to QPSK

HS-PDSCH


HS-PDSCH Configuration


HS-PDSCH Structure

• Data rate varies according Modulation Technique applied (QPSK/16QAM)

• Total no. of bits in a slot = M x 10 x 2K; – where k = 4, M = 2 or 4(QPSK or 16QAM)

• SF = 256/2k

• Max. Channel bit rate = 15 x 960 kbps = 14400 kbps, But this is the data rate after coding is applied


HS-PDSCH Channel coding

• Max. one transport block every TTI(2ms)

• CRC Length = 24 bits

• Bit Scrambling to remove continuous 0s or 1s

• Max. code block size = 5114 bits (Turbo Coding)

• Channel coding = 1/3 Rate Turbo Coding

• Hybrid-ARQ – 2 Stage Rate Matching

• Bits are mapped onto (up to 15) HS-PDSCH(s)

• Interleaving for each HS-PDSCH

• Different 16QAM constellations are defined

• Mapping to different HS-PDSCH(s)


Hybrid Automatic Repeat ReQuest (HARQ)

• Hybrid– Feed forward error correction, using turbo coding

– ARQ : Feedback error correction

• The Erroneous packets are not discarded but are combined with the retransmissions


Combining retransmissions

• To minimize the number of additional re-transmission requests

• HARQ can be operated in two different ways :– with identical retransmissions or

– with non-identical retransmissions

• Chase combining (CC) or Soft Combining – involves sending an identical version of an erroneously detected packet;

– received copies are combined by the decoder prior to decoding

• Incremental redundancy (IR) – involves sending a different set of bits incrementally to be combined with the original set,

– thus increasing the amount of redundant data and the likelihood of recovering from errors introduced on the air


Two Stage rate matching for IR


HS-PDSCH Channel coding - Example


Downlink Physical Channel: HS-SCCH

• Used to carry the control information needed for HS-DSCH decoding

• Fixed Spreading Factor of 128 (SF = 128)

• UE monitors one or more (upto 4) HS-SCCH channels to know when to receive data

• Uses QPSK modulation scheme


HS-SCCH Structure

• Slot #0 carries modulation information of HS-PDSCH:– Channelization Code Set (7 bits) (OVSF code assignment)

– Modulation Scheme: QPSK or 16QAM (1 bit)

• Slot #1, #2 carry channel-coding information of HS-DSCH:– Transport-block size (6 bits)

– Hybrid-ARQ process information (3 bits)

– Redundancy and constellation version (RV) (3 bits)

– New Data Indicator (NDI) (1 bit)

• The data of the 3 slots are covered with the 16 bit UE identity


HSDPA - Downlink Physical Channel Timing

• The NodeB transmits two slots of the HS-SCCH sub-frame before it begins transmitting the HS-PDSCH.

• The HS-SCCH and HS-PDSCH thus overlap during a slot (the third slot of the HS-SCCH).

• The 1st slot contains information of the channelization code set and modulation

• The UE must quickly decode the HS-SCCH, so that that UE does not miss the HS-PDSCH


Uplink Physical Channel: HS-DPCCH

• Used to carry the feedback information for HS-DSCH transmission– Acknowledgments (ACK) and Negative Acknowledgements (NAK)– Channel Quality Indicators to indicate quality of downlink transmission

• Fixed Spreading Factor of 256 (SF = 256)

• Uses BPSK Modulation scheme (One each for every HSDPA user)


• The 1st slot contains HARQ-ACK, 2nd and 3rd slots contains CQI• The HARQ-ACK information (1 bit) is coded to 10 bits,

– ACK coded as 1111111111– NAK coded as 0000000000

• The CQI information is coded using a (20,5) code• The feedback cycle of the CQI can be set as a network parameter in

predefined steps from 2 ms to infinity (disabled)• HARQ ACK/NAK response for every transmitted packet

HS-DPCCH Structure


HSDPA – Downlink & Uplink Physical Channel Timing

• HS-DPCCH is transmitted approximately 7.5 slots after its corresponding downlink HS-PDSCH sub-frame is received at the UE

• The timing of the ACK/NACK response from the UE on the HS-DPCCH is defined explicitly, so that the NodeB will know when to expect the response for each sub-frame it has transmitted.


Link Adaptation Techniques

• Two different Link Adaptation techniques are used

– Fast Re transmission using Hybrid Automation Repeat Request (H-ARQ)

– Adaptive Modulation and Coding (AMC)


Adaptive Modulation and Coding (AMC):

• Vary the downlink modulation and coding scheme depending on the channel conditions for each user

• 1/3 Turbo Coding is used, different effective code rates are obtained through various rate matching parameters

• Channel conditions reported by CQI in HS-DPCCH by the UE

• Modulation and coding can be adjusted Every 2 ms


Inter-TTI Interval

• Transmission Time Interval (TTI) is the time to transmit one data packet– For HSDPA its always 2 ms (=1 sub frame)

• Inter-TTI interval = number of TTIs (or subframes) between transmissions to the same UE

• All UEs must be capable of receiving transmissions at least every 3rd sub-frame (Minimum Inter-TTI interval = 3)

• More capable UE can handle Inter-TTI intervals of 2 or 1

Inter –TTI = 2


HARQ Processes

• If only 1 HARQ process is present, inter-TTI interval is at least 6 because arrival of ACK/NAK takes about 5 subframes


HARQ Processes

• Multiple parallel HARQ processes per user are possible:– Data rate to user is Increased

– Available system capacity is not wasted


HARQ Processes

• Throughput can be increased by reducing the inter-TTI interval (add more HARQ processes)

• UE w/ minimum inter-TTI = 3 supports 2 HARQ processes

• UE w/ minimum inter-TTI = 2 supports 3 HARQ processes

• UE w/ minimum inter-TTI = 1 supports the max. 6 processes

• Soft memory is partitioned across HARQ processes

• NodeB chooses no. of processes up to the max the UE can handle


UE Categories


Recap...

• Shorter Radio Frame(2ms instead of 10ms) allows to adapt quickly to channel conditions

• HS-PDSCH (Downlink Data)– Carries HS-DSCH data, SF = 16, 1/3 Turbo Coding, – Upto 15 Channelization Codes, TDM and CDM, 16QAM, – HARQ: 2Stage Rate Matching using Incremental Redundancy by varying Redundancy and Constellation Version

• HS-SCCH (Downlink Control)– Carries Control Information for HS-PDSCH, SF=128,– Channelization Code and Modulation Scheme, Transport Block Size, HARQ process no., RV and Constellation No.,

New Data Indicator, UE Identity

• HS-DPCCH (Uplink Control)– ACK/NAK, CQI

• Downlink and Uplink Physical Channel Timing– 2 slots between HS-SCCH and HS-PDSCH– Approximately 7.5 slots to receive a HS-DPCCH(ACK/NAK) at NodeB for a particular transmission

• Inter-TTI Interval – 3, 2 or 1

• HARQ Processes – 1 HARQ process: Inter-TTI = 6, so use multiple processes


Questions???

HSDPA Physical Layer Concepts






Backup Slide – Convolution Coding

Output 0G0 = 557 (octal)

InputD D D D D D D D




InputD D D D D D D D


(a) Rate 1/2 convolutional coder

(b) Rate 1/3 convolutional coder


Backup Slide – 1/3 Rate Turbo Coding

xk

xk

zk

Turbo codeinternal interleaver

x’k

z’k

D

DDD

DD

Input

OutputInput

Output

x’k

1st constituent encoder

2nd constituent encoder


Backup Slide – Interleaving

• Data of the form aaaabbbbccccddddeeeeffffgggg

will be transmittied as: abcdefgabcdefgabcdefgabcdefg

• To protect data transmission against burst errors

• Transmission without Interleaving:– Error-free message: aaaabbbbccccddddeeeeffffgggg

– Transmission with a burst error: aaaabbbbcccc____eeeeffffgggg

• Transmission with Interleaving:– Error-free transmission: aaaabbbbccccddddeeeeffffgggg

– Interleaved: abcdefgabcdefgabcdefgabcdefg

– Transmission with a burst error: abcdefgabcd____bcdefgabcdefg

– Deinterleaved with a burst error: aa_abbbbccccdddde_eef_ffg_gg


Backup Slide – HSDPA Cell Setup


Backup Slide – HSDPA Call Setup


Backup Slide – HS-SCCH 1st Slot UE id

• The HS-SCCH frame and the corresponding HS-PDSCH frame are overlapping by one timeslot

• UE has to first receive and decode the HS-SCCH frame before it knows whether the corresponding HS-PDSCH is addressed to it

• The UE identity can be resolved after reception of the first HS-SCCH timeslot

• The data in the 1st timeslot is masked using a bit string that is derived from the UE identity

• Only the correct UE identity can decode this timeslot


Backup Slide – HS-PDSCH Physical Layer HARQ function

• Two Stage Rate Matching and Incremental Redundancy

Systematicbits

Parity 1bits

Parity2bits

RM_P1_1

RM_P2_1

RM_P1_2

RM_P2_2

RM_S

First Rate Matching Second Rate MatchingVirtual IR Buffer

Nsys

Np1

Np2

Nt,sys

Nt,p1

Nt,p2

bitseparation

NTTIbit

collection

NdataC W


Backup Slide – CQI 20, 5 coding

• The code words of the (20,5) code are a linear combination of the 5 basis sequences denoted Mi,n defined in the table below

• The CQI values 0 .. 30 are converted from decimal to binary to map them to the channel quality information bits (1 0 0 0 0) to (1 1 1 1 1) respectively

• The channel quality information bits are a0 , a1 , a2 , a3 , a4 (where a0 is LSB and a4 is MSB). The output code word bits bi are given by:

where i = 0, …, 19.

i Mi,0 Mi,1 Mi,2 Mi,3 Mi,4 0 1 0 0 0 1 1 0 1 0 0 1 2 1 1 0 0 1 3 0 0 1 0 1 4 1 0 1 0 1 5 0 1 1 0 1 6 1 1 1 0 1 7 0 0 0 1 1 8 1 0 0 1 1 9 0 1 0 1 1

10 1 1 0 1 1 11 0 0 1 1 1 12 1 0 1 1 1 13 0 1 1 1 1 14 1 1 1 1 1 15 0 0 0 0 1 16 0 0 0 0 1 17 0 0 0 0 1 18 0 0 0 0 1 19 0 0 0 0 1

CQI value Transport Block Size

Number of HS-PDSCH

Modulation Reference power

adjustment NIR XRV

0 N/A Out of range

1 137 1 QPSK 0

2 173 1 QPSK 0

3 233 1 QPSK 0

4 317 1 QPSK 0

5 377 1 QPSK 0

6 461 1 QPSK 0

7 650 2 QPSK 0

8 792 2 QPSK 0

9 931 2 QPSK 0

10 1262 3 QPSK 0

11 1483 3 QPSK 0

12 1742 3 QPSK 0

13 2279 4 QPSK 0

14 2583 4 QPSK 0

15 3319 5 QPSK 0

16 3565 5 16-QAM 0

17 4189 5 16-QAM 0

18 4664 5 16-QAM 0

19 5287 5 16-QAM 0

20 5887 5 16-QAM 0

21 6554 5 16-QAM 0

22 7168 5 16-QAM 0

23 7168 5 16-QAM -1

24 7168 5 16-QAM -2

25 7168 5 16-QAM -3

26 7168 5 16-QAM -4

27 7168 5 16-QAM -5

28 7168 5 16-QAM -6

29 7168 5 16-QAM -7

30 7168 5 16-QAM -8

9600 0

2mod)(,

4

0Mab ni

nni


Backup Slide – Other topics

• F- DPCH

• Associated DPCH channels

• Mapping of RV to Puncturing scheme

•

Documents

HSDPA Physical Layer Concepts