WCDMA Physical 001

1 © NOKIA WCDMA_physical.PPT/ 31.1.2002 Kari Heiska, TLI361-WCDMA radioverkkosuunnittelu - luentosarja Jyvaskylän yliopistossa

WCDMA Physical Layer31.1.2002 Kari Heiska


What is Physical layer?• Physical layer defines how the data (controlling data and the user data

= user traffic) has been structured for the transmission over the air-interface

• In mobile cellular systems the effect of the physical layer is high because of the characteristics of the radio channel (=air interface)

• Defines the maximum capacity limits of the system (maximum allowed bit-rate, maximum number of simultaneous users)

• In practice the physical layer does not necessary limit the capacity but the implementation of the equipments and the radio channel.

• Big impact on equipment complexity• processing power, algorithms


Main 3G requirements on Physical layer

• High bit-rates

• Flexible variable bit rate both in uplink and in downlink

• Multi-service• Different services have been multiplexed on a single physical

connection

• Efficient packet data• Support for All IP-RAN

• High spectral efficiency


Functions of the physical layer• Main functions:

• Error detection• Multiplexing (TrCH→CCTrCH), demultiplexing (CCTrCH → TrCH)

• Channel coding, interleaving, rate matching• CCTrCH Mapping to physical Channels (PhCH)• Modulation, demodulation• Spreading, despreading• Combination of physical channels• Closed loop power control• Radio frequency processing (RF)• Synchronization (chip, bit, slot, frame)• Measurements

• Bit-error ratio (BER), Signal-to-Interference ratio (SIR), Transmission power (TxP), • Macrodiversity (soft(er)-handover)


Physical Channels

Superframe (720 ms)

#0 #1 #71

Radio frame (10ms)

#0

Slot (0.667 ms)

#14#2

Pilot TPC

DataDPDCH

DPCCH FBITFCI

DataTFCI PilotData TPC

DPCCH DPCCHDPDCH DPDCH

UL

DL

I/Q code multiplexed withcomplex scrambling

Time multiplexed with complex scrambling

• W=3.84 Mcps, one time slot 2560 chips• Physical channel is characterize with frequency, code, duration and in uplink with

phase shift• 1 radio frame (10 ms) includes 15 time slots (one slot equals to power control period,

1/(10ms/15)=1500 Hz). Slot structure is just for controlling the physical channel and its radio performance


Variable bit rate (dedicated channels)

• DPCCH (Dedicated physical control channel) is constant bit rate and carries all the information in order to keep physical connection running

• Reference symbols for channel estimation in coherent detection and for SIR estimation in fast power control

• Power control signalling bits (TPC)• Transport format information (TFCI) = bit rate, interleaving

• DPDCH (Dedicated physical data channel) is variable bit rate and carriers

• User data• Higher layer signalling, e.g. mobile measurements, active set

updates, packet allocations

• DPDCH bit rate is indicated with TFI bits on DPCCH


Uplink dedicated physical channel

Pilot TFCI TPC

Data (including user data and higher layer signalling: measuerements and so on)Data (including user data and higher layer signalling: measuerements and so on)

Slot 0.667 ms = 2/3 ms

Frame 1 Frame 2 Frame 72

Slot 1 Slot 2 Slot 16

Super frame 720 ms

10 ms



Procedure in the base station:• Estimate the SIR (Pilot)• Detect TPC and adjust DL Tx power • Detect the used bit-rate and interleaving (TFCI)• Detect the data (Data): needs buffering of the Data field

FBIDPDCH

DPCCH


Uplink dedicated physical channel• There can be several uplink DPDCH for one mobile but only one DPCCH• Admission Control (RRM) produces TFCS and estimates the minimum

allowed SF• DTX for speech example is possible since DPCCH is always ON and

possible discontinuous in data part dos not cause audible interference• (not the same as DTX in downlink which was for the variable data rate!!)

• TFCI = Transport Format Combination Indicator• TPC = Transmitted Power Control• FBI = Feedback information (for Tx antenna diversity, for example)• DPDCH spreading factor from 256 (15 ksps) to 4 (960 ksps)• DPCCH spreading factor from 256 (15 ksps) = constant• For example: SF = 16 � 3.84e6/16/1000=240 kbps

• Speech (AMR codec) 12.2 kbps + 64 kbps packet data + DCCH 3.4 kbps (12.2×3+64×3+3.4×3=238.8 � 240 kbps)

• I/Q modulation (QPSK): 1 bit = 1 symbol

I (DPDCH)

Q (DPCCH)


Uplink variable rate• DPDCH bit rate can change frame-by-frame (10 ms)• Higher bit rate requires more transmission power • Also DPCCH power is higher for higher bit-rates in order to enable

accurate channel estimation• Continuous transmission regardless of the bit rate

• No audible interference problems like in GSM• Admission control in RNC allocates those bit rates that can be used on

physical layer

DPCCH

DPDCH

Lower bit rateHigher bit rate Medium bit rate

10 ms frame 10 ms frame 10 ms frame

power


Dedicated uplink PhCHDPDCH fields

Slot Format #i Channel Bit Rate(kbps)

Channel SymbolRate (ksps)

SF Bits/Frame

Bits/Slot

Ndata

0 15 15 256 150 10 101 30 30 128 300 20 202 60 60 64 600 40 403 120 120 32 1200 80 804 240 240 16 2400 160 1605 480 480 8 4800 320 3206 960 960 4 9600 640 640

DPCCH fields

SlotFormat

#i

Channel BitRate (kbps)

ChannelSymbol Rate

(ksps)

SF Bits/Frame

Bits/Slot

Npilot NTPC NTFCI NFBI Transmittedslots per

radio frame0 15 15 256 150 10 6 2 2 0 15

0A 15 15 256 150 10 5 2 3 0 10-140B 15 15 256 150 10 4 2 4 0 8-91 15 15 256 150 10 8 2 0 0 8-152 15 15 256 150 10 5 2 2 1 15

2A 15 15 256 150 10 4 2 3 1 10-142B 15 15 256 150 10 3 2 4 1 8-93 15 15 256 150 10 7 2 0 1 8-154 15 15 256 150 10 6 2 0 2 8-155 15 15 256 150 10 5 1 2 2 15

5A 15 15 256 150 10 4 1 3 2 10-145B 15 15 256 150 10 3 1 4 2 8-9


Downlink dedicated physical channel



PilotData1

Super frame 720 ms

10 ms

Slot 0.667 ms = 2/3 ms

TFCITPC



• Time multiplexed DPCCH and DPDCH:

• DCH is carried by DPDCH

• Discontinuous transmission in DPDCH fields in order to handle variable data rates

DPCCH DPDCH

Data2

DPCCHDPDCH DPCCH


Downlink dedicated physical channel

• DPDCH and DPCCH are now time multiplexed (The audible interference is not a problem in DL)

• The DPDCH and DPCCH have the same power and the same SF

• DPDCH spreading factor from 512 (7.5 ksps) to 4 (960 ksps)

• For example: SF = 8 � 3.84e6/8/1000=480 ksps=960 kbps• Speech (AMR codec) 12.2 kbps + 384 kbps packet data + DCCH 3.4

kbps (12.2×3+384×3+3.4×3=1198.80 �960 (puncturing))• I/Q modulation (QPSK): 2 bit = 1 symbol

I

Q DPDCH/DPCCH


Downlink Variable Rate• DPDCH bit rate can change frame-by-frame (10 ms)

• Rate matching done to the maximum bit-rate of the connection

• Lower bit rates obtained with discontinuous transmission (no audible interference)

• The usable DL bit-rate allocated by the Radio Resource Management (RRM) algorithms (in this case Admission Control)

• Discontinuous transmission:

Lower bit-rate

DPCCH: Pilot + power control

DPDCH: Data

Maximum bit-rate Data absent


Downlink DPDCH and DPCCH fieldsBits/Frame DPDCH

Bits/SlotDPCCH Bits/SlotSlot

Format#i

ChannelBit Rate(kbps)

ChannelSymbol

Rate(ksps)

SF

DPDCH DPCCH TOT

Bits/Slot

NData1 NData2 NTFCI NTPC NPilot

0 15 7.5 512 60 90 150 10 2 2 0 2 4

1 15 7.5 512 30 120 150 10 0 2 2 2 4

2 30 15 256 240 60 300 20 2 14 0 2 2

3 30 15 256 210 90 300 20 0 14 2 2 2

4 30 15 256 210 90 300 20 2 12 0 2 4

5 30 15 256 180 120 300 20 0 12 2 2 4

6 30 15 256 150 150 300 20 2 8 0 2 8

7 30 15 256 120 180 300 20 0 8 2 2 8

8 60 30 128 510 90 600 40 6 28 0 2 4

9 60 30 128 480 120 600 40 4 28 2 2 4

10 60 30 128 450 150 600 40 6 24 0 2 8

11 60 30 128 420 180 600 40 4 24 2 2 8

12 120 60 64 900 300 1200 80 4 56 8* 4 8

13 240 120 32 2100 300 2400 160 20 120 8* 4 8

14 480 240 16 4320 480 4800 320 48 240 8* 8 16

15 960 480 8 9120 480 9600 640 112 496 8* 8 16

16 1920 960 4 18720 480 19200 1280 240 1008 8* 8 16

Half rate speech

144 kbps

384 kbps

• With spreading factor of 4 and with 3 parallel code channels, 2 Mbps can be reached• The spreading factor = the number of orthogonal spreading codes


Codes in WCDMA• Two type of codes in WCDMA:

• Channelization Codes (Spreading code, short code, orthogonal code)• Length is dependent on spreading factor • Used for channel separation from the single source• Good orthogonality properties => decreased interference • Useage have to be managed: If one code with low spreading factor is used, the code

in the same code tree branch can not be used• Same codes in every cell / mobiles and therefore the additional scrambling code is

needed• Scrambling Codes (=long code)

• Very long (38400 chips), many codes available• Uplink: to separate different mobiles• Downlink: to separate different cells/sectors• Good correlation properties: • The correlation between two codes (two mobiles) is low• The autocorrelation is low when the phase shift ≠ 0. Then the multipath propagation does not have big impact on the

interference levels

data rate Chip Rate (W)

DATA

Channelisation code Scrambling code

Chip Rate (W)


Orthogonal Codes in Downlink• Codes c1=[-1 1 1 -1], c2=[1 1 1 1] are orthogonal: c1*c2

T=0• If two downlink users use orthogonal codes (+ same scrambling code) they don’t

interfere each other in MS reception. This is the case when there is only one multipath channel. In a channel with several multipaths, the orthogonality does not hold anymore and also DL channels begin to interfere each others

• Orthogonal codes have to be reserved for • dedicated channels• common channels• soft-handover users• shared channels1

• The maximum capacity with one set of orthogonal codes under one scrambling code:• Full rate speech (SF=128): 98 channels• Half rate speech <7.95 kbps (SF=256): 196 channels• Data: 2.5 Mbps

• The code channels does not limit the capacity in practice but the interference of the network

• Second set (code tree) can be obtained by using 2nd scrambling code• increases the interference due to loss of orthogonality• Needed in smart antenna applications


Long and short codesShort Code =Channelisation Code

Long Code =Scrambling Code

Uplink Downlink Uplink Downlink

Usage Separation ofDPDCH and DPCCHfrom the same MS

Separation of different userswithin one sector

Separation ofmobiles

Separation of cells

Length 4, 8, 256 chips,depends on bit-rate

4, 8, … 512 chips, depends onbit-rate

1) 10 ms = 38400chips2) 256 chips can beused with advancedreceiver algorithms

10 ms = 38400 chips

Number ofCodes

= spreading factor (sf=4 � 4 codes) 16.8 million 512, easier to find by MS andstill easy to plan by networkplanner (different codes inadjacent cells)

Code Family OrthogonalVariable SpreadingFactor (OVSF)

Orthogonal Variable SpreadingFactor (OVSF). The codes usedhave to be orthogonal. Have tobe managed by the system

Gold code (long 10ms code)


The tree of orthogonal short codes in DL• Hierarchical selection of short codes from a "code tree" to maintain orthogonality

• Several long scrambling codes can be used within one sector to avoid shortage of short codes

Spreading factor:

SF = 1 SF = 2 SF = 4 SF = 8

C0(0) = [ 1 ]

C1(0) = [ 1 1 ]

C1(1) = [ 1 -1 ]

C2(0) = [ 1 1 1 1 ]

C2(1) = [ 1 1 -1 -1 ]

C2(2) = [ 1 -1 1 -1 ]

C2(3) = [ 1 -1 -1 1 ]

C3(0) = [ 1 1 1 1 1 1 1 1 ]

C3(1) = [ 1 1 1 1 -1 -1 -1 -1 ]

C3(2) = [ 1 1 -1 -1 1 1 -1 -1 ]

C3(3) = [ 1 1 -1 -1 -1 -1 1 1]

C3(4) = [ 1 -1 1 -1 1 -1 1 -1 ]

C3(5) = [ 1 -1 1 -1 -1 1 -1 1 ]

C3(6) = [ 1 -1 -1 1 1 -1 -1 1 ]

C3(7) = [ 1 -1 -1 1 -1 1 1 -1 ]

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Example ofcode allocation

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Two code layer scheme, downlink

LC0 LC1

LC2

Long

code

layer

(scr

ambli

ng)

Short

code

layer

SC0-SCn SC0-SCn

SC0-SCn

Cell

Orthogonal sets of SCswithin one LC phase

LC1

LC2


Downlink shared channel (DSCH) (1/2)

Slot 0.667 ms

DPCCH DPDCH

Physical channel 2 (SPCH)DSCH

Pilot TPC TFCI Data

Frame structure for the DSCH when associated to a DCH.

• The number of orthogonal codes in downlink is limited and the code is reserved according to the maximum bit rate in transport format set

• variable bit rate connections consume a lot of code resources� downlink shared channel concept which uses the same branch of the code tree

• DSCH is shared between a group of downlink users

• For packet data services

• Existence of data on DSCH for a particular user can be indicated with TFCI (frame basis) or with higher layer signalling (slower)

• Associated with DL dedicated channel (DCH): TPC, decoding info

• DSCH is not frame synchronized with the corresponding dedicated channel


Downlink shared channel (DSCH) (2/2)

SF=4

SF=8

DSCH branch

t1

SF=8 SF=16

SF=16

SF=16

SF=32

SF=8

SF=16

SF=32

t2

t3

P1

P2

P3

Basic allocation principle:P1≈P2≈P3


Uplink Common Channels


Physical Random Access Channel (PRACH)• With Random Access Channel (RACH) power ramping is needed with preambles since the

initial power level setting in the mobile is very coarse with open loop power control• Preamble: mobile sends 256 repetitions of 16 chip (1 preamble = 4096 chips) signature

sequence with increasing power• L1 acknowledgement: base station acknowledges the sequences received with high

enough power level (AICH = Acquisition Indication CH)• Mobile RACH message follows the acknowledgement • Can be used also for Data transmission• Message part length 10 or 20 ms

P2

Downlink / BS

RACHP1

L1 ACK / AICH

Uplink / MS

Preamble

Not detected

Message partPreamble

• SF=256 to 16

• short packets

• 1 or 2 frames


Physical Common Packet Channel (CPCH)• Extenssion for RACH: for data transmission, same data-rate as in UL DPDCH• RACH preamble signature sequences are used

• Initial access channels indicated by CSICH channel• Differences to RACH: Several Frames, Fast Power Control, no SHO• Collision detection preamble part: RACH preamble signature sequences used, different

scrambling code � reduces the collision probability (because of many frames and more data to loose).

• Power control information from DPCCH

• Maximum duration (N*10msec) from the network

4096 chips

P0

P1Pj Pj

Collision ResolutionPreamble

Access Preamble Control

Data

[10] msec N*10 msec

Message Part

CPCH AP-AICH

DL

UL

CPCH-CD, CA-ICH


Downlink Common Channels


Downlink Common Pilot Channel (CPICH)

Pre-defined symbol sequence

Slot #0 Slot #1 Slot #i Slot #14

Frame #0 Frame #1 Frame #i Frame #71

Tslot = 2560 chips , 20 bits = 10 symbols

Tf = 10 ms

Tsuper= 720 ms

• Primary and secondary CPICH• Primary CPICH

• Unmodulated, fixed rate, fixed power channel scrambled with the cell specific primary scrambling code• Used as a phase reference•15 kbps, SF=256 (Cch,256,0)• Used in handover measurements:

CPICH Ec/I0• Used for channel estimation

• Secondary CPICH• Used with multiple antenna beams


Primary Common Control Physical Channel (P-CCPCH)

• Carrying the Broadcast Channel (BCH)

• Contains random access codes, code channels of other common channels

• Pure DATA channel: channel estimation from Common pilot channel

• Needs to be demodulated by all the terminals in the system: High Tx power needed

• Fixed data rate (30 kbps=15ksps), channellization code length 256 Cch,256,0

• No power control

Data18 bits

Slot #0 Slot #1 Slot #i Slot #14

Frame #0 Frame #1 Frame #i Frame #71

Tslot = 2560 chips , 20 bits

Tf = 10 ms

Tsuper= 720 ms

(Tx OFF)

256 chips

•P-CCPCH and SCH are time multiplexed (SCH used in TxOFF period of above shown figure)


Synchronisation Channel (SCH)Synchronisation Channel (SCH)

PrimarySCH

SecondarySCH

256 chips

2560 chips

One 10 ms SCH radio frame

acsi,0

acp

acsi,1

acp

acsi,14

acp

Slot #0 Slot #1 Slot #14

• For initial cell search for the MS• The Primary SCH consists of a modulated code of length 256 chips which is identical for every cell of the system• The Secondary SCH (S-SCH) consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronisation Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted cs

i,k

where i = 1, 2, …, 64 is the number of the scrambling code group, and k = 0, 1, …, 14 is the slot number• SSC code of each slot is one of 16 codes out of total number of 256 orthogonal short codes


Cell Search Procedure

• MS knows the 256 chip code word of the Primary SCH (P-SCH) = system specific code

• The Matched Filter (MF) output in MS tells the time location of the P-SCH and also S-SCH (P-SCH and S-SCH are time aligned)

� Chip, symbol and slot synchronization

• MS checks in each slot position 16 possible SSC sequences (240 checkings) and selects which gives the highest correlation value � 15 codes, 1 for each slot

• This cyclic shift is unique and it gives the frame synchronization and the scrambling code group (64 scrambling code groups each having 8 codes = 512 scrambling codes)

• MS correlates each 8 scrabling codes with CPICH and finds the scrambling code of the cell by finding the maximum correlation value

• Read P-CCPCH to decode BCH: System and cell specific BCH information


Other DL Common Channels• Secondary Common Control Physical Channel (S-CCPCH)

• Carries Forward Access Channel (FACH) and Paging Channel (PCH) which can be mapped to same or different S-CCPCH

• SF=256 (15 ksps) to 4 (960 ksps)• No TPC• Active only when data available

• Acqusistion Indicator Channel (AICH)• User for RACH channel indication• For the detection of AICH MS used Common pilot channel• To all MS in the cell: high power, low data rate

• Paging Indicator channel (PICH)• A terminal registered to the network is allocated a paging group• When there are paging messages coming for any UEs of that group the Paging

Indicator will be send on PICH. • After that UE decodes the next PCH message on S-CCPCH to find out whether

there was paging messages intended for it• This procedure decreases the power consumption of the UE


Timing and synchronization• Network Synchronization

• distribution of the synchronization references to nodes• Node Synchronization

• estimation and compensation of timing between nodes (BSs)• Timing relationship between Physical Channels

• SFN transmitted on P-CCPCH used as a timing reference• SCH, CPICH, P-CCPCH and PDSCH have identical frame timing• S-CCPCH timing offset from P-CCPCH is multiple of 256• DPCH timing offset from P-CCPCH is multiple of 256

• Transport Channel Synchronization• CFN is a unique number for RRC connection and is associated with each

Transport Block Set (TBS).• BFN is the BS common frame counter (0-4095)• RFN is the RNC common frame counter (0-4095)• CFN is mapped to SFN of the first radio frame used for TBS transmission (L1↔L2)

by using Frame Offset parameter computed by RNC• In SHO the Frame Offsets of different links are selected by RNC in order to have

timed transmissions over air interface


Timing and synchronization• Radio Interface Synchronization

• Serving RNC transmit the default offset value to UE in order to inform the UE when the frames in the downlink are expected. The offset is generated so that the transmitting frames are not overlapping in order to avoid high power caused by transmitting pilot symbols

• Radio Interface synchronization is necessary to assure that the UE receives radio frames synchronously from different cells in order to minimize UE buffers

• UE measures the timing of DPCH and the target cell SFN and report it to RNC. RNC informs BSs about the timing difference which rounds it to closest 256 chip boundary and then uses it for the downlink DPCH transmission


256 chips

Radio frame timing


Uplink spreading on dedicated channels

Uplink scrambling code

Uplink channelization code

• βd is the amplitude coefficient for data channel• βc is the amplitude coefficient for control channel

DPDCH { -1,1}

DPCCH { -1,1}

QPSK modulation


Uplink spreading on dedicated channels• One DPCCH and up to 6 DPDCH channels can be transmit

simultaneously• Spread signals (after channelization code) can be multiplied by gain

factor (βd for DPDCH and βc for DPCCH )

• The scrambling code is aligned with the radio frame. First scrambling chip corresponds to the beginning of radio frame

• DPCCH is always spread by code Cch,256,0

• When only one DPDCH is to be transmitted the code is Cch,SF,k

• When more than one DPDCH is to be transmitted the code is Cch,4,k

• The scrambling code (MS specific) is complex valued. The scrambling code has been formed so that the rotation between chips within one symbol is limited to ±90° (better power amplifier efficiency because there is no zero crossnings in the constellation

• Scrambling code and minimum SF are assigned by network in the beginning of connection


Downlink spreading and modulationDownlink scrambling code

same in all DL channels (except SCH)

DPDCH bit or DPCCH bit

Downlink channelization

code

other DL channel

complex value input to modulation

↑↑↑↑ I

↓↓↓↓Q+

-sin(ωt)

cos(ωt)

QPSK modulationSum over all DL physical channels except synchronization channel (SCH)


Downlink spreading code

• Same channelization code (=short code=spreading code) as in uplink (OVSF-code). Orthogonal codes

• Typically one code tree per cell: Code tree shared between all downlink users

• Code for CPICH = Cch,256,0 and for P-CCPCH = Cch,256,1

• Resource manager assigns the channelization code for other channels

• Downlink SF does not vary on frame by frame bases, except for DSCH

• data rate variation is taken care of with rate matching or with L1 DTX

• In multicode tranmissions (high bit rates > 1 Mbps) the parallel code channels have different channellization code under the same scrambling code but the same SF


Downlink scrambling code• Long scrambling code (218-1=262143 codes)• Only 38400 chips from the beginning of the code is used • The DL scrambling code is time aligned with the scrambling code of P-

CCPCH channel which is the timing reference

• From these only 8192 codes, devidid into 512 sets, are used in WCDMA in order to speed up the cell search

• Each code set includes 1 primary and 15 secondary scrambling (other PhCH) codes.

• 512 primary scr. codes has been devided into 64 subgroups• Each cell is allocated one primary scrambling code (carrying P-CCPCH,

P-CPICH, PICH, AICH and S-CCPCH) • Other channels can use the primary scrambling code or secondary code

from the same set. If the secondary code is used the orthogonality is lost � reduction of system performance

WCDMA Physical 001

Documents

physical data channel

physical control channel

physical layer high

chips physical channel

radio channel

physical layer main

channel estimation

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