WCDMA Technology Past, Present and Future

1
WCDMA TechnologyPast, Present and Future

• Part IV
• Physical Layer on WCDMA

2
Part IVPhysical Layer on WCDMA

• Overview on WCDMA Physical Layer
• WCDMA Physical Channel
• Functions of WCDMA Physical Channel
• Spreading in WCDMA Physical Channel
• Operations in making a phone call
• Baseband Packet Formats
• Important Physical Layer Procedures and Issues

3
What is Physical Layer?

• Physical layer (PHY) 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

4
Main Requirements on WCDMA 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

5
WCDMA Radio Interface Protocol Architecture
Control Plane
User Plane
L3
RRC
Control
PDCP
BMC
L2
Radio Bearers
RLC
Logical Channels
MAC
Transport Channels
L1
PHY(W-CDMA)
Physical Channels
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Physical Channels (1/3)

• Common Physical Channels
• Synchronization Channels (SCH, DL)
• Primary Synchronization Channel (P-SCH)
• Secondary Synchronization Channel (S-SCH)
• Common Pilot Channel (CPICH, DL)
• Common Control Physical Channels (CCPCH, DL)
• Primary common physical channel (P-CCPCH)
• Secondary common physical channel (S-CCPCH)
• Indication Channel
• Acquisition Indication Channel (AICH, DL)
• Page Indication Channel (PICH, DL)
• CPCH Access Preamble Acquisition Indicator
  Channel (AP-AICH, DL)
• CPCH Collision Detection Channel Assignment
  Indicator Channel (CD/CA-ICH, DL)
• CPCH Status Indicator Channel (CSICH, DL)
• Physical Random Access Channel (PRACH, UL)
• Physical Common Packet Channel (PCPCH, UL)
• Dedicated Physical Channels
• Dedicated Physical Data Channel (DPDCH, DLUL)
• Dedicated Physical Control Channel (DPCCH, DLUL)

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Physical Channels (2/3)
8
Physical Channels (3/3)

• Chip rate 3.84 Mcps
• 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)
• one time slot 2560 chips
• Slot structure is just for controlling the
  physical channel and its radio performance

9
Timing Relationship Between Physical Channels
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Physical Layer Baseband Processing Block Diagram
Coding and Multiplexing of Transport Channels
Transport Channel 1
Transport Channel 2
Transport Channel N
To RF
CRC Attachment, Channel Coding, 1st
Interleaving, Rate Matching
CRC Attachment, Channel Coding, 1st
Interleaving, Rate Matching
Modulation
CRC Attachment, Channel Coding, 1st
Interleaving, Rate Matching
Pulse Shaping
Transport Channel MUX
Scrambling
CCTrCH
I jQ
(DPDCH)
Physical Channel Segmentation, 2nd
Interleaving, Physical Channel Mapping
Spreading Gain Weighting
PhCH 1
I
PhCH 2
PhCH 3
Q
PhCH 4
DPCCH
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Functions of WCDMA Physical Layer

• 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)

12
Spreading Codes in WCDMA

• Channelisation Codes (Spreading code, orthogonal
  code)
• Length is dependent on spreading factor
• Used for channel separation from the single
  source
• Good orthogonality properties gt decreased
  interference
• Usage 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
• 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

13
Channelisation and Scrambling codes
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Two code layer scheme, downlink
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Utilization of Channelisation and Scrambling Codes
Uplink Downlink
Channelisation Codes Identify the physical channels for the same user if multi-code is used Identify the physical channels of each user
Scrambling Codes Identify the users Identify the cells
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Channelization Codes OVSF Codes

• The OVSF (Orthogonal Variable Spreading Factor)
  code is described as Cch,SF,k, where SF is the
  spreading factor of the code and k is the code
  number, 0 ? k ? SF-1.

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Downlink scrambling code (1/2)

• Long scrambling code (218-1262143 codes)
• Only 38400 chips from the beginning of the code
  is used
• The DL scrambling code is time aligned with the
  scrambling code of PCCPCH 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 divided 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

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Downlink Scrambling Code (2/2)

• Configuration of DL scrambling code generator.

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Uplink Scrambling Code

• The definition of the nth scrambling code word
  for the in phase and quadrature components
  follows as
• C1,nltxn(0)y(0),
  xn(1)y(1),, xn(N-1)y(N-1)gt
• C2,nltxn(M)y(M),
  xn(M1)y(M1),, xn(MN-1)y(MN-1)gt
• where N is the period in chips and M
  16,777,232.

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Block Diagram of WCDMA PHY
Spreading
Multipath Fading Channel
Despreading
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Part IVPhysical Layer on WCDMA

• Overview on WCDMA Physical Layer
• WCDMA Physical Channel
• Functions of WCDMA Physical Channel
• Spreading in WCDMA Physical Channel
• Operations in making a phone call
• Baseband Packet Formats
• Important Physical Layer Procedures and Issues

22
Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
Listen Broadcast Information
Wait for Paging
Establish a dedicate connection for a call
Data Transmission and Reception
Release connection for a call
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Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
1.
Listen Broadcast Information
Wait for Paging
Establish a dedicate connection for a call
Data Transmission and Reception
Release connection for a call
24
WCDMA Cell Search
One timeslot625 ?sec
P-SCH
Base 1
P-CCPCH
S-SCH
P-SCH
Base 2
P-CCPCH
S-SCH
P-SCH
P-SCH
Base 3
P-CCPCH
P-CCPCH
S-SCH
S-SCH
Mobile Searcher
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Cell Search Procedure (1/2)

• How to do cell search? --gt Three steps fast cell
  search algorithm
• Step 1 Slot synchronization
• Step 2 Frame synchronization and code-group
  identification
• Step 3 Scrambling code identification

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Cell Search Procedure (2/2)
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Synchronisation Channel (SCH)

• For initial cell search for the MS
• The Synchronisation Channel (SCH)
• A downlink signal used for cell search
• Consists of two sub channels
• Primary SCH
• Secondary SCH

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Downlink Common Pilot Channel (CPICH)

• 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, SF256 (Cch,256,0)
• Used in handover measurements
• CPICH Ec/I0
• Used for channel estimation
• Secondary CPICH
• Used with multiple antenna beams

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Modulation Pattern for CPICH in case of Transmit
Diversity

• In case of Transmit Diversity (open or closed
  loop), the CPICH shall be transmitted from both
  antennas using the same channelization and
  scrambling code. In this case, the pre-defined
  symbol sequence of the CPICH is different for
  Antenna 1 and Antenna 2.

30
Downlink spreading code

• Typically one channelization 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 gt 1
  Mbps) the parallel code channels have different
  channelisation code under the same scrambling
  code but the same SF

31
Downlink spreading and modulation
P-CPICH S-CPICH
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Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
Listen Broadcast Information
2.
Wait for Paging
Establish a dedicate connection for a call
Data Transmission and Reception
Release connection for a call
33
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 kbps15ksps), channellization
  code length 256 Cch,256,0
• No power control
• P-CCPCH and SCH are time multiplexed (SCH used in
  TxOFF period of above shown figure)

34
Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
Listen Broadcast Information
Wait for Paging
3.
Establish a dedicate connection for a call
Data Transmission and Reception
Release connection for a call
35
Secondary CCPCH

• The S-CCPCH is used to carry the FACH and PCH.
• Forward Access Channel (FACH) and Paging Channel
  (PCH) which can be mapped to same or different
  S-CCPCH
• There are two types of S-CCPCH those that
  include TFCI and those that do not include TFCI.
  It is the UTRAN that determines if a TFCI should
  be transmitted, hence making it mandatory for all
  UEs to support the use of TFCI.
• The parameter k determines the spreading factor
  SF of the S-CCPCH as SF 256/2k. The spreading
  factor range is from 256 down to 4.
• No TPC
• Active only when data available

36
Secondary CCPCH
37
S-CCPCH Fields
38
Page 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

39
Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
Listen Broadcast Information
Wait for Paging
Establish a dedicate connection for a call
4.
Data Transmission and Reception
Release connection for a call
40
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

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Spreading and Modulation of PRACH Message Part
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Structure of the Random Access Message Part
43
Random-access Message Data and Control Fields
44
PRACH access procedure
45
Acquisition Indicator Channel (AICH)

• 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

46
Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
Listen Broadcast Information
Wait for Paging
Establish a dedicate connection for a call
Data Transmission and Reception
5.
Release connection for a call
47
Dedicated Physical Channel

• 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
• DPDCH bit rate is indicated with TFI bits on DPCCH

48
Downlink dedicated physical channel(1/3)

• Time multiplexed DPCCH and DPDCH
• DCH is carried by DPDCH
• Discontinuous transmission in DPDCH fields in
  order to handle variable data rates

49
Downlink dedicated physical channel(2/3)

• 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
• 3.84x106/8/1000480 ksps960 kbps
• I/Q modulation (QPSK) 2 bit 1 symbol
• Procedure in the UE when receiving DL-DPCH
• Estimate the SIR (Pilot)
• Detect TPC and adjust UL Tx power
• Detect the used bit-rate and interleaving (TFCI)
• Detect the data (Data) needs buffering of the
  Data field

50
Downlink DPCH Fields
Half Rate Speech
144Kbps
384Kbps
51
Downlink dedicated physical channel(3/3)

• 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

52
Downlink spreading and modulation
DPCH
53
Uplink dedicated physical channel(1/3)

• There can be several uplink DPDCH for one mobile
  but only one DPCCH
• TFCI Transport Format Combination Indicator
• TPC Transmitted Power Control
• FBI Feedback information (for Tx antenna
  diversity)

DPCCH
DPDCH
54
Uplink dedicated physical channel(2/3)

• DPDCH spreading factor from 256 (15 ksps) to 4
  (960 ksps)
• DPCCH spreading factor from 256 (15 ksps)
  constant
• For example SF 16
• 3.84x106/16/1000240 kbps
• I/Q modulation (QPSK) 1 bit 1 symbol
• Procedure in the base station when receiving
  UL-DPDCH/DPCCH
• 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

55
Uplink DPDCH Field
56
Uplink DPCCH Fields

• There are two possible compressed slot formats
  for each normal slot format. They are labelled A
  and B and the selection between them is dependent
  on the number of slots that are transmitted in
  each frame in compressed mode. The channel bit
  and symbol rates given in table 2 are the rates
  immediately before spreading.

57
Uplink dedicated physical channel(3/3)

• 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
• Admission control in RNC allocates those bit
  rates that can be used on physical layer

58
Uplink spreading on dedicated channels
59
Power Control in W-CDMA (1/4)
60
Power Control in W-CDMA (2/4)
UE
UL

Node B
Open loop
61
Power Control in W-CDMA (3/4)
62
Power Control in W-CDMA (4/4)
63
Illustration of Power Control
Inner Loop
Outer Loop
64
Transmitter Power Control Timing
65
Physical Layer Operations in making a phone call
MS
BS
Power on
Cell Search
Listen Broadcast Information
Wait for Paging
Establish a dedicate connection for a call
Data Transmission and Reception
Release connection for a call
6.