WCDMA Technology Past, Present and Future Part IV: Physical Layer on WCDMA

Part IV: Physical 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

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

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

WCDMA Radio Interface Protocol Architecture Control Plane User Plane U-plane Radio Bearers L3 RRC Signalling Radio Bearers Control BMC PDCP L2 Radio Bearers RLC Logical Channels MAC Transport Channels L1 PHY(W-CDMA) Physical Channels

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, DL&UL) Dedicated Physical Control Channel (DPCCH, DL&UL)

Physical Channels (2/3)

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 改圖

Timing Relationship Between Physical Channels k:th S - CCPCH AICH access slots Secondary SCH Primary t S CCPCH,k 10 ms PICH #0 #1 #2 #3 #14 #13 #12 #11 #10 #9 #8 #7 #6 #5 #4 Radio frame with (SFN modulo 2) = 0 (SFN modulo 2) = 1 DPCH,n P Any CPICH PICH for k:th Any PDSCH n:th DPCH

Physical Channel Segmentation, Physical Channel Mapping 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 PhCH #1 Spreading & Gain Weighting I Σ ⊕ j PhCH #2 PhCH #3 Q PhCH #4 DPCCH

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)

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 => 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

Channelisation and Scrambling codes 改圖

Two code layer scheme, downlink 改圖

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 Identify the users Identify the cells

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.

Downlink scrambling code (1/2) Long scrambling code (2^18-1=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 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

Downlink Scrambling Code (2/2) Configuration of DL scrambling code generator.

Uplink Scrambling Code The definition of the nth scrambling code word for the in phase and quadrature components follows as: C1,n=<xn(0)+y(0), xn(1)+y(1),…, xn(N-1)+y(N-1)> C2,n=<xn(M)+y(M), xn(M+1)+y(M+1),…, xn(M+N-1)+y(M+N-1)> where N is the period in chips and M = 16,777,232. Uplink Capacity

Block Diagram of WCDMA PHY Channel Encoder Rate Matching Block Interleaver Data Modulator Spreader Pulse Shaping Filter I/Q Modulator & Up-converter Spreading Multipath Fading Channel Channel decoder De-rate Matching Block De-Interleaver Data Demodulator Pulse Shaping Filter Down-converter & I/Q Demodulator RAKE Receiver Synchronizer Searcher AFC N. B. AGC SIR-Measurement W. B. AGC Despreading

Part IV: Physical 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

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

Physical Layer Operations in making a phone call MS BS Power on 1. Cell Search Listen Broadcast Information Wait for Paging Establish a dedicate connection for a call Data Transmission and Reception Release connection for a call

WCDMA Cell Search One timeslot=625 sec P-CCPCH P-CCPCH Base 1 P-CCPCH P-SCH S-SCH P-CCPCH P-SCH S-SCH P-SCH Base 1 S-SCH P-CCPCH P-CCPCH P-SCH S-SCH P-CCPCH P-SCH S-SCH P-SCH Base 2 P-CCPCH S-SCH P-CCPCH P-SCH S-SCH P-SCH P-SCH Base 3 P-CCPCH S-SCH P-CCPCH S-SCH Mobile Searcher

Cell Search Procedure (1/2) How to do cell search? --> Three steps fast cell search algorithm Step 1: Slot synchronization Step 2: Frame synchronization and code-group identification Step 3: Scrambling code identification

Cell Search Procedure (2/2)

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 改圖

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

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.

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 > 1 Mbps) the parallel code channels have different channelisation code under the same scrambling code but the same SF

Downlink spreading and modulation P-CPICH S-CPICH

Physical Layer Operations in making a phone call MS BS Power on Cell Search 2. Listen Broadcast Information Wait for Paging Establish a dedicate connection for a call Data Transmission and Reception Release connection for a call

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

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

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

Secondary CCPCH

S-CCPCH Fields

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

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

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 改圖

Spreading and Modulation of PRACH Message Part

Structure of the Random Access Message Part

Random-access Message Data and Control Fields

PRACH access procedure

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

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 5. Data Transmission and Reception Release connection for a call

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

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 改圖

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 3.84x10^6/8/1000=480 ksps=960 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

Downlink DPCH Fields Half Rate Speech 144Kbps 384Kbps

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: 改圖

Downlink spreading and modulation DPCH

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) 改圖 DPCCH DPDCH

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 3.84x10^6/16/1000=240 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

Uplink DPDCH Field 改圖

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.

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 改圖

Uplink spreading on dedicated channels

Power Control in W-CDMA (1/4)

Power Control in W-CDMA (2/4) UE UL UE Node B Close loop UL DL Node B Open loop

Power Control in W-CDMA (3/4)

Power Control in W-CDMA (4/4)

Illustration of Power Control Inner Loop Outer Loop

Transmitter Power Control Timing

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 6. Release connection for a call