ECE 5221 Personal Communication Systems

Prepared by:

Dr. Ivica Kostanic

Lecture 24 – Basics of 3G – UMTS (4)

Spring 2011

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PHY layer procedures

• Initial system acquisition (cell search)• RACH procedure• Paging• Transmit diversity• Open loop power control • Fast closed loop power control• Handover measurements • MS and UTRAN measurements

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Cell search procedure

• WCDMA – asynchronous system• Goal of search process

– Synchronize to the system– Demodulate PCCPCH (Primary Common

Control PHY Channel)

• Procedure initiated every time the phone is turned on

• Subdivided into four steps– Acquisition of slot synchronization– Acquisition of frame synchronization– Determination of the PrSC– Resolution of the PCCPCH TTI ambiguity

(TTI = 20ms)

• If the acquired system is the home system – end of the procedure

• If the acquired system in not the home system – procedure may be restarted

Note 1. To demodulate PCCPCH the UENeeds to determine proper PrSC and proper code offsetNote 2. There are 512 codes and 38400possible offsets – size of search space is~ 20 million possibilitiesNote 3. Four step process allows for quick pruning of the search space

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Step 1 – TS synchronization• Accomplished through the search for P-SCH (Primary Synchronization

Channel)• P-SCH uses 256 bit long code at the beginning of each time slot• Each TS is 0.67ms (15 TS make 10ms frame)• All cells (Node Bs) in the network use the same P-SCH code

P-SCH radio frame

UE may receive P-SCH from multiple cellsIt will “key on” the strongest one

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Step 2 – Frame synchronization

• Accomplished through acquisition of S-SCH• S-SCH: 64 codes that consists of 15 code words that

remains unique under cyclic shits• UE reads decodes 15 time slots and based on the

received code, it determines beginning of the frame• Decoded S-SCH points to one of 64 groups for PrSC

Example: Word that is unique under cyclic shift:

• Horse• Orseh• Rseho• Sehor• Ehors

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Step 3: PrSC identification

• There are 512 PrSC arranged in 64 groups with 8 codes in each group• S-SCH points to one of 64 groups reducing the search to 8 PrSC candidates• PrSC is 38400 long and it is aligned with the beginning of the radio frame• By convolving single radio frame with 8 possible candidates, the mobile

determines PrSC of the cell

PrSC establishes the cell identity. Once mobile determines the PrSC it can decode the information associated with a given cell

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Step 4: Decoding of PCCPCH

• Broadcast channel (BCH) is sent over PCCPCH in 20 ms TTI • BCH aligned with beginning of every other frame• Mobile determines the beginning if PCCPCH through simple CRC checks

Once PCCPCH is decoded, the mobile has acquired the system and it may register

Note: BCH is the only transport channel mapped to PCCPCH

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Random Access Procedure• Uses PRACH (PHY Random Access Channel)• Steps in RACH procedure

– Decode BCH to learn the available RACH sub channels and their scrambling codes and signatures (SIB Type 5)

– Select randomly the sub channel and scrambling code – signature combination– Set initial transmit power on the basis of open loop power control– Send 1 ms preamble with selected signature– Wait for the response on AICH– If there is no response, increase power and send preamble again– If the response is negative PHY informs MAC and stops the procedure– If the response if positive, send RACH message (may be 10ms or 20 ms long)

Note 1: Mobile should send several preambles before it is heard by the systemNote 2: In case of negative AIC response, UE randomizes time and starts again

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PRACH (power and timing)

• Power– Initial power determined using open loop power control– Power step and maximum number of power steps: signaled on the BCH

• Timing (signaled on BCH)– Time between preambles – Time between preamble and AI– Time between preamble and message

Note: Setting the access power is balancing between setup success rate and interference

AS = Access Slot

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RACH – priority management• The UE accesses the system through sub-channels• There are 12 sub-channels mapped on 15 access slots (per 20ms)• Depending on the UE priority class, it can be assigned one or more sub-channels• High priority users may use more than one sub-channel

Mapping between access slots and access sub-channels

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Paging procedure

• Registered terminal is assigned a paging group (144, 72, 36 or 18 groups)• Each paging group has a PI assigned on the PICH• Terminal monitors the assigned PI, and in the mean time it sleeps• If there is a page for any terminal within the paging group associate PI is set• Once terminal decodes a set PI, it decodes PCH on the SCCPCH • SCCPH is 3 timeslots after PICH

Note: location of the PICH (and SCCPCH) changes from frame to frame – randomizes paging location of the mobiles

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Transmit diversity

• Used to improve robustness of DL towards fading• Main idea: multiple copies of the signal have small probability of

simultaneous fading• Requires two transmit antennas on the base station• Net gain – DL transmit power reduced and capacity increases• There are three approaches specified in WCDMA

x Site selection transmit diversity (SSTD)x Closed loop transmit diversity– Open loop transmit diversity

• Closed loop transmit diversity – not implemented and it will be removed from the specs

• SSTD proved difficult to implement – will be removed from the specs

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Open loop diversity• Use of the Space Time Block Codes (STBC)• Open loop – no feedback required• Data sent through two antennas• Encoding applied using 4 bits at the time• Uses Alamounti Space Time Block Codes• Used on downlink DPDCH

Note: STBC do not increase symbol rate. They use special encoding scheme to provide diversity reception using a single antenna

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Power control

• Very important in CDMA• Minimizes interference –

increases capacity• Power control classification

– Open loop – no feedback– Closed loop – close to real time

feedback

• Open loop power control– UL open loop– DL open loop

• Closed loop power control– UL inner loop – UL outer loop– DL inner loop – DL outer loop

• Power control – more critical for performance of UL

PHY channel Open loop

Closed loop – Inner

Closed loop – outer

No power control

DPDCH Yes Yes Yes X

DPCCH Yes Yes Yes X

PCCPCH X X X Yes

SCCPCH (BCH and FACH)

X X X Yes

AICH X X X Yes

PICH X X X Yes

PRACH Yes X X X

CPICH X X X Yes

PSCH X X X Yes

SSCH X X X yes

Power control fir different PHY channels

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Power assignment for PHY channels without PC

• Overhead channels that need to be heard over entire cell• Overhead channels – no power control• Power allocation depends on the cell coverage requirements

Overhead channel Power range Typical power settings Comment

CPICH -10 to 50 dBm 33 dBm About 10% of the available PA power

PSCC/SSCH -35 to 15 dB relative to CPICH

-5 dB relative to CPICH

PCCPCH (BCH) -35 to 15 dB relative to CPICH

-2 dB relative to CPICH

SCCPCH (PCH) -35 to 15 dB relative to CPICH

-2 dB relative to CPICH

SCCPCH (FACH) -35 to 15 dB relative to CPICH

1 dB relative to CPICH

AICH -22 to 5 dB relative to CPICH -6 dB relative to CPICH

PICH -10 to 5 dB relative to CPICH -7dB relative to CPICH

Typical power assignments for overhead channels

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UL Open loop power control

• Necessary to prevent UL interference due to mobiles that are not in closed loop Power control

• Open loop on the UL is implemented on– PRACH – during access– UL DPDCH and UL DPCCH – before closed

loop control starts

• Based on the mobile estimates of what it should transmit

• Not very accurate – nominal accuracy is +/- 9 dB

ConstULRSCPPP ceinterferenCPICHCPICH-TX preamble

Estimate of the initial mobile TX power on PRACH or UL DPCCH. UL DPDCH is adjusted depending on transport format

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DL open loop power control

• Initial Dl transmission – before closed loop power control

• Initial power depends on requested data rate, mobile reported CPICH quality and target Eb/No

Estimate of the initial Node B TX power on a DPDCH

oc

b

NE

WR

/P

/NE/log10P

CPICHTX

ob DPCH-DL

Note: There is a direct dependence between TX rate and power

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UL closed loop power control

• UL power control is implemented in two loops

– Inner loop: • Fast loop• Instantaneous SIR of the

mobile on the uplink• Executed by Node B

– Outer loop: • Slower loop• Manages the target SIR for

the mobile on the uplink • Executed by RNC

• Mobile receives one TPC (Transmit Power Control) command per every time slot UL closed loop power control

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UL closed loop power control in handover

• Two types of handover– Soft (two different Node Bs)– Softer (two cells of the same

Node B)

• Each cell issues TPC to the mobile

• TPC bits from the same Node B are combined – one command per Node B

• TPC commands from different Node B’s – “or of the downs”

– In the case of conflicting commands the mobile powers down

UL power control for mobile in handover

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DL closed loop power control• On the DL both inner and outer loops are at the UE• UE sends one TPC command received by all Node Bs in the active set• All node B’s adjust their power in the same direction • Additional algorithms need to be implemented at RNC to make sure Node B

powers do no drift due to erroneous UP frames

DL closed loop power control