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ECE 5221 Personal Communication Systems. Prepared by: Dr . Ivica Kostanic Lecture 24 – Basics of 3G – UMTS (4). Spring 2011. PHY layer procedures. Initial system acquisition (cell search) RACH procedure Paging Transmit diversity Open loop power control Fast closed loop power control

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ECE 5221 Personal Communication Systems

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ece 5221 personal communication systems
ECE 5221 Personal Communication Systems

Prepared by:

Dr. Ivica Kostanic

Lecture 24 – Basics of 3G – UMTS (4)

Spring 2011

phy layer procedures
PHY layer procedures

Initial system acquisition (cell search)

RACH procedure


Transmit diversity

Open loop power control

Fast closed loop power control

Handover measurements

MS and UTRAN measurements

cell search procedure
Cell search procedure

Note 1. To demodulate PCCPCH the UE

Needs to determine proper PrSC and

proper code offset

Note 2. There are 512 codes and 38400

possible offsets – size of search space is

~ 20 million possibilities

Note 3. Four step process allows for quick

pruning of the search space

  • 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
step 1 ts synchronization
Step 1 – TS synchronization

UE may receive P-SCH from multiple cells

It will “key on” the strongest one

P-SCH radio frame

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

step 2 frame synchronization
Step 2 – Frame synchronization
  • Example: Word that is unique under cyclic shift:
    • Horse
    • Orseh
    • Rseho
    • Sehor
    • Ehors

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

step 3 prsc identification
Step 3: PrSC identification

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

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

step 4 decoding of pccpch
Step 4: Decoding of PCCPCH

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

Note: BCH is the only transport channel mapped to 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

random access procedure
Random Access Procedure

Note 1: Mobile should send several preambles before it is heard by the system

Note 2: In case of negative AIC response, UE randomizes time and starts again

  • 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)
prach power and timing
PRACH (power and timing)

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

AS = Access Slot

  • 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
rach priority management
RACH – priority management

Mapping between access slots and access sub-channels

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

paging procedure
Paging procedure

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

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

transmit diversity
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
    • Site selection transmit diversity (SSTD)
    • 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
open loop diversity
Open loop diversity

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

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

power control
Power control

Power control fir different PHY channels

  • 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
power assignment for phy channels without pc
Power assignment for PHY channels without PC

Typical power assignments for overhead channels

Overhead channels that need to be heard over entire cell

Overhead channels – no power control

Power allocation depends on the cell coverage requirements

ul open loop power control
UL Open loop power control

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

  • 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
dl open loop power control
DL open loop power control

Estimate of the initial Node B TX power on a DPDCH

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

Initial Dl transmission – before closed loop power control

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

ul closed loop power control
UL closed loop power control

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 in handover
UL closed loop power control in handover

UL power control for mobile 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
dl closed loop power control
DL closed loop power control

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