01 - wcdma principles and radio functionality

1

WCDMA Principles and Radio Functionality

WCDMA Principles and Radio Functionality 2

Topics1. Principles of Code Division Multiple Access2. 3GPP Standardization3. UMTS Overview4. Multiple Access, Channelisation and Scrambling Codes 5. Rake Receiver6. Power Control 7. Handovers

2


Principles of Code Division Multiple Access


Multiple Access ApproachesFrequency Division Multiple Access

Each User has a unique frequency

(1 voice channel per user)

All users transmit at the same time

AMPS, NMT, TACS

Each Transmitter has a unique spreading code

Each Data Channel has a uniqueorthogonal code

Many users share the same frequency and time

IS-95, cdma2000, WCDMA

CodeDivision Multiple Access

SpreadSpectrumMultipleAccess

Multiple Transmittersand Data Channels

Each User has a unique time slot

Each Data Channel has a uniqueposition within the time slot

Several users share the same frequency

IS-136, GSM, PDC

Time Division Multiple Access

3


Channel Capacity• In 1948, Claude Shannon of Bell Laboratories

proved the following remarkable formula:

• Capacity measured in bits/second− one bit = yes/no, or on/off, or 0/1

• Bandwidth is in hertz• Signal power and noise power measured

in same units, e.g. watts• Shannon’s result is the best that can ever be achieved—it’s the job

of engineers to design systems which approach this capacity

⎟⎟⎠

⎞⎜⎜⎝

⎛+×=

power noisepower signal1logbandwidthcapacity 2

Claude Shannon (1916—2001)


Orthogonality and Correlation of Signals• In communication receivers,

need to quantify the degree to which discrete waveforms or digital symbols 'differ' from one another.

• This can be accomplished by correlation operation

• Two signals s1(t) and s2(t) are said to be orthogonal or in the interval t1 to t2 if:

( ) ( ) 0 2

1

21 =⋅∫ dttstst

t

s (t)1

t0

1 2

1

Signallinginterval, T

t0

1 2

s (t)2

1

-1

-1

( ) ( )dttstsSt

txy 2

1

21∫ ⋅=

4


Correlation Values

( ) ( ) 1 1

0 21 +=⋅∫ dttsts

s (t)1

t0

+1

SignallingInterval, T

t0

s (t)2

+1

-1

-1

0.5 1

0.5 1

( ) ( ) 1 1

0 21 −=⋅∫ dttsts ( ) ( ) 0 1

0 21 =⋅∫ dttsts

s (t)1

t0

+1


t0

0.5 1

s (t)2

+1

-1

-1

0.5 1

s (t)1

t0

+1


t0

s (t)2

+1

-1

-10.5 1

0.5 1

Similar Signals Dissimilar Signals Orthogonal Signals


DS Spread Spectrum System

t

t

t

t

t

m (t)

m (t)p (t)

p (t)

p (t)

x (t)

T c

Tb

u (t)

LPF

Coherentdetection

2cosω t c

Correlator

p(t)

Recovereddata

signalDatasignal m(t)

Rc = 1Tc

Rb = 1Tb

p(t)s(t)

ModulatorMultiplierChannel

r(t) u(t) x(t)dt

TbT

b

)(1

0∫ v(t)

Codegenerator

Carriercosω t c

Carrierrecovery

Codegenerator

Codesync

5


OC-N

OC-2

OC-1

RFModulation

RFDemod

OC-2

Data Channel 1

Data Channel 2

Data Channel 3

Receiver1

Σ

Transmitter 1

Codes in WCDMA

SC-2

RFModulation

Transmitter 2

SC-M

RFModulation

Transmitter M

SC-1..

..

SC-1

Scrambling Code (SC)• Used to distinguish

transmission source (Base Station or Mobile Station)

• SC are NOT orthogonal codes

Orthogonal Code (OC)• allows multiple data streams

to be sent on the same RF carrier


Practical Issues with Codes• UEs near to cell edge experiences higher interference due to

− DL channels from neighbour cells (iother)− Degraded orthorgonality of own cell’s channels (iown)

• When there is little multipath with a cell, there is little mutual interference between DL channels

• Antenna downtilt is used to manage interference from other cells

UE3

PN 1

PN 2

OC 1OC 2

UE2

UE1

CPICH+CCH

6


3GPP Standardization


Why 3G and UMTS?• Evolution of circuit switched and voice centric 2G.• Needed to realize multimedia heterogeneous services for cellular

mobile communication systems. − Both packet and circuit switched data traffic.− Variable data rates.− Peak data rates around 2 Mbps.

• Improvement in spectral efficiency.− Lower spectrum cost.− Higher data rates.

• Lowering the cost as the result of over-all efficiency improvements.− Lowers cost of equipment and spectrum.− Improves users acceptance while increasing revenues of service

providers and equipment makers.

7


What is 3GPP?• 3rd Generation Partnership Project (3GPP) is a collaboration

agreement that was established in 1998.• Original scope was to prepare, approve and maintain Technical

Specifications (TSs) and Technical Reports (TRs) for a 3G mobile system.

− Universal Terrestrial Radio Access (UTRA) with Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.

• Systems was to be based on evolved GSM core networks.• The scope was extended.

− Maintenance and development of the GSM TSs and TRs.− Evolution of GSM radio access technologies.

General Packet Radio Service (GPRS).Enhanced Data rates for GSM Evolution (EDGE).


3GPP Roadmap

Release 6 (HSUPA)• Commercial in 2007.• MBMS Platform

Release 5 (HSDPA)• Trials in 2004.• Commercial in 2005.• IMS Platform

Release 99 and R4• Trials in 1999.• CS Voice, 2 Mbps PS Data• Bearer independent CS

GSM GPRS EDGE UMTS HSDPA HSUPA

3GPP Responsibility

• A number of options studied:• Wideband CDMA.• Wideband TDMA.• OFDMA.• ODMA.

• The core supporter of the research leading to the selection of 3G technology was EU.

• WCDMA was selected by ETSI in January 1998.

Release 7 (HSPA+)• HOM 64QAM• MIMO 2x2 16QAM

Release 8 (LTE)• OFDM, MINO 2x2• MIMO 2x2 16QAMRelease 8 (HSPA+)• MIMO 2x2 64QAM• 2 carrier 64QAM

Release 9 (LTE)• EnhancementsRelease 9 (HSPA+)• 2 carrier (10MHz)• MIMO 4x4 64QAM

LTEHSPA+

8


3GPP Roadmap• Release 99 (R99)

− Original UMTS system,− CS voice services with peak

data rates up to 2Mbps.− Commercial systems delivered

PS data of up to 384kbps.• Release 4 (R4)

− Defined a bearer-independent CS architecture, separating switches into gateways and controllers, and laying the groundwork for IP Multimedia Subsystem (IMS).

• Release 5 (R5)− Defined High Speed Downlink

Packet Access (HSDPA), with DL data rates up to 14Mbps.

− Completed design of IMS.

• Release 6 (R6)− Increased UL data rates to

>5Mbps with High SpeedUplink Packet Access (HSUPA)

− Introduced support formultimedia broadcast/multicastservices (MBMS).

• Release 7 (R7)− Further enhancements to

HSDPA and HSUPA, calledHSPA+.

− Higher-order modulation(64QAM)

− Multiple-Input/Multiple-Output(MIMO) antenna systems offerup to 21.6Mbps (with only64QAM modulation) and28.8Mbps (with 2x2MIMO and16QAM modulation).


3GPP Roadmap• Release 8 (R8)

− Defined the Long TermEvolution (LTE) system,starting the transition to 4Gtechnology.

− Use of OFDMA as the accessmethod and MIMO.

− HSPA+ is also enhanced in R8to support up to 42Mbps eitherusing 64QAM modulation with2x2MIMO or deploying only64QAM with dual carriers(10MHz = 2*5MHz)

• Release 9 (R9)− Further LTE enhancements,

including support for MBMSand definition of Home eNBsfor improved residential and in-building coverage.

− HSPA+ in R9 and beyond willsupport 4x4MIMO with 64QAMon dual carriers of 5MHz eachto provide data rates up to84Mbps.

• Release 10 (R10)− Includes definition of LTE

Advanced− Offers to support 1Gbps on DL

and up to 500Mbps on UL using 8x8MIMO, channelaggregation up to 100MHz,and relay repeaters.

9


UMTS Overview


Basic UMTS Features• Based on Wideband CDMA (WCDMA) air interface.

− CDMA chip rate is 3.84 MHz.− Typical channel separation is 5 MHz with 200 KHz raster. − Orthogonal Variable Spreading Factor (OVSF) codes used to

support multi-rate and heterogeneous traffic.− Full frequency reuse.

• UMTS Terrestrial Radio Access Network (UTRAN) is designed around WCDMA with a number of specific solutions.

− Power control (open and close loop).− Soft and softer handover.− Transmitter and receive antenna diversity.− Admission and load control.

• Uses the same core network as GSM/GPRS. − Packet and circuit switched data traffic are supported.

• Open interface architecture between network elements.

10


UMTS Spectrum• Specific spectrum is allocated for UMTS.

• In USA, FCC the spectrum is partitioned differently.− PCS bands are at 1.9 GHz.− Unlike in EU case, in the USA spectrum is never allocated

exclusively for a specific technology.• 3G Extension Band 2500-2690 MHz – still debated.

− Possible arrangement: FDD UL (14): 2500 MHz – 2570 MHz, FDD DL (14): 2620 MHz – 2690 MHz, TDD (10): 2570 MHz – 2620 MHz.

FDD Uplink FDD DownlinkTDD

1920 - 1980MHz 2110 - 2170 MHz

TDD

1900 - 1920MHz 2010 - 2025MHz

4 312 12


Relevant Spectrum Allocations

11


Frequency Bands

Operating Band

UL FrequenciesUE transmit, Node B receive

DL frequenciesUE receive, Node B transmit

TX-RX frequency separation

I 1920 - 1980 MHz 2110 -2170 MHz 190 MHz

II 1850 -1910 MHz 1930 -1990 MHz 80 MHz.

III 1710-1785 MHz 1805-1880 MHz 95 MHz.

IV 1710-1755 MHz 2110-2155 MHz 400 MHz

V 824 - 849MHz 869-894MHz 45 MHz

VI 830-840 MHz 875-885 MHz 45 MHz

VII 2500 - 2570 MHz 2620 - 2690 MHz 120 MHz

VIII 880 - 915 MHz 925 - 960 MHz 45 MHz

IX 1749.9 - 1784.9 MHz 1844.9 - 1879.9 MHz 95 MHz

X 1710-1770 MHz 2110-2170 MHz 400 MHz

XI 1427.9 - 1452.9 MHz 1475.9 - 1500.9 MHz 48 MHz

XII 698 - 716 MHz 728 - 746 MHz 30 MHz

XIII 777 - 787 MHz 746 - 756 MHz 31 MHz

XIV 788 - 798 MHz 758 - 768 MHz 30 MHz


Channel Number• Carrier frequency is designated by the UTRA Absolute Radio

Frequency Channel Number (UARFCN). • For each operating Band, the UARFCN values are defined as

follows:− Uplink: NU = 5 * (FUL - FUL_Offset),

for the carrier frequency range FUL_low ≤ FUL ≤ FUL_high

− Downlink: ND = 5 * (FDL - FDL_Offset),for the carrier frequency range FDL_low ≤ FDL ≤ FDL_high

12


Channel Number• For each operating Band, FUL_Offset, FUL_low, FUL_high, FDL_Offset,

FDL_low and FDL_high are defined below for the general UARFCN.

Band

UPLINK (UL)UE transmit, Node B receive

DOWNLINK (DL)UE receive, Node B transmit

UARFCN formula offset

FUL_Offset [MHz]

Carrier frequency (FUL) range [MHz]

UARFCN formula offsetFDL_Offset [MHz]

Carrier frequency (FDL) range [MHz]

FUL_low FUL_high FDL_low FDL_high

I 0 1922.4 1977.6 0 2112.4 2167.6II 0 1852.4 1907.6 0 1932.4 1987.6III 1525 1712.4 1782.6 1575 1807.4 1877.6IV 1450 1712.4 1752.6 1805 2112.4 2152.6V 0 826.4 846.6 0 871.4 891.6VI 0 832.4 837.6 0 877.4 882.6VII 2100 2502.4 2567.6 2175 2622.4 2687.6VIII 340 882.4 912.6 340 927.4 957.6IX 0 1752.4 1782.4 0 1847.4 1877.4X 1135 1712.4 1767.6 1490 2112.4 2167.6XI 733 1430.4 1450.4 736 1478.4 1498.4XII -22 700.4 713.6 -37 730.4 743.6XIII 21 779.4 784.6 -55 748.4 753.6XIV 12 790.4 795.6 -63 760.4 765.6


Identical to GPRS Core Network.

Specifically designed to support WCDMA wireless interface.

Circuit switched.

Packet switched.

MS

MSC

UMTS Network Architecture

13


UMTS Network Elements Acronyms• UE – User Equipment, i.e.,

mobile terminal or MS.• Node B – base station, or BTS.• Cell – is an area covered with a

transmission that corresponds to a unique WCDMA scrambling (this is typically a sector served by a base station).

• CN – Core Network.• RNC – Radio Network

Controller.• Uu - Interface for Node B-to-UE

Communication.• Iu - Interface for RNC-to-CN

Communication.• Iub - Interface for RNC and

Node B Communication.• Iur - Interface for RNC-to-RNC

Communication.

• SGSN - Serving GPRS Support Node.

• GGSN - Gateway GPRS Support Node.

• EIR - Equipment Identity Register.

• VLR - Visitor Location Register. • HLR - Home Location Register.• AuC - Authentication Center.• MSC - Mobile Services

Switching Centre.• VLR - Visitor Location Register.• GMSC - Gateway MSC.• PSTN - Public-Switched

Telephone Network.• SIM - Subscriber Identification

Module.


Multiple Access, Channelisation and Scrambling Codes

14


GSM900/1800 3G (WCDMA)

UMTS & GSM Network Planning

SC1

SC2

SC3SC7

SC6 SC4

SC5

SC7

SC6 SC4

PN5

SC1

SC2

SC3

SC1

PN2

SC3SC7

SC6 SC4

SC5

SC1

SC2

SC3SC7

SC6 SC4

SC5

SC1

SC2

SC3SC7

SC6 SC4

SC5

Frequency Planning

Scrambling Code Planning


Differences between WCDMA & GSM

WCDMA GSM

Carrier Spacing 5 MHz 200 kHz

Frequency Reuse Factor

1 1 - 18

Power Control Frequency

1500 Hz 2 Hz or lower

Quality Control Radio Resource Management Algorithms

Network Planning (Frequency Planning)

Frequency Diversity 5 MHz bandwidth gives Multipath Diversity with

Rake Receiver

Frequency Hopping

Packet Data Load-Based Packet Scheduling

Timeslot Based Scheduling with

GPRSDownlink Transmit Diversity

Supported for Improving Downlink Capacity

Not Supported by the standard, but can be

applied

High bit rates

Spectral efficiency

Different quality requirements

Efficient packet data

15


WCDMA Technology

3.84 MHz

5 MHz

f

Direct Sequence (DS) CDMATime

Freq

uenc

yWCDMA

5 MHz, 1 carrierTDMA (GSM)

5 MHz, 25 carriers

f


• Spreading is the basic operation in a WCDMA transmitter.• Each data symbol multiples, i.e., modulates a sequence of WCDMA

chips.

WCDMA Principles - Spreading

d(t))

c(t))

t

t

chipsym

chip

sym

TT

T

T

=

=

=

gain Spreading

period chip

period symbol

Scrambling Code Generator

Chip ClockFc >> Fd

RF Modulator

cos(ωrf*t)

Nulls @ N*Rc Frf

Filter

PN Code Mask

“Bits”

“Chips”

0 0.1 0.2 0.3 0.4 0.5 0.6-50

-40

-30

-20

-10

0

10

Frequency

Pow

er S

pect

rum

Mag

nitu

de (d

B)

0 0.1 0.2 0.3 0.4 0.5 0.6-60

-50

-40

-30

-20

-10

0

10

Frequency

Pow

er S

pect

rum

Mag

nitu

de (d

B)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 107

-40

-20

0

20

40

60

80

Frequency

Pow

er S

pect

rum

Mag

nitu

de (d

B)

)(td

)(tc)( )( )( tctdtx =

16


Spectrum Example• In this example, we present spectrum of original input signal d(t) and

output x(t) after spreading.• Spreading factor is 128.• Note that input signal

spectrum is spreaded across spectrum range that is 128 larger than original one.

• Even though spreaded spectrum appears to be ‘thinner’, energy-wise both signals are identical.


Spreading and Processing Gain

• Processing gain is defined as

where is the spreading factor

Unspread narrowband signal Spread wideband

signal

Bit rate R

Bandwidth W (3.84 Mchip/sec)

Frequency

Pow

er d

ensi

ty (W

atts

/Hz)

( ) ( )RWSFGp log10log10 ==SF

17


Processing Gain Examples• Spreading

sequences have a different length

• Processing gain value depends on the user data rate

• Higher proceesing gain gives better receiver sensitivity.

dB10=pG

dB25=pG

Voice user (R = 12,2 kbit/s)

Packet data user (R = 384kbit/s)

Frequency (Hz)

Frequency (Hz)

Pow

er d

ensi

ty (

W/H

z)Po

wer

den

sity

(W

/Hz)

R

R

Unspreadnarrowband

signal Spread wideband signal




• The symbol that is sent over a CDMA code can be retrieved with the following despreading operation.

• In other words, perform multiplication with the spreading sequence (i.e., its conjugate), and integrate the product over the symbol duration.

− Ideal synchronization is required.

WCDMA Principles - Despreading

c*(t))x(t))

Tsym

d(t))

c(t))

∫symT

dt0

18


OC-N

OC-2

OC-1

RFModulation

RFDemod

OC-1

Data Channel 1

Data Channel 2

Data Channel 3

Receiver 1Σ

Transmitter 1

Multiple Access in WCDMA

SC-2

RFModulation

Transmitter 2

SC-M

RFModulation

Transmitter M

SC-1..

..

SC-1

Scrambling Code (SC)• Used to distinguish

transmission source (Base Station or Mobile Station)

Orthogonal Code (OC)• allows multiple data

streams to be sent on the same RF carrier

RFDemod

OC-2Receiver 2

SC-1


Channelisation Codes in UMTS• Orthogonal code are used to multiplex different data streams that

are send to one or more different users. • Codes with different spreading factor can be selected such that the

orthogonality is preserved. − Orthogonal Variable Spreading Factor (OVSF) codes used to

support multi-rate and heterogeneous traffic.

c10 = (1)

c20 = (1,1)

c21 = (1,-1)

c40 = (1,1, 1,1)

c41 = (1,1, -1,-1)

c42 = (1,-1, 1,-1)

c43 = (1,-1,-1,1)

This branch must be excluded from further selection because the code c21 is selected earlier.

• The spreading factor of the OVFS codes is 2K, where K = 1, …, 9.

• RNC allocates which codes a base station uses in the downlink.

• No need to coordinate code usage between base station (because of unique scrambling).

SF=1 SF=2 SF=4

19


DL & UL Channalisation Codes• Walsh-Hadamard codes:

− Orthogonal variable spreading factor codes (OVSF codes)− SF for the DL transmission in FD mode = − SF for the UL transmission in FD mode =

• Good orthogonality properties− cross correlation value for each code pair in the code set equals 0

• Orthogonal codes are suited for channel separation, where synchronisation between different channels can be guaranteed

− e.g. DL channels under one cell, UL channels from a single user;

• Orthogonal codes have bad auto correlation properties and thus not suited in an asynchronous environment.

− UL signals from different users are not time synchronised.

{ }512 256, 128, 64, 32, 16, 8, 4,{ }256 128, 64, 32, 16, 8, 4,


• Downlink− Reuse 1 - neighboring cells use

the same frequency.− To separate different cells, in the

downlink, each cell uses a unique scrambling code.

− Long scrambling Gold codes are used with 38,400 chips (10 msec).

− There is a set of 512 codes (each to be assigned to a different cell, which is a part of cell planning procedure).

64 subsets each with 8 scrambling codes.A terminal determines the particular code using the cell search procedure.

• Uplink− Long scrambling Gold codes are used with 38,400 chips

(10 msec),− Short scrambling may be with 256 chips (to support MUD).

Complex Scrambling in UMTS

x(t))

d1(t))

c1(t))

dN(t))

cN(t))

Σ

s(t))

OVSFcodes

Scramblingcodes

20


Channelisation and Scrambling Codes

Channelisation code Scrambling CodeUsage Uplink: Separation of physical data

(DPDCH) and control channels (DPCCH) from same terminals.Downlink: Separation of downlink connections to different users within one cell

Uplink: Separation of mobileDownlink: Separation of sectors (cells)

Length 4 – 256 chips (1.0 – 66.7 us)Downlink also 512 chipsDifferent bit rates by changing the length of the code

Uplink: (1) 10 ms = 38400 chips or (2) 56.7us = 256 chipsOption (2) can be used with advanced base station receiversDownlink: 10 ms = 38400 chips

Number of Codes

Number of codes under one scrambling code = spreading factor

Uplink: 16.8 millionDownlink: 512

Code Family

Orthogonal Variable Spreading Factor Long 10 ms code: Gold CodeShort code: Extended S(2) code family

Spreading Yes, increases transmission bandwidth

No, does not affect transmission bandwidth


Inter-Code Interference • For OVSF codes to be orthogonal, they have to be received

perfectly aligned, i.e., synchronized. − In DL, codes are transmitted perfectly aligned, but due to multipath

propagation, orthogonality is violated. − In UL, transmission from multiple terminals cannot be synchronous,

which violates orthogonality between codes sent by different terminals.

• For codes that randomly interfere between each other, and are received with same power, signal-to-interference-and-noise ratio is

∑−

=

+= 1

1/

N

iNoSFP

PSINR

P - received powerSF – spreading factorNo - noise variance (power)N – number of codes that randomly interfere

21


RAKE Receiver


Wireless Channel • Path loss• Shadowing

• Multipath propagation− Small-scale (fast) fading− Frequency selectivity

• Temporal variations

3. Remote scattering with significant delay

Large scale fading

1. Direct2. Multiple scattering

with similar delay

22


Multipath• The same signal can arrive at a receiver multiple times via different

paths, with different time delays and signal strengths.• WCDMA systems can use these multipaths to its advantage by

adjusting the multipath phases and recombining them into one stronger signal.

• Both subscriber devices and RBSs are able to recombine multipaths, allowing them to further reducing transmit power.


Scrambling Codes & Multipath Propagation

1 code Scrambling

C31 Δ+C

2 code Scrambling

C11 Δ+C

21 Δ+C2C

23


Multipath Fading• Fast (Rayleigh) Fading

Time between fades is related to

• RF frequency

• Geometry of multipath vectors

• Vehicle speed: Up to 4 fades/sec per kilometer/hour

time (mSec)

CompositeReceived

Signal Strength

Deep fade caused by destructive summationof two or more multipath reflections

msec


Multipath Diversity• Each path experiences

independent small-scale fading.• Having multiple paths, there is a

greater chance to experience one or more paths with high channel magnitude, i.e., channel quality.

• More paths

PRO: more diversity.CON: higher interference because each path acts as additional interfering WCDMA code*.

*Using equalizer instead of the RAKE receiver may suppress the interference while exploiting the diversity.

Path 1 strong Path 2 weak

Path 1 weak Path 2 strong

24


Temporal Diversity• Temporal variations provide UMTS

with one more source of diversity.• We present time variations of SNR for

different velocities (3, 60 and 120 kmph), for up to 0.1 sec.

• The higher terminal velocities result in greater time diversity, i.e., there is a greater chance that the channel reaches higher magnitude.

• Higher velocity PRO: more diversity.CON: higher channel estimation noise*.


The RAKE Receiver• WCDMA Mobile Station RAKE Receiver Architecture

Each finger tracks a single multipath reflection. Also be used to track other base station’s signal during soft handoverOne finger used as a “Searcher” to identify other base stations

Finger #1

Finger #2

Finger #N

Searcher Finger

Combiner

Sum of individual multipath components

Power measurement of Neighboring Base Stations

MRC

25


Power Control


WCDMA Reception Issues• Unequal received power levels degrade SSMA performance

− Near-Far Ratio, terrain, RF obstacles, “Turn-the-Corner” effects, ...

• Multipath fading cancellation• Time of Arrival delay spread

26


Power Control• Benefits

− Works against detrimental channel fading (large and small scale, i.e., fast fading) maintaining the target SINR to ensure the required QoS.

− Lowers the interference that the transmission causes by preventing the excessive transmit power levels.

− Extends mobile terminal battery life.• Mechanisms

− Open loop power control.− Closed loop power control.− Outer power control loop.

• Applied both in the uplink and downlink.• Not all physical layer channels are power controlled.

− Some common and shared channels are not power controlled using closed loop mechanism.


Open Loop Power Control• Main idea

− Using the estimate of the received signal power, the transmitter sets its power assuming that the downlink and uplink channels are symmetric.

• Application − Used in the initial phases before the closed loop power control is

established;− Channels that are not subject to the closed loop power control use

this mechanism;• Practical problems:

− In UMTS-FDD systems, uplink and downlink channels are not identical. This causes the transmit power mismatch between true and the assumed channel.

− Usually this mechanism works well against large scale fading (which is typically identical for the uplink and downlink).

− Small scale, i.e., fast fading, cannot be mitigated using the open loop power control.

27


Closed Loop Power Control• Main idea

− The receiver measures quality of the channel. − Based on the measurements, it sends a request for higher or lower

transmit power so that the target SINR is maintained.− The feedback is just one bit, indicating whether the transmit power

should be changed.− It is sent 1500 times per second (1.5 KHz), allowing mitigation of

the fast fading.


Outer Loop• Closed loop power control needs a target SINR.• For a given communication session, the UMTS Quality-of-Service

(QoS) mechanism sets the target Block Error Rate (BLER). − For the downlink, the target BLER is sent by the RNC to the mobile

terminal.• SINR required to achieve the target BLER depends on a number of

parameters:− Multipath profile of the radio channel; − Mobile terminal speed;− Errors in the closed loop power control;

• An uplink example:− If RNC detects greater Block Error Rate than targeted, it will

elevate the target SINR. The target is passed to a base station that provides access for the particular mobile terminal.

− On the contrary, if the BLER is low, the target SINR may get lowered.

28


WCDMA Power Control

Open-Loop Power Control

Compute Initial

Transmit Power

Measure received power

from BTS

Read BTS transmit power from Broadcast

Channel

Transmit Access Preamble

Access Acknowledged?

Increase Transmit Power by 1 dB

No

Yes

UE BeginsUplink TCH

Transmission

Outer-Loop (slow) Power Control Inner-Loop (fast) Power Control

FER/BLER Acceptable

?

Raise RxPower Target

Lower RxPower Target

No

Yes

Received power

> target?

Increase UE Transmit Power

by 1 dB

Decrease UE Transmit Power

by 1 dB

No

Yes


WCDMA Power Control

Inner-loop power control(Initial receive power target)

BS Receive Power Target

Open-loop Power ControlAccess Preambles

Outer-loop power control(Update receive power target of inner-loop)

BTS Receive Power

time

800 updates/sec (IS-95, cdma2000)1500 updates/sec (WCDMA)

The PRACH is “power controlled” by means of preamble ramping i.e. UL open loop PC

Preambles DPCHRACH

29


Closed Loop Power Control• Practical problem

− Feedback rate and frequency (1.5 Kbps) can be too slow for fast varying wireless channels.

− If maximum transmit power is reached, power control fails to increase the SINR (e.g., a UE being at the edge of a cell).

− Typically, link budget needs to take account of this by adding a Power Control Headroom margin.


Soft Handover

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Inter-Cell Interference• In UMTS, the frequency reuse is 1 between the neighboring cells

and sectors - causes inter-cell interference both in the uplink and downlink.

Instead of being detrimental, this effect is applied in the soft handover procedure that is beneficial to mobile terminals at the cell edge.

For example, in the downlink a mobile terminal that is at the edge of a cell experiences comparable signal strengths of the neighboring base station downlink transmissions.


Handover Methods• Softer handover, WCDMA only (“Make-before-break”)• Soft handover, WCDMA only (“Make-before-break”)• Hard handover, all access methods (“Break-before-make”)

Sector A2 C2B2SofterHandover

SofterHandover

Soft Handoverwith neighboring cells

(same carrier) Hard Handoverbetween cells with different carriers

Hard Handoverbetween cells with different carriers

Sector A

SofterHandover

B Sector A1 C1B1

SofterHandover

SofterHandover

SofterHandover

C

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Handover MethodsSofter handovers, WCDMA only• “Make-before-break”• A new connection can be made prior to breaking the old connection• Occurs between sectors with the same carrier within each cell

Soft handover, WCDMA only• “Make-before-break”• Occurs between neighboring cells with same carrier

Hard handover, all access methods• “Break-before-make”• Mobile must disconnect (or break) its connection with old cell before

connecting to new cell• Occurs between neighboring cells with different carriers


Soft Handover Advantages• Reduction in TX power reduces interference and increases capacity.

Hard Handover

Soft Handover

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Soft Handover Advantages• Transition from one Node B sector to another is triggered by signal

strengths of the pilots. • Since WCDMA allows soft handover to occur, UE is frequently

demodulating and combining signals from multiple Node Bs.− This allows for lower transmit power on downlink.

• Furthermore, multiple Node B sectors in soft handover are demodulating multiple signals from UE and forwarding them to the RNC.

− This allows UE to lower its transmit power as well.• Lower power on both down and uplink equate to increased system

capacity and allow for longer battery life for UEs.


WCDMA Without Soft Handover

time

Trouble zone: Prior to Hard Handover, the MS causes excessive interference to BTS2

BTS2 Receive Power Target

UE responding to BS1power control bits

UE responding to BS2power control bits

time

BTS1 Receive Power Target

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WCDMA With Soft Handover

time

BS2 Receive Power Target

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 1 1 1 12 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

UE responding to BS1power control commands

UE responding to BS2power control commands

time

BS1 Receive Power Target

1 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 12 2 2 2 2 2

1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

BTS1 BTS2 Action0 0 Reduce power0 1 Reduce power1 0 Reduce power1 1 Increase power

UE responds to power control commandsfrom both BS1 and BS2


• CDMA Soft Handover

− One finger of the RAKE receiver is constantly scanning neighboring Pilot Channels.

− When a neighboring Pilot Channel reaches the w_add threshold, the new BTS is added to the active set

− When the original Base Station reaches the w_drop threshold, originating Base Station is dropped from the active set

WCDMA Soft Handover

Monitor Neighbor BTS Pilots Add Destination BTS Drop Originating BTS

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Soft Handover Procedure in UMTS• Assisted by mobile terminal – Mobile Evaluated Handover (MEHO).• Base station that initially connects to the mobile terminal, informs it

about the base stations in the neighborhood (the neighbor list).• Mobile terminal monitors strength of downlink received signal (i.e.,

common pilot) from each base station in the neighbor list.• Upon a request or if particular trigger conditions are met, mobile

terminal informs RNC (i.e., RRC) about measurements.• If the received power exceeds certain threshold, for certain duration

of time, the RNC commands the mobile terminal to get in a handover with the selected base station.

• If resources available, the RNC will command that the base station connects to the mobile terminal, i.e., joins its active set.

• Similarly, if received power is below certain threshold, mobile terminal initiates removal of corresponding base station from handover.

• Note that not all UMTS channels can can support soft handover.


• Hysteresis and trigger time (T) are used to prevent series of short term changes of cell associations i.e., to prevent ping-pong effect.

• Hysteresis characteristic - drop window is typically slightly larger than add window.

Handover Example –A Hysteresis-Based Algorithm

Connected to 1 Add 2 Replace 1 with 3 Drop 3

E c/I o

= Pi

lot p

ower

/ R

ecei

ved

pow

er [d

B]

Base station 2

Base station 3

Ec3/Io3 - Ec1/Io1 > WREPLACE

T T

[Event 1A] [Event 1C] [Event 1B]

T

Ec2/Io2 - Ec3/Io3 > WDROP Ec1/Io1 - Ec2/Io2 < WADD

Base station 1

Assume Active Set = 2

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Soft Handover and Micro-Diversity• In downlink, from a number of neighboring base stations, the same

information is sent to a mobile terminal that is in a soft handover.− Each base station uses different scrambling allowing the mobile

terminal to separate the received signals.− received signals are combined in mobile terminal (e.g., using

maximum ratio combining).• In uplink, different multipaths from mobile are received by antenna

diversity and combined in Node B using maximum ratio combining.

Node BRAKE

ReceiverMS

RAKEReceiver

Micro Diversity PointsMaximum ratio combining is used

Summed signal


Macro Diversity in the RNC• In the uplink, the signal

from a mobile terminal in a soft handover is received and decoded by a number of neighboring base stations.

• The decoded messages are sent to the serving RNC that picks a version that has a correct CRC.

• This is known as selection combining.

Macro Diversity Pointselection combining is used

Active cell set

Node B

Node B

Node BD-RNC

S_RNC

CoreNetwork

36


Soft Handover and Macro-Diversity• PRO - There is improvement in diversity because the same

message is sent to and from multiple base stations, thus experiencing different channels. This is known as macro-diversity.

• This is particularly beneficial at the cell edge where there is a little room left for the power control to mitigate channel fading.

• CON - One mobile terminal is occupying a multiplicity of resources because it is connected to a number of base stations.

• Downlink inter-cell interference is also increased.


Cell Breathing • If number of mobile terminals or data rates are increasing in

downlink, the relative power of the primary pilot (P-CPICH) is diminishing, i.e., its Ec/I0 is getting lower.

− The effective cell area is getting smaller.− Lower Ec/I0 leads to a higher probability of mobile terminal

performing handover to the neighboring cells.− This is a form of load control mechanism in WCDMA networks.

Effective coverage is getting smaller with the higher cell loading. As cell shrinks, the mobile

terminal will more likely perform a handover to one of the neighboring cells.

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Soft Handover and Cell Breathing ExampleE c

/I o=

Pilo

t pow

er /

Rec

eive

d po

wer

[dB

]

Base station 2

Handover condition: Ec2/Io2 - Ec1/Io1 > WREPLACEBase station 1Low loading

Base station 1High loading

Handover condition met later

Handover condition met earlier


Softer Handover• This is equivalent procedure involving neighboring sectors of one

base station and mobile terminals at the boundary of the sectors.• RNC is not involved thus it is consider simpler and faster, i.e., softer

than the inter-cell soft handover.• Combining is performed in one base station, and it is Maximum

Ratio Combining (MRC), providing a slightly larger gain compared to selection combining.

Cell with three 120o sectors.

In softer handover.

In soft handover.

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Types of Handovers• Soft and softer handover;

− Only intra-carrier.• Hard handover;

− The old connection is terminated before a new one is created.• Inter-carrier handover;

− Typically used for load control, i.e, capacity management.− It is hard handover.

• Inter-mode handover between TDD and FDD modes;• Inter-mode handover between GSM and UMTS

− IRAT HO – Inter Radio Access Technology Handover− When UMTS coverage is extended via GSM network.− Used by a load control mechanism

(e.g., move voice traffic from UMTS to make room for data traffic).− Same as hardhandover between different RNCs.


WCDMA Soft Handover• Key points to remember about Soft Handover

− SSMA used to distinguish all transmitters in a Cellular CDMA system

− Fast power control is required to sustain SSMA performance

− When fast power control is used, soft handover is essentialAllows MS to operate in most conservative power control mode

− Soft handover provides performance benefits“Seamless” coverage at cell fringes

− Handover may be less noticeable to the userIncreases apparent system capacity when system is lightly loaded

− Soft handover also degrades system capacityUses redundant physical layer resources from adjacent or overlapping cells

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Summary1. Principles of Code Division Multiple Access

− Code orthogonally, Scrambling and OVSF codes2. UMTS Overview

− Features, Spectrum and Network Architecture3. Multiple Access, Channelisation and Scrambling Codes

− WCDMA & GSM, Spreading & Desptreading, Processing Gain, Channalisation Codes, Inter-Code Interference

4. Rake Receiver− Multipath Effects, Rake Receiver

5. Power Control− Open and Closed Loop Power Control

6. Handovers− Handoff Methods, Soft Handover, Micro and Macro Diversity− Soft Handover Procedure, Cell Breathing− Types of Handover


01 - wcdma principles and radio functionality

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