Cdma04

1 © NOKIA Cdma05.ppt / 10.02.00 3G BASICS TRAINING B6X 067594AE, 1.0.0.

Proprietary and Confidential

For internal use only

4. CDMA Air Interface3G Basics Training

• What is WCDMA?

• CDMA comparison to TDMA and FDMA

• CDMA benefits and Spread Spectrum Communications

• WCDMA´s Key Radio system Features

• Multipath advantage and Diversity

• Power Control

• Interference and maximum load

• Handover types

• TDD Basics

• Radio Network Planning




4. CDMA Air Interface3G Basics Training

• Module objectives. The participant will be able to explain:

• How CDMA is different compared to TDMA and FDMA?

• What is the Spread Spectrum Communications principle?

• WCDMA´s Key Radio system Features• What are the multipath propagation and diversity

advantages• Power Control principles in WCDMA• How interference affects to maximum load• Handover types in WCDMA• TDD Basic features• Radio Network Planning principles




What is WCDMA?

Terms and Abbreviations

• CDMA = Code Division Multiple Access

• Multiple Access (MA) = A method of sharing a common transmission medium between multiple users (or terminals) communicating simultaneously

• Code Division MA = Separating different users by unique signature codes

What is CDMA?

• CDMA = A Multiple Access method utilizing spread spectrum

techniques

What is WCDMA?

• WCMDA = Wideband CDMA, one 3. Generation Standard (3GPP) . The access scheme is Direct-Sequence Code Division Multiple Access (DS-CDMA) with information spread over approximately 5 MHz bandwidth.




CDMA comparison to TDMA and FDMA

Frequency Division Multiple Access (FDMA)

• Users transmit simultaneously using separate frequencies

Time Division Multiple Access (TDMA)

• Users transmit at separate times

Mobile Station 1

Mobile Station 2

Mobile Station 3

Time

Frequency

Mobile Station 1

Mobile Station 2

Mobile Station 3

Time

Frequency

Mobile Station 1

Mobile Station 3

Mobile Station 2




CDMA comparison to TDMA and FDMA

Code Division Multiple Access (CDMA)

• User signals cover the whole frequency band all the time

• The Base Station (BTS) separates the users by applying spread spectrum techniques

- Code each users´ signal with a unique signature sequence

- The receiver can separate the desired signal if it knows the sequence

Mobile Station 3, Code 3



Time

Frequency




CDMA Benefits

• Improved system capacity• Increase the amount of users (information) in a given

bandwidth

• Improved system performance in severe environments

• Multipath propagation; Can benefit from multipath by using RAKE receiver

• Interference from other users and from other systems

• Simplified frequency planning• Universal frequency re- use is one

• Enables seamless soft handover• Ongoing calls are handled in the boundary region

between two cells by both Base Stations




Definition of Spread Spectrum Communications

• Signals are transmitted at much higher bandwidth than the information rate (typically at least 100 times higher)

• The spreading is achieved with a code (spreading sequence) that is independent of the transmitted information

Conventional Spread Spectrum

Frequency Frequency

Power Power




Spreading

Data xCode

Data

Code

Code(pseudonoise)

Data

+1

+1

+1

+1

+1

Symbol

-1

-1

-1

-1

-1

ChipChip

DespreadingDespreading

Spectrum

Symbol




Detecting own signal. Correlator

Code

Data aftermultiplication

+1

+1

+1

-1

-1

-1

Ownsignal

+8

-8

Data afterIntegration

Code

Data aftermultiplication

+1

+1

+1

-1

-1

-1

Othersignal

+8

-8

Data afterIntegration




Basic Methods

Spread Spectrum Concept

Direct Sequence (DS)

• Modulate the signal directly with the spreading sequence

• The spreading sequence has a much higher bit rate than the data

• Comparable to data scrambling

Data modulation

Spreading modulation

De- modulation

De- spreading

Data Data

Spreading Code

Spreading Code

Spreading codes must be synchronous !

Frequency Hopping (FH)

• Not used in WCDMA

• Let the spreading sequence change the carrier frequency

• For example, each bit is sent with a different carrier frequency




Direct Sequence (DS) Spread Spectrum

• Code generator output is a pseudo- random periodic bit (chip) sequence of rate Rc

• This chip rate Rc is typically > 100 times the data rate Rb

Spreading modulation

De- spreading

Code generator

Code generator

X X

Rc

Carrier

DataData

Carrier

Rc

Data

Code

Spreadsignal

Rb Rb




WCDMA Main Parameter Summary

• Multiple Access (MS) scheme• Direct-Sequence Code Division Multiple Access (DS-CDMA) with

information spread over approximately 5 MHz bandwidth.

• Chip rate• 3, 84 Mcps

• Spreading Factor • Uplink 4- 256• Downlink 4- 512

• Frequency bands: • Uplink 1920 - 1980 MHz, • Downlink 2110 - 2170 MHz

• Duplex scheme• Frequency Duplex Division (FDD)• TX - RX frequency separation is 190 MHz

• Channel spacing• The nominal value is 5 MHz• Channel raster is 200 kHz




WCDMA's Key Radio System Features

• Wide 3.84 Mcps bandwidth ==> good frequency & interferer diversity ==> low Eb/No

• coherent in both up- and downlink ==> low Eb/No

• fast power control (PC) ==> minimizes interference ==> high spectral efficiency

• robust RAKE diversity receiver ==> low complexity

• dynamic variable rate multiplexing ==> flexibility

= Codes with different spreading, giving 8-500 kbps

f

t

10 ms frame

5 MHz

High rate multicode userVariable rate users

....P




Multipath advantage in WCDMA

• Multipath propagation causes several peaks in matched filter (MF) output

• Allocate RAKE fingers to these peaks

• Track and monitor the peaks




UL Receiver diversity (space diversity)

Fading

= Antenna 1= Antenna 2

Time

Amplitude

Antenna RAKEcombining(MRC)

RNC




Power Control (PC) Loops in WCDMA

RNC

Open Loop Power Control (Initial Access)

Closed Loop Power Control

Outer Loop Power Control

MS BTS




P1

P2• MS1 and MS2 are transmitting in the same frequency => equalizing transmitter powers is critical ("near-far" problem)

• Optimum situation: P1 = P2 at the BTS at all times

• Different path attenuations are compensated by using power control.

• Open loop power control: MS adjusts it’s initial transmitter power according to received signal level

• Closed loop power control: BTS commands MS to increase or decrease it’s transmission power at 1.5 kHz It is based on received signal to interference ratio (SIR) estimates in BTS.

• Closed loop power control follows also the fast fading pattern at low and (lesser) medium speeds (< 50 km/h)

BTS

MS2

MS1

TPC commands

TPC commands

if SIR > (SIR)set then "down"else "up"

MS adjusts power accordingto TPC commands

Open and Closed Loop PC, Uplink




Outer Loop PC, Uplink

CNRNC

if SIR > (SIR)set then "down"else "up"

frame reliability info

(SIR) set adjustmentcommand

outer loopcontrol

if FER increase then (SIR) set "up"else (SIR) set "down"

Required (SIR)set for 1 % FER

time

MS stands still

• Outer loop power control maintains link quality

• Optimizes capacity / range




• Interference is caused by mobiles in the neighbouring cells as well as by mobiles in the same cell area (uplink)

• Total interference experienced by BTS is summary of all those

• If interference increases the needed output power from MS needs to be increased as well.

Interference in CDMA

Signal

Interference




100% 50% 0%

Load per carrier

Ou

tpu

t P

ow

er

"Safe area"

Maximum Load in CDMA

• CDMA System must be dimensioned according to estimated traffic.

• Too much load makes the system unstable.

• Dimensioning rule is that 50 % of the theoretical capacity can be used and still maintain good performance.




WCDMA Handover types

• Soft handovers• Softer handover between sectors in one BTS• Soft handover between different BTSs under one RNC• Soft handover between different BTSs in different RNCs

• Hard handovers• Intra-frequency hard handovers between RNCs when soft

handover is not possible.• Inter-frequency hard handovers

• Intra-BTS between carriers• Intra-RNC between cell layers (macro to micro or vice versa)• Inter-RNC(and CN) between different core networks

• Inter-System handovers• Handover WCDMA <--> GSM900/1800• Handover WCDMA/FDD <--> UMTS/TDD




Softer Handover

Sector/Antenna RAKEcombining

RNC

• Softer handover is handled by BTS internally

• Softer handover probability about 5 - 15 %

• No extra transmissions across Iub

• Basically same RAKE MRC processing as for multipath/antenna diversity (BTS / MS). More RAKE fingers needed.

• Provides additional diversity gain

• Softer handover does create additional interference and needs BTS LPA resources




Soft handover

CNRNC



frame selection /duplication

Except for the TPC symbolexactly the same information(symbols) is sent via air.Differential delay in order of fraction of symbol duration

• Soft handover probability about 20-40 %

• Extra transmissions across Iub

• UL / DL soft HO diversity processing very different

•MS: MRC RAKE combining•RNC: frame selection

• Soft handover does create additional interference and needs BTS LPA resources




TDD (Time Division Duplex)

Characteristics:• same frequency for up-

and downlink, time division duplex

• utilises unpaired spectrum allocation

• asymmetry between uplink and downlink capacities

• support for unlicensed operation

Difficulties:

• risk for system interference

• synchronization requirements between base stations and between operatorsM1->BS1

M2->BS2

BS1->M1

BS2->M2

M2->M1 BS1->BS2

CELL 1

CELL 2

down uplinkf1




UTRA TDD (Time Division Duplex) principles

TD/CDMA principle

• Combination of TDMA and CDMA

• Users separated inside a time slot by code

• DS-CDMA used• user data rate

granularity

Standardisation• UTRA=UMTS Terrestrial Radio

Access• UTRA is both UTRA FDD and

UTRA TDD• TDD utilises unpaired spectrum• Basic parameters are

harmonized, and the only few exceptions exist in physical layer

Bundling of FDD and TDD frequency in licensing?




UTRA/TDD complements FDD where there is shortage of spectrum due

to traffic or regulation• Additional capacity to network

• Flexible support for asymmetric traffic

• Seamless integration with UTRA/FDD using multimode terminals

• Micro/picocell applications emphasizing high data rates

• TDD technology is best suitable for indoor use where interference from base stations is manageable and lower range does not matter




TDD utilizes Dynamic Channel Allocation




TDD interference

• To fight interference scheduling, coordination and synchronization is needed.

• Adaptive system (DCA, RRM)

• Planning can alleviate problems




TDD Conclusions• Key issues for TDD implementation by customers

are• mature 3G TDD standard • seamless handover FDD/TDD/GSM • availability of multimode terminals

• There are still high market and technology uncertainties

• Competing technologies within the target market (TDD/FDD/WLAN)

• Frequency allocation is the critical issue• Frequency bundling for UMTS• Warc2000

• Nokia provides a full end-to-end solution - TDD is needed to secure/complement Nokia FDD business




WCDMA Radio Network Planning process

• Network dimensioning• Site selection (cooperation with site acquisition and

existing GSM sites)• Detailed network planning

• coverage/capacity planning• propagation model tuning• parameter planning• soft/softer handover overhead analysis & optimization

• Network testing and tuning

NETWORK REQUIREMENTS

PropagationModel Tuning

NetworkDimensioning

Site Selection Detailed Network Planning

Parameters Download OMC

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