cdma technology

CDMA TECHNOLOGY

What is CDMA?

• It is cellular wireless Air Interface technology• Developed by QUALCOMM CORPORATION• Deployed HONGKONG in 1995• Also known as Wideband Spread Spectrum

Evolution of 2G CDMA

• 1ST form of CDMA IS-95• Specified Dual Mode Operation in 800MHz for AMPS and CDMA• Defined requirement for MS & BS compatibility with AMPS &

CDMA• Additional features added with IS-95A• IS-95A defines

i. Structure of wideband 1.25mhz CDMA channelii. Operation required for power controliii. Call processingiv. Handoffsv. Reg procedure for proper operation

• Provided Circuit Switched Data service at 14.4kbps along with Voice Service

• ANSI J-STD-008 CDMA operation in PCS band

• Additional features- TIA/EIA-95-B in 1999• Compatible with IS-95A & ANSI J-STD-008• Allowed packet switched data at 64kbps- 2.5 G

CDMA• Grouped under cdmaone

Components Of cdmaone N/w

Evolution of 3G CDMA

• Cdma2000 3G CDMA system• One among the 5 proposals approved by ITU

for IMT-2000 for 3G std• Cdma2000- wideband enhanced version of

CDMA• Backward Compatible with TIA/EIA-95-B• Data service upto 2mbps• Multimedia service

Implementation of cdma2000

• Planned phases• 1st phase- 1xRTT(1x radio transmission tech)

1.25MHz• 2nd phase- 1xEV - 1xEV-DO

1xEV-DV• 1xEV-DO assym data rates -2.4mbps

downlink direction 153kbps in uplink

• 1xEV-DV supports integrated voice and data at 3mbps

CDMA Basics

• Multiple access technique• Uses wide band spread spectrum digital

technique • Separates signal concurrent in time and freq• Shares the same freq spectrum simultaneously• Signal spread over entire bandwidth &

encoded by code• Demodulation done by copy of the same code

CDMA FREQ BANDS

• Can be deployed in US for existing cellular freq band- Band Class 0 & PCS bands- Band Class 1

• 3G CDMA system will be allowed in newly released AWS bands (1710-1755 MHz & 2110-2155MHz)

• Other freq band available for CDMA use 450MHz

• Spacing btw forward and reverse channel- 45MHz

• MS transmit freq - 824-849MHz• BS transmit freq – 869-894MHz• Not all freqs in this band are used by CDMA

cellular wireless n/w• GSM, NA-TDMA,CDMA- 1900 MHz PCS band• Transmit and receive freq is 80- CDMA

80.04- NA-TDMA 90- GSM

INTERFERENCES REDUCTION

• FCC indicated availability of channels for use by CDMA

• Designated asi. Valid ii. Conditionally validiii. Not valid

Preferred CDMA channels by PCS

FREQ PLANNING ISSUES

• Freq reuse factor N=1• Freq planning simple• Requires 1 carrier per BS• Carrier chosen from list of preferred CDMA channels• Same channels to be used by all BS throughout the

system• Non uniform growth causes system operation

degradation• Problem arise when dealing with intersystem PCS

bands

N/W & System Architecture

• NA Wireless Mobile System ref architecture - STD developed by TIA

• TIA committee TR-45 developsa. System performanceb. Compatibilityc. Interoperabilityd. Service standards for cellular bands• TR-46 coordinates the same activities for PCS

band

• TR-45.3 Subcommittee- NA-TDMA• TR-45.5 Subcommittee- CDMA• Initial Ref Architecture for IS-95 CDMA similar

to GSM• Adoption of TIA/EIA-95 provided additional

n/w interfaces• Ref model developed by TR-45/46

Initial CDMA(IS-95) Ref Architecture

CDMA 2000 Architecture

Layered architecture for MSC-BSC Interface

• TIA/EIA-634-B messaging btw MSC-BSC using the A interface

Functionalities

• A interface supports 4 functional planesi. Call processingii. Mobility managementiii. Radio resource managementiv. Transmission facility management

• Call Processing & Mobility Mngmnt- MS & MSC

• Resource Mngmnt & Trans Faci Mngmnt- BS & MSC

• CALL PROCESSING supported over TIA/EIA-634-Bi. Call originated & terminated by subscriberii. Call releasesiii. Call waiting• MOBILITY MANAGEMENTi. Registration & Deregistrationii. Authenticationiii. Voice privacy

• RESOURCE MANAGEMENTi. Maintenance of radio link btw RBS &

subscriberii. Operation Needed to accomplish the aboveiii. Initiation of handoff• TRANSMISSION FACILITY MANAGEMENTi. Transport voiceii. Signaling info

Major N/W Elements Of Modern cdma2000

N/W Nodes Found In cdma2000

CDMA Channel Concept

• Cdmaone & cdma2000 uses CDMA tech• Provides additional user capacity over limited radio spectrum• Achieved using SPREAD SPECTRUM• Uses Orthogonal Walsh Spreading Codes for Channel

Encoding• Encoded channels are used to providei. Precise system timingii. Controliii. Overhead info• Other channels used to carry user traffic

Basic Structure Of Walsh Code(64-bit)

• 64 bit codes• Consist equal no of 1s & 0s• Except the all zero Walsh code• W0

64 • Used to spread the signal over the entire band• All the Walsh encoded signal are transmitted on the

same spectrum• Receiver with the same Walsh code cane demodulate

it

Basic Spread Spectrum Operation

• Forward channels encoded diff from reverse channels in CDMA system

• IS-95 CDMA system uses two more types of Pseudorandom Noise Codes (PN)

i. Short PN codes- time shifted to identify CDMA BS & provide time synchronization signals to SD

ii. Long PN codes- used to provide data scrambling on

forward traffic channel means to distinguish the reverse link channels

Forward Logical Channels

• IS-95 CDMA FC exists btw BS & SD• IS-95 signals occupy 1.25MHz bandwidth• Fig below shows the basic spreading

procedure used on forward channels

Forward Control Channels

• Pilot Channel• Synchronization Channel• Paging Channel• Traffic/Power Control Channels

a. Pilot channel

• Used to provide a Ref Signal for all SDs within a cell

b. Synchronization Channel

c. Paging channel

d. Traffic/Power Control Channel

Reverse Logical Channels

• Exists btw SD & CDMA BS• Encoding of info diff than forward channel• Data to be sent not spread initially• Mapped into a Walsh code & then transmitted• 6 bit of data to be transmitted is mapped to a

particular Walsh Code• Increases the b/w by over 10 folds

Adv & Procedure

• Reduces system error rate• One to one relation btw the twoPROCEDURE• Each RC spread by long PN seq code- derived

from 32-bit ESN• Scrambled by short PN seq code

TYPES OF CDMA RC

i. Access Channelsii. Reverse Traffic/Control Channel

i. Reverse Access Channel

• Used by mobile to i. answer pages ii. transmit control information for call set upiii. Tear down

Generation Of Access Channel

ii. Reverse Traffic/Control Channel

• Supports both voice and data at 2 rate sets RS1 & RS2

• RTC is also used to send info to BSC about i. Pilot channel signal strengthii. Control info regarding handoff operationiii. Ongoing frame error(FER) statistics

CDMA FRAME FORMAT

• Takes 20ms segments of digital samples of voice signal

• Encodes it using VOCODER into variable rate frames

i. Forward Channel Frame Formatsii. Reverse Channel Frame Formats

a. Forward Channel Frame Format

• Out of the 4 forward channel Pilot Channel does not employ frame format

• Consists of continuous transmission RF signal• It is a 20ms frame• Consists of i. Information bitsii. Frame error control check bitsiii. Tail bits depending upon Rate Set and Data Set

Rate 2 traffic channel structure

• Logically subdivided into 16 1.25 ms power control groups

• Power control bits transmitted over the forward traffic channels are inserted randomly into the data stream of each 1.25ms power control groups

• This yields a power control signal rate of 800bps

Sync Channel Frame

Super frame

Paging Channels

• Transmits system Overhead Info & MS specific msgs

• Formatted into 80ms paging slots of 8 half frames of 10ms

• Each half frame start with a Sync Capsule Indicator(SCI) similar to SOM

• Synchronized paging channel msg capsule begins after an SCI bit is set to 1

Structure Of Paging Channel

• Formatted into paging slot cycles• Provides MS battery life• Mobile may be operated in i. Slotted mode- wakes up periodically to check

for paging msg directed to it in specified preassigned slots

ii. Unslotted mode- reads all page slots while in mobile station idle state

REVERSE CHANNEL FRAME FORMAT

• Divided into 20ms traffic channel frames• Logically subdivided into 16 1.25ms Power

Control Groups• When mobile transmit data rate is max all 16

power control groups transmitted• If it is half of max rate then 8 PCG are

transmitted• Data burst randomizer ensures repeated code

to be transmitted only once

Reverse Channel Variable Data Rate Transmission

CDMA SYSTEM OPERATION

• Initialization/Registration• Call Establishment• Call Handoff• Power Control

1.INITIALIZATION

• Reg depends on the status of MS• MS may be switched off or out of

range(detached) or attached• When turned on goes through Power Up State• Selects CDMA system • Acquires pilot & sync channels to synchronize

its timing to CDMA system

MS Initialization State

3 States When MS Is ATTACHED

• Mobile station idle state- monitors paging channel

• System access state- MS comm with BS • MS control on traffic channel- MS comm with

BS using forward & reverse traffic channels

Call processing state

REGISTRATION PROCESS

• CDMA MS through BS informs cellular systemi. Its identificationii. Locationiii. Statusiv. Slot cyclev. Other info for proper operation

Diff Forms Of Reg

• Power-up• Power-down• Timer-based• Distance-based• Zone-based• Parameter-change• Ordered• Implicit• Traffic Channel• User Zone

2. Call Establishment

• Requires various system tasks includingi. Mobile initializationii. Idleiii. System accessiv. Traffic channel commv. Call termination

1.Mobile Initialization State

• Mobile powers on• Searches for pilot channel• On acquiring it synchronizes with it• Switches to WALSH CODE 32 W32 64

• Looks for SOM• Decodes this msg and aligns its timing with

the serving BS

2. IDLE

• Mobile waits toi. Receive callsii. Data msg iii. Originate call iv. Data transfer• Apart from the above mobile alsoi. Monitors radio channel qualityii. Decodes paging channel msgs to obtain access

parameters

3.Access State

• Enters this state when MS receives a mobile directed msg which requires

i. Acknowledgementii. Originate a calliii. Perform registration• MS randomly attempts to access the system• Access to system only when MS receives

response from BS on Paging Channel

Access Channel Probing

• Access protocol implemented through the use of Access Class Groups with

i. Assigned prioritiesii. Gradual increase in access request power

leveliii. Random time delays for access requestiv. Max no of automatic access attempts

3.Traffic State

• Begins to transfer user information• Info can be voice or data• In this state MS transmits voice & signaling info on RTC• Signaling is performed byi. Blank and burst process- replaces 1.25ms speech data

with signaling msg burstii. Dim and burst process- inserts signaling msg

when speech activity is low• Various modes and flag bits are used to alert the RX of

about the signaling method

a. MOBILE ORIGINATED CALL

b. Mobile Terminated Call

c. CALL TERMINATION

• Occurs at the end of a call• Initiated by MS or BS

3. CALL HANDOFF

• Handoff occurs when serving sector/cell not capable of supporting comm

• Procedures used & type of Handoff performed depends on the Present State Of Mobile

• All handoffs are Mobile Assisted• CDMA supports Soft/Softer handoff

TYPES OF HANDOFF

• Idle Handoff• Access Handoff• Soft Handoff • Hard Handoff

Idle Handoff

• Occurs when mobile is in idle state• RSS of diff Pilot Channel becomes twice the

current pilot channel• A form of Hard Handoff

Access Handoff

• When mobile is in Access State• Can occur i. Before mobile begins access probesii. During access probeiii. After receiving access probe ack

Soft Handoff

• Occurs when mobile simultaneously communicates with several new cells or new sectors of current cell over FTC while maintaining communication over FTC of current cell/sector

• Can perform Soft Handoff only in traffic state to new cell /sector that has the same carrier freq

• SH is associated with i. Near- far problemii. Power control mechanism

TYPES OF SOFT HANDOFFiii. Softer Handoffiv. Soft Handoffv. Soft-Softer Handoff

PROCEDURE

• Involves a no. of BS and their pilot channels• For Soft Handoff more than one cell simultaneously

supports the call• For Softer Handoff more than one sector of a cell

simultaneously supports the call• Mobile continuously scans for pilot and establish

comm with any sector or cell(max 3) that has pilot RSS > threshold (T_ADD)

• Similarly it drops comm if pilot RSS < threshold (T_DROP)

CATEGORIZATION OF PILOT

• Mobile categorizes the pilot it receives as well as the pilot the serving sector /cell specifies to it :

i. Active setii. Candidate setiii. Neighbor setiv. Remaining set