Basics CDMA

8/3/2019 Basics CDMA

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INFINITE SOFTCOMMINFINITE SOFTCOMM

SOLUTIONSSOLUTIONS

701,UDYOG VIHAR, PHASE701,UDYOG VIHAR, PHASE --VV

GURGAON, INDIAGURGAON, INDIATEL NO. 0124TEL NO. 0124-- 4005655, 43621504005655, 4362150--5252

FAX NO. 0124FAX NO. 0124--43621514362151


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How Did We Reach This Point?How Did We Reach This Point?


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TelegraphyTelegraphy


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TelephonyTelephony


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Radio MilestonesRadio Milestones


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Hello

Shalom Guten Tag

Time Division Frequency Division!

CHAOS

Buenos Dias

Bonjour

The SYMPHONY!The SYMPHONY!


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GSM Vs. CDMA

GSM Vs. CDMA


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7 cell re-use patternf7

f7

f2

f2

f6

f6

f1

f5

f3

f4

f1

f5

f3

f4

Frequency Reuse- GSM


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f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

FREQUENCY REUSE IN CDMAFREQUENCY REUSE IN CDMA

f1

f1


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GSM Vs CDMA

FREQUENCY REUSE IN CDMA & TDMA

TYPICAL TDMA SYSTEM

EACH CELL USESDIFFERENT FREQUENCY

THE PATTERN IS REPEATED

FOR THE NEXT SETOF

CELL SITES

TYPICAL CDMA

SYSTEM

EACH CELL USES

SAME FREQUENCY

F 1 F 1

F 1

F 1

F 1

F 1

F 1

F 1

F 2

F 5

F 4F 6

F 7 F 3


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CODE DIVISION MULTIPLE

ACCESS (CDMA)


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CDMACDMA -- Code Division Multiple AccessCode Division Multiple Access

CDMA is a "spread spectrum" technology

Spreads the information contained in aparticular signal of interest over a much

greater bandwidth than the original signal. Goal of spread spectrum

Interference mitigation

Spread spectrum helps mitigate the harmfuleffects of interference

Deliberate - Military Jammer (Use first byDOD)

Inadvertent - Co-channel users


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ShanonShanon Hartley TheoremHartley TheoremTo consider the technology used in CDMA, consider the Shannon-Hartley theorem,which is given as follows:

C = W log2 (1 + S/N)

Where, C = capacity

W = Bandwidth

S/N = Signal to Noise Ratio

According to the theorem, an increase in the bandwidth causes a decrease in the signalto noise ratio, thus requiring a trade-off between the two parameters. However,CDMA operates by maximizing the bandwidth, which brings about a correspondingreduction in the SNRThis reduction is countered by the utilization of an efficienterror correction code, which ensures optimum performance even in low SNRconditions.

Spread Spectrum Principles

Shannons work relates capacity to both bandwidth and signal to

noise ratio. It allows some simple conclusions to be drawn. In a bandwidthlimited system the only way to increase capacity (or

data rate) is to doone or both of the following:

Increase the Signal Power

Decrease the Noise Power


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Forward & Reverse LinksForward & Reverse Links

824MHz

869MHz

849MHz

894MHz

Forward linkReverse link

Uplink Downlink


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Physical ChannelPhysical Channel

The 3 dB bandwidth of a channel is the frequency range where the signal at the edges is 3 dB

lower than the peak value at the center frequency, fc


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CDMA Technology BenefitsCDMA Technology Benefits

Capacity increase

Improved call quality

Simplified system planning

use of the same frequency in every sector of everycell

Enhanced privacy

Improved coverage characteristics

possibility of fewer cell sites better performance in fading/interference prone

environment

Increased talk time for portables

Bandwidth on demand


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Capacity in CDMACapacity in CDMA

Capacity of the system depends on

interference (Total power received in the

Receiving Antenna) in the system

PowerControl Techniques are employed

to reduce the total interference in the

system.

By controlling power, the capacity can bedynamically adjusted (Capacity in CDMA

systems is soft)


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CDMA is altering the face ofTelecommCDMA is altering the face ofTelecomm

Dramatically improving the telephone traffic (Erlang)

capacity Dramatically improving the voice quality and eliminating

the audible effects of Multipath fading

Reducing the incidence of dropped calls due to handofffailures

Providing reliable transport mechanism for datacommunications, such as facsimile and internet traffic

Reducing the number of sites needed to support anygiven amount of traffic

Simplifying site selection

Reducing deployment and operating costs becausefewer cell sites are needed

Reducing average transmitted power

Reducing interference to other electronic devices

Reducing potential health risks


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All CDMA users occupy the samefrequency at the same time!Frequency and time are not usedas discriminators

CDMA operates by using CODINGto discriminate between users.

CDMA interference comes mainlyfrom nearby users

Each user is a small voice in aroaring crowd --but with a uniquelyrecoverable code

CDMA: Using A New Dimension


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Frequency-Hopping

Each users narrowband signal

hops among discrete

frequencies, and the receiver

follows in sequence

Spectrum (FHSS) CDMA is NOT

currently used in wirelesssystems, although used by the

militaryDirect Sequence

Narrowbandinput from a user is coded(spread)by a user-uniquebroadbandcode,thentransmitted. Broadbandsignalis received; receiverknows, appliesusers code, recoversusersdata

TYPES of CDMA Technology


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At OriginatingSite:

Input A: Users Data @19,200bps

Input B: Walsh Code #23 @

1.2288 Mcps

Output: Spread SignalSpectrum

At ReceiverSite:

Input A: Received spread

spectrum signal

Input B: Walsh Code #23 @

1.2288 Mcps

Output: Users Data

@19,200bps just asoriginally sent

DSSS Spreading: Time-Domain View


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Spreading from a Frequency-Domain View

CDMAuseslarger

bandwidth

but

uses

resultingprocessinggaintoincreasecapacity


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Spreading: What we do, we can undo

Sender combines data with a fast spreading sequence,

transmits spread data stream.

Receiver intercepts the stream, uses same spreading

sequence to extract original data.


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Shipping and Receiving via CDMA

Whether in shipping and receiving, or in CDMA,

packaging is extremely important!

Cargo is placed inside nested containers for

protection and to allow addressing

The shipper packs in a certain order, and the receiverunpacks in the reverse order

CDMA containers are spreading codes


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CDMAs Nested SpreadingSequences

CDMA combines three different spreading sequences

to create unique, robust channels.

The sequences are easy to generate on both sending

and receiving ends of each link.


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U1 = 0110010101001000

C1 ( 100110.10110010)*

=U1C1 ( 1001100000)

U1C1 ( 10011000000)

U1 = 0110010101001000

C1 ( 100110.10110010)*

=

UnCn

U4C4

U3C3

U2C2

UnCn*C1 = 0, UnCn*Cn = Un

U4C4*C1 = 0, U4C4*C4 = U4

U3C3*C1 = 0, U3C3*C3 = U3

U2C2*C1 = 0, U2*C2*C2 = U2

C1*C1 = 1, C2*C2 = 1. Cn*Cn = 1 BUT C1*C2 = 0C1*Cn = 0

DSSS Spreading/DespreadingDSSS Spreading/Despreading


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End to end overviewEnd to end overview


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Types ofCodeTypes ofCode


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Forward & Reverse LinkForward & Reverse Link


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The Three CDMA Spreading TechniquesThe Three CDMA Spreading Techniques


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Orthogonal SequencesOrthogonal SequencesOrthogonal SequencesOrthogonal Sequences

Definition:Orthogonal functions have zero correlation. Two binary sequences

are orthogonal if the process ofXORing them results in an equal

number of 1s and 0s. Example:Example:

00000000

((XOR) 01010101

------------01010101

0 0

0 1- Repeat: right & below

- Invert: diagonally

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0


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Walsh Codes 64 Magic Sequences, each 64 chips long

Each Walsh Code is precisely Orthogonal with respectto all other Walsh Codes

its simple to generate the codes, or

theyre small enough to use from ROM


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Short codeShort code


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Usage ofcodeUsage ofcode


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Need for SynchronisationNeed for Synchronisation


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Functions of the CDMA Forward Channels


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WALSH CODEWALSH CODE

contd.contd.

WALSH CODES ARE USED TO SEPARATE

INDIVIDUAL USERS WHILE THEY

SIMULTANEOUSLY OCCUPY THE SAME RFBANDWIDTH

THE SEQUENCE ARE ORTHOGONAL TO EACH

OTHER AND ARE GENERATED USING THEHADAMARD MATRIX

WALSH-0 IS NOT USED TO TRANSMIT ANY

BASEBAND DATA


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PN CODEPN CODE

PN SEQUENCES

USED TO SPREAD THE BANDWIDTH OF THE

MODULATED SIGNAL TO LARGER TRANSMISSION

BANDWIDTHS DISTINGUISH BETWEEN DIFFERENT USER

SIGNALS

MULTIPLICATION BY A SHORT PN SEQUENCE IS

DONE TO PROVIDE ANOTHER LAYER OF

ISOLATION ON THE FORWARD LINK

WE CAN HAVE A MAXIMUM OF 512 DIFFERENT PN

SEQUENCES EACH WITH A SEPARATION OF 64

CHIPS FROM EACH OTHER


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SecuritySecurity


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CHANNELSCHANNELS


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CDMA IS-2000 HIGH LEVEL ARCHITECTURE

FORWARD CHANNELS

PILOT CHANNEL (1)

SYNC CHANNEL (32)

FORWARD TRAFFIC

PAGING CHANNELS ( 1-7)

REVERSE CHANNELS

REVERSETRAFFIC CHANNELS

ACCESS CHANNELS


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SummarySummaryForward Reverse

Short PN Differentoffset- Each

Sector

Zero offset-Used by MS

Walsh code 64 Channel 64 Ary

modulation

Long code Use to

scramble TCH

Identify TCH

for each MS


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Information about the long code is broadcast to the mobilestation by the Sync Channel (orControl Channel) to helpthe mobile lock onto the base station, and helps provideseparation from other base stations.

One of the codes used in conjunction with the WalshCodeis the PN (pseudo-random noise) short code. The PN shortcode on the forward link is used to provide the base stationwith a unique identification that the mobile station uses toidentify the serving base station.

The user signal (or control channel) is multiplied by theWalsh code. The Walsh code provides each user or channelwith a unique identifier and, in DS spreading, may spreadthe frame across the bandwidth.


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Main process:

y(t) = b(t) c(t)

Scrambling:

If b(t) and c(t) have the same rate then y(t) has the same rate,

and the spectrum of the signal is unchanged b(t) is said to beencrypted or scrambled

Spreading:

If c(t) has a higher rate than b(t), y(t) has the faster rate and its

correspondingly wider spectrum .In addition to being scrambled, b(t)

is said to have had its spectrum spread

CDMA codes are used to perform scrambling andspreading.


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Rayleigh Fading In addition to delay

spread, the same multipath environment causes

severe local variations in signal strength as these

multipath signals are added constructively and

destructively at the receiving antenna. This typeof variation is called Rayleigh fading

This can cause large blocks of information to be

lost.If the set of reflected signals have one

dominant component, such as a line-of-sight

signal, the fading is more appropriately modeled

using the Rician model. Note that if the mobile speed is zero, there isno fading, except if signals are reflected frommoving objects.

For slow fading, CDMA uses power control to adjust the transmitted

power in order to overcome the fades. Power control is too slow for

the fast Rayleigh fading; instead, FEC encoding and bit interleaving

are used.


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Channel Contd.Channel Contd.


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Forward channelForward channel


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PILOT CHANNELPILOT CHANNEL

PILOTSIGNALS ARETRANSMITTED BYEACH CELLSITETO ASSIST MOBILERADIO IN ACQUIRING ANDTRACKING THE CELL SITE DOWNLINK SIGNAL

PILOT CHANNEL IS ASSIGNED CODE CHANNELNUMBER ZERO

THESIGNAL STRENGTH = Ec/Io

Ec/Io ISTHEENERGY PER CHIP PERINTERFERENCE DENSITY MEASURED ON THE

PILOT CHANNE

L Ec/IoEFFECTIVELY DETERMINESTHE FORWARD

COVERAGE AREA OF A CELL OR A SECTOR


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SYNC CHANNELSYNC CHANNEL

SYNC CHANNEL IS GIVEN THE CODE CHANNEL

NUMBER 32; FIXED DATA RATE 1200 KBPS

ALLOWSRECIEVERTO OBTAIN FRAME

SYNCHRONIZATION ON SIGNAL

MESSAGESSENT ON SYNCH CHANNEL ARE

SYSTEM TIME

CHARACTERISTICS OF THESYSTEM


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FORWARD TRAFFIC CHANNELSFORWARD TRAFFIC CHANNELS

PAGING CHANNELS ARE GIVEN THE CODE

CHANNEL NUMBER 1 THRU 7

FORWARD TRAFFIC CHANNELS GROUPED INTO

RATESET 1( 9.6, 4.8, 2.4 or 1.2 KBPS) AND RATE

SET2 (14.4, 7.2, 3.6 or 1.8 KBPS)

RATESET 1 ISREQUIRED FOR IS-95 WHEREAS

RATESET2 IS OPTIONAL

SPEECH ISENCODED WITH VARIABLERATEVOCODERTO GENERATE FORWARD TRAFFIC

CHANNEL DATA DEPENDING ON VOICE ACTIVITY


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REVERSE TRAFFIC CHANNELSREVERSE TRAFFIC CHANNELS

IDENTIFIED BY LONG USER CODE OFFSET

DATA TRAN

SMITT

ED ON

REVERSE

CHANNE

L IS

CONVOLUTIONALLYENCODED, BLOCK

INTERLEAVED, MODULATED BY MEANS OF 64-ary

ORTHOGONAL MODULATION, AND DIRECT

SEQUENCESPREAD PRIORTO TRANSMISSION

DATA RATE IS 9.6, 4.8, 2.4 OR 1.2 KBPS


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ACCESS CHANNELSACCESS CHANNELS

ENABLESTHE MOBILETO COMMUNICATE

NONTRAFFIC INFORMATION

DATA RATE IS FIXED AT 4.8 KBPS

IDENTIFIED BY A DISTINCT ACCESS CHANNEL

LONG-CODESEQUENCE OFFSET

A PAGING CHANNEL NUMBER IS ASSOCIATED

WITH ACCESS CHANNEL


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Forward LinkForward Link


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Putting it All Together: CDMA Channels


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How a BTS Builds the Forward Code Channels


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Code Channels in the Reverse Direction


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Functions of the CDMA Reverse Channels


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Variable rate vocoderVariable rate vocoder

Variable Rate Vocoding &


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Vocoders compress speech, reduce bit rate, greatlyincreasing capacity

CDMA uses a superior Variable Rate Vocoder

full rate during speech

low rates in speech pauses increased capacity

more natural sound

Voice, signaling, and user secondary data may be

mixed in CDMA frames.

Variable Rate Vocoding &Multiplexing

M i CDMA


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In CDMA, most call processing events are driven by messages

Some CDMA channels exist for the sole purpose of carryingmessages; they never carry users voice traffic

Sync Channel (a forward channel)

Paging Channel (a forward channel)

Access Channel (a reverse channel)

Some CDMA channels exist just to carry user traffic

Forward Traffic Channel

Reverse Traffic Channel

On these channels, most of the time is filled with traffic andmessages are sent only when there is something to do

Messages in CDMA


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Rake ReceiverRake Receiver


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Whats In a Handset? How does it work?


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The Rake Receiver Handset uses combined outputs of the three traffic correlates

(rake fingers).

Each finger can independently recover a particular PN offset and

Walsh code.

Fingers can be targeted on delayed Multipath Reflections, or on

different BTSs.

Searcher continuously checks pilots.

The Pilot Searching ProcessThe Pilot Searching Process


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The Pilot Searching ProcessThe Pilot Searching Process

Active Set

Candidate Set

Neighbor Set

Remaining Set

M M T 9 8 0 1 0 4 1 4 A c . e p s


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Handoff SignalingHandoff Signaling

(1) (2) (3) (4) (5) (6) (7)

Time

PilotStrength

T_ADD

T_DROP

Neighbor Set

Neighbor Set

Active Set

Candidate Set

M E S 9 8 0 1 0 1 5 8 A g . e p s


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Near-farProblem

Path Loss

Fading

Performance Objectives

Power Control Is Required ?


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Forward Power Control


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Forward Power Control

The BTS continually reduces the strength of each users

forward base band chip stream When a particular handset sees errors on the forward link,

it requests more energy

The complainers chip stream gets a quick boost; afterward

,continues to diminish

Reverse Power Control


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Reverse Power Control

Reverse Open Loop: Handset adjusts power up or down

based on received BTS signal (AGC). Reverse ClosedLoop: Is handset too strong? BTS tells up or

down 1 dB 800 times/second.

Reverse OuterLoop: BSC has FER trouble hearing handset?

BSC adjusts BTS set point.


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Types ofHandoffTypes ofHandoff

SOFT HANOFF

1. Soft handoff - 2 BTS are involved

2. Soft Soft - 3 BTS are involved

3. Softer - TwoSectorof same BTS

HARD HANDOFF

- Between different frequency.


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Hard & Soft handoverHard & Soft handover

CDMA Soft Handoff Mechanics


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CDMA Soft Handoff Mechanics

CDMA soft handoff is driven by the handset

Handset continuously checks available pilots Handset tells system pilots it currently sees

System assigns sectors (up to 6 max.), tells handset

Handset assigns its fingers accordingly

The Complete Rules of Soft Handoff


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The Handset considers pilots in set

ACTIVESET: Pilots of sectors in use

CANDIDATESET: Pilots mobile

requested, but not yet set up

NEIGHBOURSET: Pilots told to mobile

by system, as nearby sectors to check

REMAINING SET: Any pilots used by

system but not already in the other sets Handset sends Pilot Strength

Measurement Message to the system

whenever:

It notices a pilot in neighbor or

remaining set exceeds T_ADD

An active set pilot drops belowT_DROP forT_TDROP time

A candidate pilot exceeds an active

by T_COMP

The Complete Rules of Soft Handoff


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Softer Handoff

Handset will ask for whatever pilots it wants

Ifmultiple sectors ofone BTS simultaneously serve ahandset, this is called Softer Handoff

Handset cant tell the difference, but softer handoff

occurs in BTS in a single channel element

Handset can even use combination soft-softer handoffon multiple BTS & sectors

S ft H/O ADV & DIS ADVS ft H/O ADV & DIS ADV


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Soft H/O ADV. & DIS. ADV.Soft H/O ADV. & DIS. ADV.

R li kR li k


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Reverse linkReverse link

F d li kF d li k


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Forward linkForward link


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Processing GainProcessing Gain

Spreadinggain or processinggain is achieved when noisecomponents, or noise-like

components, remain spread when the original signal (user 1 in thefigure) is despread. The

original signal appears to have gained energy relative the noise. It canalso be seen as if the noise

has been suppressed.

By filteringout most of the wideband noise energy the original signalcan be extracted, provided

sufficient bit energyover noise ratio, Eb/NT. It can be seen that thesignal to noise ratio after

despreading will favor user 1 by a factorof G = BW/bw (or Fc

/Fb orTb/Tc). G is then called

spreadinggain or processinggain. Processinggain can also be seenas the numberof chips per

bit.

P i G iP i G i


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Energy per BitEnergy per Bit

Eb/NT is traffic channel bit energy over noise.Ec/I0 is pilot channel chip (bit) energy over interference.

Capacity EnhancementCapacity Enhancement


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Capacity EnhancementCapacity Enhancement


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Capacity FactorCapacity Factor

Idle Mode Handoff


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An idle mobile always demodulates the best available signal

In idle mode, it isnt possible to do soft handoff and listen tomultiple sectors or base stations at the same time -- the pagingchannel information stream is different on each sector, notsynchronous -- just like ABC, NBC, CBS, and CNN TV newsprograms arent in word-sync for simultaneous viewing

Since a mobile cant combine signals, the mobile must switchquickly, always enjoying the best available signal

The mobiles pilot searcher is constantly checking neighbor pilots If the searcher notices a better signal, the mobile continues on the

current paging channel until the end of the current superframe,then instantly switches to the paging channel of the new signal

The system doesnt know the mobile did this! (Does NBCs TomBrokaw know you just switched your TV toCNN?)

On the new paging channel, if the mobile learns that registration isrequired, it re-registers on the new sector


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Signal processingSignal processing


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Speech encoding. This step is only used if

speech information is transmitted. Data

transmission omits this step.

Quality indicator

Forward Error Correction (FEC) encoding

InterleavingScrambling

Spreading

Digital modulation

RF modulation

Amplification ofRF signal.


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Speech encodingSpeech encoding

In order to transmit speech over a digital system, it must be digitized and

encoded using a vocoder. Normal speech is received as an analog signal.

The analog signal is converted into a digital signal using a process called

Nyquist sampling, in which the analog input is typically sampled 8,000

times per second. The product of Nyquist sampling is a digital waveform

called PCM (pulse code modulation).The PCM output is transferred to a

vocoder (voice coder), which compresses the digitized voice signal into

eitherRate Set 1 (RS1) with an output of 8 kbps, orRate Set 2 (RS2) with

an output of 13 kbps, dependingon the type of vocoder. In CDMA,

variable rate vocoders are used.

Variable Rate Vocoder

The variable rate vocoder employs a codec (coder/decoder) that compresses

digitized speech from the analog-to-digital (A/D) converter and produces an output

that complies with the data rate to be transmitted. During these lulls in the

conversation, the vocoder can reduce its bandwidth requirements, before the FEC

encoder, from full rate (9600 bps forEVRC) to 1/2 rate, 1/4 rate, or 1/8 rate (1200

bps forEVRC). Since the transmitteronly transmits the lowest bit rate required, the required

transmit power is minimized, and the channel interference is reduced.


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Frame

Information bits are grouped into frames. A frame is the basictiming interval in the system. The

length of a frame depends on what channel on which it is

transmitted (e.g., Sync Channel,Traffic Channel), what type of information transmitted in the frame

(e.g., overhead messages,

traffic information), and what air-interface standard is used

Error Measurements

There are a numberof error measurements available in CDMAtransmission: Bit ErrorRate

(BER), Frame ErrorRate (FER), and Packet ErrorRate (PER).

The frame quality indicator is a Cyclic Redundancy Code (CRC).


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Basics CDMA

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