CDMA Call Processing, Handoffs

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1

CDMA Call Processing

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Call Processing

There are four modes of operation in

call processing in CDMA technology as



Initialization mode



Idle mode



Access mode



(3)

3 3

(4)



 The subscriber unit performs initializationThe subscriber unit performs initialization

when the phone is turned on. when the phone is turned on. During initialization, the mobile: During initialization, the mobile:



 searches for a usable pilot signal searches for a usable pilot signal acquires theacquires the

system via the Pilot code channel system via the Pilot code channel



 synchronizes with the system by reading thesynchronizes with the system by reading the

Sync code channel broadcast from that site Sync code channel broadcast from that site

Ini

(5)

5 5

(6)



 The mobile is not involved in a call The mobile is not involved in a call during idleduring idle

mode, but it must stay in

mode, but it must stay in communication withcommunication with the base station.

the base station.



 The mobile and the base stationThe mobile and the base station

communicate over the access and paging communicate over the access and paging code channels

code channels



 So in the Idle State the mobile is So in the Idle State the mobile is mainlymainly

listening to the Paging Channel for incoming listening to the Paging Channel for incoming messages. (Also monitors the strength of pilot messages. (Also monitors the strength of pilot channel in neighbouring cells)

channel in neighbouring cells)

Idl

(7)

7 7

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

The mobile accesses the network via

the Access code channel during call

origination.



The Access channel and Paging channel

carry the required call set-up

communication between the mobile

phone and the BTS until a traffic

channel is established.

s m

(9)

9 9

(10)



 Once assigned to a Traffic Channel, the mobile is noOnce assigned to a Traffic Channel, the mobile is no longer monitoring the Paging Channel.

longer monitoring the Paging Channel.



 During a land to mobile (LTM) call:During a land to mobile (LTM) call: 

 The mobile receives a page on the paging channel.The mobile receives a page on the paging channel. 

 The mobile responds on the access channel.The mobile responds on the access channel. 

 The traffic channel is established and maintained throughoutThe traffic channel is established and maintained throughout

the call. the call. 

 During a mobile to land call (MTL):During a mobile to land call (MTL): 

 The call is placed using the Access channel.The call is placed using the Access channel. 

 The base station responds on the paging channel.The base station responds on the paging channel. 

 The traffic channel is established and maintained throughoutThe traffic channel is established and maintained throughout

the call. the call.

c

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11 11 

 During the call, overhead messagingDuring the call, overhead messaging

(signaling) continues on the traffic channel in (signaling) continues on the traffic channel in a limited fashion.

a limited fashion.



 This messaging uses "Dim and Burst" orThis messaging uses "Dim and Burst" or

"Blank and Burst" signaling, which replaces "Blank and Burst" signaling, which replaces part of the voice traffic with system

part of the voice traffic with system messages.

messages.



 The user does not detect this signaling,The user does not detect this signaling,

however, due to the strong data recovery however, due to the strong data recovery schemes inherent to CDMA.

schemes inherent to CDMA.

l

sa

(12)



CDMA has three primary types of

handoff:

  hardhard   softsoft   idleidle 



The type of handoff depends on the

handoff situation.

pe

(13)

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Soft Handoff



 Soft handoff refers to the state where theSoft handoff refers to the state where the

mobile is in communication with multiple base mobile is in communication with multiple base stations at the same time.

stations at the same time.



 Soft handoff is a Soft handoff is a make-before-break type of make-before-break type of

handoff whereby a mobile acquires a target handoff whereby a mobile acquires a target code channel before breaking an existing one. code channel before breaking an existing one.



 The advantages of soft handoff are several:The advantages of soft handoff are several:



 Fewer dropped calls.Fewer dropped calls.



 Soft handoffs in general require less mobile transmitSoft handoffs in general require less mobile transmit

power. power.

(14)



 A CDMA mobile station is said to be in soft A CDMA mobile station is said to be in soft

handoff when the mobile communicates with handoff when the mobile communicates with two or more antennas (PN offsets).

two or more antennas (PN offsets).



 The PN offsets involved in softhand offs areThe PN offsets involved in softhand offs are

said to be in Mobile

said to be in Mobile Stations Active Set.Stations Active Set.



 Up to six PN offsets can be present in activeUp to six PN offsets can be present in active

set. set.

(15)

15 15

Soft Handoffs Soft Handoffs

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

 A soft handoff establishes a connection with the new BTS prior A soft handoff establishes a connection with the new BTS prior

to breaking the connection with the old one. to breaking the connection with the old one.



 This is possible because CDMA cells use the same frequency.This is possible because CDMA cells use the same frequency. 

 The mobile detects a new pilot as it travels to the next coverageThe mobile detects a new pilot as it travels to the next coverage

area. The new base station then establishes a connection with area. The new base station then establishes a connection with the mobile.

the mobile.



 This new communication link is established while the mobileThis new communication link is established while the mobile

maintains the link with the old BTS. So Soft handoff refers to maintains the link with the old BTS. So Soft handoff refers to the state where the mobile is in communication with multiple the state where the mobile is in communication with multiple base stations at the same time.

base stations at the same time.



 Soft handoffs are also called "make-before-break."Soft handoffs are also called "make-before-break." 

 If the sectors are from same physical cell site (a sectorizedIf the sectors are from same physical cell site (a sectorized

site), handoff is referred to as “softer handoff”. site), handoff is referred to as “softer handoff”.

t h

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Idle Handoff



 While in theWhile in the , the mobile may, the mobile may

move from one cell to another. move from one cell to another.



 Idle handoff arises from the transitionIdle handoff arises from the transition

between any two cells. between any two cells.



 Idle handoff is initiated by the mobile when itIdle handoff is initiated by the mobile when it

measures a pilot signal significantly stronger measures a pilot signal significantly stronger than the current serving pilot

than the current serving pilot (3 dB stronger).

(3 dB stronger).

Idle state Idle state

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Idle Handoff

Idle Handoffs Idle Handoffs

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

 CDMA soft handoff is driven by CDMA soft handoff is driven by the handsetthe handset



 Handset continuously checks available pilotsHandset continuously checks available pilots



 Handset tells system pilots it currently seesHandset tells system pilots it currently sees



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

handset handset



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21 21

The Pilot Searching Process

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

 The searching process is continuous and isThe searching process is continuous and is

conducted to find handoff candidates. conducted to find handoff candidates.



 The handoff process is “mobile assisted”:The handoff process is “mobile assisted”:

When the mobile detects a pilot of sufficient When the mobile detects a pilot of sufficient strength, it reports the event to the base

strength, it reports the event to the base station. station. Th The e e e s s r r g g e R e R

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23 23



When the base station receives a report

from the mobile, a handoff decision is

made and directions are sent to the

mobile to perform the handoff.

e

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Coding and Spreading

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g a

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

 The encoded voice information is encoded using aThe encoded voice information is encoded using a code that will separate it

code that will separate it from other encoded voicefrom other encoded voice information.

information.



 This process, known as channelization, will This process, known as channelization, will spreadspread the encoded symbols over the entire bandwidth of the encoded symbols over the entire bandwidth of the CDMA channel.

the CDMA channel.



 The code used to channelize and The code used to channelize and spread the encodedspread the encoded voice info is known to

voice info is known to the receiver.the receiver.



 The receiver will use The receiver will use the code to despread/decthe code to despread/decodeode the signal and

CDMA Call Processing, Handoffs