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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 Traffic mode

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Initialization mode The subscriber unit performs initialization

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

searches for a usable pilot signal acquires the system via the Pilot code channel

synchronizes with the system by reading the Sync code channel broadcast from that site

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Idle mode The mobile is not involved in a call

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

The mobile and the base station communicate over the access and paging code channels

So in the Idle State the mobile is mainly listening to the Paging Channel for incoming messages. (Also monitors the strength of pilot channel in neighbouring cells)

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Access mode

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.

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Traffic mode Once assigned to a Traffic Channel, the mobile

is no longer monitoring the Paging Channel. During a land to mobile (LTM) call:

The mobile receives a page on the paging channel. The mobile responds on the access channel. The traffic channel is established and maintained

throughout the call. During a mobile to land call (MTL):

The call is placed using the Access channel. The base station responds on the paging channel. The traffic channel is established and maintained

throughout the call.

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Call processing (messages) During the call, overhead messaging

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

This messaging uses "Dim and Burst" or "Blank and Burst" signaling, which replaces part of the voice traffic with system messages.

The user does not detect this signaling, however, due to the strong data recovery schemes inherent to CDMA.

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Types of CDMA handoff

CDMA has three primary types of handoff: hard soft idle

The type of handoff depends on the handoff situation.

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Soft Handoff Soft handoff refers to the state where

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

Soft handoff is a make-before-break type of handoff whereby a mobile acquires a target code channel before breaking an existing one.

The advantages of soft handoff are several:

Fewer dropped calls. Soft handoffs in general require less mobile

transmit power.

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A CDMA mobile station is said to be in soft handoff when the mobile communicates with two or more antennas (PN offsets).

The PN offsets involved in softhand offs are said to be in Mobile Stations Active Set.

Up to six PN offsets can be present in active set.

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Soft Handoffs

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Soft handoff A soft handoff establishes a connection with the new

BTS prior to breaking the connection with the old one. This is possible because CDMA cells use the same

frequency. The mobile detects a new pilot as it travels to the next

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

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

Soft handoffs are also called "make-before-break." If the sectors are from same physical cell site (a

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

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Idle Handoff While in the Idle state, the mobile may

move from one cell to another. Idle handoff arises from the transition

between any two cells. Idle handoff is initiated by the mobile

when it measures a pilot signal significantly stronger than the current serving pilot (3 dB stronger).

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Idle Handoff

Idle Handoffs

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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 All messages sent by dim-and-burst.

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The Pilot Searching Process

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The Mobile Searches for Strong Pilots

The searching process is continuous and is conducted to find handoff candidates.

Mobile Reports The handoff process is “mobile

assisted”: When the mobile detects a pilot of sufficient strength, it reports the event to the base station.

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The Base Station Directs 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.

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Coding and Spreading

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Coding and Spreading

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Coding and Spreading

The encoded voice information is encoded using a code that will separate it from other encoded voice information.

This process, known as channelization, will spread the encoded symbols over the entire bandwidth of the CDMA channel. Receiver Decoding / Despreading

The code used to channelize and spread the encoded voice info is known to the receiver.

The receiver will use the code to despread/decode the signal and recover the encoded voice data.

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Codes used in CDMA

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Two kinds of codes

CDMA uses two important types of codes to channelize users. Walsh codes channelize users on the

forward link (BTS to mobile). Pseudorandom Noise (PN) codes

channelize users on the reverse link (mobile to BTS).

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Why we have two spreading codes? Walsh codes are used to differentiate

transmissions with in a cell. PN codes are used to isolate different

cells (base stations) that are using the same frequencies.

Same PN sequence is used in all cells with different offsets.

The code is of 32768 length with 64 bits in the shift register producing 512 offsets.