CDMA Call Processing, Handoffs

CDMA Call Processing

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Call Processing

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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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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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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) 6

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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. 8

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Once assigned to a Traffic Channel, the mobile is no longer monitoring the Paging Channel. During a land to mobile (LTM) call: 

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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): 

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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. 10

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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. 11

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CDMA has three primary types of  handoff: 

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hard soft idle

The type of handoff depends on the handoff situation. 12

Soft Handoff  

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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: 

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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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 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  

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While in the , 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). 18

Idle Handoff  Idle Handoffs

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CDMA soft handoff is driven by the handset

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Handset continuously checks available pilots

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Handset tells system pilots it currently sees

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

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All messages sent by dim-and-burst.

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The Pilot Searching Process

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

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The handoff process is “mobile assisted”: When the mobile detects a pilot of sufficient strength, it reports the event to the base station. 22

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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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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. 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 despread/decode the signal and recover the encoded voice data. 26

Codes used in CDMA 

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CDMA uses two important types of  codes to channelize users. 

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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 is olate 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. 29