An Introduction to CDMA Air Interface: IS-95A. Contents Introduction Forward CDMA Channel Reverse CDMA Channel

An Introduction to CDMA Air Interface: IS-95A

Contents

• Introduction

• Forward CDMA Channel

• Reverse CDMA Channel

CDMA Around the World

IS-95 Interfaces

• A Interface (BSC-MSC) .. This interface is between the BSC and the MSC. It supports both the control plane and user plane

• Abis Interface (BTS-BSC)—This is the interface between the BSC and BTS. This is internal interface and generally proprietary

• B Interface (MSC-VLR) This interface is defined by TIA IS-41

• C Interface (MSC-HLR) This interface uses IS-41 messaging as well

• D Interface (HLR-VLR) – HLR-VLR signaling is based on IS-41 as well. It sits on top of SS7

IS-95 interface (cont..)

• E Interface (MSC-MSC)– Inter MSC signaling is defined in IS-41

• L interface (MSC-IWF) This interface allows the ability for circuit switched data in second generation networks

• Um Interface (BS-MS) – This is the air interface between the mobile and the network

Multiple Access

• CDMA – unique digital codes are used to differentiate subscribers

– codes are shared by both MS and BS

– all users share the same range of radio spectrum

• Benefits of CDMA:– Capacity increases: 4 to 5 times (GSM)

– Improved call quality

– Simplified system planning

– Enhanced privacy

– Improved coverage characteristics

– Increased talk time for portables

– Bandwidth on demand

CDMA

• There are two CDMA common air interface standards:– Cellular (824-894 MHz) - TIA/EIA/IS-95A

– PCS (1850-1990 MHz) - ANSI J-STD-008

They are very similar in their features, with exceptions of the frequency plan, mobile identities, and related message fields.

• IS-95A– 45 MHz spacing for forward & reverse channel

– Permissible frequency assignments are on 30 kHz increments

Forward CDMA Channel of IS-95A

• From BSS to MS

• It carries traffic, a pilot signal, and overhead information.

• Pilot is a spread but unmodulated

• Pilot and overhead channels establish the system timing and station identity.

• Pilot channel is also used in the mobile-assisted handoff (MAHO) process as a signal strength reference.

Overhead Channels

• There are three types of overhead channel in the forward link:– pilot, is required in every station

– sync

– paging

• Pilot channel– pure short code with no additional cover or information content

– always code channel zero

– a demodulation reference for the mobile receivers and for handoff level measurements

– carries no information

– all stations use the same short code, distinguished by the phase

Overhead Channels

• Sync Channel– sync channel carries timing and system configuration information

– data rate is always 1200 bps

Overhead Channels

• Paging channel– used to communicate with MSs when they are not assigned to a traffic ch

annel

– successful accesses are normally followed by an assignment to a dedicated traffic channel

– paging channel may run at either 4800 or 9600 bps

– each BS must have at least one paging channel per sector, on at least one of the frequencies in use

Traffic Channel

• Traffic channels – assigned dynamically, in response to MS accesses, to specific MS

– always carries data in 20 ms frames– carry variable rate traffic frames, either 1, 1/2, 1/4, or 1/8 of 9600 bps

– rate is independently variable in each 20 ms frame

– the 800 bps reverse link power control subchannel is carried on the traffic channel by puncturing 2 from every 24 symbols transmitted.

• Timing– all base stations must be synchronized within a few microseconds

Handoff

• Steps in a handoff– Starting in a state where only one cell is supporting the call in question.

– Informing the candidate cell of the imminent handoff

– Signaling the mobile to begin executing the handoff.

– New cell beginning to service the mobile

– Mobile beginning to use the new cell

– Entering the mid-handoff state (prolonged only in CDMA)

– Mobile discontinuing use of the old cell

– Old cell stopping service to the mobile

– Ending in a state where the new cell is supporting the call in question

Primary and Signaling Traffic

MM=0

MM=1

MM=1

MM=1

MM=1

TT=0

TT=0

TT=0

TT=0

TM=00

TM=01

TM=10

TM=11

Primary Traffic = 171 bits


Signaling Traffic = 88 bits









9600 bps Primary Traffic only

Dim & Burst with rate 1/2primary & signaling traffic



Blank & Burst withsignaling traffic only

4800 bps primarytraffic only

2400 bps primarytraffic only

1200 bps Primarytraffic only

172 bits

80 bits

40 bits

16 bits

Secondary Traffic

MM=1

TT=1

TM=00



MM=1

TT=1

TM=01



MM=1

TT=1

TM=10



MM=1

TT=1

TM=11


Dim & Burst with rate1/2 primary &

Secondary traffic


Secondary traffic


Secondary traffic

Blank & Burst withSecondary traffic only

172 bits

Sync Channel

• Signaling on all channels use a synchronized bit-oriented protocol.

• Sync channel is used during the system acquisition stage.

• Sync channel frame length is the length of pilot PN sequence.

• Only the Sync Channel Message is sent on the sync channel.

• MS – obtains information from Sync Channel Message

– adjusts its timing to normal system timing

– begins monitoring its Paging Channel

Paging Channel

• Data rate: 2400, 4800, 9600 bps

• One 9600 bps Paging Channel can support 180 pages/sec.

• Paging Channel conveys four major types of messages:– overhead– paging– order– channel assignment

• Configuration of the system is conveyed in four overhead messages:– System Parameter Message– Access Parameter Message– Neighbor List Message– CDMA Channel List Message

Overhead Messages

• System Parameter Message:– configuration of the Paging Channel– registration parameters– parameters to aid pilot acquisition

• Access Parameter Message– configuration of the Access Channel– control parameters used to stabilize the Access Channel

• Neighbor List Message– time offset of the pilot– basic neighbor configuration

• CDMA Channel List Message– CDMA frequency assignment that contain Paging Channels

Paging Channel Messages

• Page Message: – contains pages to one or more mobile stations.

• Order Message: – a broad class of messages used to control a particular MS.

• Channel Assignment Message:– let BS to assign a MS to the traffic channel

– change Paging Channel Assignment

Access Channel

• Access Channel provides communications from MS to BS when MS is not using a Traffic Channel.

• All Access Channel use 4800 bps mode

• Access Channel Message:– call origination

– response to pages

– orders

– registrations

• Control of Access Channel transmission is accomplished through the Access Parameter Message sent on the Paging Channel

Framing and Signaling on the Traffic Channel

• Both forward & reverse Traffic Channels use 20 ms frames.

• Frames can be sent at 9600, 4800, 2400, or 1200 bps

• Signaling– blank-and-burst signaling:

• sent at 9600 bps

• replace one or more frames of primary traffic data

– dim-and-burst signaling:

• sent at 9600 bps

• sends both signaling and primary traffic data in a frame

• degradation in voice quality is essentially undetectable

Traffic Channel Messages & Service Option

• Four types of control messages on the Traffic Channel– messages controlling the call itself

– messages controlling handoff

– messages controlling forward link power

– messages for security and authentication

• IS-95 supports different user applications, called service options

• Two different service options can be simultaneous supported:– primary traffic

– secondary traffic

• MS can specify the desired service option at call origination.

IS-95 Standard

• Forward Link– Pilot Channel

– Sync Channel

– Paging Channels (max. 7)

– Traffic Channels• Power Control Sub-Channel

• Reverse Link– Access Channels

– Traffic Channels

Documents

An Introduction to CDMA Air Interface: IS-95A. Contents Introduction Forward CDMA Channel Reverse CDMA Channel