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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 - PowerPoint PPT Presentation
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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
Primary Traffic = 80 bits
Signaling Traffic = 88 bits
Primary Traffic = 40 bits
Signaling Traffic = 128 bits
Primary Traffic = 16 bits
Signaling Traffic = 152 bits
Signaling Traffic = 168 bits
Primary Traffic = 80 bits
Primary Traffic = 40 bits
Primary Traffic = 16 bits
9600 bps Primary Traffic only
Dim & Burst with rate 1/2primary & signaling traffic
Dim & Burst with rate 1/4primary & signaling traffic
Dim & Burst with rate 1/8primary & 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
Primary Traffic = 80 bits
Signaling Traffic = 88 bits
MM=1
TT=1
TM=01
Primary Traffic = 40 bits
Signaling Traffic = 128 bits
MM=1
TT=1
TM=10
Primary Traffic = 16 bits
Signaling Traffic = 152 bits
MM=1
TT=1
TM=11
Signaling Traffic = 168 bits
Dim & Burst with rate1/2 primary &
Secondary traffic
Dim & Burst with rate1/4 primary &
Secondary traffic
Dim & Burst with rate1/8 primary &
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