Transcript An Introduction to CDMA Air Interface: IS-95A

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 IS41 as well. It sits on top of SS7
IS-95 interface (cont..)
• E Interface (MSC-MSC)– Inter MSC signaling is defined in IS41
• 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:
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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
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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
channel
– 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
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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
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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
172 bits
9600 bps Primary
Traffic only
MM
=0
Primary Traffic = 171 bits
Dim & Burst with rate 1/2 MM
primary & signaling traffic =1
TT
=0
TM
=00
Dim & Burst with rate 1/4
primary & signaling traffic
MM
=1
TT
=0
TM
=01
Dim & Burst with rate 1/8
primary & signaling traffic
MM
=1
TT TM Primary Traffic
=0 =10
= 16 bits
Blank & Burst with
signaling traffic only
MM
=1
TT
=0
TM
=11
Signaling Traffic
= 88 bits
Primary Traffic
= 80 bits
Primary Traffic
= 40 bits
Signaling Traffic
= 128 bits
Signaling Traffic
= 152 bits
Signaling Traffic = 168 bits
80 bits
4800 bps primary
traffic only
Primary Traffic = 80 bits
40 bits
2400 bps primary
traffic only
Primary Traffic = 40 bits
16 bits
1200 bps Primary
traffic only
Primary Traffic = 16 bits
Secondary Traffic
172 bits
Dim & Burst with rate
MM
1/2 primary &
=1
Secondary traffic
TT
=1
TM
=00
Dim & Burst with rate
MM
1/4 primary &
=1
Secondary traffic
TT
=1
TM
=01
Dim & Burst with rate MM
1/8 primary &
=1
Secondary traffic
TT TM Primary Traffic
=1 =10
= 16 bits
MM
Blank & Burst with
Secondary traffic only =1
TT
=1
TM
=11
Primary Traffic
= 80 bits
Primary Traffic
= 40 bits
Signaling Traffic
= 88 bits
Signaling Traffic
= 128 bits
Signaling Traffic
= 152 bits
Signaling Traffic = 168 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:
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overhead
paging
order
channel assignment
• Configuration of the system is conveyed in four overhead messages:
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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:
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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
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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
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Pilot Channel
Sync Channel
Paging Channels (max. 7)
Traffic Channels
• Power Control Sub-Channel
• Reverse Link
– Access Channels
– Traffic Channels