An Introduction to CDMA Air Interface: IS-95A

1
An Introduction to CDMA Air Interface IS-95A
2
Contents

• Introduction
• Forward CDMA Channel
• Reverse CDMA Channel

3
CDMA Around the World
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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

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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

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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

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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

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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.

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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

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Overhead Channels

• Sync Channel
• sync channel carries timing and system
  configuration information
• data rate is always 1200 bps

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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

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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

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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

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Primary and Signaling Traffic
172 bits
MM 0
9600 bps Primary Traffic only
Primary Traffic 171 bits
Signaling Traffic 88 bits
MM 1
TM 00
Primary Traffic 80 bits
TT 0
Dim Burst with rate 1/2 primary signaling
traffic
MM 1
Primary Traffic 40 bits
Signaling Traffic 128 bits
TT 0
TM 01
Dim Burst with rate 1/4 primary signaling
traffic
MM 1
Signaling Traffic 152 bits
TT 0
TM 10
Primary Traffic 16 bits
Dim Burst with rate 1/8 primary signaling
traffic
Blank Burst with signaling traffic only
MM 1
TT 0
TM 11
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
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Secondary Traffic
172 bits
Dim Burst with rate 1/2 primary Secondary
traffic
Dim Burst with rate 1/4 primary Secondary
traffic
Dim Burst with rate 1/8 primary Secondary
traffic
Blank Burst with Secondary traffic only
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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

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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

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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

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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

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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

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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

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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.

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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