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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

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An Introduction to CDMA Air Interface: IS-95A

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  1. An Introduction to CDMA Air Interface: IS-95A

  2. Contents • Introduction • Forward CDMA Channel • Reverse CDMA Channel

  3. CDMA Around the World

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.

  9. 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

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

  11. 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

  12. 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

  13. 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

  14. 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

  15. Signaling Traffic = 88 bits MM =1 TM =00 Primary Traffic = 80 bits TT =1 MM =1 Primary Traffic = 40 bits Signaling Traffic = 128 bits TT =1 TM =01 MM =1 Signaling Traffic = 152 bits TT =1 TM =10 Primary Traffic = 16 bits MM =1 TT =1 TM =11 Signaling Traffic = 168 bits 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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.

  23. 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

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