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 Benefits of 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 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 Timing 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 172 bits 9600 bps Primary Traffic only MM =0 Primary Traffic = 171 bits Dim & Burst with rate 1/2 primary & signaling traffic MM =1 TT =0 TM =00 Primary Traffic = 80 bits Signaling Traffic = 88 bits Dim & Burst with rate 1/4 primary & signaling traffic MM =1 TT =0 TM =01 Primary Traffic = 40 bits Signaling Traffic = 128 bits MM =1 Dim & Burst with rate 1/8 primary & signaling traffic TT =0 TM =10 Primary Traffic = 16 bits Signaling Traffic = 152 bits 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

Secondary Traffic 172 bits MM =1 TT TM =00 Primary Traffic = 80 bits Dim & Burst with rate 1/2 primary & Secondary traffic MM =1 TT TM =00 Primary Traffic = 80 bits Signaling Traffic = 88 bits Dim & Burst with rate 1/4 primary & Secondary traffic MM =1 TT TM =01 Primary Traffic = 40 bits Signaling Traffic = 128 bits Dim & Burst with rate 1/8 primary & Secondary traffic MM =1 TT TM =10 Primary Traffic = 16 bits Signaling Traffic = 152 bits Blank & Burst with Secondary traffic only MM =1 TT TM =11 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: 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: Access 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: 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 Reverse Link Pilot Channel Sync Channel Paging Channels (max. 7) Traffic Channels Power Control Sub-Channel Reverse Link Access Channels