IS 95 CDMA Architecture
P K Divakaran
Copyright 2003 P K Divakaran
MS
IS 95 System Architecture Base Station (BS) BTS
HLR
BSC/ MSC
PSTN Copyright 2003 P K Divakaran
VLR
AC
IS 95 System Architecture HLR – Subscriber information ESN Mobile Directory Number ( similar to MSISDN in GSM) IMSI User profiles – Current Location Copyright 2003 P K Divakaran
BTS
IS 95 CDMA Architecture MSC
BSC/
P S T N
Transcoder (XC) BTS
•XC performs Transcoding of PCM coding to CDMA voice coding and vice versa •During Soft Handoff multiple BTSs forwards speech/ data frame to XC from MS. XC selects the best from Reverse channels. In Forward Direction it distributes traffic to all BTSs involved in Soft handoff
Copyright 2003 P K Divakaran
CDMA IS 95 Traffic cases
P K Divakaran
Copyright 2003 P K Divakaran
Mobile Registration What is Mobile Registration? It is the process by which mobile stations make their locations and other particulars known to the Cellular system Location Registration enables the System to page a mobile in a particular area instead of paging the entire network. Copyright 2003 P K Divakaran
Types of Registration Different types of Registrations are specified in IS 95. The Operator has flexibility to choose depending on system size & other considerations. The
types of Registrations that are enabled are communicated to the mobiles in the Paging channels Copyright 2003 P K Divakaran
Types of Registration Autonomous
Registration (inititated by
mobile) – 1. Power up Registration When power is switched on – 2. Power down Registration
When power is switched off
Copyright 2003 P K Divakaran
Types of Registration (Autonomous) 3.Timer
based Registration
– To avoid paging mobiles which did not perform
a successful Power down registration, Timer based registration is enforced. If mobile fails to register as per this, it is deregistered. 4.Zone
based Registration
– Zones are groups of Base stations . Whenever
the mobile enters a new Zone, it registers Copyright 2003 P K Divakaran
Types of Registration (Autonomous) Distance
based Registration
– Registration after the mobile has moved a
certain distance ( as determined by the Latitude & Longitudes of the Base stations ). Used in the case of an environment where most mobiles are fairly static and a small number mobile. Rather than forcing Timer based registrations for all mobiles to register, only those mobiles moving beyond a certain distance only need to register Copyright 2003 P K Divakaran
Types of Registration Ordered
Registration
– The cellular system may become aware of a mobile
within the network for which it doesnot have all the info to connect a call (following a call origination from mobile). In this case the System orders a Registration by mobile
Implicit
Registration
– When a mobile successfully sends an origination
message or responds to paging message, it conveys sufficient info for the Base station to infer its location . This is Implicit registration Copyright 2003 P K Divakaran
Types of Registration Parameter
change Registration
– Certain parameters in the mobile station affects
the delivery of call to it and therefore should be updated in the system whenever there is a change.
Traffic
channel registration
– The method in which mobile receives
registration related information while on traffic channel.An instance is inter System Handoffs Copyright 2003 P K Divakaran
Authentication Authentication is the process of verifying that a mobile trying to access the Network is not a fraudulent one
Copyright 2003 P K Divakaran
Authentication Parameters “A”
Key
– 64 bit secret number used by mobile and the
Authentication Centre (AC) – Stored in the mobiles’ permanent security and identification memory and AC
Copyright 2003 P K Divakaran
Authentication Parameters Shared
Secret Data (SSD)
– A 128 bit secret “temporary key” calculated
and stored in mobile and the Network using: A key ESN Random Number (RAND)
SSD
comprises of SSD A and SSD B
– SSD A used for Authentication – SSD B used for encryption Copyright 2003 P K Divakaran
Authentication Parameters RAND – Random Number generated by the Network
The
essence of Authentication confidentiality is that “A “ key is never transmitted on air interface. Only the computed results are exchanged and compared at MS and Network for a decision Copyright 2003 P K Divakaran
Hand off Types – Idle State Handoff Initiated by mobile Always Hard Hand off – Traffic State Handoff Soft Hand off Hard Handoff
Copyright 2003 P K Divakaran
Soft Hand off Process
of establishing a link with a target cell before breaking the link with the serving cell Prime requirement--- Both cells to be on same frequency Mobile
and Network are involved in the process ( Mobile assisted) Copyright 2003 P K Divakaran
Soft Hand off Mobiles
search for Pilot frequencies on the current frequency to identify candidate cells for hand off. Each Pilot has the same PN code but with a different time offset (n X 64 chips delay) for Base station identification
Copyright 2003 P K Divakaran
Soft Hand off Process MS
BSC/BTS Pilot Strength measurement Message (PSMM) Handoff Direction Message (HDM)
Includes Forward Traffic ch. Assignment in new cell MS demodulates New Traffic ch. Handoff Completion Message (HCM) Copyright 2003 P K Divakaran
Benefits of Soft Handover Path
Diversity in Forward & Reverse Channels Hence less power is required in each channel Hence less “interference” and improved capacity Less power means longer battery life of MS Copyright 2003 P K Divakaran
Hard Handoff Connection with the old traffic channel is broken before connection with new is established. Possible reasons: – Base stations have different frequency Carriers – Change from one Pilot to another Pilot without
first being in soft handoff with the new Pilot(disjoint cell)
Copyright 2003 P K Divakaran
IS 95 Call Origination from MS BTS BSC MSC/ MS origination Access channel
origination
Traffic ch Assignment (Page ch) Acquires Traffic ch Reverse Traffic Ch. Preamble
Origination ack.
Speech path set up
Copyright 2003 P K Divakaran
HLR/AC
Subscribe validation
PSTN Call set u
IS 95 Call Termination to MS BTS
MS Page MS (Paging ch)
BSC Page request
Incoming call
Page response Page response (Access ch) Traffic ch. Assign Call treatment ment ( Paging ch) Acquires Tfc ch. Reverse Tfc ch Connection & Speech path set up Preamble Alert Copyright 2003 P K Divakaran
MSC PSTN
MS Base Station
ROAMING Visited MSC VLR
Call Forwarding SS7 IAM Normal call Processing. Call termination To MS
Home MSC
HLR From PSTN SS7 IAM
Query MS Location MS location info
Copyright 2003 P K Divakaran
SS7 IAM- Signalling System No.7, Initial Address Message
CDMA 2000 CDMA
2000 Radio Transmission Technology (RTT) is a spread spectrum Radio interface to satisfy the needs of 3G wireless systems It is as per the IMT 2000 (International Mobile Telecommunications 2000) standards of ITU ( International Telecommunication Union) Copyright 2003 P K Divakaran
CDMA 2000 Naming Conventions 3G1x RTT = 3rd Generation 1x 1.25 Mhz carrier 3G 3x RTT = 3rd Generation 3x 1.25 Mhz carrier ( total bandwidth 5 Mhz )
Copyright 2003 P K Divakaran
Enhancements in Cdma 2000 Backward
compatibility with IS 95 144 kbps peak data rate in Phase 1 of Cdma 2000 (Cdma 2000 1x) 2 Mbps data for fixed applications in Phase 2 (Cdma 2000 3x) More channels in forward and Reverse directions Pilot in the Reverse direction from Mobile Copyright 2003 P K Divakaran
Enhancements in Cdma 2000 Walsh
codes in Reverse link also Variable Walsh code length upto 128 bits Faster Power control in Forward direction
Copyright 2003 P K Divakaran
Advantages of Cdma 2000 Almost
twice the capacity of IS 95. Higher Data Rates Migration to 3G Lower power mobiles and longer battery life
Copyright 2003 P K Divakaran