CDMA Technology Overview

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CDMA Technology Overview

• Lesson 5 Power Control, Registration, and
  Handoffs

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Power Control
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CDMA Power Control

• CDMA is an interference-limited system based on
  the number of users
• Unlike AMPS/TDMA, CDMA has a soft capacity limit
• Each user is a noise source on the shared channel
• The noise contributed by users is cumulative
• This creates a practical limit to how many users
  a system will sustain
• Precise power control of the mobile stations is
  critical if we want to
• Maximize system capacity
• Increase battery life of the mobile stations
• The goal is to keep each mobile station at the
  absolute minimum power level necessary to ensure
  acceptable service quality
• Ideally the power received at the base station
  from each mobile station should be the same
  (minimum signal to interference)
• Mobile stations which transmit excessive power
  increase interference to other mobile stations

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Reverse Open Loop Power Control
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Estimated Reverse Open LoopOutput Power
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Estimated Reverse Open LoopOutput Power
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Reverse Closed Loop Power Control
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Power Output Estimations (Summary)
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Reverse Outer Loop Power Control
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Forward Traffic Channel Power Control
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Summary of All Power Control Mechanisms
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Registration
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Roaming

• A mobile station may be in any of the following
  roaming states
• Home mobile station is at its home location (not
  roaming)
• NID roaming mobile station is within a foreign
  NID but in the home SID
• SID roaming mobile station is within a foreign
  SID
• A mobile station maintains a list of one or more
  home pairs
• These are SID/NID combinations defining the
  mobile stations home location
• They are stored in semi-permanent memory
• The identity of current SID/NID is contained in
  the System Parameters Message (sent on the Paging
  Channel)

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HLR VLR

• Contains permanent subscriber data
• provisioning information
• service information
• features available to the subscriber
• Contains dynamic information
• mobile stations current location
• Supports call routing
• Queried by the MTX when subscriber information is
  needed, regardless of the mobile stations
  current location

• Stores a subset of the HLR information pertaining
  to the mobile stations currently registered in
  the VLRs service area

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CDMA Registration

• Registration is the means by which a mobile
  station notifies the cellular system of its
  location, status, identification, and other
  characteristics
• Balance is required between paging and
  registration
• Infrequent registration results in a high rate of
  paging
• Frequent registration places a high load on
  access channels
• Proper system design allows a base station to
  efficiently page the mobile station when
  establishing a mobile-terminated call
• Registration also provides
• The mobile stations SLOT_CYCLE_INDEX and
  SLOTTED_MODE
• The mobile station class mark and protocol
  revision number so that the base station will
  know the mobile stations capabilities
• Two types of mobile registration
• Non-Autonomous explicitly requested by the base
  station, or implied based on other types of
  messages received by the mobile station
• Autonomous triggered by some event other than
  the reception of an explicit or implicit request
  from the base station

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Forms of CDMA Registration

• Power-up registration
• Power-down registration
• Timer-based registration
• Zone-based registration
• Distance-based registration
• Parameter-change registration
• Implicit registration
• Ordered registration
• Traffic channel registration

Autonomous Registration
Non-Autonomous Registration
Registration Types NOT Supported by Nortel
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Power-Up Registration

• Access Channel

• Mobile station registers when
• Directed to power-on by the user
• Switched to an alternate serving system
• Switched from using an analog system
• Delays 20 seconds
• Preventing multiple registrations whenever power
  is quickly turned on and off

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Power-Down Registration

• Access Channel

• Mobile station registers when directed to
  power-down by the user
• Mobile station will not power down until attempt
  is completed
• Mobile station will not do power down
  registration if
• Not registered in the current system
• Prevents unnecessary attempts to reach a user
• Can be unreliable (v.gr., user powers down in
  garage)

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Timer-Based Registration

• Access Channel

• Mobile station registers when a timer expires
• Registration period is determined by the base
  station
• Allows system to de-register mobile stations that
  fail to register on power-down

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Distance-Based Registration

• Mobile Station MS registers whenever it does an
  Idle Handoff (handoff when not in a call) into
  a cell which lays outside a circle with REG_DIST
  radius and centered at the base station where MS
  last registered
• At position a MS registers with Base Station
  BS-1. BS-1 transmits its latitude and longitude,
  and the REG_DIST parameter on its paging channel
• At position b MS does an idle handoff into BS-2
  and reads the latitude and longitude of this base
  station. MS then calculates the distance between
  BS-2 and BS-1, and if the result is less than
  REG_DIST it does not have to re-register
• At position c MS is still listening to BS-2 (no
  need to re-register yet)
• At position d MS does an idle handoff into
  BS-3. MS reads the latitude and longitude of
  BS-3 and calculates the distance between BS-3 and
  BS-1. As this distance exceeds REG_DIST, MS
  re-registers

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Zone-Based Registration
NOTE These are registration zones, not TMSI
zones!

• The mobile station registers when it enters a new
  zone
• A zone is a subset of the base stations within a
  network
• The mobile station keeps a list of the zones
  where it has registered, up to a maximum
  determined by the base station
• Each zone is uniquely identified by the
  registration zone number parameter (REG_ZONE)
  plus the SID and the NID to which it belongs
• The mobile station activates a timer for every
  zone where it has registered, except the active
  one, and de-registers when the timer expires
• The mobile station will not re-register if it
  enters a zone which is already in its list

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Parameter-Change Registration
SLOT_CYCLE_INDEX SLOTTED_MODE MOB_TERM_HOME etc.

• Access Channel

• The mobile station registers after it modifies
  any of the following parameters (stored in the
  mobile station)
• the preferred slot cycle index
• the slotted mode indicator
• the call termination enabled indicators
• or the following capabilities supported by the
  mobile station
• the band classes
• the power classes
• the rates
• the operating modes

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Implicit Registration
Origination Message

• Access Channel

• Occurs when the mobile station and base station
  exchange messages not directly related to
  registration
• Messaging conveys sufficient information to
  identify mobile station and its location
• Considered successful whenever mobile station
  sends an Origination Message or Page Response
  Message
• Compatible with AMPS and IS-54 methods
• Effectiveness considered adequate to preclude use
  of ordered registration

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Ordered and Traffic Channel Registration

• Ordered Registration
• Allows the base station to order a mobile station
  to register
• mobile station can be idle or in an active call
• Traffic Channel Registration
• Allows the base station to obtain registration
  information about a mobile station that has been
  assigned to a Traffic Channel
• Information exchange occurs on the Traffic
  Channel
• Suggested use is on inter-system handoffs
• Neither one is supported by Nortels CDMA system

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Handoffs
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What is Ec/Io?

• Ec/Io
• Measures the strength of the pilot
• Foretells the readability of the associated
  traffic channels
• Guides soft handoff decisions
• Is digitally derived as the ratio of good to
  total energy seen by the search correlator at the
  desired PN offset
• Never appears higher than Pilots percentage of
  serving cells transmitted energy
• Can be degraded by strong RF from other cells,
  sectors
• Can be degraded by noise

Ec
Energy of desired pilot alone
Io
Total energy received
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Whats In a Handset?
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CDMA Handoffs

• Handoff is the process by which a mobile station
  maintains communications with the Mobile Services
  Switching Center (MSC/BSC), when traveling from
  the coverage area of one base station to that of
  another

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CDMA Soft Handoff Mechanics

• CDMA soft handoff is driven by the handset
• Handset continuously checks available pilots
• Handset tells system pilots it currently sees
• System assigns sectors (up to 6 max.), tells
  handset
• Handset assigns its fingers accordingly
• All messages sent by dim-and-burst, no muting!
• Each end of the link chooses what works best, on
  a frame-by-frame basis!
• Users are totally unaware of handoff

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Softer Handoff

• Each BTS sector has unique PN offset pilot
• Handset will ask for whatever pilots it wants
• If multiple sectors of one BTS simultaneously
  serve a handset, this is called Softer Handoff
• Handset is unaware, but softer handoff occurs in
  BTS in a single channel element
• Handset can even use combination soft-softer
  handoff on multiple BTS sectors

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Overall Handoff Perspective

• Soft Softer Handoffs are the best
• but a handset can receive BTS/sectors
  simultaneously only on one frequency
• all involved BTS/sectors must connect to a single
  BSC (the BSC must choose packets each frame)
• must be same on all BTS/sectors
• If above not possible, handoff still can occur
  but will be hard like AMPS/TDMA/GSM
• intersystem handoff hard
• change-of-frequency handoff hard
• CDMA-to-AMPS handoff hard, no handback
• auxiliary trigger mechanisms available

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CDMA-to-CDMA Hard Handoff

• Between cells operating on different frequencies

• Between cells with traffic channels whose frames
  are staggered differently

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CDMA-to-CDMA Hard Handoff

• Between cells that could be on the same frequency
  and have the same frame alignment, but which are
  subordinated to different BSCs which are not
  interconnected.
• This type of hard handoff would become a soft
  handoff if the frames received at both cells
  could be delivered quickly to the same BSC for
  comparison, by interconnecting both BSCs with a
  high-speed link (see Inter BSC Soft Handoff /
  Inter System Soft Handoff)

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Pilot Detection Trigger CELL_PILOT_BEACON
sectors

• This trigger utilizes the existing soft handoff
  algorithm in the mobile station to facilitate the
  hard handoff.
• Certain pilots in the region where hard handoff
  is desired are identified as CELL_PILOT_BEACON in
  the Pilot Database of the SBS Controller
• The cell on the left serves the mobile station on
  frequency f1
• The cell on the right operates in frequency f2
  and has a Pilot Beacon Unit that generates a
  pilot on frequency f1 (or this pilot is generated
  by a standard CDMA cell of an adjacent
  system/market)
• As the mobile station travels into the region in
  which hard handoff is desired, soft handoff
  processing is initiated when the mobile station
  reports to the network that the signal of the
  beacon cell is received with sufficient strength
• SBS software determines that the reported pilot
  corresponds to a beacon cell, and hard handoff
  processing commences

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Hard Handoff using Beacon Pilot Sectors
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Boundary Sector Trigger(CELL_BORDER Sectors)

• This is a two-stage trigger which indirectly
  utilizes the existing soft handoff algorithm in
  the mobile station to facilitate the hard handoff
• certain pilots in the region where hard handoff
  is desired are identified as CELL_BORDER in the
  Pilot Database of the SBS Controller
• as the mobile station travels from left to right,
  it enters into handoff with both sectors and
  eventually ceases communication with the sector
  on the left (CELL_STANDARD)
• when the active set contains only sectors
  datafilled as CELL_BORDER, the first-stage
  trigger is met and the second-stage trigger is
  enabled
• the SBS starts monitoring the Round Trip Delay
  (RTD) of the signals between the mobile station
  and the base station from which it derives its
  time reference
• when the RTD exceeds a certain threshold, the
  second-stage trigger is met and handoff
  processing continues with the target selection
  activity

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Hard Handoff Using Border Sectors
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CDMA-to-Analog Handoff

• The mobile station is directed from a forward
  traffic channel to an analog voice channel
• Radio link continuity is not maintained
• Two types of handoff
• Inter-system - occurs while the mobile station is
  traveling into another system that has no CDMA
  service
• Messaging will tell the mobile station to select
  AMPS
• Currently, the mobile station cannot handoff back
  from AMPS to CDMA (until the end of the call,
  when the mobile station reacquires the system)
  because the necessary signaling messages not
  supported)
• Intra-system - occurs while the mobile station is
  traveling within the system
• Load balancing
• Improve voice quality
• No CDMA service

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Lesson Review

• What is the purpose of power control?
• To maximize system capacity by minimizing
  noise/interference, and to increase battery life
  in the mobile stations.
• What is the ideal situation at the base station
  regarding power level of the received mobile
  station signals?
• To receive exactly the same (minimum) power level
  from every mobile station.
• Define CDMA registration and explain its purpose
• CDMA registration is the means in which a mobile
  station notifies the cellular system of its
  location, status, identification and other
  characteristics. The purpose of CDMA registration
  is to allow the system to efficiently page the
  mobile station when establishing a mobile
  terminated call.

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Lesson Review

• Name the forms of registration, indicating
  whether they are autonomous or not.
• Autonomous
• Power-up registration,
• Power-down registration,
• Timer-based registration,
• Distance-based registration, and
• Zone-based registration.
• Non-autonomous
• Parameter-change registration, Implicit
  registration,
• Ordered registration and Traffic
  Channel registration.
• Identify the cases of CDMA handoff that can occur
  when the mobile station is in the Traffic Channel
  State.
• Soft handoff, Softer handoff, CDMA-to-CDMA Hard
  Handoff, and CDMA-to-Analog Hard Handoff.
• Identify the message sent by the mobile station
  to report the strength of the pilots it measures
• Pilot Strength The Measurement Message.

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