Handoff Algorithms

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Handoff Algorithms

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Handoff Algorithms

• ? Introduction to Handoff
• ? Handoff Procedure in PACS

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Introduction to Handoff

• ? What is handoff
• ?The process of transferring a mobile from one
  channel or base-station to another is called
  handoff
• ?When a mobile moves into a different cell while
  a conversation is in progress, the MSC(Mobile
  Switch Center) automatically transfers the call
  to a new channel belonging to the new
  base-station

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Introduction to Handoff

• ? What is the purpose of handoff
• ?Rescue handoff
• ?To rescue the connection with poor quality
• ?If quality of a communication has become worse
  than a threshold, a decision of handoff is made
  for rescuing this connection
• ?Confinement handoff
• ?To minimize interference
• ?When the mobile can employ lower transmission
  power in another cell than in current one, a
  handoff can be triggered to confine the
  transmission power

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Introduction to Handoff

• ? What is the purpose of handoff
• ?Traffic handoff(Load sharing)
• ?It takes account of capacity and load
• ?If a new call set-up occurs, a traffic handoff
  will be triggered owing to the congestion in
  current cell
• ?A certain established call in the congested
  cell will be handed over to another cell so that
  the new call can be set up

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Introduction to Handoff

• ? When to initiate a handoff
• ?It is important to ensure that the drop in the
  measured signal level is not due to momentary
  fading and that the mobile is actually moving
  away from the serving base-station
• ?The base-station monitors the signal level for
  a certain period of time before a handoff is
  initiated

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Introduction to Handoff

• ? When to initiate a handoff
• ?The length of time needed to decide if a
  handoff is necessary depends on the speed at
  which the vehicle is moving
• ?If the slope of the short term average received
  signal level in a given time interval is steep,
  the handoff should be made quickly

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Introduction to Handoff

• ? Handoff type
• ?By destination base-station
• ?Intercell handoff
• ? Call is handed off to another base-station
• ?Intracell handoff
• ?The destination base-station is current
  base-station
• ?By connection method
• ? Soft handoff 1
• ?The connection is never suspended
• ?Hard handoff 1
• ?The connection is suspended at first and
  then recovered at the destination base-station

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Introduction to Handoff

• ? Handoff margin (Fig.1) 1
• ?This margin, given by ?Pthreshold -Pminimum,
  cannot be too large or too small
• ?If ? is too large, unnecessary handoffs which
  burden the MSC may occur
• ? If ? is too small, there may be insufficient
  time to complete handoff before a call is lost
  due to weak signal conditions

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Handoff Procedure in PACS

• ? Three kinds of radio link measurements
• ?Received Signal Strength Indicator(RSSI)
• ?The measurement of the received RF signal
  strength and the translation of this measurement
  to quantity that can be logically manipulated
• ?Quality Indicator(QI)
• ?The estimate of the eye opening of a radio
  signal that relates to the radio of signal to
  interference plus noise
• ?Word Error Indicator(WEI)
• ?An indication of whether one or more bit
  errors occur in a burst due to any radio link
  degradation

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Handoff Procedure in PACS

• ? Trigger ALT/TST Procedure(Fig.2Fig.3) 2
• ?The SU sorts 3 RPs in the short list in
  descending order according to the QI(Quality
  Indicator), and the measurements for QI and
  RSSI(Received Signal Strength Indicator) should
  be used to identify if any of the 3RPs is
  suitable to do ALT(Automatic Link
  Transfer)/TST(Time Slot Transfer)
• ?This process should run once every NS211 frames
  and then reset Cup and Cdown, where NS211 sets
  the window size used for examining short term
  word error counts to detect trouble situation on
  both the uplink and downlink

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Handoff Procedure in PACS

• ? Trigger ALT/TST Procedure(Fig.2Fig.3)
• ?A dwell timer, TS204, indicates the minimum
  time between allowable ALTs
• ?After ALT fails, a timer TS206 is set to
  prevent the target channel from being reconsidered

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Handoff Procedure in PACS

• ? Intra-RPCU ALT Procedure(Fig.4Fig.6) 2
• ?The message sequence is shown in Fig.7 while
  executing an ALT
• ?The SU temporarily suspends the conversation by
  transmitting a LINK_SUSPEND message to the old RP
  on the old channel
• ?Upon receipt of an ALT_REQ message, the RPCU
  grants access with a START_CIPHER message on the
  new RP channel within 20 msec
• ?After receiving ALT_COMP from the SU, the new
  RP resumes conversation and ALT suceeds

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Introduction to handoff
Fig.1 Handoff situations
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Handoff Procedure in PACS
Fig.2 Trigger ALT/TST process(1/2)
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Handoff Procedure in PACS
Fig.3 ALT/TST decision process
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Handoff Procedure in PACS
Fig.4 Intra-RPCU ALT(1/3)
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Handoff Procedure in PACS
Fig.5 Intra-RPCU ALT(2/3)
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Handoff Procedure in PACS
Fig.6 Intra-RPCU ALT(3/3)
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Handoff Procedure in PACS
Fig.7 Message sequence for Intra-RPCU ALT
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Handoff Procedure in PACS
Fig. 8 Receiver Block Diagram for RSSI Measurement
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Handoff Procedure in PACS
Fig. 9 Coherent demodulator structure with
selection diversity
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Handoff Procedure in PACS
Fig. 10 Digital demodulation circuit for
?/4-DQPSK (used for QI measurement)
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Handoff Procedure in PACS
Fig. 11 Cumulative distribution function of QI at
different CIR levels.
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Handoff Procedure in PACS
Fig. 12 The relation between WER (Word Error
Rate) and CIR
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Handoff Procedure in PACS
Fig. 13 Po versus n with decision making using
single-window QI measure
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Handoff Procedure in PACS
Fig. 14 Po versus n with decision making using
multiple-window QI measure
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Handoff Procedure in PACS
Fig. 15 Po versus n with decision making using
C/I-based measure
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Reference

• 1 Theodore S. Rappaport, Wireless
  Communications., New Jersey Practice Hall, 1996.
• 2 PACS Air Interface Specification,April 1995.

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