Mobile Unit Handoff

1
Mobile Unit Handoff

• Handoff when a mobile unit moves from one cell
  to another while a call is in progress, the MSC
  must transfer (handoff) the call to a new channel
  belonging to new base station
• New voice and control channel frequencies
• Very important task ? often given higher priority
  than new call!!
• Which is more annoying to customers?
• Dropping call during middle of conversation
• Blocking a new call request

2
Illustration of Handoff at Cell Boundary
3
Mobile Unit Handoff

• Minimum Useable Signal (MUS)
• Lowest acceptable voice quality
• Specified by system designers
• Typical values ? ?100 dBm 0.1 pW!
• Handoff Threshold (HT) gt MUS Level
• Handoff Margin ?
• ? PHT ? PMUS
• Carefully selected
• ? too large ? unnecessary handoff ? MSC loaded
  down
• ? too small ? not enough time to transfer ? call
  dropped!
• Fig. 3.3, pg. 63

Note
4
Mobile Unit Handoff

• Dropped Call Occurrence 3 typical causes
• Excessive delay by MSC in assigning handoff
• High traffic times and ? computational load on
  MSC
• No channels available in new serving cell
• Mobile unit moves into dead zone
• Handoff Decision
• Signal level decreasing due to
• Momentary signal fading ? do NOT handoff
• Mobile moving away from base station
• Must monitor RSS over a period of time ? moving
  average
• Time allowed to complete handoff depends on
  mobile speed
• Large negative RSS slope ? high speed ? quick
  handoff
• Statistics of small-scale (short time) fading
  signal important to making appropriate handoff
  decision ? Chpt. 5

5
Mobile Unit Handoff

• 1st Generation (1G) Cellular (Analog FM ? AMPS)
• RSS of RVC measured at base station monitored
  by MSC
• Another Rx in base station (locator Rx) monitors
  RSS of RVC in adjacent cells (controlled by MSC)
• MSC monitors RSS from all base stations decides
  on handoff
• Call handoff took 1?2 sec ? very noticeable to
  user
• Break in conversation and white noise before
  new channel established in new cell

6
Mobile Unit Handoff

• 2nd Generation (2G) Cellular w/ digital TDMA
  (USDC, GSM, IS?136)
• Mobile Assisted HandOffs (MAHO)
• Important advancement
• Mobile measures RSS of FCC from multiple adjacent
  base stations reports back to serving base
  station
• If Rx power from new base station gt Rx power from
  serving (current) base station by pre-determined
  margin for certain time period ? handoff
  initiated by MSC
• MSC makes handoff decision but is assisted by
  mobile which reports RSS values from serving and
  neighboring cells

7
Mobile Unit Handoff

• 2G MAHO
• MSC no longer monitors RSS of all channels like
  in 1G
• MSC computational load considerably reduced
• Enables rapid ( 20 msec) and efficient handoffs
• Imperceptible to user
• Improves call quality as well
• RSS from adjacent cell may be much better than
  serving cell and RSS gt PHT ? handoff not
  initiated but adjacent cell has better signal!!

8
Mobile Unit Handoff

• Prioritizing Handoffs
• Perceived Quality of Service (QoS)
• Assign higher priority to handoff vs. new call
  request
• Dropped call more aggravating than occasional
  blocked call
• Guard Channels
• of total available cell channels exclusively
  set aside for handoff requests
• Fewer channels available for new call requests!!
• Good strategy for dynamic channel allocation (not
  fixed)
• Guard channels only allocated/used in needed
  cells (high traffic)
• Not wasted on cells with low traffic

9
Mobile Unit Handoff

• Prioritizing Handoffs (continued)
• Queuing Handoff Requests
• Use time delay between handoff threshold and MUS
  level to place handoff request in a queue
• Prioritize requests (mobile speed) and handoff as
  needed
• High speed mobile user ? rapid variation of RSS ?
  strong negative slope of RSS when leaving cell ?
  short time duration before MUS level crossed ?
  high priority handoff
• Low speed mobile user can be placed lower in
  queue priority even though handoff threshold
  crossed before another high speed user

10
Mobile Unit Handoff

• Practical Handoff Considerations
• Problems occur b/c of large range of mobile
  velocities
• Pedestrian vs. vehicle user
• Small cell sizes and/or micro-cells ? larger
  handoffs
• MSC load is heavy when high speed users are
  passing between very small cells
• handoffs/time increases

11
Mobile Unit Handoff

• Practical Handoff Considerations
• Umbrella Cells
• Use different antenna heights and Tx power levels
  to provide large and small cell coverage
• Multiple antennas Tx can be co-located at
  single location if necessary
• Large cell ? high speed traffic ? fewer handoffs
• Small cell ? low speed traffic
• Example areas interstate highway passing thru
  urban center, office park, or nearby shopping
  mall
• Fig. 3.4, pg. 67

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Umbrella Cell
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Handoff Parameters

• Typical 1G Analog Cellular Parameters
• Threshold margin ? ? 6 to 12 dB
• Total time to complete handoff ? 8 to 10 sec
• Transfer time ? 1 to 2 sec

14
Handoff Parameters

• Typical 2G Digital Cellular parameters
• Threshold margin ? ? 2 to 6 dB
• Total time to complete handoff ? 1 to 2 sec
• Transfer time ? 20 msec
• Small handoff time
• Greater flexibility in handling high/low speed
  users
• Queuing handoffs prioritizing
• More time to rescue calls needing urgent
  handoff
• Fewer dropped calls ? QoS increased

15
Mobile Unit Handoff

• Soft vs. Hard Handoffs
• Hard handoff ? different radio channels assigned
  when moving from cell to cell
• All 1G analog (AMPS) 2G digital TDMA systems
  (USDC, GSM, IS-136, etc.)
• Spread spectrum users share the same channel in
  every cell
• CDMA ? IS?95, J?STD?8 (Verizon Wireless, Sprint
  PCS)
• MSC simultaneously monitors reverse link signal
  at several base stations
• Soft Handoff ? MSC chooses best signal and passes
  on to PSTN
• Advantage unique to CDMA systems

16
Cellular Interference

• Interfering Sources
• Another mobile in same cell (adjacent frequency
  channel)
• Base stations operating at same frequencies
  (co-channel)
• Other cellular carriers transmitting out-of-band
• Non-linear Tx products ? intermodulation
• More severe in urban areas with many service
  providers
• Other microwave, electronic, and wireless devices

Interference is the limiting factor in
performance of all cellular radio systems by
design!!
17
Cellular Interference

• Voice/Data Channel Interference
• Cross talk ? data interference
• Lower voice quality, lower data rates
• Dropped calls
• Control Channel Interference
• Missed, blocked, and dropped calls
• Reduced system capacity

18
Cellular Interference

• Most interference generated by same cellular
  network
• Cannot be completely controlled due to random
  propagation effects of radio signals
• Two major types of system-generated interference
  
• 1) Co-Channel Interference (CCI)
• 2) Adjacent Channel Interference (ACI)