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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

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mobile unit handoff
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

ECE 4730: Lecture #4

mobile unit handoff1
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) > 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

ECE 4730: Lecture #4

slide3

PHT

PMUS

PHT

PMUS

Illustration of

Handoff

at Cell Boundary

ECE 4730: Lecture #4

mobile unit handoff2
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 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

ECE 4730: Lecture #4

mobile unit handoff3
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

ECE 4730: Lecture #4

mobile unit handoff4
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 > 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

ECE 4730: Lecture #4

mobile unit handoff5
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 > PHT  handoff not initiated but adjacent cell has better signal!!

ECE 4730: Lecture #4

mobile unit handoff6
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

ECE 4730: Lecture #4

mobile unit handoff7
Mobile Unit Handoff
  • Prioritizing Handoffs (continued)
    • Queuing Handoff Requests
      • Use time delay between handoff threshold and MUS level to place handoff request in 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

ECE 4730: Lecture #4

mobile unit handoff8
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

ECE 4730: Lecture #4

mobile unit handoff9
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

ECE 4730: Lecture #4

umbrella cell
Umbrella Cell

ECE 4730: Lecture #4

handoff parameters
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

ECE 4730: Lecture #4

handoff parameters1
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

ECE 4730: Lecture #4

mobile unit handoff10
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

ECE 4730: Lecture #4

cellular interference
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 ***

ECE 4730: Lecture #4

cellular interference1
Cellular Interference
  • Voice Channel Interference
    • Cross “talk”  data interference
    • Lower voice quality
    • Dropped calls
  • Control Channel Interference
    • Missed, blocked, and dropped calls
    • Reduced system capacity

ECE 4730: Lecture #4

cellular interference2
Cellular Interference
  • Most interference generated by same system
    • 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)

ECE 4730: Lecture #4

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