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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


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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