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
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
Illustration of
Handoff
at Cell
Boundary
PHT
PMUS
PHT
PMUS
ECE 4730: Lecture #4
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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 

Fig. 3.3, pg. 63
Note
 P 

dBm  10 log 
 1 mW 
  = PHT  PMUS
 Carefully selected
  too large  unnecessary handoff  MSC loaded down
  too small  not enough time to transfer  call dropped!
ECE 4730: Lecture #4
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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
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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
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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
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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
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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
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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
ECE 4730: Lecture #4
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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
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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
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Umbrella Cell
ECE 4730: Lecture #4
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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
ECE 4730: Lecture #4
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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
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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
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Cellular Interference
*** Interference is the limiting factor in
performance of all cellular radio systems ***

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
ECE 4730: Lecture #4
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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
ECE 4730: Lecture #4
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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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