Cellular Systems Cellular Concepts The cellular concept was a ...

EE5401 Cellular Mobile Communications
EE5401 Cellular Mobile Communications
Handover / Handoff
Occurs as a mobile moves into a different cell during
an existing call, or when going from one cellular
system into another.
• It must be user transparent, successful and not too
frequent.
• If the slope of the short-term average received signal
is steep, handover should carry out fast. Information
about vehicle speed is important.
Handover take place
f1
f2
P HO
• Not only involves identifying a new BS, but also
requires that the voice and control signals be allocated
to channels associated with the new BS.
Time taken to
complete handover
P min
Once a particular signal level P min is specified as the
minimum usable signal for acceptable voice quality
at the BS receiver, a slightly stronger signal level
P HO is used as a threshold at which a handover is
made.
P HO
= P min
+ ∆
• If ∆ is too large ⇒ unnecessary handovers
• If ∆ is too small ⇒ insufficient time to complete
handovers, call may drop due to poor received signal
quality.
• Running average should be used to avoid unwanted
handover due to momentary fading.
Dwell Time
• The time over which a user remains within one cell is
called the dwell time.
• The statistics of the dwell time are important for the
practical design of handover algorithms.
• The statistics of the dwell time vary greatly,
depending on the speed of the user and the type of
radio coverage.
Institute for Infocomm Research 49 National University of Singapore
Institute for Infocomm Research 50 National University of Singapore