3. Handovers or Handoffs
Why Handovers Required
For Seamless mobility / connectivity , to prevent call drop.
When mobile node move away from cell centre(tower/ BTS)
towards cell edge, signal level dropped. This can be
continuously transferred through Measurement Reports to
BTS.
One of the key elements of a mobile phone or cellular
telecommunications system, is that the system is split
into many small cells to provide good frequency re-use
and coverage.
However as the mobile moves out of one cell to another it
must be possible to retain the connection. Passing of
control from one cell to another cell . The process by
which this occurs is known as handover or handoff.
Its a critical process and if performed incorrectly handoverwww.rsmangrulkar.com GSM: Handover
4. Types of Handover
Hard Handover : Used in GSM
Break before Make
Soft Handover : Used in 3G
Make before Make
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5. Factors Affecting Handover
Transmitted power: as we know that the transmission
power is different for different cells, the handoff threshold
or the power margin varies from cell to cell.
Received power: the received power mostly depends on
the Line of Sight (LoS) path between the user and the
BS.
Area and shape of the cell: Apart from the power levels,
the cell structure also a plays an important role in the
handoff process.
Mobility of users: The number of mobile users entering or
going out of a particular cell, also fixes the handoff
strategy of a cell.
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7. Types of Handover
Intra-BTS handover This form of GSM handover occurs if it
is required to change the frequency or slot being used by a
mobile because of interference, or other reasons. In this form
of GSM handover, the mobile remains attached to the same
base station transceiver, but changes the channel or slot.
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9. Inter-BTS Intra BSC handover This for of GSM handover
or GSM handoff occurs when the mobile moves out of the
coverage area of one BTS but into another controlled by the
same BSC. In this instance the BSC is able to perform the
handover and it assigns a new channel and slot to the mobile,
before releasing the old BTS from communicating with the
mobile.
Inter-BSC handover When the mobile moves out of the
range of cells controlled by one BSC, a more involved form of
handover has to be performed, handing over not only from one
BTS to another but one BSC to another. For this the
handover is controlled by the MSC.
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10. Inter-MSC handover This form of handover occurs when
changing between networks. The two MSCs involved negotiate
to control the handover.
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11. Handoffs In Different Generations
In 1G analog cellular systems, the signal strength measurements were made by the BS and in turn
supervised by the MSC. The handoffs in this generation can be termed as Network Controlled Hand-Off
(NCHO). The approximate time needed to make a handoff successful was about 5-10 s.
In 2nd generation, which started using the digital technology, handoff decisions were mobile assisted and
therefore it is called Mobile Assisted Hand-Off (MAHO). In MAHO, the mobile center measures the power
changes received from nearby base stations and notifies the two BS. Delay in handoff was reduced to 1-5 s.
In the current 3G systems, the MS measures the power from adjacent BS and automatically upgrades the
channels to its nearer BS. Hence this can be termed as Mobile Controlled Hand-Off (MCHO).
Mobility of users: The number of mobile users entering or going out of a particular cell, also fixes the
handoff strategy of a cell.
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12. All these types of handoffs are usually termed as hard handoff
as there is a shift in the channels involved. There is also
another kind of handoff, called soft handoff
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13. Handoff in CDMA: In spread spectrum cellular systems, the
mobiles share the same channels in every cell. The MSC
evaluates the signal strengths received from different BS for a
single user and then shifts the user from one BS to the other
without actually changing the channel. These types of
handoffs are called as soft handoff as there is no change in the
channel.
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