Cellular Handover and handoff
- a summary or tutorial about the basics of cellular handover or handoff
occurs with mobile phone networks, including hard, soft and softer handover or
handoff.
The concept of a cellular phone system is that it has a large
number base stations covering a small area (cells), and as a result frequencies
are able to be re-used. Cell phone systems also provide mobility. As a result it
is a very basic requirement of the system that as the mobile handset moves out
of one cell to the next, it must be possible to hand the call over from the base
station of the first cell, to that of the next with no discernable disruption to
the call. There are two terms for this process: cellular handover is used within
Europe, whereas cellular handoff is the term used in North America.
The handover or handoff process is of major importance within
any cellular telecommunications network. It is necessary to ensure it can be
performed reliably and without disruption to any calls. Failure for it to
perform reliably can result in dropped calls, and this is one of the key factors
that can lead to customer dissatisfaction, which in turn may lead to them
changing to another cellular network provider. Accordingly handover or handoff
is one of the key performance indicators monitored so that a robust cellular
handover / handoff regime is maintained on the cellular network.
Handover basics
Although the concept of cellular handover or cellular handoff
is relatively straightforward, it is not an easy process to implement in
reality. The cellular network needs to decide when handover or handoff is
necessary, and to which cell. Also when the handover occurs it is necessary to
re-route the call to the relevant base station along with changing the
communication between the mobile and the base station to a new channel. All of
this needs to be undertaken without any noticeable interruption to the call. The
process is quite complicated, and in early systems calls were often lost if the
process did not work correctly.
Different cellular standards handle hand over / handoff in
slightly different ways. Therefore for the sake of an explanation the example of
the way that GSM handles handover is given.
There are a number of parameters that need to be known to
determine whether a handover is required. The signal strength of the base
station with which communication is being made, along with the signal strengths
of the surrounding stations. Additionally the availability of channels also
needs to be known. The mobile is obviously best suited to monitor the strength
of the base stations, but only the cellular network knows the status of channel
availability and the network makes the decision about when the handover is to
take place and to which channel of which cell.
Accordingly the mobile continually monitors the signal
strengths of the base stations it can hear, including the one it is currently
using, and it feeds this information back. When the strength of the signal from
the base station that the mobile is using starts to fall to a level where action
needs to be taken the cellular network looks at the reported strength of the
signals from other cells reported by the mobile. It then checks for channel
availability, and if one is available it informs this new cell to reserve a
channel for the incoming mobile. When ready, the current base station passes the
information for the new channel to the mobile, which then makes the change. Once
there the mobile sends a message on the new channel to inform the network it has
arrived. If this message is successfully sent and received then the network
shuts down communication with the mobile on the old channel, freeing it up for
other users, and all communication takes place on the new channel.
Under some circumstances such as when one base transceiver
station is nearing its capacity, the network may decide to hand some mobiles
over to another base transceiver station they are receiving that has more
capacity, and in this way reduce the load on the base transceiver station that
is nearly running to capacity. In this way access can be opened to the maximum
number of users. In fact channel usage and capacity are very important factors
in the design of a cellular network.
Types of handover / handoff
With the advent of CDMA systems where the same channels can
be used by several mobiles, and where it is possible to adjacent cells or cell
sectors to use the same frequency channel there are a number of different types
of handover that can be performed:
- Hard handover (hard handoff)
- Soft handover (soft handoff)
- Softer handover (softer handoff)
Although all of these forms of handover or handoff enable the
cellular phone to be connected to a different cell or different cell sector,
they are performed in slightly different ways and are available under different
conditions.
Hard handover
The definition of a hard handover or handoff is one where an
existing connection must be broken before the new one is established. One
example of hard handover is when frequencies are changed. As the mobile will
normally only be able to transmit on one frequency at a time, the connection
must be broken before it can move to the new channel where the connection is
re-established. This is often termed and inter-frequency hard handover. While
this is the most common form of hard handoff, it is not the only one. It is also
possible to have intra-frequency hard handovers where the frequency channel
remains the same.
Although there is generally a short break in transmission,
this is normally short enough not to be noticed by the user.
Soft hand over
The new 3G technologies use CDMA where it is possible to have
neighbouring cells on the same frequency and this opens the possibility of
having a form of handover or handoff where it is not necessary to break the
connection. This is called soft handover or soft handoff, and it is defined as a
handover where a new connection is established before the old one is released.
In UMTS most of the handovers that are performed are intra-frequency soft
handovers.
Softer handover
The third type of hand over is termed a softer handover, or
handoff. In this instance a new signal is either added to or deleted from the
active set of signals. It may also occur when a signal is replaced by a stronger
signal from a different sector under the same base station. This type of
handover or handoff is available within UMTS as well as CDMA2000.
Summary
Cellular handover or cellular handoff are performed by all
cellular telecommunications networks, and they are a core element of the whole
concept of cellular telecommunications. There are a number of requirements for
the process. The first is that it occurs reliably and if it does not, users soon
become dissatisfied and choose another network provider in a process known as
"churn". However it needs to be accomplished in the most efficient manner.
Although softer handoff is the most reliable, it also uses more network
capacity. The reason for this is that it is communicating with more than one
sector or base station at any given instance. Soft handover is also less
efficient than hard handover, but again more reliable as the connection is never
lost.
It is therefore necessary for the cellular telecommunications
network provider to arrange the network to operate in the most efficient manner,
while still providing the most reliable service.
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