Handover

A handover (also known as a handoff) is the process by which a call in
progress is transferred from a radio channel in one cell to another radio
channel, either in the same cell or in a different cell. A handover can occur
within a cell, between cells of the same BTS, between cells of different BTSs
connected to the same BSC, between cells of different BSCs, or between
cells of different MSCs. Not only can a handover occur between TCHs, a
handover is also possible from an SDCCH on one cell to an SDCCH on another cell. It is also possible from an SDCCH on one cell to a TCH on
another cell. The most common, however, is a handover from TCH to TCH.
Depending on the source (the original cell) and the target (the destination
cell) involved in the handover, the handover may be handled completely
within a BSS or may require the involvement of an MSC. In the case where
a handover occurs between cells of the same BSC, the BSC may execute the
handover and simply inform the MSC after the handover has taken place.
If, however, the handover occurs between BSCs, then the MSC must become
involved, because no direct interface exists between BSCs.

A handover in GSM is known as a mobile-assisted handover (MAHO).
This means that it is the network that decides if, when, and how a handover
should take place.The MS, however, provides information to the network to
enable the network to make the decision.

Recall that GSM is a TDMA system, with eight timeslots per frame in
the case of full-rate speech. This means that the MS is transmitting for oneeighth
of the time and receiving for one-eighth of the time. In fact, at the
BTS, a given timeslot on the uplink is three timeslots later than the corresponding
downlink timeslot, which means that the MS is not required to
receive and transmit simultaneously.We note that this offset is specified at
the BTS rather than at the MS because the distance of the MS from the
BTS influences the exact instant at which the MS should transmit. For
example, when an MS is close to the BTS it should transmit slightly later
than if it were further from the BTS. This variation is known as time-alignment
and is controlled by the BSS. In other words, the BSS periodically
instructs the MS to change its time alignment as necessary.

Nonetheless, it is clear that for most of the time the MS is neither transmitting
nor receiving.During this time, the MS has the opportunity to tune
to other carrier frequencies and determine how well it can receive those signals.
It can then relay that information to the network to allow the network
to make a determination as to whether the MS would be better served by a
different cell. Because of frequency reuse, it is possible that a number of
nearby cells might be using the same BCCH frequency. Therefore, it is not
sufficient for the MS to simply report signal strength for specific frequencies.
Rather, the MS must be able to synchronize to the BCCH of neighboring
cells and decode the information being transmitted. Exactly which
frequencies the MS should check for are specified in system information
messages transmitted by the BTS on the BCCH and the SACCH. The MS
sends measurement reports to the BSS on the SACCH as often as possible.
These reports include information on how well the MS can “hear” the serving cell as well as information about signal strength measurements on up
to six neighboring cells. Specifically, for the serving cell, the MS reports the
RXLEV (an indication of received signal strength) and the RXQUAL (an
indication of the bit error rate on the received signal). For neighboring cells,
the MS reports the BSIC, the BCCH frequency, and the RXLEV.
In addition to the measurements reported by the MS, the BTS itself
makes measurements regarding the RXLEV and RXQUAL received from
the MS. These measurements and those from the MS are reported to the
BSC. Based on its internal algorithms, the BSC makes the decision as to
whether a handover should occur, and if so, to what cell.

Figure 3-13 shows an inter-BSC handover. In this case, it is not sufficient
for the BSC to handle the handover autonomously—it must involve the
MSC. Therefore, once the serving BSC determines that a handover should
take place, it immediately sends the message Handover Required to the
MSC. This message contains information about the desired target cell (or
the cells in the preferred order), plus information about the current channel
that the MS is using. The MSC analyzes the information and identifies
the target BSC associated with at least one of the target cells identified by
the source BSC. It then sends a Handover Request message to the target
BSC. This contains, among other items, information about the target cell,
the type of channel required, and, in the case of a speech or data call, the circuit
to be used between the MSC and the target BSC.

If the target BSC can accommodate the handover (if resources are available),
then it allocates the necessary resources and responds to the MSC
with the Handover Request Acknowledge message. This message contains
a great deal of information regarding the cell and channel to which the MS
is to be transferred, such as the cell identity, the exact channel to be used
(including the type of channel), synchronization information, the power
level to be used by the MS when accessing the new channel, and a handover
reference. The MSC then sends the Handover Command message to the
serving BSC. This message is used to relay the information received from
the target BSC. On receipt of the Handover Command message from the
MSC, the serving BSC passes the information to the MS in a Handover
Command message over the air interface.

Upon receipt of the Handover Command message, the MS releases existing
RF connections, tunes to the target channel, and attempts to access that
channel. Upon access, it may send a Handover Access message to the target
BSS. It will do so if it was commanded to do so in the Handover Command
message. If the Handover Access message is received by the target BSS, then it sends a Handover Detect message to the MSC. When the MS has
established all lower layer connections on the target channel, it sends a
Handover Complete message to the target BSC, which, in turn, sends a
Handover Complete message to the MSC. At this point, the MS again starts
taking measurements of neighboring cells. Meanwhile, the MSC instructs
the old BSC to release all radio and terrestrial resources related to the MS. 82