Page 275 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 261

for the signal strength of the active cells and neighboring cells to the eNode‐B. Details
of measurements and reporting are discussed in Section 4.7. On the basis of this input,
the eNode‐B can take a decision if a handover of the connection to a neighboring cell
with a better signal is necessary. Apart from ensuring that the connection does not fail,
a handover usually also improves the data throughput of the mobile device in the uplink
direction as well as in the downlink direction. At the same time, it also reduces the
amount of power required for uplink transmissions and hence decreases the overall
interference.
In LTE, there are two types of handover. The most efficient one is a handover where
the source eNode‐B and the target eNode‐B directly communicate with each other over
the X2 interface. This handover is referred to as an X2 handover. If for some reason the
two eNode‐Bs cannot communicate with each other, for example, because they have
not been configured for direct communication, the handover signaling will take place
over the S1 interface and the MME assists in the process. Such a handover is referred to
as an S1 handover.

X2 Handover
On the basis of the measurement reports from the mobile device on the reception level
of the current cell and the neighboring cells, the eNode‐B can take the decision to hand
over the ongoing connection to another eNode‐B. As shown in Figure 4.20, the first step
in this process is a Handover Request message from the source eNode‐B to the target
eNode‐B which contains all relevant information about the subscriber and all relevant
information about the connection to the mobile device, as described in 3GPP TS 36.423
[19]. The target eNode‐B then checks if it still has the resources required to handle the
additional subscriber. Particularly if the connection of the subscriber requires a certain
QoS, the target eNode‐B might not have enough capacity on the air interface left during
a congestion situation and might thus reject the request. In a well‐dimensioned net-
work, however, this should rarely, if at all, be the case. Also, it should be noted at this
point that in practice, no specific QoS requirements are used except for network opera-
tor‐based voice calls.
If the target eNode‐B grants access, it prepares itself by selecting a new C‐RNTI for the
mobile device and reserves resources on the uplink so that the mobile device can perform
a non‐contention‐based random access procedure once it tries to access the new cell. This
is necessary as the mobile device is not synchronized, that is, it is not yet aware of the
timing advance necessary to communicate with the new cell. Afterward, the target eNode‐
B confirms the request to the source eNode‐B with a Handover Request Acknowledge
message. The message contains all the information that the mobile device requires to
access the new cell. As the handover needs to be executed as fast as possible, the mobile
device should not be required to read the System Information messages in the target cell.
Hence, the confirmation message contains all the system parameters that the mobile
device needs to configure itself to communicate with the target cell. As was described in
more detail earlier in this chapter, the information required includes the PCI, the carrier
bandwidth, RACH parameters, the uplink shared channel configuration, reference signal
configuration, PHICH configuration, SRS parameters, and so on.
Once the source eNode‐B receives the confirmation, it immediately issues a handover
command to the mobile device and ceases to transmit user data in the downlink