Page 302 - From GMS to LTE

288 From GSM to LTE-Advanced Pro and 5G

sometimes be fixed only with additional base stations. In such a case, the equipment
could at least notify the network operator.
Energy saving. Reduction of transmission power in case it is not needed – automatic
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shutdown and reinitialization of femtocells when the user arrives in or leaves the
coverage area of a femtocell.
PCI configuration. As described above, every LTE cell has a PCI that is used during
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the cell search procedure to distinguish the transmissions of several cells on the same
carrier from each other. Only 504 IDs are available and neighboring base stations
should use a certain combination for easier detection. As it is sometimes difficult to
predict all cell neighbors, an auto‐configuration functionality is highly desirable.
Again, the mobile is required to report to the network as to which cells it looks out for
in the automated configuration process.
Handover optimization. By analyzing the causes of failure of handovers, coverage
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holes or wrong handover decisions can be detected and changed.
Load balancing. If a cell already experiences high load from many users, users at the
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cell edge can be redirected to other nearby cells.
RACH optimization. The RACH is needed for the initial communication between
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non‐synchronized mobile devices and the network. Depending on the load, the number
of resources dedicated to the RACH can be changed dynamically.
Further details and references to specifications on some of these topics can be found
in Sauter [25].

4.13 CS‐Fallback for Voice and SMS Services with LTE

One of the major design choices of LTE was to focus on the development of a packet‐based
core and access network infrastructure. The circuit‐switched core network and dedi-
cated telephony features of GSM and UMTS radio access networks have not been
adapted for LTE. This significantly reduces the overall complexity of the network and
follows the direction that was taken in fixed‐line networks many years earlier. Here, a
clear trend toward IP and voice services over IP is well underway. At the homes of cus-
tomers or in offices, multifunctional gateways that include a DSL modem, a Wi‐Fi
access point, fixed‐line Ethernet ports and also RJ‐11 ports to connect ordinary tele-
phones are now common. Inside the device, Session Initiation Protocol (SIP)‐based IP
telephony data streams and signaling are converted into the classic analog or ISDN
format and the user can thus continue to use their legacy devices.
With LTE, reuse of legacy equipment is not possible, and hence, other ways have to be
found to offer voice services. Another major complication that is not found in fixed‐line
networks is the necessity for voice and other previously circuit‐switched services such
as SMS to be backward compatible to the services offered in fixed‐line networks. For a
user, it should be invisible whether the service is offered over the circuit‐switched part
of the GSM or UMTS network or the packet‐switched IP‐based LTE network. Also, an
ongoing voice call over LTE should be seamlessly handed over to GSM or UMTS if the
user leaves the LTE coverage area. In other words, the IP‐based voice call must be con-
verted to a circuit‐switched voice call on the fly as otherwise the overall user experience
will be unsatisfactory. The system designed for LTE to tackle these challenges is referred