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A GUTI is an LTE/EPS network‐level identity that has end‐to‐end significance and is used to provide unambiguous identification of a UE. The construction, as well as its hierarchical structure of a GUTI, is shown in Figure 5.3.

Figure 5.3 Illustration: components and derivation of a GUTI.

Using a GUTI, the identification of the MME and its corresponding network can be determined. It can be used by the network and the UE to establish the UE's identity during signaling between them in the LTE/EPS network.

 Physical Layer Identity of a Cell

At the physical layer level, each cell in an LTE/E‐UTRAN network is identified through a Physical Cell Identity (PCI), which is illustrated through Example 5.3 and Figure 5.4.

As defined by the TS 36.211 [90], there are 504 unique Physical Layer Cell Identities, and they are divided into 168 (0–167) unique Physical Layer Cell Identity groups; each group has three (0, 1, 2) unique identities. A PCI has the relationships and is derived from the downlink reference signals primary synchronization signal (PSS) and secondary synchronization signal (SSS) which are transmitted from a base station. The mathematical relationship among the PCI, PSS and SSS is shown below.

PCI= (3* Physical Cell Identity Group) + Physical Layer Identity Where Physical Cell Identity Group =0 to 167; Physical Layer Identity=0 to 2.

The PSS has the link to the Physical Layer Identity, which is a Zadoff–Chu sequence, and the SSS has the link to the PCI group. A UE reads the synchronization signals during a cell search procedure in the LTE system. To avoid interference and PCI collision as well as confusion issues, the same PCI is never reused in the neighbouring cells. A PCI collision occurs when two adjacent cells have the same PCI. A PCI confusion occurs when a cell has two neighbours with the same PCI. As the number of PCIs (504) is limited, PCIs are carefully planned and repeated in other cells that are not adjacent or neighbor to each other as shown in Figure 5.4.