LTE cell search and cell selection :
1) LTE Initial Access:
Like all mobile communication systems, in LTE a terminal must perform certain steps
before it can receive or transmit data. These steps can be categorized in cell search
and cell selection, derivation of system information, and random access. The complete
procedure is known as LTE Initial Access and is shown in Figure 1. After the initial
access procedure, the terminal is able to receive and transmit its user data.
Figure 1: LTE Initial Access: cell search and cell selection
2) Initial synchronization :
Successful execution of the cell search and selection procedure as well as acquiring
initial system information is essential for the UE before taking further steps to
communicate with the network. For this reason, it is important to take a closer look at
this fundamental physical layer procedure. This section focuses on the cell-search
scheme defined for LTE and the next chapter describes reception of the essential
system information.
As in 3G (WCDMA), LTE uses a hierarchical cell-search procedure in which an LTE
radio cell is identified by a cell identity, which is comparable to the scrambling code
that is used to separate base stations and cells in WCDMA. To avoid the need for
expensive and complicated network and cell planning, 504 physical layer cell identities
of is sufficiently large. With a hierarchical cell search scheme, these identities are
divided into 168 unique cell layer identity groups in the physical layer, in which each
group consists of three physical layer identities1. To remember this hierarchic principle, consider the example of first names and surnames. According to statistics ,
the most common English surname is “Smith”, which corresponds to physical layer cell
identity group 0. The second most common surname is “Johnson”, which represents
the physical layer cell identity group 1. This example can be extended to the last group,
which would be “Rose”. The most common male first names are “James”, “John”, or
“Robert” and female names are “Mary”, “Patricia”, and “Linda”. Each first name
represents one of the three physical layer identities.
This information is now transmitted using two different signals, generated by Layer 1.
The two signals, carrying the physical layer identity and the physical layer cell identity
group, are the primary and the secondary synchronization signals respectively. This
means that the complete cell search procedure consists of two steps to identify the
cells’ identity. The process is shown graphically in Figure 2.
Step I – Primary Synchronization Signal
The UE first looks for the primary synchronization signal (PSS) which is transmitted in
the last OFDM symbol of the first time slot of the first subframe (subframe 0) in a radio
frame. This enables the UE to acquire the slot boundary independently from the
chosen cyclic prefix selected for this cell. Based on the downlink frame structure (Type
1, FDD), which is shown in Figure 6, the primary synchronization signal is transmitted
twice per radio frame, so it is repeated in subframe 5 (in time slot 11). This enables the
UE to get time synchronized on a 5 ms basis, which was selected to simplify the
required inter-frequency and inter-RAT measurements. LTE must accommodate
handover to and from other radio access technologies, such as GSM/GPRS/EDGE,
WCDMA/HSPA or CDMA®2000 1xRTT/1xEV-DO.
In the frequency domain, six resource blocks (RB) around the DC subcarrier are
reserved for transmission of the synchronization signals. In the frequency domain, an
RB is formed by 12 subcarriers. With a subcarrier spacing of 15 kHz a bandwidth of
180 kHz (12*15 kHz) is occupied, reserving a frequency range of 1.08 MHz (6*180
kHz) around the center frequency for transmission of synchronization signals (that is,
72 subcarriers). This is independent from the defined channel bandwidth that is
1) LTE Initial Access:
Like all mobile communication systems, in LTE a terminal must perform certain steps
before it can receive or transmit data. These steps can be categorized in cell search
and cell selection, derivation of system information, and random access. The complete
procedure is known as LTE Initial Access and is shown in Figure 1. After the initial
access procedure, the terminal is able to receive and transmit its user data.
Figure 1: LTE Initial Access: cell search and cell selection
2) Initial synchronization :
Successful execution of the cell search and selection procedure as well as acquiring
initial system information is essential for the UE before taking further steps to
communicate with the network. For this reason, it is important to take a closer look at
this fundamental physical layer procedure. This section focuses on the cell-search
scheme defined for LTE and the next chapter describes reception of the essential
system information.
As in 3G (WCDMA), LTE uses a hierarchical cell-search procedure in which an LTE
radio cell is identified by a cell identity, which is comparable to the scrambling code
that is used to separate base stations and cells in WCDMA. To avoid the need for
expensive and complicated network and cell planning, 504 physical layer cell identities
of is sufficiently large. With a hierarchical cell search scheme, these identities are
divided into 168 unique cell layer identity groups in the physical layer, in which each
group consists of three physical layer identities1. To remember this hierarchic principle, consider the example of first names and surnames. According to statistics ,
the most common English surname is “Smith”, which corresponds to physical layer cell
identity group 0. The second most common surname is “Johnson”, which represents
the physical layer cell identity group 1. This example can be extended to the last group,
which would be “Rose”. The most common male first names are “James”, “John”, or
“Robert” and female names are “Mary”, “Patricia”, and “Linda”. Each first name
represents one of the three physical layer identities.
This information is now transmitted using two different signals, generated by Layer 1.
The two signals, carrying the physical layer identity and the physical layer cell identity
group, are the primary and the secondary synchronization signals respectively. This
means that the complete cell search procedure consists of two steps to identify the
cells’ identity. The process is shown graphically in Figure 2.
Step I – Primary Synchronization Signal The UE first looks for the primary synchronization signal (PSS) which is transmitted in
the last OFDM symbol of the first time slot of the first subframe (subframe 0) in a radio
frame. This enables the UE to acquire the slot boundary independently from the
chosen cyclic prefix selected for this cell. Based on the downlink frame structure (Type
1, FDD), which is shown in Figure 6, the primary synchronization signal is transmitted
twice per radio frame, so it is repeated in subframe 5 (in time slot 11). This enables the
UE to get time synchronized on a 5 ms basis, which was selected to simplify the
required inter-frequency and inter-RAT measurements. LTE must accommodate
handover to and from other radio access technologies, such as GSM/GPRS/EDGE,
WCDMA/HSPA or CDMA®2000 1xRTT/1xEV-DO.
In the frequency domain, six resource blocks (RB) around the DC subcarrier are
reserved for transmission of the synchronization signals. In the frequency domain, an
RB is formed by 12 subcarriers. With a subcarrier spacing of 15 kHz a bandwidth of
180 kHz (12*15 kHz) is occupied, reserving a frequency range of 1.08 MHz (6*180
kHz) around the center frequency for transmission of synchronization signals (that is,
72 subcarriers). This is independent from the defined channel bandwidth that is
