Long Term Evolution (LTE) is the result of the standardization work done by the 3rd Generation Partnership Project (3GPP) to achieve a new high speed radio access in the mobile communications frame. Cell selection by a mobile UE is another issue in LTE. In particularly, an interesting challenge in the physical layer of LTE is how the mobile unit immediately after powering on, select a radio cell and locks on to it. More specifically, to understand how the mobile unit establishes the connection with the strongest cell station in surrounding region. To do this, the mobile unit has to overcome the challenges of estimating the channel to communicate with the cell site and frequency synchronization. To appropriately synchronize the mobile unit with the base station when multiple mobile unit are communicating with same receiver from various distances.

1. International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) ISSN: 2319-4413
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A Novel Approach for Cell Selection and Synchronization in
LTE-Advanced
Sunita Kumari, M.E. (ECE) Fellowship, Chitkara University, Chandigarh
T. L. Singal, Professor & Guide, ECE Department, Chitkara University, Chandigarh
ABSTRACT
Long Term Evolution (LTE) is the result of the
standardization work done by the 3rd Generation
Partnership Project (3GPP) to achieve a new high speed
radio access in the mobile communications frame. Cell
selection by a mobile UE is another issue in LTE. In
particularly, an interesting challenge in the physical layer
of LTE is how the mobile unit immediately after powering
on, select a radio cell and locks on to it. More specifically,
to understand how the mobile unit establishes the
connection with the strongest cell station in surrounding
region. To do this, the mobile unit has to overcome the
challenges of estimating the channel to communicate with
the cell site and frequency synchronization. To
appropriately synchronize the mobile unit with the base
station when multiple mobile unit are communicating with
same receiver from various distances.
Keywords
LTE, LTE-A, Cell Selection, Synchronization
1. INTRODUCTION
Initially, Long Term Evolution and abbreviated as LTE
was introduced in the Release 8 in 2008. 3GPP published
and introduced the various standards for IP based system
LTE. In 2010, the Release 9 was introduced to provide
enhancements to LTE and in 2011 Release 10 was brought
as LTE-Advanced. The objective was to expand the limits
and features of Release 8 and to meet the requirements of
the International Mobile Telecommunications-Advanced
(IMT-Advanced) of ITU-R for the fourth generation
mobile technologies (4G), and the future operator and end
user’s requirements. The key reason of the evident of the
LTE-A is the growing demand for network services, such
as web browsing, VoIP, video telephony, and video
streaming, with constraints on delays and bandwidth
requirements, poses new challenges in the design of the
future generation cellular networks. [1]. The overall goal
of LTE technology is to significantly increasing peak data
rates scaled linearly according to spectrum allocation,
improving spectral efficiency, lowering costs, improving
services making use of new spectrum opportunities,
improved quality of service, and better integration with
other open standards.
2. NEW TECHNIQUES USED IN LTE
i) Multicarrier Technology: The access scheme is different
for the uplink and downlink in LTE. OFDMA (Orthogonal
Frequency Division Multiple Access) is used in the
downlink; whereas, SC-FDMA (Single Carrier -
Frequency Division Multiple Access) is used in the uplink.
Refer Figure 1. OFDM technology has been incorporated
into LTE because it enables high data bandwidths to be
transmitted efficiently while still providing a high degree
of resilience to reflections and interference.
Fig. 1: OFDMA and SC-FDMA in LTE
The Peak-to-Average Power Ratio (PAPR) of an OFDM
signal is relatively high, causes difficult to tolerate for the
transmitter of the mobile terminal. SC-FDMA has a
significantly lower PAPR, the more constant power
enables high RF power amplifier efficiency in the mobile
handsets. [2].
ii) Multiple Input Multiple Output (MIMO): MIMO is used
to improve the data bit rates and spectral efficiency. It
consists of using multiple antennas in both the receiver
and transmitter in order to use the multipath effects, which
decreases the interference and leads to high transmission
rates. See Figure 2. One of the main problems that
previous telecommunications systems have encountered is
that of multiple signals arising from the many reflections
that they encountered. By using MIMO these additional
signal paths can be used to advantage and to increase the
throughput. [3].

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Fig. 2: MIMO in LTE
iii) System Architecture Evolution (SAE): As there is need
for high data rate and low latency for 3G LTE, it is
necessary to evolve the system architecture so that
improved performance can be achieved. The new SAE
network is based upon the GSM/WCDMA core networks
to enable simplified operations and easy deployment, as
shown in Figure 3.
Fig. 3: LTE Architecture System
3. SALIENT FEATURES OF LTE-A
The features of LTE are extended in LTE- Advanced in
order to exceed or at least meet the IMT-Advanced
requirements. It also must fulfil operator’s demands like a
reduced cost per Mbit transmitted, better service providing
in terms of homogeneity, constant quality of the
connection, smaller latency and compatibility with all
3GPP previous systems.
a) Support of asymmetrical bandwidths and larger
bandwidth: In LTE (release 8), the bandwidth could have
different sizes but had to be the same in the downlink and
in the uplink. In LTE-Advanced (Release 10) bandwidths
can be different because due to actual demand in mobile
networks, the load from the station to the user is more than
from the user to the station.
b) Enhanced multi-antenna transmission techniques: LTE
introduced MIMO in the data transmission. However in
LTE-Advanced, the MIMO scheme has to be extended to
gain spectrum efficiency, cell edge performance and
average data rates. LTE-Advanced uses a configuration
8x8 in the downlink and 4x4 in the uplink. LTE-Advanced
tries to get the network closer to the user to provide a
uniform user experience and to increase the capacity of the
network by using advanced topology networks. Advanced
topology networks provide the benefits and the
performance increase. Some of the characteristics of this
type of networks are:
• Self-organizing networks
• Intelligent Node Association
• Support for relays
• Adaptive Resource Allocation
• Multicarrier (spectrum aggregation)
c) Coordinated multipoint transmission and reception
(CoMP): It is used to improve the received signal of the
user terminal. Both the serving and neighbour cells are
used in a way that the co-channel interference from
neighbouring cells is reduced. It implies dynamic
coordination between geographically separated
transmission points in the downlink and reception at
separated points in the uplink. This mechanism is used to
improve the coverage of high data rates and to increase the
system bit rate.
d) Relaying: Relaying increases the coverage area and
capacity of the network. User’s mobile devices
communicate with the relay node, which communicates
with a donor eNB (enhanced Node B). See Figure 4.
Fig. 4: Relay types in LTE systems
Relay nodes can also support higher layer functionality
like decoding user data from the donor eNB and re-
encoding the data before transmitting it to the user
terminal. Type 1 relay nodes control their cells with their
own cell identity, and are used for the purpose of the
transmission of synchronization channels and reference
symbols. Type 2 relay nodes don’t own an identity, so the

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mobile user won’t be able to distinguish if a transmission
comes from the donor eNB or from the relay.
4. A COMPARISON: LTE vs. LTE-A
LTE and LTE Advanced are high speed 4G wireless
technologies. These 4G technologies give the real
experience of the triple play services such as voice, video
and high speed data. Both LTE and LTE Advanced offer
high speed access to internet equivalent to FE connection.
The key differences between LTE and LTE-Advanced are
mentioned below and in Table 1.
Table 1: Specifications of LTE and LTE-Advanced
Parameter LTE LTE-A
Peak Data Rate
Down Link
150 Mbps 1 Gbps
Peak Data Rate
Up Link
75 Mbps 500 Mbps
Transmission
Band (DL)
20 MHz 100 MHz
Transmission
Band (UL)
20 MHz 40 MHz
Scalable
Bandwidths
1.3,3,5,10, and
20MHz
Up to 20-
100 MHz
Capacity 200 active
users per cell
in 5MHz
3 times
higher than
LTE
5. CHALLLENGES IN IMPLEMENTING
LTE-A
i) Uplink scheduler limitations: OFDMA is used in
downlink. Without ordering constraints, the scheduler can
fill out the RB allocation. Due to the use of SC-FDMA,
the uplink allows the UEs to transmit only in a single
carrier mode. Thus, the scheduler for the uplink has
limited degrees of freedom. It has to allocate contiguous
RBs to each user without any choice among the best one
available. [4].
ii) Spectral efficiency: In LTE, one of the main goal is to
achieve effective utilization of radio resource. Hence,
several types of performance indicators need to be
calculated. For example, a specific policy could be
planned to maximize the number of users served in a given
time interval, or the spectral efficiency, by always serving
users that are experiencing the best channel conditions.
User throughput can be used as efficiency indicator which
is defined as the actual transmission data rate without
including layer two overheads and packet retransmissions
due to physical errors.
iii) Fairness: If overall cell throughput increases to
greatest extent, positively it enables effective utilization in
terms of spectral efficiency, but also leads to very unfair
resource sharing among users. Therefore fairness is a key
requirement that should be taken into account to guarantee
minimum performance to the cell-edge users and the users
who experiencing bad channel conditions.
iv) QoS Provisioning: It is a major feature in all-IP
architectures and very important in next generation
mobile. LTE maps QoS constrained flows to dedicated
radio bearers that depending on their QCIs enable special
RRM procedures QoS constraints may vary depending on
the application and they are usually mapped into some
parameters: maximum delivering delay, minimum
guaranteed bitrate, and packet loss rate. Hence, it is
important to define QoS-aware schedulers.
6. THE CELL SEARCH PROCEDURE
In LTE, Similar to all mobile communication systems,
terminal must perform certain steps before it can receive
or transmit data. [5]. These steps can be categorized in cell
search and cell selection, derivation of system information,
and random access. Refer Figure 5.
Fig. 5: LTE Initial Access
A se A user equipment willing to access an LTE cell
must first undertake a cell search procedure. Cell search
procedure is a group of procedures which consists of a
series of synchronization stages through which the UE
determines time and frequency synchronization parameters
that are necessary to demodulate the downlink data and to
transmit in uplink slot with the correct timing so as the
signal maintains orthogonality with other users.
7. CELL SYNCHRONIZATION PROCESS
Matching up with time and frequency parameters of the
reference or source is called synchronization. In case of
LTE networks, eNodeB is the source which controls
access to the UE. Thus, UE should adjust its frequency
and time according to the eNodeB. This is done with the
help of special Zadoff Chu (ZC) sequences. [6]. eNodeB
transmits these sequences periodically so that all UEs can

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Beginning of the
cell search
PSS Detection
Physical
Layer Cell ID
Frequency
Synchronization
Slot
Boundaries
SSS Detection
Frame
Timing
Group
Cell
End of
Cell Search
synchronize to the reference accordingly. There are three
synchronization requirements in LTE:
- symbol timing acquisition by which the correct
symbol start is determined;
- carrier frequency synchronization which mitigates
the effect of frequency errors resulting from
Doppler shift, and errors from electronics circuitry;
and
- sampling clock synchronization.
This is achieved by two types of sequences called as
primary synchronization sequences (PSS) and secondary
synchronization sequences (SSS). Refer Figure 6.
Fig. 6: Cell search and Synchronization Algorithm
PSS detection is the first step in cell search. To start
synchronization, UE should understand the time clock and
frequency on which eNodeB is working. [7]. For this UE
after powering up performs the downlink synchronization
which is detection of PSS and SSS and acquiring time,
frequency and system configuration information from
broadcast channel.
a) Uplink Synchronization: After downlink
synchronization, the UE has synchronization with eNodeB
clock and frequency. In addition, to obtain the uplink
timing advance information from eNodeB, UE performs
transmits one of the ranging code supported in the system
and waits for the ranging reply in the downlink control
channel. Once the ranging is successful, UE receives the
message and new dedicated ranging code with temporary
ID in the downlink control channel. If the ranging
successful message is not received, then UE tries again till
it succeeds.
b) New cell identification or initial synchronization: If the
UE is already registered with one eNodeB and moving out
of coverage area then new cell identification procedure is
carried out where the connected eNodeB assists the UE in
ranging and registration to the new cell. UE is assigned
with a dedicated ranging sequence and then ranging is
performed with the new cell. Whereas in case of initial
synchronization, the UE performs ranging procedure on
the contention basis.
PSS and SSS synchronization signals are used in cell
search process where slot start time, frequency offset and
physical layer ID is achieved after detecting PSS. The
detection of SSS gives radio frame timing, cell ID, cyclic
prefix length and TDD/FDD frame system configuration.
PSS sequences and SSS sequences are transmitted twice
per 10 ms radio frame.
The PSS is located in the last OFDM symbol of the first
and 11th slot of each radio frame which allows the UE to
acquire the slot boundary timing which is independent of
the type of cyclic prefix length. The PSS signal is the same
for any given cell in every sub frame in which it is
transmitted. The location of the SSS immediately precedes
the PSS – in the before to last symbol of the first and 11th
slot of each radio frame. The UE would be able to
determine the position of the 10 ms frame boundary as the
SSS signal alternates in a specific manner between two
transmissions.
8. CONCLUSION
To understand how the mobile unit establishes the
connection with the strongest cell station in surrounding
region. To do this, the mobile unit has to overcome the
challenges of estimating the channel to communicate with
the cell site and frequency synchronization to

5. International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) ISSN: 2319-4413
Volume 3, No. 5, May 2014
i-Explore International Research Journal Consortium www.irjcjournals.org
21
appropriately synchronize the mobile unit with the base
station when multiple mobile unit are communicating with
same receiver from various distances. To identify the
available cell site so that the mobile unit will connect
successfully. For this, it uses two signals, the Primary
Synchronization Signal and the Secondary
Synchronization Signal sequentially. The primary signal is
based on Zadoff-Chu sequence which is a Constant
Amplitude Zero Auto Correlation (CAZAC) sequence.
This Research simulates the mobile cell search procedure
and the challenges associated with it can be analyzed using
MATLAB tool.
REFERENCES
[1] Brian Katumba, Johannes Lindgren and Kateryna
Mariushkina “The LTE Access Procedure”, IEEE
wireless communications and networks, vol. 7, no.
3, Jan 2011.
[2] D. Supratim, P. Monogioudis, J. Miernik And P.
Seymour “Algorithms for enhanced inter cell
interference coordination in LTE Hetnets”, IEEE
wireless communications and networks, vol. 16, no.
3, Jan 2013.
[3] S. Fulani and Q. Liang, “Physical layer test trials and
analysis of call drops and real time throughput
versus channel capacity of the long term evolution
(4G) technology”, International Journal of
electronics Engineering, vol. 9, no. 3, pp.120-130,
2011.
[4] G. Berardinelli, “Air interface for next generation
mobile communication networks: physical layer
design: a LTE-A uplink”, Department of Electronic
Systems, pp. 56-60, 2010.
[5] K. Davaslioglu and E. Ayanoglu, “Interference-
based cell selection in heterogenous networks”,
Department of Electrical Engineering and
Computer Science, IEEE wireless communications,
vol. 11, no. 4, pp.13-25, Feb. 2013.
[6] K. Son, S. Chong and G. Veciana, “Dynamic
association for load balancing and Interference
avoidance in multi-cell networks”, IEEE
Transactions on wireless communications, vol. 8,
no. 7, pp. 3566- 3576, July 2009.
[7] A.Zakrzewska, Sarah Ruepp and S. Michael, “Cell
selection using recursive bipartite matching”, IEEE
International Conference on Computer
Communications (INFOCOM 2013), vol. 13, no. 6,
pp.656-666, 2013.