A Robust Cell Search Algorithm for 3GPP LTE
Bengt Lindoff
1
, Tobias Ryd
´
en
2
, David Astely
3
1
Ericsson Research, SE-223 70, Lund, Sweden
3
Ericsson Research, SE-164 80, Stockholm, Sweden
2
Department of Mathematical Sciences, Lund University, SE-221 00 Lund, Sweden
Email: {bengt.lindoff, david.astely}@ericsson.com
tobias.ryden@matstat.lu.se
Abstract—Mobility handling and performance is of major
importance for the 3GPP Long Term Evolution (LTE) system.
Therefore it is important for mobile devices to use reliable and
robust algorithms for the cell search procedure, i.e. the procedure
to detect neighbouring cells that can be used as potential
handover candidates. We present a novel, low complexity, robust
cell search scheme for LTE. The scheme uses a non-coherent
approach and is therefore robust against Doppler spread and
phase shifts due to sampling time instant mismatch, which makes
it suitable for use in synchronized LTE TDD systems, as well as in
high speed scenarios like high speed trains. This paper describes
the theory behind the algorithm, and simulation results for LTE
TDD showing its superior performance over coherent cell search
algorithms are also presented.
I. INTRODUCTION
The 3GPP Long Term Evolution (LTE) system, aiming at
commercial deployment around 2010, will support data rates
well beyond the current HSPA standard. The OFDM-based
LTE system includes support for peak rates above 100 Mb/s in
Downlink and 50 Mb/s in Uplink, low latency (10 ms round-
trip delay), improved system capacity and coverage as well
as reduced operating and maintenance costs. Furthermore the
system supports both FDD and TDD, flexible bandwidths from
1.4 to 20 MHz, MIMO as well as mobility and seamless
handover to other cellular systems. For a comprehensive
introduction to LTE and its standardization we refer to [2]
and [1] respectively.
The mobility performance of a cellular system consists of
mainly three parts; (1) Cell Search, (2) Cell Measurements,
and (3) Handover procedures. The handover procedures and
cell measurements are mainly set by the standard, and hence
mobile terminal implementations only have a small impact
on their performance. Therefore, the most fundamental and
important part of the mobility is the cell search. Just as in
WCDMA, specific synchronization signals are provided in
LTE in order to help mobile devices to detect and synchronize
to the base stations (“enode Bs”) with respect to time and
frequency.
In this paper we describe a low complexity, robust cell
search scheme for LTE. The scheme uses a non-coherent
approach and is therefore robust against Doppler spread and
phase shifts due to sampling time instant mismatch, which
makes it suitable for use in synchronized LTE TDD systems,
as well as in high speed scenarios like high speed trains.
Section 2 gives an overview of the LTE synchronization signal
generation and principles, while Section 3 provides the theory
behind the non-coherent cell search algorithm. In Section 4
this algorithm as well as a coherent one used for benchmarking
are described in detail, and the simulation results can be found
in Section 5. Finally, Section 6 concludes.
II. OVERVIEW OF SYNCHRONIZATION SIGNALS IN LTE
This section gives a brief overview of the synchronization
signals used in LTE. For a more in-depth description we refer
to [2] as well as to the 3GPP Evolved UTRAN standardization
documents [1]. The cell search is based on a hierarchical
scheme, with a primary and secondary synchronization signal
(PSS and SSS, respectively) giving a first and second set of
cell information. PSS and SSS are transmitted every 5 ms
(Figure 1). The positions of PSS and SSS within a frame are
different depending on the duplex mode, i.e. whether FDD or
TDD is employed. For FDD, PSS and SSS are transmitted in
the last and second last OFDM symbol respectively in slots 0
and 10 of each 10 ms frame, while for TDD, SSS is transmitted
in the last OFDM symbol of slots 0 and 10, while PSS is
transmitted in the third OFDM symbol of the adjacent DwPTS.
Fig. 1. Synchronization signal positions for LTE FDD and TDD. FDD (top):
PSS and SSS are transmitted in the last and second last OFDM symbols
respectively of slots 0 and 10. TDD (bottom): SSS is transmitted in the last
OFDM symbol of slots 0 and 10, while PSS is transmitted in the third symbol
of the adjacent DwPTS.
The PSS is a one of three different length 63 Zadhoff-Chu
sequences (indices 25, 29 and 34), each of which determine a
cell identity within a cell group (that is determined by the
SSS). The PSS is generated in the frequency domain and
transmitted in the six centre resource blocks of the OFDM
symbols specified above (Figure 2).
European Wireless 2009
303
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