Low complexity cell search scheme for LTE and LTE-advanced mobile technologies☆
Graphical abstract
Highlights
► We propose a novel method to accomplish cell search procedure in 3GPP-LTE. ► Initial CP detection allows for efficient detection of synchronization signals. ► Complexity detection is reduced compared to other approaches. ► The average cell search time is within the range of 30–50 ms.
Introduction
In the recent years, mobile and wireless communications are evolving towards a common objective: providing broadband capabilities to mobile users. On the roadmap to the convergence of wireless networks that is expected for the 4th generation (4G) of mobile communications, a promising family of technologies is being developed by the 3rd Generation Partnership Project (3GPP): the 3GPP Long Term Evolution (LTE), and its evolution LTE-Advanced.
LTE technology [1], [2], [3], is targeted to provide 100 Mbps in the downlink (DL) using orthogonal frequency division multiple access (OFDMA), and 50 Mbps in the uplink (UL) using single-carrier frequency division multiple access (SC-FDMA), low delays in both the user and control planes, bandwidth scalability from 1.4 up to 20 MHz, and supported terminal mobility up to 350 km/h. LTE specification was frozen by the end of 2008, and the first commercial LTE devices have recently been presented at CES 2011.
The evolution of LTE, referred to as LTE-advanced (LTE-A), is expected to allow enhanced peak data rates up to 1 Gbps for advanced services and applications. In LTE-A, the system capacity is increased through different techniques such as carrier aggregation, coordinated transmission, relaying and advanced multi-antenna configurations [4].
In a cellular system, the first step for a mobile user equipment (UE) is to perform cell search procedure, prior to any UL initial transmission. In the case of LTE and LTE-A [1], there exist dedicated physical signals in the DL frame to facilitate this process: Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). The detection of PSS is crucial for the acquisition of initial radio frame timing. Once this step is managed, SSS identification is easily carried out, thus solving the radio timing for the DL transmission.
During the development of LTE specification, many authors tackled the design of synchronization signals (SS) from different points of view: In [5], it was illustrated the design of synchronization sequences for LTE DL with the aim of optimizing the cell search time. In this line, in [6] general aspects about synchronization channel design as well as the evaluation of different solutions for cell search were considered. Moreover, [7] proposed a cell search scheme base on frequency-domain sequence hopping of synchronization channel symbols. Some mechanisms for the improvement of detection performance in neighboring cell search were introduced in [8], [9].
Less references are available when considering the definitive SS structure, according to the final LTE specification: In [10], an overview of synchronization aspects and cell search in LTE is provided. Recently, [11] makes use of additional information apart from the synchronization signals (i.e. the cyclic prefix (CP) an cell-specific reference signals) to improve the cell search detection performance. However, it is assumed that the CP size is known by the UE and performance in terms of cell search time is neglected.
In this paper, a low complexity method for the cell search procedure in LTE downlink is presented. Two main contributions are proposed: First, we perform a blind detection of the CP size prior to the SS detection, which facilitates a coarse time and frequency synchronization of the received signal. Thus, the effect of inter symbolic interference (ISI) and inter carrier interference (ICI) is reduced. Secondly, the SS detection is efficiently performed in the frequency domain: only dot-products are required instead of matched filters for PSS detection, and 1-bit processing and two matched filters are used for SSS detection. This approach allows for a complexity reduction while achieving a good detection probability and cell search time.
The remainder of this paper is organized as follows: in Section 2, synchronization signals in 3GPP LTE DL frame are presented. Section 3 is dedicated to describe the proposed mechanism for performing the cell search procedure in LTE. An statistical analysis of the average cell search time is performed in Section 4. Finally, Section 5 presents the performance results in terms of cell search time and detection probability, whereas main conclusions are exposed in Section 6.
Section snippets
Synchronization signals in 3GPP-LTE DL
Fig. 1 shows the structure of the LTE DL frame, for the case of frequency domain duplex (FDD). The radio frame length is Tframe = 10 ms long, and consists of 10 subframes of length Tsubframe = 1 ms. A subframe consists of two consecutive time slots, each of 0.5 ms duration. One slot can be seen as a time-frequency resource grid, composed by a set of OFDM subcarriers along several OFDM symbol intervals. The number of OFDM symbols per slot may be seven or six, depending on the cyclic prefix length
Average search time analysis
In this section, an statistical analysis of the proposed algorithm in terms of average cell search time is provided. For this purpose, the cell search procedure is regarded as a Markovian process [22]; hence, the flow graph model can be used to determine the acquisition performance. In Fig. 6, a simplified state diagram of the cell search process is depicted [7], [18]. The captions on the branches between two nodes indicate the probability of each transition, multiplied by a power of the
Results
In this section, we evaluate the performance of the proposed scheme in terms of probability of incorrect cell search (denoted as PI), and cell search time (CST). According to the definitions in Section 4, the correct detection probability PD is the probability of joint successful detection of CP mode, and ; hence, the incorrect cell search probability PI will be given by
In the simulations, we consider a LTE system with BW = 20 MHz, type-1 frame with extended CP
Conclusions
We have presented a novel method to perform cell search procedure in 3GPP LTE and LTE-A mobile technologies. The inclusion of a CP detection stage in the time domain allows to reduce ICI and ISI in SS detection. Besides, PSS detection is performed in the frequency domain with a reduced complexity, avoiding the necessity of matched filters. Finally, SSS can be efficiently detected by means of 1-bit based processing, and only two matched filters are required. This method has been tested in
Acknowledgements
This work is partially supported by the Spanish Government under project TEC2010-18451 and by the company AT4 Wireless.
F. Javier Lopez-Martinez received the M.Sc. and Ph.D. degrees in Telecommunication Engineering from the University of Malaga (Spain), in 2005 and 2010, respectively. Since 2005, he has been in Communication Engineering Department at the University of Malaga as an associate researcher. His current research activity is focused on the performance analysis of wireless communication systems, and digital signal processing for wireless communications (WiMAX, LTE, LTE-A) with FPGAs.
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F. Javier Lopez-Martinez received the M.Sc. and Ph.D. degrees in Telecommunication Engineering from the University of Malaga (Spain), in 2005 and 2010, respectively. Since 2005, he has been in Communication Engineering Department at the University of Malaga as an associate researcher. His current research activity is focused on the performance analysis of wireless communication systems, and digital signal processing for wireless communications (WiMAX, LTE, LTE-A) with FPGAs.
Eduardo Martos-Naya received the M.Sc. and Ph.D. degrees in Telecommunication Engineering from the University of Malaga, Spain, in 1996 and 2005, respectively. In 1997, he joined the Department of Communication Engineering, University of Malaga, where he is currently an Associate Professor. His research interests include performance analysis of communication systems and digital signal processing.
Jose Tomas Entrambasaguas Muoz received the M.Sc. and Ph.D. degrees in Telecommunication Engineering from the Polytechnic University of Madrid (UPM), Spain, in 1975 and 1990, respectively. In 1993, he joined the University of Malaga where he is now a professor in the Department of Communication Engineering. His current research interests include digital signal processing for wireless communications and methodologies for developing and testing complex communications systems.
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Reviews processed and approved for publication by Editor-in-Chief Dr. Manu Malek.