Nasir et al. EURASIP Journal on Wireless Communications and Networking (2016) 2016:180 DOI 10.1186/s13638-016-0670-9 REVIEW Open Access Timing and carrier synchronization in wireless communication systems: a survey and classification of research in the last 5 years Ali A. Nasir1, Salman Durrani2*, Hani Mehrpouyan3,StevenD.Blostein4 and Rodney A. Kennedy2 Abstract Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions. Keywords: Timing synchronization, Carrier synchronization, Channel estimation, MIMO, OFDM 1 Introduction In order to fulfill the demand for higher data rates, Motivation: The Wireless World Research Forum a critical requirement is the development of accurate (WWRF) prediction of seven trillion wireless devices and realizable synchronization techniques to enable novel serving seven billion people by 2020 [1] sums up the communication paradigms. Such synchronization tech- tremendous challenge facing existing wireless cellular niques allow communication systems to deliver higher networks: intense consumer demand for faster data rates. data rates, e.g., through the use of higher-order mod- Major theoretical advances, such as the use of multiple ulations and utilization of cooperative communication antennas at the transmitter and receiver (multiple-input schemes. Hence, there has been considerable research multiple-output (MIMO)) [2, 3], orthogonal frequency- recently in synchronization techniques for novel commu- division multiple access (OFDMA) [4], and cooperative nication strategies. relaying [5–7] have helped meet some of this demand Aim: The aim of this paper is to provide a survey and and have been quickly incorporated into communication classification of the research in the field of synchroniza- standards. These technologies also form a core part of the tion for wireless communication systems that spans the next-generation cellular standards, 5G, which is under last 5 years (2010–2014). This is not an easy task given development [8, 9]. the large number of papers dealing with synchronization and its associated challenges in both current and emerg- ing wireless communication systems. The critical need *Correspondence: [email protected] for such a survey is highlighted by the fact that the last This work was supported in part by the Australian Research Council’s Discovery Project funding scheme (project number DP140101133). comprehensive survey paper on synchronization was pub- 2Research School of Engineering, Australian National University (ANU), 2601 lished nearly a decade ago [10]. While survey papers on Canberra, Australia synchronization for wireless standardization have recently Full list of author information is available at the end of the article appeared [11–13], these surveys do not overview the © 2016 Nasir et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Nasir et al. EURASIP Journal on Wireless Communications and Networking (2016) 2016:180 Page 2 of 38 state-of-the-art published research. For an overview of the wireless networks is toward tighter accuracies, e.g., tim- state-of-the-art in synchronization research prior to 2010, ing accuracy of 200 ns, to enable location-based services see the 2009 special issue on synchronization in wire- [12]. Hence, there is a need for new and more accu- less communications of the EURASIP Journal on Wireless rate timing and carrier estimators. In general, in order Communications and Networking [14]. to quantify the performance of any proposed estimator, a In this survey, we overview the relationships within the lower bound on the mean-square estimation error can be published research in terms of system model and assump- derived. The bounds are also helpful in designing efficient tions, synchronization challenges, proposed methods, training sequences. In addition, for multiple parameters and their limitations. We also highlight future research needed, say, for the joint estimation of timing and carrier directions and important open problems in the field frequency offsets, these bounds include coupling infor- of synchronization. The main intended audience of this mation between the estimation of these parameters. For survey paper are those interested in or already work- example, if the bound suggests very-low coupling between ing in synchronization. This survey paper aims to enable the estimation of timing and carrier frequency offsets, researchers to quickly immerse themselves in the current this implies that these parameters can be estimated sepa- state of the art in this field. Moreover, by highlighting rately without any significant loss in the estimation perfor- the important open research issues and challenges, we mance. In particular, there usually exists strong coupling believe the paper would stimulate further research in between channel and carrier frequency offset estimation this field. Since this paper is not intended to be a tuto- and their joint estimation is helpful to achieve improved rial on synchronization, we deliberately avoid presenting estimation accuracy [19, 20]. mathematical details and instead focus on the big picture. Although timing and carrier synchronization are neces- Background and scope: Synchronization is a common sary for successful communication, they cannot provide a phenomenon in nature, e.g., the synchronized flashing of common notion of time across distributed nodes. Clock fireflies or the synchronous firing of neurons in the human synchronization is the process of achieving and main- brain [15, 16]. In wireless communications, timing and taining coordination among independent local clocks to carrier synchronization are fundamental requirements provide a common notion of time across the network. [17]. In general, a wireless receiver does not have prior Some wireless networks, such as worldwide interoper- knowledge of the physical wireless channel or propagation ability for microwave access (WiMAX), are synchronized delay associated with the transmitted signal. Moreover, to to the global positioning system (GPS) [12]. Others, e.g., keep the cost of the devices low, communication receivers Bluetooth, wireless fidelity (Wi-Fi), and Zigbee rely on a use low-cost oscillators which inherently have some drift. beacon strategy, where all nodes in the network follow the In this context, timing synchronization is the process by same time reference given by a master node broadcast- which a receiver node determines the correct instants of ing a reference signal [12]. For recent surveys on clock time at which to sample the incoming signal and carrier synchronization, please see [21–25]. synchronization is the process by which a receiver adapts In the literature, timing and carrier synchronization the frequency and phase of its local carrier oscillator with techniques are sometimes considered in conjunction with those of the received signal. Particularly, depending on the radio frequency (RF) front-end impairments. RF impair- specific communication systems, synchronization defini- ments arise as a result of the intrinsic imperfections tion/procedure could be very different. According to 3rd in many different hardware components that comprise Generation Partnership Project, a terminology referred the RF transceiver front ends, e.g., amplifiers, convert- to as cell search has been widely used to represent an ers, mixers, filters, and oscillators. The three main types entire synchronization procedure, which may constitute of RF impairments are I/Q imbalance, oscillator phase both initial and target cell searches. Timing synchroniza- noise, and high-power amplifier (HPA) nonlinearities tion may consist of frame/slot/symbol/chip synchroniza- [26]. I/Q imbalance refers to the amplitude and phase tions, residual timing tracking, first arrival path search mismatch between the in-phase (I)andquadrature(Q) (in terms of OFDMA), multi-path search (in terms of signal branches, i.e., the mismatch between the real and CDMA), etc. Similarly, carrier synchronization may imply imaginary parts of the complex signal. Oscillator phase integer/fractional frequency